A fractographic assessment of sulphide inclusion distributions and their influence in promoting environmentally assisted crack growth in ferritic pressure vessel steels

Abstract

The present study was aimed at fractographically assessing the extent of the sulphur concentration that is needed to initiate environmentally assisted crack (EAC) growth in fatigue crack growth tests on pressure vessel steels in a high temperature water environment. A relationship has been established between the extent of sulphide segregation within a cluster and the propensity towards the initiation of EAC growth. Also the EAC growth has been shown to be a function of K max at which the cluster occurred on the fatigue fracture surface. In terms of sulphur concentration, EAC growth occurred at sulphur concentrations of around 7000 ppm at K max values of 80 MPa√ m while at lower values (viz. 40 MPa√ m ) concentrations approaching 35 000 ppm were required to initiate EAC growth. Under an ideal sulphide distribution it was suggested that no EAC growth was possible. Finally the sulphur anion concentration levels for both EAC and non-EAC growth, assessed from the present fractographic evaluation, have exhibited good commonality with those predicted from a slip dissolution model which characterised fatigue phenomena in high temperature water environments.