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

•Nb-induced structure refinement raised diffusible H and promoted SSC propagation.•Nb-induced structure refinement prevented SSC propagation by diverting SSC path.•Microalloyed Nb contributed more to the resistance than to the assistance to SSC.•Block had a closer correlation to KISSC than Packet, offering a better impact.•Increased Nb content allowed a higher tempering temperature for increased KISSC.