Effect of Mn and Ni on the variation of the microstructure and mechanical properties of low-carbon weld metals

Low-carbon weld metals with various amounts of Mn and Ni addition were made using metal-cored wires and Ar-2%O-2 shielding gas, and their mechanical properties were evaluated. The objective of the research, aimed to develop welding consumables with better resistance to cold cracking, was to determine the optimum composition ranges of Mn and Ni, in the presence of carbon content less than 0.02 %. The hardness of weld metals were found to increase linearly with Mn and Ni, which was attributed mainly to solid solution strengthening and in part to formation of hard phases. Varying Ni content influenced Charpy impact energy, the extent of which depended on Mn content. For a low-Mn composition, Ni addition increased hardness without sacrificing impact toughness whereas for a high Mn composition, Ni deteriorated the impact toughness seriously and caused intergranular fracture. The fracture path followed columnar grain boundaries that are identical to prior austenite grain boundaries since no delta -ferrite phase formed during solidification. Accordingly, these boundaries without having ferrite phase were susceptible to cracking under dynamic loading. Based on hardness and impact resistance, the optimum levels of Mn and Ni were suggested to be 0.5-1 % and 4-5 %, respectively.