HIGH-STRENGTH STEELS WITH IMPROVED SULFIDE STRESS CRACKING RESISTANCE

The objective of this study is to improve the sulfide stress cracking (SSC) resistance of a 42CD4 (AISI 4140/4145) high-strength steel left bracket ultimate tensile strength (UTS) 1000 MPA right bracket . Two modifications have been brought to the base composition: (1) the reduction of sulfur and phosphorus contents or (2) the addition of carbide or nitride-forming elements (vanadium, niobium, and boron). The addition of carbide/nitride-forming elements was effective in providing hydrogen with numerous trapping sites. Hydrogen embrittlement (HE) (cathodic charging) and SCC (NACE TM-01-77) testing showed that both low sulfur/phosphorus and vanadium or niobium additions allowed a gain in threshold stress. (Threshold stress goes from less than 50 MPa for the base steel to almost 250 MPa with the vanadium addition, at 780 to 790 MPa yield strength). This behavior is explained in terms of beneficial trapping (by fine homogeneous carbide dispersions), reduction in potential crack sites (less and rounder inclusions in low-sulfur steel), and a refining of the ferrite/bainite microstructure (resulting from carbide dispersions).