Phase transformation and the mechanical characteristics of heat-affected zones in austenitic Fe–Mn–Al–Cr–C lightweight steel during post-weld heat treatment

Abstract

We identified the influence of post-weld heat treatment (PWHT) on the heat-affected zone (HAZ) in the Fe–29.7Mn–10.5Al–3.06Cr–0.93C (wt%) lightweight alloy based on transitions in its microstructure and mechanical properties. The investigation was performed for samples under various PWHT conditions including as-welded state. Gas tungsten arc welding (GTAW) and Gleeble simulator were used for analyzing HAZ characteristics, and the PWHT processes were carried out with a commercial furnace. For investigating detailed mechanical and microstructural characteristics, micro hardness, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were employed. According to the experimental results, it was revealed that the addition of Cr in the austenitic lightweight steel had various effects on HAZ characteristics. In the as-welded state, Cr stabilized the HAZ mechanical characteristics by suppressing the driving force of κ-carbide precipitation during the welding thermal cycle. However, in the PWHT process at 600 °C, Cr caused the phase transformation mechanism of M7C3 precipitation and allotriomorphic ferrite formation at grain boundaries both in the HAZ and the base steel. In addition, the analysis of fusion zone showed that the increased grain boundaries could accelerate the driving force of phase transformation mechanism and promote the harmful β‑manganese formation. Our results concluded that the addition of Cr might cause harmful effects in the HAZ of austenitic lightweight steels under the isothermal heat treatment such as PWHT.