Vacuum heat treatment furnace for molds

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The mold is an important forming equipment in the industry. Modern industrial production puts forward the requirements of high speed, good quality, low cost and long life for the mold. The heat treatment of the mold is a key link that affects the above four requirements. Among the various heat treatment processes of the mold, vacuum heat treatment has many unique advantages.

Mold manufacturing cost is high, and its quality and life are the key factors affecting product quality and cost. Therefore, there are high requirements for the surface quality and deformation of the mold during heat treatment. In order to prevent the surface oxidation, decarburization and carbon increase of the mold during heat treatment, the traditional mold heat treatment mainly adopts salt bath furnace heating or box furnace protection heating, and later also adopts controlled atmosphere heating, but these heating methods are used in high temperature heating. It is difficult to keep the gaseous medium, protective agent or salt bath neutral throughout the heating process to prevent them from reacting with the metal.

Vacuum acts as an ideal heating medium to treat molds. After vacuum heat treatment, the mold has a bright surface, no oxidation, decarburization, and carbon increase, small changes in shape and size, good wear resistance, excellent comprehensive performance, and long service life, which can save or reduce grinding.

Vacuum is an ideal “atmosphere” for heat treatment. Vacuum heat treatment has the functions of preventing the surface oxidation of the workpiece, purifying the surface, degreasing and degassing, etc. The workpiece has high mechanical  properties and stable quality after vacuum heat treatment. In addition, vacuum heat treatment also has the advantages of high degree of automation, no pollution, no pollution, and good operating environment.

The development trend of vacuum furnace

The implementation of vacuum heat treatment is greatly restricted by equipment. A modern vacuum heat treatment furnace is required to heat up to a high enough temperature, with sufficient cooling capacity and production efficiency to achieve workpieces of different steel grades, larger sizes and larger weights. Treated with different processes.

heating method:

At the beginning of each heat treatment, the vacuum furnace must be evacuated to 1-10Pa before heating. Traditional vacuum furnaces only have radiant heating, which makes the heating rate in the low temperature stage very slow. The modern vacuum furnace has developed a vacuum positive pressure convection heating technology. This technology is to charge an inert gas (N2) of about 1.5 × 10Pa in the low temperature stage of the vacuum furnace heating, and start the circulating fan to implement convection heating. After heating to about 750°C, the filled inert gas is pumped out (to the required vacuum degree), and vacuum radiation heating is still carried out in the high temperature stage of heating. This not only improves the heating speed in the low temperature stage, but also retains the effect of heating degassing and uniform heating during vacuum radiation in the high temperature stage. This is the characteristic of the third-generation vacuum furnace heating, which can achieve high loading and intensive furnace loading.

cooling method:

The cooling method of vacuum heat treatment is developed from oil cooling to air cooling. It has been found through practice that, compared with the oil-cooled method, in addition to the simpler structure and operation of the furnace, the air-cooled method has better surface condition and smaller deformation of the treated material, and does not require complicated subsequent degreasing and cleaning processes.

In order to improve the cooling capacity of gas quenching, the development of modern vacuum heat treatment adopts high pressure (5 ~ 10) × 105Pa, ultra-high pressure (20 × 105Pa) gas (N2 or N2 + He, H2) quenching technology.

When using (5~6)×105Pa N2 cooling, the cooling capacity is equivalent to that of general salt bath quenching, which can completely quench O40mm high-speed steel, which is suitable for heat treatment of general tool and die steel.

cooling method

During quenching, the workpiece is cooled by airflow in two directions, which is called 2R cooling. During cooling, the gas flows through the charge from top to bottom or from bottom to top, and the direction of the gas can be changed alternately, and a time interval can be pre-specified, such as 10s from the top and 10s from the bottom. This 2R system allows uniform cooling of the charge and minimal deformation.

The further 2×2R cooling method not only has the cooling airflow transformation in the up and down direction, but also the airflow transformation in the left and right direction.

Scope of application:

The emergence of vacuum convection technology has improved the heating efficiency of vacuum heat treatment and further saved energy. The practical application of vacuum ultra-high pressure gas quenching technology has expanded the material range of vacuum heat treatment from high-speed steel, stainless steel and die steel to general alloy steel.

The vacuum heat treatment of modern molds includes quenching, tempering, annealing, and vacuum carburizing and metalizing processes of various molds. Due to the importance of the tool, its compositional characteristics and the characteristics that allow for higher costs, the tool has become the most widely used and popular product for vacuum heat treatment. However, carbon steel and low alloy steel with low hardenability are generally not suitable for vacuum quenching because it is difficult to harden by vacuum quenching.

Among the various vacuum heat treatment processes of the above-mentioned molds, vacuum quenching is used more.

Hot extrusion die steels all have high hardenability, especially 4Cr5MoSiV1 and other steel grades. The use of gas quenching can effectively reduce quenching deformation and give full play to the effect of vacuum heat treatment.