Effect of High-Temperature Normalizing on the Post-Forging Microstructure of H13 Steel
H13 steel exhibits high hardenability, excellent wear and heat resistance, maintaining relatively high hardness and strength below 600°C, along with superior cold and hot fatigue resistance and good temper stability. It is widely used in die casting molds for aluminum, copper, and their alloys, yielding significant economic benefits. However, the H13 modules produced by the company exhibited surface cracking, incomplete spheroidization, and uneven microstructure during trial production. This paper introduces a high-temperature normalizing followed by spheroidizing annealing process to improve module quality and achieve uniform microstructure.
1. Issues were identified during the prototype development process.
The experimental module was fabricated from H13 steel with dimensions of 400 mm × 1000 mm × 3000 mm, and the final forging temperature was approximately 950°C. Due to the absence of timely post-forging heat treatment, surface cracking occurred after air cooling to room temperature, extending into the module interior. Analysis revealed that H13 steel is a hardenable steel, meaning its martensitic structure forms during air cooling. The extensive microstructural stress generated during the post-forging air cooling process led to surface cracking of the module.
To prevent cracking, the H13 steel modules were directly subjected to slow cooling in the furnace after forging, followed by spheroidizing annealing. After the spheroidizing annealing, the module hardness was measured using a Brinell hardness tester; three samples were tested, with hardness measurements taken at eight points each (see Table 1). As shown in Table 1, the hardness generally met the requirements, but the uniformity was unsatisfactory due to significant hardness variation, with individual points exceeding the specified limit (≤220 HB). Metallographic analysis of the modules revealed the microstructure as illustrated in Figure 1.
Figure 1 shows the microstructure after isothermal spheroidizing annealing, with the spheroidizing process consisting of an elevated-temperature stage at 870°C for 8 hours and a low-temperature stage at 730°C for 14 hours. In Figure 1(a), severe carbide network segregation and significant grain mixing are evident. Figure 1(b) reveals abundant dispersed carbide particles, indicating that the spheroidizing effect meets the intended objective; however, the segregation remains pronounced, with some carbides forming a network-like distribution that reduces spheroidization effectiveness in those regions. Networked carbides pose serious risks to mold steel, as they readily serve as crack sources during service, leading to material failure. Therefore, such carbide networks must be controlled within acceptable limits. This study subjected the H13 steel batch exhibiting severe carbide segregation to high-temperature normalizing followed by isothermal spheroidizing annealing to evaluate the improvement in its microstructure under these conditions.
2 Experimental Materials and Methods
Small samples were taken from the modules exhibiting severe segregation for spectral analysis; their chemical compositions are shown in Table 2. The chemical composition of this H13 steel complies with the standard GB/T 1299–2000 "Alloy Tool Steel." The modules were divided into three groups and subjected to high-temperature normalizing at 970 °C, with holding times of 5, 7, and 10 hours, respectively. The heat treatment process consisted of high-temperature normalizing (air-cooled) followed by isothermal spheroidizing annealing. The isothermal spheroidizing annealing parameters were: high-temperature stage at 870 °C for 8 hours, and low-temperature stage at 730 °C for 14 hours. Based on the effect of high-temperature normalizing time on the microstructure of the modules, a new heat treatment process was developed.
After 5 hours of high-temperature normalizing, the degree of segregation (see Figure 2(a)) showed significant improvement compared to Figure 1, although segregation still persisted. When the normalizing holding time was extended to 7 hours, segregation was virtually eliminated; however, the carbide network remained relatively distinct, with fine secondary carbide networks dissolving completely into the matrix during the austenitization process, while coarse carbide networks remained insoluble—falling into the non-conforming category according to the North American Die Casting Association standards. Extending the holding time to 10 hours eliminated most carbide networks, leaving only the initially coarse carbide networks. Subsequent spheroidizing annealing resulted in a more uniform spheroidized microstructure, earning the material the AS4 grade under North American Die Casting Association standards and meeting acceptance criteria. The analysis demonstrates that high-temperature normalizing effectively removes segregation and improves the morphology and distribution of carbide networks. As the holding time increases, carbide particles gradually dissolve, leading to progressive elimination of segregation and carbide networks, thereby significantly enhancing material microstructure.
After forging, H13 steel is prone to coarse grain formation due to excessively high final forging temperature and slow cooling rates. As an hypereutectoid steel, H13 exhibits network-like secondary carbide precipitation during slow cooling, and its high alloy content inevitably leads to material segregation. To improve the post-forging microstructure of H13 steel modules, high-temperature normalizing treatment is recommended to achieve uniform microstructure.
To eliminate the reticular carbides, the workpiece must be heated to a temperature sufficient to dissolve secondary carbides. However, the spheroidizing annealing temperature (870 °C) is insufficient to achieve this dissolution. Consequently, without high-temperature normalizing, the carbide network remains difficult to remove, and segregation cannot be improved, resulting in the H13 module failing to meet standards. After high-temperature normalizing, fine carbide particles are fully dissolved into the matrix, the carbide network gradually refines until it fractures, and the microstructure becomes more uniform. Following the normalizing holding period, rapid air cooling prevents the formation of reticular secondary carbides. Subsequently, spheroidizing annealing promotes the growth of uniformly dispersed carbides from the matrix into spherical structures, thereby enhancing the annealed microstructure.
In summary, post-forging slow cooling leads to coarse microstructures and the formation of networked carbides, whereas high-temperature normalizing improves both the networked carbides and material segregation. Therefore, the annealing microstructure can be optimized through two approaches: appropriate rapid cooling after forging, followed by high-temperature normalizing and final spheroidizing annealing. Based on the analysis of these examples, the optimal process was established as shown in Figure 3. After forging, the material is air-cooled and held above the Ms point for a period to prevent martensitic transformation, followed by high-temperature normalizing and isothermal spheroidizing annealing. Figure 4 illustrates the microstructure obtained under this new process, demonstrating highly significant spheroidization with a spheroidization rate exceeding 95% and a grain size of approximately grade 7. Classified as AS1 according to the North American Die Casting Association standards, the product has received positive feedback from customers.
4 Conclusion
1) Slow post-forging cooling of H13 steel leads to the formation of a network-like carbide precipitation. The slower the cooling rate, the more pronounced the carbide network becomes. Although subsequent normalizing can improve this, some abnormal microstructures persist in the matrix and are difficult to eliminate. Additionally, slow cooling results in abnormally large grain sizes, which are hereditary and challenging to refine later.
2) High-temperature normalizing improves segregation and networked carbides present in the post-forging microstructure of H13 steel. Within a certain time range, the improvement in microstructure becomes more pronounced as the normalizing holding time increases.
3) After forging, H13 steel is air-cooled above the Ms temperature and then subjected to high-temperature normalizing, resulting in a more uniform and dispersed spheroidized structure with superior spheroidization effect.