Effects of Ti/N ratio and tempering time on sulfide stress cracking resistance of B-added high-strength steel

B has been widely acknowledged to improve the hardenability of steels. However, its role on the resistance of steels for oil country tubular goods to sulfide stress cracking (SSC) has not been well understood. Considering that B protection from N is important in retaining free B in steel, in the current work, two B-added high-strength steels with various Ti/N ratios were prepared to investigate the effect of Ti/N ratio and tempering time on the microstructure and SSC resistance. The results showed that, a low Ti/N ratio did not obtain full martensite but gave rise to a small amount of upper bainite, obtaining a refined effective grain size. The introduction of upper bainite decreased the dislocation density, with little impact on the precipitation behavior of cementites. Moreover, the differences in hydrogen diffusion and trapping behaviors during electrochemical hydrogen permeation and thermal desorption spectroscopy tests were primarily associated with variations in dislocation density. After tempering for a same time, the steel with upper bainite exhibited decreased yield strength and impact toughness compared with the steel without upper bainite, and it was assumed that upper bainite also impaired the SSC resistance, given that the strength often inversely correlates with SSC resistance in most practical applications. Increasing the tempering time could greatly increase the SSC resistance without greatly sacrificing the yield strength, which was due to the decrease in reversible hydrogen traps resulting from decreasing dislocation density. This work offers both technical guidance and theoretical basis for the composition design, microstructural control and servicing performance optimization of high-quality OCTG materials.