Niobium in high strength steels for oil country tubular goods

This paper reviews the present and potential use of niobium in high strength oil country tubular goods (OCTG). High strength OCTG used in sour and gas well applications are generally microalloyed with niobium to improve both the strength and the resistance to sulphide stress cracking (SSC). One of the benefits of niobium is to produce a finer grained microstructure during austenitizing. Furthermore, niobium plays a more important role in preventing grain growth in the new heat treatment processes, such as accelerated cooling after rolling and induction quenching, which have been investigated for obtaining ultra fine grains. Another important effect of niobium is precipitation hardening due to niobium carbide. Austenitizing niobium-bearing steels at high temperature enhance the finer niobium carbide precipitation during subsequent tempering. The niobium addition, therefore, makes it possible to temper at higher temperatures giving an improvement in SSC resistance.