Combining hardness measurements of a heat-treated crankshaft bearing with cross-sectional residual stress and retained austenite distributions measured by HEXRD

Abstract

Surface hardening is commonly used to modify mechanical properties of crankshaft bearings. In this work, residual stress and hardness distributions across the crankshaft bearings cross-sections are evaluated using synchrotron high-energy X-ray diffraction and hardness testing. It is shown that the measured hardening depth correlates with a point of sudden sharp reversal of the stress gradient from compressive to tensile. This point is linked to the microstructure and does not shift with subsequent tempering or trimming of the sample. The superimposed data is used to interpret the evolution of stresses during the quenching and tempering cycle and gain understanding of the hardening process for such complex geometries. Within the hardened zone retained austenite is found to increase with depth to over 15 %, which is attributed to reduced quenching effects as the material is further away from the surface. All measured properties agree in the determined hardening depth of 3.5 mm to 4.5 mm, which in turn fits well with optical evaluation of metallographic microsections.