| An investigation was performed to determine the effects of alloy composition and heat treatment on the physical metallurgy and welding metallurgy of several 18% and 28% Cr ferritic stainless steels. Results indicate that welding produced wholly ferritic structures in all regions and caused dissolution of all second phases in the fusion zone and coarse-grained heat-affected zone of the alloy containing 28% Cr - 5% Mo, which had been heat treated to produce sigma and chi phases.;Equiaxed-dendritic grains formed near the center of welds in several alloys. It is likely that dendrite fragmentation was the source of nuclei. However, the formation of equiaxed grains was accompanied by the formation of shrinkage cavities surrounding individual grains and these cavities negate any improvement in mechanical properties.;Results of Subscale Varestraint tests reveal that VIM alloys are very resistant to hot-cracking because the melting method produced very low levels of C, N, P, and S. Hot-cracking occurred in all AOD-refined 18% Cr - 2% Mo alloys, and was attributed to segregation of the stabilizing elements niobium and titanium, along with sulfur, to intercellular regions. Most other elements analyzed (Mo, P, Al, Ni, Mn, Si) segregate to interdendritic volumes during solidification. Hot-cracking occurred in all eleven alloys welded with argon - 6% nitrogen shielding gas. This was caused by the formation of a ferritic-austenitic structure in the fusion zone, and hot-cracking occurred along the ferritic-austenitic interface.;The VIM alloys had the greatest weld depth of penetration, with depth-to-width ratios nearly twice that of the AOD-refined alloys. This difference was attributed to the greater levels of surface-active oxygen in the VIM alloys, and to niobium, titanium and other elements in the AOD-refined alloys which deoxidized the liquid weld metal, thus removing surface-active oxygen. A sample with an oxidized surface exhibited a doubling of weld penetration which was caused by dissolution of the iron oxide scale, which more than doubled the oxygen content of the fusion zone. |
