Heat treatment process for high-speed bearing

Abstract

The invention belongs to the technical field of bearing heat treatment, and particularly relates to a heat treatment process for a high-speed bearing. The heat treatment process for the high-speed bearing is characterized by comprising the following steps: (1) heating stage: placing a high-carbon steel workpiece in a heat treatment furnace, heating, controlling carbon potential in the furnace, and preserving heat; (2) a heat preservation stage: continuously preserving heat; (3) a carbon potential increasing stage: keeping the temperature, and increasing the carbon potential in the furnace; (4) a carbonitriding stage: carrying out carbonitriding treatment; (5) a carbon potential reduction stage: reducing the carbon potential; (6) diffusion stage: heat preservation; (7) quenching stage: after the diffusion is finished, transferring the workpiece into a quenching medium for quenching treatment; and (8) tempering. Through segmented carbon potential control carbonitriding, characteristic quenching medium and precise tempering control, the surface quality, the dimensional stability, the mechanical property and the service reliability of the bearing part are remarkably improved, the surface structure quality is remarkably improved, and the wear resistance and the fatigue resistance are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of bearing heat treatment, and particularly relates to a heat treatment process for high-speed bearings. BACKGROUND

[0002] As a key component in precision mechanical equipment, high-speed bearings are widely used in the fields of aerospace, high-speed motors, rail transportation, numerical control machine tools, etc. The working conditions of high-speed bearings usually have the characteristics of high speed, heavy load, long service life and high reliability. During high-speed operation, the bearing ring and the rolling body bear alternating contact stress and friction heat, which are prone to failure modes such as fatigue spalling, wear, and gluing. Therefore, the surface hardness, wear resistance, contact fatigue strength of the bearing parts and the toughness of the core are extremely demanding.

[0003] In order to meet the above performance requirements, heat treatment process is an indispensable core link in the bearing manufacturing process. Through reasonable heat treatment (especially surface strengthening treatment), the surface hardness and wear resistance of the material can be significantly improved on the basis of ensuring good toughness and strength of the core, and beneficial residual compressive stress can be introduced on the surface, thereby effectively improving the fatigue life and running stability of the bearing. At present, for high-speed bearings made of high-carbon chromium steel or carburizing bearing steel, a comprehensive process of chemical heat treatment combined with quenching and tempering is generally adopted, among which carbonitriding is widely used because it can simultaneously improve the surface hardness, anti-seizure property and fatigue strength.

[0004] In the prior art, a typical heat treatment process flow of high-speed bearings is as shown in Figure 1 , mainly including: heating, one-step carbon potential rising, carbon equalization and exhaust, carbonitriding, rapid quenching oil quenching, and low-temperature tempering. The process can realize a surface 0.4-0.6mm of the penetration layer depth, the product surface is in a compressive stress state, the residual austenite content is controlled in the range of 20%~40%, and basically meets the performance requirements of conventional bearings.

[0005] However, the process still has several key technical defects in actual application, which is difficult to meet the higher requirements of precision high-speed ball bearings on size stability, surface integrity and high reliability, which are specifically as follows:

[0006] (1) Unreasonable carbon potential control leads to surface quality decline: the process adopts "one-step carbon potential rising" method, that is, the carbon potential is raised to the target value at one time and the carbonitriding is directly carried out under high carbon potential, lacking of stepwise rising / descending carbon potential and diffusion stage. This leads to the supersaturation of carbon and nitrogen atoms on the surface, forming a thick carbonitride layer (such as Fe3C, Fe4N, etc.) in the range of 0.05mm on the surface layer. The compound layer is high in brittleness and easy to become the source of fatigue crack initiation, reducing the contact fatigue life of the bearing.

[0007] (2) The content of residual austenite is too high, which affects the dimensional stability: the existing process controls the content of residual austenite at 20% to 40% through low-temperature tempering, which can alleviate the quenching stress, but for precision high-speed bearings, the content is still too high. In actual operation, the residual austenite continues to transform into martensite due to the friction temperature rise (up to 160°C or more) caused by rapid acceleration and deceleration or continuous high-speed operation of the bearing, which causes the volume expansion of the material, resulting in the expansion of the internal size of the bearing, the reduction of the play, and the aggravation of the contact stress, and finally induces early peeling failure.

[0008] (3) The quenching method causes severe deformation and low qualification rate: the production equipment is a box-type quenching furnace, and the whole basket of products is vertically quenched. In order to prevent quenching soft spots, fast quenching oil with high cooling speed is used. However, the medium has too fast cooling speed and uneven cooling, which easily causes significant oval deformation or warping of thin-walled bearing rings, resulting in low first-time qualification rate, and a large amount of subsequent sizing or scrapping, which increases the production cost.

[0009] (4) The quenching medium is limited: although the isothermal quenching oil can reduce deformation, it has high viscosity, poor flowability and slow heat convection, which easily causes local rapid rise of oil temperature and reduction of cooling capacity during batch quenching in the whole basket, and there is a risk of incomplete quenching or soft spots, which is difficult to be popularized and applied under the existing box-type furnace production conditions.

[0010] In summary, the existing high-speed bearing heat treatment process has obvious deficiencies in surface structure control, residual austenite stability and quenching deformation control, and it is difficult to meet the comprehensive performance requirements of high precision, long service life and high reliability of precision high-speed ball bearings. Therefore, it is urgent to develop a new type of heat treatment process to solve the above technical problems and improve the overall performance and service reliability of high-speed bearings. SUMMARY

[0011] The purpose of the present application is to provide a heat treatment process for high-speed bearings.

[0012] In order to achieve the above purpose, the present application provides the following technical solutions:

[0013] A heat treatment process for high-speed bearings, comprising the following steps:

[0014] (1) The high-carbon steel workpiece is placed in a heat treatment furnace, heated, and the carbon potential in the furnace is controlled, and the temperature is kept constant;

[0015] (2) Continue to keep the temperature for 15-20 minutes;

[0016] (3) The carbon potential in the furnace is kept constant, and the carbon potential is increased;

[0017] (4) Carbonitriding treatment is carried out;

[0018] (5) Carbon potential reduction stage: reduce the carbon potential;

[0019] (6) Diffusion stage: keep temperature for 120-130 min;

[0020] (7) Quenching stage: after the diffusion, transfer the workpiece to quenching medium for quenching treatment;

[0021] (8) Tempering stage: keep temperature for a certain time at a specific temperature.

[0022] Preferably, in step (1), heat to a temperature of 800-840°C, and at the same time, control the carbon potential in the furnace to be 0.55-0.65%, and keep temperature for 5-10 min.

[0023] Preferably, in step (2), continue to keep temperature for 15-20 min at a temperature of 800-840°C and a carbon potential of 0.55-0.65%.

[0024] Preferably, in step (3), increase the carbon potential in the furnace to 1.00-1.10%.

[0025] Preferably, in step (3), the carbon potential increasing time lasts for 5-10 min.

[0026] Preferably, in step (4), carry out carbonitriding treatment at a temperature of 800-840°C and a carbon potential of 1.00-1.10% for 140-160 min.

[0027] Preferably, in step (5), reduce the carbon potential to 0.80-0.90%, and the adjusting process lasts for 5-10 min.

[0028] Preferably, in step (6), keep temperature for 120-130 min at a temperature of 800-840°C and a carbon potential of 0.80-0.90%.

[0029] Preferably, in step (7), the quenching starting temperature is 70-85°C, and isothermal fractional quenching oil is used for cooling, the viscosity of the quenching oil is 50-65 mm / s, the maximum cooling speed is 70-85°C / s, and the characteristic temperature is in the range of 710-735°C. 2

[0030] Preferably, the isothermal fractional quenching oil is KERUN® KR488 isothermal fractional quenching oil.

[0031] Preferably, in step (8), keep temperature for 180-240 min at a temperature of 180-220°C.

[0032] ​Preferably, the high carbon steel workpiece includes the following mass percentages of components: C 0.95-1.05%, Si 0.15-0.35%, Mn 0.25-0.45%, Cr 1.4-1.6%, Mo ≤0.1%, P ≤0.02%, S ≤0.015%, O ≤0.0008%, and the balance being iron.

[0033] Compared with the prior art, the application has the advantages and beneficial effects that:

[0034] 1. The application provides an optimized heat treatment process for high-speed bearings, which significantly improves the surface quality, dimensional stability, mechanical properties and service reliability of bearing parts by segmented carbon potential carbonitriding, characteristic quenching medium and precise tempering control, significantly improves the surface tissue quality, and improves the wear resistance and fatigue resistance. A multi-stage process of gradient carbon potential, high carbon potential carbonitriding, low carbon potential and diffusion is adopted to avoid the problem of surface carbon and nitrogen oversaturation caused by traditional "one-step carbon potential increase". In the diffusion stage, slightly lower carbon potential is adopted to promote the uniform inward diffusion of carbon and nitrogen atoms, effectively inhibit the formation of coarse carbon and nitrogen compound particles within 0.05mm of the surface layer, and obtain fine, dispersed and uniform carbon and nitrogen compound tissue. Ensure that the carbon content of the working surface of the product is ≥0.9%, the nitrogen content is ≥0.1%, significantly improve the surface hardness, wear resistance, tempering stability and high temperature performance, and meet the demand of high-speed heavy load working condition.

[0035] 2. The application effectively controls heat treatment deformation, improves dimensional accuracy and first pass yield, and introduces a new type of quenching medium, the kinematic viscosity, characteristic temperature and maximum cooling speed of which are between rapid quenching oil and isothermal quenching oil: the cooling capacity is strong enough to avoid quenching soft spots; good flowability and fast thermal convection reduce the rapid rise of oil temperature during whole basket quenching; small phase change stress significantly reduces the oval deformation of thin-walled parts (such as ring with wall thickness coefficient ≤1.14). The deformation after heat treatment is controlled within ≤0.15% D (D is the nominal diameter of the part), which greatly improves the first pass yield before grinding, reduces the shape correction process and reduces the production cost.

[0036] 3. Through specific tempering process parameters (such as temperature and time accurate control), the residual austenite content of the product surface is stably controlled in the ideal interval of 10%-20%: the lower limit is ≥10%: a certain amount of residual austenite is retained to absorb impact energy, improve the anti-shock and anti-pollution ability and fracture toughness; the upper limit is ≤20%: avoid the decomposition of a large amount of residual austenite into martensite due to friction temperature rise during high-speed operation, prevent the volume expansion, clearance reduction and early peeling failure caused thereby. Significantly improve the dimensional stability and running reliability of the product under high-speed and high-temperature working conditions, especially suitable for precision high-speed ball bearings. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1Flow chart of prior art heat treatment process.

[0038] Figure 2 Flow chart of heat treatment process of the present application.

[0039] Figure 3 Metallographic chart of heat treatment technology of workpiece Comparative Example 1.

[0040] Figure 4 Metallographic chart of heat treatment technology of workpiece using Example 1.

[0041] Figure 5 Test report schematic diagram of KERUN® KR218 fast bright quenching oil.

[0042] Figure 6 Test report schematic diagram of KERUN® KR488 isothermal fractional quenching oil. DETAILED DESCRIPTION

[0043] The technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative labor fall within the protection scope of the present application.

[0044] Example 1

[0045] Please refer to Figure 2 The present embodiment provides a heat treatment process for high-speed bearings, comprising the following steps:

[0046] (1) Temperature rising stage: place the high-carbon steel workpiece in a heat treatment furnace, heat to a temperature of 820°C, and at the same time control the carbon potential in the furnace to 0.60%, and keep for 5 min to achieve uniform heating and prevent surface decarburization; the high-carbon steel workpiece comprises the following components by mass percentage: C 0.98%, Si 0.24%, Mn 0.35%, Cr 1.49%, Mo 0.002%, P 0.012%, S 0.002%, O 0.0008%, and the balance is iron.

[0047] (2) Holding stage: continue to keep at a temperature of 820°C and a carbon potential of 0.60% for 15 min to ensure uniform temperature of the whole workpiece and prepare for the subsequent carburizing-nitriding process.

[0048] (3) Carbon potential rising stage: keep the temperature at 820°C, and raise the carbon potential in the furnace to 1.05%, and the carbon potential rising time lasts for 5 min to avoid sudden rise of carbon potential leading to too high surface carbon concentration.

[0049] (4) Carbonitriding stage: carbonitriding treatment is carried out at 820°C and carbon potential of 1.05% for 140 minutes, so that carbon and nitrogen atoms diffuse into the workpiece surface layer at the same time to form a strengthened layer with a certain depth.

[0050] (5) Carbon potential reduction stage: the temperature is kept at 820°C, and the carbon potential is reduced to 0.85% for 5 minutes to prepare for the diffusion stage and avoid the formation of coarse carbides on the surface.

[0051] (6) Diffusion stage: the temperature is kept at 820°C and the carbon potential is kept at 0.85% for 120 minutes, so that the carbon and nitrogen atoms that have penetrated into the surface layer diffuse uniformly inward, reducing the surface concentration gradient, reducing the generation of brittle phases, and improving the uniformity of the layer structure.

[0052] (7) Quenching stage: after the diffusion is completed, the workpiece is quickly transferred to the quenching medium for quenching treatment. The quenching starting temperature is about 80°C, and isothermal staged quenching oil is used for cooling. The isothermal staged quenching oil is KR488 isothermal staged quenching oil (see Figure 6 ), to obtain martensite structure, improve surface hardness and wear resistance.

[0053] (8) Tempering stage: the temperature is kept at 200°C for 210 minutes to eliminate quenching stress and stabilize the martensite structure.

[0054] Comparative Example 1

[0055] This comparative example is a heat treatment process of the prior art, specifically:

[0056] A heat treatment process for high-speed bearings, comprising the following steps:

[0057] (1) Heating stage: the high-carbon steel workpiece is loaded into the heat treatment furnace and heated to a temperature of 790°C, while the carbon potential in the furnace is controlled at 0.6% for 5 minutes. The high-carbon steel workpiece includes the following mass percentages of components: C 0.98%, Si 0.24%, Mn 0.35%, Cr 1.49%, Mo 0.002%, P 0.012%, S 0.002%, O 0.0008%, and the balance is iron.

[0058] (2) Uniform temperature and carbon potential stage: after the heating is completed, the furnace temperature is raised to 820°C, and the carbon potential is stabilized at 1.055% at this temperature for 10 minutes.

[0059] (3) Carbonitriding stage: carbonitriding treatment is carried out at 820°C and carbon potential of 1.05% for 270 minutes.

[0060] (4) Quenching stage: after the carbonitriding is finished, the workpiece is taken out from the furnace and quickly transferred to a quenching medium, the name of the quenching medium is KERUN® KR218 fast bright quenching oil; see Figure 5 , the initial quenching temperature is about 60°C, and the quenching cooling is performed.

[0061] (5) Tempering stage: the temperature is kept at 160°C for 210 min.

[0062] Performance test

[0063] According to the requirement of the standard JB / T7363, the contents of carbon and nitrogen of the high-carbon steel workpiece of the comparative example 1 are determined, and the results are shown in Table 1 (the carbon and nitrogen content data before improvement). The high-carbon steel workpiece is subjected to heat treatment according to the method of the example 1, and the results are shown in Table 2 (the carbon and nitrogen content data after improvement). The residual austenite of the high-carbon steel bearing after heat treatment of the comparative example 1 is determined, and the results are shown in Table 3. The high-carbon steel workpiece is subjected to heat treatment according to the method of the example 1, and the residual austenite content is determined, and the results are shown in Table 4. The metallographic photos of the workpieces of the comparative example 1 and the example 1 are shown in Figures 3-4 .

[0064] Table 1 Contents of carbon and nitrogen of high-carbon steel workpiece

[0065]

[0066] Table 2 Contents of carbon and nitrogen of high-carbon steel workpiece after heat treatment

[0067]

[0068] Table 3 Residual austenite content of comparative example 1

[0069]

[0070] Table 4 Residual austenite content of example 1

[0071]

[0072] It can be known from Tables 3-4 that, compared with the comparative example 1, the heat treatment process of the example 1 significantly reduces the residual austenite content of the bearing steel surface and the core. This shows that the process of the example 1 has more advantages in controlling the stability of the structure, so as to be able to bring more excellent dimensional stability, wear resistance and fatigue life, which is crucial for high-speed bearings.

[0073] The above is the preferred embodiment of the present application, and it should be pointed out that, for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which should also be considered as the protection scope of the present application.

Claims

1. A heat treatment process for high speed bearings, characterized in that, The method comprises the following steps: (1) a temperature rising stage: placing a high-carbon steel workpiece in a heat treatment furnace, heating, controlling carbon potential in the furnace, and holding; (2) a holding stage: continuing holding for 15-20 min; (3) a carbon potential rising stage: keeping the temperature and rising the carbon potential in the furnace; (4) a carbonitriding stage: performing carbonitriding treatment; (5) a carbon potential decreasing stage: decreasing the carbon potential; (6) a diffusion stage: holding for 120-130 min; (7) a quenching stage: after the diffusion ends, transferring the workpiece to a quenching medium to perform quenching treatment; (8) a tempering stage.

2. The heat treatment process for high speed bearings according to claim 1, characterized in that, In the step (1), the heating is performed to a temperature of 800-840 ℃, and the carbon potential in the furnace is controlled to 0.55-0.65%, and holding is performed for 5-10 min.

3. The heat treatment process for high speed bearings according to claim 1, characterized in that, In the step (2), the holding is continued for 15-20 min under the conditions of a temperature of 800-840 ℃ and a carbon potential of 0.55-0.65%.

4. The heat treatment process for high speed bearings of claim 1, wherein, In the step (3), the carbon potential in the furnace is increased to 1.00-1.10%.

5. The heat treatment process for high speed bearings of claim 1, wherein, In the step (3), the carbon potential increasing time lasts for 5-10 min.

6. The heat treatment process for high speed bearings of claim 1, wherein, In the step (4), the carbonitriding treatment is performed under the conditions of a temperature of 800-840 ℃ and a carbon potential of 1.00-1.10% for 140-160 min.

7. The heat treatment process for high speed bearings of claim 1, wherein, In the step (5), the carbon potential is decreased to 0.80-0.90%, and the adjusting process lasts for 5-10 min.

8. The heat treatment process for high speed bearings of claim 1, wherein, In the step (6), the holding is performed under the conditions of a temperature of 800-840 ℃ and a carbon potential of 0.80-0.90% for 120-130 min.

9. The heat treatment process for high speed bearings according to claim 8, characterized in that, The quenching initial temperature of the step (7) is 70-85℃, and isothermal step quenching oil is used for cooling, the quenching oil viscosity is 50~65mm 2 / s, the maximum cooling speed is 70-85℃ / s, and the characteristic temperature is in the range of 710~735℃.

10. The heat treatment process for high speed bearings of claim 1, wherein, In the step (8), the tempering stage is performed under the conditions of a temperature of 180-220 ℃ for 180-240 min.

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