Effect of Postweld Heat Treatment on HAZ Toughness of Q+T High Strength Steels

Nowadays the application ratio of high strength steels is continuously increasing. Due to their outstanding mechanical, especially strength properties significant weight reduction can be achieved. Besides the decreasing operational costs due to the energy saving in mobile structures, the thinner plates and smaller cross sections result savings in the amount of applied base and filler materials. Because of the abovementioned advantages the development of strength properties of these structural steels is in the research focus of steel and welding consumable producers [4]. In the present paper the highest steel grade of EN 10025-6, S960QL is investigated in the aspect of the microstructural changes in the heat affected zone of gas metal arc welded (GMAW) joints. Quenched and tempered (Q+T) high strength steels have a non-equilibrium tempered martensitic microstructure due to the water cooling used in the quenching cycle and to the high temperature tempering. In order to realize the quenching condition in the whole cross section, alloying components (Cr, Mo) are added to the steel, which move the CCT curves to the right. Microalloying elements (Nb, V and Ti) are also used in order to ensure and preserve a fine grain microstructure. The tempered, fine-grained microstructure results high toughness at negative temperature (even at -40 oC). The chemical composition of the investigated base material is shown in Table 1, the mechanical properties are presented in Table 2.