The mold is a crucial component in industrial production and serves as the basis for the mold industry.
Internationally, mold is recognized as a key element in metal processing and is considered a symbol of a country's economic and technological advancement.
The importance of the development of the mold industry is recognized worldwide.
However, some challenges persist in the mold industry, such as a shortage of qualified professionals, outdated technology, long production cycles, inferior quality, high costs and limited mold life.
Related reading: How to improve mold quality?
According to statistics and analysis conducted by relevant parties, material selection and heat treatment of dies are responsible for 50% of the factors contributing to die failure. This highlights the importance of proper material selection and heat treatment to ensure the longevity and effectiveness of the dies.
1. Comparison of life at home and abroad
According to the 11th edition of the 2001 China Die Information report, Table 1 compares the service life of dies in China and abroad.
Despite 20 years of progress, the overall level of fungi in China has remained relatively unchanged compared with that in foreign countries.
However, there is still a significant gap between China and foreign countries when it comes to producing large, precise, complex and long-lasting molds.
Table 1 Comparison of life at home and abroad
| Mold type | Molded parts, materials and dimensions | Mold material | Total die life (drilling times, parts) | |
| Advanced world standard | Domestic level | |||
| Blind mold | Brass, low carbon steel plate; Flat blind parts; Material thickness ≤ 1 mm, size 40 mm × 40 mm, φ 45 mm | T8, T10 carbon tool steel for concave and convex die | 4 million ~ 7 million | <1 million |
| Alloy tool steel G12, G12MoV | 8 million to 10 million | 3 million ~ 5 million | ||
| Use YG15, YG20 carbide | 600 million to 3 billion | <50 million | ||
| Silicon steel plate for motor rotor and stator, material thickness ≤ 0.5 mm, size < 200 mm | Hard alloy (multi-station continuous suppression array) | US Linina: 300 million | 38 million ~ 50 million | |
| Kuroda Seiko: 270 million | ||||
| Statomat, Switzerland: 80 million | ||||
| Stellrem, UK: 100 million | ||||
| Fine Suppression Matrix | Mild steel with w C ≤ 0.2%; pull rod, cam, base plate and other thin obturation parts with material thickness less than 3mm or 3-6mm | Alloy tool steel: Cr12MoV | 500,000 ~ 1,000,000 | <150,000 |
| Alloy tool steel: Cr12MoVHigh speed tool steel: W6Mo5G4V2 | 300,000 ~ 600,000 | 100,000 ~ 120,000 | ||
| Casting Mold | Aluminum Alloy Parts | Cr-Ni Steel, 3Cr2W8 | >450,000 | <200,000 |
| Forge die | Steel, crankshaft | CrNi Steel, 5CrNiMo | 14,000 ~ 20,000 | 5,000 ~ 7,000 |
| Injection mold | ABS, medium | alloy steel for tools | >500,000 | 200,000 ~ 300,000 |
| Polyethylene, medium | alloy steel for tools | > 2 million | 500 thousand | |
2. Technical measures to improve service life
2.1 Adopt high-performance pure steel
Materials form the base, but the base can be unstable. Tool and die steel (GB/T 1299-2014) lists the specific components of cold work die steel, hot work die steel and plastic die steel and imposes strict requirements for impurities and content.
However, the quality of commercially available die steel remains a source of discord between buyers and sellers. To avoid such disputes, it is recommended to buy from reliable sources rather than being tempted by cheap prices.
It is also important to prioritize powdered steel, spray steel and high-quality steel with high purity. When selecting 3Cr2W8V steel for hot working dies, pay attention to its carbon content.
Advanced foreign standards dictate wc=0.25%~0.35%, while the Chinese standard is wc=0.30%~0.40%. This steel follows the 3X2B8 Ø steel standard of the former Soviet Union.
The Russian TOCT 5950-2000 standard has been revised to wC=0.27%~0.33%, while the Chinese standard remains unchanged. In practice, it has been proven that the high carbon content in 3Cr2W8V steel is harmful and contributes to early failures.
2.2 Carry out strengthening and hardening treatment
When medium alloy and medium carbon hot working die steel is cooled slowly after forging or when the die section is large (diameter greater than 100 mm), chain carbides may form in the structure, leading to brittle fracture premature, hot cracking and fissures. the die.
To improve the strength, toughness and service life of the matrix, it is necessary to eliminate chain carbides through pre-treatment of the fabric.
3CrMoW2V steel is standardized at 1130°C, which can dissolve M6C carbides. If the air cooling rate exceeds 15°C/min, it exceeds the critical cooling rate, leading to the formation of chain carbides. However, subsequent spheroidizing annealing can eliminate chain carbides and result in a uniform distribution of carbides.
2.3 New thermal pretreatment process with energy conservation and consumption reduction
1）The residual heat annealing process after forging is subjected to thermomechanical treatment.
2）A new spheroidizing annealing process is used for rapid homogenization.
3) Hot working die steel undergoes a change from high temperature tempering to medium temperature tempering.
4) Quenching and tempering treatment is increased.
2.4 Heat treatment with vacuum quenching or protective atmosphere
Since the successful implementation of vacuum quenching for Cr12MoV steel dies in the late 1980s, the use of vacuum quenching for dies has gained widespread popularity, especially with the rise of high-pressure gas quenching.
2.5 Cryogenic treatment
Subjecting a hardened matrix to cryogenic treatment below -110°C results in the precipitation of fine carbide residue and the transformation of residual austenite into martensite. This increases the wear resistance, tempering resistance and dimensional stability of the matrix.
The service life of an M12 nut cold extrusion die can be increased by two times through cryogenic treatment, while the service life of an aluminum alloy hot extrusion die can be improved by one time.
2.6 Cooling and tempering
The mold is made of high-speed steel and its quenching temperature is different from that of the tool. Cooling quenching, which involves a lower quenching temperature, is typically used.
For example, the quenching temperature of W18BCrV steel is between 1180-1200°C, while that of M2 and W9 steel is 1160-1180°C.
Low temperature quenching results in good strength and toughness, reduces the risk of tool warping, cracking and breakage, and ultimately improves die performance, quality and life.
2.7 High temperature quenching
Hot working dies made from steels such as 5CrNiMo, 5CrMnMo and 3CrW8V must be quenched at a higher temperature to produce more lath martensite. This improves fracture toughness and thermal fatigue resistance, leading to better performance and longer die life.
Related Reading: 10 Types of Quenching Methods in Heat Treatment Process
2.8 Reinforcement and tempering of composites
Heating the M2 steel mold to 1180-1190°C and then isothermally treating it for 1-1.5 hours below the Ms point, followed by two cycles of nitrate tempering at 560°C for 2 hours, may result in a Bbelow+M multiphase structure. This process increases flexural strength by 56% compared to oil quenching.
By extruding 8 steel parts, the service life is significantly improved and the part suffers less wear.
In another example, changing the quenching and tempering process of the H13 steel matrix to quenching by heating at 1030°C, followed by isothermal classification at 250°C for 10 minutes, results in a 33.4% increase in the aK value and 1.6-6 times greater service. service life compared to 3CrW8V steel.
2.9 Tempering in the first type of tempering of the brittle zone
Everything in the world is relative and not absolute. The first type of tempering brittle zone for T10A steel and GCr15 steel is between 230-270℃, while tempering is normally carried out at 180-200℃.
Some individuals prefer hardened steel in the first type of brittle and tempered zone as it results in high fatigue resistance.
For cold work dies that experience low stress concentration and are subject to tension, compression and bending stress, the initiation of fatigue cracks determines their service life. Therefore, it is important to maximize your strength.
This process can produce remarkable results.
2.10 Surface reinforcement
All types of mold failures usually originate at the surface, so it is important to focus on the “surface”. This can be achieved through various treatments such as carbonitriding, nitrocarburizing, oxidation after nitriding, steam treatment, TD treatment, surface coating, boronization, metallization, sulfurization, boron-sulfur composite carburizing, surface induction heating, hardening. laser, etc.
It is important to note that not all molds can be strengthened through these treatments. Current methods for strengthening mold surfaces around the world are as follows:
Thermal method
- Induction hardening
- Flame hardening
- Extinguishing by electron beam
- Pulse extinction
- Laser remelting
- Welding
- Laser extinguishing
Thermochemical method
- Boronization
- Nitriding
- Cover hardening
- Carboamination
- Vulcanization
- Laser Remelting Alloy
- Laser reinforcement
- Oxidation
Electrochemical method
- Hard chrome plating
- Nickel Plate
- Cadmium plating
Mechanical method
- Rolling
- Air jet treatment
- Polishing
- Compaction
- Shot peening hardening
- Barrel coating
Thermodynamic method
- Spray
- Explosive coating
Chemical/physical method
- Ionic coating
- Ion transplant
- PVD Coating
- CVD Coating
- CVD Plasma Coating
2.11 Improve the thermal fatigue resistance of hot working die
Thermal cracking and thermal fatigue affect the resistance of materials to high temperatures and the condition of the die surface. EDM-induced scratches and deformations can contribute to the formation and growth of cracks, so measures are taken to resolve these problems.
1) To increase the thermal fatigue resistance for Y10 steel molds, it is recommended to increase the quenching temperature and tempering temperature appropriately.
2) Decarburization should be avoided as it expands thermal fatigue cracks and reduces thermal fatigue resistance.
3) Nitriding, especially when a composite layer is present, can prevent the formation of thermal fatigue cracks.
4) Low surface roughness and wear lines can decrease thermal fatigue resistance.
5) Increasing strength and plasticity at high temperatures can help improve thermal fatigue resistance.
6）The large deformation layer caused by EDM can negatively impact thermal fatigue resistance.
7）High temperature tempering has a lower sensitivity to thermal shock cracking compared to low temperature tempering.
8) Coating a hot working die can improve its thermal fatigue property and wear resistance.
2.12 Die heat treatment deformation correction method
Heat treatment deformation is a normal occurrence and the key is to understand the deformation patterns and make efforts to correct them. The following methods can be used for correction:
1）The superplasticity principle of martensitic transformation can be utilized for timely correction. This can be done by tempering and cooling 4 m long mechanical blades and 1.5 m long broaches to the appropriate temperature and then gently applying pressure for correction. The same approach can be used for straightening molds.
2) Pressure tempering: involves a temper that applies pressure to correct temper distortion, such as for large and thin sheets.
3) Correction of cold treatment: For stainless steel parts that have a larger amount of retained austenite, cryogenic treatment at -70℃ for 1-2 hours may cause expansion in size. The Cr12 steel matrix is the most suitable for this correction.
4) Hot spot correction: The most convex part of a bent part can be quickly heated to about 700℃ using an oxyacetylene flame or high-frequency induction heating device, quickly cooled, and then corrected.
5) High-frequency shrinkage cavity correction: The swollen workpiece can be heated to about 700℃ in an induction coil and cooled quickly, creating a shrinkage cavity. If there are multiple shrinkage cavities, stress relief treatment must be carried out.
6) Electroplating thickening correction method.
7） Chemical corrosion correction: This can be achieved by using a corrosive agent such as 40% HNO3+60% H2O or 20% HNO3+20% H2SO4. Parts that do not require corrosion must be protected with asphalt or paraffin.
8) Correction of shrinkage cavity by rapid cooling: For workpieces with enlarged cavities, they can be annealed and heated to 700℃ and then rapid cooled 1-2 times for correction.
3. Conclusion
Science and technology are the main driving forces of production. The 12 technical measures to extend the life of molds, as discussed above, are economical and practical.
By carefully studying the causes of mold failure, developing rectification plans and implementing appropriate technical measures, it is possible to create high-quality molds with a long service life.
