Epitaxial growth and phase formation of austenitic stainless steel weld metals near fusion boundaries: Study of solidification and transformation of Cr-Ni stainless steel weld metals (3rd Report)

Summary

This paper describes an investigation of the solidification behaviour of austenitic stainless steel weld metals at fusion boundaries, focusing particularly on epitaxial growth and primary phase selection. In the AF solidification mode, the planar austenite grows epitaxially from the austenite grains in the base metal. Ferrite subsequently forms at the dendrite boundaries of the primary austenite after the austenite changes from planar to dendritic. In the FA solidification mode, the planar austenite grows epitaxially from the austenite grains in the base metal as a result of competition between ferrite nucleation and austenite growth. Ferrite nucleation then ensues through enrichment of Cr in the liquid and ferrite subsequently continues to grow as the primary phase. The ferrite establishes the Kurdjumov-Sachs crystallographic orientation relationship (K-S relationship) with the planar austenite. At the stage of primary ferrite solidification, austenite solidifies as the second phase at the dendrite boundaries and has the same crystallographic orientation as the original planar austenite. During FA mode solidification of the sample with the lowest Cr-Ni ratio, it is also possible for the austenite to solidify as the primary phase because of solute fluctuations in the liquid and sub-cooling changes due to local changes in the temperature gradient in the liquid.