Influence of austenitization temperature on phase transformation features of modified high Cr ferritic heat-resistant steel

The phase transformation features of high Cr ferritic heat-resistant steel have been investigated using Differential Thermal Analysis (DTA) in accordance with different austenitization temperatures as 950 °C, 1050 °C, 1150 °C, 1200 °C and predicted by Thermo-calc calculation. The results based on DTA curves on heating showed that the magnetic transition and the Ac1 temperature occurred at 744.9 °C and 850.9 °C, respectively. The measured Ac3 temperature was around 919.5 °C, the obtained austenitization temperature range was about 69 °C. Two new phenomena were found: firstly, the onset temperature of martensite transformation increases strongly and then not obviously with the increase of austenitization temperature; secondly, the δ-ferrite abnormally forms in the specimen austenitized at 950 °C due to the existence of un-dissolved M23C6 particles.

► Ms firstly increases then decreases with increasing austenitization temperature.
► δ-Ferrite abnormally forms austenitized at 950 °C.
► Ms as a function of austenite grain size based on kinetics model.