The interphase precipitation of carbides has been studied in two microalloyed steels containing Ti and Nb, but only one of which contains molybdenum. The precipitates obtained are, therefore, (Ti,Nb)C and (Ti,Nb,Mo)C respectively. It is found that molybdenum significantly reduces the size of the carbide precipitates and also strongly retards their coarsening behaviour during subsequent heat treatment. This is because it enhances the nucleation rate by reducing the interfacial energy, but it is not thermodynamically favoured within the (Ti,Nb,Mo)C, so the later stages of growth involve its partitioning into the matrix, thus explaining the reduction in coarsening rate relative to the molybdenum free steel.
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