Precipitation of molybdenum carbide in steel: multicomponent diffusion and multicomponent capillary effects

Many steels contain substitutional solutes which have a strong affinity for carbon or nitrogen. Such steels can be hardened by heat treatment which induces the precipitation of fine alloy carbides. Examples of such carbides include MX, 2 73 , 6 and 236 , where M stands for a mixture of iron and substitutional solute atoms and X stands for interstitial solute atoms. In practice, the heat treatment has to be at a temperature which is in excess of ~500°C in order to allow the substitutional atoms to diffuse in a reasonable time period, over the dimensions necessary for precipitate growth. Cementite and other transition iron carbides can precipitate by a mechanism which requires only the carbon atoms to diffuse; such carbides can therefore precipitate at temperatures as low as 200°C. As a consequence it is inevitable that iron carbide precipitation must precede that of the alloy carbides. It follows that the steel at first softens as the precipitation of iron carbides removes carbon from solid solution, but begins to harden again as a second reaction occurs in which the alloy carbides start to precipitate at the expense of the less stable iron carbides. It is for this reason that such alloys are called secondary hardening steels. Secondary hardened steels are of great importance in the manufacture of power plant and in the petrochemical industry where creep must be avoided. Attempts have recently been made to quantify the complicated sequence of precipitation reactions as a function of alloy composition and heat treatment. Since the precipitation reactions involve a large set of reactions often occurring together, Robson and Bhadeshia1,2 adapted the classical Avrami theory to deal with simultaneous transformations. Such work is of use in the design of new alloys. However, there are a number of fundamental problems which are outside of this framework of simultaneous transformations, associated with the growth of individual phases, which prevent accurate solutions.