METHOD FOR HEAT TREATING A STEEL COMPONENT

Abstract

A method for heat treating a steel component includes heating the component to a temperature at or above the austenitizing temperature and holding the component at or above the austenitizing temperature for a time sufficient to at least partially austenitize the steel, cooling the component to a first temperature in a temperature range of 200° C. to 270° C. and holding the component in the temperature range for at most 6 hours, thereby transforming at least a portion of the austenite into bainite, cooling the component to a temperature of from −30° C. to 30° C., thereby forming martensite; and reheating the component to a second temperature in the temperature range, holding the component in the temperature range for at most 6 hours and then cooling the component to a temperature of from −30° C. to 30° C., thereby transforming a further portion of the austenite into bainite and forming tempered martensite.

Description

CROSS-REFERENCE

This application claims priority to German patent application no. 10 2022 210 928.7 filed on Oct. 17, 2023, the contents of which are fully incorporated herein by reference.

TECHNOLOGICAL FIELD

The present disclosure is directed to a method for heat treating steel components, such as, for example, bearing components such as rolling elements, rollers or balls, and/or inner or outer rings and to an inner or outer ring of a bearing. The present disclosure is particularly suited to components for those bearings conventionally known in the art as “large size bearings”. More specifically, the bearings are suitable for applications such as in wind turbine main shafts.

BACKGROUND

Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings include inner and outer raceways and a plurality of rolling elements (for example balls and/or rollers) disposed therebetween. Roller bearings are rolling element bearings in which the rolling elements are rollers, as opposed to balls, for example. Roller bearings include spherical and tapered roller bearings, for example. For long-term reliability and performance it is important that the various elements have a high resistance to rolling fatigue, wear and creep. Bearings are well known components to those skilled in the art and are described in various catalogues and technical brochures freely available from SKF, for example, the catalogue entitled “Rolling bearings”.

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. In known heat treatments of steel components for bearings, after austenitization, the components are held for bainite transformation for extended periods of time, typically in a salt bath in which the component is quenched after austenitization for over 10 hours.

Conventionally, for roller bearings such as spherical or tapered roller bearings for example, the inner and/or outer ring of the roller bearing is formed from a low-carbon steel though it is also known to form bearing components from high-carbon steel, for example as disclosed in WO 2009/045147 (family member of U.S. Pat. No. 8,562,767).

Surface hardening processes are well known and are used to locally increase the hardness of surfaces of finished or semi-finished components so as to improve, for example, wear resistance and fatigue resistance. A number of surface or case hardening processes are known for improving rolling contact fatigue resistance. SKF Nautilus bearings have been manufactured by case carburizing steels such as 18NiCrMo14-6 and by surface induction hardening steels such as 50CrMo4. Slewing bearings can also be used in large size main shaft applications.

SUMMARY

An aspect of the present disclosure is to provide a bearing component, such as an inner or outer ring or rolling elements for a roller bearing that can be manufactured at a lower cost, while still exhibiting comparable or improved wear resistance and load carrying capacity as compared with conventional roller bearing components.

A further aspect of the present disclosure is to provide a method for heat treating a steel component, for example to obtain an inner or outer ring or rolling element for a bearing, with comparable or improved wear resistance and loading capacity compared with conventional roller bearing rings, wherein the method requires a lower manufacturing cost compared with the manufacture of conventional roller bearing rings.

The present disclosure seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto.

The disclosure is directed to a method for heat treating a steel component that includes:

• • (i) providing a steel component formed of a high-carbon bearing steel;
  • (ii) heating the component to a temperature at or above the austenitizing temperature and holding the component for a time sufficient to at least partially austenitize the steel;
  • (iii) cooling the component to a temperature of from 200° C. to 270° C. and holding the component for at most 6 hours, preferably at most 4 hours, thereby transforming at least a portion of the austenite into bainite;
  • (iv) cooling and/or quenching the component to a temperature of from −30° C. to 30° C., thereby forming martensite; and
  • (v) repeating steps (iii) and (iv) by heating the component to a temperature of from 200° C. to 270° C. and holding the component for at most 6 hours before cooling and/or quenching the component to a temperature of from −30° C. to 30° C., thereby transforming a further portion of the austenite into bainite and forming tempered martensite.

The present disclosure will now be described further. In the following passages, different aspects/embodiments of the disclosure 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 disclosure relates to a method for heat treating a steel component. The term “steel component” as used herein may encompass any component, i.e. any object, made of or comprising steel. The disclosure is not limited to a particular type or grade of steel. The type of “component” as used herein is not particularly limited. Preferably the steel component is a bearing component such as an inner or outer ring, preferably for a roller bearing. In a further aspect described in greater detail herein, there is provided a bearing component, preferably an inner or outer ring, or a rolling element, for a bearing. As will be appreciated, the bearing component of the further aspect may be obtainable by the method of the first aspect, and preferably is obtained by the method. That is, the inner or outer ring is a heat-treated ring and the rolling element is a heat treated rolling element. By extension, the method is suitable for obtaining the bearing component. Furthermore, any disclosure in respect of the component may be applied equally to the disclosure of the method, and vice versa.

Preferably, the bearing component is a roller bearing, for example a cylindrical roller bearing or a tapered roller bearing. In a preferred embodiment, the roller bearing is a tapered roller bearing which may be suitable for use in a main shaft of a wind turbine. In accordance with a further aspect of the present disclosure, there is also provided a roller bearing, preferably a tapered roller bearing, comprising an inner ring and an outer ring, wherein each of the inner and outer rings are in accordance with the ring described herein.

Roller bearings feature a cup and cone assembly. The bearings further comprise steel components such as rollers as part of a cone assembly which is further formed of the inner ring and a cage. The cup is comprised of the outer ring.

The shape of the inner or outer ring of the present disclosure is not particularly limited and may take the shape of any conventional inner or outer ring for a roller bearing. The term “inner ring” as used herein may refer to a roller bearing ring that is positioned radially inwards relative to the rolling elements in the roller bearing and therefore has a raceway on a radially outer surface of the inner ring. The term “outer ring” as used herein may refer to a roller bearing ring that is positioned radially outwards relative to the rolling elements in the roller bearing and therefore has a raceway on a radially inner surface of the outer ring. The physical and/or mechanical properties of the inner or outer ring of the present disclosure are particularly desirable for use in spherical and/or tapered roller bearings due to the loading requirements of such bearings and the high wear resistance and loading capacity exhibited by the inner or outer rings of the present disclosure.

Preferably, the tapered roller bearing is for the main shaft of a wind turbine. In a tapered roller bearing, the inner ring has a generally frustoconical ring shape with a conical outer raceway for engaging with roller bearing. The outer ring has a generally complementary “cup” shape having a conical inner raceway.

The present disclosure is particularly suited to large size bearings. Typically, a preferred steel component will have a weight of greater than 50 kg, for example, from 100 kg to 1000 kg. A bearing formed from such steel bearing components may have an equivalent weight. A bearing being comprised of an inner and outer ring and a plurality of rolling elements may have a weight of up to 3500 kg. In another preferred embodiment, the ring for the bearing has a bore diameter of at least 0.5 m, preferably at least 1 m, more preferably at least 1.5 m. Generally, the bore diameter and outer diameter will be no greater than 5 m, for example, no greater than 4 m. Furthermore, the width of the ring (i.e. the thickness between the opposite faces of the ring, and therefore being parallel with the axis of rotation) is preferably from 70 mm to 250 mm. The diameter of a corresponding roller is preferably from 35 mm to 200 mm.

The heat treatment comprises a step (i) of providing a steel component formed of a high-carbon bearing steel (such bearing steel may be referred to herein as a steel composition). A high-carbon bearing steel is a well-known term in the art, though this generally refers to a steel which comprises from 0.8 to 1.15 wt. % carbon. Preferably, the steel further comprises the additional elements as outlined herein. Suitable steels include “through-hardening bearing steels” such as those described in Table 3 of ISO 683-17:2014. In combination with the other alloying elements, this results in the desired mixed bainitic and martensitic (micro)structure (BMS), load carrying capacity, core and surface hardness and impact toughness. Compared to the conventional low-carbon steels used in roller bearing components, such as spherical and tapered roller bearings, such a high-carbon steel can achieve higher hardness. Carbon also acts to lower the bainite transformation temperature so that the desired bainitic microstructure is achievable. Preferably, the steel composition comprises from 0.93 to 1.05 wt. % carbon.

The composition of such high-carbon steel may be as follows:

• • 0.8-1.15 wt. % carbon;
  • 0.1-0.9 wt. % silicon;
  • 0.1-1.8 wt. % manganese;
  • 0.8-2.2 wt. % chromium;
  • 0-0.3 wt. % nickel;
  • 0-0.7 wt. % molybdenum;
  • 0-0.03 wt. % phosphorous;
  • 0-0.03 wt. % sulfur; and
  • the balance iron, together with unavoidable impurities.

The bearing steel may comprise from 0.1 to 0.9 wt. % silicon. In combination with the other alloying elements, this results in the desired bainitic microstructure with a minimum amount of retained austenite. Silicon has negligible solubility in carbides, particularly at high temperatures where its diffusivity is sufficiently high for it not to be trapped in carbides. Silicon also helps to suppress excessive precipitation of cementite and carbide formation. In addition, silicon stabilizes transition carbides and improves the tempering resistance of the steel microstructure. However, too high a silicon content may result in undesirable surface oxides and a poor surface finish and may also result in lowering the elastic properties of the matrix. For this reason, the maximum silicon content is preferably 0.75 wt. %. When the silicon content is lower than 0.15 wt. %, the desired bainitic microstructure may be difficult to obtain without substantial retained austenite, for example greater than 10 vol. % retained austenite. Therefore, the steel composition may comprise from 0.15 to 0.75 wt. % silicon. In some embodiments, the steel composition comprises from 0.15 to 0.45 wt. % silicon and in other embodiments, the steel composition comprises from 0.45 to 0.75 wt. % silicon.

The bearing steel may comprise from 0.1 to 1.8 wt. % manganese. In combination with the other alloying elements, manganese acts to improve hardenability. Accordingly, when the manganese content is lower than 0.6 wt. %, a steel composition with both the desired bainitic microstructure and high core hardness may not be easily achievable. In addition, manganese acts to increase the stability of austenite relative to ferrite. However, higher manganese may serve to increase the amount of retained austenite and to decrease the rate of transformation to bainite. This may lead to practical metallurgical issues such as stabilizing the retained austenite too much, leading to potential problems with the dimensional stability of the bearing components. Preferably, the steel composition comprises from 0.5 to 1.5 wt. % manganese, more preferably from 0.7 to 1.2 wt. % manganese.

The bearing steel may comprise from 0.8-2.2 wt. % chromium. Chromium acts to increase hardenability and reduce the bainite start temperature. Thus, when the chromium content is lower than 0.8 wt. %, a steel composition with both the desired bainitic microstructure and high core hardness may not be easily achievable. Preferably, the steel composition comprises from 0.9 to 2.1 wt. % chromium, more preferably from 1.0 to 2.0 wt. % chromium.

The bearing steel optionally comprises from 0 to 0.3 wt. % nickel, for example 0 to 0.2 wt. % nickel. Nickel may be beneficial in terms of general toughness and/or impact properties, for example. The bearing steel optionally comprises from 0 to 0.3 wt. % copper, for example 0 to 0.2 wt. % copper.

The bearing steel may comprise from 0 to 0.7 wt. % molybdenum. Molybdenum may act to avoid austenite grain boundary embrittlement owing to impurities such as, for example, phosphorus. Molybdenum may also reduce the bainite start temperature and increases hardenability, which is important when the steel is used to manufacture e.g. a large-size bearing ring that requires hardening to a relatively large depth upon quenching from high temperature. Thus, it is preferred that the bearing steel comprises molybdenum, for example at least 0.10 wt. %. The molybdenum content in the alloy is preferably no more than about 0.7 wt. %, otherwise the austenite transformation into bainitic ferrite may cease too early, which can result in significant amounts of austenite being retained in the structure. Molybdenum may also contribute to an increased rolling contact fatigue resistance by creating finely dispersed molybdenum carbides. Preferably, the steel composition comprises from 0.1 to 0.7 wt. % molybdenum, more preferably from 0.15 to 0.5 wt. % molybdenum.

It will be appreciated that the bearing steel composition referred to herein may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.3 wt. % of the composition. Preferably, the steel composition contains unavoidable impurities in an amount of not more than 0.1 wt. % of the composition, more preferably not more than 0.05 wt. % of the composition. In particular, the steel composition may also include one or more impurity elements. A non-exhaustive list of impurities includes, for example:

• • from 0-0.03 wt. % phosphorus;
  • from 0-0.03 wt. % sulfur;
  • from 0-0.04 wt. % arsenic;
  • from 0-0.075 wt. % tin;
  • from 0-0.075 wt. % antimony;
  • from 0-0.002 wt. % lead; and
  • from 0-0.002 wt. % boron.

The steel alloy composition preferably comprises little or no phosphorous, for example from 0 to 0.015 wt. % phosphorous. The steel alloy composition preferably comprises little or no sulfur, for example from 0 to 0.015 wt. % sulfur.

The steel composition preferably comprises ≤5 ppm oxygen. Oxygen may be present as an impurity. The steel composition preferably comprises ≤30 ppm titanium. Titanium may be present as an impurity. The steel composition preferably comprises ≤20 ppm boron. The steel composition preferably comprises ≤50 ppm calcium. Calcium may be present as an impurity.

The bearing steel composition described herein may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements that are mandatory other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence. The steel composition described herein may in some cases consist of the recited elements.

The steel component provided for the method of heat treatment described herein may be obtained by any conventional means known to those skilled in the field. By way of example, a ring for the heat treatment may be provided as follows. First, the steel composition is prepared and cast. The steel composition may then be subjected to a conventional high-temperature soaking step, followed by hot-rolling, typically at a starting temperature of about 1150° C. Several hot-rolling passes may be applied as necessary. The hot-rolled steel, which can be in a bar or plate form, is then allowed to cool slowly to room temperature to avoid the formation of high-carbon martensite. A typical preferred structure in the as hot-rolled condition, at room temperature, is pearlite.

The hot-rolled material may then optionally be homogenized in a homogenization step, such as an approx. 1200° C. treatment for about 24 to 48 hours in vacuum. The material may then, optionally, be furnace-cooled to allow it to cool down slowly to room temperature, also under vacuum.

The material may then be machined in a machining step to near-net-shape components, e.g. an inner or outer ring for a bearing as described herein.

The method comprises a step (ii) of heating the component to a temperature at or above the austenitizing temperature and holding the component for a time sufficient to at least partially austenitize the steel. Such a step may be referred to as an austenitization step, or austempering, which will be known to those skilled in the art. For example, step (ii) comprises heating the component to a temperature of from 790° C. to 915° C. and/or heating the component for from 1 to 6 hours, preferably from 2 to 4 hours. Austenitic steel and the structure thereof is well known to those skilled in the field.

The method comprises step (iii) of cooling the component to a temperature of from 200° C. to 270° C. and holding the component for at most 6 hours, preferably at most 4 hours, thereby transforming at least a portion of the austenite into bainite. Whilst it is known to cool a steel component after austenitization to induce bainite transformation by holding the component at such an elevated temperature, it is known to hold the component for 10 hours or more at such temperature. The inventors have found that this unduly lengthens the heat treatment process and adds cost, at least in view of the cost of energy in heating. The inventors found that this step may be significantly reduced in length when combined with the subsequent tempering steps described below. Furthermore, the inventors were surprised to find that the quicker and more energy efficient process provides a steel with improved properties, such as hardness, and by extension, improved wear resistance and rolling contact fatigue resistance. The present heat treatment process can shorten the cycle time in a salt bath by, for example 75%. The process described herein facilities the bainite transformation and martensite formation to form a mixed bainite martensite steel improving upon the overall transformation time of known processes as well as the steel microstructure and its resulting properties.

The method also comprises step (iv) of cooling and/or quenching the component to a temperature of from −30° C. to 30° C., for example from −10° C. to 10° C., thereby forming martensite. The component is generally held at this temperature for at least 5 minutes to ensure complete quenching to a uniform temperature before the following tempering step.

The method further comprises step (v) of repeating steps (iii) and (iv) by heating the component to a temperature of from 200° C. to 270° C. and holding the component for at most 6 hours before quenching the component to a temperature of from −30° C. to 30° C., thereby transforming a further portion of the austenite into bainite and forming tempered martensite. Step (v) is essentially a first tempering whereby the component is heated after quenching. Generally, the component is heated to the temperature range as fast as practicable. As will be appreciated, by “repeating” steps (iii) and (iv) involves adjusting the temperature to from 200° C. to 270° C. whether by cooling from at or above the austenitizing temperature or heating after having been quenched.

Preferably, when the component is held within a given temperature range, it is held at a substantially constant temperature. The duration of such holding may be measured as the time held within the temperature range, or preferably, the time held at the substantially constant temperature.

In a preferred embodiment, the method further comprises after step (v), a step (vi) of repeating step (iii) by heating the component to a temperature of from 200° C. to 270° C. and holding the component for at most 6 hours before cooling or quenching the component. The further repetition of the heat holding step provides a second tempering that serves to further refine the bearing steel microstructure and reduce the amount of undesirable “retained” austenite by transforming it into bainite. As will be appreciated, desired properties, for example sufficient hardness, can be obtained without step (vi) of an additional tempering.

Preferably, the steps are carried out in order without intervening steps. The method of heat treatment may therefore include only the steps described.

Preferably, in steps (iii), (v), and when/if present step (vi), the component is held at 200° C. to 270° C. for at least 1 hour, preferably at least 2 hours. If the component is held for less than 1 hour, a reduced amount of austenite is transformed to bainite which is undesirable for providing the advantageous properties achievable in the final heat-treated bearing steel. In one preferred embodiment, in a step (iii), the component is cooled to a temperature of from 220° C. to 240° C. In step (v), and when/if present step (vi), it is preferred that the component is heated to a temperature of from 230° C. to 270° C., preferably from 245° C. to 265° C. It is preferred that in these subsequent heating steps, the temperature that the component is heated to is greater than the temperature the component is heated to in step (iii). These tempering steps are preferably carried out at a higher temperature that the initial holding step (iii) to improve the reduction in the amount of retained austenite in the heat treated steel.

Preferably, after each instance of holding the component at a temperature of from 200° C. to 270° C., the method further comprises cooling the component at a rate of from 0.5° C. to 1.0° C. per minute. Such a “soft-cooling” is preferred because it is believed that too fast a quench can lead to cracking in the ring due to the formation internal stress. It is preferred that the component is cooled to a temperature of 80° C. to 120° C. before the following quenching step. Cooling and quenching is preferred after the first holding step after austenitization and the first tempering repetition, though the method may simply comprise quenching without the slow-cooling step. In some embodiments, the method may not include a final quenching step, for example in the second tempering repetition since it is believed there is no further significant phase transformation during a final quench, in which case the component is simply cooled to about ambient temperature (e.g. 20° C.). Cooling is preferably carried out at a substantially constant rate. Quenching is a customary term in the art and relates to a very rapid cooling (e.g. greater than 10° C. per minute). Quenching therefore differs to the more temperature controlled slow-cooling.

In a particularly preferred embodiment, in step (v), and when/if present step (vi), the component is heated in a tempering furnace. Typically, the component after austenitization is quenched in a salt bath due to the high temperature of the steel component prior to quenching.

Those skilled in the art retain the component in the salt bath in order to effect the bainite transformation. The inventors have found that they can increase throughput in the manufacture of a plurality of steel components, in particular large components such a large bearing rings, by carrying out the subsequent tempering steps in a tempering furnace which allows the salt bath to be used for the initially quenching of further components.

Preferably, the heat treatment is completed within 24 hours, more preferably within 22 hours.

The inventors were surprised to find that the additional tempering step, in particular double tempering of steps (v) and (vi) allowed for the production of a steel with a desirable microstructure. For example, in an inner or outer ring for a bearing, the final microstructure of the raceway track comprises at least 60 vol. % bainite and the microstructure of the bearing steel of the ring comprises at most 5 vol. % retained austenite.

Therefore, in accordance with a further aspect of the present disclosure, there is provided an inner or outer ring for a bearing, the ring having a raceway track and being formed of a high-carbon bearing steel;

• • wherein the microstructure of the bearing steel of the raceway track comprises at least 60 vol. % bainite; and
  • wherein the microstructure of the bearing steel of the ring comprises at most 5 vol. % retained austenite.

Unlike the prior art which relates to, for example, surface hardening, the heat treatment of a high-carbon bearing steel of the present disclosure produces a steel component in which the core and surface (including the raceway track) do not have an appreciable difference in their microstructure. That is, the microstructure of the bearing steel (i.e. the component, such as a ring or rolling element, and therefore the bearing steel of the component) comprises at least 60 vol. % bainite, preferably at least 65 vol. % bainite, and at most 5 vol. % retained austenite. Preferably the microstructure includes bainite, retained austenite and tempered martensite.

Furthermore, the hardness value of the component, in particular the surface such as the raceway track of a ring, is at least 59 HRC. Such a hardness is an improvement over the known heat treatment processes which provides the component with better rolling contact fatigue resistance. The units “HRC” as used herein denotes the Rockwell ‘C’ scale and is known to those skilled in the field. Measurement of hardness on the Rockwell ‘C’ scale may be performed by any indentation method known to those skilled in the field. The Vickers hardness test may also be used to measure the core hardness of the inner or outer ring. Conventional methods are known to those skilled in the field.

The maximum dimensional growth of the ring at 150° C. after 2500 hours is less than 15 μm per 100 mm (i.e. SO dimensional stability as measured with respect to the outer diameter of the ring). Such a dimensional stability cannot be achieved by the prior process.

As a result of the heat treatment disclosed herein, the residual compressive stress of the raceway track of a bearing ring is at least 100 MPa. Residual compressive stress may be measured using an X-ray diffraction analyzer. Conventional techniques are known to those skilled in the field.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in relation to the following non-limiting drawings, in which:

FIG. 1 is a flow chart illustrating a heat treatment process of the present disclosure.

FIG. 2 is a graph illustrating an exemplary heat treatment process of a high-carbon bearing steel according to the present disclosure.

FIG. 3 is a perspective view of a heat mapping of hardness across a cross-section of a slice of a ring for a bearing, the cross-section being a plane that contains the axis of rotation.

FIG. 4 is a top view of the heat mapping of hardness across the cross-section of the slice of ring shown in FIG. 3.

DETAILED DESCRIPTION

FIG. 1 illustrates the process of the present disclosure as described herein for the heat treatment of a steel component. In a first step, the process involves providing a steel component for the heat treatment, the steel component being formed of a high-carbon bearing steel. In a second step, the component is subjected to austenitization to at least partially austenitize the steel before third step in which the component is cooled and held at a temperature for a time sufficient for transforming at least a portion of the austenite into bainite. In a fourth step, the component is then cooled and/or quenched. Finally, the third and fourth steps are repeated as a first tempering step which involves heating the component and holding the component before cooling and/or quenching.

FIG. 2 plots temperature on the y-axis against running time on the x-axis.

In the method in accordance with the present disclosure as exemplified in FIG. 2, after an austenitization step, a bearing ring is held within the range of 200° C. to 270° C. for about 3 hours before the ring is slow-cooled over about 3 hours to a temperature of about 100° C. before quenching. Shortly thereafter, the ring is tempered by repeating the heat holding step, but at a temperature greater than the initial holding step and for only about 2 hours, before a similar slow-cool to about 100° C. and quench. The tempering step is repeated for a second time, but the ring is allowed to continue to slow-cool at a slightly higher rate, and to ambient temperature of about 20° C. without quenching. The heat treatment is complete within 22 hours.

The labels for each peak of the FIG. 2 graph read, from left to right in the drawing, “Austenitization,” “Bainite transformation,” “Cooling stage,” “First tempering,” “Cooling stage,” “Second tempering,” and “Cooling to RT [room temperature] for further processing.” The X-axis represents running time, and the Y-axis is temperature in ° C.

Example 1

Large-sized high-carbon bearing rings with outer diameters in range of 0.8 m to 2.6 m were subjected to a known heat treatment process. The high-carbon bearing rings are formed of a steel composition comprising about 0.93 wt. % carbon, about 0.5 wt. % silicon, about 1.0 wt. % manganese, about 1.9 wt. % chromium, about 0.5 wt. % molybdenum, about 0.18 wt. % nickel, and about 0.15 wt. % copper.

The bearing ring is austenitized, such as by the initial austenitization steps shown in FIG. 2, and to which a bainite through-hardening step is performed to obtain a microstructure comprising bainite. In the through-hardening step, the ring is held at above 200° C. for over 10 hours before the ring is quenched to 5° C. The ring is tempered by heating to about 200° C. for about 6 hours. The ring is finally quenched to ambient temperature to complete the heat treatment after more than 26 hours.

The resulting microstructure of the steel may consist essentially of 85-90 vol. % bainite and at most 10-15 vol. % retained austenite. The hardness value of the raceway track may be at least 57 HRC. The compressive residual stress level may be about 120-140 MPa.

Example 2

Equivalent rings as those used for Example 1 are subjected to the heat treatment process of FIG. 2 in accordance with the present disclosure. The microstructure of the steel includes at least 60 vol. % bainite, tempered martensite and at most 5 vol. % retained austenite. The retained austenite content is generally about 4 vol. %±1 vol. %, including at the core and at the surface of the component. The hardness value of the raceway track is at a minimum 59 HRC. The compressive residual stress at the raceway track layer achieves minimum 100 MPa due to the percentage of bainite in the microstructure.

FIGS. 3 and 4 show the heat map of HV10 Vickers hardness of the exemplary ring. The heat map is shown in four regions from light to dark which, as shown in the scale bar, correspond to HV10 of 600.1 to 623.8, 623.8 to 647.8, 647.8 to 671.3, and 671.3 to 695.0, respectively. The hardness, when converted according to DIN EN ISO 18265 Table A1, provides a hardness in HRC. FIGS. 3 and 4 demonstrate that the hardness of the raceway track is from about 59.2 HRC to about 59.9 HRC and the hardness of the core is about 57 HRC.

As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of” (intended to mean that specific further features can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of” (intended to mean that no other feature may be included), unless the context clearly dictates otherwise.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

Claims

1. A method for heat treating a steel component, the method comprising:

(i) providing a steel component formed of a high-carbon bearing steel having an austenitizing temperature;

(ii) heating the component to a temperature at or above the austenitizing temperature and holding the component at or above the austenitizing temperature for a time sufficient to at least partially austenitize the steel;

(iii) after step (ii), cooling the component to a first temperature in a first temperature range of 200° C. to 270° C. and holding the component in the first temperature range for at most 6 hours, thereby transforming at least a portion of the austenite into bainite;

(iv) after step (iii), cooling and/or quenching the component to a temperature of from −30° C. to 30° C., thereby forming martensite; and

(v) after step (iv) reheating the component to a second temperature in the first temperature range, holding the component in the first temperature range for at most 6 hours and then cooling and/or quenching the component to a temperature of from −30° C. to 30° C., thereby transforming a further portion of the austenite into bainite and forming tempered martensite.

2. The method according to claim 1,

wherein holding the component in the first temperature range for at most 6 hours in step (iii) comprises holding the component in the first temperature range for at most 4 hours.

3. The method according to claim 1, further comprising, after step (v), reheating the component to a third temperature in the first temperature range, holding the component in the first temperature range for at most 6 hours and then cooling and/or quenching the component to a temperature of from −30° C. to 30° C., thereby transforming a further portion of the austenite into bainite.

4. The method according to claim 1, wherein the reheating the component is performed in a tempering furnace.

5. The method according to any claim 1, wherein steps (ii)— (v) are completed within 24 hours.

6. The method according to claim 1, wherein steps (ii)— (v) are completed within 22 hours.

7. The method according to claim 1,

wherein, in step (iii), the component is cooled to a temperature of from 220° C. to 240° C.

8. The method according to claim 1,

wherein in step (v) the second temperature is greater than the first temperature.

9. The method according to claim 1,

wherein, in step (iii), the first temperature is from 220° C. to 240° C. and in step (v) the second temperature is greater than the first temperature.

10. The method according to claim 1,

wherein cooling and/or quenching the component comprises cooling the component at a rate of from 0.5° C. to 1.0° C. per minute to a temperature in a second temperature range of 80° C. to 120° C. and then quenching the component.

11. The method according to claim 1,

wherein the steel component is a tapered inner bearing ring or a tapered outer bearing ring.

12. An inner bearing ring or an outer bearing ring formed by the method of claim 1.

13. A method for heat treating a steel component, the method comprising:

(i) providing a steel component formed of a high-carbon bearing steel having an austenitizing temperature;

(ii) heating the component to a temperature at or above the austenitizing temperature and holding the component at or above the austenitizing temperature for a time sufficient to at least partially austenitize the steel;

(iii) after step (ii), cooling the component to a first temperature in a first temperature range of 220° C. to 240° C. and holding the component in the first temperature range for at most 4 hours, thereby transforming at least a portion of the austenite into bainite;

(iv) after step (iii), cooling the component at a rate of from 0.5° C. to 1.0° C. per minute to a temperature in a second temperature range of 80° C. to 120° C. and then quenching the component to a temperature of from −30° C. to 30° C., thereby forming martensite; and

(v) after step (iv) reheating the component to a second temperature in the first temperature range, holding the component in the first temperature range for at most 6 hours and then cooling the component at a rate of from 0.5° C. to 1.0° C. per minute to a second temperature in the second temperature range and then quenching the component to a temperature of from −30° C. to 30° C., thereby transforming a further portion of the austenite into bainite and forming tempered martensite,

wherein the second temperature is greater than the first temperature, and

wherein steps (ii)-(v) are completed within 22 hours.

14. A bearing ring having a raceway track and being formed of a high-carbon bearing steel;

wherein a microstructure of the bearing steel forming the raceway track comprises at least 60 vol. % bainite; and

wherein a microstructure of the bearing steel away from the raceway track comprises at most 5 vol. % retained austenite.

15. The bearing ring according to claim 14, wherein the ring has a bore diameter of at least 0.5 m.

16. The bearing ring according to claim 14, wherein the ring has a bore diameter of at least 1.5 m.