EFFECTS OF STRENGTHENING MECHANISMS ON SULFIDE STRESS CRACKING RESISTANCE OF LOW STRENGTH STEELS

Sulfide stress cracking (SSC) of low strength steels are often thought as being.related to non-metallic inclusions. However, effects of intrinsic metallurgical factors such as chemical composition and micro structure have not yet been clarified. In this paper, effects of strengthening due to grain refining, precipitation, solid solution and dislocation were studied using tensile type SSC tests. Threshold stress (sigma(th)) is in proportion to intragrain hardness irrespective of strengthening mechanism and grain size. Further analysis shows that sigma(th) is relared to sigma0 defined in the Hall-Petch relationship. The ratio Of sigma(th) to yield strength therefore increases with an increase in grain size, which is contrary to the case of high strength steel. The difference originates from the fact that SSC of low strength steels is transgranular whereas SSC of high strength steel contains intergranular fractures.