Influence of shielding gases on grain refinement in welds of stabilized 21 % Cr ferritic stainless steel

The influence of shielding gas mixtures of argon, nitrogen, and oxygen was experimentally investigated in order to evaluate the possibilities of grain refinement in autogenous welds of stabilized 21 % Cr ferritic stainless steel heats that would otherwise solidify in a columnar fashion. The main focus was on metallography and weld metal compositions, but some mechanical and corrosion properties were also evaluated. According to the findings, substantial grain refinement can be achieved by using nitrogen plus oxygen additions to the argon shielding gas in gas tungsten arc welding. Nitrogen addition alone was insufficient because oxygen was needed to produce oxide inoculants for nitrides and a uniform slag to protect the solidifying weld pool surface from nitrogen outgassing. Excessive nitride precipitation in ferrite deteriorated some of the properties, but an interesting finding in some of the welds was the presence of retained austenite instead of martensite. However, laser welding experiments did not bring about similar features in the weld metals. Apparently, nitrogen absorption is too modest to have an effect in disk laser welding even when using a non-keyhole welding mode.