Papers by Keyword: α/γ Phase Transformation

Abstract: Computer simulation of the α/γ phase transformation in multipass weld of duplex stainless steel was made for predicting the distribution of the γ phase fraction in the weld metal (WM) and HAZ. The kinetic equations including rate constants of the dissolution behaviour as well as precipitation behaviour of γ phase were determined by isothermal heat treatment test. Based on the kinetic equations determined, the distribution of the γ phase fraction in multipass weld of duplex stainless steel was calculated applying the incremental method combined with the heat conduction analysis in welding process. The γ phase fraction was reduced in the higher temperature HAZ and WM, however, that in the reheated HAZ and WM was increased and recovered to the base metal level. Microstructural analysis revealed that the calculated results of the γ phase fraction in multipass weld were consistent with experimental ones. Based on the computer prediction, the microstructural improvement welding (“reheat bead welding”) process, with analogous concept to the temper bead welding technique, was newly proposed for recovering the γ phase fraction in weld even in the as-welded situation.

Abstract: In-situ observations of a/g phase transformation were made to study the effect of grain boundary microstructure of the generation of a new phase and the migration of a/g interphase boundaries in an Iron-4.2at.%Cr alloy. It was found that triple junctions with more random boundaries could be the primary nucleation sites, while triple junctions with low angle and low S coincidence boundaries did not play a role as preferential sites. The migration of a/g interphase boundaries during heating across the transformation temperature showed the two stage behaviour characterized first by a stage with a migration velocity of 0.33-0.75µm/s and secondly a stage with 3.7-7.6 µm/s. It was also found that abnormal grain growth and a high density of S3 coincidence boundaries could occur in the a/bcc phase after cycling of a/g/ a phase transformation.