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Design principle of induction hardening inductor

Time：

2022-09-21

source：

Cite the magazine Metal Working (Hot Working)

Induction heating and quenching inductor is a key heating element that uses eddy current principle to finish surface quenching and strengthening of parts. There are many types of parts for surface heating, and their shapes vary greatly. Therefore, the design forms of sensors are different. Generally, the size of sensors mainly considers the diameter, height, section shape, cooling water path, and spray hole of the induction coil. The design ideas are as follows.
1. Diameter of inductor
The shape of the inductor is determined according to the surface profile of the heating parts. It is required that there must be a relative gap between the induction coil and the parts, and all parts should be uniform.
When heating the outer circle, the inner diameter D of the inductor=D0+2a; When heating the inner hole, outer diameter D of the inductor=D0-2a. Where D0 is the outer diameter or inner diameter of the workpiece, and a is the gap between them. Take 1.5~3.5mm for shaft parts, 1.5~4.5mm for gear parts, and 1~2mm for inner hole parts. If the medium frequency heating quenching is carried out, the clearance is slightly different. Generally, the axial parts are 2.5~3mm, and the inner holes are 2~3mm.
2. Height of inductor
The height of the inductor is mainly determined according to the power P0 setting of the heating equipment, the diameter D of the workpiece and the determined specific power P setting:
(1) For the primary heating of short shaft parts, in order to prevent sharp corners from overheating, the height of the induction coil should be less than the height of the parts.
(2) When long shaft parts are heated once and cooled locally, the height of the induction coil is 1.05-1.2 times of the length of the quenching zone.
(3) When the height of the single turn induction coil is too high, the heating of the workpiece surface is uneven. The middle temperature is much higher than the temperature on both sides. The higher the frequency is, the more obvious it is. Therefore, double turn or multi turn induction coils are often used instead.
3. Cross section shape of induction coil
There are many section shapes of the induction ring, such as round, square, rectangle, plate type (externally welded cooling water pipe), etc. When the quenching area is the same, it is more economical to bend the induction ring into a rectangular section, and the diathermic layer is uniform, while the circular section is poor, but it is easy to bend. The selected materials are mostly brass tubes or copper tubes. The wall thickness of high-frequency induction coil is 0.5mm, and that of intermediate frequency induction coil is 1.5mm.
4. Cooling water circuit and spray hole
In consideration of heating due to eddy current loss, all components need to be cooled by water. Copper tubes can be directly cooled by water. Copper plate manufacturing parts can be made into interlayer or welded copper tubes to form cooling water circuit; When self spraying cooling is adopted for high-frequency continuous or simultaneous heating, the diameter of the spray hole of the induction coil is 0.8~1.0mm, and that of the medium frequency heating is 1~2mm; The spray hole angle of the induction coil for continuous heating and quenching is 35 °~45 °, and the hole spacing is 3~5mm. At the same time, the heating and quenching water spray holes shall be staggered, with uniform hole spacing. Generally, the total area of the water spray holes shall be less than the area of the water inlet pipe to ensure that the water spray pressure and water inlet pressure meet the requirements.
It should be noted that in order to solve the annular effect of inner hole heating, iron oxide (high-frequency quenching) or silicon steel (medium frequency quenching) sheets can be clamped on the induction coil to make a gate shaped magnet, which drives the current to flow along the gap of the magnet (outer layer of the induction coil). In order to prevent the parts that should not be hardened from being heated, steel rings or soft magnetic materials can be used to make magnetic short circuit rings for shielding. In addition, during induction heating, the gap near the sharp corner of the induction coil shall be appropriately increased to prevent local overheating.

Keyword: Recarburizer

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