The Characteristics of Sigma Phase Precipitated in HK40-alloy

Abstract

Concerning the sigma phase which has precipitated in HK 40 heat resistant cast steel, a study has been carried out with the object of clarifying its properties, precipitation mechanism and the effect on material strength.
The results obtained are as follows.
(1) The sigma phase precipitating in HK 40 alloy has two types in its shape and chemical composition, i. e. the massive type and the “needle type”. And it seems that the two types of sigma have different precipitation mechanism.
(2) The massive type sigma phase precipitates in the P.F.Z. (precipitate free zone) around the primary carbide and grows up to about the same size as the primary carbide. On the other hand, the “needle type sigma phase” precipitates directly from the matrix.
(3) Both types of sigma phase reduce the material strength, especially the needle type sigma phase affects it sevesely. But the effect of massive type sigma phase doesn't appear unless it precipitates more than 2 or 3% in volume.