Effect of nitrogen content on solidification behaviors and morphological characteristics of the precipitates in 55Cr17Mo1VN plastic die steel

Abstract

The effect of nitrogen content on as-cast microstructure, particularly the precipitates (M7C3, M2(C, N), Fe3C) of 55Cr17Mo1VN plastic die steel was investigated. It's found that higher nitrogen content would enlarge the solidification region, and increase the area ratio of precipitates. The eutectic M7C3 carbides and M2(C, N) carbonitride are mainly distributed in the interdendritic region. The eutectic M7C3 carbides mainly possess coarse (M7C3-C) and lamellar (M7C3-L) morphologies. With increasing nitrogen content from 0.16% to 0.31%, the M7C3 carbide transforms from M7C3-C to M7C3-L and promotes M2(C, N) carbonitride precipitates. Based on Bramfitt's two-dimensional lattice misfit theory, Al2O3 inclusion is the very potent heterogeneous nucleation site for the formation of M2(C, N). In contrast, M2(C, N) could be a poor heterogeneous nucleation site for M7C3. Eutectoid transformation γ-Fe → α-Fe + Fe3C occurred in 0.23% and 0.31% nitrogen content samples when solidifying, which caused the outer layer carbides in lamellar morphology. The excessive precipitates weaken the austenite stability and eventually promote martensite transformation.