Vacuum heat treatment process and equipment for magnetic materials

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Magnetic materials are widely used in various industrial fields because of their special physical properties. In order to achieve the design performance of parts, heat treatment is needed when magnetic materials are processed into parts. After the application of vacuum technology in heat treatment industry, a new type of heat treatment process – vacuum heat treatment has been produced. Compared with traditional process, vacuum heat treatment has obvious advantages in many aspects. In this paper, the vacuum heat treatment technology of magnetic materials will be introduced according to the specific parts of several types of vacuum heat treatment equipment developed by our company in recent years.

Magnetic materials are mainly used to make electrical and electronic parts. They are widely used in basic industries and high-tech fields such as power transmission and transformation equipment, household appliances, motors, computers and military equipment. According to the magnetic characteristics of materials, magnetic materials are divided into soft magnetic materials and hard magnetic materials (also known as permanent magnetic materials). Soft magnetic materials exhibit strong magnetism after magnetization in the magnetic field. After leaving the magnetic field, the magnetism basically disappears, mainly including permalloy, electrical silicon steel and electrical pure iron. Hard magnetic materials have permanent magnetism after magnetization and are not easy to demagnetize, mainly Nd-Fe-B alloy, Al-Ni-Co alloy and ferrite. In the process of making magnetic materials from raw materials into parts, heat treatment must be carried out to achieve the design function of the parts. SIMUWU has developed several sets of vacuum heat treatment equipment for the treatment of magnetic alloys. In the process of commissioning, we have made some explorations and experiments with users on heat treatment process, and accumulated some experience in heat treatment process and equipment structure design. This paper mainly introduces the vacuum heat treatment process and equipment of magnetic alloys, and according to some typical workpieces, combined with several typical vacuum furnaces produced by our company, the vacuum heat treatment process of soft magnetic materials is mainly introduced.

Vacuum heat treatment of soft magnetic materials
The raw materials of soft magnetic alloys are mainly bars, strips and sheets. Parts are cut, punched, punched or winded by cold working. In the process of processing, the original grain structure of the material will be destroyed, which will reduce the magnetic conductivity of the material. Meanwhile, the processing stress in the process will lead to the instability of the size of the parts. Therefore, parts made of soft magnetic alloys must be annealed after processing and forming, so that the material can regain regular equiaxed grain structure, reduce impurity content in the material, restore and improve the magnetic properties of the material, at the same time eliminate the processing stress and stabilize the size of the parts.

According to different materials and application requirements, the annealing of soft magnetic alloy parts can be carried out at high temperature (1150-1250 C) and medium temperature (800-850 C). Among them, permalloy with high permeability is usually annealed at high temperature, and sometimes annealed in magnetic field to further improve its permeability. For stacked or coiled silicon steel sheets (usually called cores) of transformers, transformers or motors, it is annealed at medium temperature.

Before the introduction of vacuum furnace, many manufacturers used well-type external heating atmosphere furnace, which was treated by nitrogen or hydrogen protection. This treatment method has obvious defects, especially when nitrogen is used as protection atmosphere. Firstly, when treated with nitrogen protection, the purity of the atmosphere obtained by nitrogen replacement in the furnace is not enough, and the flow direction of nitrogen has a great influence on the surface quality of the workpiece. There is often a phenomenon of color inconsistency after the same furnace parts are treated. This is due to the presence of nitrogen flow in the furnace area, some parts of the workpiece slightly oxidized. Secondly, the temperature uniformity of the external heating furnace is poor, and the performance of the treated parts is quite different, and the qualified rate is low. In addition, in terms of energy consumption, the lining of the external heating furnace is thicker, the power consumed on the lining in the heating process is high, and the thermal efficiency is low; in the heating process, high-purity nitrogen gas needs to be continuously filled, and the consumption of nitrogen is large; and the service life of the heat-resistant steel tank at high temperature is low, which leads to higher operating costs of the external heating furnace. If hydrogen is used as a protective atmosphere, depending on the strong decarbonization and reduction ability of hydrogen, the performance and surface quality of the workpiece can be improved, but the problems of uneven temperature and high operation cost are still inevitable.

Vacuum Heat Treatment Furnace

The use of internal heat vacuum annealing furnace is of great significance for annealing soft magnetic alloy parts. Internal heat vacuum furnace has obvious advantages not only in the aspects of workpiece treatment effect, operation cost, but also in the aspects of production efficiency and production environment. The vacuum annealing process and corresponding equipment for different kinds of parts will be introduced below.

Vacuum annealing of permalloy
Permalloy is a kind of high magnetic conductivity material. It is often used as a high sensitive control element in household appliances, electrical and military industries. The main components of permalloy are iron and nickel, and a small amount of molybdenum, tungsten, copper and other alloying elements. Carbon content in permalloy should be strictly controlled. If carbon is infiltrated into the material, its performance will be greatly reduced. Therefore, in order to avoid carbon infiltration, the existence of active carbon atoms should be avoided as far as possible in the vacuum furnace used for permalloy annealing. The annealing temperature of permalloy is usually 1150-1250 C. The graphite furnace will have a slight carburizing atmosphere at this temperature. Therefore, the vacuum furnace with molybdenum sheet is mostly used, and its heating elements and reflecting screen are made of molybdenum metal.

Permalloy parts are usually processed after grinding and polishing, and no longer processed after treatment, so the surface quality of annealed parts is required to be high. Because the size of permalloy parts is usually small, the shape is regular and easy to burn through, there is no special requirement in the heating stage of the process. Of course, for some parts with large size and special shape, the problems of heating deformation and burning through should still be considered in the process of compiling. In terms of vacuum, the annealing vacuum of Pomo alloy should be kept in the order of 10 – 2 Pa in order to reduce impurities in the material and to benefit grain growth.

In June 2002, our company provided customers with a permalloy annealing furnace. The furnace is equipped with coils and lead-in electrodes. The magnetic field can be generated by DC current, which has the function of magnetic annealing. Its main performance indicators are:
Effective working area: 300 mm *300 mm *600 mm
Maximum temperature: 1300 C
Furnace Temperature Uniformity (Nine Points Temperature Measurement): +5 C (600 C)
+3 C (800 C)
Heating power: 3 x 20 kW
Limit vacuum: 4.0 x 10 – 3 Pa
Pressure rise rate: 0.4 Pa/h
Manual/automatic operation mode

The picture is an annealing process for a circular workpiece. The workpiece’s outer circle is less than 100, the inner hole is less than 60, the thickness is 16, the material is 1J79, and the furnace load is 40kg. Before treatment, the workpiece is cleaned with detergent and dried. The operator takes clean gloves to load. After vacuum is pumped below 10 Pa by rotary vane pump and Roots pump, the high vacuum valve is opened, and the high vacuum is pumped by diffusion pump. After 8-10 minutes, the vacuum reaches 8*10-2 Pa, then the temperature controller can be started for heating.

Because the part has a certain thickness, a 20-minute average temperature platform is set up at 600 of the heating curve. After heating to 1150 and holding 250 in, the cooling time is 300 minutes from 1150 to 600 and then cooling out quickly after cooling to 300 with the furnace. The key of annealing process for Pomo alloy parts is the control of holding time and cooling rate. The heat preservation stage is a process in which alloy elements are fully dissolved and homogenized. The stage of cooling from 1150 to 600 is the stage of grain formation and growth. It is necessary to control the cooling rate in order to obtain the ideal structure. Therefore, in this cooling stage, it is necessary to adopt program control. During the cooling process, the heater still has power output.

The parts treated by the above process have been tested and all the indexes have met the required requirements. For different shape parts and different charging conditions, the time of heating stage can be adjusted accordingly, but the first cooling curve should be determined strictly in accordance with the requirements of materials.

Annealing of Large Silicon Steel Laminated Core and Silicon Steel Strip Winding Core
Silicon steel is 0.8%-4.8% silicon steel. In the transformer manufacturing industry, the core made of silicon steel (usually containing 2.8%-4.8% silicon) is widely used as the magnetic conductive element of transformer. When transformer works, the power loss caused by core heating is called “iron loss”, which is one of the important indexes of transformer. The heating of the core is mainly caused by the magnetoresistance of the core itself and the “eddy current” generated under the alternating magnetic field. The purpose of core forming by lamination or winding is to reduce the effect of “eddy current”, while the magnetoresistance of core depends on the magnetic conductivity of core. In the cold working process of punching, shearing or winding of silicon steel sheet, the original grain structure of the material will be destroyed, and the magnetic orientation of the material will be changed. As a result, the no-load loss of the transformer will be increased by 5%-30%. Therefore, the ideal fiber structure must be obtained through annealing treatment, so that the original magnetic properties of the material can be restored.

After lamination or winding of transformer core, it should be clamped by fixture and then loaded into furnace. Usually, the weight of a core varies from 200 kg to 2000 kg, and each furnace has to deal with multiple cores to reduce production costs, so the vacuum furnace used for core annealing needs a large furnace load.

Because of the large volume, heavy weight and large section size of transformer core, there are some special points in the design of annealing furnace and the preparation of annealing process compared with conventional furnace. Because of the large cross-section size of the core, poor thermal conductivity and large volume of the vacuum furnace, the overall heat storage is large. In addition to the consideration of heating power, the heater with large radiation area and high efficiency should be adopted. The vacuum furnace usually used for annealing treatment has no special requirement for cooling rate, but from the point of view of improving production efficiency, the core annealing furnace should have a certain cooling capacity.

Since 2003, our company has provided several transformer factories with internal heating vacuum core annealing furnaces of 5T, 8T and 15T overcharge capacity.

The picture shows the vacuum annealing process curve of core with cross-section diameter less than 280 mm. The furnace charge is 5T. The curve below the x-axis indicates the vacuum degree in the furnace.

In the annealing process, the basic principle of the heating stage is also to ensure the penetration, that is, to ensure that the core temperature of the workpiece is consistent with the surface temperature. The main purpose of vacuum heating below 300 C is to remove most of the impurity gases by baking and evacuation. From 300 to 450 C, nitrogen-filled protective heating is adopted to increase heating speed and reduce the temperature difference between inside and outside of the workpiece. Vacuum heating is adopted after 450 C to control the heating speed and set up a heat preservation platform to ensure uniform heating of the whole workpiece. After heat preservation at 810 C, it enters the stage of controlled cooling. At this time, the ability of atom diffusion inside the material is strong, and the grain repair is started. The lattice distortion is gradually eliminated. Under vacuum, the impurities such as carbon, nitrogen and hydrogen in the material are also continuously diffused to the surface and then eliminated. When the temperature drops to 550 C, high purity nitrogen can be filled to accelerate the cooling process. The time of opening the cooling fan can be determined according to the production needs.

After vacuum annealing, all indexes of silicon steel core meet or exceed the national standard, and the surface of silicon steel sheet keeps the original quality. The performance of the same furnace product is identical, and there is no phenomenon of rework or scrap. In addition, compared with the external annealing furnace, the internal vacuum furnace has higher thermal efficiency, and the core heat-resistant steel fixture will not be lost due to oxidation, which greatly reduces the production cost and has obvious economic benefits.

Vacuum Annealing of Small Silicon Steel Parts
Small transformers or transformers are often used in household appliances, electronic equipment, computer accessories and other industries. These devices are also made of silicon steel cores, but the size is small, the shape is regular, the number of mass production is large. Such cores are usually formed by stamping and laminating of silicon steel sheets, and also need to be annealed. The annealing process is the same as other silicon steel parts, but this type of workpiece has a special feature: the stamping oil adhered to the workpiece during the stamping process is clamped between the sheet and the sheet after lamination, which can not be removed by conventional cleaning. Punching oil is brought into the vacuum furnace, which will pollute the vacuum chamber, lead to the failure of insulators, and seriously affect the vacuum pumping. At the same time, the carbon black will be attached to the surface of the workpiece after high temperature calcination of the stamping oil. Additionally, the amount of carbon black can not be removed, which has a serious impact on the performance index and surface quality of the workpiece.

In 2006, our company provided a vacuum annealing furnace specially for the treatment of small silicon steel cores.

In the design of the furnace, the particularity of the workpiece is taken into account, and the deoiling process is added in the process of treatment. A set of deoiling device is specially installed in the system, which can be removed before the impact of stamping oil on the equipment and workpiece, thus ensuring the reliability of the operation of the equipment.

The degreasing process is carried out at a temperature lower than the decomposition temperature of the stamping oil. The pressure in the furnace is lowered below its saturated vapor pressure by vacuum pumping, and the temperature in the furnace is raised at the same time. Under the combined action of two factors, the pressure in the furnace is lowered below its saturated vapor pressure.

The evaporation rate of oil is accelerated, and the oil is discharged in the form of steam. When the oil vapor is discharged outside the furnace, it can not enter the vacuum system, otherwise it will be mixed into the vacuum pump oil, which will dramatically reduce the pumping capacity of the pump. Therefore, before the oil vapor enters the vacuum pump, it should be condensed through the condensation device, and then discharged outside the system.

Through the above treatment, the surface of the parts after annealing is bright, and the same state as before entering the furnace. The insulators in the furnace remain the original color. A large amount of oil can be released from the deoiler, which shows that the deoiling effect of the equipment is very good. Since the equipment was delivered and used in October 2006, it has been running steadily and processed parts with good indicators, which has been well received by users.

Vacuum heat treatment of hard magnetic materials
Most parts made of hard magnetic alloys are formed by powder metallurgy and then treated by solid solution or annealing. So the heat treatment of hard magnetic materials is mainly vacuum sintering, vacuum solid solution treatment and vacuum annealing. As the carbon content of Nd-Fe-B and Al-Ni-Co alloy materials needs to be strictly controlled, molybdenum sheets should be used in the production of hard magnetic alloy parts, such as sintering, solid solution treatment, heating elements of vacuum furnace and reflecting screen.

Hard magnetic alloy parts are stamped or extruded in the die after mixing powder and adhesive evenly. In the sintering process, due to the existence of adhesives, there will be a peak of deflation in the workpiece between 300 and 600 C, and a second peak of deflation in the workpiece between 900 and 1000 C. Therefore, the vacuum furnace used for sintering hard magnetic alloys should be equipped with a larger pumping speed diffusion pump to avoid oxidation.

After sintering and forming of hard magnetic alloy parts, some of them need solid solution treatment to make the material uniform in composition and stable in structure, so as to improve its permanent magnetism. Because solid solution treatment has certain requirement for cooling rate (> 100 C/m in), molybdenum sheet pressure gas quenching furnace can be used for solid solution treatment.

The normal quenching and low temperature tempering structure of GC R15 steel should be fine tempered martensite (including dark dark cryptocrystalline martensite and whiter crystalline martensite), fine granular carbide with uniform distribution and a small amount of retained austenite. Now, metallographic tests show that there are reticulated carbides. Through material inspection, it is found that the carbides in raw materials are mainly reticulated carbides, and the reticulated carbides are grade 4, which belong to unqualified organization. There are two possibilities for the occurrence of reticulated carbides: one is that the quenching heating temperature is too high and the cooling rate is too slow, which leads to the reticulated precipitation of carbides; the other is that spheroidizing annealing is not sufficient. GC R15 is a hypereutectoid steel with reticulated carbides in the original structure. If the spheroidizing is not sufficient, it will be left in the structure after quenching and tempering, which will make the steel have a layer of brittle shell on the grain boundary, and sharply. Reduce the strength and toughness of steel and increase the brittleness of material. Analyzing from metallographic structure, it should not be the former. Because if the quenching heating temperature is too high, the obtained martensite should be acicular martensite; if the cooling rate is too slow, it should not be cryptocrystalline martensite but bainite; and if the quenching heating temperature is too low, it should be tempered torrentite. According to metallographic examination, the structure after quenching and tempering is fine cryptocrystalline martensite. From this analysis, the network carbides may be caused by insufficient spheroidization, or the network carbides in raw materials are too serious to be completely eliminated by spheroidizing annealing alone.

Based on the above analysis, in order to verify the correctness of the analysis results, on the basis of strictly controlling the dot inclusions in raw materials, we modified the processing procedure of cycloidal wheel, that is, normalizing at high temperature before spheroidizing annealing to prepare for spheroidizing annealing, in order to eliminate network carbides, the remaining composition and process have not changed much. As a result, there was no fracture.

CONCLUSIONS AND SUGGESTIONS
1) From the analysis of physical and chemical test results and production verification results, the brittle fracture of the cycloidal wheels is mainly caused by insufficient spheroidizing annealing and non-metallic inclusions in raw materials exceeding the standard.
2) Normalizing the material at high temperature before spheroidizing annealing to ensure the normal structure before quenching, which can effectively avoid brittle fracture of the cycloid wheel of GC R15 steel.

Suggestions: Strengthen the inspection of raw materials to ensure that their composition, organization and various defects are within the allowable range of standards, and control the spheroidizing annealing process.

Vacuum heat treatment technology for magnetic materials has obvious advantages in improving the quality of parts, reducing production costs and improving production environment, so it is a technology worthy of promotion and long-term research. We sincerely hope that the vast number of users will understand and use it. We also hope that more heat treatment workers and researchers will devote themselves to the research of this technology so as to make it more perfect and better serve the vast number of users.