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Common 11 problems and treatment methods of fixture heat treatment

In the design, manufacture, use and repair of fixture, problems often occur due to improper selection of materials and heat treatment, which affects the quality of products. When dealing with these problems, it is not only necessary to improve the material and heat treatment, but also to change the structure of the fixture.

Question 1

When positioning circular workpiece with V-block as positioning part, if the radial clamping force is needed to be increased and 45 steel or 20Cr is used for carburizing and quenching, the positioning surface is in line contact with the workpiece, the pressure per unit area is large, and the material surface is hard and the inner part is soft, which is prone to produce pits, thus reducing the service life of V-shaped block.
Treatment method: the materials of V-shaped block can be made of carbon tool steel T8A, T10A or alloy tool steel CrWMn and CrMn, and the quenching hardness is 58-63hrc, so the service life of V-shaped block can be improved.

Question 2

For those parts with special shape or very wide thickness difference, cracks are easily produced due to uneven cooling speed during heat treatment. If the parts shown in Figure 1 are made of carbon tool steel (avoid as much as possible in design), cracks and even whole ring fracture are easily produced during quenching.

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Figure.1 parts with high susceptibility to quenching cracks (arrow refers to the crack prone parts)
Treatment method:

  • ① The sharp angle is changed to the fillet to avoid stress concentration during quenching and the thickness is increased appropriately;
  • ② Reduce the hardness requirements of parts, and use the method of heat treatment and then machining;
  • ③ Alloy steel with good hardenability and slow cooling can be used to make it;
  • ④ The parts where cracks are easy to occur and their vicinity are not hardened.

Question 3

Large positioning parts with uneven thickness and rectangular surface (large V-shaped block shown in Fig. 2a) are made with carbon tool steel T8A or alloy tool steel CrMn, and cracks are also easy to occur during quenching, as shown in the figure breaking crack line.

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Figure.2 Influence of large V-shaped block on its structure
Treatment method:

  • ① Change the right angle transition to fillet transition, as shown in Fig. 2B;
  • ② The composite structure is used. As shown in Fig. 2c, the positioning surface is equipped with high hardness gasket, so that the main body of V-block can not use high hardness material. The gasket is fixed with screws and pins, but the flatness of V-groove surface will be affected by this structure, which will cause some trouble to clean and locate the surface;
  • ③ If the clamping force of V-groove is not large, the carburizing steel (35 steel or 20Cr) can be used, which can also reduce the possibility of quenching cracking.

Question 4

For the pin hole parts that need to be drilled, if the relationship between the direction of drilling and the hardness of the material is not considered, the problem of non matching drilling will occur.

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Figure.3 selection of hardness of drilling parts
Treatment method: as shown in Figure 3, two parts need to be drilled, and the drilling direction is from top to bottom. When selecting material hardness, it is necessary to pay attention to that the hardness of the drilling part of the lower bottom plate cannot be high, otherwise, drilling matching will not be carried out. If other parts of the base plate need high hardness, the local carburizing of carbon steel can be used to meet this requirement.

Question 5

For parts requiring pressure sleeve, if the hardness of material does not affect the accuracy of position and convenience of the press sleeve, it cannot meet the manufacturing requirements and use requirements. For the pressure sleeve parts shown in Figure 4, the inner hole of the bushing needs to have a higher hardness. When the tolerance δ of its position dimension L is strict, the hardness of the main part cannot be high because the main mounting hole needs to be processed after heat treatment, otherwise it will be difficult to use coordinate boring machine for processing.

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Figure.4 selection of material hardness of pressure sleeve parts
Treatment method: if the other parts of the main body need to have higher hardness requirements, it can be made of cemented steel and local carburizing is carried out to meet the requirements of use. However, in some cases, if the main body needs to adopt all materials with high hardness, the outer circle of the bushing shall be kept low hardness to avoid the “hard hitting” pressing sleeve method; at this time, the liner can be made of carburizing carbon steel, and the internal hole of the bushing can be carburized locally to achieve high hardness, while the outer circle shall not carburize to keep the low hardness.

Question 6

For large flat plate base, as shown in Fig. 5a, when the flatness of upper and lower plane and the parallelism between them are required to be high, scraping is required to ensure, if steel material (such as 45# steel) is used, it is difficult to scrape, and it is easy to deform naturally and it is not easy to guarantee the precision requirements.

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Figure.5 material selection and corresponding structure of large flat base
Treatment method: the large flat base is better to adopt castings (such as ht21-40), so the structural shape of the base plate should be changed accordingly, as shown in Fig. 5B. It should be noted that the rigidity of castings depends on the reasonable section form, that is, the reinforcement should be arranged reasonably, rather than simply increasing the wall thickness. For the wall thickness of castings, for the fixture, it is usually 12-16mm.

Question 7

For parts with large size and high hardness and high precision requirements, the integral drilling template with two holes close to each other but uneven wall thickness as shown in Fig. 6A. Even if the alloy tool steel with small heat treatment deformation is adopted, it is easy to produce large deformation after advanced finishing and then heat treatment. The precision obtained by the finishing before the heat treatment is damaged, which can not guarantee the precision requirements of drilling formwork, and it is also used It’s not economical either. But if the material with low hardness is used, such as 45 steel, it will be easy to wear when it is used for finishing after heat treatment.

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Figure.6 heat treatment deformation of integral drilling template and application of flat drilling sleeve
Treatment method: for this drilling template, the material with low hardness can be used, and two flat grinding drilling sleeves can be inserted, as shown in Fig. 6B. Because the inner and outer circles and flat surfaces of the flat drilling sleeve can be finished after heat treatment, so that the requirements of dimension accuracy can be guaranteed.

Question 8

For the carburized surface which is easy to wear during use, the carburizing layer will be worn off when repairing, and the lower the hardness of the carburizing surface is, the service life of the fixture will be affected.
Treatment method: for the surface which is seriously worn and often needs repair, it is better to adopt the combination structure, and insert the parts made of high hardness carbon tool steel or alloy tool steel in the easily worn place.

Question 9

For large area carburizing surface, due to the large deformation after quenching, after the deformation is removed during the manufacturing process, the carburizing layer at both ends is also ground off, which affects the performance of the use.
Treatment method:

  • ① The area of carburizing surface is reduced and the surface is changed to intermittent surface, which can reduce deformation and reduce the carburizing layer.
  • ② The main parts are not cemented, but the parts made of high hardness carbon tool steel or alloy tool steel are inserted on the parts, and the composite structure is changed.

Question 10

When casting aluminum is used as the main body of fixture, for parts (such as locating pin, positioning shaft, etc.) which are interference fit with it, when subjected to alternating load or large cutting force, the structure of cast aluminum is loose, which is easy to loose.
Treatment method: the main material of fixture is changed from cast aluminum to cast iron or steel, which can increase the strength of interference fit.

Question 11

The friction surface of clamping device contacting with other parts of the fixture is not protected by wear pad due to the large force, so it will affect the normal operation of the fixture due to the fast wear, and it needs to be repaired frequently.

Treatment method: wear pad should be set for protection in the parts where the fixture is stressed and wear is serious. The wear pad shall be made of materials with high hardness, such as T8A, with hardness of 53-58hrc.

Source: Network Arrangement – China Pipe Sleeves Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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