On the chemistry and stability of ternary precipitates in microalloyed steels

Abstract: Light optical-, electron microscopy and Charpy-V-notch testing is used to characterize a number of different Ti-microalloyed 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 distribution and chemical compositions of the complex carbides and nitrides formed in these steels, and this is carried out on extracted particles using quantitative STEM-EDX microanalysis. It is found that while individual particles inevitably reflect the compositions of the steel in which they have formed, the relative amounts of the various metallic elements present in particles are very much 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. In certain cases, as in weld thermal cycling, this is not possible and it is then necessary to take the diffusional kinetics of individual elements in particles into account, if compositional changes are to be understood.