Abstract
The purpose of this study is to elucidate the effect of Cr and Mo on sulfide stress cracking (SSC) of API X70 grade steels. Six hot-rolled steel plates containing different amounts of Cr and Mo (0.15, 0.3wt% Cr, 0.15, 0.3wt% Mo, 0.15wt% Cr-0.3wt% Mo, 0.3wt% Cr-0.2wt% Mo) were prepared by the identical thermo-mechanically controlled processes (TMCP). SSC of tested steels were investigated in terms of metallurgical parameters, mechanical properties and electrochemical properties such as corrosion resistance and hydrogen permeation. Steel microstructure and distribution of precipitates were analyzed because they were key parameters affecting both mechanical and SSC properties. To determine the mechanical properties of steels, constant elongation rate tests (CERT) and hardness measurement were performed. To evaluate corrosion properties, specimens were immersed in NACE TM01-77A solution and weight loss according to immersion time was measured. SSC susceptibility of steels was evaluated in reference to NACE TM01-77-96A standard test method. Hydrogen flux through steel matrix was measured using a modified Devanathan-Stachurski cell.
As a result, 0.3wt% Cr added steel is considered to be the best candidate for API X70 grade steel among all tested steels even though some Mo containing steels have higher SSC threshold stress. It has optimum strength-toughness balance, high corrosion resistance and low SSC susceptibility.
