Decomposition behavior and evolution mechanism of MC primary carbonitride at 1473K and 1523K in die steel

Abstract

Evolution of morphology and chemical composition and decomposition behavior of MC-type primary carbonitride held at 1473K and 1523K in H13 die steel are studied through electron probe microanalyzer (EPMA) and optical microscope (OM). Result shows that the decomposition of MC-type carbonitride has a relationship with constituent, holding temperature and time and carbonitride structure. It is found that Fe would diffuse to carbonitride from steel matrix accompanied by the decomposition of carbonitride at elevated temperature. The metastable (Vx,Mo1–x)(Cy,N1–y) is transformed into fine Fe-rich (Vx,Moz,Fe1–x–z)6(Cy,N1–y) when held at 1473K, and completely disappeared when held at 1523K. During isothermal holding, the mean size of carbonitride is gradually decreased to 6.5 μm, but there are still a small quantity of (Tix,V1–x)(Cy,N1–y) when held at 1253K for 15 h. Theoretical calculation indicates that Ti content in carbonitride is directly proportional to the thermal stability of carbonitride. As a result, the (Tix,V1–x)(Cy,N1–y) is transformed into (Tix,Vz,Fe1–x–z)(Cy,N1–y) with Fe content larger than 13 wt.% and becomes jagged at the edge. The compact (Tix,V1–x)(Cy,N1–y) is then evolved into the porous fishnet structure and finally dissolved as Fe larger than 32 wt.% in carbonitride. High temperature seems more beneficial for carbonitride decomposition than extension of holding time.