Role of elongated MnS inclusions in hydrogen embrittlement of high-strength steels

Abstract

The effect of inclusion orientation and volume fraction on threshold stress intensity K TH and crack-growth rate has been studied as a function of matrix strength and hydrogen activity of the environment by employing a medium carbon hardenable steel having two different sulphur contents (0·04 and 0·15%). The presence of platelike inclusions leads to enhanced crack-growth rate and decreased K TH in the short transverse orientations. In the longitudinal and transverse orientations, inclusions result in the elevation of K TH and reduction of crack-growth rate. The extent of anisotropy in crack-growth rate and K TH caused by a given inclusion dispersion is diminished by an increase in either matrix strength or hydrogen of the environment. It is proposed that the failure for crack growth in hydrogen involves the attainment of a critical tensile stress at a microstructurally significant distance ahead of the crack-tip. The orientation dependence of K TH is attributed to inclusion-induced perturbations of the crack-tip stress field. The results of permeation experiments show that there is no significant directionalty in the diffusivity of hydrogen and it is deduced that anisotropy of crack-growth rate must also be caused primarily by the inclusions affecting the crack-tip stress field.