Effects of boronising on fatigue life of plain carbon and low alloy Cr-Mo steels

In the present paper the hardness profiles, stress versus number of cycles to failure (S-N curves), and fatigue threshold stresses of thermochemically boronised plain carbon and low alloy Cr-Mo steels have been investigated. A comparision has been made with the fatigue behaviour of the steel without any surface treatment. In addition, X-ray diffraction has been employed to determine the boride phases formed. Optical and scanning electron microscopy have been used to examine the microstructure and fracture surfaces of all boronised and fatigue tested specimens. The results obtained revealed a boronised layer with a thickness of about 100 mum, for both types of steel. In comparison with untreated specimens, a 50% increase and a 20% decrease in fatigue resistance for boronised plain carbon and Cr-Mo steel specimens, respectively, was observed. In addition, it was found that hardening and tempering the boronising specimens of both grades of steel decreased the fatigue strength, however, the fatigue strength of the boronised plain carbon steel was still greater than that of the as received plain carbon steel. (C) 2001 IoM Communications Ltd.