The influence of martensitic microstructure and oxide inclusions on the toughness of simulated reheated 10 wt% Ni steel weld metal multi-pass fusion zones

Abstract

The influence of the effective grain size of the martensitic microstructure and the presence of oxide inclusions on the toughness of a novel high strength, high toughness 10 wt% Ni steel weld metal was investigated. Previously, it was determined that welds produced with the gas tungsten arc welding (GTAW) process exhibited superior toughness to those produced using gas metal arc welding (GMAW), and differences in the martensitic microstructure and oxide inclusion content were identified between the two welds. To elucidate the effect of these two microstructural constituents on toughness, multi-pass weld reheat simulations were performed using a Gleeble 3500 thermal-mechanical simulator designed to produce identical martensitic microstructures in GTAW and GMAW specimens. The GMAW reheat specimens contained a large presence of oxide inclusions from the use of a 98% Ar/2% O2 shielding gas used during welding, whereas the GTAW specimens exhibited a smaller quantity since 100% Ar was used as the shielding gas. These reheat experiments demonstrate that even when both welds have a fine martensitic microstructure, a known toughening mechanism, the toughness of the GMAW is still significantly lower than the GTAW. Thus, the oxide inclusions are the main influence in the lower toughness of the as-welded GMAW, and microstructural refinement is the secondary influence. However, the superior toughness of the GTAW is not only from the lower quantity of oxide inclusions, because when the effective grain size of the GTAW is coarse, the toughness is very low. Thus, both influences are necessary for high toughness of the as-welded GTAW. These results are significant in that they demonstrate the necessity of developing an oxygen-free shielding gas to improve the toughness of welds produced with the GMAW process. The results also now allow for welding procedures to be developed in such a way to avoid low toughness regions in welds produced with both processes, based on the scientific foundation that has been laid here.