Modelling the effects of alloying elements on precipitation in ferritic steels

This paper reports a systematic study on the effects of different alloying elements on the precipitation of M23C6 particles in power plant steels using a modified modelling technique. MTDATA is used to calculate some of the thermodynamic parameters for the alloying system. The elements studied include tungsten, aluminium, silicon, manganese, cobalt, copper, nickel and molybdenum with varying degrees of concentration. Results show that all the elements are beneficial in reducing the coarsening rate of M23C6 precipitates at low concentrations. Coarsening rate decreases progressively with increasing concentration of Si, Co, Al and Cu. Ni can be very harmful by increasing coarsening rate when its concentration is greater than about 0.4 at.%. Both Mn and W are quite effective in reducing coarsening rate when the concentrations are low. Extremely sharp increases in coarsening rate are found when the concentrations of these two elements exceed about 0.5 at.%. Mo is significant in reducing the coarsening rate even at very low concentrations and the effect is more pronounced with increasing concentration. All these effects result from the change of solubility of chromium in the matrix caused by the alloying elements.