The Basics of Induction Heat Treatment: 12 FAQs You Should Know

1. What is electromagnetic induction phenomenon?

The essence of electromagnetic induction is that alternating magnetic field can cause alternating electric field, on the contrary, alternating electric field can also cause alternating magnetic field.

Generally speaking, an alternating current is applied to a coil, and then a conductor is placed in the coil, then an induced current will be generated in the conductor and heating will begin.

The Basics of Induction Heat Treatment: 12 FAQs You Should Know 1

2. What is skin effect?

Skin effect is also called skin effect or surface phenomenon.

When the conductor passes through direct current, the current density at each point of the conductor section is uniform;

However, when the conductor passes through the AC, the skin phenomenon will occur, that is, when the high-frequency current passes through the conductor, the current on the conductor section is not evenly distributed, but mainly concentrated on the conductor surface.

The Basics of Induction Heat Treatment: 12 FAQs You Should Know 2

3. What is proximity effect?

Proximity effect is that the distribution of alternating current in the body is affected by the alternating current in the adjacent conductor.

The specific performance is as follows:

When two parallel conductors pass current with opposite direction and equal size, the current will be concentrated to the side where the conductors are close to each other, as shown in the following fig. (a);

When two parallel conductors pass current with the same direction and the same size, the current will concentrate on the side furthest from the conductors, as shown in fig. (b) below;

The Basics of Induction Heat Treatment: 12 FAQs You Should Know 3

4. What is the skin effect when there is a magnetic circuit?

When the inductor is equipped with a strip-shaped conductive magnet, the current can flow from the conductor surface on the opening side of the conductive magnet.

This is called the skin effect when there is a magnetic circuit, also called the current tending effect of the conductive magnet.

As shown in the following figure, the current distribution of the high-frequency inductor after the effective coil is installed.

The Basics of Induction Heat Treatment: 12 FAQs You Should Know 4

5. What are power frequency, high frequency, intermediate frequency, ultra audio frequency and ultra high frequency current?

The frequency of power frequency current is 50-60Hz, and the depth of heating layer is 10-20mm. It can be used for surface quenching of large workpieces;

The frequency of medium frequency induction heating current is generally 1-10khz, and the depth of heating layer is 2-10mm.

It is mainly used for surface quenching of large modulus gears, shafts with large diameter and cold rolls;

The frequency of ultrasonic induction heating current is generally 20-100khz.

When the small module gear is heated by the ultrasonic induction current, the heating layer is roughly distributed along the tooth profile, and the service performance is good after quenching;

The frequency of high-frequency induction heating current is usually 100-500khz, and the depth of the heating layer is 0.5-2mm.

It can be used for surface quenching of gears, cylinder liners, cams, shafts and other parts;

Ultra high frequency induction heating refers to the megahertz frequency with current frequency higher than high frequency, such as 27 MHz.

The heating layer is extremely thin, only about 0.15 mm.

It can be used for thin-layer surface quenching of workpieces with complex shapes such as circular saws.

6. What is the current penetration depth?

Current penetration depth refers to a physical quantity that characterizes the skin effect of induced current.

Due to the skin effect, the current density on the surface of the conductor is the largest, and the farther away from the surface, the smaller the current density.

Theoretically, the current penetration depth is defined as the distance from the surface at the point where the current density (effective value) of the planar conductor is equal to its surface current density 1 / e ≈ 36.8% (e is the bottom of the natural logarithm) when the sinusoidal planar electromagnetic wave is vertically incident on the infinite thickness homogeneous planar conductor.

It is specified in the engineering that the depth at 1 / e of the maximum current value measured from the surface maximum current value (I0) is the current penetration depth.

7. What is penetration heating? What is conductive heating?

Penetration heating is also called heat capacity heating, that is, when the parts are heated, the penetration depth of current heat is greater than the depth of the hardening layer, which can be expressed by the following formula: dheat > Ds.

During penetration heating, the heat energy of the hardened layer is generated by eddy current, and the temperature in the whole layer is basically uniform.

There are many examples of penetrating heating. When the parts of automobiles and tractors are produced in large quantities, most of them choose penetrating heating.

For example, when the current frequency is 8kHz, the heat penetration depth of the current is about 6mm (850 °), and any hardened layer with a depth less than 6mm belongs to penetration heating.

Conductive heating is also known as surface heating, that is, when the parts are heated, the penetration depth of current heat is less than the depth of hardening layer, which can be expressed by the following formula: dheat < Ds.

When conducting heating, the thermal energy is only generated in the layer of D heat, and the temperature of the metal after exceeding D heat depends entirely on heat conduction.

In order to make the layer of DS depth reach the quenching temperature, the heating speed should not be large, otherwise the surface will be overheated.

There are many examples of conductive heating.

Due to the limitation of the power supply equipment, the parts with a layer depth greater than 1mm processed on the high-frequency equipment belong to conductive heating.

High frequency equipment f ≈ 250kHz, dheat = 1mm.

The following figure shows the temperature distribution curve on the cross section of the parts with penetration heating and conduction heating when the same hardening layer depth is reached, where 1 is conduction heating and 2 is penetration heating.

The Basics of Induction Heat Treatment: 12 FAQs You Should Know 5

The characteristics of the two heating methods are compared as follows:


Category of induction heating


Conduction heating

Penetrating heating

Heat generation of parts

On the surface

Mainly in the hard layer

Temperature distribution

According to the law of heat conduction

Steep, close to right angle

Surface superheat

Large when heating rapidly

Small (also small during rapid heating

Heating of non quenched parts of parts



Heating time

Long (in minutes), especially when the required depth is large and the superheat is small

The longer section (in seconds) is the same when the required depth is large and the superheat is small




Heating thermal efficiency of parts (n)

Low, when the surface superheat △ t = 100 ℃, n = 13%

High, when the surface superheat △ t = 100 ℃, n > 30%

8. What is magnetic permeability?

Permeability is the physical quantity that characterizes the magnetism of magnetic medium.

It indicates the resistance to generate magnetic flux or the ability to conduct magnetic lines of force in the magnetic field after a current flows through the coil in the space or in the magnetic core space.

The formula μ = B / h, where h = magnetic field strength and B = magnetic induction strength.

It is usually represented by the symbol μ, and a is the permeability of the medium, or the absolute permeability.

The relative magnetic permeability is commonly used in induction heating, expressed by μr.

It is the ratio of the permeability of any medium to the permeability in vacuum, i.e. μr=μ/μ0.

For all non-magnetic materials, μr=1, while for ferromagnetic materials, R can reach tens of thousands.

This value is also affected by the strength of the magnetic field.

Such as μr = 7500 for silicon steel sheet.

9. What is magnetic field strength?

The magnetic field strength is a physical quantity describing the properties of the magnetic field. It is a measure of the magnetic potential strength.

It has directionality and is similar to the pressure gradient at a certain place of the water flow.

Denoted by H.

The definition formula is:

The Basics of Induction Heat Treatment: 12 FAQs You Should Know 6

Where B is the magnetic induction strength and M is the magnetization strength, μ 0 is the permeability in vacuum, μ0=4π × 10-7 Tesla · M / A.

The unit of H is ampere / meter.

In the Gauss system, the unit of H is Oster.

1 A / M = 4 π × 10-3 Oster.

10. What is magnetic induction strength?

When the conducting wire is perpendicular to the direction of the magnetic field, the ratio of the ampere force F on the conducting wire to the product IL of the current I and the length L of the conducting wire is called the magnetic induction strength.

There is a magnetic field around the current (moving charge).

Its important external performance is that it has a magnetic field force on the moving tentative charge, current carrying conductor or permanent magnet introduced into the field.

Therefore, the magnetic field can be described by the effect of the magnetic field on the moving tentative charge.

Therefore, the magnetic induction intensity B is introduced as a basic physical quantity to quantitatively describe the characteristics of each point in the magnetic field, and its position is equivalent to the electric field intensity E in the electric field.

The magnetic induction intensity reflects the interaction force, which is the stress relationship between the two reference points A and B.

The magnetic field intensity is the unilateral amount of the main body, regardless of whether Party B participates or not.

The direction of B is defined as the direction in which the right hand spirals forward when the direction of the maximum force Fm received by the positive charge turns to the direction of the charge movement v.

After defining B, the force exerted by the moving charge in the magnetic field B can be expressed as F = QVB, which is the Lorentz force formula.

11. What is magnetic flux?

The distribution of magnetic field space is called magnetic flux.

In a uniform magnetic field with magnetic induction intensity B, there is a plane with area s and perpendicular to the direction of the magnetic field.

The product of magnetic induction intensity B and area s is called the magnetic flux passing through this plane, referred to as magnetic flux, and the symbol“ Φ”. The unit is WB.

12. What is magnetoresistance?

Magnetoresistance is similar to resistance.

There is resistance in the circuit and magnetoresistance in the magnetic circuit.

When the same magnetic flux is generated, the current required when the ferromagnetic substance is placed in the magnetic circuit is small, and the current required when there is no ferromagnetic substance is large.

In other words, when the same current is inserted into the coil, the magnetic flux generated by the ferromagnetic substance is large.

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