Proposal of Independent Two Phase Growth during Solidification in Austenitic Stainless Steels

Solidification morphologies of austenitic stainless steel weld metals that solidified with primary ferrite were investigated in terms of crystallography. At the fusion boundaries, austenite grows first with plane-front morphology from the base metal austenite in an epitaxial manner. Then, ferrite forms on the growing austenite by keeping the Kurdjumov-Sachs orientation relationship with the austenite. The ferrite grows, as the primary phase, more rapidly with dendritic morphology than the planar austenite. Though the phase diagram indicates that the formation of the austenite results from the eutectic reaction in the primary ferrite solidification mode, no specific orientation relationship was confirmed by crystallographic studies between the primary ferrite and the interdendritic austenite. The austenite is found to grow independently, growing along < 100 > direction, even when the primary ferrite changes its growth direction. Consequently, it is suggested that the austenite in the interdendritic regions is not crystallographically restricted by the primary ferrite during the growth. The growth manner of the primary ferrite and secondary austenite is named as "independent two-phase growth", and was confirmed not only in weld metals but in cast metals.