TWI Industrial Member Report Summary 462/1992
By B J Ginn
Background
High alloy austenitic stainless steels containing circa 20%Cr, 18%Ni, 6%Mo and 0.2%N (eg UNS S31254) are of interest for many corrosion resistant applications, due to their ability to resist pitting, crevice and stress corrosion cracking. However, weld. metals produced autogenously or with matching composition filler are susceptible to preferential corrosion. To overcome such attack, Ni-based fillers, which are overalloyed especially with respect to Mo (ie 9%), are commonly used.
For many applications the use of autogenous welding, particularly in making the root pass, would simplify welding procedures. It has been previously established, in autogenous TIG bead on plate trials, that the addition of small amounts of nitrogen to the inert shielding gas results in an enhancement of pitting corrosion resistance. Low arc energies were also found to be beneficial in this respect. Before these results can be generally applied, it is necessary to confirm that they are also valid for a practical welding situation. Accordingly a comparable series of butt welds in tube has been produced and assessed.