On the chemistry and stability of complex carbides and nitrides in microalloyed steels

Abstract

Light optical and electron microscopy, and quantitative (STEM-EDX) micro-analysis are employed to characterize a number of different Ti-micro-alloyed steels, containing various alloying additions including high and low levels of nitrogen, and a Nb-steel is used as a comparison. The steels are studied in both the as-received and weld simulated conditions. Particular emphasis is placed on measuring the particle size distributions and chemical compositions of a large number of the complex carbides and nitrides formed in these steels. It is found that while individual particles inevitably reflect the compositions of the steels in which they have formed, the relative amounts of the various metallic elements present in particles are a function of the prior thermal treatment of the alloy. Thus, weld thermal cycling treatments substantially change particle compositions compared to the as-received (rolled or normalized) conditions, and even the energy input of the welding process is of importance in this respect. As a general rule, TiN-containing steels provide for good grain growth control during weld cycling, although the presence of additional alloying elements in the particles may affect particle stability in a complicated way. It is shown that equilibrium thermodynamics can be used to make reasonable predictions of particle compositions provided the prior heat treatment is such to allow the particles to remain near their equilibrium condition, such as in slow thermal cycling. In certain cases, as in low energy weld thermal cycling, this not likely and it is then necessary to take the diffusional kinetics of individual elements in particles into account, if compositional changes are to be understood.