The Effect of High Temperature Normalization on the Microstructure of H13 Steel after Forging

H13 steel has high hardenability, good wear resistance, and heat resistance. It has relatively high hardness and strength below 600 ℃, good resistance to cold and hot fatigue, and high resistance to tempering stability. It is widely used in die-casting molds for aluminum, copper, and their alloys, and has achieved good economic benefits. During the trial production, the H13 module experienced surface cracking, incomplete spheroidization, and uneven microstructure after forging. This article proposes a high-temperature normalizing+spheroidizing annealing process to improve module quality and homogenize the structure.

In this experiment, H13 steel with severe network carbide segregation was subjected to high-temperature normalizing and isothermal spheroidization annealing treatment to study the improvement of its microstructure by high-temperature normalizing.

Experimental materials and methods

Divide the modules into three groups and normalize them at 970 ℃ for 5, 7, and 10 hours respectively. The heat treatment process includes high-temperature normalizing (air cooling) and isothermal spheroidization annealing. The isothermal spheroidization annealing process parameters are: high temperature section 870 ℃× 8 h, low temperature section 730 ℃× 14 h. Based on the effect of high temperature normalizing time on the module structure, a new heat treatment process is developed.

Experimental results and analysis

With the extension of the normalizing insulation time, the degree of carbide network segregation gradually decreases, and the small carbide network gradually dissolves.

After 5 hours of high-temperature normalizing, the degree of segregation significantly improved, but segregation still exists. When the normalizing insulation time is extended to 7 hours, segregation is basically eliminated, but the carbide network is still relatively clear. The small secondary carbide network dissolves smoothly into the matrix during austenitization, but the coarse carbide network is not dissolved. When the insulation time is extended to 10 hours, the carbide network is basically eliminated, and only the particularly coarse carbide network in the early stage remains. After spheroidization annealing in the later stage, the spheroidized structure is also relatively uniform. From the above analysis, it can be concluded that high-temperature normalizing can eliminate segregation in the organization and improve the shape and distribution of network carbides. With the extension of high-temperature normalizing insulation time, carbide particles gradually dissolve, segregation and carbide network are gradually eliminated, and the organization is significantly improved.

Conclusion

1) Slow cooling of H13 steel after forging will result in the precipitation of carbides in a network like pattern. The slower the cooling rate, the more severe the carbide network. Although it can be improved by later normalizing, some abnormal structures still remain in the matrix and are difficult to eliminate. In addition, slow cooling after forging will cause abnormally coarse grains, which are hereditary and difficult to refine in the later stage.

2) High temperature normalizing can improve the segregation and network carbides in the microstructure of H13 steel after forging. Within a certain time range, the improvement in microstructure becomes more significant with the extension of normalizing insulation time.

3) After forging H13 steel, it is air-cooled above the Ms point and then subjected to high-temperature normalizing to obtain a more uniform and dispersed spheroidized structure with better spheroidization effect.