THE ROLE OF NICKEL IN THE SULFIDE STRESS CRACKING OF LOW-ALLOY STEELS

The question of whether nickel in low-alloy steels is detrimental to their resistance to sulfide stress cracking (SSC) has been addressed extensively in the literature. Much of the confusion surrounding this question has been created by research that did not properly control important variables. Principal among these variables are strength level, hardness, chemical composition, microstructure, and heat treatment. This investigation achieved control of these variables and was able to demonstrate that Ni content of low-alloy steels does not have a direct effect on SSC. The SSC resistance of both Ni-free and Ni-bearing low-alloy steels depends on the heat treatment, microstructure, and strength, which are interdependent. The strong dependence of the lower critical temperature (Ac1) of low-alloy steels on Ni content limits the use of high tempering temperatures which are necessary to reduce hardness and affect microstructure sufficiently to produce SSC resistance. The effect of test solution composition on SSC and the effect of nickel on surface trench formation was also studied. The NACE Standard TM0177-86 solution more aggressive than the Shell solution. Ni appeared to have some effect in changing trench morphology.