Induction heat treatment high-carbon gear steel

Abstract

The invention discloses induction heat treatment high-carbon gear steel, and belongs to the technical field of machine manufacturing. The gear steel comprises the following chemical components in percentage by mass: 0.705%-0.995% of C, 0.5%-2.5% of Si, 0.1%-0.5% of Cr, 0.010%-0.3% of Mo, 0%-0.3% of Cu, 0%-0.2% of V, 0%-0.02% of Ti and O < lt >. 30 ppm, S < lt >; 0.025%, P < lt >; 0.025%, and the balance being Fe. The material is subjected to vacuum melting to obtain a cast ingot, then the cast ingot is heated to 1050-1100 DEG C for complete austenitizing and heat preservation to achieve alloy component homogenization, then the cast ingot is continuously forged into the needed size in the cooling process, and the final forging temperature is controlled to be 850-900 DEG C. The forge piece is heated at 780-820 DEG C, subjected to heat preservation for 1-8 hours, cooled to 600 DEG C at the cooling speed of 20 DEG C / min and then discharged out of a furnace, and a structure with spherical cementite distributed on a ferrite matrix can be obtained; or heating and normalizing at 850 DEG C to obtain a pearlite structure. After induction quenching, the surface hardness of the steel plate can reach 62-64 HRC, and the depth of a hardening layer is 0.5-5 mm. The surface structure is characterized by comprising carbide with the average size of 0.3-0.6 mu m, martensite with the length of 0.5-2 mu m and a fine grain structure with the length of 7-15 mu m.

Description

Technical Field

[0001] This invention belongs to the field of mechanical manufacturing technology, specifically relating to a low-hardenability high-carbon gear steel suitable for induction heat treatment and its heat treatment method. Background Technology

[0002] Gears are key mechanical components used to transmit torque and change rotational speed, widely used in transmission systems such as automotive transmissions, wind turbines, and mechanical equipment reducers. Gears transmit loads through sliding contact on their surfaces, which bear extremely high contact stresses, thus requiring high surface hardness and wear resistance. However, the root of the gear bears significant bending stress, requiring sufficient toughness in the core. To meet this structural performance requirement of a hard surface and a tough core, the industry commonly employs carburizing heat treatment. This process brings the surface carbon content to a high carbon range of 0.8-1.0%, while the core retains its original low-carbon composition. After overall quenching, the surface achieves a high hardness of HRC60 or higher, and the core obtains a low-carbon martensite or bainite structure with good toughness, thereby meeting the service performance requirements of the gear. Therefore, carburizing heat treatment has become an indispensable key process in gear manufacturing.

[0003] However, traditional carburizing heat treatment processes are extremely energy-intensive. Parts typically need to be held at 900-950°C for 8-10 hours, and for large parts, the processing time can even reach tens of hours. This makes carburizing heat treatment one of the most energy-intensive processes in the machinery industry, and there has been an urgent need to reform this process both domestically and internationally. Some foreign studies have attempted to increase the carburizing temperature to 1000°C to shorten the carburizing time and thus achieve energy savings. However, this process places more stringent requirements on carburized steel; the material must possess excellent resistance to grain growth when heated at 1000°C for an extended period, otherwise its mechanical properties will deteriorate. In addition, the carburizing process requires the decomposition of raw materials such as kerosene and methanol, which generates a large amount of CO, causing environmental pollution.

[0004] Induction heat treatment technology offers a viable alternative to traditional carburizing heat treatment. Utilizing the skin effect of electromagnetic induction, induction heat treatment heats the workpiece, offering significant advantages such as rapid heating and high thermal efficiency. The heating process begins at the surface and gradually propagates inwards. By precisely controlling parameters such as heating time and current frequency, a thin-shell structure with surface hardening and a tough core, similar to carburizing and quenching, can be obtained. Currently, induction hardening is mainly applied to medium carbon steel. For example, patent 201410082954.1, "A Heat Treatment Process for Medium Carbon Gears," discloses an induction hardening process for 45# steel, including tempering, induction hardening, and tempering steps, resulting in a hardness of 550-600 HV. Domestic companies typically use 40Cr and 42CrMo steels, achieving a hardness of 45-50 HRC after induction hardening, a level significantly lower than that achievable through carburizing and quenching. According to the *Handbook of Materials for Mechanical Engineering* (5th edition, 2007), the highest carbon content of the two low-hardenability induction heat-treated steels, 60Ti and 70Ti, is 0.7%, and their hardness after induction quenching still cannot match that of the carburized layer. The latest authorized patent, "A Low-Hardenability High-Carbon Gear Steel and Heat Treatment Process" (ZL 2017 1 1219013.8), discloses an ultra-high-carbon steel with a carbon content in the range of 1.0-1.5%, which significantly improves hardness and wear resistance after quenching, thus enhancing gear life. However, excessively high carbon content can deteriorate the material's machinability. To achieve a good balance between gear service life and machinability, this invention proposes an induction heat-treated high-carbon gear steel with a carbon content controlled in the range of 0.705-0.995%, filling the gap in existing low-hardenability gear steels within this range. Summary of the Invention

[0005] The purpose of this invention is to provide an induction heat-treated high-carbon gear steel and its heat treatment method that can replace the traditional carburizing heat treatment process. This material, through induction hardening alone, achieves high surface hardness while maintaining high core toughness, reaching a level of microstructure and mechanical properties comparable to low-carbon steel after carburizing and quenching. Existing induction heat-treated gear steels are mainly concentrated in the medium carbon range (0.4-0.5% C), resulting in insufficient hardness and limited wear resistance after quenching; even the highest carbon content induction heat-treated steels given in manuals is below 0.7%, and their wear resistance still cannot compare with carburized surfaces. Further increasing the carbon content to above 0.7% easily leads to the formation of a network of secondary cementite in the microstructure; this brittle phase severely impairs fatigue life. This invention controls the carbon content within the range of 0.705-0.995% and, by adding Si, an element that inhibits the formation of network cementite, successfully increases the critical carbon content to nearly 1.0% to prevent the formation of network cementite. Therefore, after induction heat treatment, the surface hardness of this material can reach the level of high carbon steel, while also taking into account good processing performance, truly possessing the potential to replace the traditional carburizing heat treatment process.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a high-carbon gear steel for induction heat treatment, the chemical composition of which, by mass percentage, is: 0.705-0.995% C, 0.5-2.5% Si, 0.1-0.5% Cr, 0.01-0.3% Mo, 0-0.3% Cu, 0-0.2% V, 0-0.02% Ti, with the balance being Fe. The roles and proportions of each element in this invention are as follows: a carbon content of 0.705-0.995% is the basis for ensuring high hardness and high wear resistance after quenching; a silicon content of 0.5-2.5% can effectively promote the spheroidizing annealing process and inhibit the precipitation of network carbides; the addition of chromium helps to improve the oxidation resistance of steel; the addition of molybdenum can inhibit temper brittleness; copper can improve the corrosion resistance of the material; the addition of vanadium and titanium is mainly used to refine the grains. Through microalloying and process control, a fine-grained structure with an average grain size of 7-15 μm can be obtained, which is beneficial to improving strength and toughness.

[0007] The preparation and heat treatment method of the gear steel described in this invention is as follows: First, ingots or continuously rolled profiles are obtained through vacuum melting or conventional industrial special high-quality steel melting processes, with the oxygen content controlled below 30 ppm and the sulfur and phosphorus contents below 0.025%, respectively. Then, the ingots or rolled profiles are heated to 1050~1100°C for complete austenitization and held at that temperature for an appropriate time to promote alloy composition homogenization. Then, during the cooling process, they are continuously forged into the required shape and size, with the final forging temperature controlled at 850~900°C to avoid work hardening due to excessively low temperatures or coarse grains due to excessively high temperatures. The forgings are then subjected to preliminary heat treatment: they can be heated to 780-820°C and held for 1-8 hours (this temperature range is close to Ac1, which is beneficial for cementite spheroidization), then slowly cooled to 600°C at a cooling rate of 20°C / min before being removed from the furnace and air-cooled to obtain the desired shape and size. Figure 1 The spheroidized annealed microstructure shown is characterized by cementite distributed uniformly in a spherical shape within a ferrite matrix; alternatively, a pearlitic microstructure can be obtained by normalizing at 850°C. Finally, the workpiece undergoes induction hardening.

[0008] Figure 2 Macroscopic metallographic images of small samples of the high-carbon gear steel of this invention, with a composition of 0.9% C, 1.6% Si, 0.3% Cr, 0.2% Mo, 0.2% Cu, and 0.12% V, are shown after induction hardening. The images clearly show the outline of the surface hardened layer: the outermost black area is the mounting material, the next layer of dark gray is the clearly outlined hardened layer, and the middle light gray area is the unhardened original structure, forming an ideal outline hardened layer.

[0009] Figure 3The microstructure of the surface hardened layer of the steel of this invention, which has a composition of 0.8% C, 1.5% Si, 0.2% Cr, 0.1% Mo, 0.3% Cu, and 0.15% V, after induction hardening. Due to the grain refinement to 7-15 μm, the martensite morphology is difficult to distinguish, exhibiting characteristics of acicular martensite, with fine carbides of an average size of 0.3-0.6 μm uniformly distributed on it. Figure 4 This corresponds to the subsurface structure, which is a mixed structure of pearlite and cementite. Attached Figure Description

[0010] Figure 1 This is a microstructure diagram of ferrite and spheroidal cementite obtained by spheroidizing annealing in an embodiment of the present invention.

[0011] Figure 2 This is a macroscopic morphology image of the sample after induction heat treatment in an embodiment of the present invention, showing a clearly defined hardened layer.

[0012] Figure 3 This is a microstructure diagram of the surface hardened layer after induction hardening in an embodiment of the present invention.

[0013] Figure 4 This is a microstructure diagram of the non-hardened layer on the subsurface after induction hardening in an embodiment of the present invention.

[0014] Figure 5 This is a hardness distribution curve from the surface to the core of the three embodiments of the present invention after induction hardening. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. However, it should be understood that the specific embodiments of this invention are only for explaining the invention and are not intended to limit the scope of protection of this invention. The chemical composition of the materials used in the embodiments should conform to the specifications in Table 1.

[0016] Table 1 Chemical composition (mass fraction, %) of the examples

[0017]

[0018] Example 1 The chemical composition of the materials used in this embodiment is shown in Sample 1 of Table 1. Induction hardening was performed using the same induction heating parameters (heating time 8.24 seconds) as in Examples 2 and 3. The hardness distribution curve of the treated sample is shown below. Figure 5As shown, the highest surface hardness reached 950 HV, the hardened layer depth (measured at half the peak hardness) reached 1 mm, and the total hardened layer depth reached 3 mm. The results of the surface residual stress test after quenching are shown in Table 2, with a residual compressive stress of -437 MPa.

[0019] Example 2 The chemical composition of the materials used in this embodiment is shown in Sample 2 of Table 1. The same induction heating parameters as in Example 1 (heating time 8.24 seconds) were used for treatment. The hardness distribution curve of the treated sample is shown below. Figure 5 As shown, the surface hardness also reaches 950 HV, but the depth where the hardness is maintained above 900 HV reaches approximately 1.5 mm, the half-hardened layer depth is 2.2 mm, and the total hardened layer depth reaches 4 mm. Comparing Examples 1 and 2, it can be seen that under the same induction heating parameters, due to the higher carbon content of Sample 2 (approximately 0.1% higher than Sample 1), its hardenability is better and the hardened layer is deeper. The residual stress test results in Table 2 show that the surface residual compressive stress of Sample 2 reaches -541 MPa, which is approximately 100 MPa higher than that of Sample 1. This is because the higher carbon content leads to a stronger solid solution strengthening effect, resulting in a larger residual compressive stress.

[0020] Example 3 The chemical composition of the materials used in this embodiment is shown in Sample 3 in Table 1. The induction heating time was extended to 10.3 seconds for quenching treatment. The hardness distribution curve of the treated sample is shown below. Figure 5 As shown, the depth of the hardened layer increased significantly, reaching approximately 4 mm, but the maximum surface hardness decreased slightly to approximately 900 HV, with a hardness of 800 HV at a depth of 3.5 mm. Analysis suggests that with increasing carbon content and prolonged heating time, the amount of retained austenite in the hardened layer increased, leading to a slight decrease in macroscopic hardness. The surface residual compressive stress of sample 3 further increased to -681 MPa, 140 MPa higher than that of sample 2, which is also attributed to the higher carbon content.

[0021] Table 2 summarizes the surface residual stress test results after induction hardening in the three embodiments.

[0022] Table 2 Residual surface stress after induction hardening in the examples

[0023] The above description is only a partial embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for induction heat treatment of high-carbon gear steel and the same, characterized in that, The finished gear steel composition, by mass percentage (wt.%), is: 0.705-0.995% C, 0.5-2.5% Si, 0.1-0.5% Cr, and 0.01-0.3% Cr. Mo, 0.01-0.3% Cu, 0-0.2% V, 0-0.02% Ti, balance Fe.

2. The induction heat treatment method for high-carbon gear steel according to claim 1, characterized in that, The gear steel is obtained by vacuum melting or conventional industrial special high-quality steel melting to obtain ingots or continuously rolled profiles, with an oxygen content of less than 30 ppm and S and P contents of less than 0.025%, respectively.

3. The induction heat treatment method for high-carbon gear steel according to claim 1 or 2, characterized in that, The heat treatment method includes the following steps: The obtained ingots or rolled profiles are heated to 1050~1100℃ for complete austenitization and held at that temperature to homogenize the alloy composition. Then, during the cooling process, they are continuously forged into the required shape and size, with a final forging temperature of 850~900℃. The forgings are heated to 780-820℃ and held for 1-8 hours, then cooled to 600℃ at a cooling rate of 20℃ / min before being removed from the furnace and air-cooled to obtain a ferrite and spheroidal cementite structure. Alternatively, they can be heated to 850℃ for normalizing treatment to obtain a pearlite structure.

4. The induction heat-treated high-carbon gear steel and its heat treatment method according to any one of claims 1-3, characterized in that, The depth of the hardened layer can be controlled by adjusting the heating time, current and frequency parameters; after induction hardening, the surface hardness can reach 62-64 HRC, the hardened layer depth is 0.5-5 mm, and the surface residual compressive stress is 400-600 MPa.

5. The induction heat-treated high-carbon gear steel and its heat treatment method according to any one of claims 1-4, characterized in that, The hardened layer obtained after treatment by the method has the following microstructure characteristics: average grain size of 7-15 μm, average carbide size of 0.3-0.6 μm, and martensite lamellar length of 0.5-2 μm; surface hardness of 62-65 HRC after induction hardening, and hardened layer depth of 0.5-5 mm.

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