What is induction heating ?

Clean and fast heat being supplied to the heated workpiece

meets the considerably increased requirements with regard
to environmental protection. The surroundings is not expo-
sed to any thermal and atmospheric pollution. The particular
advantage of this process is to produce the heat inside the
workpiece without the need for any external heat source.

According to the physical law of induction an alternating

magnetic field is generated around each electrical conductor
through which an alternating current is flowing. By conside-
rably increasing these magnetic fields, metals brought into
close proximity will be heated by eddy currents produced
within the metal. Heating by induction makes use of the ca-
pability of the magnetic field to transmit energy without direct
contact. This means heating is not done by contact trans-
mission such as known in resistance heating in light bulbs,
heating plates or electrical furnaces where the direct current
flow causes resistance wires to glow.

A basic problem of induction heating is to create a suf-

ficiently intense electro-magnetic field and to position the
Induction heating is one of a wide range component to be heated within the center of the field in such
a way as to obtain optimum transmission of energy from
of electrical heat used in industry and the electrical conductor to the workpiece. Normally this is
household today. The main applications achieved by forming the electrical conductor also referred
to as inductor or coil with one or more turns. The workpiece
of the process are in the steel and metal- is positioned in the centre of the coil, thus concentrating the
working industries. magnetic field onto the component. The field will then force
the electrical current to flow within the workpiece. According
to the law of transformation, the strength of the current flow
in the component is equal to that in the coil. To create a suf-
ficiently strong magnetic field, th current flow in the coil must
be very high (1000 – 10.000 A), normally a current of this
intensity would cause the coil to melt; by comparison, 10 A
is the current flow within a 2000 W heating furnace. In order
to avoid this problem, the coils are made of water cooled
copper tubing. Another method of creating a strong alterna-
ting magnetic field is to increase the frequency of the current.
Normally the electrical mains supply to both household and
industry operates at a frequency of 50 Hz, i.e. the current will
change direction 50 times per second. Depending upon the
application, an induction heating equipment will operate at a
frequency of between 50 and 1 million Hz.

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These high frequencies, which are not available from the While for melting, forging and annealing mostly medium
normal mains electrical supply, are obtained by means of frequency is used as energy source, for hardening and sol-
generators: medium frequency generators in the range up to dering applications it depends on the requirements whether
10.000 Hz and high frequency generators above this level. It high or medium frequency can or is to be used.
may be asked why such a large frequency range is neces-
sary and why not all induction heating processes cannot be
carried out at the same frequency. This is due to a physical Summary:
reason as well, i.e. the so called skin effect. The electrical
current flows into the outer skin of the workpiece only, this
means the center of the workpiece remains theoretically cold.
Induction heating provides a heat source
which is very easily controllable, can be
The thickness of the layer in which the current flows in turn is
dependent on the frequency. At low frequencies, the layer is
limited to partial heating zones and crea-
thick, i.e. the workpiece is penetrated by the current almost tes reproducible heat-up processes. This
to the centre, and consequently heated through. At very high
frequencies, the current flows at the surface only and the
provides the opportunity to build heating
penetration depth is in the range of 0 to 1 mm. This effect is equipment with a high level of automa-
made use of in order to use the frequency appropriate for the
application.
tion which allows to be integrated in a
production line, such as machine tools.
The most common applications utilising induction hea-
ting technology are:

• Melting of steel and non ferrous metals at temperatures

up to 1500 °C.
• Heating for forging to temperatures up to 1250 °C.
• Annealing and normalising of metals after cold forming
using temperatures in the range of 750 – 950 °C.
• Surface hardening of steel and cast iron workpieces at
temperatures from 850 – 930 °C (tempering 200-300 °C)
and soft and hard soldering at temperatures up to
1100 °C, moreover, special applications such as heating
for sticking, sintering.

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Induced eddy current Transferable power at different

heating processes

Power transmission
Type of heating
(W/cm²)

Convection
5 x 10 - 1
(Carrying heat, by molecular
movement)

Radiation (electric furnace,

8
box-type furnace)

Thermal conduction, touch

20
(hot plate, salt bath)

Infrared point emitters 2 x 10 2

Flame (burner) 10 3

Induction heating 10 4

Laser (CO2) 10 8

Electron jet 10 10

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Penetration depths (mm) at different materials depen-

ding on frequency and temperature (δ)

Temp. Copper Copper Steel Steel Steel Steel Ni-Cr Graphite Alu
20° C 1100° C 20° C 600° C 800° C 1500° C 20° C

μ – – 60 – 80 40 1 1 – – –

50 Hz 10 32

500 Hz 2,97 1,38 22,50 3,89

500 Hz 2,91 9,4 3,78 7,75 22,50 26 20,6 65

500 Hz 2,2 7 2,9 5,8 17,5 20 16 50

500 Hz 1,68 5,44 2,18 4,31 13 15 11,87 37,6 –

500 Hz 1,59 5,14 2,06 4,12 12,3 14,4 11,25 35,6 –

500 Hz 1,19 3,86 1,55 3,1 9,22 10,65 8,4 26,7 –

500 Hz 1,13 3,65 1,46 2,93 8,73 10 8,0 25,3 1,38

10 kHz 0,7 2,22 0,82 1,83 5,53 6,32 5,05 15,8 0,87

12 kHz 0,65 2,1 0,84 1,68 5,03 5,88 4,6 14,5 –

500 kHz 0,1 0,32 0,13 0,26 0,78 0,9 0,7 2,25 –

700 kHz 0,08 0,037 0,600 0,104

2500 kHz 0,043 0,020 0,320 0,055

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Theoretical energy requirement of various materials

( i = in kWh/kg + kcal/kg)

0,42

0,4

0,38

0,36

0,34

0,32

0,3

0,28

0,26

0,24
kWh / kg

0,22

0,2

0,18

0,16

0,14

0,12

0,1

0,08

0,06

0,04

0,02

0
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700

Temperatur in °C

kWh/kg steel
kWh/kg aluminium
kWh/kg copper
kWh/kg brass

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Current penetration depths of different frequencies in steel

10

9,5

8,5

7,5

6,5
Eindringtiefe in mm

5,5

4,5

3,5

2,5

1,5

0,5

0
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700

Temperatur in °C

Frequency 4 kHz
Frequency 10 kHz
Frequency 30 kHz
Frequency 100 kHz
Frequency 200 kHz

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Energy sources for induction heating

Depending on the current penetration depth required the ope-

rating frequency of the induction installation is determined. The
range of the applicable frequencies reaches from the value of
the mains frequency (50 Hz) to the short-wave range (3 MHz)
and is divided in three sections:

• Low frequency 50 Hz – 500 Hz

• Medium frequency 500 Hz – 50 kHz
• High frequency 50 kHz – 3 MHz

Induction equipment with higher frequencies have to generate

these frequencies from the mains frequency via converters. In
order to do so, the following processes are available:

Frequency Efficiency
Process Power in kW
in kHz in % (Volllast)

Frequency multiplier 0,15

(statical frequency converter) 0,25 88 – 93 up to 3.000
0,45

Thyristor inverter and

0,5 – 25 90 – 95 up to 15.000
transistorized inverter

HF- transistorized inverter 50 – 800 88 – 92 up to 1.000

High frequency
1.000 – 3.000 60 – 70 up to 250
(tube generator)

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Hardening process in the material

In induction heating, the process in the material is the trans- Extract from the iron-carbon diagram
forming and/or quench-hardening process known for the
iron-carbon materials. First, the steel will be heated to tem-
peratures above the GOS-line (figure 3.4). In this process, the Temperature (°C)
originally present cementite-ferrite-crystal mixture forms a
homogenous mixed crystal, the austenite. The carbon, which 1000
was bound in the cementite (Fe3C) is atomically detached Hardening temperatures
in the austenite. The following cooling down process must for induction heating
be done so fast that the carbon remains detached after the 800
crystal transformation and the transformation of the austenite
to perlite and ferrite is suppressed. This results in the har-
dening structure martensite. Martensite is the carrier of the
increased hardness. The considerable increase of hardness 600
due to the formation of martensite becomes obvious and
Furnace heating
of practical use only when the carbon content of the steel
exceeds 0,35 %. The hardening yield continues to increase
up to carbon contents of 0,7 %. Carbon contents higher than 400
0,7 % do not result in any considerable increase of hard-
ness. On the contrary, higher carbon contents, particularly in
combination with alloy elements, cause the transformation 200
of austenite to martensite to be shifted to lower temperatu-
res such way that this is not yet entirely completed at room
temperature. Due to this, a more or less large quantity of
austenite (residual austenite) remains in the structure which 0
reduces the total hardness due to its low hardness. 0 0,2 0,4 0,6 0,8 1,0

Carbon content (%)

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The martensite being a result of quench hardening is hard, compensate by diffusion in the austenite come into being.
but also very brittle. Its specific volume is larger than that of The compensation process depends on time and tempera-
the original structure. This causes unavoidable changes in ture. It goes slow closely above the transformation tempe-
the dimensions of the hardened part and internal tensions rature and faster at increased temperatures. Are in the steel
when the workpiece is only locally martensitic due to surface besides the iron carbide (cementite) any carbides from alloy
hardening. These tensions are overlapped by tensions which elements (e.g. chrome) present, the austenitizing process
are caused by the considerable differences of temperature will take longer due to the dissolution of the carbides either
in the workpiece in the heating and quenching process. The starting with delay or going slower.
totality of tensions causes the hardening distortion and pos-
sibly hardening cracks.
Steel provides the optimal requirements for the hardenability,
Tempering at temperatures of 150 – 200 ° C will change the provided the austenitizing process
martensite structure. The martensite experiences a conside-
rable stress relief without any substantial hardening reduc- 1. dissolves and transforms the perlite and ferrite
tion. This has a very positive effect on the mechanical fea- 2. largely dissolves the alloy carbides
tures (stretch and toughness). The workpiece is less sensitive 3. all differences in concentration (carbon and alloy ele-
to shock and cracks are hardly to be expected. ments) are compensated.

Although in induction hardening the same process is done

in the workpiece as in the other transformation hardening Both, a dwell time longer than required (overtimes) and a
processes, the necessarily preceding austenitizing process too high austenitizing temperature cause a coarse austeni-
is very limited in time as a result of the fast heating. When a te grain unless the dwell time is reduced at the same time
workpiece is heated in the furnace to hardening temperature, (overheating). The risk of forming a coarse grain as a result
the time required for through hardening is in general suffici- of increased hardening temperatures, as applied for a faster
ent to austenize the structure completely. On the basis of the austenitizing in induction hardening, however does not exist
usual ferrite-perlite structure of the steel, this means that with as long as there are undissolved rests of carbide present.
increasing temperature and dwell time beyond the transfor-
mation point first the perlite is transformed into austenite and
then increasingly the ferrite. Since both structure compo-
nents have a very different carbon content (perlite ≈ 0,9 and
ferrite < 0,01) this difference of concentration of carbon must

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Comparison of the induction, flame, dip, case and nitride

hardening processes

Induction hardening cannot and is not to replace those Induction hardening

surface hardening processes being generally in use. It is an
additional hardening process which is used for those appli- Advantages
cations where there is a benefit, both in technical and eco- Uniform heating of the parts of the component to be harde-
nomic respect. The advantage becomes the more obvious ned. Short heating times and as a result thereof the formation
the smaller the surface to be hardened on a workpiece is, of a minimum amount of scale. In many cases no subsequent
compared with its total surface. The following is a summary work is necessary. Due to short-time heating the formation of
of the advantages and disadvantages of the different surface coarse grain as a result of overtimes and overheating is avo-
hardening processes. The decision which hardening process ided. Safe control of heat input. The temperatures required
is advantageous for a specific workpiece can be taken by the are kept. The distortion is generally low. In comparaison with
processing company only and, in case of doubt, after having case hardening, expensive alloyed case hardening steels can
consulted experts for such processes. be replaced by cheap heat-treatable steels. Partial hardening
is mostly possible even on most difficult workpiece shapes.
The hardening machines and generators can be directly inte-
grated in the production lines. The space requirement is low,
easy and clean operation with no health hazards.

The hardening installation is always ready for operation and,

with careful routine maintenance, safe in operation. The har-
dening machines can be manufactured such way to allow for
fully automatic operation.

Disadvantages
The purchase costs for a hardening installation are high and
can only be amortized through a good utilization and/or ma-
jor quantities of workpieces to be processed. When harde-
ning heat-treatable steels a zone of low strength (soft zone)
might occur between the core and the hardened outer zone.
Different inductors have to be used for the different proces-
ses. Hardening components with large changes in sections
can be difficult.

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Flame hardening Case hardening

Advantages Advantages
Low capital costs. The heating times are relatively short. The hardened layer, although relatively thin, is uniform over the
The distortion is low.The minimum hardness depths that can component. Selective hardening can be achieved, dependent
be obtained are more limited downwards than with induc- upon the component shape. The core strength is increased at
tion hardening. Within limits, selective hardening of specific the same time when the surface is hardened. Higher efficiency
areas of the component is possible. The hardening plant and in general on parts whose whole surface is to be hardened.
equipment can be installed in a production line. Low space
requirements and simple operation. The installation is always Disadvantages
ready for operation. The hardening machines can be partly High operating costs, long annealing times. Severe distortion
automated. can occur as the whole component will be heated. Areas which
are not to be hardened must be covered or the hardened layer
Disadvantages must be removed before the hardening process. The process
Due to variations in the burner gas pressure and mixture the can only be carried out in a special hardening shop involving
heating flame temperature is not always constant causing the additional transportation cost. In order to receive a clean
hardening depth to vary. The hardening of bores is difficult surface the hardened workpieces need subsequent work.
and can only be carried out on large diameters. For harde-
ning different components different burners have to be used.
When hardening heat treatable steels, a tempering zone (soft
zone) occurs between the core and the hardened outer layer.
Nitride hardening (gas nitriding)
Advantages
Uniform hardness depth irrespective of the shape of the
component. As the process temperature is low (approx. 500
Dip hardening °C), distortion on stress-relieved annealed components is
insignificant. No quenching is necessary. Very high hardness
Advantages values can be achieved and will remain nearly the same at
Low heat treatment costs. Short process times. The distorti- temperatures above 500 °C. The resistance to wear is very
on is low. high in accordance with the high hardness. Nitrited compo-
nents do not have to be reworked after hardening.
Disadvantages
Selective hardening is only possible in certain instances. The Disadvantages
complete component is surface hardened as it is impossible High operating costs. Only special steels can be used. The
to mask areas which should not be hardened. It is not possi- annealing times are very long, depending on the hardness
ble to obtain a perfect hardened layer at points where there depth between 1 – 4 days are necessary. The whole com-
is a change in section or notches in the component. The ponent is heated through. The hardened layer is thin. The
hardening works can only be carried out in a special harde- hardness reduces considerably in the zones below 0,2 mm.
ning shop involving additional transportation cost. The fumes The surfaces do not withstand high surface pressure as they
of the dip baths are harmful to the health. The hardened tend to collapse under pressure. Sections not to be harde-
components require subsequent work. ned have to be coated by tinning or nickeling. The surface of
the component must be perfectly clean before nitriding. The
process can only be carried out in a special hardening shop,
involving additional transportation costs.

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Cooling curves of water, mineral oil and aqueous solutions

900

800

700

600
Temperatur in °

500

400

300

200

100

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Zeit in Sek.

Water
SERVISCOL 78 10% synthetic quenchant
DURIXOL 4 intense high-performance quenchant
DURIXOL W 25 vaporization-proof high-performance
quenchant
DURIXOL A 650 hot bath oil for bath temperatures
up to 250 °C
DURIXOL H 222 vacuum quench oil

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Table to compare hardness values according to

Rockwell, Vickers, Brinell, tensile strength

Rockwell Vickers Brinell tensile strength Rockwell Vickers Brinell tensile strength
Rm Rm
HRC HV HB N/mm² HRC HV HB N/mm²

20 240 228 770 44 430 409 1385

21 245 233 785 45 445 423 1450
22 250 238 800 46 460 437 1485
23 255 242 820 47 470 447 1520
24 260 247 835 48 480 456 1555
25 265 252 850 49 500 475 1630
26 270 257 865 50 510 485 1665
27 280 266 900 51 520 495 1700
28 285 271 915 52 545 515 1780
29 295 280 950 53 560 532 1845
30 300 285 965 54 580 551 1920
31 310 295 995 55 600 570 1995
32 320 304 1030 56 610 580 2030
33 330 314 1060 57 630 599 2105
34 340 323 1095 58 650 620 2180
35 345 330 1115 59 670 - -
36 355 335 1140 60 700 - -
37 365 340 1150 61 720 - -
38 370 352 1190 62 740 - -
39 380 361 1220 63 770 - -
40 390 371 1255 64 800 - -
41 400 380 1290 65 830 - -
42 410 390 1320 66 860 - -
43 420 399 1350

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Inductively hardable steels

DIN-term material- HRc- analysis INDUCTOHEAT Europe GmbH

number values C Si Mn P S Cr Mo Ostweg
Ni 5 V C
% ≤% ≤% ≤% ≤% % % D-73262
% Reichenbach/Fils
% %
heat-treatable steels Telefon +49 (0)7153 504-235
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C 35 1.0501 51 – 57 0,35 0,35 0,80 0,045 0,045
verkauf@inductoheat.eu
35 S 20 1)
1.0726 50 – 55 0,35 0,40 0,90 0,060 0,250
www.inductoheat.eu
Ck 35 1.1181 51 – 57 0,35 0,35 0,80 0,035 0,035
Cf 35 1.1183 51 – 57 0,35 0,35 0,80 0,025 0,035
C 45 1.0503 56 – 61 0,45 0,35 0,80 0,045 0,045
45 S 20 1)
1.0727 55 – 60 0,45 0,40 0,90 0,060 0,250
Ck 45 1.1191 56 – 61 0,45 0,35 0,80 0,035 0,035
Cf 45 1.1193 56 – 61 0,45 0,35 0,80 0,025 0,035
Cf 53 1.1213 58 – 63 0,53 0,35 0,70 0,025 0,035
60 S 20 1)
1.0728 58 – 62 0,60 0,40 0,90 0,060 0,250
Ck 60 1.1221 59 – 64 0,60 0,35 0,90 0,035 0,035
Cf 70 1.1249 60 – 64 0,70 0,35 0,35 0,025 0,035
79 Ni 1 1.6971 60 – 64 0,79 0,30 0,55 0,025 0,025 0,15 0,15 0,05
36 Mn 5 1.5067 52 – 56 0,36 0,35 1,50 0,035 0,035
40 Mn 4 1.5038 53 – 58 0,40 0,50 1,10 0,035 0,035
37 MnSi 5 2)
1.5122 55 – 58 0,37 1,40 1,40 0,035 0,035
38 MnSi 4 2)
1.5120 54 – 58 0,38 0,90 1,20 0,035 0,035
46 MnSi 4 2)
1.5121 57 – 60 0,46 0,90 1,20 0,035 0,035
53 MnSi 4 2)
1.5141 58 – 62 0,53 1,00 1,20 0,035 0,035
45 Cr 2 1.7005 56 – 60 0,45 0,40 0,80 0,025 0,035 0,50
34 Cr 4 1.7033 51 – 55 0,34 0,40 0,90 0,035 0,035 1,05
37 Cr 4 1.7034 53 – 58 0,37 0,40 0,90 0,035 0,035 1,05
38 Cr 4 1.7043 53 – 58 0,38 0,40 0,90 0,025 0,035 1,05
41 Cr 4 1.7035 54 – 58 0,41 0,40 0,80 0,035 0,035 1,05
42 Cr 4 1.7045 54 – 58 0,42 0,40 0,80 0,025 0,035 1,05
34 CrMo 4 1.7220 52 – 56 0,34 0,40 0,80 0,035 0,035 1,05 0,25
41 CrMo 4 1.7223 54 – 58 0,41 0,40 0,80 0,025 0,035 1,05 0,25
42 CrMo 4 1.7225 54 – 58 0,42 0,40 0,80 0,035 0,035 1,05 0,25
49 CrMo 4 1.7238 57 – 62 0,49 0,40 0,80 0,025 0,035 1,05 0,25
50 CrMo 4 1.7228 57 – 62 0,50 0,40 0,80 0,035 0,035 1,05 0,25
50 Cr V 4 1.8159 57 – 62 0,50 0,40 1,10 0,035 0,035 1,05 0,15
58 Cr V 4 1.8161 58 – 63 0,58 0,35 1,10 0,035 0,035 1,05 0,09
30 CrNiMo 8 1.6580 50 – 54 0,30 0,40 0,60 0,035 0,035 2,00 0,35 2,00
34 CrNiMo 6 1.6582 53 – 56 0,34 0,40 0,70 0,035 0,035 1,55 0,25 1,55
36 CrNiMo 4 1.6511 54 – 57 0,36 0,40 0,80 0,035 0,035 1,05 0,25 1,05
tool steels
X 41 CrMo V 5,1 1.2344 55 – 59 0,41 1,00 0,40 0,015 0,010 5,00 1,30 0,50
86 CrMo V 7 1.2327 60 – 64 0,86 0,35 0,45 0,030 0,030 1,75 0,30 0,10
X 20 Cr 13 1.2082 48 – 53 0,20 0,50 0,40 0,035 0,035 13,00
X 40 Cr 13 1.2083 55 – 58 0,40 0,50 0,40 0,030 0,030 13,00
stainless steels
X 90 CrMo V 18 1.4112 55 – 58 0,90 1,00 1,00 0,045 0,030 18,00 1,15
X 90 CrCoMo V 17 1.4535 55 – 58 0,90 1,00 1,00 0,045 0,030 16,50 0,50 0,25 0,25 ca. 1,5
X 105 CrMo 17 1.4125 56 – 60 1,05 1,00 1,00 0,045 0,030 17,00 0,60 0,10
rolling bearing steels
100 Cr 6 1.3505 62 – 65 1,00 0,35 0,40 0,030 0,025 1,55
valve steel
X 45 CrSi 9-3 1.4718 56 – 60 0,45 3,50 0,50 0,030 0,025 9,50
X 80 CrNiSi 20 1.4747 52 –55 0,80 2,75 1,00 0,030 0,030 20,00 1,50
casting material

}
GG-25 0.6025 48 – 52
GTS-45 51 – 57
GTS-65 56 – 59 Please ask for an additional instruction sheet
GGG-60 0.7060 53 – 59
GGG-70 0.7070 56 – 62

1)
higher hardening variations are possible 2)
good transmutations, but danger of cracks for strong shaped pieces

Carburized steels suitable for partial hardening, e.g. Ck 15, 16 MnCr 5, 20 MnCr 5, 15 CrNi 6, 20 MoCr 4 etc.
Dry powdered metals iron-carbon basis hardening is possible

Key for hardening depths: max. 2 mm

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über 6 mm

Leading Manufacturers of Melting, Thermal Processing &

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