A New Observation Method of σ Phase in Austenitic Stainless Steels

In order to establish an appropriate process for observation of distribution of σ-phase particles in austenitic stainless steels, which enables observation of a large numbers of specimens in a short time, extraction replica methods combined with image-processing technique were explored. Obtained results are as follows:(I) For σ-phase in SUS316H steel, any of the following three extraction method works satisfactorily : (1) electrolytic etching by 10%acetylacetone-1%HCl-metanol followed by extraction by the same electrolyte ; (2) etching by 10%Br2 -ethanol solution followed by extraction by Villella's solution : (3) etching by Villella's solution followed by extraction by the same solution.(II) For σ-phase in SUS304H and SUS321H steels, the third method above is suitable.(III) For exclusive observation of the σ-phase particles from other coexisting M23C6, TiC and Laves phase particles, image-processing is useful.