The Fatique Resistance of Surface Hardened Steels

Abstract

The initiation and propagation of fatigue cracks from notches in 10 mm thick sections of two engineering steels O80M15 and O80A47 has been studied in three point bending fatigue. A range of notch root radii varying from 0.13 mm to 3.17 mm provided different absolute distances within which the fatigue crack propagation is influenced by the notch stress field. The fatigue crack propagation data from the regions affected by the notch stress field were compared with that not influenced by the notch stress field, to obtain the modified stress intensity factor coefficient for cracks emanating from notches. It was found that the modified stress intensity factor coefficient does not fall to zero at the tip of a blunt notch. The effective stress intensity factor coefficient for blunt notches could be described as a function of notch geometry and microstructural features of steels, viz. grain size. The initiation and propagation of fatigue cracks from notches in 10 mm thick sections of gas carburised, gas carbonitrided and nitrocarburised O80M15 and induction hardened 080A47 were tested in three point bending fatigue. The number of cycles to produce a detectable fatigue crack, N., correlated well with the effective stress intensity factor range. The microstructural feature in case hardened steel was the prior austenitic grain size. The fracture surface examination did not indicate subsurface crack initiation generally associated with the case hardened steels subjected to fatigue. The results of fatigue crack initiation in carburised and carbonitrided notched geometries indicated that fatigue crack initiation is not affected by the case depth. The fatigue crack propagation data for cracks emanating from case hardened notches in O80M15 could be correlated to the effective stress intensity factor range. The results provided an adequate engineering method for design against fatigue failure from a range of stress concentration for the surface hardened steels examined.