Decomposition of ferrite to austenite in 26%Cr-5%Ni stainless steel

Abstract

The decomposition of ∆-ferrite to austenite has been studied in a 26%Cr-5%Ni stainless steel. The decomposition takes place via two mechanisms: by nucleation and growth in the high temperature range (650–1200°C) and by an athermal martensitic process in the low temperature range (300–650°C). The high temperature reaction exhibits growth kinetics which are intermediate between volume diffusion control and growth by a ledge mechanism. The morpho-logical variants of the austenite mirror those described in the Dubé morphological classification for low-alloy steels, and the steel was used as a ‘model system‘ for the study of bcc ⇄ fcc transformations. In the low temperature range (400–600°C) the formation of α′, which gives rise to 475°C embrittlement, is shown to occur by a nucleation and growth process, and the subsequent dissolution of the α′ is explained by the precipitation of austenite. Finally, it is shown by STEM microanalysis that Ni partitions to the austenite and Cr partitions to the ferrite during the high temperature reaction and that there is no composition difference between the parent and product phases in the low-temperature martensitic formation of austenite.