Microporosity formation in partially melted zone during welding of high nitrogen austenitic stainless steels

Gas tungsten arc welding experiments were conducted using austenitic stainless steel containing 0.51%N and 0.78%N. Microstructure observation and hardness measurement were made to study the loss of nitrogen. It was found that welding resulted in a considerable loss of hardness in the weld metal for the case of 0.78%N steel, but not for 0.51%N steel. We explain this in terms of a higher nitrogen content enabling a significantly smaller critical pore size and hence N2 porosity formation to be energetically more favourable. The major finding was that, for the case of 0.78%N steel, a band of microporosity was observed along and near the complete fusion boundary of the weld. It was identified that these micropores were present in the parent metal, not in the weld metal. Partial melting in the zone next to the complete fusion boundary resulted in a nitrogen content significantly higher than the solubility of nitrogen in the liquid channels or pockets. Nitrogen gas pores then formed and became trapped in that zone. Supporting this forming mechanism of microporosity band was the fact that hardness decreased in that zone due to the loss of nitrogen in γ phase matrix for solute strengthening and that nitride particles disappeared after welding.