A FRACTOGRAPHIC ASSESSMENT OF SULFIDE INCLUSION DISTRIBUTIONS AND THEIR INFLUENCE IN PROMOTING ENVIRONMENTALLY ASSISTED CRACK-GROWTH IN FERRITIC PRESSURE-VESSEL STEELS

The present study was aimed at fractographically assessing the extent of the sulphur concentration that is needed to initiate environmentall y assisted crack (EAC) growth in fatigue crack growth tests on pressur e vessel steels in a high temperature water environment. A relationshi p has been established between the extent of sulphide segregation with in 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 sul phur concentration, EAC growth occurred at sulphur concentrations of a round 7000 ppm at K(max) values of 80 MPa square-root m while at lower values (viz. 40 MPa square-root m) concentrations approaching 35000 p pm were required to initiate EAC growth. Under an ideal sulphide distr ibution it was suggested that no EAC growth was possible. Finally the sulphur anion concentration levels for both EAC and non-EAC growth, as sessed from the present fractographic evaluation, have exhibited good commonality with those predicted from a slip dissolution model which c haracterised fatigue phenomena in high temperature water environments.