Proceedings of the 28th ASM Heat Treating Society Conference
October 20–22, 2015, Detroit, Michigan, USA
Copyright © 2015 ASM International®
All rights reserved
asminternational.org
Breakthrough in Induction Hardening Shafts
Dr.Valery Rudnev, William West, Aaron Goodwin, Steve Fillip
Inductoheat Inc., An Inductotherm Group Company
32251 N. Avis Dr., Madison Heights, MI 48306 USA
rudnev@inductoheat.com; wwest@inductoheat.com; agoodwin@inductoheat.com; sfillip@inductoheat.com
Abstract
acts as the crack arrester and increases torsional and bending
fatigue life.
This presentation reviews selected innovations related to
induction hardening of various automotive powertrain
transmission and engine components, including but not limited
to induction surface hardening of complex geometry shafts.
Thanks to several innovative designs (patented and patent
pending), important goals were achieved. Process flexibility in
shaft scan hardening has been substantially enhanced thanks to
a novel inverter design that allows controlling independently
frequency and power during scanning. This innovation allows
improving quality of induction hardened components
maximizing production rate and process flexibility.
Heating inductors provide required three-dimensional (3-D)
heat generation during the heating cycle that can produce upon
quenching the desired hardness pattern. Inductors can be of
different styles and geometries to accommodate the shape of
the workpiece needed to be heat treated and process specifics.
In everyday practice, an inductor is also called an induction
coil or simply a coil. However, its geometry often does not
resemble the shape of a classical coil. Figure 1 shows
examples of various inductor designs used in induction
hardening.
When applying single-shot hardening for heat treatment of
output shafts, flanged shafts, yoke shafts, sun shafts,
intermediate shafts, drive shafts and others, coil life is often
limited due to a necessity to “squeeze” coil current in a certain
area, maximizing power density. This seemingly unavoidable
feature of the great majority of single-shot inductors
represents a “weak link”, limiting coil life expectancy. Thanks
to innovative design (patent-pending) of a single-shot
inductor, its life was increased approximately nine times.
Process sensitivity has been dramatically reduced. Other
benefits include measurable improvement in process
robustness, coil reliability and maintainability.
Introduction
Figure 1: Examples of various inductor designs used in
induction hardening. Courtesy of Inductoheat Inc.
Hardening of steels, cast irons and powder metallurgy
components represents the most popular applications of
induction heat treating. Induction hardening of parts (e.g.,
machine tools, hand tools, powertrain components, bearings,
links, pins, rods, frames, blades, springs, fasteners, gear-like
components and many others) may be done for the purpose of
obtaining certain properties that include but are not limited to
strength, fatigue, and/or wear resistance [1,2]. For example, in
torsion and in bending the stress is greatest at the surface, and
is zero at the center. For this reason, an induction surface
hardening can improve component performance with these
two load cases, as it increases the strength at the surface where
it is needed the most [1]. In addition, surface hardening
normally leaves the surface of the part with a beneficial
residual compressive stress of appreciable magnitude, which
There are four primary heating modes of induction hardening:
scanning, progressive, single-shot and static modes.
Depending upon the applied method and application specifics,
parts may or may not be rotated, moving in a linear motion or
be motionless.
Steel shafts and shaft-like components are among parts that
traditionally undergo induction hardening (Fig.2). Shaft-like
components are usually induction hardened using scanning or
single-shot heating modes.
Scan Hardening of Shafts
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With the scan hardening of cylindrical parts, using a singleturn or multi-turn coils, induced eddy currents flow
predominately circumferentially. Figure 3 shows the results of
FEA computer modeling of a scan hardening of a hollow shaft
with fillet utilizing vertical design [3].
According to scan hardening, the coil and/or part move
relative to each other. The workpiece generally rotates inside
the coil to even out the induction hardened pattern around the
circumference.
Figure 4 shows another example of FEA computer modeling
of the sequential dynamics of a horizontal induction scan
hardening system for heat treating of a hollow shaft with
diameter changes, undercut, and grooves [4]. Because the
shaft is symmetrical, only the top half was modeled.
Figure 2: Examples of various shafts and shaft-like
components. Courtesy of Inductoheat Inc.
Scan hardening can be performed on outside diameters (OD)
and/or inside diameters (ID) of cylinder components as well as
on flat surfaces. Both horizontal and vertical coil arrangements
can be used, with the vertical design being the most popular
for hardening of short and moderate-length cylinders [1, 2].
When scanning the outside surfaces of solid shafts, the
induction coil typically encircles the part. A quench ring is
positioned next to the coil in order to spray-quench the area
that has been heated. In other cases a machined integral
quench (MIQ) inductor can be used. On some machines the
coil remains stationary and the part moves, while on other
machines the opposite is true.
Figure 4: FEA computer modeling of sequential dynamics of
horizontal induction scan hardening system for heat treating
of hollow shaft with diameter changes, undercut, and grooves
[4]. Because the shaft is symmetrical, only the top half was
modeled.
The scan hardening process starts with the coil positioned at
one end of the part. The part begins to rotate, the power is
turned on and the coil may remain stationary for a period of
time to drive the heat into a certain region of the part. This is
known as the dwell and is typically used to preheat the fillet
areas. The part and/or coil then begin to move relative to each
other, which is why the term scanning is used.
As the heat front moves along the length/height of the part the
scan rate and/or coil power may be changed to affect the case
depth as needed. This is typically done where the diameter or
the configuration of the part changes, when there is a
combination of the solid and hollow sections of the shaft, or
where other geometrical irregularities (e.g., shoulders, fillets,
diameter and wall thickness changes, etc.) are present.
The great majority of commercially available inverters are
designed to provide certain frequency of output AC current
that cannot be instantly changed during the scanning
operation. Therefore, the depth of heat generation (that is
related to current penetration depth) might not be an optimal
for a particular portion of the heat treated part, its geometrical
features and/or localized case depth requirements.
Figure 3: Dynamics of temperature profiles during induction
scan hardening of hollow shaft [3].
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In some cases, available frequency might be substantially
lower than optimal, but in other cases it might be significantly
higher for obtaining particular hardness case depth (CD) in a
certain area of the part during scan hardening.
If an available frequency is noticeably higher than optimal, it
results in too small current penetration depth  that might not
be sufficient for a proper austenization of the subsurface
region at required hardness case depth. Therefore, additional
time is needed to allow the thermal conduction to provide a
sufficient heat flow from the workpiece’s surface toward
required depth to ensure its proper austenization. This is
commonly accomplished by a reduction of both: the scan rate
and power density. Otherwise, the surface might be
overheated and undesirable metallurgical structures might be
formed (for example, grain boundary liquation, severe grain
coarsening, etc.). Unfortunately, this not only adds
unnecessary cycle time, but it might still lead to some
metallurgical issues related to excessive surface temperatures.
Figure 5. Independent Frequency and Power control
capability of new inverters.
In contrast, if available frequency is noticeably lower than
optimal frequency, it might produce a much deeper austenized
layer than needed. In order to minimize the negative impact of
using lower than desirable frequency, the majority of
induction heat treating users are trying to suppress a thermal
conduction by increasing both: scan rate and power density.
Thus, conventional scan hardening technologies relay upon a
compromise between achieved quality, production rate and
process capability. Regardless of the fact that those recipe
modifications can help to minimize a negative impact of using
other-than-optimal frequencies by rely upon suppression or
enhancement of thermal conduction, they often cannot
eliminate it. Depending upon the shaft’s geometry, this could
limit the ability to meet a pattern specification and might
negatively affect the achieved metallurgical quality of heat
treated components and distortion requirements.
Figure 6. Patented Statitron-IFPTM inverter is capable in
providing Independent Frequency and Power control. It was
specifically designed for induction hardening applications
allowing instantly and independently adjust both: frequency
(within 5kHz - 60kHz) and output power (up to 450kW).
Courtesy of Inductoheat Inc.
According to theory of induction heating, applied frequency is
the most powerful parameter that directly affects the depth of
heat generation [1, 2]. Obviously, in order to address
geometrical subtleties of heat treated components in an
optimal manner, it would be much more beneficial to apply
various combinations of frequencies, power densities and scan
rates at various stages of the scan hardening operation. This
would maximize production and improve metallurgical quality
of the heat treated components. Unfortunately, the great
majority of available inverters do not have such a capability.
Single-Shot Hardening of Shafts
Statitron-IFPTM inverter technology is the long-awaited desire
of commercial heat treaters, providing the greatest process
flexibility, measurably expanding the equipment capabilities
and improving the metallurgical quality of induction heat
treated components.
With the single-shot method, neither the part nor coil move
relative to each other; however, part rotation is typically used.
The entire region that is to be hardened is heated all at once.
A single-shot inductor induces eddy currents that primarily
flow along the length of the part. An exception to this rule
would be the arc-shape (cross-over) regions of a single-shot
inductor, where eddy current flow is circumferential.
Normally, the single-shot method is better suited for hardening
relatively short parts or parts where only a relatively short
areas is to be heat treated [1,2]. This method is also better
suited to cylindrical parts having axial symmetry and
geometrical irregularities, such as diameter changes, fillets,
shoulders, and so forth. A single-shot inductor, to some
degree, must follow the contour of the entire part or the area
The new generation of patented transistorized inverters
eliminates this limitation and simplifies achieving required
hardness pattern specifications, allowing independently
controled frequency and power (Figure 5) during induction
scan hardening. This allows optimizing electromagnetic and
thermal conditions when heat treating components with
geometrical irregularities and various required hardness case
depths. As an example, Figure 6 shows the Statitron-IFPTM
inverter that provides Independent Frequency and Power
control.
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required to be hardened. As an example, Figure 7 shows a
conventional CNC-machined single-shot inductor.
There are several ways to control heat generation at different
regions of a heat treated shaft. This includes an inductor
copper profiling resulting in a variation of coil-to-shaft
electromagnetic coupling and/or machining different widths of
the current-carrying faces of the inductor. Besides that, a
magnetic flux concentrator can be attached to certain areas of
the hardening inductor to enhance localized heat intensity as it
is shown on Figure 7.
Figure 8. A tag from one of the world’s largest supplier of
automotive parts that indicates an achievement of 225,000 hits
(nine-fold increase) of a single-shot coil life compared to
conventional inductors.
Customer has even named this unique inductor design (patent
pending) as a “magic coil”. Details of this design and
explanation of its benefits will be discussed in detail in the
PowerPoint presentation.
Figure 7. Conventional single-shot inductor for induction
surface hardening of shaft-like components having fillets and
diameter changes. Courtesy of Inductoheat Inc.
Significant increase in coil life is not the only attractive
feature associated with this design. Other important benefits
include a substantially reduced sensitivity of the positioning of
a heat treated shaft in comparison to an inductor. This feature
is also highly desirable since it dramatically improves process
robustness.
As a result, certain regions of coil copper might carry
extremely high current densities resulting in an intense heat
generation. Regardless of an attempt to position water-cooling
pockets as close to the current carrying face of an inductor as
possible and utilization of high-performance pumps, a coil
copper might be still overheated, leading to an accelerated
deterioration of the copper surface and eventual premature coil
failure.
Conclusions
Presentation discusses two innovations related to induction
scan hardening and single-shot hardening of shaft-like
components (including but not limiting to induction surface
hardening of output shafts, flanged shafts, yoke shafts, sun
shafts, intermediate shafts, drive shafts, etc.). Advanced
designs (patented and patent-pending) focus on ensuring
metallurgical quality of heat treated components and
improving process robustness and flexibility. Customer
recorded a nine-fold increase in a single-shot coil life
compared to conventional inductors.
This is particularly so, because a combination of intense heat
generation within the current carrying face of coil copper with
intense heat radiation/convection from workpiece’s surface (in
particularly when relatively small coil-to-shaft air gaps are
used) may lead to a substantial copper overheating. This may
promote water vaporization and the formation of a steam
vapor barrier in that region [5]. Therefore, regardless of what
may appear as a sufficient water-cooling flow, the presence of
a steam vapor barrier essentially will act as a thermal insulator
inside the water-cooling pocket and appreciably restrict copper
cooling. Both factors lead to a premature failure shortening
coil life down to 22,000-24,000 heat cycles.
References
[1] Rudnev, V., Fett, G.A., Griebel, A., Tartaglia, J.,
“Principles of Induction Hardening and Inspection,” ASM
Handbook Vol.4C: Induction Heating and Heat Treating,
V.Rudnev and G.Totten (eds.), ASM Int’l, 2014, pp.5886.
[2] Rudnev, V., et al., Handbook of Induction Heating, CRC
Press, 2003, 800p.
Recently developed novel inductor design allows for
dramatically extending coil life. As an example, Figure 8
shows a tag from one of the world’s largest suppliers of
automotive parts that indicate an achievement of a nine-fold
increase in a single-shot coil life compared to conventional
inductors.
144
[3] Rudnev, V., “Q & A Regarding Quenching and Case
Hardening of Solid Shafts vs Hollow Shafts”, Professor
Induction Series, Heat Treat. Progress, ASM Int’l, Sept
2009, p.29-32.
[4] Rudnev, V., “Computer Modeling Helps Prevent Failures
of Heat Treated Components,” Advanced Materials and
Processes, ASM Int’l, October 2011, pp. 28-33.
[5] Rudnev, V., “Systematic Analysis of Induction Coil
Failures and Prevention,” ASM Handbook Vol.4C:
Induction Heating and Heat Treating, V.Rudnev and
G.Totten (eds.), ASM Int’l, 2014, pp.646-672.
145