Solubility and precipitation of carbides containing niobium and molybdenum in low alloy steels

Niobium and molybdenum have been critically important alloying additions in steels for several decades. Niobium typically contributes through careful control of niobium carbide precipitation in austenite and ferrite, which enables grain refinement and precipitation strengthening, although solute effects on recrystallization and transformation kinetics can also be important. Molybdenum is a powerful hardenability agent and is extensively used to influence austenite decomposition kinetics and microstructure development in many steels. Molybdenum and niobium have been employed together in steels, and more recent investigations suggest that interactions associated with combined niobium plus molybdenum additions may provide synergistic enhancements beyond the separate effects of the individual elements. Molybdenum can enhance the benefits of NbC precipitates by reducing the carbide coarsening kinetics. This behavior has been suggested to be important in fire-resistant constructional steels, and high strength, formable automotive hot-rolled sheet steels. More recently, similar effects have been obtained in austenite, where enhanced grain refinement during high temperature carburizing simulations has been confirmed in Nb+Mo modified allo ys. The objective of the present paper is to provide some perspective on the physical metallurgy fundamentals influencing a variety of such applications, including solubility of niobium and molybdenum carbides in ferrite and austenite, and the precipitation and coarsening behavior of mixed (Nb,Mo) carbides. (AU) © 2015 Companhia Brasileira de Metalurgia e Mineração (CBMM)