Abstract
18Cr-9Ni-2Mo and 20Cr-10Ni-3Mo type 316 weld metals containing FN6 and FN 18 δ-ferrite, heat treated at 850° and 625°C, have been investigated by microstructural and microanalytical techniques to provide a detailed study of the decomposition of the ferrite phase. As welded, the composition of the ferrite is enriched in chromium and molybdenum and depleted in nickel to such an extent that the ferrite composition is close to that of χ and σ intermetallic phases. On heat treatment at 850°C, the ferrite readily transforms to intermetallic phases with a subsequent enrichment of chromium and molybdenum in these phases. Aging at 625°C gives incomplete transformation to intermetallic and carbide phases, and the retained ferrite composition has been observed to be depleted in chromium and molybdenum compared with the as-welded ferrite composition. As a result of these observations on the 18Cr-9Ni-2Mo steel, a mechanism for the prevention of intermetallic-phase precipitation in the ferrite phase is suggested, based on the reaction δ → M23C6 + γ. The precipitation of carbides enriched in molybdenum and chromium causes a depletion of these elements in the ferrite which is then either retained or able to transform to austenite on further aging.
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