Heat-Treatment Process for a Steel

The present invention relates generally to the field of metallurgy and a heat-treatment process for a steel component. The process induces a compressive residual stress (CRS) in a surface region of the component with the corollary of an improvement in mechanical properties, for example fatigue performance.

Conventional techniques for manufacturing metal components involve hot-rolling or hot-forging to form a bar, rod, tube or ring, followed by a soft forming process to obtain the desired component. Surface hardening processes are well known and are used to locally increase the hardness of surfaces of finished components so as to improve, for example, wear resistance and fatigue resistance.

A number of surface hardening processes are known for improving fatigue performance. Shot peening involves bombarding the surface of the metal component with rounded shot to locally harden surface layers. However, this process results in a rough surface finish which can create other problems and therefore additional steps need to be taken to improve the surface finish. This adds to productions costs.

Case-hardening may also be achieved by heating the steel component in a carbonaceous medium to increase the carbon content, followed by quenching and tempering. This thermochemcial process is known as carburizing and results in a surface chemistry that is quite different from that of the core of the component. Alternatively, the hard surface

layer may be formed by rapidly heating the surface of a medium/high carbon steel to above the ferrite/austenite transformation temperature, followed by quenching and tempering to result in a hard surface layer. Heating of the surface has traditionally been achieved by flame hardening, although laser surface-hardening and induction hardening are now often used. Induction hardening involves heating the steel component by exposing it to an alternating magnetic field to a temperature within or above the transformation range, followed by quenching. Heating occurs primarily in the surface of the component, with the core of the component remaining essentially unaffected. The penetration of the field is inversely proportional to the frequency of the field and thus the depth of the hardening can be adjusted in a simple manner. The penetration of the field also depends on the power density and interaction time. An alternative to case-hardening is through-hardening. Through-hardened components differ from case-hardened components in that the hardness is uniform or substantially uniform throughout the component. Through-hardened components are also generally cheaper to manufacture than case-hardened components because they avoid the complex heat-treatments associated with carburizing, for example. The steel grades that are used depend on the component section thickness. For components having a wall thickness of up to about 20 mm, DIN lOOCrβ is typically used. For larger section sizes, higher alloyed grades are used such as for example, DIN 100CrMo7-3, DIN 100CrMnMo7, DIN 100CrMo7-4, or DIN 100CrMnMo8.

For through-hardened steel components, two heat-treating methods are available: martensite hardening or

austempering. Component properties such as toughness, hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat treatment employed.

The martensite through-hardening process involves austenitising the steel prior to quenching below the martensite start temperature. The steel may then be low- temperature tempered to stabilize the microstructure. The martensite through-hardening process typically results in a compressive residual stress (CRS) of from 0 to +100 MPa between the WCS (working contact surface) and down to an approximately 1.5 mm depth below the WCS.

The bainite through-hardening process involves austenitising the steel prior to quenching above the martensite start temperature. Following quenching, an isothermal bainite transformation is performed. Bainite through-hardening is sometimes preferred in steels instead of martensite through- hardening. This is because a bainitic structure may possess superior mechanical properties, for example toughness and crack propagation resistance. The bainite though-hardening process results in a CRS of from 0 to -100 MPa between the WCS and down to an approximately 1.5 mm depth below the WCS.

Numerous conventional heat-treatments are known for achieving martensite through-hardening and bainite through- hardening.

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

Accordingly, in a first aspect the present invention provides a process for inducing a compressive residual stress in a surface region of a steel component, the process comprising a heat-treatment having the following steps:

(i) providing a component comprising a steel composition;

(ii) induction heating at least a part of the component followed by quenching said at least part, wherein the hardness in a surface region of the component is increased; and

(iii) subsequently performing a martensite and/or bainite through-hardening step to obtain a microstructure comprising martensite and/or bainite.

During the induction heating, said at least part of the component is preferably heated to a depth of from 0.5 to 3 mm, more preferably from 0.75 to 2.5 mm, still more preferably from 1 to 2 mm. That is the induction heating preferably penetrates to a depth of at least about 0.5 mm and up to a maximum depth of up to about 3 mm. Induction heating to such depths, in conjunction with the other steps of the process, has been found to induce a compressive residual stress (CRS) in a surface region of the component with the corollary of an improvement in mechanical properties, for example fatigue performance.

During the induction heating, the surface of said at least part of the component preferably reaches a temperature of from 1000 to 1100 ⁰ C, more preferably from 1020 to 1080 ⁰ C. After quenching, the surface microstructure comprises martensite or at least martensite as the predominant phase.

The process may further comprising, after step (iii) : (iv) induction heating at least a part of the component followed by quenching said at least part of the component, wherein the hardness in a surface region of the component is increased.

In a second aspect the present invention provides a process for inducing a compressive residual stress in a surface region of a steel component, the process comprising a heat- treatment having the following steps:

(a) providing a component comprising a steel composition;

(b) performing a martensite and/or bainite through- hardening step to obtain a microstructure comprising martensite and/or bainite.

(c) induction heating at least a part of the component followed by quenching said at least part of the component, wherein the hardness in a surface region of the component is increased.

In the second aspect, during the induction heating, said at least part of the component is preferably heated to a depth of from 1 to 6 mm, more preferably from 2 to 5 mm.

In the second aspect, during the induction heating, the surface of said at least part of the component preferably reaches a temperature of from 900 to 1000 ⁰ C, more preferably from 920 to 980 ⁰ C After quenching, the surface

microstructure comprises rαartensite or at least martensite as the predominant phase.

In the second aspect, following the induction heating and the quenching, the component is preferably subjected to tempering, preferably low temperature tempering at a temperature of up to about 25O ⁰ C.

The present invention will now be further described. In the following passages different aspects/embodiments of the invention are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention involves either pre- or post-induction processing in relation to a through-hardening heat-treatment process in order to introduce thermal strains and/or phase transformation strains such that a large compressive residual stress (CRS) is achieved. In particular, the present invention enables a steel product to be produced with a CRS in the range of -200 to -900 MPa at the near surface, typically being maintained at -300 to - 500 MPa at 1 mm depth below the surface. The near surface is typically less than 300 microns below the heat-treated surface.

The process is applicable to all though-hardening steel grades. The steel will typically be a medium (0.3 to 0.8% carbon) or high carbon steel (> 0.8% carbon) such as a high

carbon chromium steel or a low alloy bearing steel. For example, 0.65-1.20 wt . % C, 0.05-1.70 wt . % Si, 1.1-2.2 wt . % Cr, 0.10-0.1.10 wt. % Mn, 0.02-1.0 wt . % Ni, 0.02-0.70 wt . % Mo, and the balance Fe, together with any unavoidable impurities. Suitable commercial examples include: DIN lOOCrβ (= SAE 52100), DIN 100CrMo7-3, DIN 100CrMnMo7, DIN 100CrMo7-4, and DIN 100CrMnMo8.

The induction heating is preferably medium and/or high frequency induction heating and is advantageously performed at a frequency of from 2-100 kHz. The interaction time and power level may be varied having regard to the component size and desired depth.

The inducting heating is preferably followed by quenching, for example to room temperature (20 to 25°C) or even to 0 ⁰ C or less.

In the first aspect, the induction heating step advantageously achieves rapid surface heating using medium and/or high frequency induction heating (preferably at a frequency of 2-100 kHz, more preferably 5 to 20 kHz) to a depth of typically 0.5 to 3 mm, more typically 1 to 2 mm. The surface preferably reaches a temperature of from 1000 to 1100 ⁰ C, more preferably from 1020 to 1080 ⁰ C. As noted above, after the induction heating, the component is preferably quenched using, for example, oil or a polymer solution in order to ^λ freeze' the effect of the surface conditioning .

In the second aspect, the induction heating step advantageously achieves rapid surface heating using medium

or high frequency induction heating (preferably at a frequency of 2-100 kHz, more preferably 40 to 130 kHz) to a depth of typically 1 to β mm, more typically 2 to 5 mm. The surface preferably reaches a temperature of from 900 to 1000 ⁰ C, more preferably from 920 to 980°C. As noted above, after the induction heating, the component is preferably quenched using, for example, oil or a polymer solution in order to 'freeze' the effect of the surface conditioning.

If the process of either the first or second aspects involves a martensite through-hardening step, then conventional processes may be relied on. For example, the martensite through-hardening step will typically comprise austenitising the steel and subsequently quenching the steel below the martensite start temperature (Ms is typically 180 to 22O ⁰ C, more typically 190 to 200 ⁰ C, still more typically approximately 200 ⁰ C) . Quenching may be performed using, for example, molten salt. Following the martensite through- hardening step, the component is preferably post-quenched in, for example, cold water to promote further austenite to martensite transformation. Following the post-quench, the component is preferably subjected to low temperature tempering to stabilize the microstructure .

Similarly, if the process involves a bainite through- hardening step, then conventional processes may be relied on. For example, the bainite through-hardening step will typically comprise austenitising the steel and quenching the steel above the martensite start temperature (Ms is typically 180 to 220 ⁰ C, more typically 190 to 200 ⁰ C, still more typically approximately 200 ⁰ C) . Quenching may be performed using, for example, oil or molten salt. This is

followed by an isothermal bainite transformation, which is preferably performed at a temperature in the range of from 200 to 250 ⁰ C, more preferably from 210 to 240 ⁰ C. The steel is preferably held within this temperature range for from 1 to 30 hours, more preferably from 2.5 to 20 hours depending on the steel grade and section thickness.

Irrespective of whether one or both of martensite and/or bainite are desired, the steel is preferably austenitised (prior to the quench below/above the martensite start temperature) . Austenitising is well known in the art. However, the inventors have found (particularly in relation to the first aspect) that applying through-hardening using a 10-50 ⁰ C lower hardening temperature than what would normally be used (e.g. 840 to 890 ⁰ C) further promotes the CRS buildup. This is believed to be because the core is under- austenitised in relation to the slightly over-austenitised surface portion. Therefore, the phase transformation differences will be more pronounced. The benefit of having a delayed phase transformation in the surface portion is that it will take place on a fully or partially transformed core, which will restrict the possibility for plastic deformation (phase transformations usually involve a volume increase) , and the final surface stress state will therefore become compressive. For these reasons, austenitising is preferably performed at a temperature in the range of from 790 to 89O ⁰ C, more preferably from 790 to 88O ⁰ C, still more preferably from 790 to 84O ⁰ C. The steel is preferably held within this temperature range for from 10 to 70 minutes, more preferably from 20 to 60 minutes.

The austenitisation is typically performed in an atmosphere furnace where the component can reach a homogeneous temperature throughout its cross-section. Consequently, a homogenous austenitisation and cementite dissolution is advantageously achieved.

In the process of the present invention the chemical composition of the steel remains essentially unchanged. In other words, the process does not need to involve a thermochemical enrichment process. This is in contrast conventional case-hardening treatments.

The final microstructure comprises either (tempered) martensite or bainite as the major phase or a combination of the two. Cementite may also be present. In general, the microstructure appears to be essentially homogeneous from the surface to the core. However, some inherent segregation of alloying elements (e.g. N, C, Cr, Si, Mn) may be present.

The hardness within the surface is typically 50 - 75 HRC, more typically 56-68 HRC. The retained austenite content is typically 0-30%.

The underlying core also comprises either martensite and/or bainite or mixtures thereof. The hardness of the core microstructure is typically greater than 50 HRC, more typically greater than 56 HRC. The hardness of the core generally does not exceed 67 HRC, more typically it does not exceed 64 HRC. The retained austenite content is typically 0-20%.

In the second aspect of the present invention, the heat- treatment steps result in a transition zone visible both in hardness and in rαicrostructure.

The component may be any type of steel component. For example, component for a bearing such as a raceway or a rolling element.

The present invention enables a product to be produced with a CRS in the range of -200 to -900 MPa at the near surface, being maintained at -300 to - 500 MPa at 1 mm depth below the surface. Such a CRS profile compares very favourably to conventional components.

Thus, in a third aspect the present invention provides a component formed from steel, wherein the component comprises through-hardened martensite and/or through-hardened bainite and has a substantially homogeneous chemical composition and microstructure, at least a part of the component having a compressive residual stress profile comprising -200 to -900 MPa at the near surface, and -300 to - 500 MPa at 1 mm depth below the surface.

In a fourth aspect, the present invention provides a process involving a combination of the first and second aspects.

Here, a first induction heating step, corresponding to the first aspect, introduces mainly a carbide dissolution gradient that affects the phase transformation characteristics. This is followed by martensite and/or bainite though-hardening. Next, a second induction heating step, corresponding to the second aspect, is performed to introduce thermal strains between the surface and the core.

The present invention will now be described further with reference to the following Examples and the accompanying drawings, provided by way of example, in which:

Figure 1 is a plot showing the compressive residual stress profile for the component of Example 1;

Figures 2a and 2b are micrographs showing the surface (a) and core (b) microstructures for the component of Example 1;

Figure 3 is a plot showing the hardness profile for the component of Example 1 after the induction heating step but before the bainite through-hardening step;

Figure 4 is a plot showing the hardness profile for the component of Example 1 after the induction heating and bainite through-hardening steps;

Figure 5 is a plot showing the compressive residual stress profile for the component of Example 2 after the heat-treatment and compared to standard martensite and standard bainite;

Figures 6a, 6b and 6c are micrographs showing the surface (a) , transitional zone (b) , and core (c) microstructures for the component of Example 2 after the bainite through-hardening and induction heating steps;

Figure 7 is a plot showing the hardness profile for the component of Example 2 after the bainite through- hardening and induction heating steps; and

Figure 8 is a plot showing the compressive residual stress profile for the component of Example 3 after the martensite through-hardening and induction heating steps .

Example 1 (pre-processing and bainite rehardening)

Test component: Spherical roller bearing (SRB) outer ring with OD 180 mm formed from lOOCrδ steel.

Pre-processing: Inductive surface heating using -10 kHz to reach a surface temperature of -1050 ⁰ C and a pre-processing depth of ~2 mm, followed by quenching using a 5% Aquaquench polymer solution.

Bainite-through Furnace rehardening using 820 ⁰ C and 20 hardening: minute soaking time, followed by quenching and transformation in ~230°C molten Petrofer AS140 salt for 240 minutes, followed by cooling in still air.

Figure 1 is a plot showing the compressive residual stress profile for the component of Example 1. The plot shows a near surface CRS of -300 to -800 MPa. The CRS is maintained at -300 down to at least 1.2 mm.

Figures 2a and 2b are micrographs showing the surface (a) and core (b) πiicrostructures for the component of Example 1. The micrographs show a bainite microstructure. The surface microstructure is slightly coarser with less residual carbides (cementite) than the core.

Figure 3 is a plot showing the hardness profile for the component of Example 1 after pre-induction process only

Figure 4 is a plot showing the hardness profile for the component of Example 1 after the complete process.

Example 2 (bainite through hardening and post-processing)

Test component: Cylindrical roller bearing (CRB) inner ring with OD 120 mm formed from 100Cr6 steel)

Bainite through- Furnace rehardening using 86O ⁰ C and 20 hardening: min soaking time, followed by quenching and transformation in -23O ⁰ C molten Petrofer AS140 salt for 240 min, followed by cooling in still air.

Post-processing: Inductive surface heating using ~8 kHz to reach a surface temperature of ~940°C and a case depth of ~1.8 mm, followed by quenching using a 5% Aquatensid polymer quench solution and tempering at 160 ⁰ C for 60 min

Figure 5 is a plot showing the compressive residual stress profile for the component of Example 2 after the heat- treatment and compared to standard martensite and standard bainite.

Figures 6a, βb and βc are micrographs showing the surface (a) , transitional zone (b) , and core (c) microstructures for the component of Example 2 after the bainite through- hardening and induction heating steps. The micrographs show a martensite surface microstructure, a tempered bainite microstructure in the transition zone, and a bainite core microstructure .

Figure 7 is a plot showing the hardness profile for the component of Example 2 after the bainite through-hardening and induction heating steps. The hardness profile shows a transition zone.

Example 3 (martensite through hardening and post-processing under interference fit)

Test component: Deep groove ball bearing (DGBB) inner ring with OD 62 mm formed from 100Cr6 steel

Martensite through: Furnace rehardening using 860 ⁰ C and 20 Hardening min soaking time, followed by oil quenching in 6O ⁰ C oil and tempering at

160 ⁰ C for 60 min

Post-processing: Mounting on over-sized shaft resulting in hoop-stress. Inductive surface

heating using -90 kHz to reach a surface temperature of ~940°C and a case depth of ~1.8 mm, followed by quenching using a 5% Aquatensid polymer quench solution and tempering at 160 ⁰ C for 60 min.

Removal of shaft.

Figure 8 is a plot showing the CRS profile for the component of Example 3 after the martensite through-hardening and induction heating steps with different levels of hoop stress .