PRODUCTS

• Strength Properties

Hardness Hardness is the primary technical indicator for mold steel. To maintain shape and size under high stress, mold steel must possess sufficient hardness. For cold work mold steel, hardness typically remains around HRC 60 at room temperature. For hot work mold steel, the required hardness range is usually between HRC 40-55, depending on working conditions. Within a given hardness range for the same steel grade, hardness is proportional to deformation resistance. However, steel grades with the same hardness but different compositions and structures may exhibit significantly different plastic deformation resistances.

Red Hardness Hot work molds must retain their structure and performance stability at high temperatures to maintain sufficient hardness, a property known as red hardness. Carbon tool steel and low-alloy tool steel usually maintain red hardness within the 180-250°C range, while chromium-molybdenum hot work mold steel generally maintains it within the 550-600°C range. Red hardness primarily depends on the steel’s chemical composition and heat treatment process.

Compressive Yield Strength and Compressive Bending Strength Molds often experience high pressure and bending during use, necessitating adequate compressive and bending strengths in the mold material. Compression and bending tests closely mimic the actual working conditions of molds. For example, the compressive yield strength of mold steel corresponds well with the deformation resistance exhibited by punches during use. Bending tests have the added advantage of providing a large absolute strain value, sensitively reflecting differences in deformation resistance among various steel grades and heat treatment states.

• Toughness

Molds endure impact loads during operation, requiring mold steel to have adequate toughness to minimize breakage and chipping. Factors such as chemical composition, grain size, purity, carbide and inclusion quantity, morphology, size, and distribution, as well as heat treatment procedures and resulting microstructures, greatly influence the toughness of mold steel. Particularly, the steel’s purity and hot working deformation significantly impact its transverse toughness. Balancing toughness, strength, and wear resistance involves selecting appropriate chemical compositions and employing refined processing and heat treatment techniques to achieve optimal performance.

Impact toughness measures the total energy absorbed by a specimen during a single impact until complete fracture. However, tools often fail under fatigue conditions, so conventional impact toughness does not fully reflect mold steel’s fracture performance. Testing techniques like low-energy repeated impact fracture work and fatigue life assessments are increasingly utilized.

• Wear Resistance

Wear resistance is often the most critical factor determining mold life. Molds face significant compressive and frictional stresses, requiring them to maintain dimensional accuracy under severe friction. Mold wear includes mechanical, oxidation, and melting wear types. Enhancing wear resistance involves maintaining high hardness and ensuring a reasonable composition, morphology, and distribution of carbides or other hard phases. For molds operating under heavy loads and high-speed wear conditions, the steel surface should form a thin, dense, adherent oxide film to maintain lubrication and reduce melting wear and oxidation wear. Wear resistance can be evaluated using simulated test methods, with the relative wear index indicating the wear level of different steel compositions and structures.

• Heat Fatigue Resistance

Hot work mold steel, subjected to cyclic loading and rapid thermal cycling, requires strong thermo-mechanical fatigue fracture resistance. Heat mechanical fatigue encompasses thermal fatigue performance, mechanical fatigue crack growth rate, and fracture toughness. Thermal fatigue reflects the material’s lifespan before thermal fatigue crack initiation, with higher resistance leading to more thermal cycles before crack initiation. The mechanical fatigue crack growth rate indicates the extent of crack propagation per stress cycle after crack initiation. High fracture toughness means that the material resists unstable crack growth under sufficient stress intensity factors. High heat fatigue resistance, low crack growth rates, and high fracture toughness contribute to extended mold life.

• Galling Resistance

Galling resistance measures the resistance to “cold welding.” This property is crucial for mold materials. Tests involve subjecting the tool steel sample and a material prone to galling (such as austenitic steel) to constant-speed friction motion under dry conditions. The load is gradually increased, and the corresponding torque also rises. The load at which galling occurs is the “galling critical load.” Higher critical loads indicate stronger galling resistance.

By understanding and optimizing these key performance metrics, mold steel can be selected and processed to ensure superior quality, longevity, and efficiency in various industrial applications.