Influence of cooling rate on δ-ferrite/γ-austenite formation and precipitation behavior of 18Cr–Al–Si ferritic heat-resistant stainless steel

The δ-ferrite/γ-austenite formation and precipitation behavior of 18Cr–Al–Si FHSS (ferritic heat-resistant stainless steel) during solidification has been investigated by a high temperature confocal scanning laser microscopy (CSLM) at the cooling rates from 5 °C min−1 to 100 °C min−1. The results have demonstrated that the increase of cooling rate will decrease nucleation temperature of δ-ferrite phase, which will affect δ-ferrite/γ-austenite formation process, and the δ→γ transformation process all occurred at the texted cooling rates. The γ-austenite phase first precipitated at δ-ferrite phase boundaries and then appeared within δ-ferrite phase, and in the subsequent cooling process, part of the γ-austenite begins to transform into martensite. The content of γ-austenite/martensite phase and M23C6 carbides increased with the increment of cooling rate. In addition, the solidification mechanism of the complex nucleus at the cooling rates of 5 °C min−1 and 100 °C min−1 were also discussed.