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Vacuum Heat Treatment Furnace

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Aluminum extrusion die vacuum heat treatment

In the production of aluminum extrusion, tools and dies are of great significance to the realization of the entire extrusion process. The performance of hot extrusion tools not only depends on material factors, but also has a close relationship with heat treatment, which affects the quality and service life of tools and dies. The factors are numerous and complex.

Advantages of vacuum heat treatment:

Vacuum heat treatment refers to the heat treatment process of metal workpieces in a material space with a vacuum degree of 10~1Pa. It is an environmentally friendly and pollution-free heat treatment process. This process is gradually being promoted and applied. Its main features are as follows:

(1) The deformation is small. Practice has proved that heating in a vacuum furnace, cooling in nitrogen, and quenching in ZZ-1 medium oil, the maximum deformation of the tool and die is not more than 0.05~0.1mm, and the deformation is uniform.

(2) It can eliminate the corrosion phenomenon on the surface of the workpiece. In a salt bath or an endothermic controllable atmosphere, it is usually impossible to form a strict balance with the carbon potential, and the surface of the workpiece will increase and decarburize, and heat treatment in a sufficient vacuum degree. The mold can neither increase carbon nor reduce carbon.

(3) Oxidation is suppressed, and bright heat treatment without oxidation heating can be realized.

(4) Simplify the process, which can realize less or no processing after quenching, and reduce the processing cost.

(5) Extend the service life of tools and dies. Practice has proved that the tool life of vacuum heat treatment is three times higher than that of salt bath quenching.

(6) The hardness distribution of the tools and molds after vacuum quenching is uniform, the quality is stable, safe and reliable, it can be quickly heated and cooled quickly, and it can realize the advantages of fully automatic and manual operation, energy saving and time saving.

Problems caused by improper vacuum heat treatment

Although vacuum heat treatment has many advantages over box furnace and salt bath furnace quenching, in the actual production process, due to the influence of process formulation, workpiece placement, personnel operation, improper tempering and other factors, some vacuum heat treatment defects will still be caused .

(1) Uneven heat treatment hardness

In the process of heating in vacuum, due to the thinning of gas molecules, the average degree of freedom of gas molecules increases significantly with the decrease of pressure. The heating of workpiece is mainly carried out by radiation heating. The heating rate is faster than that of the “shade” place, so the furnace loading method of the workpiece is improper, resulting in uneven heating and uneven hardness.

(2) Improper control of deformation

During the process of the workpiece coming out of the hot chamber, closing the heat insulation door, sending nitrogen gas for gas quenching and oil cooling, due to the improper process of gas quenching and oil cooling and the influence of operating factors, the deformation of the workpiece is large.

(3) The surface is not bright enough

The brightness after vacuum quenching varies greatly with the change of vacuum degree when heated in a vacuum furnace. When the vacuum degree is 1Pa, a thin oxide film will be formed on the surface of the workpiece, and the brightness can reach more than 50%. Below 10~Pa, when the oxygen content is 0.13ppm, the brightness is more than 80%. In addition, the brightness after vacuum quenching is also closely related to the purity and composition of the cooling medium, and the oil inlet temperature.

(4) pitting phenomenon

When the mold is heated in vacuum, the alloy element chromium with high vapor pressure will volatilize in the vacuum state, and the vaporized gas metal will adhere to the low temperature part of the furnace, which will not only pollute the surface of the workpiece, but also damage the vacuum heating furnace, causing the workpiece pitting.

(5) There are spots on the local surface

Before the mold is put into the furnace, the oil stains on the surface must be cleaned with alcohol and dried with high pressure air. If there are oil stains, it will cause surface defects.

Extrusion Die Manufacturing Process

All extrusion tools and dies are made of 4Cr5MoSiV steel, which belongs to Cr-Mo series hypoeutectoid austenitic steel with high quenching temperature.

The rolled 4C15MoSiV steel needs to be forged to eliminate the anisotropy. After forging, it should be annealed in time to remove the forging stress, improve the structure, and prepare for the final heat treatment. The forging temperature of steel is preferably 1100~1160℃. During forging, it should be uniformly heated and burn through. Overheating is not allowed to avoid forging cracks. The hammer should be hit quickly, and the weight will be increased as appropriate, and then lightly hit to avoid continuous heavy It is strictly forbidden to hit with a cold hammer. After forging, it is cooled slowly in the furnace and annealed in time to prevent excessive internal stress. This internal stress will not cause cracking and deformation of the blank at that time, which will eventually affect the quality of heat treatment. The edges and corners of the mold should have a smooth transition to prevent quenching cracking and deformation, and the machining accuracy and surface roughness should meet the technical requirements.

Vacuum heat treatment process:

Before opening the vacuum furnace, be sure to check the insulation resistance of each electrode and the furnace shell according to the operation procedure, which should not be less than 1kΩ, the nitrogen filling pressure should be guaranteed to be above 0.8MPa, and the cooling water pressure should be 0.1 ~ 0.3MPa.

For the variable end face extrusion die of φ225×25, it is preheated at 850℃±10℃, the holding time is 2.6min/mm, the heating time is 0.5min/mm after accelerated heating to 1050℃, and the temperature is cooled to 450℃ as soon as possible after quenching. The cooling rate should be at least 50℃/min, the surface temperature of the workpiece should be cooled to 450℃ in at least 18min, until the surface temperature is about 65℃, and the surface temperature of the mold should be cooled to 50~30℃ in still air, and the furnace should be tempered immediately. Reduce deformation.

Measures to reduce workpiece deformation

①Reasonable heating: According to the characteristics of fast heating rate of vacuum furnace and slow heating rate of workpiece, full preheating is adopted to improve the uniformity of heating temperature of vacuum furnace. The heat is transmitted evenly, the accuracy of temperature measurement and temperature control is improved, and the lower limit of the quenching temperature is used for heating and heat preservation, and the amount of retained austenite is controlled to reduce the deformation of the workpiece.

② Adopt reasonable cooling technology: For large and complex molds, some materials can be filled or some parts can be covered, so that the cooling of the mold is roughly the same. During oil quenching, the oil stirring time, speed, speed, etc. should be reasonably selected according to the size and shape of the workpiece. spacing, direction, etc. Minimize the movement of the workpiece in the hot state to avoid vibration. For parts that are easily deformed, the material basket should be placed on the material tray, and the material tray should be flat to prevent the deformation of the material basket from causing deformation of the workpiece.

Improve the brightness and eliminate the high brightness after the pitting vacuum quenching is mainly due to the absence of oxide film on the surface of the workpiece, and the oil and impurities attached to the surface of the workpiece are volatilized to make the workpiece fully exhibit metallic luster. In order to improve the surface brightness of the workpiece , First of all, the surface of the workpiece and the material basket should be cleaned, otherwise the vacuum system will not be able to remove all the organic matter on the workpiece, material basket and furnace structure due to the fast heating rate, which will affect the surface quality of the workpiece. At the same time, the heating speed should be controlled. During the heating process, the workpieces, material baskets, furnace components, etc. adsorb a large amount of water vapor. During the heating process, a large amount of water is desorbed. If the heating speed is too large, the gas will increase, the vacuum degree will decrease, and the residual If the oxygen partial pressure exceeds a certain value, the surface of the workpiece is oxidized, so the heating rate under vacuum conditions must be ensured.

In addition, the vacuum degree should be selected reasonably, and the element chromium with high vapor pressure in the steel should be prevented from evaporating, otherwise the surface will seriously lose its brightness, even in an orange peel state, causing pitting corrosion. During the heat preservation process, a proper amount of partial pressure of nitrogen gas is returned to the furnace to avoid pitting corrosion.

Compared with conventional heat treatment, vacuum heat treatment has the characteristics of environmental protection, energy saving and time saving. In production practice, although there are phenomena such as uneven hardness and low brightness, the life of the mold is greatly improved. 4Cr5M oSiV steel is heated in vacuum at 1050 °C, oil quenched under pressure of 0.133Pa ~ 80000Pa, and electric furnace at 600 °C + 580 °C twice. Tempering has the advantages of good high temperature comprehensive performance, uniform and dense, improved toughness, improved thermal wear and thermal fatigue resistance, simplified process, reduced processing procedures, improved production efficiency, reduced cost, etc. The die life has been proved by extrusion production An increase of three to five times is an ideal and recommended heat treatment process.

Vacuum heat treatment furnace equipment recommendation：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.