Heat treatment method for eliminating grain unevenness of 20CrMoH gear steel

Abstract

The invention discloses a heat treatment method for eliminating grain unevenness of 20CrMoH gear steel. The heat treatment method comprises the steps of preheating, primary quenching and secondary low-temperature critical quenching. In the primary quenching step, a preheated product is heated to 840-890 DEG C along with a furnace for heat preservation, the heat preservation time is 2-3.0 min / mm, and then the preheated product is put into mineral oil to be quenched and cooled to the room temperature; and in the secondary low-temperature critical quenching step, the primary quenching product is put into a heating furnace from the room temperature to be heated to 820-840 DEG C along with the furnace for heat preservation, the heat preservation time is 2-3.0 min / mm, and then the primary quenching product is put into mineral oil to be quenched and cooled to the room temperature. According to the method, the problems of unevenness of original austenite grains and the like caused by poor heating can be eliminated, it is guaranteed that the material has good original austenite structure uniformity, the grain size of the material can be increased from 7.5-8.5 grades to 9.5 grades or above, the hardness of the material can be guaranteed, and the product performance can meet the requirements.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment processes, and in particular relates to a heat treatment method for eliminating uneven grains of 20CrMoH gear steel. Background Art

[0002] 20CrMoH gear steel has excellent mechanical properties and heat treatment characteristics. It is a widely used gear steel material. It is widely used in fasteners, axle pins, gear products, bearing rings, high-pressure pipes, automotive parts and other automotive, container, machinery and petrochemical fields. The product forms include profiles, wires, bars, plates, strips and forgings, etc. In the subsequent processing, it is often necessary to adjust its performance through quenching and tempering processes. When the quenching and tempering temperature is high, the austenite grain growth tendency of the material increases, especially the possibility of abnormal grain growth in some parts increases. With the development of industry, the performance of the product continues to develop towards high strength, high toughness, high uniformity of organization, etc., and the requirements for the original austenite organization of the material are gradually increasing. In particular, when preparing precision parts with strong toughness matching and fatigue performance requirements, such as gear products, carburizing heat treatment is usually required. During the carburizing process of gear steel, the coarsening of the original austenite grains has become a difficult problem in the industry. As the demand for improving carburizing efficiency and increasing the depth of the carburizing layer has become increasingly prominent, the quality of carburizing furnaces has gradually improved, and high-temperature carburizing has gradually become an option. This has also posed a severe challenge to the uniformity of material structure, especially the control of the uniformity of the original austenite structure, which has become a key factor limiting the application of high-temperature carburizing of materials.

[0003] At present, mild alloying elements are mostly added to pin the grain boundaries through precipitates to prevent the growth of austenite. However, this measure usually has high requirements on element ratio, smelting and continuous casting stability, forging normalizing process, etc. The ability to resist grain coarsening cannot be stably exerted. The phenomenon of abnormal grain growth during heat treatment often occurs, which also causes troubles for the inspection of parts. There are few reports on the means of eliminating abnormal growth of formed grains and uneven organization through subsequent heat treatment. From this perspective, the present invention eliminates uneven organization and achieves grain refinement while meeting the hardness of the product through cyclic quenching, thereby improving the strength, plasticity and fatigue properties of the product as a whole. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a heat treatment method for eliminating the uneven grains of 20CrMoH gear steel.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: A heat treatment method for eliminating uneven grains of 20CrMoH gear steel includes product preheating, primary quenching and secondary low-temperature critical quenching.

[0006] The preheating of the present invention is to put the 20CrMoH gear steel from room temperature into a heating furnace and heat it to 650-800℃ for insulation, and the insulation temperature is 1-2.0min / mm. The preheating operation is a necessary process in the heating process and is also a function of most heating furnaces. Muffle furnaces, box furnaces, gas protection furnaces, continuous carburizing furnaces, multifunctional carburizing furnaces and quenching and tempering lines can all achieve this.

[0007] The one-time quenching described in the present invention heats the preheated product to 840-880°C with the furnace for insulation, and the insulation time is 2-3.0min / mm, and then put it into mineral oil for quenching and cooling to room temperature. Heating 30-50°C above Ac3 helps the product to be completely austenitized before the one-time quenching, and does not cause excessive oxidation, burning and decarburization due to excessive temperature; and at this temperature, the original austenite structure will not grow significantly, but only complete recrystallization during the heating process, and the grain boundaries will preferentially nucleate and grow, which can also make the parts achieve internal and external temperature uniformity through sufficient heat treatment, ensuring the consistency of the surface and core structure and the uniformity of the austenite grains. After the heating is completed, it is immediately placed in mineral oil for quenching and cooling to room temperature, so that the material quickly forms a martensitic structure and retains the austenite grains refined by the one-time recrystallization formed during the heating process.

[0008] The secondary low-temperature critical quenching of the present invention is to place the once quenched product from room temperature into a heating furnace and heat it to 820-840°C with the furnace for insulation, and the insulation time is 2-3.0min / mm, and then put it into mineral oil for quenching and cooling to room temperature. Heating 10-30°C above Ac3 is based on the grain refinement and homogenization of the once quenching, and heating at a faster heating rate, so that the nucleus is rapidly nucleated during the austenitization process; and because the heating temperature is low, the growth rate of austenite becomes smaller, and under a longer insulation time, the entire product has enough austenitization time to complete recrystallization and refine the grains. At the same time, the lower quenching temperature reduces the tendency of oxidation and decarburization during the heating process, and can reduce the deformation of the product in the subsequent quenching process.

[0009] The inventive principle of the technical solution of the present invention is: for the mixed crystal parts or the products with coarse original austenite grain size or the products that need to be further refined, in order to reduce the temperature difference and internal and external temperature difference of different parts of the product, as well as the heating stress caused by it, it is first preheated, and then a high temperature quenching heating is carried out; the high quenching temperature is conducive to the full austenitization of different parts of the product, and the recrystallization process is completed for the first time during the austenitization process; in addition, the higher temperature increases the nucleation power of austenite, forms a more uniform original austenite structure, refines the initial uneven grains for the first time, and ensures that the oil quenching process can be quenched. There is no preheating section insulation during the secondary low-temperature critical quenching heating process, and the product is directly heated to the quenching temperature with the furnace. The sample heating temperature is low, but the heating rate is fast. Austenite has a large nucleation driving force and recrystallizes to form fine grains at a lower critical quenching temperature. Since the first quenching makes the grains finer, the stress caused by the acceleration rate and phase change of different grain sizes is reduced, and then the lower secondary quenching temperature helps to control the decarburization, quenching deformation and quenching stress during the product quenching process.

[0010] The beneficial effect of adopting the above technical scheme is that the 20CrMoH steel parts obtained by the method of the present invention have good original austenite uniformity, can eliminate the problem of uneven original austenite grains caused by poor heating, ensure that the material has good uniformity of organization, and can increase the grain size of the material from 7.5-8.5 to above 9.5, and can also ensure the hardness of the material, so that the product performance meets the requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The microscopic morphology of austenite grain size of gear steel in Example 1 before heat treatment; Figure 2 The microscopic morphology of austenite grain size of the gear steel after heat treatment in Example 1; Figure 3 The microscopic morphology of austenite grain size of gear steel in Example 2 before heat treatment; Figure 4 The microscopic morphology of austenite grain size of gear steel after heat treatment in Example 2; Figure 5 The microscopic morphology of austenite grain size of gear steel in Example 3 before heat treatment; Figure 6 This is the microscopic morphology of the austenite grain size of the gear steel in Example 3 after heat treatment. DETAILED DESCRIPTION

[0012] The present invention is further described in detail below in conjunction with specific implementation modes. Example 1

[0013] The 20CrMoH product in this embodiment is a rod with a diameter of 30 mm and a height of 30 mm. The product is subjected to a heat treatment at 1050°C for 1 hour. The entire cross section is in a state of coarse grains and mixed crystals, and the maximum grain size exceeds 508 μm. Figure 1 shown.

[0014] It is heat treated again according to preheating, primary quenching and secondary low-temperature critical quenching, as follows: (1) Preheating: Place the product from room temperature into a heating furnace and heat it to 650°C, keeping it warm for 30 minutes; (2) Primary quenching: Heat the sample to 860°C, keep it at this temperature for 1 hour, then take it out of the heating furnace and immediately put it into mineral oil for quenching and cooling to room temperature; (3) Secondary low-temperature critical quenching: Place the once quenched product from room temperature into a heating furnace and heat it to 820°C. After keeping it warm for 1 hour, take it out of the heating furnace and immediately place it in mineral oil to quench and cool it to room temperature.

[0015] After the secondary quenching, the austenite grain size of the 20CrMoH gear steel in this embodiment is 9.5, and the entire cross section is uniform and fine. Figure 2 As shown; its hardness value is 42HRC. Example 2

[0016] The 20CrMoH product in this embodiment is a rod with a diameter of 30 mm and a height of 30 mm. The product is subjected to heat treatment at 1050°C for 2 hours. The entire cross section is in a state of coarse grains and mixed crystals, and the maximum grain size exceeds 508 μm. Figure 3 As shown. Heat treat again as follows: (1) Preheating: Place the product from room temperature into a heating furnace and heat it to 800°C, keeping it warm for 30 minutes; (2) Primary quenching: Heat the sample to 880°C, keep it at this temperature for 1 hour, then take it out of the heating furnace and immediately put it into mineral oil for quenching and cooling to room temperature; (3) Secondary low-temperature critical quenching: Place the once quenched product from room temperature into a heating furnace and heat it to 840°C. After keeping it warm for 1 hour, take it out of the heating furnace and immediately place it in mineral oil to quench and cool it to room temperature.

[0017] After the secondary quenching, the austenite grain size of the 20CrMoH gear steel in this embodiment is 9.5, and the entire cross section is uniform and fine. Figure 4 As shown; its hardness value is 48HRC. Example 3

[0018] The 20CrMoH product in this embodiment is a rod with a diameter of 30 mm and a height of 30 mm. The product is subjected to a heat treatment at 1050°C for 4 hours. The entire cross section is in a state of coarse grains and mixed crystals, and the maximum grain size exceeds 508 μm. Figure 5 shown.

[0019] It is heat treated again according to preheating, primary quenching and secondary low-temperature critical quenching, as follows: (1) Preheating: Place the product from room temperature into a heating furnace and heat it to 650°C, keeping it warm for 30 minutes; (2) Primary quenching: Heat the sample to 840°C, keep it at this temperature for 1 hour, then take it out of the heating furnace and immediately put it into mineral oil for quenching and cooling to room temperature; (3) Secondary low-temperature critical quenching: Place the once quenched product from room temperature into a heating furnace and heat it to 820°C. After keeping it warm for 1 hour, take it out of the heating furnace and immediately place it in mineral oil to quench and cool it to room temperature.

[0020] After the secondary quenching, the austenite grain size of the 20CrMoH gear steel in this embodiment is 9-9.5, and the entire cross section is uniform and fine. Figure 6 As shown; its hardness value is 43HRC. Example 4

[0021] The 20CrMoH gear product in this embodiment has a diameter of Φ142 mm and a height of 23 mm. The product is heated at 980°C for 8 hours and carburized and then oil quenched. Its cross section shows coarse grains and mixed crystals, and the maximum grain size exceeds 400 μm.

[0022] It is heat treated again according to preheating, primary quenching and secondary low-temperature critical quenching, as follows: (1) Preheating: Place the product from room temperature into a heating furnace and heat it to 750°C, keeping it warm for 30 minutes; (2) Primary quenching: Heat the sample to 840°C, keep it at that temperature for 50 min, then take it out of the heating furnace and immediately quench it in mineral oil to cool it to room temperature.

[0023] (3) Secondary low-temperature critical quenching: Place the once quenched product from room temperature into a heating furnace and heat it to 830°C. After keeping it warm for 50 minutes, take it out of the heating furnace and immediately quench it in mineral oil to cool it to room temperature.

[0024] After secondary quenching, the austenite grain size of the 20CrMoH gear steel in this embodiment is 9.5-10, and the entire cross section is uniform and fine; its hardness value is 43HRC. Example 5

[0025] The 20CrMoH gear product in this case has a diameter of Φ185mm and a height of 19mm. The product is heated at 980℃ for 8h and carburized and then oil quenched. Its cross section shows coarse grains and mixed crystals, with the maximum grain size exceeding 400μm.

[0026] It is heat treated again according to preheating, primary quenching and secondary low-temperature critical quenching, as follows: (1) Preheating: Place the product from room temperature into a heating furnace and heat it to 750°C, keeping it warm for 30 minutes; (2) Primary quenching: Heat the sample to 880°C, keep it at that temperature for 50 min, then take it out of the heating furnace and immediately quench it in mineral oil to cool it to room temperature.

[0027] (3) Secondary low-temperature critical quenching: Place the once quenched product from room temperature into a heating furnace and heat it to 835°C. After keeping it warm for 50 minutes, take it out of the heating furnace and immediately quench it in mineral oil to cool it to room temperature.

[0028] After secondary quenching, the austenite grain size of the 20CrMoH gear steel in this embodiment is 9-10, and the entire cross section is uniform and fine; its hardness value is 48HRC.

Claims

1. A heat treatment method for eliminating uneven grains of 20CrMoH gear steel, characterized in that: It includes preheating, primary quenching and secondary low-temperature critical quenching.

2. A heat treatment method for eliminating uneven grains of 20CrMoH gear steel according to claim 1, characterized in that: The preheating is to put the 20CrMoH gear steel from room temperature into a heating furnace and heat it to 650-800° C. for insulation, and the insulation time is 1-2.0 min / mm.

3. A heat treatment method for eliminating uneven grains of 20CrMoH gear steel according to claim 1, characterized in that: The primary quenching is to heat the preheated product to 840-890°C with the furnace and keep it warm for 2-3.0 min / mm, and then put it into mineral oil for quenching and cooling to room temperature.

4. A heat treatment method for eliminating uneven grains of 20CrMoH gear steel according to claim 1, characterized in that: The secondary low-temperature critical quenching is to put the once quenched product from room temperature into a heating furnace and heat it to 820-840°C for insulation for a insulation time of 2-3.0 min / mm, and then put it into mineral oil for quenching and cooling to room temperature.

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