Quenching hardening method for surface of alloy steel planetary gear formed by cold precision forging

Abstract

The invention discloses a cold precision forging formed alloy steel planetary gear surface quenching hardening method which comprises the following steps: S1, exhausting, heating and raising the temperature to 940-960 DEG C, and keeping the constant temperature for 40-50 minutes; s2, strong infiltration treatment, wherein the strong infiltration time is 11-12 h; s3, a diffusion stage; s4, cooling treatment; s5, temperature equalization treatment; s6, furnace cooling treatment is conducted, specifically, furnace cooling is conducted to room temperature; s7, high-temperature tempering is conducted, specifically, an argon protection tube furnace is adopted for high-temperature tempering treatment; s8, discharging and air-cooling to room temperature; s9, induction quenching; s10, oil quenching treatment is conducted; s11, subzero treatment is conducted; s12, low-temperature tempering is conducted; and S13, finally air-cooling to room temperature to obtain a product. After induction quenching, the metallographic structure of the surface layer of the alloy steel planetary gear is converted into fine needle-shaped martensite from a pearlite and ferritic structure, a high-hardness quenching layer is formed on the surface, the average hardness is 600 HV and is larger than the initial hardness of 270 HV, and the alloy steel planetary gear with the hardness remarkably improved is successfully obtained.

Description

Technical Field

[0001] The invention relates to the technical field of high-temperature alloy materials, in particular to a surface quenching and hardening method for a cold precision forged alloy steel planetary gear. Background Art

[0002] Cold precision forging is a casting process technology in a low temperature environment or under normal temperature conditions. It is a primary and secondary processing method that uses a mold installed on a cold forging device to make the metal blank plastically flow to form parts. As a forming process with high material utilization rate and high production efficiency, cold precision forging has the characteristics of less cutting, and its formed products have good surface quality and high dimensional accuracy. However, the surface strength of the products cannot be guaranteed, so the subsequent heat treatment of cold precision forged products is very important.

[0003] Planetary gears are an important part of the differential in the automobile transmission system. They have high precision requirements and strict safety requirements, and are often formed by cold precision forging. The service conditions require them to have excellent wear resistance, high contact fatigue strength and bending fatigue strength, as well as high impact resistance and overload resistance. Contact fatigue strength, bending fatigue strength and surface hardness are the key to ensuring the reliable and efficient service of planetary gears. While ensuring that the core of the gear has sufficient toughness, the surface is required to have high hardness and wear resistance.

[0004] 18Cr2Ni4W is an alloy steel commonly used in planetary gears. It is characterized by good wear resistance, good hardenability, good toughness, excellent wear resistance, fatigue performance and high processing accuracy. It can be used after shot peening or electric spark surface strengthening.

[0005] The composition of 18Cr2Ni4W is as follows: C: 0.14-0.18%, Si: 0.17-0.37%, Mn: 0.40-0.60%, Cr: 1.45-1.65%, Ni: 4.00-4.50%, W: 0.90-1.20%, P≤0.010%, S≤0.003%, Mo≤0.10%, Cu≤0.10%, Alt: 0.030-0.050%.

[0006] The material of the planetary gear surface will be in a high alternating stress state, and surface strengthening is required. Shot peening can only improve the normal stress but not the fracture resistance of shear stress; EDM surface strengthening has many problems such as unstable discharge, uneven strengthening layer, microcracks and hole defects. Therefore, it is urgent to develop a cold forged alloy steel planetary gear surface quenching hardening method to solve the above technical problems. Summary of the invention

[0007] The object of the present invention is to provide a surface quenching and hardening method for a cold precision forged alloy steel planetary gear to solve the problems raised in the above background technology.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] A method for surface quenching and hardening of a cold-forged alloy steel planetary gear comprises the following steps: subjecting the cold-forged alloy steel planetary gear to a carburizing heat treatment, a high-temperature tempering, an induction quenching, a cryogenic treatment, and a low-temperature tempering process. The alloy steel planetary gear is cleaned and dried and then placed in a controlled atmosphere device for treatment.

[0010] S1: Exhaust gas is heated to 940-960℃ and kept at a constant temperature for 40-50min. During this period, the carbon potential of the controlled atmosphere device is controlled to be 1.20% to 1.28%;

[0011] S2: strong infiltration treatment, strong infiltration time 11h-12h, during which the carbon potential in the controlled atmosphere device is controlled at 1.20%-1.28%;

[0012] S3: Diffusion stage, diffusion time 11h-12h, during which the carbon potential in the controlled atmosphere device is controlled at 0.9%;

[0013] S4: Cooling treatment, cooling to 910-930°C, during which the carbon potential in the controlled atmosphere device is controlled at 0.9%;

[0014] S5: Temperature equalization treatment, the temperature equalization time is 150-180min, and the carbon potential in the controlled atmosphere device is controlled at 0.9% during this period;

[0015] S6: furnace cooling treatment, the furnace is cooled to room temperature, and the carbon potential in the controlled atmosphere device is controlled at 0.9% during this period;

[0016] S7: High temperature tempering, using argon protection tube furnace for high temperature tempering treatment, tempering heating to 640-660℃, holding time 150-180min;

[0017] S8: Take out of the oven and cool to room temperature;

[0018] S9: Induction quenching, heating to 820-840℃ in an argon protective atmosphere, keeping warm for 40-50min;

[0019] S10: Oil quenching treatment, the oil quenching temperature is 60-80℃;

[0020] S11: cryogenic treatment, placed in liquid nitrogen at -196°C, holding time 11-12h;

[0021] S12: Low temperature tempering, using oil bath at 175°C, tempering time 90-120min;

[0022] S13: Finally, air cool to room temperature to obtain the product.

[0023] As a further solution of the present invention: a controllable atmosphere device, a gas device with controllable composition used to protect the metal surface and adjust the chemical composition of the metal surface when heating the metal. When the metal is heated in the controlled atmosphere device, surface oxidation or decarburization can be avoided, and the surface can also be carburized as required. The controlled atmosphere is generally composed of a mixture of two or more gases. When only surface oxidation is to be avoided, a single gas can also be used as a protective gas, such as hydrogen, argon or nitrogen. The controlled atmosphere device is used for metal heat treatment and also for low-oxidation or non-oxidation heating of forgings.

[0024] As a further solution of the present invention: the air cooling is natural cooling or wind cooling in the air.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention designs a high-temperature tempering treatment after carburizing treatment. Due to the high carburizing potential and the strong carbon solubility of the alloy system, a large amount of carbon atoms are dissolved into the martensite matrix during the long carburizing process. The high carbon content brought by the carburizing process easily causes a large amount of residual austenite on the surface, reduces the strength and increases the dimensional instability. The high-temperature tempering treatment can reduce the carbon content in the matrix, precipitate carbides, and make the surface layer a coarse martensite structure, thereby avoiding the brittleness of the subsequent quenching high-carbon martensite and the generation of quenching cracks. After induction quenching, the metallographic structure of the surface layer of the alloy steel planetary gear of the present invention is transformed from pearlite plus ferrite structure into fine needle-shaped martensite, and a high-hardness quenching layer is formed on the surface, with an average hardness of 600HV, which is greater than the initial hardness of 270HV, and an alloy steel planetary gear with significantly improved hardness is successfully obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a heat treatment process flow chart for the surface quenching hardening method of alloy steel planetary gears formed by cold precision forging.

[0028] Figure 2 This is the Vickers hardness diagram of the alloy steel planetary gear formed by cold forging after surface quenching and hardening. DETAILED DESCRIPTION

[0029] Various exemplary embodiments, features and aspects of the present application will be described in detail below with reference to the accompanying drawings. The same reference numerals in the accompanying drawings represent elements with the same or similar functions. Although various aspects of the embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless otherwise specified.

[0030] The word “exemplary” is used exclusively herein to mean “serving as an example, example, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments.

[0031] In addition, in order to better illustrate the present application, numerous specific details are provided in the specific embodiments below. It should be understood by those skilled in the art that the present application can also be implemented without certain specific details. In some examples, methods, means, and elements well known to those skilled in the art are not described in detail in order to highlight the subject matter of the present application.

[0032] Embodiment 1

[0033] See also Figure 1 A method for surface quenching and hardening of cold-forged alloy steel planetary gears, which is to subject the alloy steel planetary gears to carburizing heat treatment, high-temperature tempering, induction quenching, cryogenic treatment, and low-temperature tempering processes, wherein the alloy steel planetary gears are cleaned and dried, and then placed in a controlled atmosphere device for treatment, comprising the following steps:

[0034] S1: Exhaust gas is heated to 940-960℃ and kept at a constant temperature for 40-50min. During this period, the carbon potential of the controlled atmosphere device is controlled to be 1.20% to 1.28%;

[0035] S2: strong infiltration treatment, strong infiltration time 11h-12h, during which the carbon potential in the controlled atmosphere device is controlled at 1.20%-1.28%;

[0036] S3: Diffusion stage, diffusion time 11h-12h, during which the carbon potential in the controlled atmosphere device is controlled at 0.9%;

[0037] S4: Cooling treatment, cooling to 910-930°C, during which the carbon potential in the controlled atmosphere device is controlled at 0.9%;

[0038] S5: Temperature equalization treatment, the temperature equalization time is 150-180min, and the carbon potential in the controlled atmosphere device is controlled at 0.9% during this period;

[0039] S6: furnace cooling treatment, the furnace is cooled to room temperature, and the carbon potential in the controlled atmosphere device is controlled at 0.9% during this period;

[0040] S7: High temperature tempering, using argon protection tube furnace for high temperature tempering treatment, tempering heating to 640-660℃, holding time 150-180min;

[0041] S8: Take out of the oven and cool to room temperature;

[0042] S9: Induction quenching, heating to 820-840℃ in an argon protective atmosphere, keeping warm for 40-50min;

[0043] S10: Oil quenching treatment, the oil quenching temperature is 60-80℃;

[0044] S11: cryogenic treatment, placed in liquid nitrogen at -196°C, holding time 11-12h;

[0045] S12: Low temperature tempering, using oil bath at 175°C, tempering time 90-120min;

[0046] S13: Finally, air cool to room temperature to obtain the product.

[0047] Preferably, in this embodiment, a controlled atmosphere device is a gas device with controllable composition used to protect the metal surface and adjust the chemical composition of the metal surface when heating the metal. When the metal is heated in the controlled atmosphere device, surface oxidation or decarburization can be avoided, and the surface can also be carburized as required. The controlled atmosphere is generally composed of a mixture of two or more gases. When only surface oxidation is to be avoided, a single gas can also be used as a protective gas, such as hydrogen, argon or nitrogen. The controlled atmosphere device is used for metal heat treatment and is also used for low-oxidation or non-oxidation heating of forgings.

[0048] Preferably, in this embodiment, the air cooling is natural cooling or wind cooling in air.

[0049] It should be specifically explained that the present invention designs a high-temperature tempering treatment after carburizing treatment. Because the carburizing potential is high and the carbon solubility of the alloy system is strong, a large number of carbon atoms are dissolved into the martensite matrix during the long carburizing process. The high carbon content brought by the carburizing process easily causes a large amount of residual austenite on the surface, reduces the strength and increases the dimensional instability. The high-temperature tempering treatment can reduce the carbon content in the matrix, precipitate carbides, and make the surface layer a coarse martensite structure. The brittleness of the high-carbon martensite and the generation of quenching cracks in the subsequent quenching are avoided.

[0050] The present invention designs cryogenic treatment, because an appropriate amount of retained austenite is beneficial to improving the toughness of carburized steel, but excessive retained austenite often reduces the intrinsic strengthening and solid solution strengthening effects of martensite in steel, which is harmful to the strength, wear resistance and fatigue resistance of planetary gears. 18Cr2Ni4W steel contains a large amount of alloying elements such as Cr and Ni, which improves the stability of austenite and the hardenability of steel. After carburizing and quenching, a large amount of carbon and other alloying elements are dissolved in the matrix, which significantly reduces the Ms point. Therefore, there is retained austenite after quenching, and the surface hardness decreases. Therefore, continuing to apply cryogenic treatment after carburizing and quenching can provide further phase change driving force, promote further phase change of retained austenite, and reduce the adverse effect of excessive retained austenite on hardness. After cryogenic treatment, the surface structure of the planetary gear undergoes a significant change, the retained austenite is significantly reduced, and the surface structure is finely distributed flaky martensite and needle-shaped martensite structure.

[0051] The present invention designs low-temperature tempering because after the carburized sample is subjected to induction heating quenching and deep freezing treatment, the martensite transformation is relatively more complete, and the faster heating rate of induction heating will cause greater lattice distortion of the organization and more microscopic defects; after low-temperature tempering treatment, the organization of the carburized layer undergoes a significant transformation, consisting of needle-shaped martensite and block-shaped and strip-shaped carbides, which can effectively reduce the quenching residual stress of the surface layer.

[0052] See also Figure 2 Specifically, the 18Cr2Ni4W planetary gear parts are sent into a controllable atmosphere furnace, the exhaust temperature is raised to 950±10℃, kept warm for 11 hours, and the carbon potential in the furnace is controlled to be 1.2% for strong penetration treatment; then the carbon potential in the furnace is controlled to be 0.9%, the temperature is 950±10℃, and it is kept warm for 11 hours for diffusion treatment; then the furnace temperature is reduced to 920±10℃, the carbon potential in the furnace is controlled at 0.9%, and the furnace is cooled to room temperature after keeping warm for 3 hours; then high-temperature tempering is heated to 650±10℃, kept warm for 160min, and then air-cooled to room temperature; induction quenching is performed, and the temperature is kept at 830±10℃ for 45min; after oil quenching at 70±10℃, deep cryogenic treatment is performed, placed in liquid nitrogen -196℃, and the temperature is kept for 11h; low-temperature tempering is performed at 175℃ in an oil bath for 100min; finally, air-cooling is performed to room temperature to obtain planetary gear parts.

[0053] After testing, the surface hardness of the alloy steel planetary gear parts can reach 600HV, and the penetration layer depth is 3.5mm, which is greater than the initial hardness of 270HV after cold forging. The alloy steel planetary gear with significantly improved hardness was successfully obtained.

[0054] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be considered exemplary and non-restrictive in all respects, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present invention.

[0055] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for surface quenching and hardening of cold-forged alloy steel planetary gears, which is to subject the cold-forged alloy steel planetary gears to carburizing heat treatment, high-temperature tempering, induction quenching, cryogenic treatment, and low-temperature tempering processes, wherein the alloy steel planetary gears are cleaned and dried and then placed in a controlled atmosphere device for treatment, characterized in that: The following steps are involved: S1: Exhaust gas is heated to 940-960℃ and kept at a constant temperature for 40-50min. During this period, the carbon potential of the controlled atmosphere device is controlled to be 1.20% to 1.28%; S2: strong infiltration treatment, strong infiltration time 11h-12h, during which the carbon potential in the controlled atmosphere device is controlled at 1.20%-1.28%; S3: Diffusion stage, diffusion time 11h-12h, during which the carbon potential in the controlled atmosphere device is controlled at 0.9%; S4: Cooling treatment, cooling to 910-930°C, during which the carbon potential in the controlled atmosphere device is controlled at 0.9%; S5: Temperature equalization treatment, the temperature equalization time is 150-180min, and the carbon potential in the controlled atmosphere device is controlled at 0.9% during this period; S6: furnace cooling treatment, the furnace is cooled to room temperature, and the carbon potential in the controlled atmosphere device is controlled at 0.9% during this period; S7: High temperature tempering, using argon protection tube furnace for high temperature tempering treatment, tempering heating to 640-660℃, holding time 150-180min; S8: Take out of the oven and cool to room temperature; S9: Induction quenching, heating to 820-840℃ in an argon protective atmosphere, keeping warm for 40-50min; S10: Oil quenching treatment, the oil quenching temperature is 60-80℃; S11: cryogenic treatment, placed in liquid nitrogen at -196°C, holding time 11-12h; S12: Low temperature tempering, using oil bath at 175°C, tempering time 90-120min; S13: Finally, air cool to room temperature to obtain the product.

2. The surface quenching hardening method of cold forged alloy steel planetary gear according to claim 1, characterized in that: A controlled atmosphere device is a gas device with controllable composition used to protect the metal surface and adjust the chemical composition of the metal surface when heating the metal. When the metal is heated in a controlled atmosphere device, surface oxidation or decarburization can be avoided, and the surface can also be carburized as required. The controlled atmosphere is generally composed of a mixture of two or more gases. When it is only to avoid surface oxidation, a single gas can also be used as a protective gas, such as hydrogen, argon or nitrogen. The controlled atmosphere device is used for metal heat treatment and also for low-oxidation or non-oxidation heating of forgings.

3. The surface quenching hardening method of cold forged alloy steel planetary gear according to claim 1, characterized in that: The air cooling is natural cooling or wind cooling in the air.

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