Heat treated roller bearing ring

Abstract

An inner or outer ring for a roller bearing, the inner or outer ring being formed from a steel composition comprising: carbon: 0.30-0.45 wt.%; silicon: 0.1-0.7 wt.%; manganese: 0.6-0.9 wt.%; chromium: 0.9-1.2 wt.%; molybdenum: 0.15-0.7 wt.%; nickel: 0-2.0 wt.%; phosphorus: 0-0.02 wt.%; sulphur: 0-0.02 wt.%; and the remainder iron, together with unavoidable impurities; wherein the microstructure of the steel composition comprises bainite; and wherein a carburised and nitrided case is provided on the surface of the inner or outer ring.

Description

Technical Field

[0001] This invention generally relates to the metallurgical field and the heat treatment of roller bearing rings. More specifically, this invention relates to the heat treatment of inner or outer rings for roller bearings formed of medium carbon steel. Compared to conventional roller bearing rings made of low carbon steel, the inner or outer ring has comparable or improved wear resistance and load-bearing capacity, but at a lower manufacturing cost. Background Technology

[0002] A bearing is a device that allows constrained relative movement between two components. Rolling element bearings consist of an inner raceway and an outer raceway, and multiple rolling elements (e.g., balls and / or rollers) disposed therebetween. For example, a roller bearing is a rolling element bearing in which the rolling elements are rollers rather than balls. Rolling bearings include, for example, spherical roller bearings and tapered roller bearings. For long-term reliability and performance, it is important that the various components have high resistance to rolling fatigue, wear, and creep.

[0003] Traditional processes for manufacturing metal components involve hot rolling or hot forging to form bars, rods, tubes, or rings, followed by soft forming to obtain the desired component. Surface hardening processes are well known and are used to locally increase the surface hardness of finished or semi-finished components to improve, for example, wear resistance and fatigue resistance. Various surface or case hardening processes are known for improving resistance to rolling contact fatigue.

[0004] Typically, for roller bearings (such as spherical or tapered roller bearings), the inner and / or outer rings are formed of low-carbon steel. The composition of the low-carbon steel commonly used in conventional roller bearing rings is as follows:

[0005] Carbon: 0.17–0.23 wt.%;

[0006] Silicon: 0.15–0.40 wt.%;

[0007] Manganese: 0.40–0.70 wt.%;

[0008] Chromium: 0.35–0.64 wt.%;

[0009] Nickel: 1.60–2.00 wt.%;

[0010] Molybdenum: 0.20–0.30 wt.%;

[0011] Phosphorus: 0–0.02 wt.%;

[0012] Sulfur: 0–0.02 wt.%; and

[0013] The remaining iron, and the unavoidable impurities.

[0014] Typically, low-carbon steel used in roller bearing rings is carburized and martensitic hardened to provide a ring with suitable wear resistance and load-bearing capacity for use as the inner or outer ring of a roller bearing. Because low-carbon martensitic steel has a relatively low core hardness (typically less than 40 HRC), the carburized case layer must be deep to achieve the required level of wear resistance and load-bearing capacity. In this respect, the typical carburized case layer depth in conventional roller bearing rings is 1.8–3.5 mm. One of the many reasons for forming a carburized case layer of this depth on the surface of conventional low-carbon steel is that the heat treatment process has a long cycle time. For example, for a 3–3.5 mm carburized case layer, the heat treatment cycle time required to produce conventional roller bearing rings made of low-carbon steel is typically 25–35 hours at a temperature of 950–970 °C. The longer the heat treatment cycle time, the higher the cost of the heat treatment process.

[0015] Therefore, the object of the present invention is to provide an inner or outer ring for a roller bearing that can be manufactured at a lower cost compared to conventional roller bearing rings, while still exhibiting comparable or improved wear resistance and load-bearing capacity.

[0016] Another object of the present invention is to provide a heat treatment method to obtain an inner or outer ring for roller bearings that has comparable or improved wear resistance and load-bearing capacity compared to conventional roller bearing rings, wherein the method requires lower manufacturing costs compared to the manufacture of conventional roller bearing rings.

[0017] This invention seeks to solve at least some of the problems associated with the prior art, or at least to provide commercially acceptable alternative solutions. Summary of the Invention

[0018] According to a first aspect, an inner or outer ring for a roller bearing is provided, said inner or outer ring being formed from a steel composition comprising the following components:

[0019] Carbon: 0.30–0.45 wt.%;

[0020] Silicon: 0.1–0.7 wt.%;

[0021] Manganese: 0.6–0.9 wt.%;

[0022] Chromium: 0.9–1.2 wt.%;

[0023] Molybdenum: 0.15–0.7 wt.%;

[0024] Nickel: 0–2.0 wt.%;

[0025] Phosphorus: 0–0.02 wt.%;

[0026] Sulfur: 0–0.02 wt.%; and

[0027] The remaining iron, and the unavoidable impurities;

[0028] The microstructure of the steel composition includes bainite; and

[0029] A carbonitriding shell layer is provided on the surface of the inner or outer ring.

[0030] The invention will now be described further. In the following paragraphs, different aspects of the invention are defined in more detail. Unless expressly stated to the contrary, each aspect or embodiment as defined herein may be combined with any other aspect or embodiment. In particular, any feature indicated as preferred or advantageous may be combined with any other feature indicated as preferred or advantageous. (Additionally, the symbols wt.% and vol.% in this document have the conventional meaning in the art, referring to weight percentage and volume percentage, respectively.)

[0031] Roller bearings and their structures are known to those skilled in the art. The shape of the inner or outer ring of this invention is not particularly limited, and any conventional shape for inner or outer rings used in roller bearings can be adopted. As used herein, the term "inner ring" can refer to a roller bearing ring that is radially inwardly positioned relative to the rolling element in the roller bearing and thus has raceways on its radially outer surface. As used herein, the term "outer ring" can refer to a roller bearing ring that is radially outwardly positioned relative to the rolling element in the roller bearing and thus has raceways on its radially inner surface. The inner or outer ring is preferably used for spherical roller bearings or tapered roller bearings. Due to the load requirements of spherical roller bearings or tapered roller bearings and the high wear resistance and load-carrying capacity exhibited by the inner or outer ring of this invention, the physical and / or mechanical properties of the inner or outer ring of this invention are particularly suitable for such bearings.

[0032] The combination of carbonitriding and bainitic hardening in the steel compositions described herein is particularly suitable for applications with high interference fits, high operating temperatures, and long maintenance intervals. It is not desirable to be bound by theory; this is believed to be because heat treatment imparts good hardness retention at high operating temperatures, high compressive stresses, and under hardened cores.

[0033] The steel composition comprises 0.30 to 0.45 wt.% carbon. Therefore, this steel composition is a so-called medium-carbon steel. Combined with other alloying elements, this results in the desired bainitic microstructure, desired load-bearing capacity, core hardness, and impact toughness. This medium-carbon steel can exhibit higher hardness compared to conventional low-carbon steels used in roller bearing rings (such as spherical roller bearings and tapered roller bearings). Carbon also acts to lower the bainitic transformation temperature, thereby enabling the desired bainitic microstructure. When the carbon content is above 0.45 wt.%, the maximum volume fraction of the bainitic ferrite portion of the microstructure decreases. When the carbon content is below 0.30 wt.%, the alloy has a higher martensitic initiation temperature, and the bainitic steel composition may not exhibit the desired core hardness and impact toughness. Preferably, the steel composition comprises 0.32 to 0.43 wt.% carbon, more preferably 0.35 to 0.40 wt.% carbon. In some embodiments, the steel composition comprises 0.39 to 0.45 wt.% carbon.

[0034] The steel composition includes 0.1–0.7 wt.% silicon. Combined with other alloying elements, this results in a desired bainitic microstructure with a minimal amount of retained austenite. Silicon has negligible solubility in carbides; especially at high temperatures, its diffusivity is high enough that it is not trapped by carbides. Silicon also helps to suppress excessive precipitation of cementite and carbide formation. Additionally, silicon stabilizes transition carbides and improves the tempering resistance of the steel microstructure. However, excessive silicon content can lead to undesirable surface oxides and poor surface finish, and may also reduce the elastic properties of the matrix. Therefore, the maximum silicon content is 0.7 wt.%. When the silicon content is below 0.1 wt.%, it may be difficult to obtain the desired bainitic microstructure without a significant amount of retained austenite (e.g., more than 10 vol.%). Preferably, the steel composition comprises 0.2 to 0.6 wt.% silicon, more preferably 0.3 to 0.5 wt.% silicon, and even more preferably 0.35 to 0.45 wt.% silicon. In some embodiments, the steel composition comprises 0.1 to 0.4 wt.% silicon.

[0035] The steel composition includes 0.6–0.9 wt.% manganese. Combined with other alloying elements, manganese improves hardenability. Therefore, when the manganese content is below 0.6 wt.%, it may be difficult to easily obtain a steel composition that combines the desired bainitic microstructure and high core hardness. Furthermore, manganese increases the stability of austenite relative to ferrite. However, manganese levels above 0.9 wt.% may increase the amount of retained austenite and potentially reduce the rate of transformation to bainite. This can lead to practical metallurgical problems, such as excessive stabilization of retained austenite, resulting in potential issues with the dimensional stability of bearing assemblies. Preferably, the steel composition includes 0.65–0.85 wt.% manganese, more preferably 0.7–0.8 wt.% manganese.

[0036] The steel composition includes 0.9 to 1.2 wt.% chromium. Chromium plays a role in increasing hardenability and lowering the bainite initiation temperature. Therefore, when the chromium content is below 0.9 wt.%, it may be difficult to easily obtain a steel composition that combines the desired bainitic microstructure with high core hardness. When the chromium content is above 1.2 wt.%, there may be an increased risk of segregation problems within the steel's structure and composition. Preferably, the steel composition includes 0.95 to 1.15 wt.% chromium, more preferably 1.0 to 1.1 wt.% chromium.

[0037] The steel composition optionally includes 0 to 2.0 wt.% nickel, for example, 0.1 to 2.0 wt.% Ni. For example, nickel may be advantageous in terms of overall toughness and / or impact properties. Preferably, the steel composition includes 1.0 to 2.0 wt.% nickel. In some embodiments, the steel composition includes 0 to 0.7 wt.% nickel, for example, 0.1 to 0.7 wt.% Ni.

[0038] The steel composition includes 0.15 to 0.7 wt.% molybdenum. Molybdenum helps prevent austenitic grain boundary embrittlement caused by impurities such as phosphorus. Molybdenum also lowers the bainite initiation temperature and improves hardenability, which is important when the steel is used to manufacture, for example, large bearing rings that need to be hardened to a large depth after high-temperature quenching. Therefore, when the molybdenum content is below 0.15 wt.%, it may be difficult to readily obtain a steel composition that combines the desired bainitic microstructure with high core hardness. The molybdenum content in the alloy preferably does not exceed about 0.7 wt.%, otherwise the transformation from austenite to bainitic ferrite may stop too early, which could result in a large amount of austenite remaining in the structure. Molybdenum can also help improve resistance to rolling contact fatigue by producing finely dispersed molybdenum carbide. Preferably, the steel composition includes 0.2 to 0.6 wt.% molybdenum, more preferably 0.3 to 0.5 wt.% molybdenum. In some embodiments, the steel composition includes 0.15 to 0.30 wt.% molybdenum.

[0039] It will be understood that the steel composition referred to herein may include unavoidable impurities, although in general these impurities are unlikely to exceed 0.3 wt.% of the composition. Preferably, the steel composition includes unavoidable impurities in an amount not exceeding 0.1 wt.% of the composition, more preferably not exceeding 0.05 wt.% of the composition. In particular, the steel composition may also include one or more impurity elements. For example, a non-exhaustive list of impurities includes:

[0040] Phosphorus: 0–0.02 wt.%

[0041] Sulfur: 0–0.02 wt.%

[0042] Arsenic: 0–0.04 wt.%

[0043] Tin: 0–0.075 wt.%

[0044] Antimony: 0–0.075 wt.%

[0045] Lead: 0–0.002 wt.%

[0046] Boron: 0–0.002 wt.%

[0047] The steel alloy composition preferably contains little or no phosphorus, for example, 0 to 0.015 wt.% phosphorus.

[0048] The steel alloy composition preferably contains little or no sulfur, for example, 0 to 0.015 wt.% sulfur.

[0049] The steel composition preferably includes 15 ppm or less of oxygen. Oxygen may be present as an impurity. The steel composition preferably includes 30 ppm or less of titanium. Titanium may be present as an impurity. The steel composition preferably includes 20 ppm or less of boron. The steel composition preferably includes 50 ppm or less of calcium. Calcium may be present as an impurity.

[0050] The steel compositions described herein may consist essentially of the elements listed. Therefore, it will be understood that, in addition to those essential elements, other unspecified elements may be present in the composition, provided that their presence does not substantially affect the essential characteristics of the composition. The steel compositions described herein may consist of the elements listed.

[0051] It will be understood that the steel composition used in the inner or outer ring of the roller bearing of the present invention is a known steel composition. However, the steel composition is medium carbon steel. Traditionally, low carbon steel is used for roller bearing rings. In contrast, medium carbon steels, such as those described herein, are typically used in slewing bearings. Slewing bearings are typically used in applications requiring lower load-bearing capacity and slower operating / rotational speeds than roller bearings. Previously, the use of medium carbon steel for slewing bearings, for example, was undesirable due to its relatively low strength and wear resistance. Medium carbon steel typically used in slewing bearings without the heat treatment described herein would not meet the load-bearing capacity or the abrasive wear requirements for roller bearing applications. Therefore, surprisingly and unexpectedly, the heat-treated medium carbon steel used in the present invention exhibits the desired properties described herein and can provide suitable inner or outer rings for roller bearings with acceptable load-bearing capacity and wear resistance for use in typical roller bearing applications.

[0052] Using this medium carbon steel for roller bearing rings is advantageous, at least because it is cheaper than conventional low carbon steel. However, the microstructure of the medium carbon steel, as well as the type and depth of the shell, needs to be selected and optimized, and the heat treatment parameters need to be optimized to provide rings using this non-conventional steel suitable for use as inner or outer rings for roller bearings (i.e., roller bearing rings with suitable wear resistance and load-bearing capacity).

[0053] In this respect, in this invention, the microstructure of the steel composition comprises bainite. For example, the microstructure of the steel composition typically comprises at least 60 vol.% bainite, more typically, at least 80 vol.% bainite, even more typically, at least 90 vol.% bainite, and even more typically, at least 95 vol.% bainite. Methods of bainite hardening are well known to those skilled in the art. However, it may be necessary to optimize the heat treatment parameters for the medium carbon steel described herein in order to obtain suitable roller bearing rings with acceptable wear resistance and load-bearing capacity. The microstructure of the steel composition may also comprise a small amount of martensite, for example, less than 20 vol.% martensite, or particularly, even less than 10 vol.% martensite in the core of the inner or outer ring.

[0054] Preferably, the retained austenite content in the steel composition is kept to a minimum to improve strength, hardness, and (especially) dimensional stability. The amount of retained austenite is typically less than 10 vol.%, more typically less than 5 vol.%. In some embodiments, the microstructure of the steel composition contains no or substantially no retained austenite. However, in some embodiments, the steel composition may include a small amount of retained austenite, which can be beneficial for extending the service life of the bearing. The microstructure may also include small carbide, nitride, and / or carbonitride precipitates, such as nanoscale precipitates (typically with an average size of 5–30 nm) or microscale precipitates (typically with an average size of 1–10 μm). The presence of such precipitates can improve the wear resistance of the raceway. Any such precipitates typically constitute no more than 5 vol.%, more typically no more than 3 vol.%, for example, 0.5–3 vol.%. In one embodiment, the structure contains no or at least substantially no carbides, nitrides, and / or carbonitrides.

[0055] The microstructure of steel compositions can be determined using traditional microstructure characterization techniques, such as optical microscopy, TEM, SEM, AP-FIM, and X-ray diffraction, including combinations of two or more of these techniques.

[0056] One of the many reasons is that this microstructure imparts the desired core hardness to the steel composition, thus enabling, for example, the inner or outer ring to possess the desired load-bearing capacity. This high core hardness of bainitic carbon steel also allows for a reduction in shell depth compared to conventional roller bearing rings, thereby reducing the required heat treatment cycle time and consequently lowering manufacturing costs.

[0057] In this respect, in this invention, a carbonitriding shell layer is provided on the surface of the inner or outer ring. The carbonitriding process is well known to those skilled in the art. However, it may be necessary to optimize the carbonitriding parameters for the medium carbon steel described herein in order to obtain a suitable roller bearing ring with acceptable wear resistance and load-bearing capacity. The carbonitriding shell layer gives the surface of the inner or outer ring a high surface hardness, thereby providing high wear resistance.

[0058] The inventors have surprisingly discovered that the use of medium carbon steel described herein for inner or outer rings of roller bearings, compared to conventional roller bearing rings using low carbon steel, enables the provision of an inner or outer ring for roller bearings that can have comparable or improved wear resistance and load-bearing capacity (e.g., while being available at a lower manufacturing cost), wherein the microstructure of the steel composition comprises bainite, and wherein a carbonitriding shell is disposed on the surface of the inner or outer ring.

[0059] Typically, a carbonitriding shell is formed on at least the raceway surface of the inner or outer ring. This is at least because high surface hardness and wear resistance are particularly desirable on the raceway. However, a carbonitriding shell can also be formed on any other surface of the inner or outer ring, and is not limited to the raceway surface.

[0060] The carbonitriding shell layer preferably has a shell depth of 0.7–3.5 mm, more preferably 1.0–2.5 mm. In some embodiments, the carbonitriding shell layer has a shell depth of 1.0–1.8 mm or even 1.0–1.6 mm. The carburized shell layer depth of conventional roller bearing rings is typically 1.8 to 3.5 mm. Not wanting to be bound by theory, this reduction in shell depth is feasible as a result of using bainitic medium carbon steel for the inner or outer ring. According to the Hertzian stress model theory, for example, the carburized thickness of medium carbon steel can be reduced by one-third compared to conventional low carbon steel carburizing. The core hardness of conventional low carbon steel roller bearing rings (after carburizing and martensitic hardening) is typically 40 HRC or less. However, the core hardness of the bainitic medium carbon steel of the present invention is typically greater than 40 HRC. Therefore, the shell depth can be reduced while maintaining appropriate wear resistance and load-bearing capacity of the inner or outer ring, and thus, for example, the processing time can be reduced by 1 / 3 to 1 / 4. Advantageously, this can significantly reduce the manufacturing cost of roller bearing rings. For example, a shell depth of 1.8–2.5 mm in the medium carbon steel of the present invention exhibits performance comparable to that of a shell depth of 2.5–3.0 mm in the low carbon steel of the prior art, while bearing the same load requirements. Generally, compared to conventional low carbon steel carbonitrided rings, the thinner carbonitrided shell of the present invention provides better or comparable wear resistance on the raceway for both the outer and inner rings. Below the raceway, the inner or outer ring of the present invention exhibits the desired toughness.

[0061] The depth of the shell can be measured using conventional techniques known in the art. For example, suitable methods may include examining the cross-section of the inner or outer ring by means of optical microscopy, TEM, SEM, AP-FIM, and X-ray diffraction, or a combination of two or more of these techniques.

[0062] Preferably, the shell microstructure contains less than 3 vol% retained austenite, more preferably, substantially no retained austenite. Generally, if the bearing is well-sealed, well-lubricated, and there is no metal-to-metal contact, then having retained austenite in the shell microstructure may not offer significant advantages. However, if debris ingress into the bearing is permitted, and only partial lubrication film and / or metal-to-metal contact are present, then retaining a small amount of austenite (e.g., 10–20 vol%) in the shell microstructure may be advantageous. Depending on the bearing design, this can increase resistance to rolling contact fatigue and suppress spalling and / or pitting. Therefore, in some embodiments, the shell microstructure may include austenite, such as 10–20 vol% retained austenite. The austenite can be retained in the shell prior to the bainitic hardening step in manufacturing the inner or outer ring. This can give the inner or outer ring a high surface hardness, resulting in high wear resistance. This is particularly important for raceway surfaces.

[0063] The inner or outer ring of the present invention typically has a core hardness greater than 40 HRC, preferably greater than 45 HRC. In one embodiment, the core hardness of the inner or outer ring is 40–48 HRC. Typically, the core hardness is not greater than 50 HRC. As described above, the high hardness of carbon steel in bainitic form advantageously reduces the shell depth of the inner or outer ring, thereby reducing the manufacturing cost of the inner or outer ring.

[0064] The unit "HRC" used herein refers to the Rockwell 'C' scale and is known to those skilled in the art. Hardness measurements on the Rockwell 'C' scale can be performed using any indentation method known to those skilled in the art. The Vickers hardness test can also be used to measure the core hardness of the inner or outer ring. Conventional methods are known to those skilled in the art.

[0065] At a vertical distance of 100 μm or greater from the carbonitrided surface of the inner or outer ring, the inner or outer ring preferably has a residual compressive stress of -200 MPa or less, more preferably -300 MPa or less, and more preferably -400 MPa or less. Typically, at a vertical distance of 100 μm or greater from the carbonitrided surface of the inner or outer ring, the residual compressive stress is between -200 MPa and -900 MPa. Advantageously, a higher residual compressive stress can improve the fatigue life of the bearing. This is in contrast to the approximately -200 MPa residual compressive stress for martensitic hardening of medium carbon steel. Preferably, the raceway of the inner or outer ring has a positive residual compressive stress of 500 MPa to 600 MPa at its surface. Such residual compressive stress can improve the fatigue life of the bearing. Not wishing to be bound by theory, it is believed that the heat treatment methods used to manufacture the inner or outer rings described herein, such as bainitic hardening processes, result in the aforementioned residual compressive stresses in the rings. These residual compressive stresses can be measured using X-ray diffraction analysis. Conventional techniques are known to those skilled in the art.

[0066] Advantageously, the inner or outer ring also exhibits high impact toughness. Bainite generally has higher impact toughness than martensite with the same hardness.

[0067] Advantageously, the machinability characteristics (both hot and cold working) are not significantly different between the low-carbon and medium-carbon steels of the prior art and the low-carbon and medium-carbon steels of the present invention.

[0068] For example, the inner or outer ring of the present invention can be used in tapered roller bearings having an outer diameter of about 230 mm and an inner diameter of about 130 mm. However, it should be understood that any suitable size inner or outer ring can be provided within the scope of the claims.

[0069] According to another aspect, a heat treatment method is provided to obtain an inner or outer ring for a roller bearing, the method comprising:

[0070] (i) Preparing a ring formed from a steel composition, said steel composition comprising:

[0071] Carbon: 0.30–0.45 wt.%;

[0072] Silicon: 0.1–0.7 wt.%;

[0073] Manganese: 0.6–0.9 wt.%;

[0074] Chromium: 0.9–1.2 wt.%;

[0075] Molybdenum: 0.15–0.7 wt.%;

[0076] Nickel: 0–2.0 wt.%;

[0077] Phosphorus: 0–0.02 wt.%;

[0078] Sulfur: 0–0.02 wt.%; and

[0079] The remaining iron, and the unavoidable impurities;

[0080] (ii) The ring is subjected to a carbonitriding process to obtain a carbonitrided ring, wherein a carbonitrided shell layer is disposed on the surface of the carbonitrided ring;

[0081] (iii) Heating the carbonitriding ring to at least partially austenitize the steel composition;

[0082] (iv) Following step (iii), the carbonitriding ring is held at 210°C to 250°C for 4 to 7 hours to obtain a heat-treated ring in which the microstructure of the steel composition comprises bainite; and

[0083] (v) Recovering ( / returning / restoring) heat-treated coils.

[0084] The inner or outer ring provided by the heat treatment method described herein can be obtained by any conventional means known to those skilled in the art. For example, a ring for heat treatment can be provided as follows: First, a steel composition is prepared and cast. Then, the steel composition can be subjected to a conventional high-temperature homogenization step, followed by hot rolling, typically at an initial temperature of about 1150°C. Several hot rolling passes may be applied if necessary. The hot-rolled steel (which may be in bar or plate form) is then slowly cooled to room temperature to avoid the formation of high-carbon martensite. The generally preferred structure under hot-rolled conditions at room temperature is pearlite.

[0085] Then, optionally, the hot-rolled material can be homogenized in a homogenization step, for example, by treating it in a vacuum at approximately 1200°C for about 24 to 48 hours. Then, optionally, the material can be cooled in a furnace to allow it to cool slowly to room temperature, also under vacuum.

[0086] The material can then be machined into a near-net-shape component, i.e., the inner or outer ring used in the method described herein, during the machining step.

[0087] In step (ii) of the method described herein, the ring is subjected to a carbonitriding process to obtain a carbonitrided ring. A carbonitrided shell layer is formed on the surface of the carbonitrided ring. Generally, carbonitriding processes are known to those skilled in the art. The carbonitriding process may include, for example, heating the ring in an atmosphere containing carbon monoxide and ammonia.

[0088] The total duration of the carbonitriding process in step (ii) is preferably 5 to 35 hours, depending on the desired shell depth. More preferably, the total duration of the carbonitriding process in step (ii) is less than 25 hours. In some embodiments, the total duration of the carbonitriding process in step (ii) is 10 to 23 hours. For example, when the total duration of the carbonitriding process in step (ii) is about 22 hours, a shell depth of 2.5 to 3.0 mm can be obtained. As those skilled in the art will appreciate, the duration of the carbonitriding step will depend at least on the desired shell depth and the heating temperature.

[0089] The carbonitriding process in step (ii) preferably involves heating the coil to a temperature of 850°C to 970°C in an atmosphere having a carbon monoxide concentration of 5–25 vol.% and an ammonia concentration of 5–15 vol.%, preferably wherein the carbon potential of the atmosphere is 0.6–1.2%. In some embodiments, for example, the carbonitriding process in step (ii) includes heating the coil to a temperature of 850°C to 900°C. In other embodiments, the carbonitriding process in step (ii) includes heating the coil to a temperature of 900°C to 970°C or 870°C to 920°C. In some embodiments, for example, the carbonitriding process in step (ii) includes heating the coil in an atmosphere having a carbon monoxide concentration of 5–15 vol.%. In other embodiments, the carbonitriding process in step (ii) includes heating the coil in an atmosphere having a carbon monoxide concentration of 15–25 vol.% or 10–20 vol.%. In some embodiments, for example, the carbonitriding process in step (ii) includes heating the coil in an atmosphere having an ammonia concentration of 5–10 vol.%. In other embodiments, the carbonitriding process in step (ii) includes heating the coil in an atmosphere having an ammonia concentration of 10–15 vol.% or 7–13 vol.%. In some embodiments, for example, the carbonitriding process in step (ii) includes heating the coil in an atmosphere having a carbon potential of 0.6–0.8%. In other embodiments, the carbonitriding process in step (ii) includes heating the coil in an atmosphere having a carbon potential of 0.8–1.2% or 0.7–1.1%.

[0090] Suitable equipment and methods for maintaining a suitable atmosphere in carbonitriding processes are known to those skilled in the art. For example, oxygen probes and / or carbon controllers can be used. Suitable conventional techniques are known to those skilled in the art.

[0091] In step (iii) of the method described herein, the carbonitriding ring is heated to at least partially austenitize the steel composition. Preferably, step (iii) comprises heating the carbonitriding ring to a temperature of 860°C to 880°C for 60 to 90 minutes. More preferably, step (iii) comprises heating the carbonitriding ring to a temperature of, for example, 865°C to 875°C for 70 to 80 minutes. Austenitic steel and its structure are well known to those skilled in the art.

[0092] In step (iv) of the method described herein, the carbonitriding ring is held at a temperature of 210°C to 250°C for 4 to 7 hours after step (iii) to obtain a heat-treated ring in which the microstructure of the steel composition comprises bainite. In some embodiments, the carbonitriding ring is held at a temperature of, for example, 210°C to 230°C. In other embodiments, the carbonitriding ring is held at a temperature of 230°C to 250°C or 220°C to 240°C. In some embodiments, the carbonitriding ring is held at a temperature of 210°C to 250°C for 5 to 6 hours. Typically, a salt bath is used to hold the carbonitrided ring at such temperatures. However, other quenching and / or temperature control methods can be used. As those skilled in the art will appreciate, the shorter the duration for which the carbonitriding ring is held at the bainite transformation temperature, the more economical the overall process is. Therefore, a shorter duration is desirable.

[0093] The bainitic hardening (transformation) process may include, for example, one, two, or three transformation stages, all performed at different temperatures. The aim is to optimize hardness and total transformation time. After the bainitic transformation has ceased, the component is typically cooled to room temperature.

[0094] Advantageously, the heat treatment method described herein has a shorter heat treatment cycle time compared to conventional methods for heat treating low-carbon roller bearing rings (e.g., carburizing followed by martensitic hardening). The inventors have surprisingly discovered that the method described herein advantageously achieves this shorter heat treatment cycle time while also providing inner or outer rings with high wear resistance and high load-carrying capacity, making them suitable for roller bearings. Therefore, inner or outer rings for roller bearings can be provided at a lower manufacturing cost.

[0095] Optionally, the inner or outer ring described herein may undergo surface polishing techniques. For example, grinding, particularly grinding the raceway, followed by tempering and air cooling. The inner or outer ring may then be polished by hard turning and / or grinding operations (such as lapping and honing).

[0096] Grinding and tempering operations may lead to a sharp increase in the yield strength of the affected area, accompanied by a significant increase in hardness, residual compressive stress, and better resistance to rolling contact fatigue.

[0097] It should be understood that the heat treatment methods disclosed herein are not limited to those used for the inner or outer rings of roller bearings. For example, the methods disclosed herein can also be used for any type of bearing assembly, such as inner and / or outer rings or rolling elements, provided, for example, that the inherent hardenability is sufficient to satisfy the ring wall portion, roller diameter, and / or ball diameter. Thus, the present invention also provides a heat-treated rolling element (such as a roller or spherical rolling element) as described herein, and a heat treatment method as described herein to obtain such a rolling element.

[0098] According to another aspect, an inner or outer ring as described herein is provided, wherein the inner or outer ring has been heat-treated by a method as described herein.

[0099] According to another aspect, a roller bearing comprising an inner ring and / or an outer ring as described herein is provided.

[0100] Therefore, the present invention unexpectedly departs from the tradition of using low-carbon steel in the manufacture of roller bearing rings to provide inner or outer rings for roller bearings formed of medium-carbon steel, wherein the inner or outer rings unexpectedly all exhibit comparable or even improved physical and mechanical properties (e.g., wear resistance and load-bearing capacity) at a lower manufacturing cost. Attached Figure Description

[0101] The invention will now be described in conjunction with the following non-limiting drawings, in which:

[0102] Figure 1 A simplified schematic diagram (generally indicated by 1) of a cross-section of a roller bearing as described herein is shown, which, when viewed in the axial direction, includes an inner ring 2, an outer ring 3, and a plurality of roller bearing elements (rollers) 4 disposed therebetween.

[0103] Figure 2 The carbonitriding conditions used in Example 1 are shown.

[0104] Figure 3 A graph showing the residual stress of sample 1# of Example 1 relative to the distance from the surface is shown.

[0105] Figure 4 A graph showing the residual stress of sample 3# of Example 1 relative to the distance from the surface is shown.

[0106] Figure 5 A graph showing the residual stress of sample 2# of Example 1 relative to the distance from the surface is shown.

[0107] Figure 1A simplified schematic diagram of a cross-section of a roller bearing 1 is shown, which, viewed axially, includes an inner ring 2, an outer ring 3, and a plurality of roller bearing elements (rollers) 4 disposed therebetween. It will be understood that the invention is not limited to this configuration, and that the invention can be applied to any conventional roller bearing construction. The remaining figures are mentioned in the examples below. Detailed Implementation

[0108] The invention will now be described in conjunction with the following non-limiting examples.

[0109] Example 1

[0110] A prototype of the inner ring of a type 22220 bearing was manufactured according to the present invention. The inner ring is formed of an SKF42 grade steel composition, which is consistent with the steel composition described herein.

[0111] Carbonitriding of the inner ring of the 22220 bearing was carried out in two steps under the following conditions:

[0112] Carbonitriding parameters, step 1:

[0113] Temperature: 820℃

[0114] Time: 4 hours

[0115] Cp: ​​0.85%

[0116] CO: 15 vol.%

[0117] NH3: 15 vol.%

[0118] Carbonitriding parameters, step 2:

[0119] Temperature: 940℃

[0120] Time: 17 hours

[0121] Cp: ​​1.1%

[0122] CO: 20 vol.%

[0123] NH3: 15 vol.%

[0124] The carbonitriding coil was then cooled to 100°C in air.

[0125] The conditions for carbonitriding are summarized in Figure 2 According to the analysis of microscopic images, the case depth is approximately 1.5 mm.

[0126] The carbonitrided samples underwent a series of rehardening tests, including both martensitic and bainitic quenching. The results are summarized below:

[0127] #1: 865℃ for 75 minutes / 210℃ for 10 minutes / 215℃ for 660 minutes / 240℃ for 300 minutes

[0128] #2: 865℃ for 75 minutes / 180℃ for 10 minutes / air cooling for 30 minutes / pressure quenching at 7℃ for 30 minutes / 235℃ for 120 minutes

[0129] #3: 870℃ for 75 minutes / 215℃ for 10 minutes / 215℃ for 300 minutes / 240℃ for 160 minutes

[0130] #4: 800℃ for 80 minutes / 65℃ oil immersion for 10 minutes / 10℃ water for 10 minutes / 190℃ for 240 minutes

[0131] Samples 1# and 3# were bainitic quenched, while samples 2# and 4#, used as comparative examples, were martensitic quenched.

[0132] The hardness of the obtained inner rings was tested, and their microstructure was also tested. Table 1 shows the surface and core hardness of each sample (i.e., the hardness at depths of 0.1, 0.5, 1.0, 1.5, 2.5, and 4.5 mm below the surface).

[0133] Table 1

[0134]

[0135] Although the hardness and microstructure of all samples were acceptable, residual stress testing showed that the residual compressive stress after bainitic quenching was close to -400 MPa, while the residual compressive stress after martensitic quenching was only about -100 MPa (see sample #1). Figure 3 See sample 3# Figure 4 See sample 2# Figure 5 ).

[0136] Higher residual compressive stress is beneficial for extending the fatigue life of bearings. Comparing the heat treatment times of samples 1 and 3, the time cost during the bainitic transformation in sample 3 was 460 minutes (215℃ / 300 minutes and 240℃ / 160 minutes), while in sample 1, the time cost was 960 minutes (215℃ / 660 minutes and 240℃ / 300 minutes). Since the time cost is shorter in sample 3 than in sample 1, the heat treatment in sample 3 would be preferred. Even a shorter time cost would be better.

[0137] Example 2

[0138] Two sets of Type 197726 bearings were successfully manufactured according to the present invention and used in train cars. The inner and outer rings are formed of an SKF42 grade steel composition, which is consistent with the steel composition described herein.

[0139] The 197726 type tapered roller bearing has the following dimensions:

[0140] Outer diameter: 230mm

[0141] Inner diameter: 130mm

[0142] Roller diameter (large end): 24.74mm

[0143] Width: 150mm

[0144] Number of rollers: 42 (two inner rings)

[0145] 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 shown herein will be apparent to those skilled in the art, and remain within the scope of the appended claims and their equivalents.

Claims

1. A heat treatment method for obtaining an inner or outer ring for a roller bearing, the method comprising: (i) Providing an inner or outer ring formed of a steel composition, said steel composition comprising: Carbon: 0.30 ~ 0.45 wt.%; Silicon: 0.1 ~ 0.7 wt.%; Manganese: 0.6 ~ 0.9 wt.%; Chromium: 0.9 ~ 1.2 wt.%; Molybdenum: 0.15 ~ 0.7 wt.%; Nickel: 0 ~ 2.0 wt.%; Phosphorus: 0 ~ 0.02 wt.%; Sulfur: 0 ~ 0.02 wt.%; and The remaining iron, and the unavoidable impurities; (ii) The inner ring or the outer ring is subjected to a carbonitriding process to obtain a carbonitriding ring, wherein a carbonitriding shell layer is disposed on the surface of the carbonitriding ring; (iii) Heating the carbonitriding ring to at least partially austenitize the steel composition; (iv) Following step (iii), the carbonitriding ring is held at 210°C to 250°C for 4 to 7 hours to obtain a heat-treated ring in which the microstructure of the steel composition comprises bainite; and (v) Recycle the heat-treated ring.

2. The method according to claim 1, characterized in that, The total duration of the carbonitriding process in step (ii) is 5 to 35 hours.

3. The method according to claim 1 or 2, characterized in that, The conditions for the carbonitriding process in step (ii) include heating the inner or outer ring to a temperature of 850°C to 970°C in an atmosphere having a carbon monoxide concentration of 5 to 25 vol.% and an ammonia concentration of 5 to 15 vol.%.

4. The method according to claim 3, characterized in that, The carbon potential of the atmosphere is 0.6 to 1.2%.

5. The method according to claim 1 or 2, characterized in that, Step (iii) involves heating the carbonitriding coil to a temperature of 860°C to 880°C for 60 to 90 minutes.

6. An inner or outer ring for a roller bearing, said inner or outer ring being formed of a steel composition comprising: Carbon: 0.30 ~ 0.45 wt.%; Silicon: 0.1 ~ 0.7 wt.%; Manganese: 0.6 ~ 0.9 wt.%; Chromium: 0.9 ~ 1.2 wt.%; Molybdenum: 0.15 ~ 0.7 wt.%; Nickel: 0 ~ 2.0 wt.%; Phosphorus: 0 ~ 0.02 wt.%; Sulfur: 0 ~ 0.02 wt.%; and The remaining iron, and the unavoidable impurities; The microstructure of the steel composition includes bainite; and A carbonitriding shell layer is provided on the surface of the inner or outer ring.

7. The inner or outer ring according to claim 6, characterized in that, The carbonitrided shell has a shell depth of 0.7 to 3.5 mm.

8. The inner or outer ring according to claim 6 or 7, characterized in that, The inner ring or the outer ring has a core hardness greater than 45 HRC.

9. The inner or outer ring according to claim 6 or 7, characterized in that, At a location 100 µm or greater vertically from the carbonitrided surface of the inner or outer ring, the inner or outer ring has a residual compressive stress of -200 MPa or less.

10. The inner or outer ring according to claim 7, characterized in that, The carbonitrided shell has a shell depth of 1.0 to 2.5 mm.

11. The inner or outer ring according to claim 9, characterized in that, The inner ring or the outer ring has a residual compressive stress of -900 MPa to -200 MPa.

12. The inner or outer ring according to claim 9, characterized in that, The inner ring or the outer ring has a residual compressive stress of -300 MPa or less.

13. The inner or outer ring according to claim 6 or 7, characterized in that, The inner ring or the outer ring has been heat-treated by the method according to any one of claims 1 to 5.

14. A roller bearing comprising an inner ring and / or an outer ring as described in any one of claims 6 to 12 or claim 13.

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