http://fluxtrol.com In induction hardening, thermal fatigue is one of the main failure modes of induction heating coils. There have been papers published that describe this failure mode and others that describe some good design practices [1-3]. The variables previously identified as the sources of thermal fatigue include radiation from the part surface, frequency, current, concentrator losses, water pressure and coil wall thickness. However, there is very little quantitative data on the factors that influence thermal fatigue in induction coils available in the public domain. By using finite element analysis software this study analyzes the effect of common design variables of inductor cooling, and quantifies the relative importance of these variables. A comprehensive case study for a single shot induction coil with Fluxtrol A concentrator applied is used for the analysis.

1. Innovations in Soft Magnetic Composites and their Applications in
Induction Systems
R. Ruffini, N. Vyshinskaya, V. Nemkov, R. Goldstein, C.J. Yakey
Fluxtrol Inc., Auburn Hills, MI USA
fluxtrol@fluxtrol.com, 248-393-2000, www.fluxtrol.com
Abstract
New soft magnetic composites have been added to the current family of materials produced by Fluxtrol Inc. which
allows users to increase their range of magnetic flux controller applications and improve their overall inductor
performance. A new material (Fluxtrol 100) is a substitution for a current well known material (Fluxtrol A). This
new material has better mechanical properties, machinability and low anisotropy. The formable materials of
Alphaform are effective on I.D. induction coils and various small coils of complex geometries. These materials may
be used at any frequency, up to several megahertz. Along with the description of new materials, this presentation
contains information about recent improvements in application of Fluxtrol materials including preparation, forming
and gluing. One of new methods is impregnation of magnetic concentrators. This advanced technology consists in
vacuum treatment of magnetic controllers or whole induction coils with subsequent placing them into a bath of a
special resin. Resin penetrates into the material pores and gaps between the concentrators and copper turns and
polymerizes inside of them. This treatment increases mechanical strength of the material and total assembly and
improved corrosion resistance. Induction coils for axle and crankshaft hardening as well as small ID coils are
selected for illustration.
Introduction
Magnetic Controllers on Heat Treating Inductors
Modification of magnetic field distribution and control of its intensity on the surface of the parts to be heated may be
accomplished by different methods: by variation of the coil turn shape and positioning, by insertion of non-magnetic
shields and magnetic templates that may be called magnetic controllers. Non-magnetic shields, typically made in the
form of copper rings or massive copper blocks, are often called “flux robber rings” [1]. Their use leads to reduction
of the coil power factor and efficiency and they are not considered in this paper.
Magnetic flux controllers are made of soft magnetic materials: steel laminations, ferrites and magnetic composites.
Magnetic controllers can concentrate field in required areas (field concentration), change field distribution, shield
certain areas from unintended heating and strongly reduce the magnetic field outside the treatment area. The team of
Fluxtrol Inc. has developed the basics of magnetic flux control including the theory, methods of simulation and
design, application technique guidelines. A course that contains these topics as well as the basics of induction
heating may be found on the company website under the tab Training [2]. A role of magnetic flux control and
methods of computer design of induction coils with magnetic controllers are presented also in multiple papers, e.g.
in [3-5].
The use of magnetic controllers on heat treating induction coils can provide accurate control heat pattern,
improvement of the coil efficiency and power factor, better utilization of power transferred to the part in local
heating processes. It can also result in reduction of the coil current demand thus improving performance of the
whole induction system and protect machine or the part components from unintended heating
Technical and economic effects of magnetic flux controllers are the following: better part quality, higher production
rate or energy savings, reduction of required power of the heating equipment.
The most effective design method of the induction coil with magnetic controllers is to use computer simulation [3].
In this way both the coil copper and concentrator may be optimized for the best performance in a particular
application.
It is important to state that the controller design, selection of material and application technique can strongly
influence performance and lifetime of heavy loaded induction coils. The goal of this paper is to inform the induction
community about the latest improvement in development and application of magnetic controllers.

2. Materials for Magnetic Controllers on Heat Treating Inductors
There
are
three
groups
of
materials
that
can
be
used
for
magnetic
flux
controllers:
laminations,
ferrites
and
Soft
Magnetic
Composites
(SMC),
aka
MagnetoDielectrics
Materials
(MDM).
Laminations
are
thin
sheets
of
electrical
steel
with
thin
electrical
insulation
on
their
surface.
They
are
working
well
in
plane-­‐parallel
(2D)
magnetic
fields
at
frequencies
up
to
20
kHz,
sometimes
even
at
30
kHz.
Advantages
of
laminations
are:
very
high
permeability,
high
temperature
resistance,
high
thermal
conductivity
in
the
plane
of
sheets,
low
magnetic
losses
at
low
frequencies.
Lamination
drawbacks
are:
overheating
in
3D
magnetic
fields,
limited
frequency
range,
difficulty
in
machining
and
installation,
resulting
in
high
labor
costs
in
the
case
of
complex
coil
geometry.
Ferrites
are
glass-­‐like
materials
made
of
oxides
of
iron,
manganese,
zinc
and
other
elements.
In
spite
of
high
permeability
(in
weak
magnetic
fields
only!)
and
relatively
low
losses,
they
are
used
in
rare
cases
of
high
frequency
coils
of
small
sizes
due
to
the
following
drawbacks:
- They
are
very
hard
and
brittle
and
practically
non-­‐machinable
- Saturation
flux
density
is
low
(up
to
0.3-­‐0.4
T)
- Low
service
temperature
for
majority
of
types
due
to
low
Curie
point
- Low
thermal
conductivity
SMCs are made from ferrous particles (iron and its alloys), covered with very thin insulation layer, mixed with
organic or inorganic binder, pressed at high pressure (up to 720 MPa and even higher) and cured according to a
special technology. Majority of SMC used in induction industry have organic binders, which provides good
machinability. All pressed materials have certain anisotropy (up to 1.5-2 times in permeability depending upon
structure) but all of them work well in 3D fields. High frequency materials have low anisotropy. Possibility to work
in 3D fields and good machinability are highly valued by the coil manufacturers.
Different types of SMC cover the whole range of frequencies used in induction heating (50 Hz – 13.56 MHz).
Losses at low frequency are comparable to laminations and at high frequencies – to ferrites. Temperature resistance
is lower than for laminations but usually sufficient for induction applications. High thermal conductivity (up to 0.23
W/cmK, i.e. 35% higher than solid stainless steel material) and possibility of effective thermal management using
external or internal cooling can keep controllers safe in heavy loaded cases. The main drawbacks of SMC are limited
dimensions (up to 220 mm long plates at present time and higher price of material. However with account for labour
cost and possible improvement in coil life time, use of SMC in many cases occurs cheaper that laminations. It is
especially correct when using net-shape manufactured or machined controllers, fig.1. Technical and economic
analyses show that in some cases a combination of different materials give excellent results. For example,
laminations may be used for the regular part of controllers and SMC for areas with complex shape and 3D field,
such as the end zones of seam annealing coils.
Figure 1: SMC blocks painted for identification (left) and different machined magnetic controllers (right)

3. SMC is a class of materials that was significantly improved during the last decade. There are newer materials with
improved properties such as Fluxtrol 100, Fluxtrol LF designed for low frequency applications (shielding of melting
furnaces, low frequency heat treating, etc.) and formable materials of Alphaform type, which can be applied to
inductors of irregular shape manufactured with low tolerance. Several studies have been performed in order to
improve technique for application of magnetic controllers to the coils and to develop corresponding guidelines for
users.
New SMC Materials
Properties of new materials are presented in Table 1 in comparison with traditional material Fluxtrol A.
Table 1: Properties of Fluxtrol 100, Fluxtrol LF and Alphaform materials in comparison with Fluxtrol A
Fluxtrol 100
Fluxtrol 100 is a new material with different insulation and binder complex than traditional Fluxtrol materials
(Fluxtrol A, 50 and Ferrotron 559 and 119). It is designed for use in a wide range of frequencies up to 50 kHz
instead of Fluxtrol A. Material has lower anisotropy than Fluxtrol A and better mechanical properties, which allows
the users to machine parts with sharp corners and thin walls. Magnetic properties of Fluxtrol 100 and A are very
similar in favorable direction perpendicular to direction of pressing, fig.2. Permeability of Fluxtrol 100 in direction
of pressing is much higher than of Fluxtrol A and it does not require the user to care about material orientation when
designing the controllers. Thermal conductivity of Fluxtrol 100 is also higher and it allows us reduce the rated
temperature of material to 200 C. However material can work for a long time at temperature 250 C with the same
magnetic properties and reduced electric resistivity.
Figure 2: Magnetic permeabilities of Fluxtrol A and 100 in two directions
Alphaform materials

4. These materials are manufactured from magnetic particles of different dimensions for “lower” (LF), middle (MF)
and high frequencies (HF) mixed with a special epoxy compound. Alphaform material is supplied in tins, which is
advised to keep refrigerated for longer life time, fig. 3, left. Materials may be manually formed/shaped when warm.
After that the coil must be heated for curing. During heating the material passes through the transient stage when it
becomes relatively thin to flow out and special coating or wrapping is necessary.
Figure 3: Tins with Alphaform materials (left) and ID induction coil with magnetic core (right)
Alphaform materials may be effectively used on ID induction coils and wrapped tubing coils of complex or
irregular geometries due to its flexibility during application. Material sticks to copper tubing resulting in good
mechanical integrity and very good thermal contact even for non-machined coils with significant tolerances, fig.3,
right. Due to the ease of installation (and removal when needed) this SMC are also great for lab and development
projects where immediate results are needed.
SMC Controllers on Crankshaft Hardening Coils
Over the last 5 years a big progress has been made in use of Soft Magnetic Composites in the Elotherm (rotational)
style and clamshell (non-rotational) style crankshaft induction hardening coils. In rotational style inductors
laminations have been the norm for decades, but SMC have proven to be more cost effective in coil assembly
techniques and overall coil performance, including inductor lifetime. The ease of installation and modification make
for easier adjustments at setup. More complex coil designs can be achieved to deal with more challenging aspects of
crankshaft hardening such as fillets and undercuts due to the flexibility/machinability of SMC materials, fig.4.
Figure 4: Hardness pattern (left) and induction coil with Fluxtrol 100 concentrators, right
In clamshell or non-rotational inductors the application of SMC controllers provides excellent heat pattern control in
journal circumference and width while reducing the required amount of power needed to achieve pattern
specifications. Along with improved heat pattern uniformity we now have industry feedback confirming increased
coil life due to the application of side shields in these types of inductors. As these types of applications grow, more
and more data is being gathered for analysis and comparisons to older styles of crankshaft magnetic controllers.

5. Recommended Application Techniques for Soft Magnetic Composites
Fluxtrol Inc. is constantly pursuing the best ways to not only adhere our material to inductors, but to make it easier
for our customers to access this technology and apply it themselves with ease and confidence. This way insuring our
material is performing at its peak, is structurally sound and being cooled to the best of its applications ability. All of
which leads to the best performing induction systems available. Many factors come into play when attaching
Fluxtrol material to an inductor. First and foremost is the use of the proper grade of Fluxtrol for your application
Conclusions
Fluxtrol, Inc. continues to improve existing and introduce new composite materials to meet industry demands.
Magnetic flux controllers can improve heat pattern, prevent unintended heating of the part or hardening machine,
improve induction coil parameters and performance of the whole induction installation. As a result, proper
application of magnetic flux controllers can strongly improve heat pattern control, increase production rates, save
energy and cut manufacturing costs.
Soft Magnetic Composites manufactured by Fluxtrol Inc. are the primary choice for magnetic controllers. They
cover the whole range of frequencies used for induction heat treating (from line frequency up to several megahertz),
may be easily machined to any desirable shape and used as constructive elements of the coil. Magnetic permeability
of these SMC reaches 120, which is sufficient for almost all induction heating applications.
The most effective way to design induction coils with magnetic controllers is to use computer simulation, which can
predict the coil performance prior to its manufacturing.

6. References
[1] Nemkov, V., “Magnetic Flux Control in Induction Installations,” Proc. of Int. Symposium HES-13 “Heating by
Electromagnetic Sources”, Padua, Italy, May 2013
[2] Website www.fluxtrol.com
[3] Goldstein, R. et al., “Virtual Prototyping of Induction Heat Treating”, Proc. of the 25th Conf. ASM Heat
Treating Society, Indianapolis, September 2009
[4] Nemkov, V., Goldstein, R., “Design Principles for Induction Heating and Hardening”, in Handbook of
Metallurgical Process Design. Chapter 15. Marcel Dekker; New York, NY-USA. 2004; pp. 591–640
[5] Nemkov V., Goldstein R., Ruffini R., “Optimal Design of Induction Coils with Magnetic Flux Controllers,” ”
Proc. of Int. Symposium HES-07 “Heating by Electromagnetic Sources”, Padua, Italy, 2007
[6] Ruffini, R., Nemkov, V., Vyshinskaya, N., “New Magnetodielectric Materials for Magnetic Flux Control.
”Proc. of Int. Symposium HES-04, “Heating by Electromagnetic Sources”, Padua, Italy, June 2004
[7] Nemkov, V., Goldstein, R., “Optimal Design of Internal Induction Coils,” Proc. of Int. Symposium HES-04
“Heating by Electromagnetic Sources”, Padua, Italy, 2004
[8] Myers, C. et al., “Optimizing Performance of Crankshaft Hardening Inductors,” Industrial Heating, December,
2006