Abstract
The objective of this study is to improve the sulfide stress cracking (SSC) resistance of a 42CD4 (AISI 4140/4145) high-strength steel [ultimate tensile strength (UTS) ∼ 1000 MPa]. 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). Permeation data showed that the addition of carbide/nitride-forming elements was effective in providing hydrogen with numerous trapping sites. Hydrogen embrittlement (HE) (cathodic charging) and SSC (NACE TM-01-77) testing showed that, for equivalent mechanical characteristics, both low sulfur/phosphorus and vanadium or niobium additions allowed a definite 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 better 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).
