Effect of Shield Gas Composition on Cold Cracking Susceptibility of YS 600 MPa Flux Cored Arc Weld Metal

Cold cracking is one of the most common catastrophic defects in welds. In this study, we investigated the effect of the shield gas composition (specifically Ar+5 similar to 20% CO2) on the cold-crack ratio on the preheating temperature of 20 degrees C similar to 100 degrees C in flux-cored arc welds. Critical assessment of the cold-crack ratio was achieved by observing the weld cross-section, appearance of surface cracks, and fractography. The cold-crack ratio decreased as the preheating temperature increased from 20 to 100 degrees C. For the preheating temperature of 50 degrees C, the shield gas (containing 20% CO2) provided higher oxygen content in the weld metal and increased the number of nucleant inclusions for acicular ferrite; thereby eliminating cold cracks. The shield gas with 15% CO2 produced welds that had nearly the same amount of acicular ferrite as compared with the 5% and 10% CO2 cases. However, the 15% CO2 shield gas produced the greatest hardness and largest amount of non-nucleant inclusions in the weld metal, therefore exhibiting the largest cold-crack ratio. If the volume fraction of acicular ferrite remains unchanged, hardness and non-nucleant inclusions should be considered roughly for assessment of the cold-crack ratio.