Interface microstructure across cladding of super duplex stainless steel with austenitic stainless steel buffer layer

The evolution of microstructure and texture across the fusion boundaries of a cladding produced by gas tungsten arc welding process was examined by optical microscopy, X-ray diffraction, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy. The cladding comprised of high strength low alloy steel substrate, austenitic stainless steel buffer layer, and super duplex stainless steel top coating. Results indicated that the heat affected zone of the substrate mainly consisted of coarse polygonal ferrite grains. The transition zone, which was separated from the buffer layer by Type II boundaries, was comprised of a narrow austenitic-martensitic band. The microstructure of the buffer layer was characterized by vermicular ferrite which was replaced by lathy morphology adjacent the top coating. The buffer layer grains had <001>║ND, i.e. normal direction, orientation near their interface with the substrate. The top coating was separated from the buffer layer by a distinct epitaxially grown austenitic layer. In none the cladding layers, the texture of the austenite was as strong as that of the ferrite. While in the buffer layer, the austenite grains had close Nishiyama-Wasserman orientation relationship with respect to the parent ferrite, Kurdjumov-Sachs relationship was observed across the top coating. Both the ferrite and austenite phases had significant fraction of low angle grain boundaries across the cladding, but the mount of austenite grain boundaries with high angle grain boundaries with low Σ special character was higher. Moreover, the austenite showed higher levels of stored energy in both layers. The total fraction of low Σ boundaries was also higher in the top coating compared to the buffer layer.