I

Distance below surface |i m)

Fig. 9 Variation of microhardness with depth in a hardened bearing steel ground under "burn" and "no burn" conditions. Source: Ref 9

The avoidance of workpiece burn during the grinding of hardened steels is critical for better tribological performance. For example, workpiece burn in bearing steels is found to affect their fatigue life adversely (Ref 30). Figure 10 shows the L10 fatigue life of hardened bearing rings finished under various grinding conditions (L10 is the fatigue life exceeded by 90% of the specimens). The fatigue life decreases dramatically when the grinding specific energy, u, exceeds the threshold limit for burn, u* (i.e., u/u* > 1).

Fig. 10 Variation of L10 fatigue life with specific energy ratio (u/u*) for a ground bearing steel. The critical value is u/u* = 1, beyond which burn is observed to occur typically. Source: Ref 9

Besides workpiece burn in steels, the occurrence of high temperatures usually leads to tensile residual stresses on ground steel surfaces. This is again undesirable for enhanced tribological performance. However, control of the residual stress distribution on ground surfaces of steels is possible by ensuring that surface temperatures are kept low during grinding. Figure 11 shows the variation of measured surface residual stress with calculated grinding temperature for 52100 bearing steel ground with Al2O3 and CBN wheels. Under most conditions reported in Fig. 11, CBN grinding is found to produce compressive residual stresses on the surface, in contrast to Al2O3 grinding. This is due to the lower temperatures at the ground surface in CBN grinding. It has been suggested that lower grinding temperatures lead to reduced tensile stresses on ground surfaces, and Fig. 11 provides perhaps the most striking experimental illustration in support of this speculation.

Fig. 11 Variation of surface residual stresses with grinding temperature in ground surfaces of 52100 steel. • , aluminum oxide 60 J6; •, aluminum oxide 100 H6; Mcubic boron nitride, (a) Residual stress perpendicular to grinding. (b) Residual stress parallel to grinding
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