Rolling bearing component and method for heat-treating a metal workpiece

Abstract

The invention relates to a rolling bearing component (6, 7, 8) made of a through-hardening steel, which contains 0.89% to 1.05% carbon, 0.50 to 0.9% silicon, 0.5 to 1.3% manganese, 1.40 to 2.05% chromium, ≤ 0.35% nickel, 0.05 to 0.6% molybdenum, ≤ 0.1% vanadium, ≤ 0.060% aluminium, ≤ 0.08% sulphur, ≤ 0.025% phosphorus, ≤ 0.03% titanium, ≤ 0.015% nitrogen, ≤ 0.007% oxygen, ≤ 0.0035% calcium, and ≤ 0.3% copper (each in percent by weight), which is present as a multiphase structure (9), characterised in that the structure (9) - is made of bainite as the main constituent, - contains > 20 to 35%, in particular > 25% to 35% (volumetric proportion) residual austenite, wherein the grain size thereof is at most 3 μm, - contains 3 to 10% (volumetric proportion) carbide, wherein the grain size thereof is less than 5 μm. The rolling bearing component (6, 7, 8) is particularly suitable for use in a wind turbine (10).

Description

[0001] P240551

[0002] - 1 -

[0003] Rolling bearing component and method for the heat treatment of a metallic workpiece

[0004] The invention relates to a rolling bearing component, particularly in the form of a bearing ring or a rolling element, suitable for use in a wind turbine, among other applications. The invention further relates to a method for the heat treatment of a metallic workpiece, particularly a rolling bearing component.

[0005] WO 2008 / 071157 A2 discloses a method for the heat treatment of rolling bearing components made of through-hardened, bainitic bearing steel. Using this method, rolling bearing components with compressive residual stresses in the surface region and a martensite content of at most 5% and a retained austenite content of at most 3% can be produced. The achievable hardness in the surface regions of the treated workpiece, i.e., the rolling bearing component, is specified in WO 2008 / 071157 A2 as at least 60 HRC. This value can be achieved with a two-stage heat treatment, which includes, among other things, quenching the workpiece in a salt bath to which at least 0.5% water has been added.

[0006] WO 2013 / 075953 A1 concerns a rolling bearing designed specifically for use in industrial trucks. At least one raceway of the rolling bearing is made of through-hardened steel with a bainitic microstructure. According to WO 2013 / 075953 A1, sufficient hardness of the raceway is related to the formation of ferrite needles or ferrite plates layered with carbides.

[0007] DE 10 2007 044 950 B3 discloses a workpiece made of through-hardened steel designed for rolling stress and a heat treatment method. The workpiece designed for rolling stress has a core zone and a carbonitrided surface zone. A bainite microstructure forms the main component of the core zone. The surface zone has a mixed microstructure of martensite and bainite, with the volume fraction of martensite being at least 20 vol.%. The core zone can be P240551

[0008] - 2 - contains bainite carbides and / or retained austenite. A retained austenite content of a maximum of 10 vol.%, in particular a maximum of 3 vol.%, is highlighted as advantageous with regard to dimensional stability in DE 102007 044 950 B3.

[0009] DE 10 2004 038 159 B3 details a process for the heat treatment of workpieces made of steel or cast iron. In particular, it addresses the conversion of austenite to bainite. 100Cr6 is mentioned as a possible material subjected to the heat treatment.

[0010] A radial rolling bearing disclosed in DE 10 2006 055 028 A1 is intended in particular for supporting shafts in wind turbine gearboxes. In this case, too, 100Cr6 is mentioned as a possible material. A special feature of the rolling bearing according to DE 10 2006 055 028 A1 is that several rolling elements are replaced by hollow rollers, which have a slightly larger diameter and a lower modulus of elasticity than the other rolling elements.

[0011] A heat treatment process for steel workpieces described in DE 10 2004 037 074 B3 involves preheating the workpieces for up to 120 seconds so that at least some areas reach an annealing temperature of at least 1000 °C. The workpieces are then annealed at this temperature for between 0.5 and 20 seconds. Afterward, they undergo at least partial bainitic treatment. DE 10 2004 037 074 B3 assumes that high quality requirements for rolling bearing components can be met if the heat treatment of through-hardening bearing steels results in the formation of the finest and most uniformly distributed carbides in the surface layer.

[0012] A method disclosed in DE 102 16 492 A1 for manufacturing a metal rolling bearing component comprises process steps that go beyond heat treatment. Among other things, vibratory finishing of the component under external heat input at a temperature above 80 °C and below the tempering temperature or P240551 is included.

[0013] - 3 -

[0014] A microstructure transformation temperature is specified. The aim is to reduce dislocation mobility in the highly stressed surface layer while simultaneously stabilizing a crack-inhibiting compressive residual stress state. Overall, the process according to DE 102 16 492 A1 should achieve an effect comparable to that of hot shot peening.

[0015] DE 10 2022 131 948 A1 discloses a process for manufacturing high-toughness rolling bearing components, using a high-purity steel material. Two different heat treatment processes are used to produce retained austenite contents in a bainitic matrix of between 2 and 18 vol.% or retained austenite contents in a martensitic matrix of between 15 and 25 vol.%.

[0016] The invention is based on the objective of achieving progress compared to the aforementioned prior art with regard to the toughness and load-bearing capacity of rolling bearing components, for example for wind turbines.

[0017] This problem is solved according to the invention by a rolling bearing component made of steel with the features of claim 1. Likewise, the problem is solved by a method designed according to claim 11 for the heat treatment of a workpiece intended for use as a rolling bearing component.

[0018] The invention is based on the consideration that, in principle, martensitic through-hardened, bainitic through-hardened, case-hardened, or induction-surface-hardened bearing rings are suitable as rolling bearing rings for wind turbines, as well as for other applications. Martensitic through-hardened bearing rings can, for example, be used in the stabilized state, i.e., highly tempered, with a retained austenite content of less than 5%. In the case of bainitic through-hardening, complete bainitic through-hardening is carried out, whereby a retained austenite content of less than 3% can be achieved. P240551

[0019] - 4 -

[0020] Case-hardened bearing components differ from through-hardened bearing components, particularly rolling bearing rings, in that they are only hardened in the surface area, i.e., to a defined depth, which may include carburizing and / or nitriding. The core of the workpiece retains a low carbon content and, depending on the hardenability of the case-hardening steel used, can have a martensitic, bainitic, pearlitic, or multiphase structure.

[0021] Inductively surface-hardened rolling bearing rings, for example, have a carbon content of 0.43 to 0.5 wt.%. During the inductive hardening process, the entire workpiece can undergo thermal pretreatment to adjust the microstructure and mechanical properties. As a subsequent step, a surface area, particularly a raceway area of ​​the future rolling bearing, is subjected to partial inductive surface hardening. This creates a martensitic microstructure in the surface layer with a proportion of retained austenite, which is limited by the base carbon content and the desired hardness.

[0022] Therefore, all common hardening processes have specific advantages and disadvantages. Surface hardening, compared to through-hardening of workpieces, results in increased manufacturing effort.

[0023] The solution as described in the application overcomes disadvantages described in the prior art, which are considered unavoidable, by producing a specific multiphase microstructure based on lower bainite in a hypereutectoid through-hardening steel of defined composition.

[0024] The steel which, according to claim 9, is subjected to heat treatment and constitutes the material from which the rolling bearing component according to the application is manufactured, has the following composition, specified in weight percent: P240551

[0025] 0.89% to 1.05% Carbon, 0.50% to 0.9% Silicon, 0.5% to 1.3% Manganese, 1.40% to 2.05% Chromium, maximum 0.35% Nickel, 0.05% to 0.6% Molybdenum, maximum 0.1% Vanadium, maximum 0.060% Aluminum, maximum 0.08% Sulfur, maximum 0.025% Phosphorus, maximum 0.03% Titanium, maximum 0.015% Nitrogen, maximum 0.007% Oxygen, maximum 0.0035% Calcium, maximum 0.3% Copper. Balance: Iron, unavoidable trace elements and smelting-related impurities.

[0026] In the rolling bearing component, the steel exists as a multiphase structure, which

[0027] - is composed of bainite as its main component,

[0028] - > 20 to 35%, in particular > 25 to 35% (by volume) stabilized retained austenite, the grain size of which is a maximum of 3 pm,

[0029] - Contains 3 to 10% (by volume) carbides with a grain size of less than 5 pm. P240551

[0030] - 6 -

[0031] The bainite content is therefore the main component in the microstructure and supplements the retained austenite and carbides to 100% (by volume). The hardness of the rolling bearing component, which is in particular a bearing ring or a rolling element, can be at least 655 HV10 and at most 750 HV10 in various configurations.

[0032] Regarding the elements carbon, chromium and molybdenum, the following restricted ranges may be given, individually or in any combination, each in wt.%:

[0033] 0.90% to 0.95% carbon

[0034] 1.85% to 1.98% chromium

[0035] 0.50% to 0.60% molybdenum

[0036] The sum of the content of the elements manganese, chromium and silicon corresponds, for example, to more than 2.95 wt.%, in particular more than 3.05 wt.%.

[0037] The method according to the application starts with the provision of a workpiece made of a steel which contains the elements mentioned in claim 1 in the proportions specified therein. In particular, it is a steel in which at least one of the elements carbon, chromium and molybdenum is contained in a narrowed range as described above.

[0038] The heat treatment provided for in the application process is carried out by first austenitizing at a temperature range of > 870 °C to 900 °C. The austenitizing temperature is important for achieving the required hardness of the rolling bearing component. Subsequently, the workpiece, i.e., the future rolling bearing component, is quenched in a salt bath at a temperature of > 220 °C to 260 °C, in particular at a temperature of 230 °C to 260 °C, which initiates a bainitic transformation. P240551

[0039] - 7 -

[0040] The duration of the bainitic transformation in the salt bath influences the amount of retained austenite that ultimately remains in the rolling bearing component. After a duration of, for example, 15 to 30 minutes, the workpiece is removed from the salt bath and maintained at least approximately at the specified temperature level, for example, a temperature in the range of > 220 °C to 240 °C. A tempering furnace is suitable for this purpose. The bainitic transformation continues in the tempering furnace or other chamber where the specified temperature level of > 220 °C to 260 °C, in particular a temperature in the range of 230 °C to 260 °C, is maintained. In total, the approximately uniform temperature level prevailing firstly in the salt bath and secondly in the tempering furnace or other heated, non-liquid-filled volume is maintained for a total duration of at least 120 minutes and at most 900 minutes.Then the workpiece is cooled to room temperature.

[0041] This results in a robust component, namely a rolling bearing component, with a multiphase microstructure consisting of a dominant bainitic structure, stabilized retained austenite, and carbide inclusions, with the size of the retained austenite grains and carbides falling within the previously mentioned ranges. The microstructure can, in particular, exhibit a minimum hardness of 675 HV10 and is therefore suitable for rolling loads. Regarding its suitability for rolling loads, whether as a bearing ring or a rolling element, the high proportion of finely structured retained austenite compared to known steels intended for the same or similar applications is particularly noteworthy. The supersaturated retained austenite contained in the rolling bearing component is thus not present in a block-like form, unlike in a martensitic heat treatment.Especially in contaminated environments, the finely structured, stabilized retained austenite contributes to the high robustness of the rolling bearing component.

[0042] A compressive residual stress measurement on an A220 lattice plane of retained austenite preferably reveals a compressive residual stress greater than 140 MPa. The compressive residual stress is determined by X-ray diffractometry according to DIN EN 15305:2009-1 from 2009. P240551

[0043] - 8 -

[0044] The retained austenite content of > 20 to 35%, and especially of > 25 to 35% (by volume), is thus stabilized against stress-induced transformation by compressive residual stresses. As a result, this retained austenite only transforms under increased load stresses, i.e., in the case of localized stress concentrations. The service life of the rolling bearing component is therefore significantly higher.

[0045] Furthermore, it has proven beneficial if the half-value width at an A220 lattice plane of the retained austenite is greater than 4°. The half-value width provides a measure of the existing stress in the steel microstructure and indicates the presence and frequency of dislocations in the microstructure.

[0046] The rolling bearing component according to the application is suitable not only for use in wind turbines, whether as a rotor main bearing or gearbox bearing, but also for numerous other stationary and mobile applications in a wide variety of dimensions. The rolling bearing, which comprises at least one component heat-treated according to the application's method, can be a single-row or multi-row bearing, configured as a radial bearing, an axial bearing, or a combined radial-axial bearing. The rolling elements of the bearing can be in the form of balls or rollers, for example, cylindrical rollers, tapered rollers, or barrel rollers.

[0047] An embodiment of the invention is explained in more detail below with reference to a drawing. The drawing shows:

[0048] Fig. 1 shows a schematic representation of a rolling bearing in a wind turbine.

[0049] Fig. 2 shows a multiphase structure of a component of the rolling bearing according to Figure 1.

[0050] A wind turbine, designated in its entirety by 10 and only indicated in Figure 1, comprises a shaft 2 which carries a rotor 3, the rotor blades of which are designated by 4. The shaft 2 is supported by a rolling bearing 1, in which there is P240551

[0051] - 9 - thus, it is a main rotor bearing. Additionally, a further rolling or sliding bearing, not visible in Figure 1, may be provided for supporting the shaft 2. In Figure 1, the rolling bearing 1 is depicted as a cylindrical roller bearing. Alternatively, the rolling bearing 1 may also be designed, for example, as a spherical roller bearing or a tapered roller bearing.

[0052] The rolling bearing 1 comprises an inner ring 6, a plurality of rolling elements 7, and an outer ring 8 as rolling bearing components 6, 7, 8. The outer ring 8 is installed in a housing 5, which is part of a nacelle of the wind turbine 10.

[0053] In the exemplary embodiment, both the inner ring 6 and the outer ring 8, i.e., the bearing rings 6, 8, as well as the rolling elements 7, i.e., rollers, have a microstructure 9 as shown in Figure 2. In the exemplary embodiment, the microstructure 9 is formed from a steel with the following composition (given in wt.%):

[0054] 0.90% to 0.95% Carbon 0.50% to 0.9% Silicon 0.5% to 1.3% Manganese 1.85% to 1.98% Chromium (maximum) 0.35% Nickel 0.50% to 0.60% Molybdenum (maximum) 0.1% Vanadium (maximum) 0.060% Aluminum (maximum) 0.08% Sulfur (maximum) 0.025% Phosphorus (maximum) 0.03% Titanium (maximum) 0.015% Nitrogen (maximum) 0.007% Oxygen P240551

[0055] - 10 - maximum 0.0035% calcium, maximum 0.3% copper

[0056] Residual iron, unavoidable trace elements and smelting-related impurities

[0057] Within structure 9, the sum of the content of the elements manganese, chromium and silicon is more than 2.95 percent by weight.

[0058] The microstructure 9 is a multiphase microstructure, which is created in the following heat treatment process:

[0059] First, the workpiece, i.e., the bearing ring 6, 8 or the rolling element 7, is austenitized at a temperature in the range of > 870 °C to 900 °C to achieve a dissolved carbon content of approximately 0.7 to 0.85%. A neutral to slightly carburizing atmosphere is maintained during the austenitization process.

[0060] The workpiece, i.e., the rolling bearing component 6, 7, 8 undergoing the manufacturing process, is then quenched in a salt bath at a temperature of > 220 °C to 260 °C, specifically at a temperature of 230 °C to 260 °C. The workpiece 6, 7, 8 remains in the salt bath for 15 to 30 minutes. During this time, bainitic quenching begins. After removal from the salt bath, the workpiece 6, 7, 8 is kept at approximately the same temperature level, for which a tempering furnace is used in the exemplary embodiment. The bainitic quenching continues in the tempering furnace. In total, the workpiece 6, 7, 8 remains at the aforementioned temperature level of > 220 °C to 260 °C, specifically at a temperature of 230 °C to 260 °C, for a total duration of at least 120 minutes and at most 900 minutes before being removed from the tempering furnace and cooled to room temperature. P240551

[0061] - 11 -

[0062] The specified temperature range of > 220 °C to 260 °C represents a bainite retention stage, in which an isothermal transformation of the austenite into a bainitic microstructure begins. Simultaneously, carbon is precipitated from the transformed microstructure, migrating into the remaining austenite. This process continues until the desired amount of austenite has been transformed, which, depending on the exact alloy composition, takes between 120 and 900 minutes. The retained austenite contained in microstructure 9 is thereby further stabilized against transformation, as the carbon content in the retained austenite increases, thus significantly lowering the martensite start temperature of this retained austenite.

[0063] The resulting multiphase microstructure 9, shown in Figure 2, consists of a dominant bainitic structure, stabilized retained austenite, and carbide inclusions. The retained austenite grains are no larger than 3 pm, and the carbides are smaller than 5 pm. Microstructure 9 has a minimum hardness of 675 HV10.

[0064] With its volume fraction, bainite constitutes the main component of the multiphase microstructure 9 of the rolling bearing components 6, 7, 8. Furthermore, the microstructure 9 contains more than 20% to 35%, in particular more than 25% to 35% (by volume), of stabilized retained austenite. The grain size of the retained austenite is a maximum of 3 pm. Additionally, the microstructure 9 contains carbides with a proportion of 3% to 10%, the grain size of which is less than 5 pm.

[0065] P240551

[0066] - 12 -

[0067] List of reference signs

[0068] 1 rolling bearing

[0069] 2nd wave

[0070] 3 Rotor

[0071] 4 rotor blades

[0072] 5 cases

[0073] 6 Inner ring, rolling bearing component

[0074] 7 rolling elements, rolling bearing component

[0075] 8 Outer ring, rolling bearing component

[0076] 9 Structure, multiphase structures

[0077] 10 wind turbines

Claims

P240551 - 13 - Patent claims 1. Rolling bearing component (6, 7, 8) made of steel with the following composition (each in wt.%): 0.89% to 1.05% carbon, 0.50% to 0.9% silicon, 0.5% to 1.3% manganese, 1.40% to 2.05% chromium, maximum 0.35% nickel, 0.05% to 0.6% molybdenum, maximum 0.1% vanadium, maximum 0.060% aluminum, maximum 0.08% sulfur, maximum 0.025% phosphorus, maximum 0.03% titanium, maximum 0.015% nitrogen, maximum 0.007% oxygen, maximum 0.0035% calcium, maximum 0.3% copper, balance iron, unavoidable trace elements and smelting-related impurities, the steel being a multiphase structure (9) which - is composed of bainite as its main component, - > 20% to 35%, in particular > 25% to 35% (by volume) retained austenite, the grain size of which is a maximum of 3 pm, - Contains 3% to 10% (by volume) carbides, the grain size of which is less than 5 pm.

2. Rolling bearing component (6, 7, 8) according to claim 1 , characterized in that the carbon content is 0.90 to 0.95 wt.%. P240551 - 14 - 3. Rolling bearing component (6, 7, 8) according to claim 1 or 2, characterized in that the chromium content is 1.85 to 1.98% wt.%.

4. Rolling bearing component (6, 7, 8) according to one of claims 1 to 3, characterized in that the molybdenum content is 0.50 to 0.60 wt.%.

5. Rolling bearing component (6, 7, 8) according to one of claims 1 to 4, characterized in that the sum of the content of the elements manganese, chromium and silicon is more than 2.95 wt.%.

6. Rolling bearing component (6, 7, 8) according to one of claims 1 to 5, characterized in that it has a hardness of at least 655 HV10 and at most 750 HV10.

7. Rolling bearing component (6, 8) according to one of claims 1 to 6, characterized in that it is designed as a bearing ring.

8. Rolling bearing component (7) according to one of claims 1 to 6, characterized in that it is designed as a rolling element.

9. Rolling bearing component (7) according to one of claims 1 to 8, characterized in that a compressive residual stress of greater than 140 MPa is present at an A220 lattice plane of the retained austenite.

10. Rolling bearing component (6, 7, 8) according to one of claims 1 to 9, characterized in that a half-width greater than 4° is present at an A220 lattice plane of the retained austenite.

11. Method for heat-treating a workpiece intended for use as a rolling bearing component (6, 7, 8), in particular according to one of claims 1 to 10, comprising the following steps: P240551 - Provision of a workpiece made of a steel of the following composition (all values ​​in wt.%): 0.89% to 1.05% Carbon, 0.50% to 0.9% Silicon, 0.5% to 1.3% Manganese, 1.40% to 2.05% Chromium, maximum 0.35% Nickel, 0.05% to 0.6% Molybdenum, maximum 0.1% Vanadium, maximum 0.060% Aluminum, maximum 0.08% Sulfur, maximum 0.025% Phosphorus, maximum 0.03% Titanium, maximum 0.015% Nitrogen, maximum 0.007% Oxygen, maximum 0.0035% Calcium, maximum 0.3% Copper. Balance: Iron, unavoidable trace elements and smelting-related impurities. - Austenitization in the range of > 870 °C to 900 °C, - Quenching in a salt bath at a temperature of > 220 °C to 260 °C, removal of the workpiece from the salt bath and maintenance of the aforementioned temperature level for a total duration of at least 120 min and at most 900 min, - Cooling the workpiece to room temperature.

12. Method according to claim 11, characterized in that the quenching takes place in a temperature range of 230 °C to 260 °C.

13. Use of a rolling bearing component (6, 7, 8) according to one of claims 1 to 10 in a wind turbine (10).

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