Phase decomposition of stainless steel weldment by thermal aging

A SUS308 stainless steel weldment of which main phases are ferrite and aust enite was aged at 450 degrees C for a time period up to 11000 h in order to investigate the compositional evolution of the ferrite by Mossbauer spectr oscopy. Changes in the internal magnetic held of Fe with aging time suggest that the ferrite decomposes by spinodal process finally to produce the Fe- rich alpha and the Cr-rich alpha' phase. According to the mean internal mag netic field measurement, the Cr content of the former is about 9 at% Cr whi ch is consistent with a calculated phase diagram. The Cr content of the a' phase is evaluated to be about 83 at% Cr by the isomer shift measurement, w hich shows a slight deviation from the calculated phase diagram. The phase decomposition is accompanied by an increase in the hardness of ferrite, whi le that of austenite is remained unchanged.