Effect of niobium on sulfide stress cracking behavior of tempered martensitic steel

Abstract Martensitic steels with increasing niobium (Nb) content from 0.003-0.060 wt.% were tempered at corresponding temperatures of 675−710 °C, consequently acquiring an almost identical yield strength of ∼780 MPa. The effect of Nb on the sulfide stress cracking (SSC) behavior was assessed via double cantilever beam testing, electrochemical permeation testing and microstructure characterization. The high Nb content and corresponding high-temperature tempering caused a more significant refinement of the martensitic packet/block with high-angle boundaries, which evidently inhibited SSC propagation; despite increased mobile hydrogen concentration which slightly promoted SSC propagation. The overall SSC resistance was therefore enhanced by increasing Nb content.