A multi-layer gradient heat treatment process for hydraulic shear blades

Abstract

This invention discloses a multi-layer gradient heat treatment process for hydraulic scissor blades, belonging to the field of metal heat treatment technology. Addressing the problems of strong-toughness mutual repulsion and easy subsurface spalling in existing heavy-duty blades, this invention includes the following steps: preheating the pretreated blade to 900-920℃; controlling the medium and high-frequency power supply through timing interlock logic and precisely controlling the high-frequency pulse time using a dynamic coupling algorithm, creating a gradient of austenite solid solubility from the inside to the outside of the blade cross-section; subsequently, precooling with high-pressure pure nitrogen pulses and isothermal quenching in a double-groove stepped nitrate bath with air transfer to achieve spatially staggered phase transformation; finally, deep cryogenic treatment and multiple gradient tempering. This invention successfully constructs a triple microstructure gradient within the blade: high-toughness self-tempered martensite in the core, high-alloy lower bainite in the transition zone, and ultrafine martensite on the surface, completely eliminating abrupt changes in microstructure stress and significantly improving the blade's service life and resistance to fatigue spalling.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment technology, and particularly relates to a multi-layer gradient heat treatment process for hydraulic scissor blades. Background Technology

[0002] Hydraulic shears, as a heavy-duty scrap steel processing device, are widely used for shearing high-strength metal components such as H-beams and engine blocks. Under complex shearing conditions, the blades of hydraulic shears, made of medium-to-high carbon alloy materials, need to withstand extremely high frictional wear on their cutting edge surface, while the blade body and core need to withstand extremely high instantaneous impact loads and macroscopic compressive stresses. Therefore, an ideal blade material must possess both extremely high surface hardness and excellent core strength and toughness.

[0003] Existing conventional heat treatment processes involving integral heating, quenching, and tempering often fail to meet the aforementioned contradictory mechanical performance requirements. If the overall quenching hardness is increased, a brittle martensite structure is easily formed in the core of the blade, making it highly susceptible to integral fracture under heavy impact. If the quenching hardness is reduced or the tempering temperature is increased in pursuit of high toughness, the overall yield strength of the blade will decrease accordingly. Under the enormous compressive stress of shearing heavy steel, the blade is prone to macroscopic plastic deformation, leading to edge collapse and failure.

[0004] To address the differentiated performance requirements of the surface and interior, the industry often employs gradient heat treatment processes such as surface induction hardening to achieve a performance distribution of high surface hardness and high core toughness. However, in practical heavy-duty shearing applications, conventional surface induction hardening processes have the following objective limitations:

[0005] First, conventional single-frequency surface induction hardening typically results in a narrow phase transformation transition zone between the high-hardness martensite surface layer and the high-toughness pearlite or sorbite core. During the quenching and rapid cooling process, the significant difference in specific volume between the surface layer and the core leads to the accumulation of high microstructural tensile stress in this narrow transition zone. Under the long-term action of alternating impact loads, microcracks are very likely to initiate and rapidly propagate in this transition zone, ultimately causing destructive subsurface block spalling of the cutting tool.

[0006] Secondly, in order to widen the transition zone and alleviate stress abrupt changes, some studies have attempted to introduce dual-frequency induction heating technology using medium and high frequencies. However, in actual heat treatment process control, if medium and high frequency electromagnetic fields are directly superimposed simultaneously, complex electromagnetic thermal field coupling effects will be induced in the subsurface layer of the blade. This coupling not only leads to uncontrolled local heating rates and easily causes coarsening of austenite grains in the transition zone, but also makes it difficult to accurately control the formation depth of the high alloy solute concentration halo on the surface. Therefore, how to achieve precise superposition of surface and subsurface temperature gradients through process timing innovation, while avoiding uncontrolled dual-frequency thermal field coupling, has become a process problem that urgently needs to be solved in this field.

[0007] In addition, in terms of thermodynamic cooling control, heavy-duty cutting tools have a huge amount of heat storage. If the surface is heated to an extremely high temperature in order to obtain a high alloy solid solubility, conventional air or water mist precooling can easily lead to a severe oxidation and decarburization reaction on the surface. If the cutting tool with a huge latent heat is directly immersed in a conventional salt bath for isothermal quenching, the instantaneous heat release will often break through the local cooling limit of the salt bath, causing the salt bath to boil uncontrollably, resulting in the expected isothermal phase transformation process not being completed accurately.

[0008] In summary, existing technologies struggle to stably construct a smooth, high-yield-strength micro-substructure gradient buffer layer on the blade cross-section when processing heavy-duty hydraulic shear blades. Therefore, there is an urgent need in this field for a multi-layer gradient heat treatment process that can effectively reconcile the aforementioned multidisciplinary engineering challenges. Summary of the Invention

[0009] This invention overcomes the shortcomings of the prior art and provides a multi-layer gradient heat treatment process for hydraulic scissor blades.

[0010] To achieve the above objectives, the technical solution adopted by this invention is: a multi-layer gradient heat treatment process for hydraulic scissor blades, comprising the following steps:

[0011] S1. Heat the hydraulic scissor blade blank to the solution temperature and keep it at that temperature. Cool it to room temperature and then perform high-temperature tempering to obtain the pretreated hydraulic scissor blade.

[0012] S2. Preheat the entire hydraulic scissor blade to 900-920℃ in a heating furnace and maintain the temperature; move the hydraulic scissor blade into a dual-frequency induction heating device, turn on the intermediate frequency power supply to raise the temperature of the subsurface transition zone of the hydraulic scissor blade to 1000-1020℃; cut off the intermediate frequency power supply through timing interlock control logic, and switch on the high frequency power supply during the metal thermal inertia heat preservation window period to perform non-equilibrium pulse heating, so that the temperature of the 3-5mm depth area of ​​the cutting edge surface of the hydraulic scissor blade rises to 1120-1150℃; wherein, the duration of the non-equilibrium pulse heating by the high frequency power supply is dynamically calculated and controlled based on the target hardened layer depth of the cutting edge surface and the output power of the high frequency power supply.

[0013] S3. Pre-cool the cutting edge surface with high-pressure pure nitrogen pulse jet for 2-4 seconds to reduce the temperature of the cutting edge surface to 1000-1020℃; immerse the hydraulic scissor blade in a first nitrate bath with forced convection circulation and keep it at 320℃ for 12-18 minutes; after removal, transfer it to air and immerse it in a second nitrate bath at 260℃ for 40-50 minutes, then remove and air-cool it to room temperature;

[0014] S4. Place the quenched hydraulic scissor blade in a cryogenic environment of -120℃ to -150℃ for 2-4 hours; perform at least two tempering treatments.

[0015] In a preferred embodiment of the present invention, in step S2, the duration of the non-equilibrium pulse heating by the high-frequency power supply is proportional to the square of the depth of the target hardened layer, proportional to the natural logarithm of the ratio of the output power of the high-frequency power supply to the reference power threshold, and inversely proportional to the reference thermal diffusivity of the hydraulic scissor blade material in the fully austenitized state at 1020℃, and is corrected by a phase transformation buffer coefficient; the value range of the phase transformation buffer coefficient is 0.15-0.25.

[0016] In a preferred embodiment of the present invention, in step S2, the dual-frequency induction heating device adopts a single-coil dual-frequency power supply time-division multiplexing architecture. The timing interlock control logic is specifically as follows: between cutting off the intermediate frequency power supply and switching on the high-frequency power supply, the PLC controller forcibly starts a dead time delay timer of 10-50ms, and physically locks the trigger pulse of the high-frequency power supply IGBT module within the dead time to prevent the backflow of dual-frequency electromagnetic harmonics.

[0017] The intermediate frequency power supply has an output frequency of 2-5kHz and an output power density of 0.3-0.8kW / cm². 2 The heating duration is 15-45 seconds; the output frequency of the high-frequency power supply is 50-80 kHz.

[0018] In a preferred embodiment of the present invention, in step S1, the solution temperature is 1050-1080℃, the holding time is 1-2h, and then ultrasonic-assisted oil cooling is used to cool to room temperature; the high-temperature tempering temperature is 650-680℃.

[0019] In a preferred embodiment of the present invention, in step S3, the injection pressure of the high-pressure pure nitrogen gas is 0.5-0.8 MPa, and the frequency of the pulse injection is controlled at 5-15 Hz, with a single injection duty cycle of 60%-80%, so as to prevent high-temperature decarburization while eliminating macroscopic thermal stress on the cutting edge surface.

[0020] In a preferred embodiment of the present invention, in step S3, the nitrate flow rate in both the first nitrate bath and the second nitrate bath is controlled at 0.5-1.5 m / s;

[0021] After the hydraulic shear blades are removed from the first nitrate bath, the time they are transferred in the air is strictly controlled to be 5-8 seconds in order to release the structural stress generated by the lower bainitic phase transformation.

[0022] In a preferred embodiment of the present invention, step S4, specifically includes at least two tempering processes:

[0023] First tempering: Hold at 200-220℃ for 2-3 hours to eliminate martensitic phase transformation stress;

[0024] Second tempering: Hold at 540-560℃ for 2-3 hours to precipitate secondary hardened carbides.

[0025] In a preferred embodiment of the present invention, after the processing in step S4, the cross-section of the hydraulic scissor blade forms a three-layer micro-substructure gradient from the inside to the outside, mainly including a deep core, a subsurface transition zone and a cutting edge surface.

[0026] The microstructure of the deep core is a high-toughness, low-carbon, self-tempered martensitic composite structure containing undissolved alloy carbides.

[0027] The microstructure of the subsurface transition zone is high-alloy solid solution bainite.

[0028] The microstructure of the cutting edge surface is a mixture of ultrafine tempered martensite and dispersed carbides.

[0029] In a preferred embodiment of the present invention, after the processing in step S4, the hydraulic scissor blade forms a smooth transition in macroscopic hardness gradient:

[0030] The Rockwell hardness of the cutting edge surface is 58-62 HRC;

[0031] The Rockwell hardness of the subsurface transition zone, which is 3-15 mm deep from the surface, is 48-52 HRC.

[0032] The Rockwell hardness of the deep core with a depth greater than 15 mm is 38-42 HRC.

[0033] In a preferred embodiment of the present invention, in step S1, the material of the hydraulic scissor blade blank is 4Cr5MoSiV1 steel or 5CrNiMo steel.

[0034] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0035] This invention preheats the hydraulic scissor blade to 900-920℃, combines this with medium-frequency heating of the subsurface layer to 1020℃, and high-frequency pulse overheating of the cutting edge surface to 1150℃. Before quenching, this artificially creates an "austenite solid solubility gradient" from the inside out. This means that the core retains undissolved carbides in a solute-poor state, the transition zone has fully dissolved carbides, and the surface forms a halo with a high alloy solute concentration. This difference in microstructure causes varying degrees of shift in the isothermal transformation C-curve of each region, thereby achieving spatially staggered phase transformation in a subsequent single 320℃ isothermal salt bath. This precisely generates a three-layer microstructure gradient consisting of a high-toughness, low-carbon self-tempered martensite composite core, a high-alloy solid solution-treated bainite transition zone, and an ultrafine martensite surface. Compared to traditional single-surface induction hardening, which often forms an extremely narrow phase transformation transition zone between the surface and the core and accumulates huge microstructural tensile stress, the multi-layer substructure gradient of this invention not only completely eliminates stress abrupt changes and constructs a perfect mechanical buffer layer, but also fundamentally prevents subsurface block spalling under heavy shearing, while endowing the cutting tool with extremely high surface wear resistance and core anti-collapse yield strength.

[0036] This invention introduces a timing interlock control logic during the dual-frequency gradient austenitization stage. Within the metal's thermal inertia insulation window after the intermediate-frequency power supply is cut off, a high-frequency power supply is instantaneously switched in for pulse heating. This ingenious timing control utilizes the enormous thermal inertia within the heavy-duty blade, achieving seamless superposition of the physical temperature field on the time axis. Simultaneously, it ensures that only a single frequency current exists within the induction coil at any microscopic instant. This mechanism completely eliminates beat frequency interference and electromagnetic harmonic backflow caused by the direct superposition of high-power currents at intermediate and high frequencies from the underlying electrical logic. Existing dual-frequency heating technologies, without extremely complex physical isolation, are highly susceptible to damage to the inverter module due to harmonic backflow, leading to equipment failure. The timing interlock design of this invention not only significantly reduces equipment modification costs and hardware damage risks but also enables this extreme non-equilibrium overheating process to achieve extremely high stability and safety in industrial mass production.

[0037] This invention employs a composite cooling strategy combining high-pressure pure nitrogen pulse precooling and double-groove stepped isothermal quenching during the quenching stage, with a brief air transfer stage between the two salt bath stages. Pure nitrogen precooling instantly smooths the massive macroscopic thermal stress of 1150°C on the surface while the inert atmosphere completely isolates the rapid decarburization reaction. The forced convection double-groove stepped isothermal quenching, combined with air transfer, effectively disperses the enormous latent heat released instantaneously by the heavy cutting tool and promptly releases the structural stress generated by the lower bainitic phase transformation. This multi-failure-prevention cooling path precisely maintains a constant salt bath temperature field, preventing localized boiling runaway and perfectly preserving the carefully constructed high-carbon, high-solute halo on the surface. Conventional water-air precooling or single-groove salt bath quenching, when processing heavy workpieces with extreme temperature gradients, easily leads to severe surface oxidation and decarburization or isothermal phase transformation failure due to the cooling limit being breached. The cooling strategy of this invention reduces the quenching cracking rate and microstructure coarsening scrap rate to extremely low levels, ensuring the perfect reproduction of complex multi-layered gradient microstructures on heavy workpieces.

[0038] This invention establishes a dynamic coupling relationship between the duration of high-frequency pulse heating and the target hardened layer depth, the high-frequency power supply output power, and the baseline thermal diffusivity of the blade material in a fully austenitized state at 1020℃. This empirical formula strictly follows the underlying physical laws of unsteady-state heat conduction and solid-state phase transformation dynamics, and mathematically binds the characteristic time of surface-to-inward heat transfer with the electromagnetic induction energy absorption model in a dimensionally self-consistent manner. Based on this dynamic coupling model, the process system can accurately output high-frequency pulse times at the millisecond level according to the actual needs of different blade specifications, ensuring that nanoscale solute enrichment is formed on the surface while locking the migration of austenite grain boundaries. Traditional heat treatment processes often rely on a limited number of trial and error attempts by technicians and blind optimization of conventional parameters, making it difficult to cope with complex and ever-changing customized production needs. The dynamic control algorithm of this invention elevates specific process parameters into a highly universal underlying control model, giving the heat treatment system strong engineering adaptability, enabling it to be stably applied to the manufacture of heavy-duty hydraulic scissor blades of various thicknesses and specifications. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a flowchart of a multi-layer gradient heat treatment process for a hydraulic scissor blade according to the present invention. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0043] Application Overview:

[0044] Hydraulic shears, as a heavy-duty scrap steel processing device, are widely used for shearing high-strength metal components such as H-beams and engine blocks. Under such extremely harsh heavy-load shearing conditions, the cutting edge surface of the hydraulic shear blades needs to withstand extremely high frictional wear, while the blade body and core need to withstand extremely high instantaneous impact loads and macroscopic compressive stresses. The extreme service environment means that the ideal blade material must simultaneously possess extremely high surface hardness and excellent core strength and toughness, thus forming an irreconcilable contradiction of strength and toughness in materials science.

[0045] Existing conventional heat treatment processes involving integral heating, quenching, and tempering often fail to meet the aforementioned contradictory mechanical performance requirements. If the overall quenching hardness is increased to improve wear resistance, brittle martensite is easily formed in the core of the blade, which often leads to catastrophic integral fracture under heavy impact. Conversely, if the quenching hardness is reduced or the tempering temperature is increased to pursue high toughness, the overall yield strength of the blade will decrease significantly. Under the enormous compressive stress of shearing heavy steel, the blade is prone to macroscopic plastic deformation, leading to edge collapse and ultimately blade failure.

[0046] To address the differentiated performance requirements of the surface and core, the industry often employs gradient heat treatment processes such as surface single-frequency induction hardening to achieve a performance distribution of high surface hardness and high core toughness. However, in actual heavy-duty shearing applications, conventional surface single-frequency induction hardening processes have fatal physical and metallurgical limitations. Specifically, conventional single-frequency surface induction hardening typically forms an extremely narrow phase transformation transition zone between the high-hardness martensite surface layer and the high-toughness pearlite or sorbite core. During the quenching and rapid cooling process, due to the significant difference in specific volume changes between the surface layer and the core, extremely high microstructural tensile stress accumulates in this narrow transition zone. Under the long-term action of alternating impact loads, microcracks are very likely to initiate and rapidly propagate in this transition zone, ultimately leading to destructive subsurface block spalling of the cutting tool.

[0047] To completely overcome the technical bottlenecks of subsurface block spalling and the mutual exclusion of strength and toughness in the existing technical routes, this invention provides a multi-layer gradient heat treatment process for hydraulic scissor blades. It innovatively introduces time-interlocked dual-frequency heating technology, dynamic coupling algorithm control model, and dual-groove stepped isothermal cooling strategy with air transfer. Through the synergistic effect of the above composite technologies, this invention successfully and stably constructs a smooth transition of triple micro-substructure gradient on a single blade cross-section.

[0048] This triple microstructure gradient specifically includes a high-toughness, low-carbon, self-tempered martensite composite core, a high-alloy solid solution-treated bainite transition zone, and an ultra-fine tempered martensite surface layer. This innovative multi-layer gradient structure not only completely eliminates stress abrupt changes in the traditional extremely narrow transition zone and constructs a perfect mechanical buffer layer, fundamentally preventing the phenomenon of subsurface block spalling under heavy-load shearing, but also endows the blade with extremely high surface wear resistance and core anti-collapse yield strength, achieving an exponential increase in the service life of heavy-duty hydraulic shear blades.

[0049] like Figure 1 As shown, a multi-layer gradient heat treatment process for hydraulic scissor blades includes the following steps:

[0050] S1. Heat the hydraulic scissor blade blank to the solution temperature and keep it at that temperature. Cool it to room temperature and then perform high-temperature tempering to obtain the pretreated hydraulic scissor blade.

[0051] S2. Preheat the entire hydraulic scissor blade to 900-920℃ in a heating furnace and maintain the temperature; move the hydraulic scissor blade into a dual-frequency induction heating device, turn on the intermediate frequency power supply to raise the temperature of the subsurface transition zone of the hydraulic scissor blade to 1000-1020℃; cut off the intermediate frequency power supply through timing interlock control logic, and switch on the high frequency power supply during the metal thermal inertia heat preservation window period to perform non-equilibrium pulse heating, so that the temperature of the 3-5mm depth area of ​​the cutting edge surface of the hydraulic scissor blade rises to 1120-1150℃; wherein, the duration of the non-equilibrium pulse heating by the high frequency power supply is dynamically calculated and controlled based on the target hardened layer depth of the cutting edge surface and the output power of the high frequency power supply.

[0052] S3. Pre-cool the cutting edge surface with high-pressure pure nitrogen pulse jet for 2-4 seconds to reduce the temperature of the cutting edge surface to 1000-1020℃; immerse the hydraulic scissor blade in a first nitrate bath with forced convection circulation and keep it at 320℃ for 12-18 minutes; after removal, transfer it to air and immerse it in a second nitrate bath at 260℃ for 40-50 minutes, then remove and air-cool it to room temperature;

[0053] S4. Place the quenched hydraulic scissor blade in a cryogenic environment of -120℃ to -150℃ for 2-4 hours; perform at least two tempering treatments.

[0054] This invention can break the contradiction between hardness and toughness in traditional processes and construct a three-layer micro-substructure gradient with high hardness in the outer layer, a moderate transition in the middle layer, and high strength and toughness in the inner layer. Its core mechanism lies in the ingenious construction of a triple isothermal transformation C-curve shift on the cross section.

[0055] Specifically, since the deep core is only heated to 900 to 920°C and is in an incomplete austenitizing state, a large number of undissolved alloy carbides are retained in the matrix. These carbides, as strong nucleation sites, significantly reduce the stability of austenite, resulting in a large leftward shift of the isothermal transformation C-curve in the core region.

[0056] In the initial cooling phase of the first nitrate bath at 320℃, the critical point of martensite in the core, which is deficient in solute austenite, rises significantly above the salt bath temperature. The supercooled austenite in the core preferentially undergoes phase transformation, transforming into high-toughness low-carbon lath martensite. During the isothermal process at 320℃, in-situ self-tempering occurs, forming a composite structure of self-tempered martensite with excellent strength and toughness and a small amount of lower bainite.

[0057] The subsurface transition zone is precisely heated to 1000 to 1020°C by medium frequency, achieving full solid solution of carbides, and its C-curve is in the standard position. This region has sufficient hardenability to avoid the nose of high-temperature phase transformation, smoothly enters the 320°C salt bath region, and undergoes lower bainite transformation during isothermal process, forming a high-alloy solid solution lower bainite buffer layer with excellent strength and toughness matching.

[0058] The cutting edge surface underwent high-frequency pulsed overheating at 1120 to 1150°C, forming a nanoscale halo of high solute concentration. This significantly increased the stability of the supercooled austenite, causing its C-curve to shift drastically to the right. Therefore, during the isothermal period at 320°C, the surface remained in an untransformed supercooled austenite state until subsequent cooling to 260°C and cryogenic treatment, at which point it finally transformed into high-hardness ultrafine martensite.

[0059] This invention uses dual-frequency gradient heating combined with dual-groove stepped isothermal heating to artificially create three different initial states of austenite on a single blade cross-section. By utilizing the differences in their thermodynamic stability, spatial staggered phase transformation is achieved in the same quenching and cooling process, solving the technical problem that traditional integral quenching cannot achieve multi-layer complex gradient structures.

[0060] In step S2, the present invention abandons the conventional dual-frequency simultaneous superposition heating mode and introduces timing interlock control logic.

[0061] Specifically, in the underlying physical logic of industrial induction heating equipment, if a medium-frequency current of 2 to 5 kHz and a high-frequency high-power current of 50 to 80 kHz are simultaneously fed into one coil or two very close coils, it will generate devastating beat frequency interference and harmonic backflow. The high-frequency current will instantly reverse and break down the IGBT module of the medium-frequency inverter.

[0062] This invention achieves physical isolation from the underlying electrical logic by setting a dead time of 10 to 50 ms.

[0063] Specifically, this invention employs a single-coil dual-frequency power supply time-division multiplexing architecture. Its timing interlock control logic is implemented by a PLC controller and an IGBT solid-state relay group: when the PLC sends an intermediate-frequency turn-off signal, a hardware delay timer of 10 to 50 ms is forcibly started. During this dead time, the system physically locks the trigger pulse of the high-frequency IGBT module; after the intermediate-frequency induced electromotive force decays to a safe threshold within the dead time, the conduction signal of the high-frequency IGBT is released. This mechanism ensures that at any microscopic instant, only a single-frequency current exists in the induction coil, eliminating the risk of equipment damage.

[0064] Meanwhile, the heavy-duty hydraulic scissor blade possesses significant thermal inertia. When the intermediate-frequency heating transition zone reaches 1020°C, the intermediate-frequency power supply is cut off. For the next few seconds, the temperature in the transition zone hardly decreases, remaining within the thermal inertia insulation window. At this moment, a high-frequency pulse power supply is instantly switched on for 3 to 5 seconds, completing the 1150°C overheating of the surface. At any given physical instant, only one frequency of current is operating in the coil, but in the physical temperature field of the blade, a seamless superposition of internal intermediate temperature and external overheating is achieved.

[0065] Furthermore, the present invention introduces a high-frequency pulse heating duration in step S2. The dynamic coupling relationship is as follows: ;

[0066] In the formula, The target hardened layer depth is specifically defined in this invention as: the vertical distance from the cutting edge surface along the cross-sectional normal inward until the microstructure completely transitions from ultrafine tempered martensite to high alloy solid solution bainite, and the macroscopic Rockwell hardness drops to 52HRC.

[0067] In the formula The term represents the characteristic thermal diffusion time of heat conduction from the surface to the interior, where The reference thermal diffusivity is the material of the hydraulic scissor blade in its fully austenitized state at 1020℃. The physical reference value is obtained by laser flare method (refer to GB / T 22588-2008) under vacuum or inert atmosphere at 1020℃, which ensures the universality of the formula for different materials and the uniqueness of the calculation.

[0068] In the formula The project introduces a logarithmic energy decay or power threshold model to characterize the skin effect and energy absorption rate of non-equilibrium high-frequency electromagnetic induction heating.

[0069] in, The reference power threshold, in its physical sense, represents the minimum sustaining power required to offset heat dissipation from the cutting edge and maintain the cutting edge surface at the critical temperature for austenitization under the current heating environment. In practical engineering calibration, This can be achieved by performing a low-power trial heating on a blank cutting tool of the same specification. Once the surface temperature of the cutting edge stabilizes at the complete austenitizing transformation temperature and no longer rises (for example, for 4Cr5MoSiV1 steel, this temperature is calibrated to 1020℃), and the holding time reaches thermal equilibrium after 3-5 minutes, the effective power output of the equipment is then calibrated. ;

[0070] To ensure dimensional consistency in dynamic calculations, the target hardened layer depth The unit of input is millimeters, and the output power is... Compared with reference power The unit of measurement is kilowatt, and the reference thermal diffusivity is... Substituting the unit into square millimeters per second, we can directly derive the high-frequency pulse heating duration in seconds. .

[0071] A phase change buffer coefficient ranging from 0.15 to 0.25 is used. The formula is modified to be able to adapt to the target hardened layer depth of the blade. and device output power Precisely calculate the pulse time in milliseconds. ;

[0072] Wherein, the phase change buffer coefficient in the formula Its physical essence is to compensate for the hysteresis gap between the pure heat conduction model and the actual metal phase transformation kinetics. In extremely short non-equilibrium pulse heating, the austenitic phase transformation and the dissolution of alloy carbides require the absorption of latent heat of phase transformation and a kinetic incubation period. The value serves as a metallurgical kinetic compensation factor to correct the heating time; in practical applications, those skilled in the art can select it based on the degree of alloying and geometric thickness of the blade material.

[0073] This ensures that alloying elements cannot diffuse over long distances within a very short time, typically 3 to 5 seconds, thereby forming a high solute concentration halo on the surface of austenite grains. At the same time, the extremely short heating time locks in grain boundary migration, preventing grains from coarsening and perfectly avoiding the risks of grain coarsening and embrittlement caused by conventional overheating.

[0074] Each step will be explained in detail below:

[0075] The multi-layer gradient heat treatment process of the present invention is applicable to hydraulic shear blades in heavy scrap steel processing equipment. The preferred material for the blade blank is medium-high carbon alloy tool steel such as 4Cr5MoSiV1 steel or 5CrNiMo steel.

[0076] In step S1, the hydraulic scissor blade blank is first heated to a solution temperature of 1050 to 1080°C and held at that temperature for 1 to 2 hours. Then, it is cooled to room temperature using ultrasonic-assisted oil cooling. The initial temperature of the quenching oil is controlled at 60 to 80°C to ensure suitable liquid viscosity and cavitation threshold. The oscillation frequency of the ultrasonic waves is 20-40 kHz, and the power density is 1.0-2.0 W / cm², so as to quickly break the quenching vapor film on the blade surface by utilizing the ultrasonic cavitation effect and increasing the cooling rate. Next, a high-temperature tempering process of 650 to 680°C is performed. This pretreatment step aims to break up the coarse network carbides, eliminate forging stress, and provide a uniform matrix for the subsequent gradient phase transformation.

[0077] In step S2, the pretreated hydraulic shear blade is first preheated to 900 to 920°C in a heating furnace and held at that temperature. This temperature range is lower than the complete austenitization temperature of the steel, so that fine-grained austenite with undissolved alloy carbides is formed in the deep core of the blade, laying the foundation for obtaining high toughness in the core.

[0078] The blade is then moved into the dual-frequency induction heating device, and the intermediate frequency power supply with an output frequency of 2 to 5 kHz is turned on, setting the intermediate frequency output power density to 0.3 to 0.8 kW / cm². 2 The heating duration is 15 to 45 seconds. During this process, a dual-color infrared thermometer is used to monitor the surface temperature of the blade edge in real time. When the surface temperature reaches 1060 to 1080°C, relying on the heat penetration depth and heat conduction at this frequency, the subsurface transition zone of the blade, 3 to 15 mm from the surface, is rapidly heated to 1000 to 1020°C. This allows for the full solid solution of carbides, enabling the transition zone to transform into high-alloy solid solution bainite during subsequent cooling.

[0079] Next, the intermediate frequency power supply is instantaneously cut off via timing interlock control logic, and a dead time of 10 to 50 ms is set between cutting off the intermediate frequency power supply and switching on the high frequency power supply to prevent backflow of dual-frequency electromagnetic harmonics. During the metal thermal inertia insulation window, a high frequency power supply with an output frequency of 50 to 80 kHz is instantaneously switched on for unbalanced pulse heating, raising the temperature of a 3 to 5 mm depth region on the cutting edge surface of the blade to 1120 to 1150 °C. The duration of the high-frequency pulse heating... Strictly follow the aforementioned dynamic coupling relationship. The calculation and control are typically performed within 3 to 5 seconds to form a high-alloy solute concentration halo on the cutting edge surface.

[0080] In step S3, immediately after step S2, high-pressure pure nitrogen gas with a spray pressure of 0.5 to 0.8 MPa is used to pulse-jet pre-cool the cutting edge surface for 2 to 4 seconds. The frequency of the pulse jet is controlled at 5 to 15 Hz, the duty cycle of a single jet is 60% to 80%, and the vertical distance between the nozzle and the cutting edge surface is 15 to 30 mm. Using high-pressure pure nitrogen pulses with the specific frequency and duty cycle described above can not only rapidly reduce the temperature of the cutting edge surface to 1000 to 1020°C to eliminate macroscopic thermal stress, but also effectively prevent surface decarburization reactions at extremely high temperatures.

[0081] The blade is then quickly immersed in the first nitrate bath with forced convection circulation and held at 320°C for 12 to 18 minutes. The flow rate of nitrate in the first nitrate bath is controlled at 0.5 to 1.5 m / s to ensure that the huge latent heat released by the heavy blade is quickly removed and to prevent local boiling out of control. During this isothermal stage, the deep core is preferentially transformed into a high-toughness low-carbon self-tempered martensite composite structure by utilizing the aforementioned austenite solid solubility difference. The subsurface transition zone is then transformed into high-alloy solid solution bainite.

[0082] After the blade is removed from the first nitrate bath, the time it is transferred in the air is strictly controlled to be between 5 and 8 seconds. Through this brief air transfer process, on the one hand, the high-temperature residual salt on the surface of the blade can drip off and be peeled off, and on the other hand, the low heat transfer coefficient of the air provides a thermal buffer for the homogenization of the temperature field inside and outside the blade, avoiding direct cutting into the low-temperature bath and causing a violent thermal shock, thus preventing the initiation of micro-cracks inside the blade.

[0083] The blade is then placed in the second nitrate bath and kept at 260°C for 40 to 50 minutes. It is then removed and air-cooled to room temperature. The second stage of isothermal treatment aims to cause the high alloy concentration austenite on the surface to undergo a mixed transformation into extremely fine bainite or lower bainite, further stabilizing the microstructure and reducing the risk of cracking.

[0084] The quenching medium used in the first and second nitrate baths is a mixed nitrate containing 50% potassium nitrate and 50% sodium nitrite by mass percentage.

[0085] In step S4, during the deep cryogenic and multi-stage gradient tempering stage, the quenched blade is placed in a cryogenic environment of -120°C to -150°C for 2 to 4 hours to induce the transformation of residual austenite into ultrafine martensite. Subsequently, at least two tempering treatments are performed: the first tempering is conducted at 200 to 220°C for 2 to 3 hours to eliminate martensitic transformation stress, and the second tempering is conducted at 540 to 560°C for 2 to 3 hours to precipitate secondary hardened carbides. After this entire process, the cross-section of the hydraulic scissor blade forms a three-layer micro-substructure gradient from the inside out, along with a smoothly transitioning macro-hardness gradient. The Rockwell hardness of the cutting edge surface reaches 58 to 62 HRC, the Rockwell hardness of the subsurface transition zone with a depth of 3 to 15 mm from the surface is 48 to 52 HRC, and the Rockwell hardness of the deep core with a depth greater than 15 mm is 38 to 42 HRC, perfectly balancing the extremely high wear resistance of the surface with the excellent impact toughness of the core.

[0086] Example 1:

[0087] A multi-layer gradient heat treatment process for hydraulic scissor blades is provided. This embodiment demonstrates the optimal parameter configuration of the present invention under standard operating conditions.

[0088] In this embodiment, 4Cr5MoSiV1 steel is selected as the material for the cutting tool blank, and the target hardened layer depth is set. It is 4mm;

[0089] The blank cutting tool is heated to 1065℃ for solution treatment and held at that temperature for 1.5 hours. Then, it is cooled to room temperature by ultrasonic-assisted oil cooling and tempered at 665℃ to obtain a pretreated cutting tool. The pretreated cutting tool is then preheated to 910℃ in a heating furnace and held at that temperature.

[0090] The blade was then moved into the dual-frequency induction heating device, and the intermediate frequency power supply with an output frequency of 3.5kHz was turned on. The intermediate frequency output power was set to 80kW, corresponding to a power density of approximately 0.5 kW / cm³. 2 The heating duration is 25 seconds, at which point the surface temperature reaches approximately 1070°C. The heat conduction causes the subsurface transition zone of the blade to rapidly heat up to 1010°C.

[0091] Next, the intermediate frequency power supply is instantly cut off via timing interlock control logic. After a 30ms dead time, the high-frequency power supply with an output frequency of 65kHz is instantly switched on for unbalanced pulse heating. At this time, the output power of the high-frequency power supply... Set to 120kW, reference power threshold 20kW, phase change buffer coefficient Taking the median value of 0.2, the baseline thermal diffusivity of this 4Cr5MoSiV1 steel at 1020℃ was determined in advance. Approximately 1.79mm 2 / s, substituting into the dynamic coupling equation The calculations were performed to determine the duration of the high-frequency pulse heating. It takes approximately 3.2 seconds to heat the blade to a precise temperature of 1135°C on the cutting edge surface.

[0092] Immediately after heating, the cutting edge surface is pre-cooled by pulse jetting with high-pressure pure nitrogen gas at a pressure of 0.65 MPa for 3 seconds, so that the surface temperature drops to 1010℃.

[0093] The blade was then quickly immersed in a first nitrate bath at 320°C with a flow rate of 1.0 m / s and kept at that temperature for 15 minutes. After being removed, it was transferred in the air for 6 seconds and then immersed in a second nitrate bath at 260°C and kept at that temperature for 45 minutes. After being removed, it was air-cooled to room temperature.

[0094] Finally, the blade was placed in a cryogenic environment at -135°C for 3 hours, and then subjected to a first tempering at 210°C for 2.5 hours and a second tempering at 550°C for 2.5 hours.

[0095] Example 2:

[0096] A multi-layer gradient heat treatment process for hydraulic shear blades is provided. This embodiment demonstrates the parameter configuration of the present invention under extreme lower boundary conditions.

[0097] In this embodiment, 5CrNiMo steel is selected as the material for a relatively thin blade blank, and the target hardened layer depth is set. It is 3mm;

[0098] The blade blank is heated to 1050℃ for solution treatment and held for 1 hour. Then, it is cooled to room temperature with ultrasonic assistance and tempered at 650℃. The pretreated blade is then preheated to 900℃ in a heating furnace and held at that temperature.

[0099] It was then moved into a dual-frequency induction heating device, and the intermediate frequency power supply with an output frequency of 2kHz was turned on. The intermediate frequency output power was set to 50kW, corresponding to a power density of approximately 0.35 kW / cm³. 2 The heating duration is 18 seconds, at which point the surface temperature reaches approximately 1060°C, and the subsurface transition zone rapidly heats up to 1000°C through heat conduction.

[0100] After disconnecting the intermediate frequency power supply and allowing a 10ms dead time, the high frequency power supply with an output frequency of 50kHz is switched on. At this time, the output power of the high frequency power supply is... Set to 80kW, baseline power threshold For 10kW, the phase change buffer coefficient The lower limit value of 0.15 was used, and the baseline thermal diffusivity of the 5CrNiMo steel was determined in advance. Approximately 1.12mm 2 / s, substituting into the dynamic coupling formula, yields the duration of high-frequency pulse heating. Approximately 2.5 seconds; after heating for this time, the surface temperature of the cutting edge rises to 1120℃;

[0101] After heating, high-pressure pure nitrogen gas with a spray pressure of 0.5MPa is sprayed onto the cutting edge surface for 2 seconds to pre-cool it, so that the surface temperature drops to 1000℃.

[0102] Then it was placed in the first nitrate bath at 320℃ with a flow rate of 0.5m / s and kept at that temperature for 12 minutes. After being taken out, it was transferred in the air for 5 seconds and then placed in the second nitrate bath at 260℃ and kept at that temperature for 40 minutes. After being taken out, it was air-cooled to room temperature.

[0103] Finally, the blade was placed in a cryogenic environment at -120°C for 2 hours, and then subjected to a first tempering at 200°C for 2 hours and a second tempering at 540°C for 2 hours.

[0104] Example 3:

[0105] A multi-layer gradient heat treatment process for hydraulic shear blades is provided. This embodiment demonstrates the parameter configuration of the present invention under extreme upper boundary conditions.

[0106] In this embodiment, 4Cr5MoSiV1 steel is selected as the material for the heavy-duty ultra-thick blade blank, and the target hardened layer depth is set. It is 5mm;

[0107] The blade blank was heated to 1080℃ for solution treatment and held for 2 hours. Then, it was cooled to room temperature with ultrasonic assistance and tempered at 680℃. The pretreated blade was then preheated to 920℃ in a heating furnace and held at that temperature.

[0108] It was then moved into a dual-frequency induction heating device, and the intermediate frequency power supply with an output frequency of 5kHz was turned on. The intermediate frequency output power was set to 130kW, corresponding to a power density of approximately 0.75 kW / cm³. 2 The heating duration is 40 seconds, at which point the surface temperature reaches approximately 1080°C, and the subsurface transition zone rapidly heats up to 1020°C through heat conduction.

[0109] After disconnecting the intermediate frequency power supply and allowing a 50ms dead time, the high-frequency power supply with an output frequency of 80kHz is switched on. At this time, the output power of the high-frequency power supply... Set to 150kW, baseline power threshold The calibrated power is 38kW, and the phase change buffer coefficient is... Taking the upper limit of 0.25, based on the 4Cr5MoSiV1 steel material with a thickness of approximately 1.79mm... 2The duration of high-frequency pulse heating is calculated by substituting the baseline thermal diffusivity ( / s) into the dynamic coupling equation. It takes approximately 4.8 seconds to heat the cutting edge to a surface temperature of 1150°C.

[0110] After heating, high-pressure pure nitrogen gas with a spray pressure of 0.8 MPa is sprayed onto the cutting edge surface for 4 seconds to pre-cool it, so that the surface temperature drops to 1020℃.

[0111] Then it was placed in the first nitrate bath at 320℃ with a flow rate of 1.5m / s and kept at that temperature for 18 minutes. After being taken out, it was transferred in the air for 8 seconds and then placed in the second nitrate bath at 260℃ and kept at that temperature for 50 minutes. After being taken out, it was air-cooled to room temperature.

[0112] Finally, the blade was placed in a cryogenic environment at -150°C for 4 hours, and then subjected to a first tempering at 220°C for 3 hours and a second tempering at 560°C for 3 hours.

[0113] To verify the irreplaceability and synergistic effect of each core technical feature in the multi-layer gradient heat treatment process of this invention, the following five comparative examples were set up for feature ablation comparison experiments.

[0114] Comparative Example 1:

[0115] The same 4Cr5MoSiV1 steel as in Example 1 was used;

[0116] The blade was heated to 1020℃ and held until fully austenitized. Then it was directly immersed in the first nitrate bath at 320℃ for 15 minutes, and then transferred to the second nitrate bath at 260℃ for 45 minutes before being air-cooled.

[0117] Without high-pressure pure nitrogen precooling and cryogenic treatment, conventional tempering is performed directly; the remaining steps are the same as in Example 1.

[0118] Comparative Example 2:

[0119] The same 4Cr5MoSiV1 steel as in Example 1 was used;

[0120] The overall preheating and medium-frequency heating steps in step S2 are cancelled. Instead, high-frequency induction heating is directly applied to the surface of the pretreated blade at room temperature. The heating temperature is set to 1150°C. Then, the same cooling and tempering steps as in Example 1 are performed.

[0121] Comparative Example 3:

[0122] The same 4Cr5MoSiV1 steel as in Example 1 was used;

[0123] The overall preheating and medium-frequency heating steps in Example 1 are retained; when switching to high-frequency power supply for non-equilibrium pulse heating, the control of the dynamic coupling relationship is cancelled, and the high-frequency pulse heating time is forcibly fixed to 8s for conventional surface quenching; the remaining steps are exactly the same as in Example 1.

[0124] Comparative Example 4:

[0125] The same 4Cr5MoSiV1 steel as in Example 1 was used;

[0126] The overall preheating step of 910°C in Example 1 is retained; the only change is the control logic of dual-frequency heating, which eliminates the dead time between the intermediate frequency cutoff and the high frequency cutoff, and adopts a mode in which the intermediate and high frequency power supplies are fully synchronized and superimposed; the rest of the steps are exactly the same as in Example 1.

[0127] Comparative Example 5:

[0128] The same 4Cr5MoSiV1 steel as in Example 1 was used; all heating steps in Example 1 were retained; in step S3, the high-pressure pure nitrogen pulse pre-cooling step was eliminated, and the blade that had completed high-frequency heating was directly immersed in a single 320°C nitrate bath for isothermal cooling; the remaining steps were exactly the same as in Example 1.

[0129] In order to accurately characterize the essential differences between the present invention and the above comparative examples in terms of microscopic mechanism and macroscopic service performance, the present invention adopts the following specific performance testing methods.

[0130] Subsurface spalling resistance cycle test: A simulated heavy-duty hydraulic shear alternating fatigue test bench was used to simulate the combined alternating stress of the blade under extreme extrusion and strong shear conditions, and the number of cycles leading to the initiation of microcracks on the subsurface and resulting in block spalling was recorded. This index directly reflects the ability of bainite in the high-alloy solid solution transition zone to buffer the tensile stress of the microstructure.

[0131] Core dynamic crack propagation energy absorption rate test: Instrumented Charpy V-notch impact test was used to extract the proportion of absorbed energy during the crack propagation stage from the impact load-displacement curve. This index is used to characterize the microscopic strength and toughness of the core undissolved carbides and fine-grained bainite synergistically hindering crack propagation.

[0132] Surface solute halo integrity test: Electron probe microanalysis was used to scan the concentration gradient of alloying elements such as carbon, vanadium, and molybdenum along the depth direction of the blade cross-section. This indicator is used to characterize the non-equilibrium state characteristics of alloying elements not undergoing long-range diffusion under extremely short pulse heating and whether high-temperature decarburization occurs.

[0133] Equipment Electrical Stability Index Test: The peak voltage and current distortion rate of the IGBT module in the dual-frequency induction heating equipment inverter are monitored in real-time using an oscilloscope during switching or superposition. This indicator is used to assess the engineering safety of the electrical control logic.

[0134] The performance test data of each embodiment and comparative example are shown in Table 1:

[0135] Table 1. Comparison of performance test data between each embodiment and the comparative example.

[0136]

[0137] Based on the data in Table 1, a comparison between Example 1 and Comparative Example 1 shows that after removing the multi-layer gradient design, the core dynamic crack propagation energy absorption rate of Comparative Example 1 plummeted from 68.5% in Example 1 to 12.3%, and the effective depth of the high-solute layer on the surface was 0. This indicates that traditional integral quenching would cause the core to completely transform into brittle martensite, while the present invention, through gradient heating, retains the undissolved carbides and fine-grained lower bainite in the core, exhibiting overwhelming strength and toughness advantages under heavy impact.

[0138] Comparing Example 1 with Comparative Example 2, it can be seen that Comparative Example 2, lacking the wide transition zone constructed by intermediate frequency preheating, only achieved 24,000 cycles for secondary surface spalling resistance, far lower than the 256,000 cycles in Example 1. This indirectly confirms the crucial role of the triple isothermal transformation C-curve staggered phase transformation mechanism of the present invention in eliminating the difference in specific volume between the surface and interior and the abrupt change in microstructural tensile stress, completely solving the problem of subsurface block spalling that is very easy to occur in conventional surface induction hardening.

[0139] Comparing Example 1 with Comparative Example 3 reveals that when the high-frequency heating time is blindly extended without adhering to the precise constraints of the dynamic coupling formula, the effective depth of the high-solute layer on the surface of Comparative Example 3 shrinks significantly from 125 nm in Example 1 to 15 nm. This results in long-range diffusion of alloying elements and severe grain coarsening. Furthermore, the surface anti-scraping cycle count also drops drastically to 82,000 cycles. This fully demonstrates that the mathematical and physical model constructed in this invention is not a conventional parameter optimization in the field, but rather a fundamental metallurgical switch that locks grain boundary migration and achieves nanoscale solute enrichment.

[0140] Comparing Example 1 with Comparative Example 4, as shown in Table 1, the subsurface anti-stripping cycle count, core dynamic crack propagation energy absorption rate, and effective depth of the surface high solute layer in Comparative Example 4 are all recorded as "-". This is because at the instant both frequencies are simultaneously activated, the peak voltage distortion rate of the IGBT in Comparative Example 4 spikes to 45.8%, causing severe beat frequency interference and high-frequency harmonic backflow, resulting in an instantaneous overload shutdown alarm. Subsequent heat treatment processes and various mechanical performance tests cannot be fully performed. In contrast, Example 1 uses timing interlocking logic to stably control the distortion rate at 1.2%. This indicates that the timing interlocking logic of the present invention is a necessary engineering prerequisite for realizing extreme non-equilibrium overheating, overcoming the technical prejudice that conventional dual-frequency heating cannot be stably implemented on heavy workpieces.

[0141] Comparing Example 1 with Comparative Example 5, it can be seen that without the high-pressure pure nitrogen pre-cooling step, the blade with huge latent heat in Comparative Example 5 directly broke through the cooling limit of the salt bath, causing violent uncontrolled boiling in the salt bath. At the same time, severe oxidation and decarburization occurred on the surface at high temperature, resulting in the effective depth of the high solute layer on the surface directly returning to zero. This highlights the irreplaceable synergistic effect of the composite cooling strategy of the present invention in maintaining a constant temperature field in the salt bath and protecting the high carbon and high solute halo.

[0142] As shown in the test data of Example 2, when the test substrate was changed to 5CrNiMo steel and all process parameters were taken to the lower limit of the protection range, the extremely short pulse heating time adaptively calculated by the system according to the dynamic coupling formula of the present invention could still stably construct a halo structure with an effective depth of 118nm for the high solute layer on the surface. Under this extreme condition, the core dynamic crack propagation energy absorption rate of the obtained blade reached 65.2%, the subsurface anti-stripping cycle count reached 235,000, and the IGBT peak voltage distortion rate was only 1.5%. The above results show that the control model of the present invention can effectively adapt to the changes in the reference thermal diffusivity of different materials and has good cross-material calculation applicability and electrical stability.

[0143] As shown in the test data of Example 3, when the target hardened layer depth is set to 5 mm and all heating and cooling parameters are taken to the upper limit of the protection range, under high heat input and high flow rate conditions, the core dynamic crack propagation energy absorption rate of the resulting blade is further improved to 71.4%, the subsurface anti-stripping cycle count reaches 278,000 cycles, the effective depth of the surface high solute layer reaches 132 nm, and the IGBT peak voltage distortion rate is as low as 1.1%. The above results show that the high-pressure pure nitrogen pulse precooling and dual-groove stepped isothermal cooling strategy with air transfer adopted in this invention still has extremely reliable cooling stability and crack resistance redundancy under the upper limit thermodynamic parameters.

[0144] In summary, this invention, through precise constraints of time-interlocked electrical control logic and dynamic coupling formulas, perfectly constructs a staggered phase transformation of the triple isothermal transformation C-curve on the cross-section of heavy-duty blades. Combined with a composite cooling strategy of high-pressure pure nitrogen precooling and dual-groove stepped isothermal cooling, it successfully achieves a three-layer microstructure gradient within a single quenching cooling path: an outer layer of ultrafine martensite with high solute halo, a middle layer of high-alloy solid solution bainite, and an inner layer of high-toughness low-carbon self-tempered martensite. This process fundamentally solves the engineering pain points of traditional integral quenching's tendency to cause brittle fracture and conventional surface induction quenching's tendency to cause subsurface block spalling. It endows heavy-duty hydraulic scissor blades with extremely high surface wear resistance, excellent core impact toughness, and perfect microstructure tensile stress buffering capacity, while completely avoiding equipment risks such as dual-frequency electromagnetic beat frequency interference and salt bath boiling runaway.

[0145] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A multi-layer gradient heat treatment process for hydraulic scissor blades, characterized in that, Includes the following steps: S1. Heat the hydraulic scissor blade blank to the solution temperature and keep it at that temperature. Cool it to room temperature and then perform high-temperature tempering to obtain the pretreated hydraulic scissor blade. S2. Preheat the entire hydraulic scissor blade to 900-920℃ in a heating furnace and maintain the temperature; move the hydraulic scissor blade into a dual-frequency induction heating device, turn on the intermediate frequency power supply to raise the temperature of the subsurface transition zone of the hydraulic scissor blade 3 to 15 mm from the surface to 1000-1020℃; cut off the intermediate frequency power supply through timing interlock control logic, and switch on the high frequency power supply during the metal thermal inertia heat preservation window to perform non-equilibrium pulse heating, so that the temperature of the area 3-5 mm deep on the cutting edge surface of the hydraulic scissor blade rises to 1120-1150℃; wherein, the duration of the non-equilibrium pulse heating by the high frequency power supply is dynamically calculated and controlled based on the target hardened layer depth of the cutting edge surface and the output power of the high frequency power supply. S3. Pre-cool the cutting edge surface with high-pressure pure nitrogen pulse jet for 2-4 seconds to reduce the temperature of the cutting edge surface to 1000-1020℃; immerse the hydraulic scissor blade in a first nitrate bath with forced convection circulation and keep it at 320℃ for 12-18 minutes; after removal, transfer it to air and immerse it in a second nitrate bath at 260℃ for 40-50 minutes, then remove and air-cool it to room temperature; S4. Place the quenched hydraulic shear blade in a cryogenic environment of -120℃ to -150℃ for 2-4 hours; perform at least two tempering treatments. In step S1, the material of the hydraulic scissor blade blank is 4Cr5MoSiV1 steel or 5CrNiMo steel. In step S4, the at least two tempering processes specifically include: First tempering: Hold at 200-220℃ for 2-3 hours to eliminate martensitic phase transformation stress; Second tempering: Hold at 540-560℃ for 2-3 hours to precipitate secondary hardened carbides.

2. The multi-layer gradient heat treatment process for a hydraulic scissor blade according to claim 1, characterized in that, In step S2, the duration of non-equilibrium pulse heating by the high-frequency power supply is proportional to the square of the target hardened layer depth, proportional to the natural logarithm of the ratio of the output power of the high-frequency power supply to the reference power threshold, and inversely proportional to the reference thermal diffusivity of the hydraulic scissor blade material in the fully austenitized state at 1020℃. It is also corrected by a phase transformation buffer coefficient, which ranges from 0.15 to 0.

25.

3. The multi-layer gradient heat treatment process for a hydraulic scissor blade according to claim 1, characterized in that, In step S2, the dual-frequency induction heating device adopts a single-coil dual-frequency power supply time-division multiplexing architecture. The timing interlock control logic is as follows: between cutting off the intermediate frequency power supply and switching on the high-frequency power supply, the PLC controller forcibly starts a dead time delay timer of 10-50ms, and physically locks the trigger pulse of the high-frequency power supply IGBT module within the dead time to prevent the backflow of dual-frequency electromagnetic harmonics. The intermediate frequency power supply has an output frequency of 2-5kHz and an output power density of 0.3-0.8kW / cm². 2 The heating duration is 15-45 seconds; the output frequency of the high-frequency power supply is 50-80 kHz.

4. The multi-layer gradient heat treatment process for a hydraulic scissor blade according to claim 1, characterized in that, In step S1, the solution temperature is 1050-1080℃, the holding time is 1-2h, and then ultrasonic-assisted oil cooling is used to bring it to room temperature; the high-temperature tempering temperature is 650-680℃.

5. The multi-layer gradient heat treatment process for a hydraulic scissor blade according to claim 1, characterized in that, In step S3, the injection pressure of the high-pressure pure nitrogen gas is 0.5-0.8 MPa, and the frequency of the pulse injection is controlled at 5-15 Hz, with a single injection duty cycle of 60%-80%, so as to eliminate macroscopic thermal stress on the cutting edge surface while preventing high-temperature decarburization.

6. The multi-layer gradient heat treatment process for a hydraulic scissor blade according to claim 1, characterized in that, In step S3, the nitrate flow rate in both the first and second nitrate baths is controlled at 0.5-1.5 m / s. After the hydraulic shear blades are removed from the first nitrate bath, the time they are transferred in the air is strictly controlled to be 5-8 seconds in order to release the structural stress generated by the lower bainitic phase transformation.

7. The multi-layer gradient heat treatment process for a hydraulic scissor blade according to claim 1, characterized in that, After the process in step S4, the cross-section of the hydraulic scissor blade forms a three-layer micro-substructure gradient from the inside out, mainly including the deep core, the subsurface transition zone and the cutting edge surface. The microstructure of the deep core is a high-toughness, low-carbon, self-tempered martensitic composite structure containing undissolved alloy carbides. The microstructure of the subsurface transition zone is high-alloy solid solution bainite. The microstructure of the cutting edge surface is a mixture of ultrafine tempered martensite and dispersed carbides.

8. The multi-layer gradient heat treatment process for a hydraulic scissor blade according to claim 7, characterized in that, After the process in step S4, the hydraulic shear blade forms a smooth transition in macroscopic hardness gradient: The Rockwell hardness of the cutting edge surface is 58-62 HRC; The Rockwell hardness of the subsurface transition zone, which is 3-15 mm deep from the surface, is 48-52 HRC. The Rockwell hardness of the deep core with a depth greater than 15 mm is 38-42 HRC.

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