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Bearing Steels

• satyendra
• August 6, 2020
• 1 Comment
• Bainitic structure, ball bearing, Bearing steel, Carburizing steel, cleanliness, Fatigue, heat treatment, high carbon steel, martensitic structure, Non metallic inclusions, Roller bearing, Thrust bearing,

Bearing Steels

The Oxford English Dictionary defines a bearing ‘as a part of a machine that allows one part to rotate or move in contact with another part with as little friction as possible’. Additional functions include the transmission of loads and enabling the accurate location of the components. A bearing is required to sustain severe static as well as cyclic loads while serving reliably in difficult environments. Steels are well–suited in this context, and in their many forms, represent the material of choice in the production of bearings. Bearings consist of rolling elements (balls, cylinders or barrel shapes) and rings. The rings form the raceways.

Bearings are highly engineered, precision-made machine elements which enable relative movement between machine components with minimum friction. They carry high loads with ease and efficiency and are able to offer high precision, reliability and durability. There are several types of bearings, each used for different purposes. These include ball bearings, roller bearings, ball thrust bearings, roller thrust bearings, and tapered roller thrust bearings. Because different applications require bearings to handle diverse specifications, the differences between types of bearings concern load type and ability to handle weight. Ball bearings are extremely common since they can handle both radial and thrust loads, but can only handle a small amount of weight. Roller bearings use cylinders instead of balls and have a greater load bearing capacity because of the greater contact between the rolling element and the rings.

The manufacturing process for the rolling elements involves the high reduction-rate plastic deformation of raw, cast material, into billets with square sections. The deformation helps to break up the cast structure and to close porosity. The billets are then reduced in section by further rolling or drawing heat-treated to a softened state and cut into lengths suitable for the manufacture of balls. The finished rolling elements are then quenched and tempered, or isothermally transformed, to the required hardness. Bearing rings can be made from seamless tube produced by hot rolling and similarly hardened, followed by careful machining and grinding to the final dimensions and surface finish. The vast majority of rolling elements and raceways are made using steel.

Incorrect fitting, corrosion, and inadequate lubrication etc. can lead to early failure of the bearings. However, even well maintained bearings can eventually fail by fatigue of the contacting surfaces. A failure mode which is widely accepted consists of sub-surface crack nucleation at a pre-existing defect in the region of the highest shear stress beneath a contact zone followed by propagation of the crack to eventually form a pit in the surface. Hence, the material to be used for the production of bearing has a great significance.

As a bearing is to sustain severe static and cyclic loads while serving reliably in difficult environments, steels are well-suited materials for this purpose. There are two categories of steels which find application in the majority of bearings. The first category consists of those steels which are hardened throughout their sections into a martensitic or bainitic condition, and other category consists of those steels which have soft cores but tenacious surface layers introduced using processes such as case or induction hardening.

Bearing steels can also be divided in a broad sense into classes such as intended for normal service, high-temperature service, or service under corrosive conditions. Bearings for normal service conditions, a category which includes a majority of all rolling-element bearings, are applicable when (i) maximum temperatures are of the order of 120 deg C to 150 deg C, although brief excursions to 175 deg C can be tolerated, (ii) minimum ambient temperatures are around -50  deg C, (iii) the contact surfaces are lubricated with such materials as oil, grease, or mist, and (iv) the maximum Hertzian contact stresses are of the order of 2.1 GPa to 3.1 GPa. Bearings used under normal service conditions also experience the effects of vibration, shock, misalignment, debris, and handling. Hence, the fabrication material is to provide toughness, a degree of temper resistance, and micro-structural stability under temperature extremes. The material is also to show the obvious requirement of surface hardness for wear and fatigue resistance.

Steels with carbon concentrations in the range 0.8 % to 1.1 % and the total substitutional solute content of less than 3 %, originally designed for machine tools, have historically dominated the type of steel for bearings. These can be made martensitic by quenching in oil or salt, from a temperature where the material is mostly austenite. The martensite is then subjected to a low temperature tempering in order to balance conflicting properties.

Small bearings are normally through hardened, i.e., the steels have sufficient hardenability to become martensitic throughout the section of the bearing. This is not so with large bearings where the surface layer is to be carburized to produce a martensitic case. Alternatively, large bearings can be through hardened by increasing the hardenability of the steel using larger concentrations of alloying elements.

Traditionally, bearings have been produced from both high carbon (1 %) and low carbon (0.2 %) steels. The high carbon steels are used in either a through-hardened or a surface induction-hardened condition in special integral bearing configurations, such as the automotive wheel spindle. Low-carbon bearing steels are carburized to provide the necessary surface hardness while maintaining other desirable properties in the core. The parallel development of both high carbon and low carbon steels for bearing applications is rooted in history. The early European manufacturers chose to use familiar chromium-type tool steels. The American bearing manufacturers, on the other hand, added a carburized case to their soft, plain steel bearings to meet the higher hardness requirements of more highly loaded rolling element bearings.

Both high carbon and low carbon steel materials have survived since each of them offers a unique combination of properties which best suits the intended service conditions. For example, high carbon steels (i) can carry somewhat higher contact stresses, such as those encountered in point contact loading in ball bearings, (ii) can be quenched and tempered, which is a simpler heat treatment than carburizing, and (iii) can offer greater dimensional stability under temperature extremes because of their characteristically lower content of retained austenite. Carburizing steels, on the other hand, present (i) greater surface ductility (because of their retained austenite content) to better resist the stress-raising effects of asperities, misalignment, and debris particles, (ii) a higher level of core toughness to resist through-section fracture under severe service conditions, (iii) a compressive residual surface stress condition to resist bending loads imposed on the ribs of roller bearings and reduce the rate of fatigue crack propagation through the cross section, and (iv) easier machining of the base material in manufacturing.

In rolling-contact bearings, it is essential to maintain an adequate strength throughout the region of maximum subsurface shear stresses. Fig 1 shows an estimated relationship between hardness and shear yield strength which is applicable to either steel type. The success of given steel in a bearing application is not as much a function of the steel type as how it is treated. Fatigue resistance generally increases with hardness and the maximum depends on the steel type. Fig 1 also compares the bending fatigue lives of through-carbon and carburized steels as a function of surface hardness. In bending fatigue, the combination of compressive residual surface stresses with higher composite section toughness gives the advantage to the carburized steel.

Fig 1 Properties of bearing steels

Microstructure characteristics 

High carbon bearing steels – In the 1950s through the 1960s the bearing industry assumed that materials with higher alloy content have better hardness retention at high temperatures. It has been reasoned that this also results in higher ambient-temperature hardness as well as longer bearing life. Based on this assumption steel producers began to develop bearing steels with higher content of alloying elements. It is necessary to compare these steel and processing variables in rolling-element fatigue tests and / or actual bearing tests. Standard mechanical tests, such as tension and compression tests or rotating beam tests, cannot be correlated with rolling-element fatigue results.

Fig 2 shows a typical microstructure of a hardened and tempered high carbon bearing steel such as AISI 52100. The matrix is high carbon martensite, containing primary carbides and 5 % to 10 % retained austenite. The hardness throughout the section is typically 60 HRC to 64 HRC. Tab 1 gives the compositions of some of the high carbon bearing steels. The first three grades are given in order of increasing hardenability. They are applied to bearing sections of increasing thickness to ensure freedom from non-martensitic transformations during hardening. Grade TBS-9 is a lower chromium bearing steel, which, because of its residual alloy content, has hardenability similar to that of AISI 52100. The grade SUJ 1 is a Japanese steel grade while grade 105Cr6 is a German steel grade used for bearing components.

Fig 2 Microstructure of high carbon and carburizing bearing steels

Carburizing bearing steels – Carburizing-grade bearing steels have reduced carbon content so that heat treatment normally results in moderate hardness and high toughness. High surface hardness, which is required for rolling-element bearing performance, is achieved by diffusing carbon into the surface, a process called carburizing, and prior to heat treatment. Locally the steel is then a high carbon alloy and is heat treatable to full hardness. The resulting structure has a surface layer with mechanical properties which are equivalent to those of traditional through-hardened bearing steels and a core that remains at low hardness, with corresponding high ductility and high fracture toughness. Surface-initiated defects (e.g. a spall) propagate cracks into the tough core before they reach critical size. The tough core prevents rapid and catastrophic fracture.

Fig 2 shows typical case and core microstructures of carburized bearing components. The case microstructure consists of high carbon martensite with retained austenite in the range of 15 % to 40 %. Case hardness is typically 58 HRC to 64 HRC. In the core of carburized bearings, the microstructure consists of low carbon martensite. It also frequently contains variable amounts of bainite and ferrite. The core hardness can vary from 25 HRC to 48 HRC.

Tab 1 gives the compositions of typical carburizing bearing steels. The AISI grades are shown in approximate order of increasing hardenability or section size applicability. SCM420 and 20MnCr5 are Japanese and German grades respectively, found in carburized bearing components. In addition to standard AISI grades, bearing steels can also be designed so that their hardenability matches the requirements of specific section thicknesses. Alloy conservation and a more consistent heat-treating response are benefits of using specially designed bearing steels.

The selection of carburizing steel for a specific bearing section is based on the heat-treating practice of the producer, either direct quenching from carburizing or reheating for quenching, and on the characteristics of the quenching equipment. The importance of a proper case microstructure to the ability of a bearing to resist pitting fatigue is shown in Fig 3, In particular, the presence of pearlite, resulting from a mismatch of quenching conditions and case hardenability, have a detrimental effect.

Fig 3 Effect of surface microstructure on surface fatigue

The most popular bearing steel is which has got around 1 % carbon and 1.5 % of chromium. Ball bearing tests conducted on such steel in 1901 by Stribeck indicated its suitability for the application and was apparently adopted some 120 years ago for bearings by Fichtel & Sachs of Schweinfurt in 1905, and has persisted to this day as a key steel in the production of bearings, with progressive improvements in fatigue performance achieved primarily by improvements in cleanliness with respect to non–metallic inclusions. It represents the majority of the bearing steel produced annually.

This bearing steel is part of several national and international standards. It is usual for the steel to be supplied in a hot rolled condition with a pearlitic microstructure including some pro-eutectoid cementite at the prior austenite grain boundaries (Fig 4a). The pro-eutectoid cementite, when it forms networks at the austenite grain boundaries, is undesirable because it has been shown to adversely affect the rolling contact fatigue life in accelerated tests conducted with contact stresses in excess of 5 GPa. The networks can be minimized by sufficiently rapid cooling from the final hot deformation temperature, or by annealing to spheroidize the cementite (Fig 4b). The relatively large carbon concentration of the this steel speeds up the spheroidization process and is one of the reasons for the success of the steel, that it can be soft annealed with relative ease. After the appropriate machining or forming, the bearing components made from the steel are subjected to hardening heat treatments.

Fig 4 Microstructures of bearing steels with 1 % carbon and 1.5 % chromium

Quality   of bearing steel

Apart from a satisfactory microstructure, which is obtained through the proper combination of steel grade and heat treatment, the single most important factor in achieving high levels of rolling-contact fatigue life in bearings is the cleanliness, or freedom from harmful non-metallic inclusions, of the steel. Bearing steels can be produced by one of these techniques namely (i) clean-steel air-melt practices, (ii) electroslag remelting, (iii) air melting followed by vacuum are remelting, and (iv) vacuum induction melting followed by vacuum arc remelting. Cleanliness, cost and reliability can increase depending on which practice is chosen.

Bearing steel cleanliness is most commonly rated by using microscopic techniques, such as those defined in ASTM A 295 for high carbon steels and ASTM A 534 for carburizing steels. The worst fields found in metallographically prepared sections of the steel can be compared with rating charts (J-K charts) according to the type of inclusion such as sulphides, stringer-type oxides, silicates, and globular-type oxides. Tab 2 gives the present levels of each of the inclusion types allowed for air-melted bearing quality steels. Several producers produce bearings with significantly lower levels of non-metallic inclusion content than allowed by rating charts.

 Bearing steel cleanliness can also be rated by oxygen analysis, the magnetic particle method, and ultra-sonic methods. Of these, the ultra-sonic method appears to show a superior correlation with bearing fatigue life when the oxygen content of the steel is less than 20 ppm. This is due to the larger volume of material sampled by the technique.

Effect of heat treatment

The matching of the hardenability and quenching of a bearing steel is important. However, within this restriction, other heat treatment variables have been found to affect the performance of bearings, particularly under the less-than-ideal conditions of debris contamination. Retained austenite in the microstructure is known to reduce the surface hardness of high carbon steels. The ductility of the surface, as expressed by the ratio of yield strength to fracture strength is improved by increasing amounts of retained austenite. This improved ductility frequently results in improved rolling-contact fatigue performance. The retained austenite level can be raised by adding nitrogen to the case or by increasing carburizing cycles. Improved performance has been shown in the high carbon steels when carburizing increased the carbon in solution and the austenite content. Fig 5 shows the improved performance of carburized AISI 52100 steel bars in comparison with the uncarburized bars of the same material. Other means of increasing surface ductility, such as optimizing the austenitizing temperature (Fig 5), controlling the quenching rate (Fig 5), and austempering to produce a completely bainitic microstructure, have been shown to improve the fatigue performance of high-carbon bearing steels for certain applications.

Fig 5 Effect of heat treatment on the properties of 52100 bearing steel

Special purpose bearing steels

When bearing service temperatures exceed about 150 deg C, common low alloy steels cannot maintain the necessary surface hardness to provide satisfactory fatigue life. The low corrosion resistance of these steels makes them susceptible to attack by environmental moisture, as well as aggressive gaseous or liquid contaminants. Hence, specialized steels are frequently applied when these service conditions exist.

Certain bearing steels are suited for high temperature service. These steels are typically alloyed with carbide-stabilizing elements such as chromium, molybdenum, vanadium, and silicon to improve their hot hardness and temper resistance. The maximum operating temperatures (around 230 deg C to 300 deg C) are those at which the hardness at temperature falls below a minimum of 58 HRC.

An important application of the high-temperature bearing steels is aircraft and stationary turbine engines. Bearings made from M50 steel have been used in engine applications for many years. Jet engine speeds are being continually increased in order to improve performance and efficiency and hence, the bearing steel used in these engines is to have increased section toughness to withstand the stresses which result from higher centrifugal forces. For this reason, the carburizing high-temperature bearing steels, such as M50-NiL and CBS-1000M, are receiving much attention. The core toughness of these steels is more than twice that of the through-hardening steels.

Carburized races have the compressive residual stress gradient while the tensile residual stresses are found in through-hardened races. The presence of compressive residual stresses can help to retard the propagation of radial fatigue cracks through the races cross-section.

Charpy-sized specimens have been carburized, hardened, tempered, and pre-cracked to several depths in case and core regions before testing. As cracks progress inward, the fracture resistance of carburized composites improves significantly.

In general, high temperature carburizing steels need more care in the carburizing process than conventional low alloy carburizing steels. Because of the high content of chromium and silicon in the high temperature steels, some pre-carburizing treatment, such as pre-oxidation, is always necessary to promote satisfactory carburizing.

Bearings which need the highest corrosion resistance necessitate the use of stainless grades with greater than 12 % chromium. At present, there is no satisfactory carburizing technique for these grades. Thus, all corrosion-resistant bearing steels are of the through-hardening type.