This paper presents the effect of magnesium on the morphology of carbides and on M7C3 → M23C6 (M = Cr and/or Fe) carbides transformation in the 21-4N heat-resistant steel. 21-4N heat-resistant steel was calculated by using the FactSage. The equilibrium carbide phase is M23C6 and is stable up to 1492 K in Mg-added 21-4N heat-resistant steel. The studies involve a wide range of experimental techniques to characterize the investigated steels in the as-cast and hot-rolled state, including X-ray diffractometer, confocal scanning laser microscope (CSLM), scanning electron microscope, transmission electron microscopes (TEM) and microhardness testers. In the as-cast and hot-rolled samples with different magnesium contents, M7C3 was detected due to nonequilibrium solidification, while the carbide types are M23C6 in the prediction from equilibrium phase diagram. Lamellar structure in the as-cast sample is composed of FCC austenite and M23C6. And blocky shape carbide M7C3 is wrapped by the lamellar structure M23C6.
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