| The weldability of two commercial ultra-low carbon bainitic steel,HQ500DB and HQ620DB offered by Anshan steel was evaluated using acombination of calculation of cracking susceptibility index Pcm,restrained Y-groove depositing test, HAZ maximum hardness evaluation test.The microstructure and mechanical property examinations on the Gleeblesimulated coarse -grain heat-affected zones (CGHAZ) of universal arcwelding, as well as, high power energy laser welding and thegas-metal-arc-welded (MIG) joint was also carried out.It is shown that these two steels had a low hardenability andcold-cracking susceptibility during welding under a preheatingtemperature of 50℃ or above and a large restraint. When a energy inputas low as 14-25 kJ·cm-1 was imposed, the CGHAZ was mainly composed of finelath-like bainitic ferrite pockets with different directions inoccupation each other, a considerable amount of retained austeniteinterlayer and a small amount of M-A constituent in the prior austenitegrain, which resulted in a tough microstructure with high resistance tobrittle cracking. Excellent mechanical properties can be obtained byusing YS800 welding wire as the filler metal and 18 kJ·cm-1 or so as theheat input energy. The high power laser welding is suitable for this typeof steel for the reason a tougher and a little harder HAZ could be obtained,as well as, the maximum hardness and the steep harness distribution couldbe soothed by both preheat treatment and laser tempering as an accessorialoperation.
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