Characterization and Prediction of Austenite Formation and Decomposition in Steel Welds

Austenite formation and its decomposition control the final microstructure and performance of steel welds. This paper presents an in-situ characterization of austenite formation and its decomposition in both the fusion zone (FZ) and heat-affected zone (HAZ) of an Fe-C-Al-Mn steel using time-resolved X-ray diffraction (TRXRD) with synchrotron radiation. Measurement of X-ray diffraction spectra at a time resolution of 0.05 s allowed monitoring of the sequence of phase evolution in welds. In steel containing 1.7 wt.% Al, incomplete austenite formation from the ferrite microstructure was observed in the HAZ during weld heating, which is in agreement with equilibrium thermodynamic predictions. In the case of the FZ, nonequilibrium austenite solidification was observed at high cooling rates, followed by its decomposition during the latter stages of the cooling cycle. A theoretical treatment of both austenite formation and its decomposition in these welds, based on existing thermodynamic and kinetic models is used to explain these experimental results.