Effect of Molybdenum on Sulfide Stress Cracking Resistance of Low-Alloy Steels Available to Purchase

The effect of molybdenum (Mo) content on the sulfide stress cracking (SSC) resistance of low-carbon (C), low-alloy steels in hydrogen sulfide (H₂S)-saturated sodium chloride (NaCl) solution was investigated by conducting a constant-load test on four steels containing 0% to 0.45% Mo. The microstructures consisted of second phases dispersed in a matrix of ferrite grains. Mo additions altered the second phases from pearlite bands to dispersed martensite/austenite (M/A) islands through suppression of the transformation temperatures of second phases. SSC was found to incorporate two components. One component was parallel to the loading stress and frequently at interfaces of manganese sulfide (MnS)/ferrite and banded second phase/ferrite. The other was perpendicular to the loading stress and initiated preferentially at M/A islands. The first component diminished with increasing Mo as a result of the breakup of the banded structures. The second component was enhanced by increasing Mo because of the formation of more M/A islands. By optimizing the two effects, the highest SSC resistance was obtained with 0.3% Mo.