Kinetics of transformation of delta ferrite during creep in a type 316(N) stainless steel weld metal

A quantitative study on the transformation behaviour of δ ferrite in a type 316(N) stainless steel weld metal during creep in the temperature range 823-923 K was carried out. The fraction of δ ferrite transformed was determined from magne-gage measurements on the samples after creep fracture at various stress levels, i.e. after different durations of creep exposure. The transformation was found to follow the Avrami kinetics, X=1-exp(-btn) where X is the fraction of δ ferrite transformed, t is the time to rupture and b and n are constants. n was found to be independent of temperature. An apparent activation energy for transformation, Q, was determined from the temperature dependence of the constant b expressed as b0 exp(-Q/RT) as about 152 kJ mol-1 which is similar to that for Mo diffusion and for boundary diffusion in 316 SS. X-ray diffraction analysis of the precipitates extracted from the creep tested samples indicated the presence of Mo-rich phases η and χ. It is proposed that the transformation is controlled initially by diffusion of Cr through δ/γ boundaries to form carbides, followed by matrix diffusion of Mo to form the intermetallic phases