25-01-2013, 03:03 PM
THE CORRECT CHOICE OF AN INDUCTION HARDENING INSTALLATION
THE CORRECT CHOICE OF AN INDUCTION.pdf (Size: 701.75 KB / Downloads: 24)
Power represents the energy in the unit of time that the installation is to be able to transfer
onto the piece.
In an induction heat treatment the heating power is that determined by the oscillating circuit
and more correctly called reactive power.
According to physical geometrical characteristics of the piece and to the temperature to be
reached, the choice of an optimal merit factor makes it possible to maximize generator
performances minimizing consumptions.
All right; but seeing as I am not an engineer, how can I “more or less” understand the required
power to harden a given piece? Let Physics help:
Tp = theoretical power KW/H/KG
Tc = thermal capacity in KW/H/KG taken from the graph
P = hourly quantity of the material to be processed in KG
Ce = absolute inductor efficiency
Take a practical example:
We have to harden this pictured C 40 steel shaft with Ø 30 mm and length 300 mm
Let us calculate its weight: 1,660 kg
We have to harden 60 pieces/h
The cycle time of 60 seconds will be composed of different passive times (load, unload,
inductor positioning, closing of shields, additional cooling at cycle end) and of an active
time to dedicate to heating
On a total of 60 seconds, only 30 will be dedicate to heating
Let us calculate the hourly quantity of product being heated up that will be of:
3,600 sec divided by 30 = 120 cycles/h multiplied by 1.660 kg = 199.2 kg
We know that hardening ideal temperature for this steel is 900 °C
From the graph we read on the “steel” curve that we need 0.18 kW/h for each kilogram of
material; hence, the needed power is of 36 kW.
Now we have to divide the theoretical power by the piece inductor efficiency. This depends
upon various factors and it changes also as function of the piece temperature during heating
(at the Curie Point = 720° the material looses magnetism).
In order not to get too technical, you should calculate a yield of 0.5 and you will never get it
wrong!
Therefore the correct power becomes 36 kW divided by 0.5 = 72 kW
Considering that the piece is not to be core hardened, more power is needed to heat rapidly
and take the sole surface layer concerned to the transformation temperature.
Accept another piece of advice that we know from experience:
If you require hardening with a 4-6 depth, multiply the power by 2
That is 72 x 2 = 144 kW
If you require hardening with a 2-3 depth, multiply the power by 1.5
That is 72 x 1.5 = 108 kW
This is, naturally, a calculation that will appal the induction theoretician because it is
irrespective of the work frequency and of many other important factors….but it will allow
you to understand if your supplier has offered you the proper installation.
What work frequency is to be chosen?
The frequency generated through the oscillating circuit affects the characteristics of the
heat treatment in two aspects:
1) Penetration depth for “skin effect”
2) Intensity of the electromagnetic field lines
(1) defines the induction heating depth; it is independent of time duration and of the power
applied to treatment.
Combined with thermal conduction (function of the heating time, of the applied power and of
heat conductivity of the treated material) it specifies the effective depth of the heat
treatment.
Variable factors that may affect the choice of an induction hardening installation
Now that you have learned how to size installation power, to choose the more appropriate
frequency and to understand what the most suitable inductor is, the problem posed is to
choose the supplier it is advisable to buy from.
What and how many are the elements that should influence the choice?
Our considerations follow.
Management of the cooling liquid
The generator heats but hardening is obtained from cooling the piece!
Suitable management of the hardening liquid is essential; it is necessary to cool (in some
cases also to heat at the beginning of the shift) the liquid, to filter it thoroughly, and to
manage its flow rate and pressure.