Precipitation behavior and its effect on mechanical properties in weld heat-affected zone in age hardened FeMnAlC lightweight steels

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.