Effects of Mn,P, and mo on sulfide stress cracking resistance of high strength low alloy steels

Effects of phosphorus, manganese, and molybdenum on sulfide stress cracking (SSC) resistance were investigated by the constant load SSC test for low alloy steels with tempered martensite struc-ture and yield strength of 690 MPa to 820 MPa. The SSC threshold stress begins to decrease with an increase in yield strength at a certain yield strength (an SSC critical strength). The fraction of inter-granular fracture surface begins to increase with an increase in yield strength at the SSC critical strength, while the fracture mode becomes transgranular below this strength. Furthermore, there ex-ists a relationship between an increase in the fraction of intergranular fracture surface and a decrease in the SSC threshold stress. A decrease in phosphorus and manganese content and an increase in molybdenum content make the SSC critical strength higher. These changes in content of chemical elements, therefore, lead to an increase in the SSC threshold stress due to a decrease in the amount of intergranular fracture surface above the SSC critical strength. Below the SSC critical strength, however, the change of the SSC threshold stress is not observed because the fracture mode remains fully transgranular. The SSC critical strength could be represented as a function of a parameter com-posed of manganese and phosphorus contents.