Effect of Carbon on the Solidification Cracking Behavior in the Austenitic Fe-Mn-Al-C Lightweight Steel Welds

In this study, the solidification cracking behavior was investigated in Fe-30Mn-10.5Al-xC (x = 0.7, 0.9, 1.1 wt.%) austenitic lightweight steels. To evaluate the solidification cracking susceptibility of each alloy, a longitudinal Varestraint test was conducted with an applied strain ranging from 1 to 4%. The experimental results indicate that all the austenitic lightweight steels exhibit great resistance to solidification cracking due to the beneficial effects of high Al content, as evidenced by maximum crack length (MCL) values below 380 μm, despite their high alloying element content. However, the MCL increased directly as a function of carbon content. Microstructural analysis confirmed that the increased carbon content destabilized δ-ferrite in the fusion zone during solidification, consequently accelerating the generation and propagation of cracks. In addition, excessive carbon content also led to the formation of a eutectic (Fe, Mn)<sub>3</sub>C phase along the grain boundaries during the terminal stage of solidification, resulting in a drastic increase of solidification crack susceptibility due to its low melting point. The high content of Al and C also promoted κ-carbide precipitation in the fusion zone. However, it was confirmed that κ-carbide formation had an insignificant influence in the viewpoint of solidification crack susceptibility due to its narrow precipitation temperature range. Hence, we suggest further investigation through Varestraint test with various austenitic lightweight steels to confirm the influence of some alloying elements, for understanding the phase transition behavior in welds and improving the usability of austenitic lightweight steel in the industrial fields.