The material selection and heat treatment quality of die-casting molds directly determine the mold’s service life, molding accuracy, and production stability. They must be specifically matched based on the die-casting alloy type, production batch, and complexity of the die-cast part. During the die-casting process, the mold cavity must withstand high-temperature molten metal (400-1100°C), high pressure (5-150MPa), and repeated hot and cold cycles. Therefore, the material must possess high strength, high wear resistance, good thermal fatigue resistance, and good thermal conductivity.
Common die-casting mold materials can be divided into three categories: carbon tool steel (such as T10A), alloy tool steel (such as Cr12MoV), and hot work die steel (such as H13 and 3Cr2W8V). Hot work die steel is the mainstream choice. H13 steel (4Cr5MoSiV1) is widely used due to its excellent overall performance. Its chromium content of 5% forms a dense oxide film that resists high-temperature oxidation, while molybdenum and vanadium enhance wear resistance and red hardness. It is suitable for aluminum and zinc alloy die castings, with production batches of 100,000 to 500,000 molds. For die casting of high-melting-point alloys such as copper alloys, the more heat-resistant 3Cr2W8V steel is used. Its tungsten content of 8% maintains sufficient strength at temperatures between 600°C and 650°C, but its toughness is relatively low, requiring process optimization to avoid brittle fracture.
Material selection should be tailored to the specific working conditions. For small, simple molds (such as zinc alloy toy parts), Cr12MoV is an option, offering lower cost and better processing performance. For large, complex molds (such as automotive engine blocks), H13 steel must be selected, employing an electroslag remelting process to reduce sulfur and phosphorus content and improve material purity. For molds with batch sizes exceeding 500,000, high-quality H13 steel (such as ASSAB 8407) is essential. Its impact toughness is ≥25J/cm², over 30% higher than that of standard H13 steel, effectively reducing the risk of premature cracking.
Heat treatment is crucial for unlocking the material’s properties, requiring three core processes: annealing, quenching, and tempering. Annealing eliminates forging stresses, reduces hardness, and facilitates machining. The annealing process for H13 steel involves holding at 850-880°C for 2-4 hours, followed by furnace cooling to below 500°C before removal. After annealing, the hardness must be controlled between 207 and 255 HBW. Quenching is a key step in enhancing strength. H13 steel is heated in a salt bath furnace at 1020-1050°C for 1.5-2 minutes per mm to ensure full dissolution of carbides. It is then oil- or air-cooled to 50-100°C. After quenching, the hardness can reach 50-55 HRC.
The tempering process is tailored to the operating temperature. Aluminum alloy die-casting molds are typically tempered two to three times at 520-560°C, each hold time for two to three hours, to achieve a final hardness of 42-46 HRC, achieving the optimal balance of strength and toughness. Copper alloy die-casting molds, due to their high operating temperatures, require a higher tempering temperature of 580-620°C, reducing the hardness to 38-42 HRC to enhance thermal fatigue resistance. After tempering, slow cooling is required to reduce residual stress. For large molds, aging treatment (at 200-250°C for four to six hours) is recommended after tempering to further stabilize the microstructure. After heat treatment, the mold cavity should be inspected for deformation, requiring a flatness of ≤0.02 mm/m. Otherwise, corrections such as straightening or grinding are necessary to ensure molding accuracy.
