The evolution of MC-type primary carbonitrides (M=V, Ti, Mo; C=C, N) in terms of morphology, quantity, size and composition was systematically investigated in commercial H13 die steels with different Ti and N contents during thermal holding at 1250 °C for 5 h to 15 h. Results showed that the mean size and quantity of carbonitrides in the four samples had decreased during thermal holding. However, the mean size and quantity of MC carbonitrides had increased with increasing Ti contents when held at 1250 °C while the addition of N increased the quantity but decreased the sizes of the stable MC carbonitrides. It was concluded that the compact carbonitrides could be decomposed and changed into a fishnet structure when held at 1250 °C, especially in samples #1 and #2 containing lower Ti and N contents. The decomposition mechanism was illustrated considering the changes in Ti and Fe elements in carbonitrides. On the basis of the thermodynamic model, the thermal stability of (Ti_x,V_1−x)(C_y,N_1−y), with a larger x value, in samples #3 and #4 containing more Ti and N contents was generally higher than those in samples #1 and #2. To control the Ti-containing MC carbonitrides, the low Ti and N contents and high holding temperature should be taken into consideration.

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 (V_x,Mo_1–x)(C_y,N_1–y) is transformed into fine Fe-rich (V_x,Mo_z,Fe_1–x–z)₆(C_y,N_1–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 (Ti_x,V_1–x)(C_y,N_1–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 (Ti_x,V_1–x)(C_y,N_1–y) is transformed into (Ti_x,V_z,Fe_1–x–z)(C_y,N_1–y) with Fe content larger than 13 wt.% and becomes jagged at the edge. The compact (Ti_x,V_1–x)(C_y,N_1–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.