Physical simulation based characterization of HAZ properties in high-strength steels.

Abstract

Nowadays there is a growing interest for the wider application of high strength steels.  Advanced 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 formability. However, the weldability of high strength steels has still challenges which are as follows: cold cracking sensitivity; reduction of strength and toughness in weld and heat-affected zone (HAZ); matching phenomena. 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 the HAZ properties can be limitedly analysed by conventional material tests, therefore physical simulators were developed for the examination of different HAZ areas. Physical simulation provides time and material saving compare to real welding experiments. In our research work the weldability, especially the HAZ characteristics 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 the HAZ simulations and the effect of cooling time on the critical HAZ areas was analysed. The properties of the selected coarse grained (CGHAZ), fine grained (FGHAZ), intercritical (ICHAZ) and subcritical (SCHAZ) zones were investigated by optical microscope tests and hardness tests.