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Abstract In this study, we investigated the effect of precipitation during the thermal cycles of age hardening and the welding process in hot rolled austenitic FeMnAlC lightweight steel. After solution treatment, samples were prepared under different age hardening conditions, which was conducted at a temperature of 550 °C in a furnace. Samples representing heat affected zones (HAZs) with three different peak temperature conditions were prepared via a Gleeble simulator. For microstructural analyses, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy (TEM) were carried out. The mechanical characteristics were investigated by tensile, hardness, and cryogenic Charpy impact tests. The results showed that precipitation behavior in the lightweight steels had specific influence in mechanical properties in age hardening and welding process. The age hardening effect was shown with increased tensile strength with loss of ductility. In the solution treated specimen, increased hardness appeared in HAZ compared to the base steel, while HAZ softening occurred in every age-hardened specimen. In the cryogenic Charpy impact test, all of the samples except the solution-treated base steel showed negligible energy absorption with inter-granular fracturing. The trends of the experimented HAZ mechanical properties revealed that the precipitation behavior of κ-carbide was strongly related to the overall mechanical properties. Moreover, we found that the welding process removed the hardening effect of aging as a result of drastic κ-carbide dissolution in the high-temperature HAZ regardless of age hardening conditions. The TEM analysis confirmed that all HAZ simulated specimens showed similar precipitation conditions for all cases. Based on our analysis, we confirmed that the precipitation behavior of κ-carbide was the main factor of different HAZ mechanical properties. We also suggested that discontinuous HAZ mechanical properties should be considered during the welding process because of κ-carbide dissolution in FeMnAlC lightweight steels. Highlights • FeMnAlC alloy was analyzed for various age hardening and HAZ conditions. • Mechanical properties had specific relationship with κ-carbide in all cases. • Inter-granular κ-carbide rapidly formed in all heat treatment conditions. [ABSTRACT FROM AUTHOR]

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 M 7 C 3 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. • Cr suppressed κ-carbide precipitation in HAZ in austenitic lightweight steel. • Post-weld heat treatment at 600 °C caused M 7 C 3 and allotriomorphic ferrite formation. • Phase transformations were accelerated in fusion zone due to increased boundaries. [ABSTRACT FROM AUTHOR]