The influence of the oxygen equivalent in a gas-mixture on the structure and toughness of microalloyed steel weldments

Testings were carried out on two steels. The first was microalloyed with Nb and second with Ti, Nb and V. The impact toughness of weld metals of these steels was evaluated using an instrumented Charpy pendulum. Five different gas mixtures (Ar, CO2, O2) were used to determine the optimal gas shielded metal arc process for both steels. The oxygen equivalent was used as a representative parameter of a mixture to follow, in particularly, its effect on the microstructure, toughness and crack propagation energy of the weld metal. For these investigated steels, the optimum gas mixture was established (5 % CO2, 0.91 % O2, balance Ar), which provided the maximum crack propagation energy, due to the microstructurewhich consisted dominantly of acicular ferrite