252 Steels: Metallurgy and Applicationsinclusion assessment. For many years, both the steelmakers and bearing manufac-
turers have employed the Jerkontoret (JK) method which is specified in ASTM
Practice for Determining the Inclusion Content of Steel-E45. This rates a steel
in terms of the worst field of the 100 fields that are examined against stan-
dard charts. However, as stated by Hampshire and King, 26 this technique is
capable of ignoring the fact that a steel with 100 equally bad fields would be
ranked the same as a steel with only one bad field. Therefore a new method
was required that took into account both inclusion severity and frequency and
such a procedure is the SAM counting technique. This is detailed in Supplemen-
tary Requirements $2 of ASTM A 295-84. The SAM procedure concentrates
on the frequency of Type B aluminates and Type D globular oxides because
these tend to be the most damaging inclusions to bearing fatigue performance.
Type A manganese sulphides are disregarded as being insignificant to fatigue
life and Type C silicates are ignored because they occur infrequently in bearing
steels.
The SAM count is regarded as a major improvement over the JK method but
major strides have been made in introducing cleaner steels and a large number
of fields must now be examined in order to obtain a statistically valid assessment
of inclusion content. Attention is being given to the use of automatic image
analysis techniques but these require very careful polishing procedures and, again,
problems are being experienced with these techniques in the accurate assessment
of steels with low oxygen and sulphur contents.
Relatively little work has been carded out on the role of primary carbides in
bearing steels. In general, it is accepted that these particles are very beneficial
in increasing the wear resistance of the material and they are also instrumental
in inhibiting grain growth during solution treatment. However, it is considered
that large primary carbides, particularly when present in segregated bands, act as
stress raisers and are detrimental to fatigue life.
The high carbon content of through-hardening steels such as SAE 52100
depresses the Mf temperature of the material below room temperature, resulting
in the formation of retained austenite. A similar situation also exists in the case
of carbudzed bearings, the high alloy content augmenting the effect of carbon
in depressing the Ms-Mf transformation range. However, the retained austenite
will transform to martensite under the high Hertzian stresses induced in service
and will tend to introduce dimensional changes in the bearing component. For
this reason, a low level of retained austenite is desirable for critical bearing appli-
cations and is generally limited to a maximum of about 5%. The formation of
strain-induced martensite will create a compressive residual stress and should
therefore be beneficial to the fatigue performance. The white bands observed
in the microstructure of SAE 52100 bearings after service are thought to be
associated with the residual stresses created by the formation of strain-induced
martensite.Modern steelmaking methods
During the 1950s, vacuum degassing facilities were introduced which resulted in
a marked improvement in the cleanness of bearing steels. By the early 1960s,