Corrosion Failure of Stainless Steel in Sensitized HAZ of Assembly Weld
Two aircraft-engine tailpipes of 19-9 DL stainless steel (AISI type 651) developed cracks along longitudinal gas tungsten arc butt welds after being in service for more than 1000 h. Binocular-microscope examination of the cracks in both tailpipes revealed granular, brittle-appearing surfaces confined to the HAZs of the welds. Microscopic examination of sections transverse to the weld cracks showed severe intergranular corrosion in the HAZ. The fractures appeared to be caused by loss of corrosion resistance due to sensitization, that could have been induced by the temperatures attained during gas tungsten arc welding. Tests demonstrated the presence of sensitization in the HAZ of the gas tungsten arc weld. The aircraft engine tailpipe failures were due to intergranular corrosion in service of the sensitized structure of the HAZs produced during gas tungsten arc welding. All gas tungsten arc welded tailpipes should be postweld annealed by re-solution treatment to redissolve all particles of carbide in the HAZ. Also, it was suggested that resistance seam welding be used, because there would be no corrosion problem with the faster cooling rate characteristic of this technique.