Effect of Molybdenum and Chromium Addition on the Susceptibility to Sulfide Stress Cracking of High-Strength, Low-Alloy Steels

Abstract

The purpose of this study is to elucidate the effect of Mo and Cr addition on corrosion and sulfide stress cracking (SSC) of high-strength, low-alloy (HSLA) linepipe steel. Six hot-rolled API X70/X80 grade steel plates containing 0.04 wt% C with different amounts of Mo and Cr (0.2, 0.3 wt% Mo, 0.15, 0.3 wt% Cr, 0.1 wt% Mo-0.3 wt% Cr, 0.2 wt% Mo-0.3 wt% Cr) were prepared by identical thermomechanically controlled processes (TMCPs). The SSC of the tested steels was investigated in terms of microstructure, mechanical properties, and electrochemical properties such as corrosion resistance and hydrogen permeation. A single addition of 0.3 wt% Mo or a combined addition of Cr and Mo increased SSC resistance of the steels by modifying the microstructure from ferrite to acicu-lar ferrite (AF) and secondary phases from grain boundary iron carbides to SSC-resistant degenerated pearlite or small martensite/austenite (M/A) constituents. A single addition of Cr increased the local corrosion of steels under external loading, which limits the application of Cr-bearing steels for linepipe steels.