Experiments were carried out to determine the properties of the welded joints in 8 mm thick high-strength steels produced by quenching and tempering and thermomechanical rolling with accelerated cooling (tensile strength 821-835 MPa). The dependence of the strength, elongation, hardness, impact energy and crack opening displacement on the heat input in the range 1.0 -0.7 kJ mm 21 was determined. The results show that the dependence of the strength of the welded joints decreases and that of the elongation increases. The heat input has only a slight effect on the impact energy and crack opening displacement in the heat-affected zone.

To cite this article: M. Pirinen, Yu. Martikainen, S.Yu. Ivanov & V.A. Karkhin (2015) Comparative analysis of the microstructure of the heat-affected zone metal in welding of high-strength steels, Welding International, 29:4, 301-305,The welding of joints 8 mm thick in high-strength steels, produced by the quenching and tempering method and thermomechanical rolling with accelerated cooling, was investigated. It is shown that at the same strength of the steels (821 -835 MPa), the microstructure of the heat-affected zone (HAZ) greatly differs for the same welding conditions. The distribution of hardness in all zones of the welded joints was determined. The results show that in the steel produced by thermomechanical rolling with accelerated cooling, the softened zone of the HAZ is wider than in the steel produced by QT and equals 5 mm for two-pass arc welding.At the present time, high-strength steels are steels with a yield limit higher than 550 MPa [1]. There are three main methods of production of these steels: quenching and tempering (QT), thermomechanical rolling with accelerated cooling (or thermomechanically controlled processed [TMCP] steels) and direct quenching.The technological cycle of QT of steel includes initial hot rolling with cooling in air, heating to temperature Ac 3 followed by rapid cooling with water. The high cooling rates suppress the diffusion processes of formation of ferrite and pearlite, and this results in the formation of a mainly martensitic structure with low impact toughness. Subsequent annealing reduces the internal stresses and results in the required mechanical properties [2].In TMCP of steels, the optimum level of the mechanical properties is obtained by producing a microstructure with very fine grains. The controlled rolling parameters are carefully selected in relation to the thickness of the sheet and chemical composition. The conditions include the high degree of deformation in the temperature range of inhibited recrystallization, which results in marked refining of the final microstructure [3].Steels of the same strength grade may greatly differ in chemical composition and, consequently, the microstructure and mechanical properties of the HAZ. The mechanical behaviour of the HAZ determines the efficiency of the entire welded joint. Therefore, investigation of the microstructure and local mechanical properties of the HAZ metal in welding of high-strength steels is of interest for practice [4 -8].The aim of the present work is the analysis of the microstructure and hardness of the HAZ metal in arc welding of sheets of high-strength steels with a tensile strength of 821-835 MPa, produced by quenching and thermomechanical rolling with accelerated cooling.Two QT and TMCP steels greatly differ in the chemical composition (Table 1) but have similar mechanical properties (Table 2); 8 mm sheets with V-shaped edge preparation (angle 608, root face 1 mm, gap 1.5 mm) were welded with OK Autrod 12.51 wire with a diameter of 1.2 mm in a shielding gas (15% CO 2 and 85% Ar)