Effect of Nitrogen‐Substituted Carbon on Thermal Stability of Cr‐Mo‐V Hot‐Working Die Steel

Abstract

To extend service life of Cr‐Mo‐V hot‐working die steel, a method of alloying with nitrogen‐substituted carbon is proposed. The mechanical properties of the test steels in the long‐time heating and holding process are measured. The effect of nitrogen‐substituted carbon on thermal stability is characterized and analyzed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Thermo‐Calc. The results show that the ratio of N in V(C,N) can be increased by nitrogen‐substituted carbon at high temperature, which significantly improves its stability. As a result, it can not only refine the grain but also broaden the austenitizing temperature range and promote the complete solution of alloy elements such as Cr, Mo. In the process of long‐term heating and insulation, nitrogen‐substituted carbon can effectively reduce the precipitation of harmful carbides and the coarsening rate by raising the activation energy of tempering transformation with 20.0 kJ mol−1, and increase the precipitation of the main strengthening phase V(C,N). Nitrogen‐substituted carbon, as an effective alloying method, innovatively improves thermal stability and service life of Cr‐Mo‐V hot‐working die steel.

Softening Behavior and Hardness Prediction of H13 Hot‐Work Die Steel during Isothermal Tempering

“…The LSW model [8,25] based on the Ostwald ripening mechanism was used to describe the evolution of the mean radius of secondary carbides during isothermal tempering, as defined by Equation ( 5):…”

“…As shown in Figure 2, input layer (2) included all the input factors. Subsequently, the information in the input layer was processed in the course of hidden layers (8) and then computed in the output layer (1). The collected database was normalized to prevent the possible influence of large deviations among the temperature, time, and hardness.…”

“…The values of lnD were calculated as -20.82, -7.62, -4.07, and -1.29 for tempering temperatures of 500, 550, 600, and 650 °C, respectively. Based on Equation (8), the average values of Q for the JMA and LSW model were calculated as 250 and 281 kJ mol À1 . In addition, the hardness for different heat-treatment conditions was accurately predicted by the JMA model, as shown in Figure 10d.…”

“…The JMA model was constructed according to the theory of solid-state phasetransition diffusion proposed by Johnson and Avrami, [7] which was based on the diffusion-driven mechanism of precipitation and growth of secondary carbides. The LSW model based on the Ostwald ripening mechanism [8,9] described the dissolution, precipitation, and coarsening of secondary carbides at a constant volume fraction, and related the softening behavior to the evolution of the average size of secondary carbides. However, the aforementioned three models describing the softening behavior have not been compared in detail, especially the applicability of the models.…”

Nitrogen Effects on Microstructural Evolution, Fracture Behavior of Cr–Mo–V Hot‐Working Die Steel during High‐Temperature Deformation at 700 °C

“…In the previous work, newly developed nitrogen-alloyed and nitrogen-substituted carbon hot-working die steel with high room-temperature strength were reported, [17,18] whereas the role of nitrogen on the high-temperature strength is still not well understood. Thus, this research is devoted to studying the effects of nitrogen addition and nitrogen-substituting carbon on fracture behavior of Cr-Mo-V hot-working die steel during hightemperature deformation at 700 °C.…”