Performance of moulding materials has a decisive influence on the life of mold. According to structure and use of mold, reasonable selection of moulding materials is one of important tasks for mold engineer.
Heat treatment for mold and surface strengthening are key processes in mold manufacturing, which are an important part of ensuring mold quality and service life. Actual use proves that a large proportion is caused by improper heat treatment in mold failure.
Molds are widely used, work conditions vary widely, and range of materials used to make molds is wide. At present, materials of stamping die, plastic die, die-casting die and powder metallurgy die are mainly steel, some die can also use low melting point alloy and non-metal material.

 

1, hardness and wear resistance (the most important form of mold failure, determine mold life)
2, machinability (shape of mold parts is complex, requiring heat treatment deformation is small)
3, strength and toughness (sufficient strength to withstand high pressure, impact load, etc, require high toughness)
4, hardenability, polishing, corrosion resistance (corrosion of plastics and additives).

1. Cold work die steel: A mold for deforming metal in a cold state, including: cold die, cold extrusion die, powder compact die. High hardness, high wear resistance, sufficient strength and toughness are required.
2. Hot work die steel: Production of heated solid or liquid metal mold under pressure molding, including: hot forging die, die-casting die. Requires sufficient strength, toughness and wear resistance at high temperatures, high thermal fatigue resistance and thermal conductivity.
3. Plastic mold steel: manufacturing a variety of plastic molds. There are many varieties of plastics, requirements are large, and range of mould materials is wide. Mainly requires high process performance (small heat treatment deformation, good polishing, corrosion resistance).
General principle: meet performance requirements, good process performance, and appropriate economic considerations.

Heat treatment for mold includes heat treatment of mould materials and mold parts.

Completed in steel plant to ensure quality of steel, such as basic mechanical properties, metallographic organization should comply with national standards or industry standards. It is characterized by mass production in large industrial furnaces.

completed at mold manufacturing plant, or completed by a professional heat treatment plant. It is characterized by small batch or single piece production, complex and diverse processes, excellent equipment.
Heat treatment process is divided into preliminary heat treatment and final heat treatment. Common methods are: normalizing, annealing, quenching, tempering, carburizing and nitriding, see as followed.

Cold working molds are mainly used for punching, drawing and bending of metal or non-metal materials. Working parts are subjected to various mechanical forces such as tension, bending, impact, fatigue and friction.
Main forms of failure: fracture, collapse, wear, adhesion (bite), bruises, softening, etc.
Main performance requirements of cold work die steel: high deformation resistance, fracture resistance, wear resistance, fatigue resistance, anti-adhesion performance
Common materials include:
Carbon tool steel, T10A, T12A
Low alloy steel, 9Mn2V, CrWMn, 9SiCr, GCr15
High alloy steel, Cr12, Cr12MoV
Main features of alloy steel: high carbon content, containing more alloying elements. Alloying elements reduce martensitic transformation point Ms of steel.

Heating method (preheating or step heating), heating medium (controlled atmosphere furnace, vacuum furnace), cooling method (smooth cooling method to reduce deformation), surface treatment (cryogenic treatment, nitriding), inter-process protection (timely cleaning)

used for small and simple cold work die parts that are not required to be high (T8A, T10A). T10A is the most common application with high strength and toughness. Carbon tool steel has poor hardenability and hardened layer is 1.5 to 3.0 mm after conventional quenching.

commonly known as oil-quenched steel, this type of steel generally has a low quenching temperature and is cooled in oil, has small deformation and good dimensional stability.
Main materials are: chromium tungsten manganese steel (CrWMn, 9CrWMn), manganese vanadium steel (9Mn2V), high carbon chromium steel (Cr2), chromium silicon steel (9SiCr)

Mainly high-chromium and high-carbon steel, with high hardenability and micro-deformation, also called micro-deformed steel and is a high-grade cold-worked steel.
Represents steel grades Cr12, Cr12MoV, Cr12W, Cr5MoV, etc.

Specially developed for simplifying production process of large and extra large cold work die, its heat treatment characteristics are: wide quenching temperature range (880--1150℃), suitable for manual operation; High hardenability, high surface hardness and good core performance after air cooling; quenching deformation tendency is small.

cold work die steel focuses on hardness and wear resistance. Carbon content is high, alloying elements are mainly for increasing hardenability and improving wear resistance. Hot work die steel has appropriate hardness requirements, focusing on red hardness, thermal conductivity and wear resistance. Therefore, carbon content is low, alloying elements are mainly for increasing hardenability, improving wear resistance and red hardness.

1, a wide range: from ordinary steel to special steel. Special steel has developed a system of its own with steel number add SM
2, strict performance requirements, heat treatment is difficult, requiring polishability and mirror quality
3, avoid deformation
4, corrosion resistance
5, easy machinability

Heat treatment of pre-hardened plastic mold steel
Heat treatment of easy-cut pre-hardened plastic mold steel
Heat treatment of carbon plastic mold steel in non-alloy
Heat treatment of carburized plastic mold steel
Heat treatment of corrosion-resistant plastic mold steel
Heat treatment of age hardening plastic mold steel
● Pre-hardened steel refers to steel that has been subjected to final heat treatment (quenching and tempering).
● Typical pre-hardened plastic mold steel is: SM3Cr2Mo. Its heat treatment process is: 840--880℃ for quenching, oil cooling, 600--650℃ for tempering, air cooling, supply hardness 32HRC.
● Corresponding US steel P20, German steel 40CrMnMo7, Japan HPM2. Post-carburization heat treatment process of P20 steel is: 820--870℃ for quenching, 150--260℃ for tempering, air cooling, carburizing layer hardness 58-64HRC. German steel 40CrMnMo7 has better hardenability than SM3Cr2Mo with better processability and polishing properties.
● Easy-cut pre-hardened steel adds one or several free-cutting alloying elements under premise of ensuring original performance.
● Typical materials are: 8CrMnWMoVS (8CrS), sulfur-containing free-cutting steel. When heat treatment hardness is 40-42HRC, machinability is equivalent to annealed T10A steel (200HBS), which is good in polishability and lithography. Typical heat treatment: 800±10℃ for annealing, heat preservation 2-4h, cooling to 700-720℃, heat preservation 4-6h, furnace cooled, hardness <=229HBS, 880-920℃ for quenching, air cooling, hardness 63HRC, 160-200℃ for tempering, air cooling, hardness >= 58HRC.
●High toughness free cutting steel: 5CrNiMnMoVSCa
● Non-alloy medium carbon plastic mold steel is a medium carbon carbon structural steel with higher metallurgical quality (purity) and carbon content of 0.4%-0.6%.
● Typical materials are: SM45, SM50, and same Japanese steel is S45C-S58C. Heat treatment process is same as conventional process. For small plastic mold parts with low precision requirements, this steel grade is preferred.
● Carburized plastic mold steel: For plastic mold parts with large impact, surface is required to be hard and core is tough, and carburized steel is used. General parts are made of 20Cr. Special steel grades are used for parts with high surface quality requirements: 12CrNi3A.
● Corrosion-resistant plastic mold steel is used to make mold parts for plastic products that are corrosive to metals.
● Typical steel grades are: Cr13, 9Cr18 and other reinforced martensitic stainless steel. Heat treatment is same as general stainless steel products.
● Age-hardening plastic mold steel: For plastic mold parts with complex shape, high precision and long life, age hardening steel is used to reduce heat treatment deformation and improve precision retention of mold parts. Mold parts become soft after solution treatment (28-34HRC), which is easy to process and then age hardened to obtain required comprehensive mechanical properties.
● Martensitic age hardening steel has high yield ratio, good cutting performance and welding performance. Typical steel grades have 18Ni hardness of 28HRC in solid solution state, 3h aging at 480℃, hardness can be increased to 43HRC, holding time is longer, hardness can reach 52HRC.
● Precipitation hardening steel is strengthened by solution treatment and precipitation of second phase, and hardness can reach 37-43HRC. Representative steel numbers are: 25CrNi3MoAl. This kind of steel has high metallurgical quality, and it is generally used for special smelting with good purity and good mirror surface grinding. Heat treatment is divided into: first solid solution (quenching), second solution treatment (tempering) and aging treatment.

● 718 plastic mold steel: delivery state has been pre-hardened to HB285-320 (HRC30-35) and does not need to be hardened again, never deformed; can be hardened to HRC40. Performance description: After electric furnace refining or electroslag remeltingl, suitable for electro-erosion (electric spark) processing, easy to mirror polishing; good hardenability and comprehensive mechanical properties. Mainly used:
(1) Long-term production or large plastic molds;
(2) Large plastic molds with mirror polishing requirements.
●718H is a vacuum-melting chrome-nickel-molybdenum alloy steel. It has been hardened and tempered before leaving factory. It has following two advantages: no risk of quenching and heat treatment deformation, no need for heat treatment, shortening mold production cycle and reducing cost of mold (for example, no need to correct deformation). Mold is easy to be modified, can be applied by nitriding and flame hardening treatment to improve surface hardness and wear resistance of mold. 718H main use: large long-life plastic molds, such as home appliances, computer casings and other molds, high surface finish plastic molds, blow molds. Due to high hardness, it can be used in slider forming tool in plastic molds, compression-brake mold(Bending machine model), construction parts and shafts(can be flame hardened or nitrided depending on production requirements).
●718H steel features: good polishing and processing, high steel purity.
●Factory state: pre-hardened HB 330-370

●S136 - High wear-resisting and anti-corrosion pre-hardened mirror steel. ASSAB (Sweden) STAVAX ESR (S-136) is a high-grade stainless steel tool steel with following characteristics:
● Excellent corrosion resistance
● Excellent polishability
● Excellent wear resistance
● Excellent machinability
● Excellent stability in quenching. Advantages listed above make this steel grade have excellent production characteristics. Due to its excellent corrosion resistance, benefits of its use in plastic molds are summarized as follows:
● Lower maintenance cost. After long-term use of mold, surface of cavity still maintains original smooth state.

High wear-resisting and anti-corrosion pre-hardened mirror steel HPM38 is made of 13Cr Mo-containing stainless steel. It has good corrosion resistance after special dissolution. Therefore, it is most suitable for plastic molds that require high hardness and high requirements for corrosion-resistant mirror surface. Moreover, HPM38 heat treatment has a very small dimensional change and is suitable for precision heat treatment. In addition, its etchability is good, which is conducive to maintenance of mold. Mainly used for: high-polishing molds for daily-use transparent products(home appliances, medical, food) and rubber molding for corrosion resistance, such as: communication equipment parts, medical equipment parts.
Features: very good polishing, good corrosion resistance, very small heat treatment size.

Hot work die mainly includes forging die, hot extrusion die, die casting die, etc.
Main failure modes: fracture (cracking or mechanical fatigue crack), plastic deformation or cavity collapse, thermal fatigue, thermal wear and thermal melting loss.
Main features of hot work die steel: In addition to good process performance, it also requires high strength, toughness, hardness and thermal stability at different temperatures. Generally, low carbon content, multi-alloyed (including Cr, W, Mo, Ni, V, Si, Co, etc.)
Higher toughness hot work die steel: used to manufacture hot work die parts with large impact loads.
Typical steel numbers: 5CrNiMo and 5CrMnMo.
High heat-strength hot work die steel: mainly produces working parts for die-casting die and machine forging die. It is required to have high heat strength, thermal fatigue, thermal melting loss, tempering resistance, and thermal stability. Generally, it contains more alloying elements such as Cr, W, Si, Mo, and V. Representative steel grades are: 4Cr5MoSiV, 4Cr5MoSiV1 (foreign grade H13), 4Cr3Mo3SiV.
High wear resistance hot work die steel: has red hardness, good thermal wear, high temperature strength, high temperature toughness and thermal stability. Representative steel number: 3Cr2W8V, high tungsten content. High quenching temperature: 1100-1150℃, in order to reduce deformation, it should be slowly heated, complex and large parts should be preheated first.

Heat treatment of cast iron for mold: manufacturing large and complex molds, generally using ductile iron and alloy cast iron. Typical heat treatments are normalizing and high temperature tempering.
Heat treatment of cast steel for mold: large forging die, typically representing ZG5CrMnMo, ZG5CrNiMo, etc. Heat treatment is generally normalized and tempered.
Heat treatment of steel-bonded cemented carbide: steel matrix is used as a bonded phase, metal carbide is used as a hard phase, which is pressed and sintered by a powder metallurgy process. Heat treatment is basically same as that of alloy tool steel.

Alloy steels that can increase hardness by aging heat treatment are referred to as age hardened alloy steels. Typical steel grades are maraging steel and precipitation hardened stainless steel. Basic composition of former is less than 0.03% carbon and less than 18%-25% nickel, and various alloying elements capable of age hardening are added. It is mainly used for certain parts on aircraft, rocket engine casing, etc. It is also used in die-casting molds. Precipitation hardening stainless steel is further divided into austenite-martensitic precipitation hardening stainless steel and martensitic precipitation hardening stainless steel. Austenitic-martensitic precipitation hardening stainless steel can be used as a thin-wall structure and honeycomb structure of an aircraft body, and can also be used as a fuel storage tank, a rocket engine casing, a high-pressure vessel, and so on. Martensitic precipitation hardening stainless steel is resistant to atmospheric, water and other media corrosion, it is used in the manufacture of liquid rocket shells and certain parts in nuclear power plants.