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Vacuum Heat Treatment Furnace

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Vacuum heat treatment process of rolling die

The thread rolling die, also known as the rolling wheel, is used to thread the workpiece on the thread rolling machine. The most important characteristics in selecting a rolling die material are the hardness, toughness and wear resistance of the die steel. There must be sufficient hardness and toughness so that the die can withstand the forces acting on it in use; good wear resistance is also necessary, because the main reason for removing the rolling die from use is due to the thread on the die The peak and tip are worn, so that the rolled thread no longer meets the requirements of the overall size or function, and can not continue to be used. Therefore, a good heat treatment level is very important to prompt its quality and life.

Thread rolling dies often fail early in the process of use. The failure modes include tooth pattern peeling, chipping, stacking teeth, premature tooth pattern wear, and tooth type peeling. The average service life is less than 200,000 pieces. The main reason is that the common defects such as surface decarburization, overheating and corrosion in conventional heat treatment seriously reduce the surface hardness, strength, wear resistance, fatigue resistance and other properties.

The study found that if general quenching (such as salt bath furnace quenching) is used, it is difficult to avoid the surface decarburization of the working face of the rolling die, and the surface decarburization (or carbon depletion) will cause the service life of the rolling die to be greatly reduced. Therefore, vacuum quenching is the best process method. Vacuum heat treatment is carried out in a very thin atmosphere. The pressure in the furnace is generally in the range of 1.33 to 0.13 Pa. The quenching heating and cooling are completed under non-oxidizing conditions. Decarburization, no carbon increase, bright surface of workpiece after treatment, small deformation (generally 1/5 of salt bath treatment), long life (generally 3 to 4 times longer than salt bath treatment), and due to the whole quenching process of the wire rolling die All operations are completed in a closed furnace, with high degree of mechanization and automation, low labor intensity of workers, and no pollution to the environment.

Vacuum heat treatment has the advantages of high quality, low energy consumption, no pollution (or less pollution), etc. It can effectively prevent the defects of the rolling die, improve the surface roughness and reduce the friction factor. Vacuum heating is carried out in the form of radiation, the heating is slow, the furnace temperature is uniform, the product deformation is small, and it also has a dehydrogenation effect, which is beneficial to improve the cutting life of the rolling die. times (average number of rolled pieces). The wire rolling dies quenched by vacuum furnace, box furnace and salt bath furnace quenched by cutting comparison test, and their lifespans from high to low are: vacuum furnace, box furnace, salt bath furnace. It can be seen that the advantages of vacuum heat treatment in the treatment of rolling dies.

Thread rolling die processing technology:

The process route of rolling die processing: forging → isothermal annealing → machining → stress relief annealing → machining → vacuum quenching, tempering → machining.

Isothermal annealing

In general, after CrWMn steel is forged, a good spherical structure can be obtained as long as it undergoes isothermal annealing. The process is: heating at 770～790℃, holding for 2～3h, cooling to 680～700℃ at ≤30℃/h for isothermal 4～5h, then furnace cooling to 500℃ at ≤30℃/h, air cooling, isothermal annealing The hardness of the latter steel is generally 207-255 HBS, and the pearlite structure is 2-4 grades, which not only reduces the hardness, but also changes the machining performance.

Stress relief annealing

The process is heating to 640-660°C, then holding for 2-3 hours, furnace cooling or air cooling, and the internal stress of machining can be reduced or eliminated after stress relief annealing.

The isothermal annealing and stress relief annealing process curves of CrWMn steel rolling die are shown in the figure.

Vacuum quenching and tempering

According to the workpiece size and heat treatment requirements, the SIMUWU double-chamber vacuum furnace is selected for the quenching and tempering of the wire rolling die. The size of the effective heating area of the equipment is 600mm×400mm×400mm, and the maximum loading capacity of the workpiece is 100kg. The grid-type stainless steel material frame (500 mm×300mm×100mm) is selected, and each furnace can load up to 18 workpieces. The wire rolling die is vacuum quenched. , The tempering process is as follows.

Vacuum heat treatment operation process:

Preparation before processing

1) Check the surface of the workpiece. The surface of the workpiece should be free of bumps, rust spots and unacceptable surface defects.

2) Check the equipment before opening the furnace, such as valves, water pressure, etc., so that the equipment is in a normal state.

3) Clean and dry workpieces and fixtures. 3.

Workpiece heat treatment operation

1) Open the front furnace cover, send the workpiece and material basket into the cooling chamber, and close the front furnace cover.

2) Turn on the power switch, and after detecting the vacuum degree in the furnace, start the mechanical vacuum pump and the mechanical booster pump in sequence, and vacuumize the heating chamber and cooling chamber at the same time.

3) When the vacuum degrees of the cooling chamber and the heating chamber tend to be the same and are below 13.3 Pa, open the vacuum insulation door, send the workpiece into the heating chamber with the material basket, and close the vacuum insulation door.

4) When the vacuum degree in the furnace reaches below 6.67Pa, turn on the heating power and heat up. Note that the heating time of the vacuum furnace is twice as long as that of the air furnace. For workpieces with a heating temperature below 900 °C, it must be preheated at about 600 °C and then raised to the heating temperature. When the workpiece is heated and kept warm, the holding time (60-100) min is determined according to the size of the furnace. During the heating process of the workpiece, the vacuum degree in the furnace is generally maintained at about 1.33Pa.

5) At the end of the heating and heat preservation stage, open the vacuum insulation door, transfer the workpiece to the cooling chamber with the material basket, close the vacuum insulation door and open the backfill nitrogen and fill the cooling chamber with 99.999% pure nitrogen, so that the cooling chamber pressure reaches 6.65×10 *Pa, start the vacuum quenching oil stirring device, and sink the workpiece into the vacuum quenching oil with the material basket to cool.

6) After the workpiece is quenched, it is cleaned and then sent to the furnace for tempering. When the vacuum degree in the furnace reaches 1.33Pa, 99.999% pure nitrogen is introduced into the furnace, and the electric fan is used for circulation after the pressure reaches 6.65×10°Pa. to the end of the tempering process.

Selection of vacuum heat treatment equipment: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.