Weldability of direct-quenched and tempered (DQT) high strength steel HT80

The report gives and briefly discusses the test results obtained in welding and weldability tests of a direct-quenched and tempered steel DQT HT80. The tests comprised of mechanical testing of joints welded with arc energies of 20 and 45 kJ/cm using SAW and FCAW processes. Also the results of weld thermal simulation and Implant weldability tests for the steel are reported and discussed. The results demonstrate the improved weldability of the DQT HT80 steel, as compared to conventional QT steels of similar strength level. The need for preheat was found 70-90°C lower for the DQT steel. Cracking of weld metal instead of HAZ, however, suggested that increasing preheat and interpass temperatures even up to 150°C become necessary with heavier plate thicknesses of 40 mm and above. With both arc energies (20, 45 kJ/cm), SAW yielded higher weld metal toughness than FCAW. In SA-welds, impact energy constantly and considerably exceeded the required level of 40 J at -40°C. This was not the case with FCA weld metals, as in the majority of welds the impact energy level of 40 J at -40°C was hardly met. The HAZ toughness properties of DQT HT80 steel were found to deteriorate as the arc energy increased from 20 to 45 kJ/cm. The Charpy-V impact energy requirement of 40 J was fulfilled at -40°C in all weldments with an arc energy of 20 kJ/cm, whilst none of the weldments qualified with an arc energy of 45 kJ/cm. The Charpy-V tests of simulated HAZ confirmed the tendency of decreasing HAZ toughness with prolonged cooling times. Whilst adequate impact toughness values were obtained with t8/5 < 24 sec, a deterioration in toughness was evident with t8/5 > 30 sec.