In almost all fields of machine manufacturing, forging, especially die forging and precision forging, is an important hot working process. For many years the forged blanks have been heated in a variety of flame stoves. From the 1950s to 1960s, induction heating technology expanded from smelting and heat treatment to the forging field, bringing revolutionary changes to the forging industry. Forging induction heating is one of the main applications of induction diathermy furnaces. The induction diathermy device is mainly used for the heating of metal thermoplastic processing (forging, extrusion, rolling, etc.) and the overall heat treatment of various profiles. Their design and production are basically the same, but they have their own characteristics only because of different materials and physical parameters and different heating process requirements.
Due to the different process requirements, forging induction furnace (diathermy furnace) also has a variety of forms and most of the non-standard products. The heating proportion of solid cylindrical charge, especially steel, is the largest, so the examples are the focus of the discussion.
INDUCTION FORGING EQUIPMENT
Forging is generally classified by induction heating furnace
(1) In terms of heating mode, it can be divided into three types: periodic, sequential, and continuous:
Periodic formula. Place one charge at a time in the furnace. After heating, place another cold charge.
Sequential. There are multiple charges in the furnace at the same time. When a new charge is sent to the inlet end, a hot charge that meets the temperature requirement is given at the outlet end, and heating is carried out in a certain beat sequence.
Continuous. The charge passes through the furnace at a constant speed to meet the heating requirements.
(2) According to the charge material, it can be divided into non-magnetic material heating and magnetic material heating:
(1) Heating of non-magnetic materials, such as heating before extrusion of aluminum, heating before forging of copper and induction heating of steel after heating by fuel furnace to above Curie point, etc.
(2) magnetic material heating, mainly for iron, nickel, cobalt-based alloy material, the temperature of the steel in the Curie point below the heating is common, such as the blue brittle blanking of steel, surface anti-oxidation treatment, induction annealing (low temperature), and tempering of materials and machine parts.
The whole heating process from magnetic to non-magnetic mainly refers to the heating of steel from normal temperature to forging temperature or quenching temperature, the low-temperature section heated by high-power furnace section and the low-temperature section heated by steel in the dual-frequency heating (the relatively low-frequency section below Curie point), etc.
(3) From the shape of the charge can be divided into cylindrical, rectangular section, hollow, plate, strip, profile, and other structures and shapes.
(4) From the furnace structure can be divided into a vertical furnace and a horizontal furnace, the vertical furnace is mostly used for the heating of large heavy burden, generally for the cycle type.
(5) It can be divided into high frequency, intermediate frequency, and power frequency heating from the power source. At present, intermediate frequency heating is in the majority. Due to the development of power supply, the frequency band is increased, in order to improve the rationality of heating, now the use of dual-frequency and three-frequency power heating began to be popularized.
(6) The heating mode can be divided into conventional heating (equal turn coil) and rapid heating (variable turn coil).
Main advantages of induction forging furnace over the flame furnace
(1) Saving energy. General induction heating furnace of total efficiency (steel) more than 60%, local heating, compared with other types of furnace heating effect will be more obvious. For example, in the 1980s, a larger crankshaft was forged in a certain factory, the original oil furnace was repeatedly sent to the furnace for overall heating, and then the induction ditherm furnace was used for local heating of each forging section, and the energy consumption was reduced by 20 ~ 30 times.
(2) The products produced are of good quality and uniform. Induction heating is of good quality due to its fast heating rate, fine grain, and light oxidation and decarburization. Because of easy to achieve mechanization, automation production, so the product performance is consistent, good repeatability. An obvious example is that in the 20th century, an Arsenal in China imported a high-speed precision forging equipment from an advanced country in Europe, and the other party suggested that an induction diathermy furnace should be equipped at the same time so that the finished products could be forged at one time. Due to the unfamiliar technology, our staff thought it was very simple to heat the furnace to more than 1000 degrees. In order to save the limited foreign exchange, they refused. After returning home, they equipped themselves with a flame furnace. Results Because the heating time is too long, the oxidation burns badly, after forging, not only have to go through grinding and another machining, but also have to carry on pickling. Increased the amount of material reserved, processing hours, and production costs. The capacity of the advanced forging machine is less than 20%.It was not until later that we realized the key to the problem and developed the corresponding induction heating furnace that we could solve it well.
(3) Low processing amount of materials, saving raw materials; Good heating performance can improve the service life of the mold.
(4) high level of mechanization and automation equipment, small volume, less floor space, so easy on the assembly line and automatic line.
(5) Clean energy, good working conditions, basically no high temperature and little environmental pollution.
(6) Fast start and stop, easy to use and maintain.
According to the preliminary statistics of some enterprises, induction heating can save about 70% of the total production time, the oxide scale is 1/2 ~ 1/4 of the flame furnace, the die life can be increased by 20% in die forging, and the metal saving is 10% ~ 15%.
Correct selection and design, production of high – quality induction forging furnace
This of course must first meet the customer’s the production and process requirements. Mainly is productivity, its main is decided by the power of variable frequency power supply, then the energy saving, it depends on the power supply frequency and the rationality of the design of the sensor, the main is the heating temperature on the process requirement, temperature distribution, etc., want to rely on impedance matching with the power of the sensor design and mechanization and automation level, according to user requirements in consultation; The second is good product quality; Advanced technical and economic indicators of the equipment, such as low energy consumption, material saving, low operating costs, good working conditions, etc.;High reliability of equipment;Easy to use and maintain, safe and reliable to operate.
The main disadvantage of the induction forging machines is poor generality. If the size and specification of the burden vary greatly, several sensors should be designed in groups. Therefore, it is necessary to propose representative varieties for the multi-specification heating furnace as the main basis for design and product acceptance.
(1) Whether the heating frequency is reasonable or not is directly related to the electrical efficiency and processing quality of the heater. The selection of power frequency mainly considers two factors.
First, to ensure the electrical efficiency, and second, to improve the temperature uniformity of the cross-section.
It can be seen that a further reduction in frequency will not deepen the heating layer, but will affect the electrical efficiency. Further uniformization depends only on the heat conduction of the charge itself. So 0.4R2 is the maximum heating depth at this frequency.
Frequencies should be selected to the high end within this range. Of course, also should be flexible according to the specific situation, such as in the heating rate is slower (small unit power), can choose a higher frequency, heat transfer to make up for the lack of shallow heating layer, the high thermal conductivity of the material can also choose a higher frequency, etc. When the user’s economic investment allows, for the larger diathermy furnace, it is suggested to adopt more reasonable double-frequency or three-frequency heating in the technology, namely divided into low-temperature section (magnetic, low-frequency), high-temperature section (non-magnetic, high-frequency) even temperature section (or not).
(2) Determine the estimation of the average heating power of the power capacity inductor. In general, take the power Py > Pg, and try to use the value given in the standard series. In the case of periodic heating of magnetic materials, if there is no automatic control function, the power capacity should be increased to make Py≈ (1.5-1.7) Pg, and in the case of periodic heating of non-magnetic materials, Py≈ (1.05~1.10) Pg. In this way, we know the power and frequency of the power supply, we can combine the specific conditions and requirements of the user and the manufacturer to select the power supply reasonably.
(3) The determination of the scale of the inductor of the core components of the heating furnace to find out the geometry size of the inductor, you can roughly estimate the size of the furnace. First, find out the induction coil length A1. Forge heating furnace (including all diathermy furnace), of course, hope the heart table temperature delta T smaller the better. The minimum heating time tK is required to ensure △T in order to determine the total length of the coil a1 (continuous) or the number of furnace charge n (sequential) or the number of furnace table N (periodic).
Of course, the diathermy furnace prefers a smaller core temperature differential but from the above discussion, it is known that although induction heating is self-heating, its effective heating layer is only 0.4 ≤ 0.4r2, and the rest still needs to be uniform-temperature by heat transfer with an electrical efficiency d. The proper value of coil inner diameter ensures the efficiency and reliability of the furnace. Too large diameter, increase magnetic flux leakage, will reduce the electrical efficiency; And get too small, will make the lining is too thin, not only reduce its thermal efficiency, will also affect the lining strength, such as clearance is too small will hinder the operation of the burden. In principle, there is an optimal value of D1/D2.
From the above discussion, it can be seen that electrical efficiency is related to two factors: the relative frequency m2 and the air gap between the coil and the charge, i.e., their diameter ratio D1/D2.The electric efficiency increases with the rapid increase in frequency. After the inflection point, the rising speed becomes slow and gradually approaches the limit value. As for the air gap, of course, the smaller the air gap is, the better the electromagnetic coupling is, the less magnetic flux leakage, and the higher the electrical efficiency is. When D1/D2 increases from 1 to 2, the electrical efficiency drops from about 95% to 76%.
(4) Combining the above two points, its total efficiency can qualitatively make a curve.
The intersection point of the two curves is the optimal point for the selection of refractory and thermal insulation materials. For the steel forging heating of the main heating object, from the comprehensive consideration of the electro-thermal efficiency, it is suggested to take D1/D2=1.4 ~ 1.8, but D1/D2≈1.2 ~ 2.0 is also acceptable. When the diameter is large, the value is slightly small; when the diameter is small, the value is slightly large. If D2 is too thick or too thin, it may exceed this range. The final determination of D1 should be based on practicality, and the following factors should be considered
D1, D2 = + delta D1.1 + delta D1.2 + delta D1.3 + delta D1.4 + delta D1.5
Where △D1.1 — the gap (mm) necessary for the charge to run in the furnace;
D1.2 — Thickness of refractory lining (mm);
D1.3 — Thickness of furnace lining insulation layer (mm);
D1.4 — charge thermal expansion size (mm);
D1.5 — machining tolerance (mm).
From the above estimates, we know the power and frequency of the equipment needed to power, has selected the power, we know the size of the induction coil, considering its working height installation method is given and the shell and the frame material is the basic idea of furnace body, water cooling water can be learned from table 1 the total efficiency of the stove and energy consumption, which need to take away the heat water, thus can also conduct a preliminary estimate.
Some of China’s existing major manufacturers of induction heating equipment have the ability to provide different technical levels of mechanical and electrical matching requirements according to the user’s process needs. According to the mechanization and automation requirements agreed with the user, the connection with the main machine, the mechanization of its own feeding and automation of the operating system, can put forward a technical and economic reasonable plan, and according to the elaborate design and production of inexpensive, durable products.
A few additional comments
(1) the main formula of rectangular section charge first, frequency selection formula, the second, power estimation, third, to ensure the shortest heating time △T.Carbon steel from room temperature heating to 1200 ~ 1300℃, the fourth, the determination of coil size. Line coil cavity height D1.When the rectangular blank (b2/D2 > 1) is heated, the height of the feed mouth has little influence on the electrical efficiency, so D1/D2=1.25 ~ 3.0. When the charge is large and the heating temperature is low, take a small value. On the other hand, it takes a larger value. Of course, also want via practical arrangement and compound just can. The other points are the same as the cylindrical charge. Line coil cavity width B1
When b2/D2 is less than or equal to 5, b1 is equal to B2 plus (d1-D2).
When b2/D2 > 5, b1=b2+(1.05 ~ 1.15)(d1-d2)
(2) the main formula of pipe heating so-called pipe, generally refers to the ratio of outside diameter and wall thickness, namely D2/ D2 > 5 and D2/ △2.First, the frequency selection formula K2≈ F (D2p/ A2), the corresponding curve can be consulted, and the value K2≈0.8 ~ 0.9 can be temporarily taken for estimation. There is an optimal frequency value for pipe heating: second, power estimation, other items can be referred to as cylindrical furnace charge heating.
(3) Dual-frequency heating With the development and improvement of semiconductor frequency conversion power supply and the popularity of electric energy, according to the difference of induction heating for magnetic and non-magnetic materials, the adoption of dual-frequency power supply for segmenting heating of steel parts from room temperature to forging temperature has been popularized. A low-frequency power supply was used before the magnetic point, and relatively high-frequency heating was used after the Curie point.Its main advantage is: save electricity.Due to the reasonable power frequency configuration, the power can be fully used, generally can save 15%~20% of electricity. (2) to save time. In the case of single-frequency heating, the frequency selection is based on the hot state. For the cold state, the frequency is too high, the heating layer is shallow, and the unit power given is relatively small, which reduces the heating speed and extends the heating time. It increases energy consumption and reduces productivity.(3) Good product quality.As the heating time is shortened, energy consumption is reduced and oxidation is reduced. Meanwhile, the reasonable frequency also ensures the low-temperature difference, so good heating quality can be obtained.
(4) Rapid heating (variable-turn heating) This is the reason that the heat transfer to the center is fast when the temperature difference is large, and the heating time is shortened when the same temperature difference is guaranteed.
For the periodic heating, the x-coordinate can be regarded as the heating time, and for the continuous and sequential heating, the length of the induction coil. In fact, when the blank surface rises to the final temperature, it accounts for 10%~30% of the total heating time (or 10%~30% of the total length at the sensor inlet), which can increase the heating speed and shorten the average temperature time. This heating specification is known as (impact) rapid heating.
(5) Comprehensive energy Heating Due to the different energy conditions in different regions, dual-energy comprehensive heating should be considered when necessary. For example, natural gas is abundant and cheap in a certain region. Can consider in 700~800℃ below with gas furnace heating, after entering the rapid induction heating furnace. In this way, it can not only make use of cheap air sources when oxidation is light in the low-temperature heating section but also adopt high-quality rapid induction heating in the high-temperature areas. It makes both economic and technological sense. To sum up, the design and production of induction heating equipment with high quality, low price, and economic durability should be based on local conditions and take full consideration of technological needs.
In the end, although the problem is very small, it is often ignored by some people, resulting in big mistakes. That is when using the existing formula to calculate, be sure to pay attention to the physical quantity in the formula must use the integral average value of the heating temperature segment, the unit in the formula should be clear, put in the correct unit. In addition to forging reheating furnace, this information is also applicable to other kinds of induction diathermy furnace.
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