JP5535922B2 - Heat treatment process for steel - Google Patents

Description

  The present invention relates generally to heat treatment processes for metallurgy and steel parts. This process induces a compressive residual stress (CRS) in the surface area of the part, resulting in a significant improvement in mechanical properties such as fatigue resistance.

  Prior art techniques for manufacturing metal components include hot rolling or hot forging to form bars, rods, materials, tubes, or rings, followed by the desired composition. A soft forming process is performed. Surface hardening processes are well known and are used to locally increase the surface hardness of the final finished component, for example to improve fracture strength and fatigue resistance.

  Many surface hardening processes are known for improving anti-rotation contact fatigue resistance. Shot peening involves locally curing the surface layer by hitting a round shot against the surface of the metal component. However, this process results in a rough surface finish and creates another problem, thus necessitating the need to take additional steps to improve the surface finish. This increases production costs.

  Carburizing and quenching can also be accomplished by heating the steel components in a carbon medium to increase the carbon content, followed by quenching and tempering. This thermochemical process, known as carburization, results in a surface chemical composition that is completely different from the chemical composition of the central part of the component. Alternatively, the hard surface layer is formed by rapidly heating the surface of the medium carbon steel / high carbon steel to above the ferrite / austenite transformation temperature, followed by quenching and tempering to provide a hard surface layer. Can do. Surface heating has traditionally been achieved by flame quenching, but laser surface quenching and induction quenching are now used. Induction hardening involves the steps of heating a steel component to a temperature in or above the transformation zone and subsequent quenching by exposure to an interleaved magnetic field. Heating occurs primarily on the surface of the component, leaving the central part of the component essentially unaffected. The permeability of the magnetic field is inversely proportional to the frequency of the magnetic field, so that the quenching depth can be adjusted in a simple way. The magnetic field transmittance also depends on the power density and the operation time.

  An alternative to carburizing and quenching is coreless quenching. In contrast to a carburized and hardened component, the self-hardened component is uniform or substantially uniform in hardness throughout the component. In addition, corelessly quenched components are typically manufactured at a lower cost than carburized and quenched components, for example, to avoid complex heat treatments associated with carburizing. The grade of steel used depends on the thickness of the component section. For components having wall thicknesses up to about 20 mm, DIN100Cr6 is usually used. For larger segment sizes higher grades of alloying are used, for example DIN100CrMo7-3, DIN100CrMnMo7, DIN100CuMo7-4 or DIN100CrMnMo8.

  For heat-hardened steel components, two heat treatment methods can be used, namely martensitic quenching or austempering. Component properties such as toughness, hardness, microstructure, residual austenite content, and dimensional stability are related to or affected by the particular type of heat treatment employed.

  The martensite coreless quenching process involves austenitizing the steel before quenching to a temperature below the martensite start temperature. The steel is then subjected to low temperature tempering to stabilize the microstructure. The martensitic coreless quenching process typically results in a residual compressive stress of 0 to +100 MPa between the WCS (working contact surface) and about 1.5 m below the WCS.

  The bainite coreless quenching process involves austenitizing the steel prior to quenching to a temperature above the martensite start temperature. Following quenching, an isothermal bainite transformation is performed. The bainite coreless quenching process may be preferred as an alternative to the martensite coreless quenching process in steel. This is because the bainite structure may have excellent mechanical properties such as toughness and crack propagation resistance. The bainite coreless quenching process results in a CRS of 0-100 MPa between WCS and about 1.5 mm deep below WCS.

  Many conventional heat treatments are known to achieve martensitic and bainite coreless quenching.

  The present invention aims to solve at least some of the problems associated with the prior art.

Accordingly, in a first aspect, the present invention provides a process for inducing compressive residual stress in the surface area of a steel part, the process comprising:
(I) providing a part comprising a steel composition;
(Ii) performing induction heating on at least a portion of the part and subsequently quenching the at least one part, increasing the hardness in the surface area of the part;
(Iii) subsequently performing martensite and / or bainite coreless quenching to obtain a microstructure comprising martensite and / or bainite;
A heat treatment is provided.

  During induction heating, the at least one part of the part is heated to a depth of preferably 0.5-3 mm, more preferably 0.75-2.5 mm, and even more preferably 1-2 mm. That is, induction heating preferably penetrates to a depth of at least about 0.5 mm and a maximum depth of about 3 mm. Induction heating to such depths induces compressive residual stress (CRS) in the surface area of the component in conjunction with other steps in the process, resulting in improved mechanical properties such as fatigue resistance, for example. I understood it.

  During induction heating, it is preferred that the at least part of the part reaches a temperature of 1000-1100 ° C, more preferably 1020-1080 ° C. After quenching, the surface microstructure comprises martensite or at least martensite as the main phase. The process comprises the step of (iv) after step (iii): (iv) subjecting at least a part of the part to induction heating and subsequently quenching at least a part of the part, wherein the hardness in the surface area of the part is increased. Furthermore, it can be provided.

In a second aspect, the present invention provides a process for inducing residual compressive stress in the surface region of a steel part, the process comprising a heat treatment comprising the following steps.
(A) providing a part having a steel composition;
(B) performing a martensite and / or bainite coreless quenching step to obtain a microstructure comprising martensite and / or bainite;
(C) Inductively heating the at least one part of the part and subsequently quenching at least the part of the part, wherein the hardness in the surface area of the part is increased.

  In a second aspect, it is preferred that during induction heating said at least one part of the part is heated to a depth of 1-6 mm, more preferably 2-5 mm.

  In a second aspect, it is preferred that during induction heating, said at least one part of the part reaches a temperature of 900-1000 ° C, more preferably 920-980 ° C. After quenching, the surface microstructure comprises martensite or at least martensite as the main phase.

  In the second aspect, following induction heating and quenching, the part is preferably subjected to tempering, preferably low temperature tempering at temperatures up to about 250 ° C.

  The invention will be further described hereinafter. In the following sections, different features / embodiments of the invention are defined in more detail. Each of the features / embodiments so defined can be combined with any other feature / embodiment, unless expressly indicated to the contrary. In particular, any feature described as being preferred or advantageous can be combined with any other feature described as being preferred or advantageous.

  The present invention includes either a high frequency pre-process or a high frequency post-process in connection with a coreless quenching heat treatment so that a large residual compressive stress (CRS) is achieved so that thermal strain and / or phase transition strain is achieved. Is introduced. In particular, the present invention has a CRS in the range of -200 to -900 in the vicinity of the surface, and manufactures steel products so as to maintain normally -300 to -500 MPa at a depth of 1 mm below the surface. to enable. Near the surface is usually less than 300 microns below the heat treated surface.

  This process can be applied to all coreless hardened steel grades. Steel is usually medium carbon steel (0.3-0.8% carbon content) or high carbon steel (> 0.8% carbon content) such as high carbon chromium steel or low alloyed bearing steel It is. Examples include 0.65-1.20 wt% C, 0.05-1.70 wt% Si, 1.1-2.2 wt% Cr, 0.10-1.10 wt% Mn, 0.02 The composition of -1.0 wt% Ni, 0.02-0.70 wt% Mo, unavoidable impurities, and the balance Fe is raised. Suitable commercially available examples include DIN100Cr6 (= SAE52100), DIN100CrMo7-3, DIN100CrMnMo7, DIN100CrMo7-4, and DIN100CrMnMo8.

  Induction heating is preferably medium frequency induction heating and / or high frequency induction heating and is conveniently performed at a frequency of 2-100 KHz. The interaction time and power level can be varied depending on the part size and design depth.

  The induction heating is preferably followed by quenching to, for example, room temperature (20-25 ° C.) or a temperature of 0 ° C. or lower.

  In the first aspect, the step of induction heating is typically 0.5-3 mm using medium frequency induction heating and / or high frequency heating (preferably 2-100 KHz, more preferably 5-20 KHz), and more. Rapid surface heating is conveniently performed, typically to a depth of 1-2 mm. The surface preferably reaches a temperature of 1000-1100 ° C, more preferably 1020-1080 ° C. As mentioned above, after induction heating, the part is preferably quenched, for example with an oil or polymer solution, to “freeze” the effect of surface conditioning.

  In a second aspect, the step of induction heating is typically 1-6 mm, more typically using medium frequency induction heating and / or high frequency heating (preferably 2-100 KHz, more preferably 40-130 KHz). For expeditious surface heating to a depth of 2-5 mm. It is preferred that the surface reaches a temperature of 900-1000 ° C, more preferably 920-980 ° C. As mentioned above, after induction heating, the part is preferably quenched, for example with an oil or polymer solution, to “freeze” the effect of surface conditioning.

  If either the first or second process includes a martensite coreless quenching step, a conventional process can be used. For example, the martensite coreless quenching step typically austenites the steel, and subsequently converts the steel to the martensite start temperature (Ms is typically 180-220 ° C, more typically 190-200 ° C, even more typically. Quenching to a lower temperature (approximately 200 ° C.). Quenching can be performed using, for example, a molten salt. Following the martensite coreless quenching step, the part is preferably post-quenched, for example in cold water, and the austenite is martensitic transformed. Following post-quenching, the part is preferably subjected to low temperature tempering to stabilize the microstructure.

  Similarly, existing processes can be used if the process includes a bainite coreless quenching step. For example, the bainite coreless quenching step typically austenizes the steel and converts the steel to martensite onset temperature (Ms is usually 180-220 ° C, more typically 190-200 ° C, even more typically about 200 ° C. A step of quenching to a higher temperature. Quenching can be performed using, for example, oil or molten salt. This step is followed by isothermal bainite transformation, which is preferably performed at a temperature in the range of 200-250 ° C, more preferably 210-240 ° C. The steel is preferably held in this temperature range for 1 to 30 hours, more preferably 2.5 to 20 hours, depending on the steel grade and section thickness.

  Regardless of whether one or both of martensite and / or bainite is desired, the steel is preferably austenitized (prior to quenching to a lower / higher temperature than the martensite start temperature). Austenitization is known in the art. However, the inventor has performed quenchless quenching using a quenching temperature that is 10-50 ° C. lower than the commonly used quenching temperature (eg, 840-890 ° C.) (particularly in connection with the first feature), We have found that an increase in CRS is conducive. This is believed to be due to the low austenitization of the core relative to the slightly over-austenitized surface portion. Therefore, the difference in phase transformation becomes more prominent. The advantage of a slow phase transformation at the surface is that the phase transformation occurs in a fully or partially transformed core, and the possibility of plastic deformation (phase transformation usually involves an increase in volume) Therefore, the final surface stress state becomes a compression state. For these reasons, austenitization is preferably performed at a temperature in the range of 790 to 890 ° C, more preferably 790 to 880 ° C, and even more preferably 790 to 840 ° C. The steel is preferably held within this temperature range for 10 to 70 minutes, more preferably 20 to 60 minutes.

  Austenitization is usually performed in an atmospheric furnace where the part can reach a uniform temperature throughout its cross section. Subsequently, uniform austenitization and dissolution of cementite are advantageously achieved.

  In the process of the present invention, the chemical composition of the steel is essentially unchanged. In other words, the process does not need to include a thermochemical enrichment process. This is in contrast to existing carburizing and quenching processes.

  The final microstructure comprises either the main phase (tempered) martensite or bainite, or a combination of these two phases. Cementite can also be present. Usually, the microstructure appears essentially uniformly from the surface to the core. However, there may be unique segregation of alloy elements (eg, N, C, Cr, Si, Mn).

  The hardness in the surface is usually 50 to 75 HRC, more typically 56 to 68 HRC. The retained austenite concentration is usually 0-30%.

  Further, the lower core part includes martensite and / or bainite, or a mixture thereof. The hardness of the core microstructure is usually higher than 50 HRC, more typically higher than 56 HRC. The hardness of the core generally does not exceed 67 HRC, and more typically does not exceed 64 HRC. The retained austenite concentration is usually 0-20%.

  In a second aspect of the invention, the heat treatment step results in a transition region that is found in both hardness and microstructure.

  The part can be any type of steel part. For example, the part can be a part for a bearing such as a race or a rotating element.

  The present invention allows products to be produced with a CRS in the range of -200 to -900 MPa near the surface and to maintain -300 to -500 MPa at a depth of 1 mm below the surface. Such a CRS profile is very suitable compared to existing parts.

  Accordingly, in a third aspect, the present invention is a component formed from steel, the component comprising martensite and / or corelessly quenched bainite, which has a substantially uniform chemical composition and fineness. Provided is a component having a structure, wherein at least a portion of the component has a compressive residual stress comprising -200 to -900 MPa near the surface and -300 to -500 MPa at 1 mm below the surface.

In a fourth aspect, the present invention provides a process that includes a combination of the first and second characteristics. Here, the first induction heating step corresponding to the first feature mainly leads to a gradient of carbide dissolution affecting the properties of the phase transformation. This induction heating step is followed by martensite and / or bainite coreless quenching. Next, a second induction heating step corresponding to the second feature is performed to cause thermal strain between the surface and the core. The invention will be further described with reference to the accompanying drawings provided by the following examples and exemplary methods.

FIG. 4 is a diagram showing a residual compressive stress profile for the part of Example 1. 2 is a photograph showing the surface microstructure of the part of Example 1. FIG. 3 is a photograph showing the core fine structure of the part of Example 1. FIG. It is a figure which shows the profile of the hardness of the components of Example 1 after induction heating and before a bainite coreless quenching step. 2 is a diagram showing the hardness profile of the part of Example 1 after induction heating and a bainite coreless quenching step. FIG. FIG. 3 shows the compressive residual stress profile after heat treatment for the part of Example 2 in comparison with standard martensite and standard bainite. FIG. 4 is a photograph showing the surface microstructure for the part of Example 2 after a bainite coreless quenching and induction heating step. FIG. 4 is a photograph showing the transition region microstructure for the Example 2 part after bainite coreless quenching and induction heating steps. FIG. 6 is a photograph showing the core microstructure for the part of Example 2 after bainite coreless quenching and induction heating steps. FIG. 4 shows the hardness profile after bainite quenching and induction heating steps for the part of Example 2. FIG. 4 shows the compressive residual stress after martensite coreless quenching and induction heating steps for the part of Example 3.

Example 1 (Pretreatment and bainite re-quenching)
Test Part: Outer Diameter (OD) 180mm Spherical Roller Bearing (SRB) Outer Ring formed from 100Cr6 Steel Pretreatment: Achieving a surface temperature of 1050 ° C. using ˜10 KHz and ˜2 mm depth of pretreatment Inductive surface heating followed by quenching with 5% water-quenched polymer solution Bainite coreless quenching: Re-quenching in furnace using 820 ° C, 20 minutes soaking time, followed by quenching and 240 minutes Transformation in molten Petrofer AS140 salt at ˜230 ° C. followed by cooling in still air

  FIG. 1 is a plot showing the compressive residual stress profile for the Example 1 part. This plot shows the CRS near the surface of -300 to -800 MPa. The CRS remains at −300 or less up to at least 1.2 mm.

  2a and 2b are photographs showing the surface microstructure (a) and core surface microstructure (b) for the part of Example 1. FIG. These pictures show the bainite microstructure. The surface microstructure is slightly rougher than the core and has less residual carbide.

  FIG. 3 is a plot showing the hardness profile for the part of Example 1 after only the preliminary induction process.

  FIG. 4 is a plot showing the hardness profile for the part of Example 1 after all processes.

Example 2 (Bainite coreless quenching and post-treatment)
Test parts: Inner ring of cylindrical roller bearing (CRB) with an outer diameter (OD) of 120 mm formed from 100Cr6 steel. Bainite coreless quenching: Re-quenching in furnace using 820 ° C., 20 minutes soaking time, followed by Quenching and transformation in molten Petrofer AS140 salt at ˜230 ° C. for 240 minutes followed by cooling after treatment in still air: achievement of surface temperature of ˜940 ° C. and carburization depth of ˜1.8 mm using ˜8 KHz Induction surface heating for heat treatment, followed by quenching with 5% water-quenched polymer solution and tempering at 160 ° C. for 60 minutes

  FIG. 5 is a plot showing the compressive residual stress of the part of Example 2 after heat treatment compared to standard martensite and standard bainite.

  6a, 6b and 6c are photomicrographs showing the surface microstructure (a), transition region microstructure (b), and core surface microstructure (c) for the part of Example 2 after the bainite coreless quenching and induction heating steps. It is. These micrographs show the martensite surface microstructure, the tempered bainite microstructure in the transition region, and the bainite core microstructure.

  FIG. 7 is a plot showing the hardness profile for the part of Example 2 after the bainite coreless quenching and induction heating steps. The hardness profile shows the transition region.

Example 3 (Martensite coreless quenching and post-processing under interference fit)
Test parts: Inner ring of deep groove roller bearing (DGRB) with outer diameter (OD) 62 mm formed from 100Cr6 steel Martensite quenching: 860 ° C. Re-quenching in furnace using 20 minutes soaking time followed by 60 ° C. Oil quenching in oil and tempering at 160 ° C for 60 minutes Post-treatment: Mounting on a shaft that is too large causes hoop stress. Perform induction surface heating to achieve a surface temperature of ˜940 ° C. and a carburization depth of ˜1.8 mm using ˜90 KHz, followed by quenching with a 5% water quenching polymer solution and 160 ° C., 60 minutes tempering. Removal from the shaft.

  FIG. 8 is a plot showing the CRS profile for the part of Example 3 with different levels of hoop stress after the martensite coreless quenching and induction heating steps.

Claims (21)

A process for inducing compressive residual stress in the surface area of a steel part,
(1) providing a part having a steel composition;
(2) performing induction heating on at least a portion of the component, and subsequently quenching the at least one portion to increase the hardness in the surface area of the component;
(3) Subsequently, performing martensite and / or bainite coreless quenching to obtain a microstructure comprising martensite and / or bainite;
A process comprising a heat treatment.   The process according to claim 1, wherein during the induction heating, at least a part of the part is heated to 0.5-3 mm. The process of claim 1, wherein during the induction heating, at least a portion of the part is heated to 1-2 mm. 4. Process according to any one of claims 1 to 3, characterized in that during the induction heating, the surface of at least a part of the part reaches a temperature of 1000-1100C. 4. Process according to any one of claims 1 to 3, characterized in that during the induction heating, the surface of at least a part of the part reaches a temperature of 1020 to 1080 [deg.] C. Process according to any one of claims 1 to 5, characterized in that said steel is a medium carbon steel or high carbon steel. The process of claim 6 , wherein the steel is a high carbon chromium steel. The process according to any one of claims 1 to 7 , wherein the induction heating is medium frequency and / or high frequency induction heating. The process according to claim 8 , wherein the induction heating is performed at a frequency of 2 to 100 kHz. Process according to any one of claims 1 to 9, characterized in that quenching is carried out subsequently to the induction heating. Step of the martensite through-hardening is the steel was austenitized followed according to any one of claims 1 to 10, characterized by the step of quenching the steel to a temperature lower than the martensite start temperature process. 12. The process of claim 11 , wherein following the martensite coreless quenching, the part is post-quenched to accelerate further austenite to martensite transformation. 13. The process of claim 12 , wherein the part is tempered after post-quenching. The bainite coreless quenching step comprises austenitizing the steel, quenching the steel to a temperature higher than the martensite start temperature, and subsequently performing isothermal bainite transformation. 14. Process according to any one of claims 1 to 13 , characterized in that The process of claim 14 , wherein the isothermal bainite transformation is performed at a temperature in the range of 210-240 ° C. The process according to claim 15, wherein the isothermal bainite transformation is carried out for 2.5 to 20 hours. The process according to any one of claims 11 to 16 , wherein the steel is austenitized at a temperature in the range of 790 to 890 ° C. The process according to any one of claims 11 to 16, wherein the steel is austenitized at a temperature in the range of 790 to 880 ° C. The process according to any one of claims 11 to 16, wherein the steel is austenitized at a temperature in the range of 790 to 840 ° C. The process according to any one of claims 17 to 19, wherein the steel is austenitized for 20 to 60 minutes. After step ( 3 )
( 4 ) The method further comprises the step of subjecting the at least one part of the part to induction heating, and subsequently quenching the at least one part of the part to increase the hardness in the surface area of the part. The process according to any one of claims 1 to 20 .