TWI Industrial Member Report Summary 622/1997
By P Woollin
Background
Austenitic stainless steels have excellent toughness, ductility and corrosion resistance. The highest alloy grades currently available, sometimes referred to as "superaustenitics", contain up to 7-8% molybdenum and 0.5% nitrogen. Production of mechanically sound joints in the superaustenitic alloys is straightforward but obtaining weld area corrosion resistance matching that of the parent steel may be problematic. Causes of poor corrosion resistance include (i) molybdenum segregation in weld metal and fusion boundary unmixed zones, (ii) intermetallic phase formation, and (iii) loss of nitrogen from the weld pool. Previous work has shown that the established 6% molybdenum superaustenitic grades typically require use of overalloyed filler to obtain acceptable corrosion resistance. However, work at TWI has shown that more recently developed grades with 4.5% and 7.5% molybdenum and 0.4 to 0.5% nitrogen may be welded autogenously at low arc energy to give acceptable corrosion performance. The present work was undertaken to examine the corrosion resistance of TIG weldments made with overalloyed filler in two of the recently developed superaustenitic stainless steels with 0.4-0.5% nitrogen, i.e. UNS S32654 (7.5%Mo, 0.5%N) and S34565 (4.5%Mo, 0.4%N).
Objective
To quantify and optimise the corrosion resistance of weldments in new high nitrogen superaustenitic stainless steels made using the TIG process with addition of overalloyed nickel-based filler metal.