12 Technical Measures to Improve Mold Life

Mold is the main process unit of industrial production, and mold industry is the basic industry.

Mold is internationally known as the “emperor” of metal processing, and the mold industry is a symbol and barometer to measure a country’s comprehensive economic and technological level.

All countries in the world attach great importance to the development of the mold industry.

However, due to lack of talents and backward technology, mold manufacturing cycle is long, quality is poor, cost is high, and mold life is unsatisfactory.

Related reading: How to Improve Mold Quality?

According to the statistics and analysis of relevant parties, among many factors of die failure, material and heat treatment account for 50%, which shows how important the selection of materials and heat treatment of dies are.

12 Technical Measures to Improve Mold Life 1

1. Comparison of die life at home and abroad

According to the report of the 11th issue of China Die Information in 2001, the comparison of die life at home and abroad is shown in Table 1.

In the past 20 years, the overall level of molds in China has not changed compared with that of foreign countries.

However, on the whole, there is still a big gap between China and foreign countries in terms of large, precise, and complex long-life molds.

Table 1 Comparison of die life at home and abroad

Mold type

Molded parts, materials and dimensions

Mold material

Total life of die (punching times, parts)

Advanced world standard

Domestic level

Blanking die

Brass, low carbon steel plate;

Flat blanking parts;

Material thickness ≤ 1mm, size 40mm × 40mm, φ 45mm

Carbon tool steel T8, T10 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 cemented carbide YG15, YG20

600 million to 3 billion

<50 million

Silicon steel plate for motor rotor and stator, material thickness ≤ 0.5mm, size < 200mm

Hard alloy (multi station continuous blanking die)

US Linina: 300 million

38 million~50 million

Kuroda Seiko: 270 million

Statomat, Switzerland: 80 million

Stellrem, UK: 100 million

Fine blanking die

 

 

Mild steel with wC ≤ 0.2%;

Pull rod, cam, base plate and other fine blanking parts with material thickness less than 3mm or 3-6mm

Alloy tool steel: Cr12MoV

500000~1000000

<150000

Alloy tool steel: Cr12MoV

High speed tool steel: W6Mo5G4V2

300000~600000

100000~120000

Die-casting die

Aluminum alloy parts

Cr-Ni steel, 3Cr2W8

> 450000

<200000

Forging die

Steel, crankshaft

Cr Ni steel, 5CrNiMo

14000~20000

5000~7000

Injection Mold

ABS, medium

alloy tool steel

> 500000

200000~300000

Polyethylene, medium

alloy tool steel

> 2 million

500 thousand

2. Technical measures to improve die life

2.1 Adopt pure high-performance die steel

Materials are the foundation, and the foundation is unstable.

The specific components of cold work die steel, hot work die steel and plastic die steel are listed in the Tool and Die Steel (GB/T 1299-2014), and strict requirements are put forward for impurities and content.

However, the quality disputes between the seller and the buyer of commercially available die steel are ongoing.

We should purchase from regular channels instead of being greedy for cheap.

We should give priority to powder steel, spray steel and high-quality steel with high purity.

In addition, when selecting 3Cr2W8V steel to manufacture hot working dies, attention should be paid to its carbon content.

The advanced foreign standards are all wc=0.25%~0.35%, while the Chinese standard is wc=0.30%~0.40%.

This steel has always followed the 3X2B8 Ø steel standard of the former Soviet Union.

The Russian standard TOCT 5950-2000 has been changed to wC=0.27%~0.33%, while the Chinese standard has not been modified.

The practice has proved that the high carbon content of 3Cr2W8V steel is harmful and useless, and many early failures are caused by it.

2.2 Conduct strengthening and toughening treatment

When the medium carbon medium alloy hot work die steel cools slowly after forging or the die blank section is large( φ> 100mm), the chain carbide is easy to appear in the structure, leading to early brittle fracture, hot cracking and crazing failure of the die.

Therefore, it is necessary to eliminate it by means of tissue pretreatment to improve the strength, toughness and service life of the die.

3CrMoW2V steel is normalized at 1130 ℃, which can dissolve M6C carbides.

When the air cooling rate is greater than 15 ℃/min, it exceeds the critical cooling rate to form chain carbides, which can eliminate chain carbides, and obtain carbides with uniform particle distribution after subsequent spheroidizing annealing.

2.3 New pretreatment heat treatment process with energy conservation and consumption reduction

1) The thermomechanical treatment of residual heat annealing after forging is adopted.

2) A new process of rapid homogenizing spheroidizing annealing is adopted.

3) Hot work die steel is changed from conventional high temperature tempering to medium temperature tempering.

4) Increase quenching and tempering treatment.

2.4 Heat treatment with vacuum quenching or protective atmosphere

Since the successful vacuum quenching of Cr12MoV steel dies in the late 1980s, the application of vacuum quenching of dies has become increasingly popular, especially high-pressure gas quenching.

2.5 Cryogenic treatment

When the quenched die is subjected to cryogenic treatment below – 110 ℃, fine carbide residues are precipitated, and the residual austenite is transformed into martensite, which can improve the wear resistance, tempering resistance and dimensional stability.

The service life of M12 nut cold heading die is increased by two times after cryogenic treatment, and the service life of aluminum alloy hot extrusion die is increased by one time.

2.6 Cooling and quenching

The mold is made of high-speed steel. Its quenching temperature is different from that of the tool. The lower quenching heating temperature, i.e. cooling quenching, is generally used.

For example, the quenching temperature of W18BCrV steel is 1180~1200 ℃, and that of M2 and W9 steel is 1160~1180 ℃.

Low temperature quenching can obtain good strength and toughness, reduce the tendency of deformation, cracking and tool breakage, and improve the performance, quality and life of the die.

2.7 High temperature quenching

Hot working dies made of 5CrNiMo, 5CrMnMo, 3CrW8V and other steels should be quenched at a higher temperature to obtain more lath martensite, improve fracture toughness and thermal fatigue resistance, and improve their performance and life.

Related reading: 10 Types of Quenching Methods in Heat Treatment Process

2.8 Composite strengthening and toughening

M2 steel mould, heated at 1180~1190 ℃, isothermal for 1~1.5h below Ms point, 560 ℃ × 2h × 2 times of nitrate tempering can obtain Bbelow+M multiphase structure.

Compared with quenching oil, the bending strength is increased by 56%.

When extruding 08 steel workpiece, the service life is greatly improved, and the workpiece is worn out.

For another example, H13 steel die is changed from conventional quenching+tempering to 1030 ℃ heating quenching.

After isothermal classification at 250℃×10min, the aK value is increased by 33.4%, and the service life is 1.6~6 times higher than that of 3CrW8V steel.

12 Technical Measures to Improve Mold Life 2

2.9 Temper in the first type of tempering brittle zone

Everything in the world is relative, not absolute. The first type of tempering brittle zone of T10A steel and GCr15 steel is 230~270 ℃, and generally 180~200 ℃ is used for tempering.

Some people prefer to temper the steel in the first kind of temper brittle zone, which can obtain high fatigue resistance.

For cold working dies with low stress concentration and bearing tension compression bending stress, since their life mainly depends on the initiation of fatigue cracks, their strength should be improved as much as possible.

This process can achieve remarkable results.

2.10 Surface strengthening

All kinds of mold failures are mostly started from the surface, so we should do a good job of “superficial”.

These include carbonitriding, nitrocarburizing, oxidation after nitriding, steam treatment, TD treatment, surface coating, boronizing, metallizing, sulfurizing, boron sulfur composite carburizing, surface induction heating, laser quenching, etc.

Not all molds can be strengthened and targeted.

At present, the mold surface strengthening methods in the world are shown below:

Thermal method

  • Induction hardening
  • Flame hardening
  • Electron beam quenching
  • Pulse quenching
  • Laser remelting
  • Welding
  • Laser quenching

Thermochemical method

  • Boronizing
  • Nitriding
  • Case hardening
  • Carboamination
  • Vulcanization
  • Laser remelting alloy
  • Laser reinforcement
  • Oxidation

Electrochemical method

  • Hard chromium plating
  • Plate with nickel
  • Cadmium plating

Mechanical method

  • Rolling
  • Air-jet treatment
  • Polishing
  • Compaction
  • Shot peening hardening
  • Barrel plating

Thermodynamic method

  • Spray
  • Explosive coating

Chemical/physical method

  • Ion plating
  • Ion transplantation
  • PVD coating
  • CVD coating
  • Plasma CVD coating

2.11 Improve the thermal fatigue resistance of hot working die

Thermal cracking and thermal fatigue determine the high temperature strength of materials and the state of die surface.

Scratches and EDM deformation layers will promote the generation and expansion of cracks, for which a series of measures have been taken.

1) For Y10 steel mould, the quenching temperature and tempering temperature shall be appropriately increased to enhance the thermal fatigue resistance.

2) Avoid decarburization, because decarburization will expand thermal fatigue cracks and reduce thermal fatigue strength.

3) Nitriding, especially when there is a compound layer, can prevent the generation of thermal fatigue cracks.

4) Poor surface roughness and wear lines will reduce the thermal fatigue resistance.

5) Increasing the high temperature strength and plasticity is beneficial to improving the thermal fatigue strength.

6) The large deformation layer in EDM will damage the thermal fatigue strength.

7) High temperature tempering has lower thermal shock crack sensitivity than low temperature tempering.

8) The coating of hot working die can improve the thermal fatigue property and wear resistance.

12 Technical Measures to Improve Mold Life 3

2.12 Correction method of die heat treatment deformation

Heat treatment deformation is normal, and the key is to master the deformation law and try to correct it.

The methods of deformation correction are introduced as follows:

1) The principle of martensitic transformation superplasticity is used for timely correction.

The 4m mechanical blade and 1.5m long broach are quenched and cooled to the appropriate temperature.

The correction can be completed by gently applying pressure, and the same is true for mold straightening.

2) Pressure tempering: It refers to the tempering that applies pressure to correct quenching distortion, such as large and thin blades.

3) Cold treatment correction: for stainless steel quenching parts with more retained austenite, the size expands during cryogenic treatment at – 70 ℃×1~2h, making Cr12 steel die is the most appropriate.

4) Hot spot correction: at the most convex part of the bending piece, use the oxyacetylene flame or high-frequency induction heating device to quickly heat it to about 700 ℃, quickly cool and shrink it, and then correct it.

5) High frequency shrinkage cavity correction:

The swollen workpiece can be heated to about 700 ℃ in the induction coil and cooled quickly to play the role of shrinkage cavity. In case of multiple shrinkage cavities, stress relief treatment shall be carried out.

6) Electroplating thickening correction method.

7) Chemical corrosion correction: the corrosive agent includes 40% HNO3+60% H2O or 20% HNO3+20% H2SO4.

Asphalt or paraffin shall be used to protect the parts that do not need corrosion.

8) Correction of rapid cooling shrinkage cavity:

For the workpiece with enlarged cavity, it can be annealed and heated to 700 ℃ and then rapidly cooled for 1-2 times for correction.

3. Conclusion

Science and technology are the primary productive forces, the 12 technical measures to improve the service life of moulds shown above are economical and practical.

As long as we carefully study the causes of mold failure, formulate rectification plans, and take corresponding technical measures, we will be able to create high-quality, long-life molds.

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