Studies on activated flux tungsten inert gas electron beam and friction stir welding of SAF2507 super duplex stainless steel

Abstract

newline Duplex and Super Duplex Stainless steels have been developed to overcome the vulnarabilites faced by the widely used austenitic stainless steels such as pitting corrosion, stress corrosion cracking and lower tensile strength. Due to the presence of approximately equal amounts of ferrite and austenite phases in the duplex stainless steels, they offer excellent strength, resistance to pitting, stress corrosion cracking. However, while welding these alloys, care needs to be taken to maintain the austenite-ferrite phase balance to retain the advantages in terms of strength and corrosion resistance. These alloys are generally welded using fillers containing higher Nickel content to maintain good phase balance and properties. Welding of thicker plates requires multipass welding with suitable application of interpass temperatures making it costlier and cumbersome. Tungsten Inert Gas Welding using upto 8 multiple passes is the general industry practice. This work explores the possibility of producing single pass, full penetration, defect free welds on 6mm thick plates of Super duplex stainless steel SAF2507. Four different welding processes were chosen for the above task: two of them are fusion welding processes i.e Tungsten Inert Gas (TIG) welding and its variant Activated flux Tungsten Inert Gas (ATIG) welding; one high energy density process i.e Electron Beam Welding (EBW) and the last is a solid state welding process i.e Friction Stir Welding (FSW) process. In this work, process parameters were optimized using design of experiments and Taguchi method for TIG and ATIG processes, to obtain single pass full penetration welds. Process parameters considered for optimization study were welding current, Welding speed, Shielding gas flow rate and Arc gap. Regression equations have been developed to predict the depth of penentration for any given set of process parameters. The EBW and FSW joints were made using parameters evolved through a trial and error study to obtain defect free single pass full penetration