How to Control the Surface Strain of Stamping Parts?

As a common quality defect in the production process, the stamping parts’ strain is common in the major automobile factories.

On the one hand, it reduces the stability and production efficiency of the production process and increases the scrap rate of parts.

On the other hand, it will cause more serious wear of the die, reduce the life of the die and the precision of the stamping parts, and increase the number of die repairs and production downtime.

stamping parts' strain

The essence of galling is that the workpiece and the die surface are partially adhered (occluded).

There are many ways to improve the galling problem.

The basic principle is to change the nature of the friction pair between the die and the machined parts, so that the friction pair is replaced by materials that are not easy to adhere.

After the mold enters the production site commissioning stage, the following methods are generally used to improve the galling problem:

1. Change die material and increase die hardness;

2. The surface of the mold is treated, such as hard chromium plating, PVD and TD, etc;

3. Coating the mold cavity with nano coating, such as RNT technology;

4. Add a layer of other substances between the mold and the processed parts to separate the processed parts from the mold (for example, apply lubricant or special lubricant or add a layer of PVC and other materials);

5. Use self-lubricating coated steel plate.

In terms of mold materials, mold steels SKD11, Cr12MoV, etc. are recognized as wear-resistant and anti bite materials.

The hardness after heat treatment can reach about HRC58-63 degrees of chromium hardness. Such materials can be used when the mold is small and the shape of the parts is relatively simple.

However, the material is difficult to process after heat treatment, brittle, easy to crack, high cost, limited size, large deformation after heat treatment, and huge research and matching work after heat treatment.

The shape of automobile inner panels is more complex and more and more high-strength steel plates are used.

Such products have higher requirements on the overall performance of the mold.

They usually adopt the inlay structure.

At present, the surface treatment processes of inlays include TD, hard chromium plating, nitriding, PVD, etc.

TD treatment is the abbreviation of thermal diffusion carbide coating process.

This technology was first developed and patented by Toyota Central Research Institute of Japan in the 1970s.

It is also called Toyota diffusion process, or TD Process for short.

It is also called molten salt infiltration in China.

Regardless of its name, the principle is to place the workpiece in a molten borax mixture and form a metal carbide coating on the surface of the workpiece through high-temperature diffusion.

The main features of TD cladding treatment are:

The coating has high hardness, HV up to about 3000, with high wear resistance, tensile damage resistance, corrosion resistance and other properties.

The service life of TD coating is about 100000 sets;

However, TD coating treatment has high requirements for mold materials, and the changes of thermal stress, phase transformation stress and specific volume during high-temperature treatment will make the mold easy to deform or even crack during heat treatment.

Generally, the repaired mold will also crack at the weld.

TD coating treatment has high requirements for the processing quality and shape of the mold;

In addition, it is difficult to reprocess after TD coating treatment, which cannot meet the needs of design change and mold adjustment and repair.

For molds that have been subject to other surface treatment, the original surface treatment shall be completely removed, otherwise the surface quality of TD coating will be affected.

In addition, the service life of TD coating treatment technology will be reduced after 3-4 times of treatment.

TD coating treatment

Fig. 2 TD processing

PVD (physical vapor deposition) is the physical vapor deposition method, and PVD coating is the surface coating manufactured by the physical vapor deposition method.

It has good tensile damage resistance.

The hardness of the coating can be as high as HV2000-3000 or even higher, so it has excellent wear resistance.

In addition, it has the advantages of low treatment temperature, small deformation of the treated workpiece, and can be treated for many times without affecting the service life.

However, the bonding force between the coating and the substrate is poor.

When it is used on deep drawing dies and dies with high forming pressure, it is easy to make the coating fall off, so it can not play its tensile damage resistance and wear resistance effect.

PVD coating

Fig. 3 PVD coating

The size of the outer plate mold is generally large.

If the mosaic block structure is adopted, there will be strain at the seam, so the overall structure is mostly adopted, and the materials are generally ductile iron and other cast iron materials.

The hardness of the formed feeding part after flame quenching can reach about HRC50-55 degrees.

Hard chromium plating process is mostly used for the surface treatment of the outer plate mold of the overall structure, but its surface hardening effect is limited, and the surface hardness is about 1000HV.

In addition, the hard chromium plated coating is mechanically combined with the base metal of the die, and it is easy to fall off when the forming pressure is large.

Once the coating falls off, the tensile damage resistance will be lost.

When the surface hardening layer is worn, galling will occur again, and the service life of the surface hardening layer is generally about 5-10 years.

chromium plating

Fig. 4 chromium plating

RNT is a new technology in recent years.

Its working principle is that after the RNT coating solution is applied to the mold cavity, the nano molecules of the coating are diffused by pressure and act on the mold surface to form a nano metal carbide coating.

The process expands from inside to outside.

The thickness and hardness increase with the increase of the working time of the mold.

The coating thickness is 0.1-1μm, and the coating hardness is HV1100-1600.

Even when the die bears a large load, the coating layer on the surface will not fall off due to the plastic deformation of the substrate, and its thickness and hardness will increase with the increase of the working time and coating times of the die from the inside to the outside.

Applying RNT coating once can generally ensure 100-500 pieces without galling.

However, the application of this technology to parts with serious roughening, parts heated in the production process and ultra-high strength plates is not mature, and the use cost is high.

roughening before using RNT

Fig. 5 roughening before using RNT

roughening after RNT

Fig. 6 roughening after RNT

The use of reasonable lubricant in the production process can effectively improve the friction conditions and reduce galling.

Its main function is to separate the contact pairs with lubricating oil film.

Generally, manual oiling or automatic equipment oiling is used for the wire head.

In addition, the use of lubricant can effectively reduce the problems of hidden injury and cracking. However, the use of lubricants will make the environment dirty and slippery.

In order to improve the impact of oiling on the working environment, Baosteel, WISCO, Masteel and other iron and steel enterprises have developed self-lubricating steel plates in recent years.

The steel plates with self-lubricating coatings have excellent self-lubricating, corrosion resistance, fingerprint resistance, processing formability and painting performance.

The main thing is to roll a layer of organic coating on the steel plates, and no more lubricating oil is required during the stamping process.

However, the use cost is slightly higher and has not been widely used.

Because the forming load and the formed materials are very different, what or several measures should be taken to solve the problem of workpiece strain, in addition to the effectiveness of the effect, we must also consider the batch size of the product, the difficulty of realization and its economy, and finally select the most appropriate method.

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