Principles
1. Physical Basics of Induction
1.1 Prinziple of Induction Heating
1.2 Current penetration depth in dependency of the frequency
1.3 Frequency / Penetrationdepth - Diagram
1.4 Calculation of the penetration depth at carbon-steel
2. Priciple layout of an induction hardening machine
2.1 Basic layout
2.2 Induktion hardening in praxis
4. ZTU - Graph
4. Examples for hardening zones
5. Coil design in Theory and praxis
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1. Physical Basics of Inducion
1.1 Prinziple of Induction Heating
All material, which may conduct electricity have the a bility to be heated by induction. In general, all metallic materials are heatable by induction. Induction heating itselve is based on the priciple, that an electric current is induced in a metall-part which is placed into a magnetic field. This current flow effects that the metal is heated.
According to the Law of Joule electrical Power is created by the current over the resistance of the metal. This power is transformed into heating-power.
Power P = R (Resistance) * I² (Current to the square)
This current is only running a certain time „t“ through the metalpart. So the electrical energy is transformed durning the time “t” into the heating-quantity “Q”
W (Energy) = R * I² * t  es so it is: W = Q (quantity of heat)
So it is obvious, that induction heating is a direct heating source. The heat is created inside the work- piece and is not applied from outside and brought into the workpiece by heat-transfer or radiation
The basics to this are the inventions from r. Faraday and the physical law of Lenz.
Is an electric conductor (workpiece) influenced by a magnetic field changing by time, so in this conductor is induced a voltage. This voltage leads to a current flow which is in opposite direction to the magnetic creation.
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1.2 Current penetration depth in dependency of the frequency
The way how induction current is appearing and affecting the workpiece is dependent on the speed of change of the magnetic field. The measure of changing is called frequency with its dimension unit “Hertz” (Hz). 50 Hz means 50 changings per second.
The current penetration depth ? (Delta) discribes the thickness of the surface layer in which the current is mainly flowing. The penetration depth discribes the mathematic value where the current density in the workpiece has 37% of ist value of the surface.
• How deep the current can affect is depending from the frequency. The higher the frequency, the smaller the penetration depth.
• But the penetration depth is also depending on the material and the actual temperature. Here we have to consider the dependency from the permeability (µ) and the specific electrical resistance (p=rho) .
Approximation :
• At last also the different components of the metalurgiacal alloy and the quality of the metal mikro-sructure have its influence to the heating or hardening result.
The magnetic characteristics and properties of the material have a big influence of the need of induction power. (power contribution in dependency of time
At the begining the losses due to change of magnetic-direction ( Hysteresys-losses).are overballanced
From the Curiepoint ( 600 - 770 °C) Only losses due to the eddy-current ( Joul-losses)are affecting
Austenitic steel is in most cases antimagnetic!
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1.3 Frequency / Penetrationdepth - Diagram
The previous explaned physical facts are displayed in the graph below:
? zum Diagramm
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1.4
Calculation of the penetration depth at carbon-steel:
Carbon steel at 950°C has a permeability of µ=1 and a specific resistance p=1,2. At a steetemperature of 400°C is the permeability µ=30 and p(rho)=0,45.
Daraus ergeben sich unter Anwendung der Formel
folgende Richtwerte:
| Frequenzy in kHz | Penedration depth at 950°C | Penedration depth at 400°C |
| 1 | 18,0 | 11,0mm |
| 4 | 9,0 | 5,6mm |
| 10 | 6,0 | 3,6 |
| 20 | 4,1 | 2,5 |
| 100 | 1,8 | 1,2mm |
| 400 | 0,9 | 0,5mm |
| 700 | 0,7 | 0,3mm |
The interaction of the magnetic and electrical influences result to the relation between the workpiece diameter and the penedration depth (dependent on the frequency). This relation is D / ? > 4 .From this follows, that it is more difficult to find the right matching for small workpieces and it is difficult to get the energy into the workpiece. Economical reasonable limits are given below:
1.5 Naming of the frequency area and recommded frequencies
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2. Priciple layout of an induction hardening machine
2.1 Basic layout
According to the requirements of the workpiece it is necessary to choose the right tenerator type right hardening-machine. Due to the high need of energy the induction generators and the induction coils are water cooled. In most cases the quenching medium is a mixture of water and polymer. So the cooling speed is controllable and adjustable
2.2 Induktion hardening in praxis
Verticalhardeningmachines
Cut-out of horizontical machine
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3. ZTU-Graph
For all workpieces discribes the ZTU- Graph the temperatur-status. First comes the heating up to austenitisationtemperature. After that the part is quenched with a water-polymer mixture, oil, nigrogen gas or air. During this down cooling the material runs trough different transformation-phases. The chromology of these phases are having a big influence on the result of the hardening. As long as during the down-cooling time the Perlitphase (P) and Bainit-Phase is not reached (red curve), the desired Martensit-structure is achieved. Is the cooling speed too slow, different mix-structures will occure.
? ZTU - Diagramm
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4. Examples for hardening zones
Depending on the workpiece geometry and the used hardening technic different hardening profiles my be reached.
| Unterschieden wird bei der Härtung in 2 Hauptgruppen | |
| Standhärtung mit Forminduktor für die komplette Härtefläche. Üblich mit Werkstückrotation (Gesamtflächenhärtung) | Vorschubhärtung. Die Härtezone entsteht durch gezielter Erwärmung entlang des Werkstücks. Üblich mit Werkstückrotation. |
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5. Coil design in Theory and praxis
? Coil Design
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