High-temperature phase transformations in strongly metastable austenitic-martensitic CrMnNi-N-C cast steels

The influence of carbon on the melting behavior and solid-solid state phase transformations in highly alloyed cast stainless steels (Fe-15Cr-3Mn-3Ni-0.1N-(0.05-0.25)C) was studied using differential thermal analysis (DTA). The temperatures of the transformations from α′-martensite to γ-austenite and from γ-austenite to δ-ferrite as well as the solidus temperatures were determined. The investigations using scanning electron microscopy combined with energy dispersive X-ray spectrometry (SEM/EDX) showed that at high carbon content (0.25 mass.%) the character of crystallization changes from ferritic-austenitic to austenitic. According to the thermodynamic calculations, the character of the crystallization should be ferritic-austenitic for all studied alloys. The calculated temperature of the α′ to γ transformation is in reasonable agreement with the DTA results, but only in the two samples with the lowest C content α′-martensite fraction was sufficiently large to detect this transformation predicted by calculations. Both, γ to δ transformation temperatures and solidus temperatures were lower in the calculation than experimentally measured. The probable reasons for the inconsistency between the results of calculations and experiments are discussed.