Effect of magnesium on the morphology of carbides and on M ₇ C ₃ →M ₂₃ C ₆ carbides transformation in the 21-4N heat-resistant steel

Abstract

This paper presents the effect of magnesium on the morphology of carbides and on M 7 C 3 → M 23 C 6 (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 M 23 C 6 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, M 7 C 3 was detected due to nonequilibrium solidification, while the carbide types are M 23 C 6 in the prediction from equilibrium phase diagram. Lamellar structure in the as-cast sample is composed of FCC austenite and M 23 C 6 . And blocky shape carbide M 7 C 3 is wrapped by the lamellar structure M 23 C 6 .