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σ-phase precipitation in 347HFG stainless steel

Abstract The microstructural evolution of an 18Cr–12Ni stainless steel (347HFG), with a fine grain size (ASTM 7–10) has been investigated. In particular, σ-phase precipitation and growth have been evaluated. Results show that σ-phase is precipitated after as little as 1000 h aging at 750°C and M23C6 is present in all aged samples. The mean area fractions of σ-phase have been found to be 2·7 and 3·4% at 700 and 750°C respectively after 15 000 h aging. This is a higher area fraction of σ-phase than has been previously observed in standard 347H under these conditions. It is thought that the reduced grain size has provided an increase in nucleation sites and diffusion paths, therefore enhancing the precipitation of σ-phase. The results from this study are discussed with regards to the effect of precipitation on the service life of a 347HFG stainless steel tube operating in advanced supercritical boilers.