Evaporation of elements from 20Cr-25Ni-Nb stainless steel during vacuum annealing

Abstract

Coupon specimens of 20Cr-25Ni-Nb-stabilized stainless steel have been annealed at 1273 K in dynamic vacuum for periods of from 1 to 1000 h. It is shown that manganese and chromium both evaporate readily over this period with kinetics which are consistent with the rate of loss from the surface being proportional to the surface concentration of the respective element. Iron is also lost but in detectable amounts only after long anneals (∼ 1000 h). The surface grain size increases with time and it is demonstrated that a significant fraction of the chromium loss in the early stages of annealing occurs by preferential evaporation from migrating grain boundaries. On the other hand, manganese is lost much more readily from regions remote from grain boundaries and this is commensurate with its higher vapour pressure. The theoretical calculations of weight loss consistently underestimate those observed gravimetrically at intermediate and long anneals and this is accounted for by evaporation from grain boundary regions.