Mo-containing ductile cast iron for heavy-duty vehicle and preparation method thereof

Abstract

This invention relates to the field of heat treatment and preparation control technology for cast iron materials used in heavy-duty vehicles, particularly to the composite quenching control and preparation method for Mo-containing ductile iron used in heavy-duty vehicles. The method includes the following steps: S1, homogenization pretreatment of the billet; S2, placing the pretreated billet in a protective atmosphere furnace and performing austenitization using a segmented heating process; S3, after austenitization, gradient precooling of the billet, controlling the stepped cooling rate and the final precooling temperature; S4, performing a cooling phase transformation treatment on the precooled billet using a graded quenching process; S5, performing segmented tempering treatment on the billet after the quenching phase transformation; S6, performing low-temperature aging treatment on the tempered billet. This invention achieves precise and uniform control of the initial dislocation density of the cast billet through the synergy of a three-stage stepped heating-holding-controlled cooling process and quantitative control of dislocation density, avoiding surface oxidation and decarburization of the billet, and improving the machinability of the billet and the controllability of subsequent heat treatment.

Description

Technical Field

[0001] This invention relates to the field of heat treatment and preparation control technology for cast iron materials for heavy-duty vehicles, and in particular to the composite quenching control and preparation method of Mo-containing ductile iron for heavy-duty vehicles. Background Technology

[0002] Heavy-duty trucks are mostly used for long-distance trunk transportation, heavy-duty engineering and mountain road operations. Their key load-bearing braking components are subjected to complex service environments with high loads, strong road impacts, continuous braking temperature rise and dusty gravel roads for a long time. This places stringent requirements on the strength and toughness matching, high temperature stability and wear resistance and fatigue resistance of the materials.

[0003] Currently, the industry generally uses ductile iron with added molybdenum as the material for core components such as brake drums, wheel hubs, and axle housings, and improves the overall performance of the base material through composite quenching process.

[0004] Existing production and heat treatment processes mainly focus on controlling macroscopic parameters such as chemical composition ratio, quenching temperature, holding time, and cooling rate, with particular attention to graphite spheroidization effect, overall hardness level, and conventional mechanical properties.

[0005] However, in actual production and service, this type of technology still has obvious limitations:

[0006] On the one hand, existing control methods mostly focus on macroscopic structure and conventional performance indicators, and pay insufficient attention to the microscopic influencing factors such as the distribution of microscopic stress formed during solidification and phase transformation of cast iron, the micro-regional distribution of alloying elements at grain boundaries and phase interfaces, and the evolution law of phase transformation stress. The lack of systematic regulation of the microscopic state makes the material prone to stress concentration and weak interfacial bonding under heavy load and alternating impact.

[0007] On the other hand, due to the lack of effective control over the microstructure evolution and interface state, components are prone to uneven local performance, early fatigue cracking, and abnormal wear under long-term high-temperature braking and road abrasive action, making it difficult to stably adapt to the harsh and continuous high-intensity service conditions of heavy-duty vehicles.

[0008] Therefore, due to the lack of control over the microscopic mechanisms in existing technologies, the service reliability and lifespan stability of molybdenum-containing ductile iron components are insufficient, resulting in high failure rates and replacement costs.

[0009] Therefore, it is of great practical necessity to develop a method for controlling and preparing Mo-containing ductile iron composite quenching that can achieve precise control of microstructure and stress state in the harsh service environment of heavy-duty vehicles. Summary of the Invention

[0010] To solve one of the aforementioned technical problems, the present invention provides a method for controlling and preparing Mo-containing ductile iron composite quenching for heavy-duty vehicles, characterized by comprising the following steps:

[0011] S1. Homogenization pretreatment is carried out on the as-cast billet of Mo-containing ductile iron for heavy-duty vehicles. The initial dislocation distribution of the as-cast matrix is ​​adjusted by controlling the heating rate, holding time and cooling path, thereby eliminating residual stress and microstructure dispersion.

[0012] The clamping posture and support status of the billet are kept consistent throughout the process to avoid generating additional structural stress during the pretreatment process;

[0013] S2. Place the pretreated billet in a protective atmosphere heating furnace and complete the austenitization process using a segmented heating process. By controlling the heating rate and corresponding holding time of each heating segment, the micro-regional distribution of alloying elements at grain boundaries and phase interfaces is adjusted. The uniformity of the atmosphere in the furnace is controlled synchronously throughout the process to avoid local abnormal oxidation of alloying elements.

[0014] S3. After austenitization, the billet is subjected to gradient precooling. The step-down cooling rate and the final precooling temperature are controlled to adjust the austenite grain size and the stress state of the graphite-matrix interface. The pressure of the protective atmosphere in the furnace is kept stable throughout the precooling process.

[0015] Avoid decarburization and oxidation on the surface of the billet;

[0016] S4. The pre-cooled billet is subjected to cooling phase transformation treatment by a staged quenching process. The austenite phase transformation evolution rate and the stress relaxation rate inside the matrix are matched by controlling the medium temperature, residence time and cooling rate of each stage of staged cooling.

[0017] The flow rate of the cooling medium is kept stable throughout the cooling process to ensure uniform cooling of all parts of the billet.

[0018] S5. Perform segmented tempering on the billet that has completed the quenching phase transformation, control the heating rate, tempering temperature and holding time of each tempering segment, and adjust the distribution state of the matrix precipitates and the residual stress level.

[0019] Throughout the tempering process, the temperature uniformity deviation inside the furnace should not exceed ±5℃.

[0020] S6. Perform low-temperature aging treatment on the tempered billet, control the aging heating rate, aging temperature and holding time, stabilize the matrix structure and interface bonding state, and cool the billet to room temperature with the furnace after aging.

[0021] As a preferred option, the homogenization pretreatment in step S1 adopts a three-stage stepped heating-holding-controlled cooling process, specifically as follows:

[0022] Starting from room temperature, the first heating stage heats up to the first holding temperature at a constant linear rate. After holding at a constant temperature, the second heating stage begins. The second heating stage heats up to the second holding temperature at a rate lower than that of the first heating stage. After holding at a constant temperature, the third heating stage begins. The third heating stage heats up to the third holding temperature at a rate lower than that of the second heating stage. After holding at a constant temperature, the billet is cooled to room temperature with the furnace at a constant rate.

[0023] A high-purity inert protective atmosphere is introduced throughout the pretreatment process to control the oxygen content inside the furnace to remain stable;

[0024] By adjusting the heating rate, holding time, and cooling rate at each stage, the initial dislocation density distribution of the pretreated as-cast matrix satisfies the following equation:

[0025] In the formula:

[0026] The initial dislocation density of the matrix after pretreatment, in units of ;

[0027] This is the baseline value for the initial dislocation density of the as-cast billet, in units of... The values ​​are consistent with the general specifications for testing and controlling dislocation density in metallic materials;

[0028] The heating-holding coupling coefficient is selected in accordance with the industry standard for homogenization heat treatment of cast iron.

[0029] The heating rate of the m-th heating segment is expressed in K / s.

[0030] This represents the heat preservation time corresponding to the m-th heating segment, in seconds.

[0031] This is the regularization coefficient, and its value conforms to the control guidelines for the uniformity of microstructure in metallic materials.

[0032] L1 regularization term for the spatial distribution of dislocation density, used to control local discrete deviations in dislocation density.

[0033] As a preferred option, the inert protective atmosphere uses argon gas with a purity of not less than 99.99%, which is introduced in a symmetrical double-sided gas inlet manner. At least two sets of gas circulation equipment are configured in the furnace. The protective atmosphere is continuously introduced throughout the pretreatment process to control the oxygen content in the furnace to not exceed 50 ppm. The start and stop of the gas circulation equipment in the furnace are synchronized with the heating section to ensure the uniformity of the furnace atmosphere and the accuracy of oxygen content control in each heating section.

[0034] As a preferred option, step S2 is a segmented heating austenitizing treatment, which is divided into a low-temperature preheating section, a medium-temperature uniform heating section, and a high-temperature constant temperature uniform temperature section.

[0035] The temperature of the low-temperature preheating section is below the critical point of austenite phase transformation; the medium-temperature uniform heating section heats up to the target austenitization temperature at a constant linear rate; and the high-temperature isothermal homogenization section maintains a constant temperature at the target temperature. Throughout the process, the oxygen potential, sulfur potential, and hydrogen activity of the furnace atmosphere are kept stable. By adjusting the heating rate and holding time at each stage, the relative deviation of the Mo element concentration between grain boundaries and intragranular microregions is made to satisfy the following equation:

[0036] In the formula:

[0037] The relative deviation of Mo concentration between grain boundaries and intragranular microregions;

[0038] The average mass fraction of Mo in the grain boundary region;

[0039] The average mass fraction of Mo in the intracrystalline region;

[0040] The overall average mass fraction of Mo is taken, and the value conforms to the alloy composition design specifications for ductile iron used in heavy-duty vehicles.

[0041] This is the element diffusion coupling coefficient, and its value conforms to the grain boundary segregation control guidelines for alloying elements in metallic materials.

[0042] The absolute temperature of the austenitizing isothermal isothermal isothermal range, in K. The holding time for the austenitizing isothermal homogenization section is expressed in seconds (s).

[0043] The diffusion coefficient of Mo in an austenitic matrix is ​​given by . The value conforms to the general calculation standard for the diffusion coefficient of metallic materials;

[0044] This is the furnace gas sulfur potential correction coefficient, and its value conforms to the technical specifications for heat treatment atmosphere control of cast iron.

[0045] This represents the measured sulfur potential of the atmosphere inside the furnace.

[0046] This is the correction factor for hydrogen activity in furnace gas, and its value conforms to the general specifications for hydrogen embrittlement control of metallic materials.

[0047] This represents the measured value of hydrogen activity in the furnace atmosphere.

[0048] As a preferred option, step S3 gradient precooling adopts a three-stage stepped cooling process. Each stage of cooling is carried out at a constant linear rate to the corresponding precooling temperature and then held at a constant temperature. The final temperature of precooling is higher than the starting point of martensitic transformation. Step S4 graded quenching process adopts a dual-medium graded cooling mode. The first stage of cooling uses a salt bath medium with a temperature higher than the starting point of martensitic transformation. The billet is held at a constant temperature in the medium until the internal and external temperatures are uniform. The second stage of cooling uses an oil-based medium with a temperature lower than the termination point of martensitic transformation. The billet is cooled until the matrix transformation is completely completed.

[0049] By adjusting the cooling rate and isothermal dwell time of the gradient precooling, the stress field gradient in the micro-region around the graphite spheres is made to satisfy the following equation:

[0050] In the formula:

[0051] The gradient of the micro-region stress field around the graphite sphere is expressed in MPa / μm.

[0052] is the coefficient of thermal expansion of the austenitic matrix, with units of 1 / K, and its value conforms to the specifications for thermophysical property parameters of metallic materials;

[0053] This is the elastic modulus of the graphite phase, expressed in GPa, and its value conforms to the general specifications for the physical properties of cast iron materials.

[0054] This is the elastic modulus of the austenitic matrix, expressed in GPa, and its value conforms to the test specifications for the elastic modulus of metallic materials.

[0055] This represents the temperature drop during the nth precooling stage, expressed in K.

[0056] This is the regularization coefficient, and its value conforms to the guidelines for interfacial stress control technology of metallic materials.

[0057] This is the L2 regularization term for the interface stress distribution, used to control the discreteness of the interface stress concentration.

[0058] This is the wall thickness difference correction factor, and its value conforms to the heat treatment specifications for thick-walled cast iron parts for heavy-duty vehicles.

[0059] This represents the difference in wall thickness between the measured area and the reference area of ​​the billet, in mm.

[0060] As a preferred option, step S6, low-temperature aging treatment, is carried out after the tempered billet is air-cooled to room temperature. The aging temperature is lower than the second tempering temperature, and the aging holding time is not less than 1.5 times the total tempering holding time. After aging, the billet is cooled to room temperature with the furnace. The aging heating rate is controlled at 3-8℃ / min, which forms a gradient match with the tempering heating rate in step S5.

[0061] As a preferred option, the graphite spheroidization rate of the as-cast Mo-containing ductile iron billet is not less than 85%, the pearlite content in the as-cast matrix is ​​not less than 70%, the mass fraction of Mo is 0.2%-0.8%, the billet surface is free of cracks, pores, shrinkage defects, and the wall thickness difference of the billet is not greater than 30% of the maximum wall thickness, which matches the process parameters of gradient precooling in step S3 and graded quenching in step S4.

[0062] As a preferred embodiment, in steps S1 to S6, the billet is clamped with multi-point symmetrical support. The number and position of the support points are determined according to the wall thickness distribution and structure of the billet. The deformation of the billet during the heat treatment process is controlled to be no more than 0.05 mm / m. High-temperature resistant buffer pads are set at the support points to avoid local stress concentration of the billet during the heating and cooling process.

[0063] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0064] 1. This invention effectively solves the industry problem of large dispersion of the as-cast microstructure of Mo-containing ductile iron for heavy-duty vehicles. Through the synergistic effect of the three-stage stepped heating-holding-controlled cooling process in the homogenization pretreatment stage and the dislocation density quantitative control formula, the initial dislocation density of the as-cast billet is precisely and uniformly controlled. At the same time, combined with the stable control of the inert protective atmosphere throughout the process, the oxidation and decarburization of the billet surface are avoided, ensuring the consistency of the matrix microstructure. This lays a uniform microstructure foundation for the subsequent full-process heat treatment, significantly reducing the microstructure difference of the same batch of billets, improving the stability of batch production from the source, and solving the defect of existing technologies that homogenization pretreatment can only relieve stress and cannot accurately control the microstructure. This significantly improves the machinability of the billet and the controllability of subsequent heat treatment.

[0065] 2. This invention achieves synergistic closed-loop control of microstructure and stress throughout the entire process of Mo-containing ductile iron. By quantitatively controlling the concentration deviation of Mo element at grain boundaries and within micro-regions during the austenitization stage, combined with stress matching design of gradient precooling and staged quenching, stress concentration and microcrack initiation at the graphite spheroid-matrix interface are effectively suppressed. At the same time, through the synergistic effect of segmented tempering and low-temperature aging, residual stress is completely eliminated and the uniform dispersion of Mo-enhanced precipitates is achieved. This solves the problems of easy cracking during quenching and poor matching of strength and toughness during tempering in the prior art, enabling the prepared cast iron parts to have high strength, high toughness and high impact resistance, perfectly adapting to the heavy-load and high-impact service conditions of heavy-duty vehicles, and significantly reducing the risk of early fatigue failure of the parts.

[0066] 3. This invention innovatively leverages the secondary hardening effect of Mo. By precisely matching the segmented tempering process with the precipitate volume fraction control formula, combined with the uniform control of Mo micro-area concentration, it achieves precise control of Mo-enhanced precipitates. This avoids the increased material brittleness caused by local agglomeration of precipitates, and the uniformly dispersed precipitates suppress the chemical adsorption effect between Mo and SiO2 abrasive particles on the road surface, significantly improving the wear resistance of cast iron components. At the same time, the uniformly distributed high-temperature stable precipitates effectively prevent the matrix from softening at high temperatures, significantly improving the high-temperature strength and thermal stability of the material. This solves the problem of easy thermal deformation and thermal degradation of components under high-temperature conditions during long downhill braking of heavy-duty vehicles, extending the service life of the components.

[0067] 4. This invention significantly improves the dimensional accuracy and batch production qualification rate of Mo-containing ductile iron components for heavy-duty vehicles through full-process process coordination and precise control. By strictly controlling the multi-dimensional quality indicators of the cast billet and combining it with a multi-point symmetrical support clamping design throughout the process, the deformation during heat treatment is controlled within 0.05mm / m, avoiding deformation and cracking caused by the superposition of clamping stress and heat treatment stress. At the same time, the coordinated matching of process parameters in each process reduces the impact of process fluctuations on product performance, significantly reduces the performance dispersion of components in the same batch, reduces the scrap rate, lowers the cost of industrial production, and improves the assembly accuracy of components, ensuring the driving safety and operational stability of heavy-duty vehicles.

[0068] 5. This invention has significant industrial application value and economic benefits. Its process design does not require additional customized testing and production equipment. The three-stage stepped heating, dual-medium graded quenching, and two consecutive tempering processes can all be achieved based on conventional heat treatment equipment. The control of each process parameter has clear quantitative formulas and industry standard basis. It is easy to operate, highly repeatable, and convenient for large-scale mass production.

[0069] Meanwhile, the Mo-containing ductile iron components prepared by this invention have comprehensive performance far exceeding that of products prepared by existing conventional processing processes. They can meet the high reliability requirements of the core load-bearing and braking components of heavy-duty vehicles, reduce the frequency of component replacement, lower the operation and maintenance costs of heavy-duty vehicles, and improve the service safety and service life of heavy-duty vehicles, thus having broad prospects for promotion and application. Attached Figure Description

[0070] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or components are generally identified by similar reference numerals. In the drawings, the elements or components are not necessarily drawn to scale.

[0071] Figure 1 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0072] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and are therefore merely examples and should not be used to limit the scope of protection of the present invention. The specific process of the present invention is as follows: Figure 1 As shown in the image.

[0073] A method for controlling and preparing Mo-containing ductile iron composite quenching for heavy-duty vehicles, comprising the following steps:

[0074] S1. Homogenization pretreatment is carried out on the as-cast billet of Mo-containing ductile iron for heavy-duty vehicles. The initial dislocation distribution of the as-cast matrix is ​​adjusted by controlling the heating rate, holding time and cooling path, thereby eliminating residual stress and microstructure dispersion.

[0075] The clamping posture and support status of the billet are kept consistent throughout the process to avoid generating additional structural stress during the pretreatment process;

[0076] S2. Place the pretreated billet in a protective atmosphere heating furnace and complete the austenitization process using a segmented heating process. By controlling the heating rate and corresponding holding time of each heating segment, the micro-regional distribution of alloying elements at grain boundaries and phase interfaces is adjusted. The uniformity of the atmosphere in the furnace is controlled synchronously throughout the process to avoid local abnormal oxidation of alloying elements.

[0077] S3. After austenitization, the billet is subjected to gradient precooling. The step-down cooling rate and the final precooling temperature are controlled to adjust the austenite grain size and the stress state of the graphite-matrix interface. The pressure of the protective atmosphere in the furnace is kept stable throughout the precooling process.

[0078] Avoid decarburization and oxidation on the surface of the billet;

[0079] S4. The pre-cooled billet is subjected to cooling phase transformation treatment by a staged quenching process. The austenite phase transformation evolution rate and the stress relaxation rate inside the matrix are matched by controlling the medium temperature, residence time and cooling rate of each stage of staged cooling.

[0080] The flow rate of the cooling medium is kept stable throughout the cooling process to ensure uniform cooling of all parts of the billet.

[0081] S5. Perform segmented tempering on the billet that has completed the quenching phase transformation, control the heating rate, tempering temperature and holding time of each tempering segment, and adjust the distribution state of the matrix precipitates and the residual stress level.

[0082] Throughout the tempering process, the temperature uniformity deviation inside the furnace should not exceed ±5℃.

[0083] S6. Perform low-temperature aging treatment on the tempered billet, control the aging heating rate, aging temperature and holding time, stabilize the matrix structure and interface bonding state, and cool the billet to room temperature with the furnace after aging to avoid the generation of secondary residual stress by rapid cooling.

[0084] Firstly, regarding the homogenization pretreatment in step S1, the heating rate, holding time, and cooling path in this scheme are all determined based on the conventional wall thickness range and as-cast microstructure characteristics of the as-cast Mo-containing ductile iron billet for heavy-duty vehicles. Specifically, the consistency control of the clamping posture and support state refers to the use of a horizontally placed multi-point symmetrical support method on the heat treatment rack. The position of the support points does not shift throughout the pretreatment process, avoiding the transfer of bending stress generated by the billet's own weight to the interior of the matrix during heating and cooling, thus preventing the formation of additional structural stress. Those skilled in the art can determine the specific number and position of support points according to the specific structural dimensions of the billet and with reference to the general specifications for clamping heat treatment of cast iron parts for heavy-duty vehicles.

[0085] The segmented heating austenitizing treatment in step S2 is specifically divided into three continuous stages: low-temperature preheating, medium-temperature heating, and high-temperature homogenization. The heating rate and holding time of each stage can be adapted and adjusted according to the wall thickness and Mo content of the billet. The uniformity of the atmosphere in the furnace is controlled throughout the process by using a constant speed of the circulating fan in the furnace and a symmetrical air intake method on both sides to avoid the formation of dead zones in the atmosphere in the furnace, which would lead to local abnormal oxidation of alloying elements. The uniformity of the atmosphere is monitored in real time.

[0086] The gradient precooling in step S3, the step-like cooling rate and the precooling end temperature are determined based on the critical point of austenitic phase transformation and the starting point of martensitic phase transformation of Mo-containing ductile iron. The precooling end temperature must always be higher than the martensitic phase transformation start point to avoid unexpected martensitic phase transformation during the precooling process. The stable protective atmosphere pressure throughout the process means that the pressure inside the furnace is maintained in a slightly positive pressure state to prevent external air from penetrating and causing decarburization and oxidation on the billet surface.

[0087] In the dual-medium staged cooling mode of the staged quenching process, the temperature and residence time of the first-stage salt bath medium are determined based on the requirement of uniform temperature inside and outside the billet, ensuring that the temperature difference between the core and the surface of the billet is controlled within a reasonable range. The cooling rate of the second-stage oil-based medium is controlled by adjusting the flow rate of the medium to ensure the matching of the austenite phase transformation evolution rate and the stress relaxation rate inside the matrix, avoiding excessive internal stress during the phase transformation process that could lead to the initiation of microcracks.

[0088] The segmented tempering in step S5 is specifically a two-stage continuous tempering process. The temperature and holding time of the two temperings are determined based on the secondary hardening characteristics of Mo. The temperature uniformity deviation in the furnace throughout the process does not exceed ±5℃. This is achieved through the multi-zone temperature control system of the furnace body, ensuring that the billet in each position in the furnace is heated uniformly and the distribution of precipitated phases is consistent.

[0089] The aging temperature of the low-temperature aging treatment is lower than the second tempering temperature to avoid abnormal growth of precipitates during the aging process. The furnace cooling after aging means that the billet is cooled to room temperature naturally in the aging furnace without forced cooling to avoid secondary residual stress during rapid cooling, which would affect the stability of the matrix structure.

[0090] This technical solution addresses the special service conditions of Mo-containing ductile iron components for heavy-duty vehicles under heavy loads, strong impacts, high-temperature braking, and multi-abrasive environments. First, the homogenization pretreatment in step S1 is used to adjust the initial dislocation distribution of the as-cast matrix, eliminating residual stress and microstructure dispersion in the as-cast state. This provides a matrix with a uniform microstructure for subsequent austenitization treatment, avoiding abnormalities in the subsequent phase transformation process caused by the dispersion of the as-cast microstructure. This control step, together with the austenitization grain control in step S2 and the interface stress control in step S3, forms a pre-treatment synergistic effect.

[0091] Secondly, the segmented heating austenitization treatment in step S2, by adjusting the micro-regional distribution of alloying elements at grain boundaries and phase interfaces, provides a uniform austenitic matrix for subsequent phase transformation processes and precipitate control. This step forms a direct synergistic relationship with the segmented tempering precipitate control in step S5. The uniformity of alloying element distribution at grain boundaries and within grains directly determines the degree of dispersion and uniformity of precipitates during tempering.

[0092] Furthermore, the gradient precooling in step S3 and the staged quenching process in step S4 form a synergistic control of stress and microstructure during the phase transformation process. Gradient precooling adjusts the austenite grain size and the stress state at the graphite-matrix interface, providing a stable nucleation basis for the subsequent staged quenching phase transformation. The matching of the austenite phase transformation evolution rate and the matrix stress relaxation rate during staged quenching directly solves the problems of stress concentration and microcrack initiation that are prone to occur during the quenching process of thick-walled cast iron parts in the prior art. The synergistic effect of these two steps, together with the initial microstructure control in step S1 and the residual stress elimination in step S5, forms a closed-loop stress control throughout the entire process.

[0093] Finally, the segmented tempering treatment in step S5 and the low-temperature aging treatment in step S6 form a stable control of the microstructure and properties. The segmented tempering adjusts the distribution of the matrix precipitates and the residual stress level, while the low-temperature aging further stabilizes the matrix microstructure and the interface bonding state. These two steps, together with all the previous steps, form the final performance realization, ensuring the material's strength and toughness matching, high-temperature stability, and wear resistance and fatigue resistance under the special service conditions of heavy-duty vehicles.

[0094] By controlling the initial dislocation distribution in step S1, the uniformity of nucleation sites during austenitization is controlled. By controlling the micro-area distribution of alloying elements, the compositional uniformity of the austenitic matrix is ​​controlled. By controlling the phase transformation process, the microstructure uniformity of the quenched matrix is ​​controlled. By controlling the precipitated phases in steps S5 and S6, the performance stability of the final matrix is ​​controlled. This ensures that the dispersion of material properties during mass production is controlled within a very small range, significantly improving product consistency and reliability. For the high-temperature conditions of long downhill braking, the high-temperature strength and thermal stability of the material are ensured by controlling the micro-area distribution of Mo and the precipitated phases. For the abrasive wear conditions of gravel roads, the chemical adsorption effect of Mo and SiO2 abrasive particles is suppressed by the control of uniformly dispersed precipitated phases, improving the wear resistance of the material. This achieves a one-to-one precise adaptation of material properties to service conditions, rather than the generalized heat treatment process in existing technologies.

[0095] In addition, this solution significantly improves the feasibility and stability of heat treatment processes, and solves the problems of low process parameter tolerance and high difficulty in mass production in existing technologies. All process steps in this solution can be achieved using conventional heat treatment equipment in this field.

[0096] As a preferred option, the homogenization pretreatment in step S1 adopts a three-stage stepped heating-holding-controlled cooling process, specifically as follows:

[0097] Starting from room temperature, the first heating stage heats up to the first holding temperature at a constant linear rate. After holding at a constant temperature, the second heating stage begins. The second heating stage heats up to the second holding temperature at a rate lower than that of the first heating stage. After holding at a constant temperature, the third heating stage begins. The third heating stage heats up to the third holding temperature at a rate lower than that of the second heating stage. After holding at a constant temperature, the billet is cooled to room temperature with the furnace at a constant rate.

[0098] A high-purity inert protective atmosphere is introduced throughout the pretreatment process to control the oxygen content inside the furnace to remain stable;

[0099] By adjusting the heating rate, holding time, and cooling rate at each stage, the initial dislocation density distribution of the pretreated as-cast matrix satisfies the following equation:

[0100] In the formula:

[0101] The initial dislocation density of the matrix after pretreatment, in units of ;

[0102] This is the baseline value for the initial dislocation density of the as-cast billet, in units of... The values ​​are consistent with the general specifications for testing and controlling dislocation density in metallic materials;

[0103] The heating-holding coupling coefficient is selected in accordance with the industry standard for homogenization heat treatment of cast iron.

[0104] The heating rate of the m-th heating segment is expressed in K / s.

[0105] This represents the heat preservation time corresponding to the m-th heating segment, in seconds.

[0106] This is the regularization coefficient, and its value conforms to the control guidelines for the uniformity of microstructure in metallic materials.

[0107] L1 regularization term for the spatial distribution of dislocation density, used to control local discrete deviations in dislocation density.

[0108] The result of the dislocation density control is directly used as the nucleation basis input for the austenitization treatment in step S2, and it works synergistically with the austenite grain homogenization control to avoid local structural abnormalities in the subsequent phase transformation process and ensure the consistency of the structure of the entire heat treatment process.

[0109] Regarding the implementation details of the three-stage stepped heating-holding-controlled cooling process, the three heating stages of this process are continuously connected. The end time of the holding of the previous heating stage is the start time of the heating of the next heating stage. There is no intermediate cooling or air cooling process, which ensures the continuity of matrix tissue evolution during the pretreatment process. The first heating stage has the highest heating rate and the lowest holding temperature. Its main function is to eliminate macroscopic residual stress in the cast billet and prevent billet cracking caused by rapid heating. The second heating stage has a lower heating rate than the first heating stage and a higher holding temperature. Its main function is to achieve preliminary homogenization of the matrix structure and adjust the overall level of dislocation density. The third heating stage has the lowest heating rate and the highest holding temperature. Its main function is to achieve precise control of dislocation density and full homogenization of the structure. The temperature and rate settings of the three heating stages are determined based on the phase transformation critical point and microstructure evolution law of Mo-containing ductile iron. Those skilled in the art can determine the specific temperature and rate parameters according to the specific wall thickness and as-cast microstructure of the billet, referring to the industry specifications for homogenization heat treatment of cast iron.

[0110] The pretreatment process is characterized by an inert protective atmosphere and controlled oxygen content. The high-purity inert protective atmosphere is argon gas with a purity of not less than 99.99%, which is continuously introduced through a double-sided symmetrical gas inlet method. The oxygen content in the furnace is monitored in real time by an online oxygen analyzer and controlled within a stable low-oxygen range to avoid oxidation and decarburization of the billet surface during the pretreatment process and ensure the stability of the matrix structure.

[0111] The heating-holding coupling coefficient β in the dislocation density control formula is determined based on the correspondence between heating rate, holding time, and dislocation density change in the industry standard for homogenization heat treatment of cast iron. Its value ranges from 1×10⁻⁶. 12 Up to 5×10 12 The specific value of β can be adjusted according to the Mo content and as-cast microstructure of the billet. The higher the Mo content, the larger the value of β, ensuring the accuracy of dislocation density control. The regularization coefficient γ is determined based on the microstructure uniformity control guidelines of metallic materials, and its value ranges from 1×10⁻⁶. 10 Up to 3×10 10 The specific value is determined based on the wall thickness difference of the billet. The larger the wall thickness difference, the larger the value of γ, which is used to suppress the local dispersion deviation of dislocation density.

[0112] The corresponding control model for the formula is a hierarchical single-input single-output calculation model, which is divided into four levels. The first level is the basic parameter input layer, and the input data includes the initial dislocation density reference value of the cast billet. Heating rates of the three heating stages Corresponding heat preservation time The first layer consists of a coupling coefficient β and a regularization coefficient γ that conform to industry standards. All input parameters can be directly obtained through conventional dislocation density detection methods and heat treatment equipment parameter control methods in this field, without the need for additional customized detection equipment. The second layer is the coupling calculation layer, which performs coupling calculations on the heating-holding process parameters and the change in dislocation density based on the input heating rate and holding time, obtaining the influence value of the process parameters on the dislocation density. The third layer is the regularization constraint layer, which constrains the spatial distribution discrepancy of dislocation density through the L1 regularization term. The L1 regularization algorithm is a well-known numerical smoothing algorithm in this field, used to suppress local abnormal fluctuations in dislocation density and ensure the uniformity of the matrix structure. The fourth layer is the result output layer, which outputs the target initial dislocation density of the preprocessed matrix. Those skilled in the art can adjust the process parameters of homogenization pretreatment based on the output target value to achieve precise control of dislocation density.

[0113] The dislocation density control result is directly used as the nucleation basis input for the austenitization treatment in step S2, and forms a synergistic effect with the austenite grain homogenization control: during the austenitization process, the nucleation of the new phase occurs preferentially at the dislocations. The uniform distribution of the initial dislocation density ensures the uniform distribution of austenite nucleation sites, thereby ensuring the uniformity of austenite grain size, avoiding local structural anomalies in the subsequent phase transformation process, and ensuring the consistency of the structure of the entire heat treatment process.

[0114] This technical solution incorporates precise control of dislocation density into the control objective of homogenization pretreatment, forming a synergistic control logic with subsequent austenitization treatment. The rapid heating and low-temperature holding in the first heating stage are primarily used to eliminate macroscopic residual stress in the as-cast state and prevent abnormal dislocation annihilation. The medium-speed heating and medium-temperature holding in the second heating stage are primarily used to adjust the overall level of dislocation density, achieving uniform dislocation rearrangement. The slow heating and high-temperature holding in the third heating stage are primarily used to achieve precise control of dislocation density, ensuring the uniformity of dislocation distribution. The process parameters of the three heating stages directly correspond to the dislocation density control formula. and The parameters, process features, and algorithm features form a one-to-one support relationship. The algorithm features provide a quantitative control basis for adjusting the process parameters, while the process features provide an implementable process path for implementing the algorithm features. Together, they achieve precise control of dislocation density.

[0115] Secondly, the design of the dislocation density control formula works closely with the grain homogenization control of the subsequent austenitization treatment. This scheme does not control the dislocation density for the sake of controlling the dislocation density, but directly uses the control result of the dislocation density as the nucleation basis input for the subsequent austenitization treatment. By ensuring the uniform distribution of the initial dislocation density, the uniform distribution of austenite nucleation sites is guaranteed, thereby achieving the homogenization control of austenite grains. This avoids the problem of uneven austenite grain size caused by the discreteness of the as-cast structure in the prior art, which leads to the discreteness of the final structure and properties.

[0116] Furthermore, the use of inert protective gas and oxygen content control throughout the pretreatment process, along with the regulation of dislocation density, creates a synergistic protective effect. The stable low-oxygen atmosphere prevents oxidation and decarburization on the billet surface, ensuring consistency between the microstructure of the matrix surface and the core. This, in turn, ensures a uniform distribution of dislocation density throughout the billet, avoiding subsequent abnormal phase transformations caused by differences in microstructure between the surface and the core.

[0117] This solution incorporates the quantitative control of dislocation density into the goal of homogenization pretreatment. Through the mutual support of process features and algorithm features, it achieves coordinated control with subsequent heat treatment steps, solving the problem of unstable final performance caused by the discreteness of as-cast structure in the existing technology. This improves the stability of mass production of Mo-containing ductile iron parts for heavy-duty vehicles.

[0118] This solution achieves synergistic control between homogenization pretreatment and subsequent austenitization, solving the problem in existing technologies where pretreatment and subsequent heat treatment steps are disconnected, making it impossible to achieve full-process microstructure control. This solution fully utilizes the nucleation role of dislocations during the austenite phase transformation process. By ensuring a uniform distribution of initial dislocation density, it guarantees a uniform distribution of austenite nucleation sites, thereby achieving homogenization control of austenite grain size. The effect of pretreatment is directly transferred to the subsequent austenitization process, forming synergistic control between the preceding and following steps, rather than the existing technology where pretreatment exists only as an independent stress-relief step.

[0119] Mo-containing ductile iron billets for heavy-duty vehicles inevitably exhibit microstructural dispersion due to significant differences in wall thickness and uneven cooling rates during casting. This dispersion includes local variations in dislocation density and uneven microstructure distribution. Existing homogenization processes cannot eliminate this microstructural dispersion, leading to substantial differences in microstructure and properties within the same batch of billets during subsequent heat treatment. However, by using L1 regularization to constrain the local dispersion deviation of dislocation density and employing a three-stage step process to achieve uniform dislocation rearrangement, the microstructural dispersion of the as-cast microstructure is eliminated. This ensures a consistent initial microstructure within the same batch of billets, thereby guaranteeing consistent final heat treatment performance and significantly reducing the defect rate in mass production.

[0120] This solution effectively reduces the risk of microcrack initiation during subsequent heat treatment, improves the impact resistance and fatigue life of heavy-duty vehicle components. Abnormal dislocation density in the as-cast matrix can lead to asynchronous phase transformation during subsequent austenitic transformation, resulting in localized stress concentration and becoming the source of microcrack initiation. By homogenizing and controlling the dislocation density, the synchronicity of the phase transformation process is ensured, avoiding localized stress concentration and reducing the risk of microcrack initiation. This, in turn, improves the material's crack resistance and fatigue life under heavy-load and high-impact conditions in heavy-duty vehicles, demonstrating significant practical application value.

[0121] As a preferred option, the inert protective atmosphere uses argon gas with a purity of not less than 99.99%, which is introduced in a symmetrical double-sided gas inlet manner. At least two sets of gas circulation equipment are configured in the furnace. The protective atmosphere is continuously introduced throughout the pretreatment process, and the oxygen content in the furnace is controlled to be no higher than 50 ppm. The start and stop of the gas circulation equipment in the furnace are synchronized with the heating section to ensure the uniformity of the furnace atmosphere and the accuracy of oxygen content control in each heating section, forming a full-process synergy with the austenitizing atmosphere control in step S2.

[0122] The argon gas used in this solution has a purity of not less than 99.99%, which is a commonly used high-purity inert protective gas source in the heat treatment process in this field. Those skilled in the art can obtain it through conventional industrial high-purity argon gas supply systems. The purity of the gas source can be confirmed by the quality inspection report attached to the gas cylinder, or it can be tested on-site by conventional gas purity analyzers in this field to ensure that the gas source meets the requirements.

[0123] In this scheme, the double-sided symmetrical air intake means that air intake ports are set at symmetrical positions on both sides of the furnace chamber of the heat treatment furnace. The air intake flow rate of the two air intake ports is consistent to avoid uneven atmosphere distribution in the furnace caused by single-sided air intake. The height of the air intake port is matched with the placement height of the billet in the furnace chamber to ensure that the introduced argon gas can directly cover the area where the billet is located, rather than just flowing at the top or bottom of the furnace chamber, thereby improving the atmosphere protection effect.

[0124] This solution includes at least two sets of gas circulation equipment, specifically high-temperature resistant circulating fans installed inside the furnace. The two sets of fans are symmetrically arranged at both ends of the furnace, and the start and stop of the fans are synchronized with the heating stage. Specifically, when the first heating stage starts, both sets of circulating fans start at low speeds simultaneously. As the heating stage progresses, the fan speed increases synchronously. During the heat preservation stage of the third heating stage, the fans maintain operation at their maximum rated speed, ensuring that the atmosphere inside the furnace is always in a state of circulation throughout the entire pretreatment process, avoiding local atmosphere dead zones, and thus ensuring the uniformity of the atmosphere and the accuracy of oxygen content control at all locations inside the furnace. Regarding the control of oxygen content in the furnace, this scheme requires that the oxygen content in the furnace be controlled to be no higher than 50 ppm throughout the pretreatment process. Specifically, this means that from the room temperature heating stage at the beginning of the pretreatment, argon gas is continuously introduced for atmosphere replacement. Once the oxygen content in the furnace drops below 50 ppm, the heating program is restarted. Throughout the pretreatment process, the oxygen content in the furnace is monitored in real time by an online oxygen analyzer. By adjusting the argon gas inlet flow rate, the oxygen content is maintained to be no higher than 50 ppm to avoid oxidation and decarburization of the billet surface during the pretreatment process.

[0125] The atmosphere control method in the pretreatment stage is consistent with that in the subsequent austenitizing stage, ensuring that the billet remains in a uniform and stable low-oxygen protective atmosphere throughout the entire process from pretreatment to austenitizing. This avoids changes in the billet surface state caused by atmospheric changes between the two stages, thus ensuring the consistency of the entire process's microstructure control. The combination of the symmetrical double-sided air intake and multiple sets of gas circulation equipment provides a double guarantee for the uniformity of the furnace atmosphere. The symmetrical double-sided air intake ensures the initial uniformity of the air intake, while the synchronously operating gas circulation equipment ensures the continuous circulation of the atmosphere within the furnace, avoiding the occurrence of localized atmospheric dead zones. The two work together to achieve uniformity of the atmosphere at all locations within the furnace, solving the problem of uneven atmosphere distribution within the furnace caused by single-sided air intake and lack of forced circulation in existing technologies, which leads to localized oxidation and decarburization of the billet.

[0126] The design of synchronizing the start-up and shutdown of the gas circulation equipment with the heating section provides direct functional support for the three-stage stepped heating-holding process. The heating rates and holding temperatures of the three heating sections are different, and the surface oxidation activity of the billet is also different. The faster the heating rate and the higher the temperature, the stronger the surface oxidation activity of the billet, and the higher the corresponding atmosphere circulation requirement. By synchronizing the fan speed with the heating section, a low-speed circulation is used in the first heating section with low temperature and low oxidation activity, and a high-speed circulation is used in the third heating section with high temperature and high oxidation activity. This ensures the uniformity of the atmosphere and the accuracy of oxygen content control in each heating section, while avoiding energy waste caused by the fan running at high speed throughout the process.

[0127] The atmosphere control method and oxygen content control index of the pretreatment stage in this scheme are consistent with the atmosphere control requirements of the subsequent austenitizing treatment stage. This ensures that the billet is always in the same low-oxygen protective atmosphere environment throughout the entire process from pretreatment to austenitizing treatment, avoiding changes in the surface state of the billet caused by abrupt changes in the atmosphere environment between the two stages, and thus ensuring the continuity and uniformity of the matrix evolution during the austenitizing process.

[0128] This solution achieves dynamic and precise control of atmosphere uniformity during the stepped heating process, solving the problems of excessive atmosphere circulation in the low-temperature section and insufficient atmosphere circulation in the high-temperature section caused by the fixed speed circulation in the existing technology. Based on the difference in oxidation activity of the billet at different temperatures, this solution synchronizes the speed of the gas circulation equipment with the heating section. Low-speed circulation is used in the low-temperature section with low oxidation activity, and high-speed circulation is used in the high-temperature section with high oxidation activity. This achieves dynamic adaptation of atmosphere circulation intensity and heating process, ensuring atmosphere uniformity and oxygen content control accuracy in each heating section, while avoiding unnecessary energy consumption. Compared with the fixed speed circulation control method in the existing technology, it has higher control accuracy and economy.

[0129] The pretreatment atmosphere control method and oxygen content control index of this scheme are consistent with the atmosphere control requirements of the subsequent austenitizing treatment stage. This ensures that the billet is always in the same low-oxygen protective atmosphere environment throughout the entire heat treatment process, avoiding abrupt changes in the atmosphere environment between different stages. This, in turn, ensures the stability of the billet surface state and avoids the impact of surface state changes on the subsequent austenitizing phase transformation process, thus achieving synergy in atmosphere control throughout the entire process.

[0130] Mo-containing ductile iron components for heavy-duty vehicles are mostly thick-walled, irregularly shaped structures. During pretreatment, billets in different locations within the furnace, and even different parts of the same billet, are prone to localized oxidation and decarburization due to uneven atmosphere distribution. This leads to differences in the microstructure between the surface and the core, thus affecting the final heat treatment performance. However, by using symmetrical air intake on both sides and multiple sets of synchronously matched circulation devices, the uniformity of the atmosphere in all locations within the furnace is ensured, and the oxygen content is consistently maintained below 50 ppm. This avoids localized oxidation and decarburization, ensuring a consistent microstructure between the billet surface and the core, thereby improving the uniformity of the final product's performance.

[0131] The dual-sided symmetrical air intake and synchronous circulation control design of this scheme significantly reduces the impact of the amount and position of the billet charge in the furnace on the uniformity of the atmosphere. Even in the case of full-furnace batch production, it can ensure the uniformity of the atmosphere and the accuracy of oxygen content control in various positions in the furnace, thereby ensuring the consistent protection effect of different billets in the same batch, greatly improving the stability of batch production and the product qualification rate, and has significant industrial application value.

[0132] As a preferred option, step S2 is a segmented heating austenitizing treatment, which is divided into a low-temperature preheating section, a medium-temperature uniform heating section, and a high-temperature constant temperature uniform temperature section.

[0133] The temperature of the low-temperature preheating section is below the critical point of austenite phase transformation; the medium-temperature uniform heating section heats up to the target austenitization temperature at a constant linear rate; and the high-temperature isothermal homogenization section maintains a constant temperature at the target temperature. Throughout the process, the oxygen potential, sulfur potential, and hydrogen activity of the furnace atmosphere are kept stable. By adjusting the heating rate and holding time at each stage, the relative deviation of the Mo element concentration between grain boundaries and intragranular microregions is made to satisfy the following equation:

[0134] In the formula:

[0135] The relative deviation of Mo concentration between grain boundaries and intragranular microregions;

[0136] The average mass fraction of Mo in the grain boundary region;

[0137] The average mass fraction of Mo in the intracrystalline region;

[0138] The overall average mass fraction of Mo is taken, and the value conforms to the alloy composition design specifications for ductile iron used in heavy-duty vehicles.

[0139] This is the element diffusion coupling coefficient, and its value conforms to the grain boundary segregation control guidelines for alloying elements in metallic materials.

[0140] The absolute temperature of the austenitizing isothermal isothermal isothermal range, in K. The holding time for the austenitizing isothermal homogenization section is expressed in seconds (s).

[0141] The diffusion coefficient of Mo in an austenitic matrix is ​​given by . The value conforms to the general calculation standard for the diffusion coefficient of metallic materials;

[0142] This is the furnace gas sulfur potential correction coefficient, and its value conforms to the technical specifications for heat treatment atmosphere control of cast iron.

[0143] This represents the measured sulfur potential of the atmosphere inside the furnace.

[0144] This is the correction factor for hydrogen activity in furnace gas, and its value conforms to the general specifications for hydrogen embrittlement control of metallic materials.

[0145] This represents the measured value of hydrogen activity in the furnace atmosphere.

[0146] The result of the Mo element concentration deviation control is directly used as the basis for the control of precipitates in the segmented tempering process of step S5. It works synergistically with the Mo element to strengthen the uniform distribution of precipitates, avoids the generation of brittle phases at grain boundaries and local precipitation agglomeration, and at the same time suppresses the risk of hydrogen-induced embrittlement at grain boundaries, thereby improving the material reliability under heavy-load impact conditions of heavy-duty vehicles.

[0147] The three stages of this process are continuous. After the low-temperature preheating stage is completed, it directly enters the medium-temperature uniform heating stage. After the medium-temperature uniform heating stage reaches the target temperature, it directly enters the high-temperature constant temperature homogenization stage. There is no intermediate cooling or air cooling process, which ensures the continuity of matrix microstructure evolution during austenitization.

[0148] The low-temperature preheating section operates below the austenitic transformation critical point. The specific temperature range can be determined based on the transformation critical point of Mo-containing ductile iron. Its main function is to ensure uniform temperature inside and outside the billet, preventing deformation and cracking caused by rapid heating, and preparing the microstructure for subsequent austenitizing transformation. The medium-temperature uniform heating section crosses the austenitic transformation critical point at a constant linear rate. The heating rate is determined based on the billet wall thickness; the greater the wall thickness, the lower the heating rate, ensuring that the transformation process in the core and surface of the billet is synchronized. The high-temperature isothermal homogenization section maintains a constant temperature at the austenitizing target temperature. The holding time is determined based on the billet wall thickness and Mo content. Its main function is to achieve sufficient diffusion of alloying elements and ensure the compositional uniformity of the austenitic matrix. For the overall furnace atmosphere control during the processing, the stable control of oxygen potential, sulfur potential, and hydrogen activity is achieved through online detection and precise regulation of the furnace atmosphere. Specifically, oxygen potential is monitored in real time by an online oxygen analyzer, and stability is maintained by adjusting the mixing ratio of argon and trace oxygen. Sulfur potential is monitored in real time by an online sulfur analyzer, and stability is maintained by adjusting the content of trace sulfides in the atmosphere. Hydrogen activity is monitored in real time by an online hydrogen analyzer, and stability is maintained by adjusting the dryness of argon and the mixing ratio of trace hydrogen. Those skilled in the art can achieve stable control of the above parameters using conventional heat treatment furnace atmosphere detection and control systems.

[0149] Regarding the formula for controlling the concentration deviation of Mo, the determination methods and basis of each coefficient in the formula are explained in detail first. Among them, the element diffusion coupling coefficient α is determined based on the diffusion law and grain boundary segregation characteristics of Mo in the austenitic matrix in the guidelines for controlling grain boundary segregation of alloying elements in metallic materials, and its value ranges from 5 × 10⁻⁶. -8 Up to 2×10 -7， The specific value can be adjusted according to the austenitizing temperature and the carbon content of the matrix. The higher the temperature, the smaller the value of α should be to ensure the accuracy of concentration deviation control. The furnace gas sulfur potential correction coefficient δ is determined based on the technical specifications for cast iron heat treatment atmosphere control, and its value range is 1×10. -3 Up to 5×10 -3The specific value is determined based on the baseline sulfur content of the furnace atmosphere; the furnace gas hydrogen activity correction coefficient ε is determined based on the general specifications for hydrogen embrittlement control of metallic materials, and its value range is 5 × 10⁻⁶. -4 Up to 2×10 -3 The specific value is determined based on the alloy element content of the matrix.

[0150] The control model in this scheme is a hierarchical multi-input single-output computation model, which is divided into four levels. The input-output relationship of each level is clear and explicit. The algorithms not described are all numerical calculation methods known in the field. The first layer is the basic parameter input layer. The input data includes austenitizing temperature, holding time, diffusion coefficient of Mo in the austenitic matrix, measured sulfur potential S in the furnace, measured hydrogen activity H, and coupling coefficients and correction coefficients conforming to industry standards. All input parameters can be directly obtained through conventional heat treatment equipment control and atmosphere detection methods in this field. The second layer is the diffusion behavior calculation layer. Based on the austenitizing temperature, holding time, and diffusion coefficient, it completes the basic calculation of the Mo grain boundary segregation trend and obtains the basic influence values ​​of temperature and time on concentration deviation. The third layer is the atmosphere influence correction layer. Through sulfur potential and hydrogen activity correction terms, it completes the correction of the influence of the furnace atmosphere on the Mo grain boundary segregation behavior and obtains the influence value of atmosphere factors on concentration deviation. The fourth layer is the result output layer. It outputs the target control value of the relative deviation of Mo grain boundary-intragranular micro-region concentration. Based on the output target value, those skilled in the art can adjust the process parameters and atmosphere control parameters of austenitizing treatment to achieve precise control of Mo micro-region concentration distribution.

[0151] The results of the Mo element concentration deviation control directly serve as the basis for the precipitate control in step S5 segmented tempering, forming a synergistic effect with the uniform distribution of Mo-enhanced precipitates. During tempering, the nucleation and growth of Mo-enhanced precipitates depend on the distribution state of Mo in the matrix. The uniform distribution of Mo element concentration at grain boundaries and within grains ensures the uniform dispersion of precipitates at grain boundaries and within grains, avoiding the formation of brittle phases and local precipitation agglomeration caused by excessive enrichment of Mo at grain boundaries. At the same time, by controlling hydrogen activity, hydrogen enrichment at grain boundaries is suppressed, reducing the risk of hydrogen-induced embrittlement, thereby improving the reliability of the material under heavy-load impact conditions of heavy-duty vehicles.

[0152] The three-stage process design of segmented heating austenitization treatment directly supports the diffusion law of Mo. The low-temperature preheating stage ensures uniform temperature inside and outside the billet, laying the temperature foundation for the uniform diffusion of subsequent alloying elements. The medium-temperature uniform heating stage ensures the synchronicity of the austenitic phase transformation process, avoiding local enrichment of alloying elements caused by asynchronous phase transformation. The high-temperature isothermal homogenization stage provides sufficient temperature and time for the full diffusion of Mo, ensuring the compositional uniformity of the austenitic matrix. The process parameters of the three stages directly correspond to the formula for controlling the Mo element concentration deviation. and The parameters, together, enable precise control of the concentration distribution of Mo in micro-regions.

[0153] The design of the Mo element concentration deviation control formula forms a close synergy with the subsequent segmented tempering treatment for precipitate control. This scheme is not for the purpose of controlling Mo element segregation, but rather uses the control result of Mo element concentration deviation as the basic input for subsequent tempering precipitate control. By ensuring the uniform distribution of Mo element concentration at grain boundaries and within grains, the uniform dispersion of Mo element-strengthened precipitates during tempering is guaranteed, avoiding the formation of brittle phases at grain boundaries and local precipitation agglomeration. This achieves the synergistic effect of austenitization composition control and tempering precipitate control.

[0154] The stable control of oxygen potential, sulfur potential, and hydrogen activity in the furnace atmosphere throughout the process, together with the effect of Mo concentration deviation regulation, forms a synergistic guarantee. The sulfur potential in the furnace gas affects the formation of Mo sulfides at grain boundaries, the hydrogen activity affects the diffusion behavior of Mo and the tendency of grain boundary segregation, and the oxygen potential affects the surface oxidation of alloying elements. By stably controlling these three atmosphere parameters, the interference of the furnace gas environment on the grain boundary segregation behavior of Mo is eliminated, ensuring the accuracy of concentration deviation regulation, while suppressing the risk of hydrogen-induced embrittlement of grain boundaries, thus forming a dual guarantee with the Mo concentration deviation regulation.

[0155] This solution incorporates the quantitative control of the concentration deviation of Mo elements at grain boundaries and within grains into the goal of austenitization treatment. At the same time, it introduces coupled control of the furnace gas environment, realizing coordinated control with subsequent tempering treatment. This solves the problems of high material brittleness, poor impact performance, and short fatigue life caused by Mo element grain boundary segregation in the prior art. It has significant progress and non-obvious technical effects in improving the service reliability of Mo-containing ductile iron components for heavy-duty vehicles.

[0156] This solution achieves precise quantitative control of the concentration deviation of Mo in grain boundaries and intragranular microregions, solving the problem that existing austenitization treatments can only control the overall Mo content. Based on Fick's diffusion law and the theory of grain boundary segregation of alloying elements, this solution precisely controls the diffusion process of Mo in the austenitic matrix through a segmented heating austenitization process. At the same time, the quantitative concentration deviation control formula provides a precise calculation basis for adjusting process parameters and atmosphere parameters. Those skilled in the art can directly adjust the austenitization process parameters through the calculation results of the formula to obtain the grain boundary-intragranular concentration deviation within the target range, greatly improving the controllability and repeatability of the process.

[0157] This solution achieves synergistic control of austenitization composition and tempering precipitate control, solving the problem of disconnect between austenitization and tempering processes in existing technologies, which makes it impossible to achieve uniform and dispersed distribution of precipitates. The distribution of Mo-enhanced precipitates during tempering directly depends on the uniformity of Mo distribution in the matrix during austenitization. Only when the concentration distribution of Mo at grain boundaries and within grains in the austenitic matrix is ​​uniform can uniformly dispersed enhanced precipitates be formed during tempering, avoiding agglomeration of precipitates at grain boundaries and the formation of brittle phases. By precisely controlling the concentration of Mo in micro-regions, the composition control of austenitization and the precipitate control of tempering are directly linked, forming a synergistic effect between the preceding and following processes, rather than austenitization being solely for obtaining uniform austenite in existing technologies.

[0158] This solution introduces coupled control of furnace gas sulfur potential and hydrogen activity, solving the problem of insufficient control precision caused by neglecting the influence of the furnace gas environment on the grain boundary segregation behavior of Mo in existing technologies. Existing technologies only focus on controlling the oxygen content in the furnace, ignoring the influence of sulfur potential and hydrogen activity in the furnace gas on the diffusion behavior and grain boundary segregation trend of Mo. Sulfur in the furnace gas will form low-melting-point sulfides with Mo at the grain boundaries, promoting the enrichment of Mo at the grain boundaries, while hydrogen in the furnace gas will increase the diffusion rate of Mo and exacerbate grain boundary segregation. By stably controlling sulfur potential and hydrogen activity, the interference of the furnace gas environment on the segregation behavior of Mo is eliminated, significantly improving the accuracy of concentration deviation control. At the same time, by controlling hydrogen activity, the risk of hydrogen-induced embrittlement of grain boundaries is suppressed, further improving the crack resistance of the material under heavy load impact conditions.

[0159] This solution significantly improves the high-temperature stability and wear resistance of heavy-duty vehicle components, solving the problem of thermal deformation and wear in existing Mo-containing ductile iron components under high-temperature conditions during long downhill braking. Mo is a core alloying element for improving the high-temperature strength and thermal stability of cast iron. In existing technologies, due to Mo grain boundary segregation, the Mo content within the grains is insufficient, making it impossible to form a stable strengthening precipitate at high temperatures, thus leading to a decrease in high-temperature strength. However, by uniformly controlling the Mo concentration at grain boundaries and within the grains, sufficient Mo is ensured within the grains, forming a uniformly dispersed, high-temperature stable precipitate during tempering. At the same time, the chemical adsorption effect of Mo on SiO2 abrasive particles on the road surface is suppressed, significantly improving the high-temperature strength, thermal stability, and wear resistance of the material. It is suitable for the harsh service conditions of heavy-duty vehicles and has significant practical application value.

[0160] As a preferred option, step S3 gradient precooling adopts a three-stage stepped cooling process. Each stage of cooling is carried out at a constant linear rate to the corresponding precooling temperature and then held at a constant temperature. The final temperature of precooling is higher than the starting point of martensitic transformation. Step S4 graded quenching process adopts a dual-medium graded cooling mode. The first stage of cooling uses a salt bath medium with a temperature higher than the starting point of martensitic transformation. The billet is held at a constant temperature in the medium until the internal and external temperatures are uniform. The second stage of cooling uses an oil-based medium with a temperature lower than the termination point of martensitic transformation. The billet is cooled until the matrix transformation is completely completed.

[0161] By adjusting the cooling rate and isothermal dwell time of the gradient precooling, the stress field gradient in the micro-region around the graphite spheres is made to satisfy the following equation:

[0162] In the formula:

[0163] The gradient of the micro-region stress field around the graphite sphere is expressed in MPa / μm.

[0164] is the coefficient of thermal expansion of the austenitic matrix, with units of 1 / K, and its value conforms to the specifications for thermophysical property parameters of metallic materials;

[0165] This is the elastic modulus of the graphite phase, expressed in GPa, and its value conforms to the general specifications for the physical properties of cast iron materials.

[0166] This is the elastic modulus of the austenitic matrix, expressed in GPa, and its value conforms to the test specifications for the elastic modulus of metallic materials.

[0167] This represents the temperature drop during the nth precooling stage, expressed in K.

[0168] This is the regularization coefficient, and its value conforms to the guidelines for interfacial stress control technology of metallic materials.

[0169] This is the L2 regularization term for the interface stress distribution, used to control the discreteness of the interface stress concentration.

[0170] This is the wall thickness difference correction factor, and its value conforms to the heat treatment specifications for thick-walled cast iron parts for heavy-duty vehicles.

[0171] This represents the difference in wall thickness between the measured area and the reference area of ​​the billet, in mm.

[0172] By adjusting the medium temperature, dwell time, and cooling rate of the staged quenching process, the difference between the austenite phase transformation evolution rate and the matrix stress relaxation rate is controlled within 10%. The gradient control of the stress field around the graphite spheres and the matching of the stress relaxation rate of the staged quenching form a synergistic effect, jointly suppressing stress concentration and microcrack initiation during the phase transformation process. This provides a matrix structure with a uniform stress state for subsequent tempering treatment, and at the same time, it forms a synergistic control of the microstructure and stress throughout the entire process with the dislocation density control in step S1.

[0173] This process employs a three-stage stepped cooling method, which is carried out continuously within the same protective atmosphere furnace after austenitization treatment. This eliminates the need to remove the billet from the furnace, avoiding oxidation and sudden temperature changes caused by contact with air. Each stage of the three-stage stepped cooling process cools from the current temperature to the corresponding pre-cooling temperature at a constant linear rate. After reaching the pre-cooling temperature, the billet is held at a constant temperature to ensure uniform temperature throughout before proceeding to the next stage. The final pre-cooling temperature is always higher than the martensitic transformation initiation point. The specific temperature range can be determined based on the martensitic transformation initiation point of Mo-containing ductile iron, preventing unexpected martensitic transformations during pre-cooling and ensuring the controllability of the subsequent staged quenching transformation process.

[0174] Regarding the implementation details of the graded quenching process, this process adopts a dual-medium graded cooling mode. After gradient precooling, the billet is quickly transferred from the heating furnace to the first-stage salt bath medium. The temperature of the first-stage salt bath medium is higher than the martensitic transformation initiation point. The billet is kept at a constant temperature in the salt bath until the internal and external temperatures are uniform, eliminating the temperature difference between the billet surface and the core. Then, the billet is quickly transferred to the second-stage oil-based medium. The temperature of the second-stage oil-based medium is lower than the martensitic transformation termination point. The billet is cooled in the oil-based medium until the matrix martensitic transformation is completely completed. Then, it is taken out and air-cooled to room temperature. The time of the entire transfer process is controlled within the conventional safe range in this field, avoiding sudden temperature drops and unexpected phase transformations of the billet during the transfer process.

[0175] The formula for controlling the gradient of the perimeter stress field of graphite spheres first provides a detailed explanation of the methods and basis for determining each coefficient in the formula. Among them, the regularization coefficient λ is determined based on the guidelines for interfacial stress control technology of metallic materials, and its value ranges from 1×10. -3 Up to 5×10 -3 The specific value of λ is determined based on the average size of the graphite spheres. The smaller the size of the graphite spheres, the larger the value of λ is, which is used to suppress the dispersion of stress concentration at the interface. The wall thickness difference correction coefficient μ is determined based on the heat treatment specifications for thick-walled cast iron parts for heavy-duty vehicles. Its value ranges from 0.1 to 0.5. The specific value is determined based on the difference between the maximum and minimum wall thickness of the billet. The larger the wall thickness difference, the larger the value of μ is, which is used to correct the influence of uneven cooling rate caused by wall thickness difference on the stress field gradient. The control model in this scheme is a hierarchical multi-input single-output calculation model, divided into five levels. The first level is the basic parameter input layer, which inputs data including temperature drop values ​​at each precooling stage, wall thickness differences of the billet, and material thermophysical parameters, elastic modulus parameters, and correction coefficients conforming to industry standards. The second level is the thermal stress basic calculation layer, which calculates the basic thermal stress values ​​at the interface between the graphite spheres and the matrix based on the elastic modulus mismatch, the difference in thermal expansion coefficients, and the temperature drop values ​​at the precooling stage. The third level is the wall thickness difference correction layer, which uses wall thickness difference correction coefficients to calculate the basic values ​​at the interface between the graphite spheres and the matrix. The first layer, along with the wall thickness difference, corrects the stress deviation caused by uneven cooling rates in different parts of the thick-walled component. The second layer, a regularization constraint layer, uses L2 regularization to constrain the discreteness of the interface stress distribution. The L2 regularization algorithm is a well-known numerical smoothing algorithm in the field, used to suppress local abnormal concentrations of interface stress. The third layer, a result output layer, outputs the target control value of the stress field gradient in the micro-region of the graphite sphere perimeter. Based on the output target value, those skilled in the art can adjust the cooling rate and the isothermal dwell time of the gradient precooling to achieve precise control of the interface stress field gradient.

[0176] The gradient control of the stress field around the graphite spheres and the matching of the stress relaxation rate during graded quenching form a synergistic effect. The precise control of the stress field gradient at the graphite sphere-matrix interface during gradient precooling eliminates the initial stress concentration at the interface in advance, providing a stable stress basis for the martensitic transformation during the subsequent graded quenching process. Meanwhile, the matching of the austenitic transformation evolution rate and the matrix stress relaxation rate during graded quenching ensures that the transformation stress generated during the transformation process can be relaxed in time, avoiding stress concentration and microcrack initiation. The two work together to achieve closed-loop stress control of the transformation process. At the same time, together with the dislocation density control in step S1, they form a synergistic control of the microstructure and stress throughout the entire process, providing a matrix microstructure with a uniform stress state for the subsequent tempering treatment.

[0177] This scheme combines precise control of the interfacial stress field gradient during gradient precooling with phase transformation-stress rate matching during staged quenching, forming a stress-coordinated control logic for the entire phase transformation process. The design of the three-stage stepped gradient precooling process directly supports the control requirements of the stress field gradient around the graphite spheres. The temperature drop, cooling rate, and isothermal dwell time at each stage of the three-stage cooling process directly correspond to the stress field gradient control formula. By combining graded cooling with constant temperature holding, the parameters achieve phased release and precise control of thermal stress at the graphite sphere-matrix interface, avoiding the sharp increase and concentration of interface stress caused by rapid cooling in one go. The process features and algorithm features form a one-to-one mutual support relationship. The algorithm features provide a quantitative control basis for adjusting the process parameters, while the process features provide an implementable process path for the implementation of the algorithm features. Together, they achieve precise control of the interface stress field gradient.

[0178] The interfacial stress regulation of gradient precooling and the phase transformation-stress rate matching of staged quenching form a synergistic effect. Gradient precooling adjusts the stress state of the graphite sphere-matrix interface before the phase transformation occurs, eliminating initial stress concentration and providing a stable stress foundation for the subsequent martensitic phase transformation. The dual-medium cooling mode of staged quenching eliminates the temperature difference between the inside and outside of the billet through the isothermal residence of the first-stage salt bath, ensuring the synchronicity of the phase transformation process. Through the rate control of the second-stage oil-based cooling, the austenitic phase transformation evolution rate and the matrix stress relaxation rate are matched, ensuring that the phase transformation stress generated during the phase transformation can be relaxed in time. The two work together to form full-process stress control from before the phase transformation to the phase transformation process, jointly suppressing stress concentration and microcrack initiation during the phase transformation process.

[0179] The stress control design of this scheme, together with the dislocation density regulation in step S1, forms a synergistic control of the microstructure and stress throughout the entire process. The dislocation density homogenization regulation in step S1 ensures the uniform distribution of nucleation sites during the phase transformation, thereby ensuring the synchronicity of the phase transformation process and avoiding local stress concentration caused by asynchronous phase transformation. Furthermore, the interface stress regulation and phase transformation-stress rate matching of this scheme further control the stress evolution during the phase transformation process. The two work together to form a closed loop of synergistic microstructure and stress control throughout the entire process, from initial microstructure control to phase transformation process control, providing a matrix microstructure with uniform stress state and no microstructure defects for subsequent tempering treatment.

[0180] In ductile iron, the graphite phase and the matrix exhibit significant differences in elastic modulus and coefficient of thermal expansion. During cooling, thermal stress inevitably arises at the interface. Existing technologies can only control the overall macroscopic stress, failing to precisely regulate the micro-stress gradient around the graphite spheres. This solution, based on elasticity mechanics and thermal stress theory, employs a three-stage stepped gradient pre-cooling process to control the generation and release of interfacial thermal stress in stages. Simultaneously, a quantified stress gradient regulation formula provides a precise calculation basis for adjusting process parameters, controlling the risk of microcrack initiation caused by interfacial stress concentration.

[0181] This solution achieves a precise match between the austenitic phase transformation evolution rate and the matrix stress relaxation rate, solving the cracking problem caused by the inability to release phase transformation stress in a timely manner during quenching in existing technologies. Martensitic phase transformation is accompanied by significant volume expansion, generating a large amount of phase transformation stress. Existing technologies only adjust the phase transformation rate by controlling the cooling rate, neglecting the matching relationship between the phase transformation rate and the matrix stress relaxation rate. When the phase transformation rate is much greater than the stress relaxation rate, the phase transformation stress accumulates rapidly, leading to stress concentration and microcrack initiation. However, by using a staged quenching dual-medium cooling mode, the evolution rate of the martensitic phase transformation is precisely controlled, matching it with the matrix stress relaxation rate. This ensures that the stress generated during the phase transformation can be relaxed in a timely manner, avoiding rapid stress accumulation and reducing the risk of quenching cracks in thick-walled cast iron parts. This design is completely absent in existing conventional quenching processes and possesses outstanding non-obviousness.

[0182] By controlling the interface stress through gradient precooling, the initial stress concentration at the interface is eliminated before the phase transformation occurs. Then, by matching the phase transformation-stress rate through staged quenching, the stress evolution during the phase transformation process is controlled, forming a closed loop of stress synergistic control throughout the entire process from before the phase transformation to the phase transformation process.

[0183] This solution significantly improves the impact resistance and fatigue life of heavy-duty vehicle components, solving the problem of early fatigue failure of ductile iron components under heavy-load impact conditions in existing technologies. The graphite spheroid-matrix interface is the most important source of fatigue crack initiation in ductile iron, and stress concentration at the interface greatly reduces the fatigue strength and impact resistance of the material. Existing technologies cannot accurately control the micro-stress at the interface, causing fatigue cracks to easily initiate at the interface under heavy-load and high-impact service conditions, leading to early failure. However, by precisely controlling the interface stress field gradient, stress concentration at the interface is eliminated. At the same time, by matching the phase transformation and stress rate, the initiation of microcracks during quenching is avoided, thus improving the impact resistance and fatigue life of the material. It is suitable for the harsh service conditions of heavy-duty vehicles and has significant industrial application value.

[0184] As a preferred embodiment, step S5, segmented tempering, employs a two-stage continuous tempering process. The first stage is high-temperature tempering, where the temperature is raised to the first tempering temperature at a constant rate and then held at that temperature. After holding, the billet is air-cooled to room temperature. The second stage is medium-temperature tempering, where the temperature is raised to a second tempering temperature lower than the first tempering temperature at the same rate and then held at that temperature. After holding, the billet is air-cooled to room temperature. A static air atmosphere is used throughout the process. By adjusting the temperature and holding time of the two tempering stages, the volume fraction of the Mo-enhanced precipitates is made to satisfy the following formula:

[0185] In the formula:

[0186] Volume fraction of the precipitated phase enhanced by Mo element;

[0187] These are the weighting coefficients for the first and second tempering processes, and their values ​​conform to the technical specifications for controlling precipitates during the tempering of cast iron.

[0188] These are the absolute temperatures of the first and second tempering processes, respectively, in K.

[0189] These are the holding times for the first and second tempering processes, respectively, in seconds.

[0190] The value represents the temperature hysteresis characteristic value of Mo secondary hardening, in K, and the value conforms to the general specification of secondary hardening behavior of Mo-containing alloys.

[0191] This is the sparsity regularization coefficient, and its value conforms to the control guidelines for the uniformity of precipitates in metallic materials.

[0192] L0 is the sparse regularization term for the spatial distribution of the precipitated phase, used to control the local aggregation of the precipitated phase.

[0193] The result of the volume fraction control of the precipitated phase, together with the dislocation density control in step S1, the interface stress state control in step S3, and the phase transformation stress matching in step S4, forms a synergistic effect throughout the entire process. While ensuring the matching of the strength and toughness of the matrix, the uniformly dispersed precipitated phase inhibits the chemical adsorption effect of Mo and SiO2 abrasive particles on the road surface, thereby improving the wear resistance and service life of the heavy-duty vehicle under complex working conditions. At the same time, it meets the high-temperature working conditions required for the long downhill braking of the heavy-duty vehicle, ensuring the high-temperature strength and thermal stability of the material.

[0194] Regarding the implementation details of segmented tempering, the two-stage continuous tempering process adopted in this process is carried out continuously after the staged quenching is completed and the billet is air-cooled to room temperature. After the first high-temperature tempering is completed and the billet is air-cooled to room temperature, the second medium-temperature tempering is carried out directly. The heating rate of the two tempering processes is kept consistent to avoid uneven microstructure evolution caused by differences in heating rate.

[0195] The temperature of the first high-temperature tempering is determined based on the secondary hardening temperature range of Mo-containing ductile iron. Its main function is to eliminate residual stress generated during quenching and initiate the nucleation process of Mo-enhanced precipitates, laying the foundation for the growth and uniform distribution of precipitates in the second tempering. The temperature of the second medium-temperature tempering is lower than that of the first tempering, and the specific temperature range is determined based on the temperature hysteresis characteristic value of Mo secondary hardening. Its main function is to achieve uniform and dispersed growth of Mo-enhanced precipitates, further eliminate residual stress, and optimize the strength and toughness matching of the matrix. Static air atmosphere is used throughout both tempering processes, without the need for a protective atmosphere, avoiding temperature fluctuations in the furnace caused by atmosphere flow and ensuring the uniformity of tempering temperature.

[0196] Regarding the formula for controlling the volume fraction of Mo-enhanced precipitates, the determination methods and basis of each coefficient in the formula are explained in detail first. Among them, the weighting coefficients for the first and second tempering are determined based on the technical specifications for controlling precipitates during cast iron tempering. The value range is from 0.3 to 0.5. The value of ω2 ranges from 0.5 to 0.7, and the sum of the two is 1. The specific value can be determined according to the Mo content of the matrix. The higher the Mo content, the larger the value of ω2, ensuring the precision of precipitate control; the sparsity regularization coefficient It is determined based on the guidelines for controlling the uniformity of precipitated phases in metallic materials, and its value range is 1×10. -2 Up to 5×10 -2 The specific value is determined based on the wall thickness difference of the billet; the larger the wall thickness difference, the better. The value of is increased accordingly to suppress local aggregation of the precipitated phase.

[0197] The control model in this scheme is a hierarchical multi-input single-output calculation model, divided into five levels. The first level is the basic parameter input layer. The input data includes the temperatures of the two tempering processes, the holding time, the temperature hysteresis characteristic value of Mo secondary hardening, and weighting and regularization coefficients conforming to industry standards. All input parameters can be directly obtained through conventional heat treatment equipment control methods in this field. Among them, the temperature hysteresis characteristic value of Mo secondary hardening can be directly determined according to the general specifications for the secondary hardening behavior of Mo-containing alloys, combined with the Mo element content and alloy composition of the matrix. The second level is the secondary hardening behavior calculation layer. Based on the temperature hysteresis characteristic value of Mo secondary hardening, it completes the calculation of the matrix for the nucleation and growth of precipitated phases during the two tempering processes. The calculation consists of five layers: the first layer is the basic influence calculation layer; the second layer is the double tempering coupled calculation layer, which calculates the coupled influence of the two tempering processes on the volume fraction of precipitates by weighting the coefficients of the two tempering processes, reflecting the synergistic effect of the two tempering processes; the third layer is the regularization constraint layer, which constrains the spatial distribution discreteness of precipitates by using the L0 sparse regularization term. The L0 sparse regularization algorithm is a well-known numerical optimization algorithm in the field, used to suppress the local aggregation of precipitates and ensure the uniform dispersion of precipitates; the fifth layer is the result output layer, which outputs the target volume fraction of the Mo-enhanced precipitates. Based on the output target value, those skilled in the art can adjust the temperature and holding time of the two tempering processes to achieve precise control of the volume fraction and distribution state of the precipitates.

[0198] The precipitate volume fraction control result, together with the dislocation density control in step S1, the interface stress state control in step S3, and the phase transformation stress matching in step S4, forms a synergistic process throughout the entire process. The dislocation density homogenization control in step S1 provides uniform nucleation sites for the nucleation of the precipitate; the Mo element micro-region concentration homogenization control in step S2 provides a compositional basis for the uniform growth of the precipitate; the stress control throughout the entire process in steps S3 and S4 ensures the integrity of the matrix structure and avoids abnormal agglomeration of the precipitate in the stress concentration area. The precipitate control in this scheme, together with the structure and stress control in all the previous steps, forms a synergistic process throughout the entire process, and finally achieves a uniformly dispersed Mo element-reinforced precipitate distribution. While ensuring the strength and toughness matching of the matrix, the uniformly dispersed precipitate suppresses the chemical adsorption effect of Mo and SiO2 abrasive particles on the road surface, thereby improving the wear resistance and service life of the material. At the same time, the uniformly dispersed high-temperature stable precipitate ensures the high-temperature strength and thermal stability of the material under the high-temperature conditions of long downhill braking.

[0199] The design of this scheme, involving two consecutive tempering processes, directly supports the temperature hysteresis effect of Mo's secondary hardening. The first high-temperature tempering eliminates residual quenching stress and simultaneously initiates the nucleation process of Mo-enhanced precipitates, avoiding the temperature hysteresis range of secondary hardening and laying the foundation for uniform nucleation of the precipitates. The second medium-temperature tempering, based on the temperature hysteresis characteristic value of Mo's secondary hardening, determines the temperature to achieve uniform dispersion and growth of the precipitates, ensuring the full realization of the secondary hardening effect. The tempering temperature and holding time of the two tempering processes directly correspond to the formula for precipitate control. , parameter.

[0200] The design of the precipitate control formula forms a synergistic effect with all the preceding heat treatment steps. This scheme is not for the sake of controlling the precipitate, but rather to form a synergistic closed loop with the precipitate control results, dislocation density control, Mo micro-region concentration control, interface stress control, and phase transformation stress matching of the preceding steps. The control results of each preceding step provide a basic guarantee for the uniform dispersion distribution of the precipitate, while the precise control of the precipitate transforms the organizational and stress control effects of all preceding steps into the final strength, toughness, high-temperature stability, and wear resistance of the material.

[0201] The precipitate phase control design of this scheme is precisely adapted to the special service conditions of heavy-duty vehicles. The uniformly dispersed Mo element strengthens the precipitate phase, which not only optimizes the strength and toughness matching of the matrix and improves the impact resistance and fatigue life of the material, but also inhibits the chemical adsorption effect of Mo and SiO2 abrasive particles on the road surface, thereby improving the wear resistance of the material. At the same time, it ensures the strength and thermal stability of the material under high-temperature conditions, making it suitable for the harsh service conditions of heavy-duty vehicles with heavy loads, strong impacts, high-temperature braking, and multiple abrasive particles. This makes the design of the tempering process no longer a general parameter setting, but a customized control for specific service conditions.

[0202] As a preferred option, the ratio of the holding time of the first tempering to the holding time of the second tempering is 1:0.8-1:1.2, the heating rate of both temperings is controlled at 510-520℃ / min, and the temperature difference between the two temperings is controlled at 80-150℃. This matches the staged quenching cooling parameters in step S4, ensuring the synergistic effect of residual stress elimination and precipitate control during the tempering process.

[0203] Regarding the control of the holding time ratio for the two tempering processes, this scheme requires the ratio of the holding time for the first tempering to the holding time for the second tempering to be 1:0.8-1:1.2. The specific ratio can be determined based on the billet wall thickness and Mo content. When the billet wall thickness is large and the Mo content is high, the holding time ratio for the second tempering can be appropriately increased to ensure uniform growth of the precipitated phase and sufficient elimination of residual stress. When the billet wall thickness is small and the Mo content is low, the holding time ratio for the second tempering can be appropriately decreased to avoid excessive growth and coarsening of the precipitated phase.

[0204] Regarding the control of the heating rate during the two tempering processes, this scheme requires that the heating rate be controlled at 510-520℃ / min for both tempering processes, and that the heating rate be consistent for both tempering processes. The specific heating rate value can be determined according to the wall thickness of the billet. The larger the wall thickness, the lower the heating rate should be to avoid excessive temperature difference between the inside and outside of the billet caused by rapid heating, which could lead to uneven residual stress elimination and abnormal microstructure evolution. Using the same heating rate for both tempering processes ensures the consistency of the matrix microstructure evolution during the two tempering processes, avoids the dispersion of the tempering effect caused by the difference in heating rate, and ensures the synergistic effect of residual stress elimination and precipitate control.

[0205] Regarding the temperature difference control of the two tempering processes, this scheme requires the temperature difference between the two tempering processes to be controlled between 80-150℃. The specific temperature difference value can be determined based on the Mo element content and the temperature hysteresis characteristic value of the secondary hardening. The higher the Mo element content, the greater the temperature hysteresis characteristic value of the secondary hardening, and the corresponding temperature difference value should be increased accordingly. This ensures that the temperature of the first tempering can effectively initiate the nucleation process of the precipitated phase, and the temperature of the second tempering can accurately match the peak range of the secondary hardening, so as to achieve uniform dispersion and growth of the precipitated phase and fully realize the effect of the secondary hardening.

[0206] The matching design of the holding time ratio, heating rate, and temperature difference between the two tempering processes creates an internal synergistic effect between them. Controlling the holding time ratio ensures the time matching between the nucleation process of the precipitated phase in the first tempering and the growth process of the precipitated phase in the second tempering, keeping the nucleation and growth times within a reasonable ratio. This avoids insufficient nucleation leading to insufficient precipitated phase quantity, or excessive growth time leading to coarsening of the precipitated phase. Controlling the same heating rate for both tempering processes ensures the consistency of the matrix microstructure's thermal evolution during both tempering processes, avoiding the dispersion of microstructure evolution caused by differences in heating rates, and providing a stable foundation for the synergistic effect of the two tempering processes. Controlling the temperature difference between the two tempering processes ensures a reasonable match between the nucleation temperature of the first tempering and the growth temperature of the second tempering, adapting to the temperature hysteresis characteristics of Mo element secondary hardening, and fully realizing the secondary hardening effect. The matching design of these three parameters supports and cooperates with each other, jointly achieving the synergistic effect of the two tempering processes.

[0207] Secondly, the goal of tempering is to eliminate residual stress from quenching and to achieve precipitate strengthening. The effectiveness of these two goals directly depends on the initial microstructure of the quenched matrix. By precisely controlling the process parameters of two tempering processes and matching them with the cooling parameters of staged quenching, the rate of residual stress elimination during tempering is adapted to the rate of nucleation and growth of precipitates. This achieves the synergistic advancement of the two goals, ensuring the full elimination of residual stress and the uniform dispersion of precipitates, thus forming a synergistic effect between the tempering and quenching processes.

[0208] This solution achieves the synergistic advancement of residual stress elimination and precipitate control through precise matching of tempering process parameters in two stages. It resolves the problem of conflicting and mutually exclusive objectives in existing technologies. During tempering, residual stress elimination and precipitate nucleation and growth are two simultaneous and mutually influential processes. Residual stress elimination promotes dislocation movement and annihilation, and dislocations are the primary nucleation sites for precipitates. If residual stress elimination is too rapid, the number of nucleation sites for precipitates will decrease rapidly, leading to insufficient and uneven precipitate nucleation. Conversely, if residual stress is controlled too quickly to ensure precipitate nucleation, the solution may fail to achieve the desired balance. The rate of residual stress elimination can lead to insufficient elimination of the final residual stress, affecting the material's impact resistance and dimensional stability. Existing technologies cannot resolve this contradiction and can only sacrifice one of the objectives. However, by precisely matching the holding time, heating rate, and temperature difference of the two tempering processes, the rate of residual stress elimination and the nucleation and growth rate of the precipitated phase can be matched. In the first tempering, the residual stress is slowly eliminated while the uniform nucleation of the precipitated phase is completed. In the second tempering, the residual stress is fully eliminated while the uniform and diffuse growth of the precipitated phase is completed, thus achieving a synergistic balance between the two objectives.

[0209] By controlling the consistent heating rate of the two tempering processes, the uniformity of the tempered microstructure of thick-walled cast iron parts is significantly improved. This solves the problem of uneven microstructure evolution between the surface and core during the tempering process of thick-walled parts in existing technologies. Heavy-duty vehicle Mo-containing ductile iron parts are mostly thick-walled structures. During the tempering process, a temperature difference inevitably occurs between the surface and the core. If the heating rates of the two tempering processes are different, the temperature difference between the surface and the core will increase, leading to asynchronous microstructure evolution and ultimately differences in microstructure and properties between the surface and the core. By using the same heating rate for both tempering processes, the heating patterns of the billet surface and core are consistent, and the microstructure evolution process is synchronized. This ensures the uniformity of the microstructure and the consistency of properties between the surface and the core of the thick-walled parts, significantly improving the product qualification rate in mass production.

[0210] As a preferred option, step S6, low-temperature aging treatment, is carried out after the tempered billet has been air-cooled to room temperature. The aging temperature is lower than the second tempering temperature, and the aging holding time is not less than 1.5 times the total tempering holding time. After aging, the billet is cooled to room temperature with the furnace. The aging heating rate is controlled at 3-8℃ / min, which forms a gradient match with the tempering heating rate in step S5, so as to avoid the generation of additional structural stress and abnormal growth of precipitates during the aging process.

[0211] The design of various process parameters for low-temperature aging treatment forms an internal synergistic effect. The aging temperature is lower than the second tempering temperature, which ensures the stability of the matrix structure and precipitates during the aging process and prevents abnormal growth of precipitates. The sufficiently long aging holding time ensures the full stabilization of the matrix structure and the complete elimination of residual stress. The low aging heating rate avoids the generation of additional structural stress during the aging process. The furnace cooling method avoids secondary residual stress generated by rapid cooling. The design of the four process parameters supports and cooperates with each other to achieve the goal of low-temperature aging treatment.

[0212] The low-temperature aging process in this scheme forms a synergistic match with the tempering process in step S5. In the prior art, low-temperature aging is usually treated as an independent supplementary process, and its process parameters are designed completely independently of the tempering process. The influence of the tempering process on the aging effect is not considered, which leads to the aging process failing to achieve the desired microstructure stabilization effect and may even have a negative impact on the already optimized matrix microstructure. However, by matching the aging temperature with the second tempering temperature, the gradient matching of the aging heating rate with the tempering heating rate, and the matching of the aging holding time with the total tempering holding time, the aging process and the tempering process form a close synergistic cooperation. The aging process is a continuation and stabilization of the tempering process, rather than an independent process. This ensures that the uniformly dispersed precipitates formed during the tempering process do not grow abnormally, and also achieves sufficient stabilization of the matrix microstructure and complete elimination of residual stress, forming a synergistic control logic of tempering-aging.

[0213] Furthermore, the low-temperature aging treatment design of this solution is precisely adapted to the long-term service requirements of heavy-duty vehicle components. Core components of heavy-duty vehicles, such as brake drums and wheel hubs, have extremely high requirements for dimensional accuracy. Even slight changes in dimensions can affect braking performance and driving safety. In existing technologies, components treated with low-temperature aging treatment will undergo dimensional deformation during long-term service due to the natural aging changes in the matrix structure and the slow release of residual stress, leading to component failure. However, through low-temperature aging treatment, the matrix structure is fully stabilized and residual stress is completely eliminated, avoiding dimensional deformation of components during long-term service, ensuring driving safety and the long service life of components, and meeting the high reliability requirements of heavy-duty vehicle components.

[0214] By designing a sufficiently long aging and holding period, the microscopic residual stress inside the thick-walled cast iron parts is eliminated, solving the problem of dimensional deformation that easily occurs during long-term service in existing technologies. Although quenching and tempering can eliminate most of the macroscopic residual stress, a large number of microscopic residual stresses still exist inside the matrix of thick-walled cast iron parts. These microscopic residual stresses will slowly release during long-term service, leading to dimensional deformation. For components with extremely high dimensional accuracy requirements, such as brake drums and wheel hubs of heavy-duty vehicles, this dimensional deformation will directly affect braking performance and bring serious safety hazards. By using furnace cooling after aging, the billet is cooled slowly and uniformly to room temperature, completely avoiding secondary residual stress caused by rapid cooling. This further improves the stability of the matrix structure and the stress relief effect, not only ensuring the long-term dimensional stability of the parts, but also eliminating fatigue crack initiation sources caused by microscopic stress concentration. This significantly improves the fatigue resistance and service life of the material, meeting the long-term high-intensity service requirements of heavy-duty vehicles and having significant practical application value.

[0215] As a preferred option, the graphite spheroidization rate of the as-cast Mo-containing ductile iron billet is not less than 85%, the pearlite content in the as-cast matrix is ​​not less than 70%, the mass fraction of Mo is 0.2%-0.8%, the billet surface is free of cracks, pores, shrinkage defects, and the wall thickness difference of the billet is not greater than 30% of the maximum wall thickness. This matches the process parameters of gradient precooling in step S3 and graded quenching in step S4, ensuring the uniformity of microstructure and temperature during the heat treatment process.

[0216] The various quality indicators of the cast billet form an internal synergistic effect. The requirement of graphite spheroidization rate of not less than 85% ensures the basic toughness and impact resistance of the cast billet; the requirement of pearlite content of not less than 70% ensures the uniformity of microstructure transformation during austenitization; the Mo content range of 0.2%-0.8% ensures the full utilization of the secondary hardening effect during subsequent heat treatment; the requirement of no surface defects avoids the expansion of defects during heat treatment; and the requirement of controlling wall thickness difference ensures the uniformity of cooling during heat treatment.

[0217] The quality requirements for the cast billet in this scheme are synergistically matched with the process parameters of gradient precooling in step S3 and graded quenching in step S4. By formulating precise quality control requirements for the cast billet, the initial state of the billet entering the heat treatment process is ensured to be consistent. At the same time, the various quality indicators of the billet are matched with the subsequent precooling and quenching process parameters. Those skilled in the art can adjust the heat treatment process parameters according to the initial state of the billet, ensuring the stability and repeatability of the heat treatment effect and solving the problem of large performance dispersion in mass production in the prior art.

[0218] This scheme requires that the graphite spheroidization rate of the as-cast billet be no less than 85% and the pearlite content be no less than 70%. This uniform as-cast structure ensures that austenite grains can nucleate and grow uniformly during the austenitization process, avoiding the austenite grain size unevenness caused by the uneven as-cast structure, which leads to abnormal structure after quenching. At the same time, the uniform as-cast structure ensures the initial uniformity of Mo element distribution in the matrix, laying the foundation for the uniform dispersion distribution of precipitates during the subsequent tempering process, maximizing the secondary hardening effect of Mo element, and enabling the subsequent heat treatment process to achieve the optimal strengthening effect, rather than the defects of the as-cast structure in the prior art that limit the strengthening effect of the heat treatment process.

[0219] By precisely controlling the wall thickness difference, the problem of microstructure and property differences caused by uneven cooling during the heat treatment of thick-walled cast iron parts has been solved. Heavy-duty vehicle components made of Mo-containing ductile iron are mostly irregularly shaped and thick-walled structures with significant differences in wall thickness across different parts. During the cooling process of heat treatment, thin-walled areas cool faster than thick-walled areas. This difference in cooling rate leads to differences in microstructure and properties in different parts of the same component, and may even result in defects such as deformation and cracking. Existing technologies only alleviate this problem by adjusting the cooling rate, neglecting the fact that controlling the wall thickness difference of the billet itself is the root cause of the problem. By controlling the wall thickness difference of the billet to no more than 30% of the maximum wall thickness, the difference in cooling rate between different parts is reduced from the source. Combined with subsequent gradient precooling and staged quenching processes, the problem of uneven cooling of thick-walled parts is perfectly solved, ensuring the uniformity of microstructure and properties in different parts of the same component. This significantly improves the overall reliability and service life of the component, demonstrating significant practical application value.

[0220] As a preferred embodiment, in steps S1 to S6, the billet is clamped with multi-point symmetrical support. The number and position of the support points are determined according to the wall thickness distribution and structure of the billet. The deformation of the billet during the heat treatment process is controlled to be no more than 0.05 mm / m. High-temperature resistant buffer pads are set at the support points to avoid local stress concentration in the billet during heating and cooling. This forms a synergistic process with the homogenization pretreatment in step S1 and the quenching phase transformation stress control in step S4.

[0221] The billet is always clamped using a multi-point symmetrical support method. Specifically, from the homogenization pretreatment to the austenitization treatment, gradient precooling, staged quenching, segmented tempering, and low-temperature aging, the clamping method, the number and position of the support points of the billet are always kept consistent. The clamping method and support position are not changed midway to avoid billet deformation and stress changes caused by changes in the clamping method.

[0222] The number and location of support points in this scheme are determined based on the wall thickness distribution and structure of the billet. The specific principles for determination are as follows: support points should be preferentially placed on thick-walled and rigid parts of the billet, and should be avoided on thin-walled and weak parts to prevent the billet from bending and deforming due to its own weight during heating and cooling; the support points should be symmetrically distributed to ensure that the center of gravity of the billet falls on the center of the support surface formed by all the support points, so as to avoid the billet tilting and uneven loading; for billets with a length greater than 1m, the number of support points should not be less than 4, and for billets with a length less than 1m, the number of support points should not be less than 3. Those skilled in the art can determine the specific number and location of support points based on the specific structure, size and weight of the billet, referring to the general specifications for heat treatment clamping of cast iron parts for heavy-duty vehicles.

[0223] Regarding the control of deformation during the heat treatment of billets, this solution requires that the deformation of the billets during the heat treatment process be no greater than 0.05 mm / m. Specifically, this means that after the entire heat treatment process is completed, the deformation per meter of the billet relative to the original size of the cast billet should not exceed 0.05 mm. The deformation is detected using conventional dimensional inspection tools in this field, such as dial indicators and coordinate measuring machines. This deformation control requirement is based on the dimensional accuracy requirements of core components such as brake drums and wheel hubs of heavy-duty vehicles, ensuring that the heat-treated billets can meet the subsequent machining and assembly accuracy requirements.

[0224] Regarding the installation of high-temperature resistant buffer pads at the support points, this solution requires the installation of high-temperature resistant buffer pads at each contact point between the support point and the billet. The pads are made of high-temperature resistant ceramic fiber or asbestos material, with a long-term operating temperature of not less than 1000℃, and can adapt to the highest temperature requirements of the entire heat treatment process. The thickness of the buffer pads is 3-10mm, with the specific thickness determined according to the weight of the billet; the heavier the billet, the greater the pad thickness. The installation of high-temperature resistant buffer pads can, on the one hand, prevent direct contact between the billet and the rigid support points, thus avoiding localized stress concentration at the contact points; on the other hand, it can buffer the displacement and deformation of the billet caused by thermal expansion and contraction during heating and cooling, preventing billet deformation and cracking caused by jamming at the support points.

[0225] In this scheme, the clamping design of the entire process works synergistically with the homogenization pretreatment in step S1 and the quenching phase transformation stress control in step S4. One of the goals of the homogenization pretreatment in step S1 is to eliminate residual stress in the casting state. The goal of the quenching phase transformation stress control in step S4 is to control stress concentration during the phase transformation process. However, an unreasonable clamping method will apply additional clamping stress and local stress concentration to the billet during the heat treatment process, which will negate the effect of stress elimination and stress control, and may even lead to deformation and cracking of the billet. By using consistent multi-point symmetrical support clamping and high-temperature resistant buffer pads throughout the entire process, the generation of additional stress during the heat treatment process is avoided. This forms a synergistic stress control throughout the entire process with the residual stress elimination in step S1 and the phase transformation stress control in step S4, ensuring the achievement of the stress control goal of the entire heat treatment process.

[0226] This design ensures that the billet remains under stable stress throughout the entire heat treatment process, avoiding stress changes and deformations caused by changing the clamping method midway. The multi-point symmetrical support design ensures that the billet's weight is evenly distributed across all support points, preventing bending deformation caused by excessive local loads. The support point locations, determined based on the wall thickness distribution and structure, ensure that the support points are placed at the most rigid parts, preventing deformation in thin-walled areas. The use of high-temperature resistant buffer pads prevents local stress concentration and jamming deformation at the support points.

[0227] The clamping design of this solution is precisely matched to the high-precision requirements of the core components of heavy-duty vehicles. Core components such as brake drums and wheel hubs of heavy-duty vehicles have extremely high requirements for dimensional accuracy and geometric tolerances. Even slight deformation during the heat treatment process can lead to insufficient machining allowance in subsequent machining, or even scrapping of the component, while also affecting assembly accuracy and braking performance. Existing clamping methods cannot control the deformation after heat treatment within an extremely high precision range, resulting in low machining pass rate and poor dimensional stability of the components. However, by controlling the clamping process throughout the entire process, the deformation of the blank after heat treatment is controlled to no more than 0.05 mm / m, perfectly meeting the high-precision requirements of the core components of heavy-duty vehicles and significantly improving the machining pass rate and dimensional stability of the product.

[0228] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. For those skilled in the art, any alternative improvements or transformations made to the implementation of the present invention fall within the protection scope of the present invention.

[0229] Any aspects of this invention not described in detail are well-known to those skilled in the art.

Claims

1. A method for controlling and preparing Mo-containing ductile iron composite quenching for heavy-duty vehicles, characterized in that, Includes the following steps: S1. Homogenize the billet by controlling the heating rate, holding time and cooling path to adjust the initial dislocation distribution of the as-cast matrix and eliminate residual stress and microstructure dispersion. S2. Place the billet in a protective atmosphere heating furnace and complete the austenitization process using a segmented heating process. By controlling the heating rate and corresponding holding time of each heating segment, adjust the micro-region distribution of alloying elements at grain boundaries and phase interfaces. S3. After austenitization, the billet is pre-cooled in a gradient manner, and the step cooling rate and the final temperature of pre-cooling are controlled to adjust the austenite grain size and the stress state of the graphite-matrix interface. The pressure of the protective atmosphere in the furnace is kept stable throughout the pre-cooling process. S4. The pre-cooled billet is subjected to cooling phase transformation treatment by a staged quenching process. The austenite phase transformation evolution rate and the stress relaxation rate inside the matrix are matched by controlling the medium temperature, residence time and cooling rate of each stage of staged cooling. S5. Perform segmented tempering of the billet, control the heating rate, tempering temperature and holding time of each tempering segment, and adjust the distribution of matrix precipitates and residual stress level. S6. Perform low-temperature aging treatment on the tempered billet, control the aging heating rate, aging temperature and holding time, stabilize the matrix structure and interface bonding state, and cool the billet to room temperature with the furnace after aging.

2. The method according to claim 1, characterized in that, The homogenization pretreatment in step S1 adopts a three-stage stepped heating-holding-controlled cooling process, specifically as follows: Starting from room temperature, the first heating stage heats up to the first holding temperature at a constant linear rate. After holding at a constant temperature, the second heating stage begins. The second heating stage heats up to the second holding temperature at a rate lower than that of the first heating stage. After holding at a constant temperature, the third heating stage begins. The third heating stage heats up to the third holding temperature at a rate lower than that of the second heating stage. After holding at a constant temperature, the billet is cooled to room temperature with the furnace at a constant rate. A high-purity inert protective atmosphere is introduced throughout the pretreatment process to control the oxygen content inside the furnace to remain stable; By adjusting the heating rate, holding time, and cooling rate at each stage, the initial dislocation density distribution of the pretreated as-cast matrix satisfies the following equation: In the formula: The initial dislocation density of the matrix after pretreatment; This is the initial dislocation density reference value for the as-cast billet; The heating-insulation coupling coefficient; Let m be the heating rate of the m-th heating segment; This refers to the heat preservation time corresponding to the m-th heating segment; The regularization coefficient is used. This is the L1 regularization term for the spatial distribution of dislocation density.

3. The method according to claim 2, characterized in that, The inert protective atmosphere uses argon gas with a purity of not less than 99.99%, which is introduced in a symmetrical double-sided gas inlet manner. At least two sets of gas circulation equipment are installed in the furnace. The protective atmosphere is continuously introduced throughout the pretreatment process to control the oxygen content in the furnace to not exceed 50 ppm. The start and stop of the gas circulation equipment in the furnace are synchronized with the heating section to ensure the uniformity of the atmosphere in the furnace and the accuracy of oxygen content control in each heating section.

4. The method according to claim 1, characterized in that, Step S2 is a segmented heating austenitizing treatment, which is divided into a low-temperature preheating section, a medium-temperature uniform heating section, and a high-temperature constant temperature homogenization section. The temperature of the low-temperature preheating section is below the critical point of austenite phase transformation; the medium-temperature uniform heating section heats up to the target austenitization temperature at a constant linear rate; and the high-temperature isothermal homogenization section maintains a constant temperature at the target temperature. Throughout the process, the oxygen potential, sulfur potential, and hydrogen activity of the furnace atmosphere are kept stable. By adjusting the heating rate and holding time at each stage, the relative deviation of the Mo element concentration between grain boundaries and intragranular microregions is made to satisfy the following equation: In the formula: The relative deviation of Mo concentration between grain boundaries and intragranular microregions; The average mass fraction of Mo in the grain boundary region; The average mass fraction of Mo in the intracrystalline region; The overall average mass fraction of Mo; The element diffusion coupling coefficient; The absolute temperature of the austenitizing isothermal isothermal homogenization section; The holding time for the austenitizing isothermal homogenization section; The diffusion coefficient of Mo in the austenitic matrix; This is the sulfur potential correction factor for the furnace gas; This represents the measured sulfur potential of the atmosphere inside the furnace. This is the correction factor for the hydrogen activity of the furnace gas; This represents the measured value of hydrogen activity in the furnace atmosphere.

5. The method according to claim 1, characterized in that, Step S3, gradient precooling, adopts a three-stage stepped cooling process. Each stage of cooling is carried out at a constant linear rate to the corresponding precooling temperature and then held at a constant temperature. The final temperature of precooling is higher than the starting point of martensitic phase transformation. Step S4, staged quenching, adopts a dual-medium staged cooling mode. The first stage of cooling uses a salt bath medium with a temperature higher than the starting point of martensitic phase transformation. The billet is held at a constant temperature in the medium until the internal and external temperatures are uniform. The second stage of cooling uses an oil-based medium with a temperature lower than the termination point of martensitic phase transformation. The billet is cooled until the matrix phase transformation is completely completed. By adjusting the cooling rate and isothermal dwell time of the gradient precooling, the stress field gradient in the micro-region around the graphite spheres is made to satisfy the following equation: In the formula: The gradient of the micro-region stress field around the graphite sphere; The coefficient of thermal expansion of the austenitic matrix; The elastic modulus of the graphite phase; The elastic modulus of the austenitic matrix; This represents the temperature drop during the nth pre-cooling stage. The regularization coefficient is used. This is the L2 regularization term for the interface stress distribution; This is the wall thickness difference correction factor; This represents the difference in wall thickness between the measured area and the reference area of ​​the billet.

6. The method according to claim 1, characterized in that, Step S6, low-temperature aging treatment, is carried out after the tempered billet is air-cooled to room temperature. The aging temperature is lower than the second tempering temperature, and the aging holding time is not less than 1.5 times the total tempering holding time. After aging, the billet is cooled to room temperature with the furnace. The aging heating rate is controlled at 3-8℃ / min, forming a gradient match with the tempering heating rate in step S5.

7. The method according to claim 1, characterized in that, The graphite spheroidization rate of the as-cast billet containing Mo is not less than 85%, the proportion of pearlite in the as-cast matrix is ​​not less than 70%, the mass fraction of Mo is 0.2%-0.8%, the billet surface is free of cracks, pores, shrinkage defects, and the wall thickness difference of the billet is not greater than 30% of the maximum wall thickness, which matches the process parameters of gradient precooling in step S3 and graded quenching in step S4.

8. The method according to claim 1, characterized in that, In steps S1 to S6, the billet is clamped with multi-point symmetrical support. The number and position of the support points are determined according to the wall thickness distribution and structure of the billet. The deformation of the billet during the heat treatment process is controlled to be no more than 0.05 mm / m. High-temperature resistant buffer pads are set at the support points.

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