Martensite formation in Fe–9Cr alloys exposed to low carbon activity gas

Model Fe–9Cr alloys were exposed to Ar–20CO2 at 799 and 818 °C, then quenched in argon. Binary Fe–9Cr formed Cr-rich carbides at 799 °C and a mixture of carbide, ferrite and martensite after reaction at 818 °C. Addition of 2 wt.% Mn to Fe–9Cr led to 100% martensite formation after carburization at 818 °C. Dilute cerium additions also increased martensite formation significantly. These results are consistent with calculated phase diagrams and a carbon activity related to oxide-alloy local equilibrium.