Sulphur segregation and high-temperature brittle intergranular fracture in alloy steels

Abstract

High temperature brittle intergranular fracture has recently been identified as a mode of failure in alloy steels. It is associated with the dynamic segregation of sulphur to cracks in hard microstructures stressed at elevated temperatures in a manner analogous to hydrogen embrittlement at ambient temperature. Several models have been proposed to describe the action of sulphur, but insufficient experimental data have been available for their evaluation. The present study characterises sulphur enrichment at cracks and on free surfaces at high temperature in detail using scanning Auger spectroscopy. Both intergranular and transgranular surfaces were studied at pressures of air from 10 −9 to 10 −3 torr. Two types of sulphur enrichment at cracks were identified; general segregation to crack faces and local enrichment close to crack tips. The source of sulphur was largely that dissolved in the ferrite matrix. Large sulphides, intersecting grain boundaries, made a minor contribution, while small “overheated” intergranular sulphides were inoperative as sulphur sources. The role of stress in encouraging sulphur segregation was confirmed. In addition, an intermediate pressure of air was found to enhance sulphur enrichment, but only at surface oxygen coverages of 15–25 at.%. These observations were generally consistent with the influence of the crack tip stress field on migration of the sulphur solute, described by the “pure drift” model of high temperature brittle intergranular fracture. Refinement of the model, using finite element stress analysis, is included.