Effects of Ni Addition on Sulfide Stress Cracking Resistance of Low Alloy Tempered Martensite Steels.

Abstract

The detrimental effect of Ni addition on sulfide stress cracking (SSC) resistance has been well known since the beginning of the study on SSC. However, effects of Ni itself and untempered martensite which can be typically formed for Ni bearing steel have not been clearly established. In this paper, the effects of Ni addition on SSC resistance are investigated using constant load tests and DCB tests for low alloy tempered martensite steels. SSC threshold stress (σth) decreases with an increase in Ni content at a constant yield strength. This is probably caused by the fact that fissures work as initiation sites of SSC. On the other hand, an addition of Ni raises slightly the value of KISSC. Each KISSC value is supposed to be determined by the hydrogen content. Hydrogen content when crack propagation stops decreases with an increase in Ni content, while hydrogen content increases in Ni content then the steel starts being exposed to a sour environment. Untempered martensite is cormed during tempering at a temperature just below AC1. Steels with untempered martensite show lower values of σth and KISSC than those without untempered martensite at a constant yield strength. So an addition of Ni promotes the formation of fissures and accelerates SSC initiation even without untempered martensite. Furthermore, untempered martensite decreases even more SSC resistance.