Effects of cooling rates on δ-ferrite/γ-austenite formation and martensitic transformation in modified ferritic heat resistant steel

Aimed at clarifying δ-ferrite formation in fusion welding of ferritic/martensitic steels, effects of cooling rates on δ/γ transformation and martensitic transformation in modified ferritic heat resistant steel are investigated by high-speed quenching dilatometer equipment and microstructure examination. In heating stage, γ-austenite starts to transform into δ-ferrite at 1250 ± 20 °C. With respect to cooling stage, at cooling rate of 1 °C/s, γ → δ transformation takes place at temperatures above 1250 °C. Stripe-shaped δ-ferrite is produced at 1 °C/s, and under higher cooling rates (30, 50 and 200 °C/s), δ-ferrite exhibits block morphology. The different levels to which the δ → γ transformation proceeds would result in the variation in austenite grain size and the amount of δ-ferrite, affecting the subsequent martensitic formation. Effects of cooling rates on martensite start temperature have been discussed in terms of austenite grain size and austenite chemical stability.