The weldability assessment of thermally hardened S540Q steel sheets

High strength thermally hardened steels are more difficult to weld than low carbon construction steels. There is a danger of cold cracking caused by the presence of martensite in welded joints. The thermally hardened 12 mm sheets of S540Q steel underwent research. The sheets were welded semi-automatically by MAG method, in the welding conditions usually applied in industry for welding normalized sheets of this steel. The welding technology was presented. The X-ray tests were performer, which confirmed the achievement of metallically continuous joints. Samples were taken from the joints and the joints hardness distribution measurements were conducted. The microstructure was examined and the static tensile tests as well as technological bend tests were carried out. Hardness measurements were performed in three measurement lines for each joint according to PN-EN 1043-1:2000 requirements. Joints microstructures were examined in the joints axis, in HAZ and in native materials. Steel microstructures were classified as low carbon tempered matrensite of layered construction. Mean values of the selected joints tensile strengths were also presented. The tensile tests values of native materials of metallurgic certificates were given for the sake of comparison. The research results presented in the paper made it possible to determine the weldability of S540Q steel.