Predicting and Evaluating Ferrite Content in Austenitic Stainless Steel Castings

The amount of ferrite in a stainless steel casting depends not only on the average chemical composition but also to a great extent on the segregation of γ-formers and α-formers, which is controlled by the solidification and cooling rates in the mold, and hence on the thickness of the casting.

The study first demonstrates the combined influence of chemical composition and thickness on the ferrite content measured in stepped bars of 5- to 60-mm thickness made of steels with compositions similar to that of the French Z6 CND 18 12-M (or American CF8M) grade, leading to an improved empirical prediction formula taking thickness into account.

The second part of the study proposes a dilatometric method for ferrite measurement that offers the advantages of performing the measurement on a volume and of not being affected by phase morphology. The ferrite content is calculated from the measurement of the linear expansion coefficient of the specimen and prior knowledge of the respective expansion coefficients of the phases present (austenite and ferrite) which are related to the composition of each of these phases. On the basis of microprobe analyses and measurements of the expansion coefficients of single-phase specimens, we have prepared nomographs making it possible to predict these coefficients on the basis of the average composition.