Improving the HAZ toughness of Q+T high strength steels by post weld heat treatment

Abstract

Steel producers are continuously developing the mechanical properties and improving the weldability of high strength steels. Quenched and tempered steels belong to the one of the highest strength grades of structural steels with outstanding toughness characteristics due to the high temperature tempering. During fusion welding the thermal cycle irreversibly changes the microstructure and the mechanical properties of the base material, therefore an inhomogeneous heat-affected zone (HAZ) forms. In the HAZ hardened and softened zones occur. Due to the thermal cycles experiences during welding, these HAZs can exhibit significant losses in toughness when compared to the base metal. In real welded joints the HAZ properties can be analysed by conventional material tests to a limited degree; therefore physical simulators have been developed for the detailed examination of different HAZ areas. In the present research work the HAZ properties of a 960 MPa yield strength quenched and tempered steel (S960QL according to EN 10025-6) are investigated. Since the toughness reduction can be only partially handled by the adjustment of welding parameters, the possibility of local post-weld heat treatment was examined. Based on preliminary physical simulations and welding experiments a medium heat input gas metal arc welding technology (t8/5 = 15 s) was selected for the HAZ simulations. The welding and the post-weld thermal cycles were determined according to the Rykalin 3D model. The effect of post weld heat treatment on the properties of the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones were investigated by electron microscopic, hardness tests and instrumented Charpy V-notch impact tests. The materials tests showed significant improvement of the toughness properties especially in ICHAZ due to the post-weld tempering, whilst the softening was acceptable.