Steels_ Metallurgy and Applications, Third Edition

Engineering steels 251

Factors affecting fatigue performance

Johnson and Sewell u were among the earliest investigators to establish a good

quantitative relationship between the bearing fatigue performance of SAE 52100

and inclusion content. The inclusions act as stress raisers, forming incipient

cracks which then propagate under stress reversals until a fatigue pit (or spall) is

formed on the surface of the component. As illustrated in Figure 3.34, Johnson

and Sewell showed that oxide inclusions such as alumina and silicates have an

adverse effect on fatigue performance, whereas sulphides appear to be benefi-

cial. These authors showed that a better correlation between fatigue performance

and inclusion content was obtained when titanium nitride was included in the

inclusion count such that the inclusion parameter was based on:

TiN

Number of alumina 4- silicate -F -~

Oxides are considered to be detrimental because they are brittle and, as illustrated

by Brooksbank and Andrews, 25 they also become surrounded by tensile stresses

on cooling from elevated temperatures due to differences in the thermal expansion

characteristics between the oxide particles and the matrix. The beneficial effect

of sulphides is generally ascribed to the fact that they tend to encapsulate the

more angular oxide inclusions, thereby reducing the detrimental tensile stresses.

However, the beneficial effects of sulphides are often disputed and some hold

the view that they are non-detrimental rather than positively beneficial. Other

workers have indicated that TiN inclusions have a relatively small effect on

bearing fatigue performance and such an effect is consistent with the tesselated

stress calculations of Brooksbank and Andrews.

Given the importance of inclusion content in relation to bearing fatigue perfor-

mance, major attention has been given to the development of reliable methods of

~" ,p- 25

X

20

~ 15

"- 10

<

(a) (b)

o o o o o

+

o

I ........ t i ..... I I, , i i , ,i I

100 200 300 400 500 200 300 400 500 600 700

Total alumina + silicates Total sulphldes

Figure 3.34 Relationship between average life and inclusion content: counts based on

total inclusions observed (x750) in 516fields representing a total area of"9 mm 2 (After

Johnson and Sewel124 )