THE SOLIDIFICATION AND WELDING METALLURGY OF GALLING-RESISTANT STAINLESS-STEELS

The autogenous welding behavior of two commercial galling-resistant au stenitic stainless steels, Nitronic 60 and Gall-Tough, was evaluated a nd compared. The solidification behavior and fusion zone hot-cracking tendency of the alloys was evaluated by using differential thermal ana lysis, Varestraint testing and laser spot-welding trials. Gleeble ther mal cycle simulations were used to assess the hot ductility of the all oys during both on-heating and on-cooling portions of weld thermal cyc les. Solidification microstructures were characterized by and the soli dification modes and phases light optical and electron microscopy, wer e identified. Gas tungsten are (GTA) welds in both alloys solidified b y the ferritic-austenitic mode, and their behavior was best described using chromium and nickel equivalents developed specifically for the N itronic series of alloys. Both alloys were found to be somewhat more s usceptible to solidification hot cracking than conventional austenitic stainless steels, although the cracking resistance of Nitronic 60 was somewhat superior to Gall-Tough. Laser spot-welding trials resulted i n both fusion and heat-affected zone cracking in the Nitronic 60, whil e Gall-Tough was resistant to cracking in these high-solidification-ra te welds. Comparison of the laser weld microstructures indicated that Nitronic 60 shifts to fully austenitic solidification, while Gall-Toug h shifts to an austenitic-ferritic solidification mode in high-energy- density processing. The hot ductility measurements indicated that Gall -Tough is generally superior to Nitronic 60 in both on-heating and of differences in grain size and the on-cooling tests, apparently as a re sult mechanism of ferrite formation at high temperatures.