Characterization of fusion boundary between low alloy steel and stainless steel cladding

The microstructure and hardness of fusion boundary (FB) and near FB of a low alloy steel (LAS) 16MND5 welding cladded with stainless steel 308L for nuclear reactor pressure vessel were investigated by using modern electron microscopes and nano-indenter. The wavy morphology of fusion line results in changes of nearby microstructures of both cladding 308L and substrate 16MND5 along the FB. The microstructure of the cladding 308L is composed of columnar grains adjacent most of the FB, but it is equiaxed grains at the valley of the wavy fusion line with large curvature. The microstructures of 16MND5 near the FB are ferrite at the wavy fusion line with large curvature, martensite at the wavy fusion line with small curvature, and transition mixed structure between them. A carbide-depleted zone adjacent to the FB was observed in the 16MND5 side, of which the hardness is lower than both FB and 16MND5 base metal. Three types of FB can be observed according to the width, being narrow or sharp FB, needle-like martensite FB, and partially-mixed zone FB. The FB includes high density of carbides of M7C3 and M3C, leading to a hardness peak at FB.