The Effect of Non-metallic Inclusions on the Fatigue Characteristic of Martensitic Steel

Abstract

Fatigue tests have been carried out with vacuum degassed (DH-treated) and not-DH-treated martensitic steels by repeated bending with tension side amplitude.
It was found that fatigue microcracks nucleated on the edge of specimen in the DH-treated steel, but from non-metallic inclusion in the not-D H-treated steel. It was also found that microcracks nucleated from inclusions were of 3 types; initiated by the fracture of inclusions themselves, by the separation at the inner boundary of duplicated inclusions, and also by the separation at the interface with matrix. From the observation of surfaces and microfractographs, a fatigue crack propagation mechanism is suggested as follows. The inclusions are considered to nucleate sub-cracks that grow into shear mode fracture, at high plastic zone in front of the main crack tip. As the main crack approaches, they join and link by cleavage or quasi-cleavage fracture wi th the main crack. This process is repeated to make the crack propagate.
An attempt has been made to represent the rate of crack propagation by the following relationship:
dl/dN=Bl
where l is length of the main crack and B is considered as a function of stress and material.
It was evident that the vacuum degassed clean material was excellent compared with the ordinary meterial in view of fatigue crack nucleation and propagation.