Influence of Peak Temperature on the Hardness in the Heat Affected Zone of S700MC, CK45 and S355W Welds

Abstract

Welding of carbon steels induces significant microstructural modifications within the heat-affected zone (HAZ), which influence the material’s performance during service. These changes are typically associated with increased hardness and reduced toughness. Although often remaining within acceptable limits, they must still be considered, especially when the weld is part of a highly demanding application.

Continuous Cooling Transformation (CCT) diagrams, which are typically used to describe heattreatment behaviour, are not fully suitable for welding applications. This is because welding heat cycles differ substantially from quenching: they reach much higher temperatures and include no soaking time. As a result, HAZ hardness depends not only on the cooling rate but also on the heating rate and the peak temperature reached at each point within the HAZ. These parameters govern the transformation of pearlite into austenite and the homogenization of the austenitic phase.

This paper investigates the influence of the peak temperature within the HAZ on the hardness of three different steels (S355, C45, and S700MC) under conditions of relatively slow cooling. Samples were heated in a furnace to predefined maximum temperatures and then immediately cooled in an air stream. The results indicate that, beyond a certain temperature, a sharp transition from low to high hardness occurs. This transition is associated with grain coarsening and completion of the austenitization process. Compared to oil quenching, additional features such as the formation of Widmanstätten microstructures were also observed.