The Effect of Phosphorus Additions and Cooling Rate on the Precipitation of M 23 C 6 in Austenite

The addition of 0·1% P to an austenitic alloy containing 16% Cr, 16% Ni, and 0·25% C accelerated the precipitation of M₂₃C₆ on ageing in spite of the absence of marked structural changes in the precipitate distribution. An addition of 0·3% P to a 16% Cr 9% Ni 0·23% C alloy markedly accelerated precipitation, partly as a result of the farmation of matrix precipitation (MDP). The kinetic data obtained indicated that the diffusian rate of chromium is increased in the presence of phosphorus. Air-cooling after solution-treatment, as campared with water-quenching, promoted MDP formatian in both phosphorus-containing and phosphorus-free alloys; this is attributed to the farmation of complexes during the cooling. The MDP nucleated dislocation loops, which then grew and continued to nucleate M₂₃C₆. Thus, the rate of precipitation was similar after either water-quenching or air-cooling awing to the nucleated loops compensating for the initial lower dislocatian density of air-cooled alloys.