Users of rolling-element bearings are generally not concerned about the steel from which bearings are made, or how the bearings are manufactured. They assume that all it takes to choose a satisfactory bearing for an application is matching catalog performance specifications to the application requirements.
But this approach is often inadequate for applications where reliability and long life are essential, such as power-dense industrial gear drives, power generation plants, and construction equipment. Such cases may require optimizing one or more elements of the steel manufacturing and bearing manufacturing processes, Table 1. To achieve the best results, work closely with the bearing manufacturer.
During recent years, manufacturers have developed steel-making advances that minimize nonmetallic inclusion stringers in wrought (rolled) steel. These stringers are elongated clusters of oxide particles that act as stress concentrations at which subsurface fatigue cracks initiate, Figure 1. The oxide inclusions form a poor interface with the steel because they are hard and brittle. Aluminum oxide stringers, in particular, degrade fatigue life.
In steel making, deoxidizing agents added to the molten steel cause chemical reactions that produce oxide inclusions, which float to the surface so they can be skimmed off. The number of inclusions in the solidified steel depends on how closely the chemical reactions and floatation phenomena approach completion. Mechanical erosion of refractory materials in the furnace and ladle also add to the inclusion content.
Severe hot working of steel (to produce shapes from which bearings are manufactured) tends to break up stringers and reduce their size, thus reducing their effect as stress risers. But hot working alone contributes only marginally to improved bearing life.
Several air-melt processes for making bearing steel, as well as the vacuum-arc remelt process, have produced progressively cleaner steels, Figure 2. The vacuum- arc remelt process provides the cleanest steel, but at the highest cost. The most advanced air-melt process produces very clean steel, which provides a level of fatigue resistance approaching that of vacuum-arc remelt steel at a cost that is only slightly above that of other processes and well below that of vacuum-arc remelting.
What is “cleanness?”
No steel component, regardless of how stringently controlled its manufacturing process, is completely free of inclusions and other internal discontinuities. However, the effect of these discontinuities depends largely on the operating parameters and risk associated with the application. In some cases, they have no significant effect on reliability. But with critical applications and severe loading conditions, discontinuities in the steel become increasingly more important.
Steel cleanness is often associated with the number of nonmetallic inclusions present. But this definition is incomplete. The effect of inclusions on fatigue resistance depends not only on the number of inclusions, but also on their dispersion and size.
Thus, a large number of small inclusions in a steel part is not necessarily detrimental. Under moderate loads with little shock, small, evenly dispersed inclusions may not create a significant stress-concentration site and probably will not affect fatigue life.
Oxygen content has been used as an indicator of cleanness and, hence, fatigue life of bearing steels. But low oxygen content alone does not guarantee improved fatigue life.
Aluminum oxide inclusion stringer content, rather than oxygen content, is what really affects service life. Test data shows that inclusion stringer length correlates to bearing fatigue life, Figure 2. Though there is a general correlation of oxygen content to oxide inclusion content, there is no correlation with the number of large inclusion stringers present. Therefore, oxygen content cannot be used to predict bearing life.
Through-hardened or case carburized
The durability of a bearing depends largely on how it is heat treated. Hardness due to heat treatment generally increases fatigue resistance. Other characteristics related to heat treatment, such as strength and ductility, help to withstand bending loads.
Both case-hardened low-carbon steel (0.20%) and through-hardened highcarbon steel (1.00%) are appropriate for bearing applications. High-carbon steels can also be surface inductionhardened. Choosing between throughhardened and case-carburized steel is based largely on application requirements and cost.
Through hardening is by far the most prevalent choice for ball and roller bearings, mainly because the hardening process is simpler and generally less costly. Through-hardened bearings made from high-carbon steel work well for light loads, for heavy loads with adequate support of the bearing races, and for applications that do not require a heavy press fit (large interference), providing that hardness, microstructure, and cleanness are adequate.
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Case-carburized bearing components possess a hard, fatigue-resistant surface and a tough, ductile, crack-resistant core, Figure 3. In general, case-hardened, low-carbon steel bearings are best for demanding application requirements such as heavy loads, shock loading, and heavy press fits.
Case-carburizing also imparts residual compressive stresses into the surface of the bearing component. These residual compressive stresses counteract at least part of the tensile stresses that occur in service, thereby improving the bending fatigue resistance of bearing components. The material composition and microstructure of case-carburizing also minimizes surface damage caused by debris.
Retained austenite effect
The heat-treatment of steel produces austenite, a high-temperature ductile constituent of the steel microstructure, at about 1,450 F. During a subsequent rapid cooling process called quenching, austenite transforms to martensite — the hardest and highest-strength steel microstructure. Depending on several factors, such as the cooling cycle and carbon content of the austenite, some austenite does not transform to martensite but is retained in the cooled-steel microstructure.
The case region of a case-carburized and hardened, low-carbon steel component consists primarily of tempered martensite and retained austenite. Moderate levels of retained austenite in the case region inhibit both surface and subsurface fatigue, thereby improving rolling-contact fatigue resistance. Retained austenite also aids in the run-in cycle of new bearings because it is ductile and can plastically deform under load. This relieves the effects of stress concentrations arising from inclusions, handling nicks, surface roughness, and edge stresses in roller bearings.
Through-hardened, high-carbon steels primarily consist of tempered martensite and fine carbide with a small percentage of retained austenite. These steels are well suited for point-contact ball bearings because the higher amount of martensite and lower amount of retained austenite helps to resist the high contact stresses inherent in such applications.
In the absence of unusual circumstances, rolling-element bearing failures usually occur in the fatigue mode. As a bearing rotates, it is subjected to cyclic stresses in the raceways and rolling elements (balls or rollers). This repeated stressing eventually causes fatigue damage. When this damage impairs performance, the bearing is considered to be failed.
Fatigue pitting starts at either surface or sub-surface origins. The sub-surface mode usually results from nonmetallic inclusions. Therefore, using bearing steels with smaller and fewer inclusions is an effective way to minimize this type of fatigue.
But fatigue resistance is not necessarily the most important criteria for all bearing applications. Depending on the application, other factors may be more important, such as noise, vibration, overload, corrosion, and wear. Such cases may require enhancing the design, rather than using a material with optimum fatigue resistance. If your application seems to fit this category, consult the bearing manufacturer.
For example, the bearing manufacturer may be able to improve the service life of highly loaded bearings by modifying raceway profiles and smoothly finishing rolling-contact surfaces. Where edge loading between the roller and raceway of a tapered roller bearing is anticipated, engineers can specify a modified internal geometry, with a relief at each end of the roller-race contact area to eliminate stress concentrations.
Improved internal geometry can also result from super-finishing, which produces more uniform rolling element sizes, improves component roundness, and enhances lubricant film effectiveness. Each of these factors plays a part in improving fatigue life.
John Ross is manager, steel market advancement, The Timken Co., Canton, Ohio.