Effect of Al content on the δ→γ transformation and M23C6 precipitation behavior in 18Cr-xAl-Si ferritic heat-resistant stainless steel during solidification process

Abstract

Aimed at clarifying δ→γ transformation and M23C6 precipitation behavior in18Cr-xAl-Si (x = 0, 0.43wt.%, 0.91wt.% and 1.82wt.%) ferritic heat-resistant stainless steel (FHSS), high temperature confocal scanning laser microscopy (CSLM) was used for in-situ observation during solidification. The results demonstrated that increasing of Al content reduced the nucleation temperature of δ-ferrite, and at the same time reduced the free energy difference between δ-ferrite and γ-austenite, which increased the difficulty of δ→γ transformation, in turn reduced the temperature of δ→γ transformation. The γ-austenite preferentially precipitated at the δ-ferrite phase boundaries and gradually along the sides of the δ-ferrite phase boundaries as the temperature decreased, and part of the martensite transformed inside the γ-austenite phases. M23C6 carbides mainly precipitated at the phase boundaries between δ-ferrite and γ-austenite/martensite. With the increase of Al content, the amount of γ-austenite/martensite and M23C6 carbides decreased. In addition, the solidification mechanism of the complex nucleus was also discussed.