Influence of κ-carbide precipitation on the microstructure and mechanical properties in the weld heat-affected zone in various FeMnAlC alloys

This study aims to investigate the effects of κ-carbide precipitation behavior in the heat affected zone (HAZ) in FeMnAlC lightweight steels. Three alloys with different amounts of Al were prepared by vacuum induction melting and hot rolling. After solution treatment, the HAZ samples were simulated by a Gleeble simulator with two heat inputs of 10 and 30 kJ/cm. Microstructural analysis with XRD and TEM were carried out while sub-sized tensile test, hardness test, and V-notched Charpy impact test were performed for investigating the mechanical properties of the base steels and HAZ. The results showed that the mechanical properties and precipitation of κ-carbide within the HAZ were strongly related to the Al content and heat input; the tensile strength and hardness of the HAZ increased as the Al content and heat input increased while elongation decreased. On the other hand, in the Charpy impact test, fracture mode transitions in the HAZ (ranging from ductile fracture to brittle inter-granular fracture) occurred in accordance with the Al content and heat input. The different fracture behavior was explained by TEM results, which showed precipitation behavior of κ-carbide in HAZ. Coherent intra-granular κ-carbide was found to cause a transition from ductile fracture to trans-granular cleavage, and we observed that a severe drop of the impact toughness occurred when partially coherent inter-granular κ-carbide appeared. Therefore, our results illustrate that the HAZ of lightweight steels with the proper Al content can be strengthened with minimal loss of impact toughness due to κ-carbide precipitation during the welding process.