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.
Steels subject to sour gas environments containing hydrogen sulfide (HzS) are sensitive to cracking failure by hydrogen ingression. For typical industrial applications, steels with a Rockwell hardness above HRC 22 (equivalent to 550MPa yield strength) are considered to have a high susceptibility to SSC,[ 1] which proceeds perpendicular to the loading axis in the form of intergranular or transgranular cracking under external loads. The main cause of SSC is the high defect density formed during various manufacturing processes such as heat treatment, cold working, and so on.[3, 4] It has been reported that SSC in sour gas environments occurs by the combined action of external tensile stress and active path corrosion. Bastien et al, however, have suggested that hydrogen embrittlement (HE) is responsible for SSC occurring in low alloy steels containing no nickel. This point of view is generally accepted as the SSC mechanism. In SSC, hydrogen sulfide retards the recombination reaction of hydrogen atoms to molecular hydrogen gas. This poisonous effect accelerates diffusion of atomic hydrogen into the steel leading to catastrophic linepipe failures.
As the requirement of strength and toughness for linepipe steel products increases, SSC resistance of steel becomes ever more important. Since the strength and toughness of steel are dependent mainly upon microstructure, understanding on the effects of steel micro structure on the SSC susceptibility is very important. In steel making processes, optimum microstructure with high strength, toughness and high SSC resistance can be achieved by a proper alloy design and thermo-mechanically controlled process (TMCP). As the grade number of linepipe steel increases to meet high strength and toughness, the addition of alloying elements such as Cr and Mo is inevitable. Addition of Cr and Mo in steel increases A3 temperature, decreases bainite transformation start (Bs)and martensite transformation start (Ms) temperature and retards ferrite-pearlite transformation. Therefore, addition of Cr and Mo will promote the formation of low temperature transformation structures and increase the strength of steels.