Multicriteria optimization of the weld characteristics of keyhole plasma arc welding on super austenitic stainless steel: a mathematical approach

The superaustenitic stainless steel AISI 304HCu tubes, preferable in crucial parts like reheater and superheater panels of Advanced Ultra-Super Critical (AUSC) boilers working at temperature range near 700 to 750 °C. The thickness of the tubes is to a minimum of 9 mm thickness which requires multiple passes by Tungsten Inert Gas (TIG) welding with excess addition of filler material. This leads to deterioration of mechanical properties due to the changes in their metallurgical features and their geometry. Keyhole Plasma Arc Welding (KPAW) a high energy density process wipes the issues of the traditional technique. To obtain a slender weld bead, the parameters of KPAW welding current, plasma gas flow rate and stand-off distance was optimized by Response Surface Methodology (RSM). The joint fabricated with optimized parameters resulted minimum bead width of 6.4 mm, maximum strength of 609 MPa and hardness of 228 HV. The improvement in the mechanical properties than base material is because of the formation of delta ferrite in the austenitic matrix formed by supercooling of stainless steel. The slenderness of bead is by the combined action of current, gas flow and standoff distance.