This study aims to reduce the diffusible hydrogen content in deposited metal during gas metal arc welding (GMAW) and flux-cored arc welding (FCAW) which induces cold cracking. To achieve this, a novel welding torch with a dual gas nozzle has been developed. This special welding torch decreases the hydrogen source gas evaporated from a welding wire by the suction from the inner gas nozzle. In order to improve the suction efficiency of this evaporated gas, precise control of the suction gas flow is indispensable. In this paper, a simplified numerical simulation model of this process has been described. This model can take account of the evaporation of the hydrogen source gas from the wire while simulating the behavior of the shielding gas and the arc. Using this model, the effect of suction nozzle structure and torch operating conditions on suction gas flow pattern and suction efficiency was also investigated to understand the process mechanism. Furthermore, the diffusible hydrogen content in deposited metal was measured by chromatography as a validation step. Results show that some of the shielding gas introduced from a shielding nozzle was drawn inward and also branched into an upward flow that was sucked into the suction nozzle and a downward flow to a base metal. This branching height was defined as the suction limit height, which decisively governed the suction efficiency. As a result, in order to reduce the diffusible hydrogen, it was suggested that the suction limit height should be controlled towards below the wire position, where the evaporation rate of the hydrogen source gas peaks through optimization of the suction nozzle design and the torch operating conditions.

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Cold cracking is a serious concern in the welding of large-scale steel structures. And, diffusible hydrogen is a major factor affecting to it. Several general techniques are known for preventing cold cracking, however there are no examples of approach in the past from welding apparatus side. The new welding process was developed to reduce diffusible hydrogen content in the weld metal with special welding torch in GMAW and FCAW. Furthermore, the factors were researched that affecting the effectiveness of the new welding process. The special welding torch was designed by calculation results of wire temperature distribution and analysis results of hydrogen source behavior during welding. It slightly sucks the gas around the wire through the suction nozzle. The effectiveness of developed process was showed by comparative experiment with conventional process, namely confirmed the correctness of the structural design. Meanwhile, it was revealed that wire structure (i.e. seamed or seamless) affects strongly the effectiveness of the process. This process was very effective with seamed flux cored wire.

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