The role of niobium in austenitic and duplex stainless steels

The two principal roles of niobium in cast and wrought austenitic stainless steels are as a stabilizing agent to reduce the risk of intergranular corrosion and as a strengthening agent. To date there appears to be no parallel roles in the duplex, austenitic-ferritic stainless steels. The principles for designing an austenitic steel composition are outlined and the effects of niobium as an alloying element summarized. The characteristics of the principal precipitate phases and their occurrence are reviewed in the context of service requirements, to illustrate how compositions have evolved. Metallurgical developments covered include the benefits derived from a balanced reduction in the levels of niobium and carbon in the 347 composition. Application of controlled heat treatments combined with cold working has allowed optimization of both steam-side oxidation and creep strength for this widely used grade. The combination of low levels of niobium with enhanced nitrogen additions has led to alternative stabilization mechanisms. Enhanced precipitation strengthening for high temperature applications derives from multiple microalloying additions of, principally, niobium, titanium and vanadium and recognition of the role of the chromium-niobium nitride. Families of high strength creep resisting steels have been developed for both fossil and nuclear power generation applications, based on the synergistic effects of multiple alloying additions of molybdenum, niobium, titanium, nitrogen, copper and tungsten. These steels exploit knowledge of alloy design to combine effective solid solution and precipitation strengthening with optimum grain boundary precipitation. (AU)