Predicting the occurrence of grind hardening in cubic boron nitride grinding of crankshaft steel

Grind hardening is a stock removal grinding operation in which the heat generated in the grinding zone is used to heat-treat the workpiece surface to a specified depth. A thermal model to describe this process has been developed from the original Jaeger modelling and has been used to predict subsurface time–temperature profiles in the dry cylindrical grinding of 42CrMo4 crankshaft steel using cubic boron nitride (CBN) wheels. By comparing with experimental hardness depths, partition ratios were calculated and a polynomial fit applied to the proportion of specific energy entering the workpiece as heat, as a function of specific removal rate. By using this polynomial to provide the heat input to the thermal model, a theoretical chart of workpiece surface speed against depth of cut has been produced, which shows the regions in which grind hardening can be achieved to specified depths. The chart shows lower limits where the martensitic transformation temperature has been reached at certain specified depths and an upper limit where melting starts at the contact surface between the CBN grinding wheel and the workpiece. This chart has then been compared with experimental results for the specific case of a hardened depth of 0.5 mm or greater, with good agreement. The predictions indicate that there is an upper limit on workpiece speed if grind hardening is to be achieved, and that the limiting speed reduces as the required hardness depth increases. The predictions also indicate that there is an optimum work speed of around 10 mm/s for hardness depths below 1.5 mm and an upper limit below 4 mm/s for hardness depths around 2 mm.