Formation of σ phase in wrought 310 stainless steel

Abstract

The formation of σ phase has been studied in a low carbon 310 stainless steel. The σ forms initially by ‘in situ– transformation of the δ ferrite platelets present in the wrought material, and the transformation is complete within minutes at temperatures in excess of 700°C. Initially the σ has essentially the same bulk composition as the δ, but its composition changes with time at temperature. The formation of σ within the initially austenite matrix is observed at higher temperatures (about 900°C) after about 5 h and is preceded by the formation of α′ ferrite, from which the σ eventually appears to form. The composition of the α′ readjusts before its transformation to σ, becoming virtually identical to the σ which is iriferred to form from it. At all temperatures σ formation was preceded by the formation of a distribution of fine precipitates, some of which appeared to be G phase, others niobium carbides. The results are discussed in the context of previous work on σ formation, notably that of Vitek and David on the role of compositional change and σ nucleation rate.

MST/2092