Induction hardening is an efficient surface hardening process that uses electromagnetic induction to generate eddy currents and rapidly heat metal components. It produces a martensitic microstructure upon quenching that increases hardness, strength, and wear resistance while minimizing distortion compared to traditional furnace treatments. Induction hardening takes less than a minute, whereas nitriding and carburizing can take hours, and it induces higher compressive residual stresses in the surface.

1. Ashley Lane
MSE 440
January 24, 2012
Image taken from http://www.contourhardening.com/images/753_GearGlowing.jpg

2. Transmission Components
 Components include:
 Gears & sprockets
 Shafts
 Connecting rods
 Several of each component in
every transmission
 Failure of any component will
cause vehicle to fail
 Most common failure is
caused by crack propagation
Image of failed gear due to cracking taken from
http://www.dragtimes.com/blog/2009/02.
Need: To strengthen transmission components quickly to
prevent cracks which lead to part failure.
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3. Popular Hardening Treatments*
 Flame hardening (~50 HRC) (a) (b)
 Only for gray cast iron
 Anneals core material during process
 Nitriding (~69 HRC)
 Uses ammonia or cyanide salt baths Images of gear teeth hardened by (a)
nitriding and (b) carburizing taken from
 Depth of 1 mm
http://www.gearsolutions.com/media//uplo
 Roughly 4 hours per work piece ads/assets//PDF/Articles/Jan_10/0110_Boeing
.pdf
 Carburizing (~50 HRC)
 Used on low carbon content steel
(<0.2%C) Need more efficient
 Depth up to 6 mm process with required
 Typically 12-72 hrs. per work piece product properties with
shorter process time!
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*Davis, et. al, Surface Engineering of Cast Irons, Surface Engineering, ASM Handbook [5] 683-700 (1994)

4. Why Choose Induction?
 Produces ideal properties
 Hardness (~67 HRC)
 Strength and wear resistance of part
 Has much faster heating rate than traditional furnace
treatments
 Provides for more control over outcome
 Less distortion of work piece*
 Warpage is roughly 0.03mm compared to 0.3mm from furnace treatments*
 Heating can be localized for surface hardening
 Allows the core metal to be unaffected
 Creates higher residual stresses
4
*Rudnev, Simulation of Induction Heat Treating, Metals Process Simulation, ASM Handbook, [22B]501–546 (2009).

5. Induction Hardening
 Heat treatment used for metal
(typically steel)
 Uses electromagnetic induction*
 Eddy currents are generated in
metal
 Resistive heating is proportional to
resistance in metal and currents
produced
 Hardening may be done on the
surface or throughout entire work Image of hardened gear teeth taken
piece from
http://www.ersengine.com/gains-
 Utilizes localized heating
ground-through-advances-in-
 Does not affect properties of the technology/.
part as a whole
5
*Rudnev, Simulation of Induction Heat Treating, Metals Process Simulation, ASM Handbook, [22B]501–546 (2009).

6. Currents and Magnetic Field
 Eddy currents
 Current induced in conductors
when exposed to a
changing magnetic field
 Due to variations of the field with
time
 Generates resistive heating in metal
 Can reaustenitize metal in 0.5 s*
 Induced as result of Faraday’s law
of induction*
𝑑Φ
𝑒 = −𝑁 Image of currents and magnetic field
𝑑𝑡 induced during hardening process.*
e =induced voltage
N=number of coil turns
Φ= magnetic field
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*Hassell, et. al, Induction Heat Treating of Steel, Heat Treating, ASM Handbook [4] 164–202 (1991).

7. Electrical Properties of Steel
 Only adjustable parameter is the
frequency
 High frequencies are used for
surface hardening
Frequency (kHz)
 Lower frequencies are used for
through-hardening
ρ
𝑑=
πμ0 μ f
d=depth of hardening
ρ= resistivity Diameter (mm)
μ0= 4π×10−7 V·s/(A·m) (magnetic permeability of vacuum)
μ =magnetic permeability of part Plot of diameter versus frequency in
f=frequency of magnetic field plain carbon steel.*
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*Hassell, et. al, Induction Heat Treating of Steel, Heat Treating, ASM Handbook [4] 164–202 (1991).

8. Typical Heat Treatment Procedure
 An induction
heater consists of
an electromagnet
 Creates a high-
frequency alternating
current (AC)
 Heats the component to
the austenitizing
temperature Image of electromagnetic coil taken from
http://www.contourhardening.com/image
 Holds it at temperature s/753_GearGlowing.jpg.
long enough to complete
the formation of austenite
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9. Quenching Stage of Heat Cycle
 Rapidly cools the metal
until martensitic
transformation occurs
 Changes structure from
FCC to BCT*
 Causes a shear-type
transformation of the
initial structure of steel
into martensite.*
 Translations of slip and
twinning occur* Images of gears taken from
http://www.contourhardening.com/imag
es/753_GearGlowing.jpg.
http://www.youtube.com/watch?feature=endscreen&NR=1&v=6Kjk45kqRdo 9
*Pense, et. al, Structure and Properties of Engineering Materials. 141-145 (1977).

10. Microstructure
 Induction hardening creates a
martensitic transformation of the
base austenite
 Martensite is characterized by*
 Needle shaped grains
 Metastable crystal structure
(BCT)
 Diffusionless Transformation
 Maintaining composition of original Micrograph of martensite (darkened
austenite areas) and austenite (white areas) taken
from Callister, et. al, Materials Science
 Having larger specific volume and Engineering: an Introduction. 331-
than austenite 333 (2007).
 Hard and brittle
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*Pense, et. al, Structure and Properties of Engineering Materials. 141-145 (1977).

11. Microstructural Relations
 Hardness of austenitized
parts depends mainly on
chemical composition
and quench medium*
 Desired Microstructures
for reaustenization*
 Finer bainite and
martensite> pearlite>
Effect of carbon on hardness in plain carbon
spheroidite steels for (A) induction hardening, (B) furnace
hardening with water quench and (C) furnace
hardening with water quench and temper. The
quenched-and-tempered steels were treated in
liquid nitrogen following water quenching
prior to tempering at 100 °C for 2 hrs.*
11
*Hassell, et. al, Induction Heat Treating of Steel, Heat Treating, ASM Handbook [4] 164–202 (1991).

12. Induced Stresses
 Treatment induces compressive
stresses at surface (>205 Mpa)*
 Surface has volume expansion
 Non-treated core remains
unchanged
 Martensite retains residual
stresses**
Typical hardness and residual stress profile of
Image of gear teeth being heated taken from
induced-hardened steels using general
http://www.ersengine.com/gains-ground-through-
dissection method. **
advances-in-technology/.
*Sinha, et. al, Defects and Distortion in Heat-Treated Parts, Heat Treating, ASM Handbook [4] 601-619 (1991). 12
**Davis, et. al, Surface Engineering of Cast Irons, Surface Engineering, ASM Handbook [5] 683-700 (1994)

13. Maximum Residual Stresses in
Surface-treated Steel*
Heat treatment Residual Stress (MPa)
Carburized and quenched 340
Nitrided to case depth of about 0.5
600
mm (0.02 in.)
Induction hardened, untempered 1000
Stress measurements taken at 0.05mm from surface
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*Sinha, et. al, Defects and Distortion in Heat-Treated Parts, Heat Treating, ASM Handbook [4] 601-619 (1991).

14. Review of Surface Hardening *
 Induction hardening (~67 HRC) (a)
(b)
 Can be used on any type of steel
 Utilizes localized heating
 Has clean transition pattern
 Process takes less than 1 minute
 Nitriding (~69 HRC) (c)
 Uses ammonia or cyanide salt baths
 Depth of 1 mm
 Roughly 4 hours per work piece
 Carburizing (~50 HRC)
Images of gear teeth hardened by (a)
 Used on low carbon content steel nitriding (b) carburizing and (c) induction
(<0.2%C) hardening found at
http://www.gearsolutions.com/media//uplo
 Depth up to 6 mm
ads/assets//PDF/Articles/Jan_10/0110_Boeing
 Typically 12-72 hrs. per work piece .pdf
14
*Davis, et. al, Surface Engineering of Cast Irons, Surface Engineering, ASM Handbook [5] 683-700 (1994)

15. Summary
 Induction is an efficient means of heat treatment
 Produces eddy currents for resistive heating in metal
 Rapid quenching to induce martensitic transformation
 Has much faster heating rate than traditional furnace
treatments
 Creates increases in:
 Residual Stresses
 Hardness
 Strength
 Wear resistance
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16. Questions
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