Physical Simulation-Based Characterization of HAZ Properties in Steels. Part 1. High-Strength Steels and Their Hardness Profiling

In the vehicle industry, there is an increasing demand for wider application of high-strength steels. New generations of high-strength steels, with higher strength and toughness properties, are continuously developed by the steel producers. They provide good strength-to-weight ratios, acceptable weldability, improved toughness, and sufficient deformation capacity. However, the weldability of high-strength steels has still challenges which are as follows: cold cracking sensitivity; reduction of strength and toughness of heat affected zone (HAZ); filler material selection. In the HAZ of high-strength steels, hardened and softened zones can be found, where the base material can significantly lose its outstanding mechanical properties. In real welded joints, HAZ properties can be limitedly analyzed by conventional material tests. Therefore, physical simulators (i.e., Gleeble) were developed for the examination of different HAZ areas. Another motivation for the application of physical simulators is the time- and material-saving, as compared to real welding experiments. In this study, the weldability, especially HAZ properties of two high-strength structural steels (S960QL and S960M) from the same strength category (R_p0.2 = 960 MPa) and thickness (t = 15 mm) were compared and discussed. Two relevant technological variants for gas metal arc welding (GMAW), t_8.5/5 = 5 and 30 s were applied during HAZ simulations and the effect of cooling time on the critical HAZ areas was analyzed. The properties of the selected coarse-grained, fine-grained, intercritical, and subcritical zones were investigated by the optical microscopy and hardness tests.