Research on the Optimization of Manufacturing Process for H13 Mold Steel

H13 die steel is a hot work die steel suitable for die manufacturing in high-stress and high-temperature environments. It contains high levels of carbon and vanadium, along with appropriate amounts of alloying elements such as chromium and molybdenum, offering excellent hardenability, wear resistance, and resistance to thermal cracking.

These properties make it an ideal material for manufacturing plastic dies, aluminum alloy die casting dies, hot stamping dies, and precision cold stamping dies. However, large-section H13 die steel is prone to segregation and internal defects, which compromise die quality and service life.

Currently, the largest die steel specification in China is Φ650 mm; larger specifications require imports or forging, increasing production costs and extending production cycles. To address this challenge, precise control of H13 die steel's chemical composition combined with optimized smelting, forging, and heat treatment processes has successfully improved microstructural uniformity and grain refinement, thereby enhancing the overall performance of large-scale H13 die steel.

The composition design of H13 die steel follows the standards of the North American Data Center Alliance (NADCA), which provides precise control over alloy elements. The chemical composition comparison between the NADCA standards and H13 die steel specified in GB/T 1299—2014 is presented in Table 1.

The chemical composition design of H13 die steel is presented in Table 2. The carbon (C) content is set at the upper limit of the standard to achieve high hardness and wear resistance; the chromium (Cr) content aligns with the standard to balance hardenability, corrosion resistance, and heat resistance; the manganese (Mn) content matches the standard to enhance hardenability and strength while maintaining good toughness; the molybdenum (Mo) content is selected at the lower standard limit to moderately improve thermal strength and toughness without excessive cost increase; the vanadium (V) content is also set at the lower standard limit to refine grain structure, enhance material strength and toughness, and control costs; sulfur (S) and phosphorus (P) contents are increased to reduce material brittleness and plasticity; strict control over gaseous elements such as nitrogen (N), hydrogen (H), and oxygen (O) minimizes porosity and inclusions, ensuring material purity and stable performance. Only through precise control of the H13 die steel's chemical composition can high-performance, stable ultra-large-sized H13 die steel be produced to meet the stringent requirements of the premium market.

2. Optimization of the Manufacturing Process

The manufacturing process for H13 die steel involves initial electroslag remelting, followed by heating forging, then cooling annealing, and finally rough machining.

2.1 Industrial Smelting

Following the preparation method proposed by Ni Zhuowen et al., a prototype of H13 die steel was fabricated using a production process that included pre-melting the slag charge, arc initiation, smelting, shrinkage compensation treatment, power-off cooling, and demolding. The mold had a crucible diameter of 480 mm and a nominal capacity of 2 tons. The slag system consisted of a four-component mixture containing 65% calcium fluoride, 15% alumina, 15% calcium oxide, 5% magnesium oxide, and no more than 0.8% silica, with a molten slag height of approximately 155 mm.

2.2 Heating Forging

The core process of H13 die steel casting involves utilizing plastic deformation to fragment carbide dendrites within the steel ingot, eliminating their chain-like arrangement in segregation zones, thereby achieving structural equilibrium and enhancing lateral and longitudinal impact resistance. During the pre-forging heating stage, uniform heating of the cast slab must be ensured; therefore, heating duration and average zone temperature should be strictly controlled within 1220–1240 °C. Any surface cracks on the forgemate must be promptly removed. A four-stage coarse drawing followed by KD drawing process is employed, with a forging ratio exceeding 6 to enhance core deformation and ensure material density and uniform microstructure. The final forging temperature must be rigorously maintained at no less than 850 °C to prevent surface cracks, particularly at edges and corners. A stepped cooling method is applied to reduce forging temperature to room temperature, with strict control over the cooling process to minimize internal stress and deformation, thereby extending die service life.

2.3 Annealing and Heat Treatment

To prepare for subsequent heat treatment processes and prevent stress generation during forging, an annealing procedure should be performed first. Prior to spheroidizing annealing, the forging temperature must be maintained above 500°C. A normalizing and ultra-fine-graining treatment is required before annealing, with the holding temperature controlled between 1020–1040°C and the cooling rate appropriately regulated. This process aims to refine the grain structure while effectively reducing segregation and networked carbides in the forging blank, ensuring a uniform microstructure. During spheroidizing annealing, the temperature should be set between 850–870°C to facilitate carbide regulation and microstructure refinement. A stepwise cooling and temperature-controlled annealing approach is employed to prepare for the final heat treatment.

The heat treatment process includes quenching and secondary tempering. First, the material is preheated for 10 minutes at 790°C ± 15°C, then heated for 10 minutes at 1010°C ± 5°C, followed by oil cooling. Subsequently, it is held at 550°C ± 6°C for 2 hours before undergoing two tempering cycles. For the annealed samples of mold steel, hardness measurements were taken at five points each on the core and surface using Brinell and Rockwell hardness testers, with the average values recorded; the results are presented in Table 3. As shown in Table 3, after annealing, the samples achieved hardness values compliant with both the national standard (≤ 229 HBW) and the NADCA standard (≤ 235 HBW), with minimal hardness difference between the surface and core regions.

2.4 Rough machining treatment

After annealing treatment, inspect the product to ensure its microstructure remains intact without cracks or other defects. Subsequently, perform turning machining using a large cutting depth and feed rate—typically 3–5 mm in cutting depth and 0.3–0.5 mm in feed rate (r^-1), with a cutting speed of 100–150 m/min. Post-machining cleaning and rust prevention are required.

3. Organization Uniformity

3.1 High-magnification tissue

One sample was collected at the center of the mold steel and another at half its radius; these three samples represent all directions of forging. The samples were corroded using a 4% nitric acid-alcohol solution, followed by 500 annealing microstructure analyses and 50 strip segregation tests, with the testing surfaces kept parallel to the primary deformation direction. This orientation ensures that post-corrosion grain boundaries lie lower relative to grain centers, creating grooves conducive to thorough microstructural analysis. The annealing segregation tests demonstrated high microstructural uniformity, indicating that the mold steel billets underwent effective electroslag remelting, forging, and heat treatment. Microstructural analysis revealed that all three samples exhibited spheroidized pearlite structures without eutectic carbides on the matrix, with fine and uniformly distributed secondary carbide particles. Due to the large dimensions of the forgings, varying cooling rates during post-forging cooling resulted in distinct microstructural variations: faster-cooled surfaces formed fewer and finer carbides, exhibiting lower spheroidization compared to the core region.

3.2 Low-magnification tissue

According to national standards, the cold acid etching method must be employed to treat hot work die steel, followed by low-magnification microstructure examination. The samples were immersed in a 30% ammonium persulfate aqueous etchant solution for approximately 5 minutes, then thoroughly rinsed with water and immediately cleaned with medical-grade alcohol, before undergoing magnifying glass and visual inspection. No metallurgical defects such as visible stratification, bubbles, white spots, or cracks were observed, nor were issues like central porosity or ingot segregation detected, as shown in Figure 1. This is attributed to the extremely rapid solidification rate of molten steel during forging, which results in fine dendrites that ensure uniform microstructure, while variations in crystal growth orientation minimize central porosity and segregation. Consequently, die steel subjected to electroslag remelting exhibits superior microstructural uniformity and density compared to conventional cast ingots.

3.3 Grain Size Analysis

The H13 die steel samples underwent graded quenching heat treatment at a temperature of 1010 °C. Subsequently, grain size corrosion testing was performed to conduct grading. The grading process employed image analysis techniques using specialized software to process and interpret images obtained from metallographic and electron microscopes, calculating key parameters such as grain area and perimeter to determine grain size.

Figure 2 shows images obtained by metallographic and electron microscopes. Observations reveal that the mold steel samples exhibit predominantly fine grains, with a small number of larger grains present. The grain size in the fine-grained regions reaches Grade 8, meeting the NADCA standard's minimum requirement of Grade 7 for grain size. This improvement results from the electroslag remelting process, followed by high-forge ratio forging and heat treatment, which significantly reduced the grain size and thoroughly refined the carbide structures.

4 Conclusion

To enhance the microstructural uniformity of H13 mold steel after forging, its chemical composition was optimized and the manufacturing process refined. The forging procedure for H13 molds was meticulously designed, utilizing an electroslag remelting furnace to produce the mold steel, followed by annealing and heat treatment. Subsequent testing revealed that the electroslag remelted H13 mold steel exhibits superior microstructural uniformity and density compared to conventional mold cast steel ingots, with significant improvements observed in the microstructure at low magnification.