A crankshaft blank forming process

Abstract

The application discloses a crankshaft blank forming process and relates to the technical field of crankshaft blank forming. Medium-carbon alloy steel is selected as raw material, and a rare earth mixture is added. After vacuum degassing smelting, low-pressure vacuum pouring is combined with electromagnetic rotary stirring to realize steel level stable filling and composition homogenization. Through the conformal cooling structure of the casting mold combined with gradient cooling technology, and by synchronously applying a static magnetic field in the cooling process, the solidification process of the casting is synergistically controlled, and thermal stress concentration and organizational defects are reduced. After the initial forging step, local pressure compensation is applied to the forging blank to close micro-pores and optimize the organizational streamline distribution. After final forging, a nanocrystalline strengthening layer is formed on the surface structure of the crankshaft forging by laser shock peening, and post-processing and multi-stage flaw detection ensure the integrity of the internal and surface structure of the forging. The application combines the advantages of casting and forging processes, reduces the porosity, surface cracks of the crankshaft blank, and improves the material utilization rate in the forming process, and simultaneously improves the mechanical properties of the crankshaft blank.

Description

Technical Field

[0001] The present invention relates to the technical field of crankshaft blank forming, in particular to a crankshaft blank forming process. Background Art

[0002] Crankshafts are key power conversion or transmission components in mechanical equipment such as forging and punching machines. They are widely used in automobiles, ships, aviation, power generation equipment and other fields. Their working environment is complex and they usually need to withstand high-intensity torque, bending force and fatigue loads. Therefore, the mechanical properties, surface quality and overall reliability of the crankshaft have a decisive influence on the operating stability, lifespan and energy efficiency of the mechanical equipment.

[0003] Currently, crankshaft blank forming processes primarily include casting, forging, and powder metallurgy. Forging, as it improves the material's internal structure and mechanical properties, is widely used in the manufacturing of high-performance crankshafts. While traditional crankshaft forging processes can meet the needs of mass production, the increasing market demand for high-performance crankshafts has exposed numerous limitations.

[0004] (1) Surface crack problem

[0005] In existing crankshaft blank forming processes, surface cracks are one of the most common quality defects. These cracks typically occur during forging, heat treatment, or cooling. The main causes include uneven material stress, improper die design, and uneven temperature control during heating or cooling. Surface cracks not only reduce the crankshaft's fatigue resistance but can also expand during subsequent processing or use, leading to premature crankshaft failure and compromising the reliability and safety of the machine.

[0006] (2) Porosity and internal defects

[0007] In addition to surface cracks, pores and internal defects are also important factors affecting the quality of crankshafts. Porosity is mostly caused by the failure to completely discharge gas during the forging process, high impurity content in raw materials, or improper heating temperature control. Porosity can lead to discontinuous structural lines in the metal structure, reducing the mechanical strength, wear resistance, and fatigue resistance of the crankshaft. After high-intensity use, large cracks often occur at the location where pores are formed, thereby affecting the overall stability of the mechanical equipment.

[0008] (3) Low material utilization

[0009] Traditional forging processes require a large machining allowance on the forging blank, resulting in material waste and increasing subsequent processing steps and costs. Furthermore, limitations in die design and uneven metal flow during the forging process can lead to excessive scrap, further reducing material utilization and increasing production costs.

[0010] (4) Limitations of heat treatment and cooling processes

[0011] After forging, crankshaft forgings need to undergo heat treatment to improve their mechanical properties. However, in the existing process, the temperature control in the heating and cooling links is uneven, which can easily lead to internal residual stress, uneven surface hardness and other problems, thereby affecting the wear resistance and crack resistance of the crankshaft; especially in the manufacturing process of large-size crankshafts, the temperature difference problem in the heat treatment process is more prominent, which can easily generate large internal stress, which will directly lead to cracking or deformation of the blank.

[0012] To meet future demands for high-performance crankshafts, existing processes must be improved and optimized to enhance the overall quality of crankshaft blank forming, improve yield rates, extend service life, and reduce production costs. This is not only crucial for improving the power performance of mechanical equipment but also lays a solid foundation for the sustainable development of the crankshaft manufacturing industry.

[0013] In summary, it is found that the existing technology has at least the following technical problems:

[0014] The existing crankshaft blank forming process has process and technical problems such as surface cracks, porosity defects and material waste. Summary of the Invention

[0015] The purpose of the present invention is to provide a crankshaft blank forming process to solve the process and technical problems of surface cracks, porosity defects and material waste in the existing crankshaft blank forming process.

[0016] The various technical effects that can be produced by the preferred technical solutions among the various technical solutions provided by the present invention are described in detail below.

[0017] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0018] The present invention provides a crankshaft blank forming process, comprising the following steps: S1, raw material pretreatment: selecting alloy steel as raw material, adding rare earth mixture, and performing vacuum degassing and smelting;

[0019] S2. Melting: The melting temperature is controlled at 1500-1550℃ and the melting time is controlled at 25-30 minutes;

[0020] S3. Pouring: The casting mold is preheated in advance, and molten steel is poured in a low-pressure vacuum. While pouring, electromagnetic rotation is used to stir the molten steel in the mold;

[0021] S4. Solidification control: In conjunction with the mold-conforming cooling structure, gradient control is adopted for the cooling temperature and cooling rate, with the connecting rod journal and crank parts being cooled first, while the main journal area is kept warm and cooled slowly. A static magnetic field of 0.8-1T is applied to the entire mold during the entire cooling process.

[0022] S5. Take out the crankshaft casting, heat it to the initial die forging temperature, and keep it warm for later use;

[0023] S6. Initial forging: Preheat the forging die in advance and put the crankshaft casting into it for initial die forging;

[0024] S7, local pressure replenishment: perform local pressure replenishment on the connecting rod journal and main journal area, control the temperature in the austenite formation area, control the pressure at 125-135MPa, and maintain the pressure for at least 30s;

[0025] S8. After final forging, the crankshaft forging is formed and placed in the room for air cooling;

[0026] S9, laser shock strengthening: laser shock strengthening is performed on the entire peripheral wall of the forging;

[0027] S10, performing post-processing on the entire forging to form a crankshaft blank;

[0028] S11. After local milling of the main journal area of ​​the crankshaft blank, medium frequency quenching is performed to control the hardness to HRC52-56, and then magnetic particle inspection is performed;

[0029] S12. After ultrasonic testing, the crankshaft blank is inspected and accepted.

[0030] In one embodiment, in step S1, the raw material is medium carbon alloy steel; the added rare earth mixture is a rare earth mixture containing La and Ce, and 0.12Wt%-0.15Wt% of the rare earth mixture is added according to mass percentage during the raw material smelting to refine the grains.

[0031] In one embodiment, in step S3, the casting mold preheating temperature is 900-1000°C, the molten steel pouring temperature is controlled at 1620-1680°C; the filling pressure is controlled at 0.6-0.8 MPa, and the filling speed is controlled at 3-5 m / s.

[0032] In one embodiment, the vacuum degree of the pouring environment of vacuum pouring is controlled at ≤10 -3 Pa is used to eliminate the pores generated when the molten steel is filling the mold.

[0033] In one embodiment, the stirring frequency of the electromagnetic rotary stirring is controlled at 35-50 Hz, and the stirring time is ≥10 min, so as to uniformly distribute the composition of the molten steel in the mold and expel the internal gas.

[0034] In one embodiment, in step S4, the connecting rod journal and crank portion of the crankshaft casting are preferentially water-cooled and the cooling rate is controlled at 10-12°C / s, the cooling rate of the main journal area is controlled at ≤2°C / s, and the cooling rate of the front and rear ends is controlled at 3-5°C / s.

[0035] In one embodiment, the crankshaft casting is heated to 1250°C in step S5; and the temperature of the forging blank is maintained between 1150-1220°C in the initial forging step S6; the temperature is controlled between 920-1000°C in the local pressurization step S7; and after the final forging in step S8, the crankshaft forging is placed in an air cooling temperature zone in the room and cooled to 300°C.

[0036] In one embodiment, in step S9, the surface temperature of the crankshaft forging is raised to 320-330°C, and the surface of the crankshaft forging is laser-shock-hardened; the laser energy density is controlled at 5-8 GW / cm 2 The pulse width is 20ns, which is used to make the average surface residual stress of the entire circumferential wall of the crankshaft forging reach at least -400MPa.

[0037] In one embodiment, in step S10, after the crankshaft forging has passed step S9, it is placed in an air cooling temperature zone and air-cooled to a surface temperature of 300°C, and then the crankshaft forging is placed in a furnace for normalizing treatment; after normalizing treatment, the crankshaft forging is placed in an air cooling temperature zone and air-cooled to 300°C, and then placed in a furnace for tempering treatment; after tempering treatment, the crankshaft forging is placed in a furnace and kept at 650°C for 2 hours. After the holding period, the crankshaft forging is placed in a furnace and cooled to 300°C, and then the crankshaft forging is air-cooled to room temperature.

[0038] In one embodiment, after the journal area of ​​the crankshaft blank is locally magnetic particle inspected and qualified in step S11, the process proceeds to step S12 for ultrasonic inspection and acceptance of the entire crankshaft blank; in step S12, the ultrasonic inspection quality of the crankshaft blank must reach level three before it can be accepted.

[0039] The present invention addresses the problems of surface cracks, porosity defects and material waste that occur during the existing crankshaft blank forming process and proposes a composite process solution. The beneficial effects of the solution are mainly reflected in the following aspects:

[0040] First, defects in crankshaft forming metal are suppressed by optimizing the materials. Adding 0.12-0.15wt% of a rare earth mixture containing La and Ce can significantly refine the grain size inside the casting metal by adsorbing grain boundary impurities (such as sulfur and oxygen) and forming high-melting-point compounds, thereby suppressing the generation of internal or surface cracks in the casting from the root of the overall metal structure.

[0041] Combined with vacuum degassing smelting, vacuum environment pouring and electromagnetic rotary stirring filling, the hydrogen and oxygen contents in the molten steel are reduced, and the porosity is reduced by more than 70% compared with the traditional crankshaft forming process. The subcutaneous pore size of the casting surface is controlled within a diameter of 0.2mm.

[0042] Thirdly, the processes during pouring work together to improve density. During pouring, low-pressure vacuum pouring and electromagnetic stirring are superimposed, and low-pressure filling is used during pouring, combined with electromagnetic stirring, to increase the Reynolds number of the molten metal flow in the casting mold, thereby avoiding the generation of air turbulence during filling. At the same time, the interdendritic component segregation of the metal structure is eliminated, and the shrinkage defect rate is reduced to below 0.3%. The grain distribution is further optimized, the stress field is uniform, and the occurrence of internal and surface defects of the casting is reduced.

[0043] During cooling, gradient cooling and static magnetic field are implemented by controlling the temperature, and a conformal cooling structure is used to give priority to water cooling of the high-stress bend area in the crankshaft casting. The main journal area is subjected to thermal insulation and slow cooling to stagger the cooling rate intervals of the two areas, forming a cooling and forming sequence. Combined with the 0.8-1T static magnetic field applied during the entire cooling process, the dendrite coarsening of the metal structure is jointly suppressed, the thermal stress peak of the casting is reduced by 40%, and the surface crack incidence rate is controlled below 5%; among them, the thermal insulation and slow cooling of the main journal area combined with the static magnetic field treatment significantly improve the fatigue resistance of the main journal area of ​​the casting.

[0044] Furthermore, composite plastic strengthening and laser shock surface modification are combined with die forging to improve the tensile strength and fatigue life of the forgings. After the initial die forging, the temperature of the forging is controlled in the temperature zone where austenite is formed. A local pressure-compensating process is used to apply a high pressure of 125-135 MPa to key parts such as the connecting rod journal and the main journal to promote the closure of micro-pores in the internal structure of the metal, form a continuous streamline structure, optimize the streamline distribution of the metal structure, and improve the tensile strength. The local pressure-compensating process can also ensure higher dimensional accuracy of key parts.

[0045] Then through laser shock peening (LSP), 5-8GW / cm 2 High-energy laser pulses impact the surface of the forging, inducing a nanocrystalline layer with a depth of at least 0.3 mm, causing the surface residual compressive stress to reach at least -400 MPa, and increasing the fatigue life of the forging by more than 50%.

[0046] Furthermore, for the main journal area, through the combination of laser shock strengthening and medium frequency quenching, a gradient strengthening layer is formed on the periphery of the main journal perpendicular to the axis, thereby improving the fatigue limit of the main journal area.

[0047] Green manufacturing and cost optimization are achieved by saving the metal materials used in forming the crankshaft blank. A near-net-shape process design is adopted to cast a crankshaft casting with a shape close to the finished product through a casting mold. The casting is then precision-forged to form a crankshaft forging with enhanced performance. Finally, local milling is combined to form the finished crankshaft blank, thereby increasing the metal material utilization rate of the entire crankshaft blank forming process to more than 85%, reducing machining allowance by at least 30% compared with traditional forging processes.

[0048] Magnetic particle testing is used to detect the key main journal area of ​​the crankshaft forging, and ultrasonic testing is used to detect the entire forging. Through dual-stage testing, online control of crankshaft blank defects is achieved, the defect detection rate is improved to ensure the quality grade of the finished crankshaft blank, and the scrap rate in subsequent crankshaft blank processing is reduced to below 0.2%, saving overall costs.

[0049] In summary, the comprehensive performance of the crankshaft blank is improved. After the crankshaft blank is treated by this process, the hardness of the main journal area reaches HRC52-56, the overall core impact toughness of the crankshaft blank is maintained at AKV≥40J, and the dynamic stiffness is increased by 25%, which can meet the extreme transmission working conditions of high-power density mechanical equipment and greatly improve the comprehensive mechanical properties and operating conditions of the crankshaft blank. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solution of the present invention, the following is a brief introduction to the drawings required for use in the implementation. Obviously, the drawings described below are only some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0051] Fig. 1 It is a structural schematic diagram of the crankshaft blank of the present invention;

[0052] Fig. 2 Schematic diagram of the internal fiber structure of the crankshaft blank of the present invention;

[0053] Fig. 3 It is a flow chart of the crankshaft blank forming process of the present invention.

[0054] The accompanying drawings are numerals as follows:

[0055] 1. Crankshaft blank; 11. Front end; 12. Rear end; 13. Main journal; 14. Connecting rod journal; 15. Crank. DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0057] A specific embodiment provides a crankshaft blank forming process, in which medium-carbon alloy steel is selected as raw material, a rare earth mixture is added, and after vacuum degassing and smelting, low-pressure vacuum pouring combined with electromagnetic rotary stirring is used to achieve smooth filling of molten steel and uniform composition; the conformal cooling structure of the casting mold is combined with gradient cooling technology, and a static magnetic field is simultaneously applied during the cooling process to synergistically control the solidification process of the casting, reduce thermal stress concentration and structural defects; after the initial forging step, local compensating pressure is applied to the forging blank to close microscopic pores and optimize the streamline distribution of the metal structure; after final forging, the surface structure of the crankshaft forging is strengthened by laser shock to form a nanocrystalline strengthening layer, and the integrity of the internal and surface structure of the forging is ensured by combining post-processing and multi-stage flaw detection; the present invention combines the advantages of casting and forging processes, reduces the pores and surface cracks of the crankshaft blank, and improves the material utilization rate in the forming process, while improving the mechanical properties of the crankshaft blank; effectively solves the process and technical problems of surface cracks, pore defects and material waste in the existing crankshaft blank forming process.

[0058] The first embodiment of the crankshaft blank forming process is as follows Figs. 1 to 3 As shown, the process comprises the following steps: S1, raw material pretreatment: alloy steel is selected as raw material, rare earth mixture is added, and vacuum degassing and smelting is performed;

[0059] S2. Melting: The melting temperature is controlled at 1500-1550℃ and the melting time is controlled at 25-30 minutes;

[0060] S3. Pouring: The casting mold is preheated in advance, and molten steel is poured in a low-pressure vacuum. While pouring, electromagnetic rotation is used to stir the molten steel in the mold;

[0061] S4. Solidification control: In conjunction with the mold-conforming cooling structure, gradient control is adopted for the cooling temperature and cooling rate, with the connecting rod journal and crank parts being cooled first, while the main journal area is kept warm and cooled slowly. A static magnetic field of 0.8-1T is applied to the entire mold during the entire cooling process.

[0062] S5. Take out the crankshaft casting, heat it to the initial die forging temperature, and keep it warm for later use;

[0063] S6. Initial forging: Preheat the forging die in advance and put the crankshaft casting into it for initial die forging;

[0064] S7, local pressure replenishment: perform local pressure replenishment on the connecting rod journal and main journal area, control the temperature in the austenite formation area, control the pressure at 125-135MPa, and maintain the pressure for at least 30s;

[0065] S8. After final forging, the crankshaft forging is formed and placed in the room for air cooling;

[0066] S9, laser shock strengthening: laser shock strengthening is performed on the entire peripheral wall of the forging;

[0067] S10, performing post-processing on the entire forging to form a crankshaft blank;

[0068] S11. After local milling of the main journal area of ​​the crankshaft blank, medium frequency quenching is performed to control the hardness to HRC52-56, and then magnetic particle inspection is performed;

[0069] S12. After ultrasonic testing, the crankshaft blank is inspected and accepted.

[0070] The crankshaft blank forming process provided by the present invention can effectively solve the common problems of surface cracks, porosity defects and material waste in the existing process through multiple process steps, and significantly improve the overall quality level of the crankshaft blank. The specific beneficial effects are analyzed as follows: (1) First, the defects of the crankshaft forming metal are suppressed by optimizing the material; 0.12-0.15wt% of a rare earth mixture containing La and Ce is added, which can significantly refine the grain size inside the casting metal by adsorbing grain boundary impurities (such as sulfur and oxygen) and forming high-melting-point compounds, thereby suppressing the generation of internal or surface cracks in the casting from the root of the overall metal structure.

[0071] Combined with vacuum degassing smelting, vacuum environment pouring and electromagnetic rotary stirring filling, the hydrogen and oxygen contents in the molten steel are reduced, and the porosity is reduced by more than 70% compared with the traditional crankshaft forming process. The subcutaneous pore size of the casting surface is controlled within a diameter of 0.2mm.

[0072] (2) Again, the processes in casting work together to improve the density; during casting, low-pressure vacuum casting and electromagnetic stirring are superimposed, and low-pressure filling is adopted during casting, combined with electromagnetic stirring, so that the Reynolds number of the metal liquid flow in the casting mold is increased, thereby avoiding the generation of air turbulence during filling, and at the same time eliminating the interdendritic component segregation of the metal structure, reducing the shrinkage defect rate to below 0.3%, further optimizing the grain distribution, uniformizing the stress field, and reducing the occurrence of internal and surface defects of the casting.

[0073] During cooling, gradient cooling and static magnetic field are implemented by controlling the temperature, and a conformal cooling structure is used to give priority to water cooling of the high-stress bend area in the crankshaft casting. The main journal area is subjected to thermal insulation and slow cooling to stagger the cooling rate intervals of the two areas, forming a cooling and forming sequence. Combined with the 0.8-1T static magnetic field applied during the entire cooling process, the dendrite coarsening of the metal structure is jointly suppressed, the thermal stress peak of the casting is reduced by 40%, and the surface crack incidence rate is controlled below 5%; among them, the thermal insulation and slow cooling of the main journal area combined with the static magnetic field treatment significantly improve the fatigue resistance of the main journal area of ​​the casting.

[0074] (3) Furthermore, the composite plastic strengthening and laser shock surface modification are combined with die forging to improve the tensile strength and fatigue life of the forgings; after the initial die forging, the temperature of the forging is controlled in the temperature zone where austenite is formed, and a local pressure-compensating process is used to apply a high pressure of 125-135 MPa to key parts such as the connecting rod journal and the main journal to promote the closure of micro-pores in the internal structure of the metal, and form a continuous streamline structure to optimize the streamline distribution of the metal structure and improve the tensile strength; and the local pressure-compensating process can ensure higher dimensional accuracy of key parts.

[0075] Then through laser shock peening (LSP), 5-8GW / cm 2 High-energy laser pulses impact the surface of the forging, inducing a nanocrystalline layer with a depth of at least 0.3 mm, causing the surface residual compressive stress to reach at least -400 MPa, and increasing the fatigue life of the forging by more than 50%.

[0076] Furthermore, for the main journal area, through the combination of laser shock strengthening and medium frequency quenching, a gradient strengthening layer is formed on the periphery of the main journal perpendicular to the axis, thereby improving the fatigue limit of the main journal area.

[0077] (4) Save the metal materials used in forming the crankshaft blank, realize green manufacturing and cost optimization; by adopting the process design of near net shape, the crankshaft casting with a shape close to the finished product is cast through the casting mold, and then the casting is precision forged to form a crankshaft forging with enhanced performance, and finally combined with local milling to form the finished crankshaft blank, thereby increasing the metal material utilization rate of the entire crankshaft blank forming process to more than 85%, and reducing the machining allowance by at least 30% compared with the traditional forging process.

[0078] Magnetic particle testing is used to detect the key main journal area of ​​the crankshaft forging, and ultrasonic testing is used to detect the entire forging. Through dual-stage testing, online control of crankshaft blank defects is achieved, the defect detection rate is improved to ensure the quality grade of the finished crankshaft blank, and the scrap rate in subsequent crankshaft blank processing is reduced to below 0.2%, saving overall costs.

[0079] (5) Improve the comprehensive performance of the crankshaft blank. After the crankshaft blank is treated by this process, the hardness of the main journal area reaches HRC52-56, the core toughness of the crankshaft blank as a whole remains AKV≥40J, and the dynamic stiffness is increased by 25%, which can meet the extreme transmission working conditions of high-power density mechanical equipment and greatly improve the comprehensive mechanical properties and operating conditions of the crankshaft blank.

[0080] As one optional implementation method,

[0081] In step S1, the raw material is medium-carbon alloy steel; the added rare earth mixture is a rare earth mixture containing La and Ce, and 0.12Wt%-0.15Wt% of the rare earth mixture is added in the raw material smelting for grain refinement.

[0082] Specifically, the medium-carbon alloy steel is selected as 42CrMo.

[0083] Regarding step S3: In step S3, the preheating temperature of the casting mold is 900-1000℃, and the pouring temperature of the molten steel is controlled at 1620-1680℃; the filling pressure is controlled at 0.6-0.8MPa, and the filling speed is controlled at 3-5m / s.

[0084] In addition, the vacuum degree of the pouring environment of vacuum pouring is controlled at ≤10 -3 Pa, for eliminating the pores generated by the molten steel during filling.

[0085] At the same time of pouring, electromagnetic rotary stirring is used for the whole casting mold, and the stirring frequency is controlled at 35-50Hz, and the stirring time is ≥10min.

[0086] In application, the preheating of the casting mold can reduce the temperature difference between the molten steel and the casting mold, improve the flowability of the molten steel on the mold, and effectively reduce the pores generated by pouring through vacuum pouring; the electromagnetic rotary stirring is used for uniformly discharging the internal gas of the molten steel in the mold.

[0087] Regarding step S4: In step S4, the connecting rod journal and the crank part of the crankshaft casting are preferentially water-cooled, and the cooling rate is controlled at 10-12℃ / s, the cooling rate of the main journal area is controlled at ≤2℃ / s, and the cooling rate of the front and rear ends is controlled at 3-5℃ / s.

[0088] In application, the cooling structure of the casting mold adopts a type-following cooling structure, so that the temperature distribution of each structural area of the casting is uniform during cooling, thereby improving the effect of temperature gradient control.

[0089] Regarding the control of the die forging step: In step S5, the crankshaft casting is heated to 1250℃; in the preliminary forging step of step S6, the forging blank temperature is maintained at 1150-1220℃; in step S7, the local pressure compensation temperature is controlled at 920-1000℃; after step S8, the crankshaft forging is placed in the air cooling temperature zone in the room, and the crankshaft forging is cooled to 300℃.

[0090] In application, the temperature of the crankshaft forging in step S8 must be controlled at 800 degrees.

[0091] Regarding step S9: in step S9, the surface temperature of the crank forging is raised to 320-330 DEG C, and the crank forging is subjected to surface laser shock peening; the laser energy density is controlled to be 5-8 GW / cm 2 , and the pulse width is 20 ns.

[0092] In application, the crank forging is subjected to surface laser shock peening (LSP), so that the overall peripheral wall of the crank forging, i.e., the surface layer, is plastically deformed, thereby generating a compressive residual stress in the impact area, and the average residual stress of the surface of the crank forging is at least -400 MPa, which can effectively offset the tensile stress generated in the use of the crank, thereby improving the fatigue strength and fatigue life of the material, and also refining the surface layer grains of the crank and increasing the surface hardness, thereby improving the wear resistance of the crank.

[0093] Regarding the post-processing step: in step S10, after the crank forging is subjected to step S9, the crank forging is cooled to a surface temperature of 300 DEG C in an air cooling temperature zone, and then is subjected to normalizing treatment in a furnace; after the normalizing treatment, the crank forging is air-cooled to 300 DEG C in an air cooling temperature zone, and then is subjected to tempering treatment in a furnace; after the tempering treatment, the crank forging is kept at 650 DEG C for 2 hours in a furnace, and then is furnace-cooled to 300 DEG C in the furnace, and then is air-cooled to room temperature.

[0094] Regarding the quality inspection step: in step S11, after the crankshaft journal area of the crank forging is subjected to local magnetic particle inspection and is qualified, the crank forging is subjected to overall ultrasonic inspection in step S12; in step S12, the ultrasonic inspection quality of the crank forging is required to be at least level three to be accepted.

[0095] In application, the crank forging is subjected to ultrasonic inspection according to GB / T6402-08, and is accepted when the quality is at least level three.

[0096] The technical features of the above embodiments can be combined in any manner. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described.

Claims

1. A crankshaft blank forming process, characterized in that: The following steps are involved: S1. Raw material pretreatment: alloy steel is selected as raw material, rare earth mixture is added, and vacuum degassing and smelting are carried out; S2. Melting: The melting temperature is controlled at 1500-1550℃ and the melting time is controlled at 25-30 minutes; S3. Pouring: The casting mold is preheated in advance, and molten steel is poured in a low-pressure vacuum. While pouring, electromagnetic rotation is used to stir the molten steel in the mold; S4. Solidification control: In conjunction with the mold-conforming cooling structure, gradient control is adopted for the cooling temperature and cooling rate, with the connecting rod journal and crank parts being cooled first, while the main journal area is kept warm and cooled slowly. A static magnetic field of 0.8-1T is applied to the entire mold during the entire cooling process. S5. Take out the crankshaft casting, heat it to the initial die forging temperature, and keep it warm for later use; S6. Initial forging: Preheat the forging die in advance and put the crankshaft casting into it for initial die forging; S7, local pressure replenishment: local pressure replenishment is performed on the connecting rod journal and main journal area, the temperature is controlled in the austenite formation area, the pressure is controlled at 125-135MPa, and the pressure is maintained for at least 30s; S8. After final forging, the crankshaft forging is formed and placed in the room for air cooling; S9, laser shock strengthening: laser shock strengthening is performed on the entire peripheral wall of the forging; S10, performing post-processing on the entire forging to form a crankshaft blank; S11. After local milling of the main journal area of ​​the crankshaft blank, medium frequency quenching is performed to control the hardness to HRC52-56, and then magnetic particle inspection is performed; S12. After ultrasonic testing, the crankshaft blank is inspected and accepted.

2. The crankshaft blank forming process according to claim 1, characterized in that: In step S1 , the raw material is medium carbon alloy steel; the added rare earth mixture is a rare earth mixture containing La and Ce, and 0.12Wt%-0.15Wt% of the rare earth mixture is added according to mass percentage during the raw material smelting to refine the grains.

3. The crankshaft blank forming process according to claim 1, characterized in that: In step S3, the casting mold is preheated to a temperature of 900-1000°C, the molten steel pouring temperature is controlled at 1620-1680°C; the filling pressure is controlled at 0.6-0.8 MPa, and the filling speed is controlled at 3-5 m / s.

4. The crankshaft blank forming process according to claim 3, characterized in that: The vacuum degree of the pouring environment of vacuum pouring is controlled at ≤10 -3 Pa is used to eliminate the pores generated when the molten steel is filling the mold.

5. The crankshaft blank forming process according to claim 3, characterized in that: The stirring frequency of the electromagnetic rotary stirring is controlled at 35-50Hz, and the stirring time is ≥10min, which is used to uniformly distribute the composition of the molten steel in the mold and discharge the internal gas.

6. The crankshaft blank forming process according to claim 1, characterized in that: In step S4, the connecting rod journal and crank part of the crankshaft casting are preferentially water-cooled and the cooling rate is controlled at 10-12°C / s, the cooling rate of the main journal area is controlled at ≤2°C / s, and the cooling rate of the front and rear ends is controlled at 3-5°C / s.

7. The crankshaft blank forming process according to claim 1, characterized in that: In step S5, the crankshaft casting is heated to 1250°C; and in step S6, the temperature of the forging blank is maintained between 1150°C and 1220°C; In step S7, the temperature is controlled between 920°C and 1000°C during the local pressure replenishment; After the final forging in step S8, the crankshaft forging is placed in an air cooling zone in the room and cooled to 300°C.

8. The crankshaft blank forming process according to claim 1, characterized in that: In step S9, the surface temperature of the crankshaft forging is raised to 320-330°C, and the surface of the crankshaft forging is laser shock strengthened; the laser energy density is controlled at 5-8GW / cm 2 The pulse width is 20ns, which is used to make the average surface residual stress of the entire circumferential wall of the crankshaft forging reach at least -400MPa.

9. The crankshaft blank forming process according to claim 1, characterized in that: In step S10, the crankshaft forging is placed in an air cooling zone to be air-cooled to a surface temperature of 300° C. after step S9, and then placed in a furnace for normalizing treatment. After normalizing, the crankshaft forgings are placed in an air-cooling zone and cooled to 300°C, and then put into a furnace for tempering. After tempering treatment, the crankshaft forging is placed in a furnace and kept at 650°C for 2 hours. After the insulation is completed, the crankshaft forging is placed in a furnace and cooled to 300°C, and then air-cooled to room temperature.

10. The crankshaft blank forming process according to claim 1, characterized in that: After the crankshaft blank is subjected to local magnetic particle inspection in the journal area and passes the inspection in step S11, the entire crankshaft blank is subjected to ultrasonic inspection and acceptance in step S12; In step S12, the ultrasonic flaw detection quality of the crankshaft blank must reach level three before it can be accepted.

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