Microstructure and mechanical properties of gas shielded arc welded joints of hot stamping steel

Objective The purpose was to study microstructure and mechanical properties of gas shielded welded joints of 1 500 MPa hot stamping steel. Methods NB-350 IGBT inverter CO₂ gas shielded arc welding machine was used to carry out welding test on uncoated hot stamping steel and Al-Si coated hot stamping steel, and microstructure, strength, hardness and fracture morphology of welded joints were analyzed. Results The results indicated that weld’s microstructure of both welded joints were composed of ferrite and bainite, and there were more ferrite in the weld of Al-Si coated hot stamping steel plate. Microstructure of heat affected zone (HAZ) of the two kinds of welded joints were similar, HAZ above A_c3 temperature was composed of martensite, and HAZ between A_c1 and A_c3 temperature was composed of ferrite and tempered martensite. Variation law of vickers hardness of welded joints was that hardness of weld was the lowest, hardness reached the highest in HAZ above A_c3 temperature, and hardness of the two-phase zone was between that of weld and base metal. Welded joints of the two kinds of hot stamping steel with ER50-6 welding wire were undermatching, strength of welded joints was only about 1/2 that of base metal. Fracture of welded joints was a mixture of brittleness and toughness, and fusion zone was the weakest position of welded joints. Conclusion Due to relatively high content of soft ferrite in the weld of Al-Si coated hot stamping steel, its strength (805 MPa) of welded joints was lower than that (866 MPa) of welded joints of uncoated hot stamping steel.