The thermal processing and heat treatment of plastic mold steel are key factors in determining mold performance, directly impacting its hardness, wear resistance, toughness, and service life. Commonly used plastic mold steels include pre-hardened mold steels (such as 718H and NAK80), age-hardened mold steels (such as P20 and 45# steel), and cold-work mold steels (such as Cr12MoV). Different types of steel require different thermal processing and heat treatment processes due to their compositional differences. Thermal processing primarily involves forging, rolling, and annealing, aiming to improve the steel’s internal structure and eliminate casting defects. Heat treatment, through processes such as quenching and tempering, adjusts the steel’s mechanical properties to suit the specific mold requirements. For example, pre-hardened mold steel requires controlled forging temperature and cooling rate to achieve a uniform pearlite structure, while cold-work mold steel requires high-temperature quenching to enhance hardness and wear resistance.
The specifications for hot working processes must be tailored to the steel’s chemical composition and forging characteristics. For example, for Cr12MoV steel, the forging temperature range should be controlled between 1050-850°C. Heating should be slowly raised to 800°C before rapidly returning to the forging temperature to avoid cracking caused by excessive thermal stress. A “light hammer, fast strike” approach should be employed during the forging process to ensure uniform deformation of the steel. Reduction per forging should not exceed 20% to prevent grain coarsening. For large mold steel billets, intermediate annealing is required: heating to 750-800°C, holding for 3-4 hours, and then cooling to below 500°C before air cooling. This eliminates forging stresses and improves cutting performance. The rolling process, suitable for thin plate mold steel, requires a temperature of 900-1000°C. After rolling, spheroidizing annealing is required to spheroidize carbides, reduce hardness, and facilitate subsequent processing.
Annealing is a crucial post-heat processing step, designed to eliminate internal stresses, refine grain size, and reduce hardness. Annealing processes vary for different types of plastic mold steel. For pre-hardened mold steel, spheroidizing annealing requires heating to 780-820°C, holding for 4-6 hours, and then cooling to below 600°C at a rate of 20-30°C/h before air cooling to ensure the hardness drops to 220-250 HBW. Age-hardening mold steel utilizes low-temperature annealing, heating to 650-700°C, holding for 2-3 hours, and then furnace cooling. Hardness can be controlled within a range of 180-220 HBW. After annealing, the steel undergoes hardness testing and metallographic analysis to ensure uniform pearlite structure and the absence of network carbides. Otherwise, re-annealing is required. For mold steels with specialized requirements, an isothermal annealing process can be employed, holding at 680-700°C for 3-4 hours to further improve structural stability.
Quenching and tempering are the core heat treatment processes that determine the ultimate properties of tool steel. Pre-hardened tool steel is typically quenched at 840-880°C, with a holding time of 1.5-2 minutes per mm depending on the thickness of the steel. Oil or air cooling is used after exiting the furnace to ensure a uniform martensitic structure. The tempering temperature is adjusted according to the desired hardness. For example, for a desired hardness of 30-35 HRC, the tempering temperature is 580-620°C, held for 2-3 hours, and then air cooled. For higher hardness requirements (38-42 HRC), the tempering temperature is lowered to 500-550°C. Cold-work tool steels such as Cr12MoV are quenched at 950-1000°C, held for 1-1.5 minutes per mm, and then oil cooled. Tempering is repeated two to three times, each holding for two hours, at a temperature of 180-200°C to eliminate residual stresses and achieve a high hardness of 60-62 HRC. Age-hardening mold steel is aged after processing and forming, heated to 500-550℃ and kept warm for 4-6 hours to increase the hardness by precipitating intermetallic compounds.
Quality control and performance testing after heat treatment are key to ensuring mold steel meets quality standards. Hardness testing is required for mold steel after quenching. Hardness variations within the same workpiece should not exceed 3 HRC; otherwise, re-tempering is required. Metallographic examinations should ensure that the martensite needles are fine and uniform, that there is no overheated or underheated structure, and that carbides are evenly distributed. For large molds, ultrasonic testing is also required to detect internal defects such as cracks and porosity. Furthermore, mold steel after heat treatment requires aging and aging for 24 hours before finishing to minimize subsequent deformation. In actual production, the appropriate heat treatment process should be selected based on the mold’s operating conditions (such as the injection material and production batch size). For example, molds for producing corrosive plastics (such as PVC) require chrome plating or corrosion-resistant mold steel (such as 2Cr13), combined with appropriate heat treatment processes, to improve the mold’s corrosion resistance and service life.
