Softening Effect in the Heat-Affected Zone of Laser-Welded Joints of High-Strength Low-Alloyed steels

Abstract

The softening effect, which occurs as an undesirable consequence of microstructural changes in the heat affected zone (HAZ) as a result of the welding process, is an inherent aspect of welding high-strength low-alloyed (HSLA) steels. One of the recommended ways to minimize these changes is the application of laser beam welding (LBW) as a lower heat input technology. Hence, this work compares and investigates the effects of laser beam welding on the properties of weld joints made of the steels S690QL, S960QL, S1100QL, S700MC, S960MC, and S1100MC. The main objective of this work is to examine the variations in the mechanical properties, grain size, and microstructure of the HAZ of different structural materials welded under various conditions. For laser-welded HSLA steels, the results showed no significant changes in the weld joint properties when the heat input was increased. When individual steels are compared, more obvious changes in the softening effect, grain growth, and the extent of the coarse-grain HAZ (CGHAZ) are evident. According to measured values it has been demonstrated that as the amount of Nb, V, and Ti in material decreases, the grain size in the coarse-grain HAZ (CGHAZ) and the zones’ overall width increases. It was also further demonstrated that the Cr and Mo content had a significant impact on the weld metal's hardness in laser-welded weld joints. These variations can be explained by the initial microstructure and the different chemical composition, which have an impact on the softening characteristics, austenitic grain growth, and the size of the CGHAZ.