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Warm extrusion die vacuum heat treatment

Warm extrusion is a new process developed on the basis of cold extrusion. It heats the blank to an appropriate temperature for extrusion. Due to the heating of the metal, the deformation resistance of the blank is reduced, the forming is easy, the tonnage of the press can also be reduced, and the life of the mold can be extended. Especially the materials that are difficult to be processed by cold extrusion at room temperature, such as stainless steel, medium and high carbon steel, high alloy steel, and even bearing steel, high-speed steel, etc., can adopt the warm extrusion process. Although warm extrusion needs to heat the metal, the total processing cost is still relatively cheap, especially when manufacturing complex non-axisymmetric special-shaped parts, warm extrusion can play its advantages. It is widely used in the manufacture of auto parts, bearings and other metal parts.

Features of warm extrusion die:

The warm extrusion die is subjected to high pressure and deformation heat during the extrusion process. The maximum unit deformation force can be as high as 2000~2500 MPa, and the die temperature can reach 300℃~500℃ or higher during continuous production. Therefore, as a warm extrusion die, it should have the following characteristics:

1) Sufficient hardness, strength and toughness to resist damage at room temperature and medium temperature;

2) Under the action of repeated deformation force and heat, it must have high wear resistance and fatigue resistance;

3) The selected mold material should have good processing technology.

W6Mo5Cr4V2 high-speed steel is the representative steel grade of tungsten-molybdenum series general high-speed steel. This steel has the advantages of fine and uniform carbide, high toughness and good thermoplasticity. However, there are a lot of eutectic carbides in this type of steel, which will reduce the strength, toughness and hot hardness of the steel. In addition, this type of steel is easy to oxidize and decarburize, and the reduction of carbon content will significantly reduce the quenching hardness, wear resistance, and fatigue properties of the steel, and the decarburization of high-speed steel will reduce the red hardness. Therefore, a reasonable forging process and heat treatment process must be adopted.

Forging process:

Although the raw material of die steel has undergone billeting and rolling, the carbides have been broken and refined to a certain extent, but it still cannot meet the requirements of warm extrusion die, especially the impact toughness index, and can only be broken by changing forging. Coarse carbides are uniformly distributed and carbide segregation is improved. In order to meet the use requirements of the warm extrusion die, the variable-direction forging method is adopted. The characteristics of this forging method are that the direction of upsetting deformation is continuously changed, the particles of each part of the blank are fully deformed, and the carbide segregation level in the entire upsetting volume is relatively uniform.

Heat treatment process:

1.Post-forging heat treatment

Isothermal spheroidizing annealing is as shown in the figure, heating at 850~870℃, holding for 3~4h, rapidly cooling to 730~750℃, holding for 6~8h, furnace cooling to below 550℃, and air cooling, the austenite transforms to pearlite to obtain granular Pearlite structure + carbide. The purpose of isothermal spheroidizing annealing is to eliminate the coarse and uneven structure and Widmanrite structure caused by the forging process, to refine the grains and facilitate machining. It is mainly suitable for alloy steel with a long annealing cycle. In addition, the pearlite after isothermal annealing is obtained at a constant temperature, and the forging can obtain a consistent internal and external structure.

2.Vacuum heat treatment

During heat treatment, the effects of oxidation, decarburization and carbon increase on the surface of the treated mold parts will have a serious impact on the service life of the mold. In order to prevent oxidation, decarburization and carbon increase.

Using vacuum heat treatment technology, it is especially suitable for heat treatment of molds. Die steel has a good surface state after vacuum heat treatment, its surface is not oxidized, not decarburized, and the quenching deformation is small. Compared with the quenching process in the atmosphere, after vacuum quenching, the surface hardness of the mold is relatively uniform and slightly higher. During vacuum heating, the surface of the die steel is in an active state, without decarburization and without an oxide film that hinders cooling. After vacuum quenching, the fracture toughness of steel has been improved, and the life of the mold has been increased by more than 40% or even higher than that of the conventional process. The mold vacuum quenching technology has been widely used in my country.

1) Quenching temperature

With the increase of quenching heating temperature, the content of carbon and alloying elements increases, and the hardness and red hardness of steel increase. The higher the quenching heating temperature, the higher the hardness and red hardness of the steel, but the hardness of the steel decreases after transition quenching. Therefore, quenching at 1210~1230℃ is selected.

2) Tempering temperature

When the temperature rises to 400°C, the hardness of the quenched high-speed steel decreases. When the tempering temperature exceeds 400°C, special carbides begin to form, and precipitation begins at 500-600°C. Due to the high dispersity of the carbides that begin to precipitate, the hardness of these steels gradually increases, and the hardness reaches the highest value at about 650 °C, so the high-speed steel adopts a temperature of 540~560 °C with the best hardening effect.

3) Tempering times

Due to the large amount of retained austenite in high-speed steel, about 10% of the retained austenite remains untransformed after one tempering, so tempering is generally performed 2 to 3 times.

3.Process curve:

The final vacuum heat treatment process curve is as follows:

Advantages of vacuum heat treatment

Vacuum heat treatment is a comprehensive technology combining two specialties of vacuum technology and heat treatment, which means that all and part of the heat treatment process is carried out in a vacuum state. Compared with ordinary heat treatment, it has the following advantages:

1) No oxidation, no decarburization, no carbon increase, and has a good protective effect on the inside and surface of the workpiece. In traditional heat treatment, the oxidation and decarburization of steel are carried out at the same time. Due to the oxidation of the decarburized layer, the reduction of carbon content will significantly reduce the quenching hardness, wear resistance, and fatigue properties of the steel. The decarburization of high-speed steel will reduce the red hardness. Vacuum heat treatment Because the metal is heated under a certain degree of vacuum, the workpiece avoids contact with oxygen, and a bright surface and better heat treatment quality can be obtained. Carbonization phenomenon. A uniform steel structure can be obtained.

2) Improve overall mechanical properties, degassing and promote purification of metal surfaces. When vacuum heating is used, hydrogen in metals and alloys can be rapidly reduced to a minimum, and hydrogen embrittlement is eliminated, thereby improving the plasticity, toughness and fatigue strength of the material, and improving the overall mechanical properties of the workpiece.

3) When metals and alloys are heated in vacuum, if the vacuum degree is lower than the decomposition pressure of the corresponding oxides, the oxides will decompose, and the free oxygen formed is immediately discharged from the vacuum chamber, which further improves the surface quality of the metal, even Make the surface reach the activated state and play a purifying role.

Selection of vacuum heat treatment equipment: The RVGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of the warm extrusion die. The 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.