Changes of microstructure and mechanical properties of the HAZ of the S960MC steel sheet weld joint

Abstract

The TMCP (thermomechanically controlled processed) steels belong to the group of ultra-high-strength steels, which exhibit an exceptional combination of high tensile and yield strength, toughness, and ductility. These steels were introduced in heavy machinery constructions, such as heavy mobile cranes, chassis trucks, and others, to reduce their weight, which increases their loading capacity and ecology of transport. The high tensile and yield strength of this type of steel is obtained by the combination of the chemical composition, heat treatment, and mechanical processing. However, the heat input into the material during the welding significantly affects the properties of the steel and the whole joint. In this paper are presented results of mechanical properties evaluation and structural analysis of the welds of the thin sheets made of the S960MC steel, which were welded using the GMAW procedure. The microstructural evaluation referred to significant changes in the HAZ. This area contains the three subzones: coarse grain (CGHAZ), fine grain (FGHAZ), and intercritical zone (ICHAZ). Analysis of microhardness and the tensile test results showed that ICHAZ is the most critical area of the whole welded joint.