design and fabrication of inductors for induction heat treating

Design and Fabrication of Inductors forInduction Heat TreatingRob Goldstein, FluxtrolWilliam Stuehr, Induction ToolingMicah Black, Tucker Induction Systems

FOR INDUCTION MELTING AND MASSHEATING, the early induction heating coilswere manufactured from copper tubing wrappedin multiple turns around a mandrel. The firstinduction heat treating coils were developed forcrankshaft hardening in the 1930s (Fig. 1, 2)(Ref 1–4). Unlike the melting and mass heatingcoils, the heat treating induction coils weremachined. These inductors consisted of twoparts with a hinge on one side that would openand shut around the crankshaft journal. Quenchholes were drilled on the inner diameter of theinduction coil to deliver quench to the part afterheating. This pioneering development was theculmination of many years of hard work by alarge team and clearly demonstrated the differentrequirements for induction heat treating as com-pared to melting and mass heating.During the infancy of induction heat treating

coil development, the specific features of the in-duction coil primarily were determined throughan analytical-, experience-, and experimental-

based method. Concepts for induction coils werecreated by physicists. Calculations of coil para-meters were determined by manually solvingcomplex equations. Induction coil designs initi-ally were made by draftsmen on blueprints.Most inductors were manufactured by crafts-men using a combination of copper tubing,manually machined components, and copperplates brazed together. Coil optimization wasmade based on experimental results throughthe trial-and-error method, because tests andmodifications took much less time than thecalculations did (Ref 1, 5).While many induction heat treating coils are

still manufactured in this way, the tools forthe design and fabrication of complex inductionheat treating inductors have evolved greatlyover the years. For many types of applications,the preferred induction coil style already isknown based on years of empirical data. Inmany cases, comparisons to known results caneven determine induction coil dimensions thatwill lead to a successful heat treating pattern,greatly limiting experimental testing times andshortening development cycles (Ref 1, 5–9).For newer applications or optimization of

existing ones, computer modeling tools are beingincorporated into the induction coil designprocedure. Sophisticated software running on

personal computers is capable of making cal-culations in seconds, minutes, or hours thatwould have taken days, weeks, or months tosolve manually. In many cases, no analyticalformula exists for complex coils, and the onlyway to make calculations prior to tests is witha computer modeling program. Induction coildesigns and process recipes are “virtually”tested to determine resulting heat patterns. Inmany cases today (2013), virtual tests and eva-luations can be made more quickly and at alower cost than physical ones (Ref 10–12).The process of mechanical induction coil

design also has evolved over the years. Whilethe initial drawings weremade by hand, inductioncoils now are drawn and detailed using computer-aided design (CAD) packages. Over the past sev-eral years, CAD packages have increased theirinteractivity with computer-aided manufacturing(CAM) software packages. In many instances,induction coil drawings can be transferred froma CAD drawing to a CAM program into the com-puter numerically controlled machine. This cre-ates the possibility for greater repeatability ofcomplex machined induction coils (Ref 13, 14).

Methods of Induction Heat Treating

Today (2014), there are many different typesof induction heat treating coils. The inductionheat treating coil style depends on the inductionheat treating process. All induction heat treatingprocesses can be classified in two categories:single shot and scanning, based on whether theinduction coil is moving (excluding rotation)relative to the part during the heating process.A brief description is given subsequently, becausemore in-depth information on induction heattreating equipment is discussed in other sectionsin this article.In single-shot (Fig. 3) induction heat treating

applications, the position of the inductioncoil relative to the heated length of the partdoes not move. In many single-shot heating

ASM Handbook, Volume 4C, Induction Heating and Heat TreatmentV. Rudnev and G.E. Totten, editors

Copyright # 2014 ASM InternationalW

All rights reservedwww.asminternational.org

Fig. 1 Tocco coil. Source: Ref 1 Fig. 2 Vologdin coil

applications, the part rotates while heating andquenching occur to ensure uniformity of pattern.Within the family of single-shot heat treatingapplications, there are several varieties of in-duction coil configurations, depending on thequench-delivery mechanism.Machined integral quench (MIQ) installations

have quench integrated right into the inductioncoil itself. The most common method is for thequench to be delivered through the coil copperitself. These inductors typically have separatepockets for water cooling and quench delivery.In some other cases, the quench may be deliv-ered through a magnetic flux concentrator.Another type of single-shot heat treating instal-

lation is quenching in place (Fig. 4). Quenching inplace is similar toMIQ, because the quench ring isa part of the induction coil assembly. What differ-entiates quenching in place from MIQ installa-tions is that the quench is not designed to be anactive component of the induction circuit.The final single-shot induction heat treating

coil style is separate quenching (Fig. 5). Inthese installations, the quench is not part ofthe induction coil assembly. In separate-quenchinstallations, the quenching often is delivered ina different station or position in the machine.These systems are common when a delaybetween heating and quenching for heat soak-ing is desirable.

Besides single shot, the other type of induc-tion machine is scanning (Fig. 6) (Ref 5, 9). Inscan-hardening applications, the induction coilmoves relative to the part. Similar to single-shotinductors, scanning inductors can be either MIQor quenching in place. They also may be sepa-rate quenching, but this is less common.

Considerations for Inductor Design

Induction heat treating coils are available inmany shapes and sizes and must perform a vari-ety of tasks in a given induction heat treatingapplication. Depending on the application, theinduction coil design requirements include:

� Meet heat treatment specifications in desiredproduction rates

� Be robust enough to tolerate manufacturingvariations

� Mount into the induction machine� Have electrical parameters that match the

induction power supply� Deliver quench� Have a satisfactory lifetime� Have satisfactory efficiency� Be repeatable from inductor to inductor

In developing a new induction heat treatingcoil and process, the first question is whether

the component will be produced on an existingsystem or if a new machine must be built. Inmany cases, the part producer’s desire is todevelop new tooling for an existing machinewith spare capacity. This reduces the degreeof freedom and can make the induction coildesign procedure more complicated, becausea less-than-optimal frequency or coil style willbe necessitated to fit the existing machine(Ref 16).To determine the ability to use existing

equipment, it is necessary to make an analysisof the part to be heat treated. Part material,prior processing, geometry, production rate,and heat treatment specifications all play roles.The part material and prior processing deter-mine what the minimum heat treatment temper-ature should be, along with how much time isallowed for cooling. The part geometry andheat treatment specifications indicate howmuch energy is required, what the preferredfrequency ranges are, and what type of induc-tion method (i.e., single shot, scanning) is bestsuited for the application. Finally, the produc-tion rate determines how much power and/orhow many spindles or stations are required.More details on this topic are given later in

this and other articles in this Volume. The dis-cussion in this article is limited to the relation-ship between these factors and the inductioncoil design.

Current Flow in the Part

Eddy currents are the primary source ofpower dissipation in most induction heat treat-ing applications. Eddy currents, just like allother electrical currents, must form a closedcontour. In most cases, the current flow in theworkpiece follows the shape of the inductionheating coil, due to the proximity effect. Thepower density in a given section of the work-piece depends on the current density. Thecurrent density can be influenced by electro-magnetic effects (end effect, edge effect, etc.),the presence of magnetic flux concentrators,the width of the copper, the geometry of thepart, and the distance between the coil and theworkpiece (coupling gap).The next step in the coil design process is

determining how the current will flow in thepart. This is critical, especially in cases wherethe geometry changes. Some common geometrychanges encountered in induction heat treatingare fillets, undercuts, corners, shoulders, cham-fers, splines, keyways, and oil holes. The heat-ing of these critical areas relative to adjacentones is strongly dependent on the induction coilstyle and frequency of heating.The first primary choices for current flow,

either in the plane of the geometry change orperpendicular to the geometry change, are dis-cussed here. When current flow is perpendicu-lar to the direction of a geometry change, thenatural tendency is for the current to concen-trate on the part surface that is closer to the

Fig. 3 Single-shot induction heat treating application

Fig. 4 Quench-in-place inductor forhardeningof a spindleFig. 6 Machined integral quench scanning inductor

with magnetic flux concentrator

Fig. 5 Separate quenching. Source: Ref 15

590 / Equipment

induction coil, due to the proximity effect. Anadditional consideration is that near the ends ofthe closer section to where the distance increases,there is either a decrease or an increase in heat-ing, depending on whether the part is magneticor nonmagnetic due to the electromagnetic endeffect. For hardening applications, there is someincrease, and for tempering there is a decrease.The magnitude of the change depends on thefrequency (Fig. 7) (Ref 12).When the current flow is in the direction of a

geometry change, the current flows under theinductor in a width approximating that of theinduction coil. When the dimension changeoccurs, the current flow follows the contour ofthe part through this dimensional change. Atthe point of the dimension change, the changein heating is governed by the electromagneticedge effect. The edge effect tends to be smallerthan the end effect, hence meaning a smallertemperature differential in this area. As the dis-tance between the coil and part increases, theamount of current remains nearly the same,

but the current begins to flow over a widerlength. The less concentrated heating leads tolower power densities (Fig. 8). The differencebetween the power densities tends to be smallerthan when current flow is in the direction of thegeometry change than when it is perpendicular.To illustrate the concept, consider the case of

a simple spindle with one bearing race and ashoulder (Fig. 9). The heat treating pattern isthe light-gray area. This part could be heatedeither by scanning or single shot. The scanninginductor almost certainly would be an encir-cling inductor with a length less than the pat-tern length. Within single shot, it also couldbe hardened by a machined encircling inductoror a so-called encircling/nonencircling inductor.For an encircling inductor (scanning or single

shot), the current flow in the coil and part tendto take the shortest path and flow along theinductor inside diameter and bearing surfacediameter. To compensate for this tendency,it is necessary to vary the gap between thecoil and part to use the proximity effect to

compensate for the shorter length for currentflow. In the area of the radius, there is a transi-tion from the smaller gaps on the flange to thelarger gaps on the diameter. For a single-turnmachined inductor, the coil design in this areainvolves a delicate balance to achieve sufficientdepth in the fillet without heating too deeply inthe bearing area just above the fillet. For casedepths that are large relative to the cross sec-tion, this becomes more difficult because coretemperatures increase, resulting in reduced con-ductive heat extraction. For a shorter scanningcoil, this task is made easier by using a mag-netic flux concentrator to drive current downfrom the shaft and into the radius of the part(Fig. 10).Encircling/nonencircling inductors consist of

multiple partial loops connected by copper railsthat are contoured to the part surface (Fig. 11).For these inductors, the part must rotate toensure even heating. For a simple part such asthis, likely only one top and one bottom partialloop would be required. In this case, currentflows under the coil turns and follows the con-tour of the coil in the part. In this way, the topand bottom of the pattern are controlled by theend loops, and the rails determine the pattern inthe central area. By following the rails along thepart surface, the current flows through the radius.

Fig. 7 End-effect drawing

Fig. 8 Edge-effect drawing

Fig. 9 Heat pattern for spindle

Fig. 10 Encircling drawing for spindle

Design and Fabrication of Inductors for Induction Heat Treating / 591

This makes achieving a more uniform contoureasier with a linear inductor than with an encir-cling one for this type of part (Fig. 12).

The Influence of Frequency

The frequency of the induction heating cur-rent has a strong influence on the induction coiland process design. It affects the powerrequired, heating time, coil losses, local distri-bution of power in the part, and the structuralsupport required. A brief overview of the effectof frequency is given in this section, because itis covered in more depth in other articles in thisVolume.Often, the same case depth can be achieved

over a wide range of frequencies. For largeparts in either the completely magnetic (coldsteel) or nonmagnetic (hot steel) state, currentdensity is highest at the surface and falls off

exponentially in depth. There is a value usedby induction heating practitioners called thereference or skin depth related to this behavior.The formula for reference depth is:

d ¼ k

ffiffiffiffiffi

rmf

r

(Eq 1)

where d is the reference depth, k is a constantdepending on units, r is the electrical resis-tivity, m is the magnetic permeability, and f isfrequency.In large bodies, 63% of the current and 86%

of the power reside in the first reference depth(Fig. 13) (Ref 6, 17). This leads to the assump-tion commonly made in estimating parametersthat all of the power is generated in the d layer.In induction hardening applications, the

distribution of current and power density is dif-ferent than that predicted by the referencedepth, because the magnetic steel below the

Curie point still interacts with the magneticfield behind the nonmagnetic hot surface layer(Ref 17). To achieve the same case depth onparts with lower frequencies, one can expecthigher power levels, shorter heating times, andlower surface temperatures. As the frequencyis increased, power levels must be decreasedand heating times increased to achieve the samecase depth without overheating the surface. Theamount of energy required depends on the bal-ance between thermal losses (heat soaking), coilefficiency, and power induced in the subcase-depth material.Besides the process, the frequency also influ-

ences how the current is flowing in the induc-tion coil itself. In practice, the most effectivethickness of the conductive part of the tubingwall (d1) is assumed to be d1 ffi 1.6d1. Thisratio is based on solution of a one-dimensionalproblem. An induction coil tubing wall smallerthan 1.6d1 results in reduction in coil effi-ciency; furthermore, in some cases the tubingwall can be much thicker compared to theaforementioned ratio. This is because it maynot be mechanically practical to use a tubingwall thickness of 0.25 mm (0.01 in.). Someguidelines for wall thickness are shown inTable 1 (Ref 9).The frequency also affects the electrody-

namic forces, which influence the mechanicaldesign of the induction coil. Electrodynamicforces in induction heating have two compo-nents: static and dynamic. The static componentinitially pulls the inductor in toward the part inthe cold magnetic state and then pushes it awayfrom the part when it is nonmagnetic. Thedynamic component oscillates around the staticcomponent at double the electric frequency(Fig. 14). The level of force in the system isproportional to the square of the coil current;that is, the forces are directly proportional tothe power (Ref 17).When the part is electrically thick (i.e., the

diameter or thickness is much greater thanthe reference depth), the current required forthe same power is proportional to the inversefourth root of the ratio of frequencies (Eq 2).From this equation, it can be seen that the cur-rent required for the same amount of powerdecreases with increasing frequency:

I2 ¼ I1

ffiffiffiffi

f1f2

4

s

(Eq 2)

Fig. 11 Encircling/nonencircling drawing for spindle

Fig. 12 Linear/single-shot drawing for spindle

Fig. 13 Distribution of current (S) and power density(Pv) in the depth of a large workpiece.Source: Ref 17

Table 1 Standard wall thickness of coppertubing

Copper wall thickness

Frequency, kHzmm in.

0.75–1 0.032–0.048 450–501–2 0.048–0.090 25–8.31.5–4 0.062–0.156 10–34–6.5 0.156–0.250 3–1

Source: Ref 7

592 / Equipment

Due to this, the electrodynamic forces arehigher at lower frequencies. Additionally,power levels generally tend to be higher atlower frequencies, increasing the electrody-namic forces even more. Therefore, the lowerthe frequency, the more robust the mechanicaldesign of the induction coil should be. In addi-tion to heavier copper wall thicknesses, the coilsupporting structure also must be stronger towithstand the mechanical loading to whichthe coil is exposed. This is especially true onsingle-shot coils.

Control of Heating in DifferentAreas of the Part

In many induction heat treating applications,there is a requirement to selectively hardensome areas of the workpiece and not hardenothers. In other instances, different case depthsare desired in different areas of the part. Often,it is necessary to achieve these various targetsin the same induction heat treating operation.For scan hardening applications, these varia-

tions typically are compensated for by adjustingthe power and scan speed as the part traversesthrough the induction coil. For single-shotinduction coils, it is necessary to accommodatethese changes in the coil design. There are threemain tools for adjustment of the coil design:coupling gap, coil copper profile, and magneticflux controllers.The coupling gap is the most straightforward

way to control the heat pattern. The closer thecoil is to the part, the greater the intensity of heat-ing in this area relative to other areas. This isbecause the current in the part flows in a nar-rower band more closely approximating that inthe induction heating coil. With increasing cou-pling gap, the current spreads out over a longerlength, resulting in lower current density andpower density.Another method to control the heat pattern is

to change the cross-sectional profile of the cop-per coil. The section(s) of the coil turn thatfaces the part and carries the majority of the

current is referred to as the heat face of thecoil. For induction coils with a single heatface, power profiling is achieved by varyingthe geometry of the heat face (Fig. 15). Forinstance, to shorten the transition zones at theends of the coil, it is common to have a largergap in the middle of the coil than on the ends.This method is commonly used for controllingthe end of a heat treating pattern where it is de-sirable to keep the pattern out of an undercut ora snap ring groove. This helps to compensatefor the divergence of magnetic flux and theadditional thermal losses near the end of thecoil.An example of this is hardening the bearing

of a crankshaft using an encircling coil (clam-shell or sharp-C technology, Ref 9). Using theFlux 2D computer simulation program, it ispossible to make a comparison to see the effectof the copper profiling on the temperature dis-tribution. Figure 16 shows a simplified exampleof a coil for this technology. The lines on thepart represent the minimum and maximum casedepth. The frequency for this comparison is50 kHz. The coupling gap is 2 mm (0.08 in.),and the coil has thin plates of Fluxtrol 50 con-centrator on both sides to prevent coupling withthe sidewalls. At 2.5 mm (0.10 in.) from eachend of the coil, the copper is recessed 1 mm(0.04 in.). This coil is compared to a coil with

the exact same geometry, except the centralsection of the inductor is not recessed. Theheating time for both cases is 4 s.The temperature distribution in the coil

without profiling is shown in Fig. 17. The areaof high temperature begins to taper off wellbefore the end of the heat face and the mini-mum required length of the pattern. The surfacetemperature already is high, so it is not possibleto increase heating to lengthen the pattern.Figure 18 shows the temperature distribution

for the coil with a profiled face. It is clear fromthis image that the area of high temperatureextends significantly longer than for the coilwithout profiling. The minimum heat treatmentwidth is met with a lower maximum surfacetemperature. For better comparison betweenthe two cases, Fig. 19 compares the surfacetemperature in length.For coils with multiple heat faces or sec-

tions, in addition to the coupling gap, the heatpattern also can be controlled by changing thewidth of the heat face from section to section.Power density in the part is proportional tothe current density squared. If it is assumedthat all of the current in the coil is transferredto the part and is flowing directly under theheat face of the induction coil, then the cur-rent density will be inversely proportionalto the width of the heat face. This means

Fig. 15 Drawing showing inductor heat face

Fig. 16 Nonrotational inductor cross section used for simulation

Fig. 14 Diagram showing relationship between current(i), static component of force (Fc), and dynamicforce (F ). Source: Ref 17


that power density will be proportional to theinverse square of the width of the heat face.The real effect is significantly less, becausenot all of the current is on the heat face, andsome of the flux outside the coil couples withthe part.For heating of a round part with a linear coil,

the effect is not as strong due to the nonuniformgap between the rectangular coil and the roundpart.To demonstrate the effect of the heat face

width, a comparison is made for heating a

12.7 mm (0.5 in.) thick plate with a 9.5 by12.7 mm (⅜ by 0.5 in.) tube and a 12.7 mm(0.5 in.) square copper tube. Computer simula-tion program Flux 2D is used for calculationof the electromagnetic problem. The top halfof the plate has the properties of steel abovethe Curie point, and the bottom half hasthe properties of cold steel. The frequency is10 kHz, and the coupling gap is set to 3.2 mm(⅛ in.). The same current, 3000 A, is used forboth coils. The magnetic field and power den-sity distributions for both cases are shown in

Fig. 20. Ideally, the power density would be78% higher for the 9.5 mm (⅜ in.) heat facecompared to the 12.7 mm (0.5 in.) one. In real-ity, this difference in maximum power densityis only approximately 17%. This is due to thelarge amount of current that is flowing in thepart outside the heat face of the coil.A more effective way to control the distribu-

tion of heat in the length of an inductor is to usemagnetic flux controllers. For induction heattreating applications, the most common materi-als used as magnetic flux controllers are siliconsteel laminations and soft magnetic compositematerials. Magnetic flux concentrators providea low reluctance path for the magnetic field toflow. The result is that the magnetic field goesfrom being widely distributed in space to beingconcentrated around the coil. This causes nearlyall of the current flowing on the sidewalls of thecopper coil to be pushed down to the heat face.This results in the current in the part being con-centrated more strongly underneath the heatface of the coil.To demonstrate the effect of the concentrator,

a comparison is made for heating a 12.7 mm(0.5 in.) thick plate with a 12.7 mm (0.5 in.)square copper tube, with and without a C-shapedconcentrator applied. The flux-concentratingmaterial used is Fluxtrol A. The other condi-tions are the same as in the case for the heatface variation study. The magnetic field andpower density distributions for the coil withconcentrator are shown in Fig. 21.The magnetic field and power density are

much more concentrated when the magneticflux controller is applied. The surface powerdensity curves from the left to the right sideof the part are shown in Fig. 22 for all threecases. It is clear that the application of a mag-netic flux controller is a stronger method ofcontrolling the heat pattern in the length of acoil than the use of copper profiling. It also ispossible to more smoothly distribute the patternbetween different areas by profiling the poles ofthe concentrator rather than having steps in theinduction coil. For linear coils heating a roundpart, the influence of both heat face width anda magnetic flux controller is less dramatic thanin the case of a flat plate, due to the divergenceof the part from the coil. More information onmagnetic flux controller selection and uses iscontained in the article “Magnetic Flux Con-trollers in Induction Heating and Melting” inthis Volume.

Presentation of the Part to theInductor

During the initial inductor design, a method ofholding the part and presenting it to the inductormust be addressed. Some considerations are:

� What is the part size and weight?� What is the process hardening or tempering?� Is it to be scanned or single shot?

Color Shade ResultsQuantity : Temperature Deg. Celsius

Time (s.) : 4 Phase (Deg): 0

Scale / Color

20.06464 / 86.92299

86.92299 / 153.78134

153.78134 / 220.63969

220.63969 / 287.49805

287.49805 / 354.35638

354.35638 / 421.21478

421.21478 / 488.07312

488.07312 / 554.93146

554.93146 / 621.78986

621.78986 / 688.64819

688.64819 / 755.50653

755.50653 / 822.36487

822.36487 / 889.22321

889.22321 / 956.0816

956.0816 / 1.02294E3

1.02294E3 / 1.0898E3

Fig. 17 Temperature distribution with a nonprofiled heat face

Color Shade Results

Quantity : Temperature Deg. Celsius


Scale / Color

20.08923 / 83.79922

83.79922 / 147.50919

147.50919 / 211.21918

211.21918 / 274.92917

274.92917 / 338.63913

338.63913 / 402.34912

402.34912 / 466.05911

466.05911 / 529.7691

529.7691 / 593.47913

593.47913 / 657.18909

657.18909 / 720.89905

720.89905 / 784.60907

784.60907 / 848.31903

848.31903 / 912.02905

912.02905 / 975.73901

975.73901 / 1.03945E3

Fig. 18 Temperature distribution with a profiled heat face

594 / Equipment

� Is the part to be rotated during processing?� What is the rate of production?� What type of materials handling equipment

is needed?� How and where on the part can it be held?� What is the kilowatt requirement and power

supply frequency?� How is it to be loaded into the inductor:

manually or mechanically?� What is the quench method: spray, dunk,

agitated bath, still air, or air blast?� What types of holding devices are to be used:

centers, chuck, or special grippers?

Depending on the process, there could beseveral mechanical methods to grip and presenta part to the inductor. In high-production appli-cations, an entire machine may be designed andbuilt for a specific materials handling process.In limited-production runs, however, a simplelift-and-rotate device within a quench tank maybe employed. Either way, a decision must bemade on how to hold the part.Part holding and presentation can be as easy

as chucking the part or as complicated as hav-ing to hold and rotate the part while it is heatedentirely. In any scenario, the electromagnetic

fields from the induction coil should be consid-ered in the design of the holding device and itseffect on the part. Magnetic steels in close pro-ximity to the inductor will heat; therefore, suffi-cient air space around the inductor is prudent.Common nonmagnetic part-holding materials,such as 300-series stainless steels, aluminum,and brass, can be heated when in close prox-imity to the inductor, but with a less significanteffect. When designed specific to an applica-tion, certain ceramics also make excellent part-holding devices.For example, typical to induction hardening

applications is the lift-and-rotate method withina quench tank. A part of reasonable size andshape, such as an automotive wheel spindle, ismounted onto a part holder made from nonmag-netic stainless steel. An upper center of thesame material is used to stabilize the part dur-ing rotation. An inductor designed to heat treatthe wheel spindle is installed onto the powersupply mounting foot within the tank. A sprayquench is integral to the inductor. The wheelspindle is loaded in the holder and raised tothe inductor. Rotation starts, the inductionpower is applied, the wheel spindle is heatedand then immediately quenched, and the spin-dle is lowered and unloaded. This entire processtakes less than 30 s. In a high-production appli-cation, throughput would be multiplied withadditional heating stations employing variousmaterials handling systems.While many examples of heat treating are

typical, when parts are large and/or geometri-cally difficult, the presentation to the inductormay require engineering. For many induction-hardened parts, rotation during heating equalizesthe heat treating pattern. Some parts, suchas truck steering arms in the shape of the letter“J,” cannot be rotated 360� due to the J-hookFig. 19 Comparison of temperature distributions with and without profiling for nonrotational heating of a crankshaft

Color Shade Results

(a) (b)

Quantity : Power density W/(cubic m)

Scale / Color0 / 35.01103E6

Phase (Deg) 0

35.01103E6 / 70.02206E670.02206E6 / 105.03309E6105.03309E6 / 140.04411E6140.04411E6 / 175.05515E6175.05515E6 / 210.06618E6210.06618E6 / 245.0772E6245.0772E6 / 280.08822E6280.08822E6 / 315.09923E6315.09923E6 / 350.11027E6350.11027E6 / 385.12131E6385.12131E6 / 420.13232E6420.13232E6 / 455.14336E6455.14336E6 / 490.15437E6490.15437E6 / 525.16541E6525.16541E6 / 560.17645E6

Color Shade ResultsQuantity : Power density W/(cubic m)

Scale / Color0 / 29.92064E6

Phase (Deg) 0

29.92064E6 / 59.84129E659.84129E6 / 89.76194E689.76194E6 / 119.68258E6119.68258E6 / 149.60323E6149.60323E6 / 139.52387E6139.52387E6 / 209.44451E6209.44451E6 / 239.36515E6239.36515E6 / 269.28579E6269.28579E6 / 299.20643E6299.20643E6 / 329.1271E6329.1271E6 / 359.04774E6359.04774E6 / 388.96830E6388.96830E6 / 418.88902E6418.88902E6 / 448.80966E6448.80966E6 / 478.7303E6

10 / 865.19256E-6

11 / 970.83917E-6

12 / 1.07649E-3

8 / 03383339E-6

9 / 759.54595E-6

Fig. 20 Comparison of (a) 9.5 mm (⅜ in.) and (b) 12.7 mm (0.5 in.) heat faces for heating of a flat plate


interfering with the inductor; yet, to equalize thehardened pattern, some form of rotation is neededto equally heat the bearing journal on the end ofthe J-hook. To accomplish this, a part holder isconnected to a crank and made to oscillate

through 210� during the hardening cycle, closelymimicking full rotation.Large, heavy parts may be application spe-

cific as to their presentation to the inductor,requiring thoughtful design, materials selection,

and testing to verify the heating results. In someinstallations, it is necessary to present the induc-tor to the part. Complex designs involve robotsmoving the inductor along the path to be hard-ened. The inductor and transformer sometimesare robotically moved together, while othertimes, depending on the power and frequency,a flexible cable connected to the inductor maybe employed. Other heating applications mayuse a handheld induction heating apparatus.All applications are dependent on the powerrequired, the size of the part, and the areas tobe heated.

Inductor Structure

For low-frequency and/or high-power-den-sity induction heat treating applications, theelectrodynamic forces are strong and the induc-tion coil requires a supplemental mechanicalsupport structure. A mechanical support struc-ture also is required if the induction coil isheavy or manufactured from thin-walled cop-per tubing. Some common types of inductorswhere mechanical supports are used includesingle-shot inductors for heating shafts orstems, multiturn induction coils, inductioncoils without rotational symmetry, and applica-tions where the inductor is heating a surfacethat is perpendicular to the buss mountingplates.For induction heat treating coils, the

mechanical support components include studboards, retainer rings, mounting plates, andcomponents to connect the coil to quench rings.In the vast majority of cases, the materials usedfor the inductor structure are made from high-temperature fiber-reinforced plastic compo-nents. In some extreme cases, such as long sin-gle-shot coils for axle shaft hardening, a fiber-reinforced stud board may be additionally sup-ported by an aluminum housing or channel. Inthis case, care should be taken to insulate theinductor and any studs from the conductivesupport housing.One of the more challenging induction appli-

cations that requires a strong mechanical sup-port system is rotational crankshaft hardeningwith U-shaped inductors (Fig. 23). The distancefrom the output of the transformer to the heat-ing area of the coil must be relatively long toensure that the counterweights of the crankshaftwill not hit the transformer. This is a challenge,because in this style of coil the electrodynamicforces are high and act in multiple directions.There also is a mechanical load placed on theinduction coil, because the inductor assemblyis in contact with the part during rotation.When the crankshaft is cold, the turns are

pulled toward the bearing journal and the side-walls of the crankshaft. Once the part is abovethe Curie point, the forces are reversed, andthe inductor is pushed away from the sidewallsand away from the bearing diameter.Due to these multiple challenges, the struc-

ture consists of several parts. Outside the

Color Shade Results

Quantity : Power density W/(cubic m)

Phase (Deg): 0

Scale/Color

0 / 115.86056E6

115.86056E6 / 231.72114E6

231.72114E6 / 347.5817E6

347.5817E6 / 463.44224E6

463.44224E6 / 579.30285E6

579.30285E6 / 695.16339E6

695.16339E6 / 811.02394E6

811.02394E6 / 926.88448E6

926.88448E6 / 1.04275E9

1.04275E9 / 1.15861E9

1.15861E9 / 1.27447E9

1.27447E9 / 1.39033E9

1.39033E9 / 1.50619E9

1.50619E9 / 1.62205E9

1.62205E9 / 1.73791E9

1.73791E9 / 1.85377E9

Fig. 21 Magnetic field and power density distribution for a 12.7 mm (0.5 in.) heat face with Fluxtrol A for heating ofa flat plate

Fig. 22 Surface power density curves for all three cases of flat plate heating

596 / Equipment

inductor there typically are two brass or alumi-num side plates. All of the components areanchored to the side plates, including the induc-tion coil. To prevent contact of the inductorwith the part, ceramic or carbide guide shoesare placed in three places (just after the endsof the loops and between the two loops) aroundthe perimeter on both sides of the inductor. Theceramic guide shoes typically are mounted tobrass or aluminum side plates.In addition to the guide shoes, the side plates

also help to keep the induction coil in position.The inductor typically is held in at least threeplaces: the ends of each loop by a stud andG-11 component, and the leads area by a G-11component. Other components that may be apart of the structure in crankshaft coils includeleaders to facilitate the coil loading, quenchheads, and side support composites to increasethe rigidity of the assembly.

Coil Leads/Busswork and Contacts

With induction coils, there is an intermediatebody between the coil head (the part where theinduction heat treating process is conducted)and the heat station, power supply, or quick-change adapter. This section of the inductor isreferred to as the coil leads or busswork. Therole of the coil leads or busswork is to mechan-ically and electrically connect the induction coilhead to the power supply. Often, the coolingwater for the coil head also is supplied throughthe leads.Good design practice is that the leads should

be close together, with a piece of syntheticfluorine-containing resin between them toelectrically separate the two leads from oneanother. It also is good practice to make theheight of the busswork significantly longer thanthe length of the heat face of the induction coil.In most cases, the coil leads or busswork

must carry the same current that is in the induc-tion coil head itself. Because the coil leads/

busswork carry a high-frequency current, thecoil leads generate heat from eddy currents.The current is drawn to the side of the bussbarthat faces the opposing bussbar by the proxim-ity effect. The current density in the leads isinversely proportional to the length of the inter-face, and hence, the losses are approximatelyproportional to the length squared. The proxim-ity of the two leads to each other primarily influ-ences the voltage drop in the leads. The closerthe leads are, the lower the inductance of theleads and hence the lower the voltage drop.The coil contacts range from simple flared or

compression fittings to machined blocks withkeys to ensure location, depending on the mat-ing surface. For heat treating environments,the machined blocks with keys are preferredfor high-power-density and low- or medium-frequency environments. Compression or flaredfittings should be used only in the case of high-frequency, lower-power-density applications orlong solenoids with high turn numbers andlow currents.For the machined block contacts, these sys-

tems can be separated into two classes: standardbolting connections and quick-change adapters.For standard bolting connections, there are twomain contact styles: fishtails or Jackson type,and flat plate. Fishtails consist of a slotted cop-per bar for adjustment of the vertical height,with a female key on the back to fix the hori-zontal position (Fig. 24a). This type of contactis common for the frequency range of 1 to 30 kHzin heat treating applications where the coil ismounted to a transformer. For this style, watertypically is fed from separate water lines to theinduction coil rather than through the heat stationelectrical contacts, because the vertical height ofthe coil is not always in the same place.Flat plate contacts typically consist of a flat

plate with two holes for bolts and a hole betweenthe two bolts for water (Fig. 24b). The water-cooling hole commonly is slightly recessedfor placement of an O-ring. Flat plate contactstypically are used for mounting to power supply

or heat station outputs for higher frequencies(50 to 450 kHz).Besides bolting contacts, there are several

types of quick-change adapters used in industry(Fig. 25). All of these systems involve the useof mechanical keys and a lever to press the con-tacts flush and fix the position of the coil in aquick motion. Coil water cooling may or maynot be incorporated into the quick-changeadapter. Separate water cooling typically isprovided to the induction coil in the case ofhigh power densities or where the quick-changeadapter allows for a variable vertical position.

Quenching Considerations inInduction Coil Design

In many cases, quenching is done to finalizethe hardening process of steel components,and it typically is done with water that has asmall percentage of a quench medium. This isthe second half of heat treating; this stage is justas important as heating. After the part surface israised above the austenitizing temperature, itshould be quenched (or rapidly cooled). In mostcases, this should be done immediately after theaustenitizing temperature is reached. The timebefore quenching can vary depending on thecomposition of the steel. The quench designmust not only remove heat rapidly but uni-formly as well. The results of nonuniform orslow quench are nonuniform hardness andpossible overall distortion in the part (or work-piece). Nonuniform hardness can lead to pre-mature part failure or cracking. Because of theimportance of uniformity when quenching,round or symmetrical parts almost always are

Fig. 23 U-shaped crankshaft hardening coil components and assemblyFig. 24 (a) Fishtail-type and (b) flat plate machined

coil contact types


rotated during the quenching process. Otherimportant considerations for quench design arethe part geometry and how the part is presentedto the quench (i.e., horizontal, vertical, etc.). Insome irregular-shaped parts, rotation is not pos-sible, or the rotation rate is slowed to help con-trol the quench. When quenching for surfacehardness, a vigorous or intensive quench usu-ally is required. Because of this, the quenchusually is done with a series of relatively smallholes. These holes usually have a staggered pat-tern. The small holes and staggered patternresult in an intense quenching that helps toavoid the development of steam pockets. If asubmerge (or dunk) quench is used, a fine layerof steam usually is developed on the surface;this steam can act as a barrier or insulator, caus-ing a slower quench. The size of the individualquench holes can vary from 1.5 to 5 mm (1=16 to3=16 in.) diameter, depending on the part sizeand geometry. It usually is desirable to keepthese holes as small as possible to keep thequenching pressure higher, which helps reducethe steam pocket described earlier. This alsohelps to lock in the hardness by fully transform-ing the martensite. One disadvantage to smallquench holes is that they are more likely to fillwith debris or quench residue; for this reason,many quench designs for heat treating havequench holes between 2.5 and 3.2 mm (3=32and ⅛ in.) diameter.

Figure 26 shows a typical scanning quenchhole arrangement. The hole size is related to theshaft or workpiece diameter, as well as a quench-ing area, and the air gap between the quenchblock and the workpiece surface (Table 2)(Ref 9).The quench design, like the inductor design,

should conform around the part. In some cases,the quench can be incorporated into the induc-tor design; in other cases, it is in a separatequench ring or quench block. This is furtherdiscussed in the section “Styles of Heat Treat-ing Inductors” in this article. With regard tothe duration of quenching time, quenching typ-ically takes longer than heating; a rule of thumbis that the quench should be 1.5 the time ofthe heating. With static heating or single-shotheating, this can be controlled by a timer, eitherin the heating station or moved to a quenchingstation. To save cycle time, some equipmenthas a short quench in the heating station fol-lowed by another station to finalize the quench-ing process. With regard to scan heating, thequench should be designed to have directimpingement for at least 1.5 the distance ofthe heating face of the inductor. When scan-ning, the quench holes are angled away fromthe coil in the same direction that the part istraveling. This helps to keep the water fromwashing back into the inductor, and allows thewater to stay on the part as long as possible.

In either case (static heating or scanning), thequenching should be done before the surfaceof the part falls below the hardening tem-perature, to ensure that the material is fullytransformed.

Induction Coil Cooling

As discussed earlier, induction heating useshigh currents and high power levels. Becauseof this, most heat treating inductors are watercooled. Some inductors are constructed ofcopper tubing that the water flows through.Other types of inductors are constructed frommachined copper details, with cooling passagesintegrated within them. In either case, the pas-sages must be designed for the intended powerlevels and must have maximum water flow withminimal restrictions.Thermal degradation is caused by local or

total overheating of the coil head due to eddycurrent losses in the copper, magnetic lossesin the flux concentrator, and heat transfer fromthe hot surface of the part via convection andradiation. Overheating can result in coppercracking or deformation as well as concentratormaterial degradation. Copper cracking usuallyoccurs in heat treating coils with a short cycledue to thermomechanical stresses, while a grad-ual coil deformation is more typical for contin-uous processes (Ref 13, 14, 18, 19). Thermaleffects can strongly increase the effects of elec-tromagnetic forces and accelerate electrical insu-lation aging.Copper overheating is the leading cause of

failure in heavy-loaded heat treating inductors;this section focuses mainly on this mode of fail-ure. For consistently manufactured inductors,copper cracks occur in nearly the same placeon an inductor each time in a certain range ofparts produced. There are several approachesto increasing coil life: provide additional cooling,reduce the density of heat sources, or change thecoil design completely.Additional cooling can be obtained by

increasing the water flow rate, reducing the watercooling temperature, adjusting the water pocket,or introducing additional cooling circuits. Waterflow rate is the first step, and it is increaseduntil the pump output limit is reached. Inductormanufacturers generally have internal guidelinesrelated to best practices for water pocket andcooling circuit design, which have been devel-oped over the years (Table 1). These guidelinesgenerally are based on frequency and powerdensity. Once these standard options have beenexhausted, the next step is to replace the existingpump with a larger one or to introduce a boosterpump to the system.Sometimes on very high-power-density coils,

a limit is reached where, even with the bestcooling circuit design and very large pumps,coil lifetime is still unsatisfactory. At this point,it is necessary to try to minimize the localizedpower density in the weak point of the inductor.This often is challenging in complex inductors,

Fig. 25 Styles of quick-change adapters. Courtesy of Capital Induction

Fig. 26 Quench hole arrangement for scanning coil.Source: Ref 9

Table 2 As workpiece diameter increases,orifice size should increase to help providea more uniform quench

Shaft diameter Orifice size

mm in. mm in.

6.5–13 0.25–0.50 1–1.5 0.046–0.06313–38 0.50–1.50 1.5–2.5 0.063–0.094>38 >1.50 3.5–4 0.125–0.156

Source: Ref 9

598 / Equipment

because changing this section may have someeffect on the heat pattern in this area of the part,as well as in the rest of the part.To determine what high power density is, it is

important to consider the cause of the failure.The failure mode that can be resolved with watercooling in high-power induction heat treatingcoils is mechanical fatigue caused by thermalcycling. During each heat treating cycle, thecopper temperature rises as the power is turnedon, primarily due to current in the induction coil.As the copper temperature increases, the copperexpands. When the power is turned off, the cop-per rapidly cools and contracts.Because the eddy currents are distributed non-

uniformly in the copper cross section, the tem-perature distribution in the copper coil is alsononuniform. Water in the induction coil removesheat through forced convection. This means thatthe rate of heat removal is directly proportionalto the heat-transfer coefficient and the tempera-ture difference between the copper and the bulkwater. The heat-transfer coefficient depends onthe velocity of the water, the pressure of thewater (in addition to the effect of pressure onwater flow), and the temperature of the water.In recent years, computer simulation has pro-

ven an effective tool for estimating thermalcycles in induction coils during a heat treatingprocess (Ref 19–21). These recent studies weremade using some combination of electromag-netic, thermal, and fluid dynamics codes or cor-relations. With this method, the temperaturesthat are the source of the dimensional movementand stress are predicted. Certain guidelines formaximum allowable temperatures are used toensure good life. It is envisioned that in thefuture, these codes could be coupled with struc-tural, stress, and distortion simulation.

Case Study: Single-Shot Coil CopperTemperature Prediction

Acase study using computer simulation to showtypical temperature distributions in a single-shotinduction hardening coil is discussed in this sec-tion. Flux 2D is used for electromagnetic andthermal simulation, and analytical calculationsare made for the heat-transfer coefficients. Theinfluence of pressure (high and low), wall thick-ness (1, 1.5, and 3 mm, or 0.048, 0.062, and0.125 in.), current in the coil (5,000, 7,500, and10,000A), and frequency (1, 3, and 10 kHz) is con-sidered. The correlations for heat-transfer coeffi-cient hold only to approximately 250 �C (480 �F)on the interface of the copper wall with the coolingwater, so if this consideration is not met, the tem-perature in the coil will be less accurate.The geometry for the study is a 19 mm

(3/4 in.) square copper tube with Fluxtrol A mag-netic flux concentrator for heating of a 38 mm(1.5 in.) diameter shaft. The coupling gap usedfor the study is 3.2 mm (⅛ in.). To limit thenumber of variables, the part is not simulatedduring the study. The part is approximated by

a nonmagnetic layer 6.3 mm (¼ in.) thick, withthe properties of steel above the Curie point andthe 25 mm (1 in.) diameter core, which has theproperties of cold low-carbon steel.Figure 27 shows the temperature distribution

in the coil cross section if the current is 5000 Aat 10 kHzwith low pressure and a wall thicknessof 1 mm (0.048 in.) after 10 s of heating. Thetemperature is highest in the corners of the heatface and somewhat lower in the center of the turn.The concentrator is at a significantly lower tem-perature, and the localized higher-temperatureareas are due to conduction through the glue tothe concentrator. The maximum temperature is117 �C (243 �F). Figure 28 shows the tempera-tures in the corner and center of the tube versustime for this case. Nearly the entire temperaturerise in the copper occurs in the first 2 s.If pressure is doubled, the temperature distri-

bution is that shown in Fig. 29(a). The maxi-mum temperature in the corner is reducedto 99 �C (210 �F). The difference between thetemperature in the corner of the copper and thecenter of the heat face is significantly higher.Figure 29(b) shows the temperature distributionif the wall thickness is increased to 1.5 mm(0.062 in.). The maximum copper temperatureis slightly higher, 102 �C (216 �F). If the wallthickness is increased to 3 mm (0.125 in.), themaximum temperature increases to approxi-mately 111 �C (232 �F) (Fig. 29c).All of these temperatures are what could

be considered reasonable, and a long copper

lifetime would be expected based on the tem-peratures. If the current is increased to 7500 A,the differences between the results are muchmore apparent. Figure 30 shows the tempera-ture distributions for the same cases shown inFig. 27 and 29 with the higher current. Nowthe temperature of the copper wall contactingthe water for the low-pressure, 1 mm (0.048 in.)wall (Fig. 30a) is above the threshold where thecorrelations hold. This means that there is dangerof vapor blanket formation and very short coillifetime. It also is important to note that themaximum temperature shifts to the center ofthe tube instead of the corners. This is a sign thatthere is insufficient cooling, and increasing thewater pressure can significantly increase thecoil lifetime.When pressure is increased, the temperature

on the internal walls of the copper tubingdrops to manageable levels (Fig. 30b). The max-imum temperature again is in the corners of thecopper tube instead of the center. However, thelevel of the copper temperature in the cornersremains elevated (225 �C, or 437 �F). For thethicker walls, the temperatures are 230 and 263 �C(446 and 505 �F), respectively (Fig. 30c, d).This means that in each cycle, the copper in the

corners of the tubing expands more than twice asmuch as for the case of the lower current. Whenpower is turned off, the tubing contracts to itsoriginal shape. At these temperatures, there isthe potential that the coil lifetime may not besatisfactory.

Color Shade Results



Scale / Color

22.72733 / 28.59668

28.59668 / 34.46604

34.46604 / 40.3354

40.3354 / 46.20475

46.20475 / 52.0741

52.0741 / 57.94347

57.94347 / 63.81282

63.81282 / 69.68217

69.68217 / 75.55153

75.55153 / 81.42089

81.42089 / 87.29025

87.29025 / 93.1596

93.1596 / 99.02896

99.02896 / 104.89832

104.89832 / 110.76767

110.76767 / 116.63702

Fig. 27 Temperature distribution in the induction coil with 1 mm (0.048 in.) wall thickness, 10 kHz frequency, 5000 Acurrent, and low-pressure cooling


If frequency is reduced but the current re-mains 7500 A, the coil temperatures are reducedsignificantly (Fig. 31). For the case of the 1 mm(0.048 in.) wall with low pressure, the maximumtemperature is reduced to 178 �C (352 �F)(Fig. 31a). The temperature in the corners issignificantly lower than in the center. If pres-sure is increased, the copper temperature isreduced to 133 �C (271 �F) (Fig. 31b). How-ever, the maximum temperature is still in thecentral sections of the copper rather thanthe corners. This is due to the wall thicknessof the copper being less than optimal (1.6d).The low copper thickness increases resistanceand losses in the central section of the coil.When the wall thickness is increased to

1.5 mm (0.062 in.), the copper temperature isreduced to 114 �C (237 �F) (Fig. 31c). Thecentral temperature of the winding also isreduced relative to the corners. The maximumin temperature lies just inside the edge of thewater cooling, implying slightly less than opti-mal wall thickness. For the case of the 3 mm(0.125 in.) wall, the maximum temperaturegoes up again to 125 �C (257 �F) (Fig. 31d).The maximum now is clearly in the corners.The temperature in the center of the tubeactually is lower for this case. The rise in max-imum temperature is due to it being a fartherdistance from the water cooling.The losses in the copper tubing are nearly the

same for these two cases (3 kHz; 1.5 and 3 mm,or 0.062 and 0.125 in., wall thickness) and justunder 20% lower than for the 1 mm (0.048 in.)wall thickness. The optimal wall thickness andhence the maximum current-carrying capacity for3 kHz would lie between these two thicknesses.At 1 kHz, 10,000 A was used for compari-

son, because at lower frequencies it is naturalto have higher currents (Fig. 32). For the1 mm (0.048 in.) wall thickness and low pres-sure, temperature in the copper clearly is exces-sive, and there is a significant gradient from thecenter to the corners (Fig. 32a). As pressure is

increased, the temperature is marginal (230 �C,or 446 �F, wall temperature), and there is stilla significant gradient from center to edge(Fig. 32b). This is because the wall thicknessis significantly less than optimal.For the 1.5 mm (0.062 in.) wall thickness, the

maximum temperature declines to approxi-mately 144 �C (291 �F) (Fig. 32c). Losses inthe copper are nearly 40% lower than for the1 mm (0.048 in.) wall thickness. The tempera-ture distribution across the face is nearly flat,implying that the wall thickness is less thanoptimal. For the 3 mm (0.125 in.) wall thick-ness, the maximum temperature is only 100 �C(212 �F), and the location is the corners(Fig. 32d). The losses in the 3 mm (0.125 in.)wall thickness are approximately 30% lowerthan for the 1.5 mm (0.062 in.) wall thickness.Another factor related to water cooling of

induction coils is the water temperature. Thetemperature of the induction coil is influencedin two ways by the water temperature. It is clearthat the lower the water temperature, the coolerthe induction coil, because the cooling methodis convection. If the water temperature waslowered by 10 �C (18 �F), then it would beexpected that the maximum temperature in thecopper also would be 10 �C (18 �F) lower.Because the temperature change (DT) of the

copper would be the same, one would expectthat the magnitude of the copper movementwould be the same, and coil lifetime would bealso. However, this is not true, because thecoefficient of thermal expansion of copper isnot constant with temperature (Fig. 33). Astemperature rises, the coefficient of thermalexpansion of copper continues to rise (Ref 22).This means that the higher the temperature, thelarger the movement for the same DT.In reality, the DT of the copper is not the

same. It actually is slightly less, because the cop-per electrical resistivity rises with temperature.Therefore, the losses in the copper are lower,and the resulting DT smaller. This overrides the

fact that the heat-transfer coefficient of water isslightly lower at lower temperature (Ref 23).The last factor to consider with water cooling

of inductors is the water quality. The best fluidfor cooling induction coils in heat treatingapplications is pure, demineralized water. Purewater has a naturally high heat-transfer coeffi-cient. Almost all additives have a negativeeffect on the heat-transfer capability.Minerals are especially bad. Over time, they

tend to fall out of solution and build up on thewall of the copper tubing. This has two nega-tive effects. First, the mineral layer reducesthe cross section for water flow. The minerallayer also creates a barrier for heat transfer,causing the temperature of the copper to rise.If water containing minerals is used, over timethe copper temperature continues to rise andreach a critical level, resulting in an inductorfailure. With this type of cooling fluid, theinduction coil must be cleaned and flushed ona regular interval.

Styles of Heat Treating Inductors

As stated previously, induction heat treatingcoils come in many shapes and sizes. An excel-lent source for photos of induction heat treat-ing coils is the book Practical Induction HeatTreating by Richard Haimbaugh (Ref 5). Thesephotos are included in Steel Heat Treating Fun-damentals and Processes, Volume 4A of ASMHandbook, 2013, so they are not repeated here.

Skills for Heat Treating InductorConstruction

Copper is a unique metal associated withan ancient skills trade—coppersmithing. Whiletraining as either a craftsman or an artisan, astudent coppersmith would apprentice under ajourneyman to learn the character and manipu-lation of this metal. Although the United Stateshas assimilated the coppersmith into other skilledtrades, coppersmithing is still a recognized tradethroughout the industrialized world.The tools and skills of a coppersmith are ide-

ally matched to the metal; however, is a copper-smith by trade needed to build a copperinductor? The answer is no; yet, the skills of acoppersmith must be learned in the process ofbuilding heat treating inductors, along with afamiliarity of high-power electrical circuits.

Basic Skills

Because copper has a characteristic of beingeasily formed into complex shapes, a craftsmanhandy with tools, a soldering iron, and a propanetorch is able to build a good basic inductor. Inthe process of forming, copper work hardens;yet, when heated to a dull red and quenched inwater, it again forms easily. Basic inductors aremade of readily available copper tubing wound

Fig. 28 Temperature dynamics during heating at critical points with 1 mm (0.048 in.) wall thickness, 10 kHz frequency,5000 A current, and high-pressure cooling

600 / Equipment

around a forming mandrel. Copper tubing pro-vides a convenient path necessary to cool thecopper. Depending on the induction power supplyconfiguration, simple electrical connections maybe brass pipe fittings—either flair or compres-sion, although flair is preferred—or copper tabsthat are soldered to the tube, as mentionedpreviously. Electrical soft solder is sufficientat power requirements of 10 kW or less.

Advanced Skills

A classically trained machinist is the usualtradesman employed to build heat treatinginductors. A suitable candidate has the ability,dexterity, and patience for modelmaking. Most

will have difficulty machining copper unlessinstruction is provided in the use of specialcutting tools, speeds, feeds, and coolant specificto this metal. Much of the work in buildingcomplex inductors involves a quantity of smallparts and various materials that require machin-ing, forming, silver brazing, and assembly. Heattreating inductors usually are built one at atime, and one machinist may do all of the work.A simple machine shop consisting of a floormodel milling machine, lathe, band-type tablesaw, a ventilated brazing table with torches,and assembly bench with a variety of hand toolsmakes a suitable work area.Most inductors are best built from design

prints. A good print package has a bill of material,

details of all components, and an assembly andinspection form. In some cases, the inductor maybe built by reverse engineering the original. Thefollowing should be considered during the con-struction of heat treating inductors:

� Machining: Most of the raw material in atypical heat treating inductor is in the formof electrolytic copper sheet, plate or round.Due to the gummy nature of the metal, cutters,feeds, speeds, and coolant specifically design-ed for copper are advised for milling, drilling,and turning operations.

� Fabrication: Many parts may be neededto complete a heat treating inductor as-sembly. Some of these parts are fit to provide



Scale / Color21.3703 / 26.2284626.22846 / 31.0866231.08662 / 35.9447735.94477 / 40.8029340.80293 / 45.6610945.66109 / 50.5192550.51925 / 55.3774155.37741 / 60.2355660.23556 / 65.0937265.09372 / 69.9518969.95189 / 74.8100474.81004 / 79.668279.6682 / 84.5263584.52635 / 89.3845289.38452 / 94.2426894.24268 / 99.10083



Scale / Color

22.26987 / 27.27455

27.27455 / 32.27923

32.27923 / 37.28391

37.28391 / 42.28858

42.28858 / 47.29326

47.29326 / 52.29794

52.29794 / 57.30262

57.30262 / 62.3073

62.3073 / 67.31197

67.31197 / 72.31666

72.31666 / 77.32133

77.32133 / 82.32601

82.32601 / 87.3307

87.3307 / 92.33537

92.33537 / 97.34005

97.34005 / 102.34473


Time (s.) : 10 Phase (Deg): 0Scale / Color29.39038 / 34.4786134.47861 / 39.5668439.56684 / 44.6550744.65507 / 49.7432949.74329 / 54.8315254.83152 / 59.9197559.91975 / 65.0079865.00798 / 70.0962170.09621 / 75.1844375.18443 / 80.2726780.27267 / 85.3608985.36089 / 90.4491290.44912 / 95.5373595.53735 / 100.62558100.62558 / 105.71381105.71381 / 110.80203

(a) (b)

(c)

Fig. 29 Temperature distribution in the induction coil with wall thicknesses of (a) 1 mm (0.048 in.), (b) 1.5 mm (0.062 in.), and (c) 3 mm (0.125 in.) with 10 kHz frequency, 5000A current, and high-pressure cooling


covers for cooling chambers and ancillary con-nections that involve cooling or quench orboth. Integrity of fit is necessary to providewatertight and quality electrical connections.

� Silver brazing: While some fuel gases arebest suited for silver brazing, choosing anappropriate fuel, such as methyl-acetylene-propadiene gas, to mix with compressedoxygen may be subject to limited availabil-ity. Torch heat must be sufficient to maintainsilver brazing temperatures throughout thecopper mass. A variety of torch tips areneeded, along with flux and 1.5 mm (0.06 in.)silver brazing rod. The silver content of thebrazing rod best suited for inductor construc-tion is at 5 to 45%. Silver brazing is a skill

that can be difficult; however, with practice,patience, and perseverance, the technique canbe mastered.

Repair Skills

Copper inductors sometimes are subjected tothe extreme conditions of induction heat treat-ing: electromagnetic stress, intense heat, oilysmoke, steam, dirt, and grime. The effects oncopper can be severe, resulting in deteriorationand fatigue of the inductor and, ultimately, theheat treating process. Inductor maintenanceis essential. Cleaning the inductor with soap,water, and a soft plastic brush, along with regu-lar inspection of the copper and components,

alerts the induction operator to deteriorationand performance issues.Repair skills are advanced. The inductor is

cleaned, disassembled, and inspected for deterio-ration of copper as well as for leaks and cracks.Cosmetic repairs such as straightening bent com-ponents usually can be made without annealingthe copper. Ancillary components such as fluxintensifiers, brackets, guides, and quench unitsare cleaned and inspected for damage and arerepaired or replaced.Cracks in a copper inductor can result from

any of the following: cyclic fatigue, differen-tial cooling of the copper, resistive brazejoints, and abuse. Cracks often are repairedwith silver braze. Although this patch may be a



Scale / Color26.80051 / 42.9855542.98555 / 59.1705859.17058 / 75.3556275.35562 / 91.5406691.54066 / 107.72571107.72571 / 123.91074123.91074 / 140.09578140.09578 / 156.28082156.28082 / 172.46587172.46587 / 188.65089188.65089 / 204.83594204.83594 / 221.02098221.02098 / 237.20601237.20601 / 253.39107253.39107 / 269.57611269.57611 / 285.76114



Scale / Color23.08636 / 35.7435235.74352 / 48.4006948.40069 / 61.0578561.05785 / 73.71573.715 / 86.3721786.37217 / 99.0293399.02933 / 111.68649111.68649 / 124.34365124.34365 / 137.00079137.00079 / 149.65796149.65796 / 162.31512162.31512 / 174.97227174.97227 / 187.62944187.62944 / 200.28659200.28659 / 212.94376212.94376 / 225.60092

Color Shade Results

(a) (b)

(d)(c)



Scale / Color22.30582 / 35.2828835.28288 / 48.2599448.25994 / 61.236161.2361 / 74.2140674.21406 / 87.1911287.19112 / 100.16818100.16818 / 113.14524113.14524 / 126.12231126.12231 / 139.09935139.09935 / 152.07642152.07642 / 165.05348165.05348 / 178.03053178.03053 / 191.0076191.0076 / 203.98465203.98465 / 216.96172216.96172 / 229.93878



Scale / Color38.29016 / 52.334452.3344 / 66.3786266.37862 / 80.4228680.42286 / 94.4670994.46709 / 108.51132108.51132 / 122.55556122.55556 / 136.59979136.59979 / 150.64403150.64403 / 164.68826164.68826 / 178.7325178.7325 / 192.77672192.77672 / 206.82095206.82095 / 220.86519220.86519 / 234.90942234.90942 / 248.95366248.95366 / 262.99789

Fig. 30 Temperature distribution in the induction coil with (a) 1 mm (0.048 in.) wall thickness, 10 kHz frequency, 7500 A current, and low-pressure cooling; (b) 1 mm (0.048 in.)wall thickness, 10 kHz frequency, 7500 A current, and high-pressure cooling; (c) 1.5 mm (0.062 in.) wall thickness, 10 kHz frequency, 7500 A current, and high-pressurecooling; and (d) 3 mm (0.125 in.) wall thickness, 10 kHz frequency, 7500 A current, and high-pressure cooling

602 / Equipment

cost-effective repair, it is seldom permanent. Ina robust process, the copper inductor should bereplaced at an appropriate time/life interval.

Attachment of Magnetic FluxControllers

It has been explained that flux controllers areneeded to direct the current flow for specificinduction heating applications. Attaching themto the inductor requires careful handling sothat they are properly placed and secure.

Misplacement or loss of any portion of amagneticflux controller may result in a nonconformingprocessed part. Magnetic flux controllers are fre-quency dependent and made from either a siliconsteel, soft magnetic ferrite, or a soft magneticcomposite. Attaching these materials to an induc-tor can present a challenge to the designer/builder.The following are a few examples.

Silicon Steel Laminations

This material is supplied in a variety ofgrades, thicknesses, and finishes. Its usual formis a thin sheet with an insulating layer on both

sides. It is first designed to a specific shape, thendie stamped or machined and to fit the inductor.The most common shape is a rectangular orsquare “C” form. The laminations are stackedside by side onto the inductor in such a way asto expose that portion of the copper to the partbeing heated. Copper tabs of the same shapeare silver brazed onto the inductor at variousintervals in order to support the stack. Securingthe stack is achieved by either gluing with ahigh-temperature epoxy or by mechanicallyclamping it to the inductor. Various clampingmethods are used. The most common is theuse of a semirigid glass epoxy circuit board

(a) (b)

(d)(c)



Scale / Color23.992 / 33.64118

33.64118 / 43.29035

43.29035 / 52.93953

52.93953 / 62.58871

62.58871 / 72.23788

72.23788 / 81.88705

81.88705 / 91.53623

91.53623 / 101.18541

101.18541 / 110.83459

110.83459 / 120.48376

120.48376 / 130.13293

130.13293 / 139.78212

139.78212 / 149.43129

149.43129 / 159.08047

159.08047 / 168.72964

168.72964 / 178.37881

Color Shade Results



Scale / Color

21.77783 / 28.7492

28.7492 / 35.72057

35.72057 / 42.69193

42.69193 / 49.6633

49.6633 / 56.63467

56.63467 / 63.60603

63.60603 / 70.5774

70.5774 / 77.54877

77.54877 / 84.52013

84.52013 / 91.4915

91.4915 / 98.46287

98.46287 / 105.43423

105.43423 / 112.4056

112.4056 / 119.37697

119.37697 / 126.34834

126.34834 / 133.3197



Scale / Color

22.64523 / 28.33354

28.33354 / 34.02185

34.02185 / 39.71016

39.71016 / 45.39847

45.39847 / 51.08678

51.08678 / 56.77509

56.77509 / 62.46339

62.46339 / 68.1517

68.1517 / 73.84001

73.84001 / 79.52832

79.52832 / 85.21663

85.21663 / 90.90494

90.90494 / 96.59325

96.59325 / 102.28156

102.28156 / 107.96986

107.96986 / 113.65817



Scale / Color

29.76266 / 35.69722

35.69722 / 41.63177

41.63177 / 47.56632

47.56632 / 53.50088

53.50088 / 59.43542

59.43542 / 65.36998

65.36998 / 71.30453

71.30453 / 77.23909

77.23909 / 83.17365

83.17365 / 89.10819

89.10819 / 95.04275

95.04275 / 100.9773

100.9773 / 106.91185

106.91185 / 112.84641

112.84641 / 118.78096

118.78096 / 124.71552

Fig. 31 Temperature distribution in the induction coil with (a) 1 mm (0.048 in.) wall thickness, 3 kHz frequency, 7500 A current, and low-pressure cooling; (b) 1 mm (0.048 in.)wall thickness, 3 kHz frequency, 7500 A current, and high-pressure cooling; (c) 1.5 mm (0.062 in.) wall thickness, 3 kHz frequency, 7500 A current, and high-pressurecooling; and (d) 3 mm (0.125 in.) wall thickness, 3 kHz frequency, 7500 A current, and high-pressure cooling


material, such as G-10, that is cut and fit tothe back of the stack and is fastened at both ends.It is practical to stack laminations aroundthe outside or inside of an inductor loop. Installedonto circular inductors, the laminations radiatefrom the center point, leaving noticeable gaps,especially on small, circular inductors; yet, withcareful design, this can be minimized.

Soft Magnetic Composites andSintered Ferrites

These products are engineered and manufac-tured for specific induction applications. Because

sintered ferrites are brittle and difficult tomachine, their use mainly has been limited totransformer applications. Soft magnetic compo-sites, however, have been designed specificallyfor the induction heating industry. These productsare engineered to be used at the common power-supply frequencies. They are manufactured invarious block sizes, are easily machined, andfit to the inductor. Both iron ferrites andsoft magnetic composites are attached to theinductor, either mechanically with bolts andscrews or with glue. The product literaturedescribes their use in induction heating applica-tions (Ref 13).

Integrity Procedures prior toProduction

This section includes discussion on flux andoxide removal, leak and flow checking, silverplating, and electrical parameter measurement.

Flux and Oxide Removal

After brazing, the induction coil copper has fluxnear the braze joints, along with oxidation fromthe high-temperature brazing process. Prior toassembly, the flux and oxides should be removed.

(a) (b)

(d)(c)



Scale / Color

28.14111 / 49.29983

49.29983 / 70.45855

70.45855 / 91.61726

91.61726 / 112.77598

112.77598 / 133.93469

133.93469 / 155.0934

155.0934 / 176.25212

176.25212 / 197.41084

197.41084 / 218.56955

218.56955 / 239.72827

239.72827 / 260.88699

260.88699 / 282.04572

282.04572 / 303.20441

303.20441 / 324.36313

324.36313 / 345.52185

345.52185 / 366.68057



Scale / Color

23.12088 / 36.80721

36.80721 / 50.49354

50.49354 / 64.17987

64.17987 / 77.8662

77.8662 / 91.55252

91.55252 / 105.23886

105.23886 / 118.92519

118.92519 / 132.61151

132.61151 / 146.29784

146.29784 / 159.98418

159.98418 / 173.6705

173.6705 / 187.35683

187.35683 / 201.04317

201.04317 / 214.72949

214.72949 / 228.41582

228.41582 / 242.10214



Scale / Color

23.82666 / 31.32733

31.32733 / 38.8271

38.8271 / 46.32866

46.32866 / 53.82932

53.82932 / 61.32999

61.32999 / 68.83066

68.83066 / 76.33132

76.33132 / 83.83199

83.83199 / 91.33265

91.33265 / 98.83332

98.83332 / 106.33398

106.33398 / 113.83465

113.83465 / 121.33532

121.33532 / 128.83597

128.83597 / 136.33664

136.33664 / 143.83731

Color Shade Results



Scale / Color

29.53604 / 34.54433

34.54433 / 39.5526

39.5526 / 44.56089

44.56089 / 49.56918

49.56918 / 54.57746

54.57746 / 59.58574

59.58574 / 64.59402

64.59402 / 69.60231

69.60231 / 74.6106

74.6106 / 79.61887

79.61887 / 84.62716

84.62716 / 89.63544

89.63544 / 94.64372

94.64372 / 99.65201

99.65201 / 104.66029

104.66029 / 109.66858

Fig. 32 Temperature distribution in the induction coil with (a) 1 mm (0.048 in.) wall thickness, 1 kHz frequency, 10,000 A current, and low-pressure cooling; (b) 1 mm (0.048 in.)wall thickness, 1 kHz frequency, 10,000 A current, and high-pressure cooling; (c) 1.5 mm (0.062 in.) wall thickness, 1 kHz frequency, 10,000 A current, and high-pressurecooling; and (d) 3 mm (0.125 in.) wall thickness, 1 kHz frequency, 10,000 A current, and high-pressure cooling

604 / Equipment

The first step in the process is to place theinduction coil under hot (70 to 80 �C, or 150 to180 �F) running water. Hot water helps toremove the bulk of the flux remaining from thebrazing process. It also is recommended to flowthe hot water through the water-cooling passagesof the inductor to remove flux that could be builtup on the inside of the tube and obstructing thepassage.After the hot water cleaning, the next step

in the process is to sandblast the inductor toremove the oxides. The sandblasting processalso removes the remaining flux from the induc-tor. After sandblasting, a fine glass beading pro-cess should be used to improve the surfacecondition of the inductor.After glass beading, the copper is clean. If left

this way, the inductor oxidizes in air and changecolor. It is good practice to spray the surface ofthe induction coil (with the exception of theelectrical contacts) with a clear coat varnish.The varnish inhibits copper oxidation.In some cases with heavily loaded heat treat-

ing inductors, a low-temperature tempering at200 �C (400 �F) for several hours or a higher-temperature annealing process in air is per-formed. This tempering process removes thestresses from manufacturing and oxidizes thecopper to a dark orange (tempering) or a darkbrown (annealing). From this state, the copperdoes not oxidize further, and it is not necessaryto apply a clear coat varnish.

Leak and Flow Checking

After cleaning the induction coil, it is neces-sary to ensure that it is leaktight and that thewater passages are not clogged. Leak checkingof the induction coil typically is done by sub-merging the induction coil in a tank of water

and applying high-pressure shop air. The testshould be conducted for several minutes. Ifbubbles appear, the coil must be sent back to thecoil builder to repair the leak. In some specialatmosphere heat treating applications, a waterleak is especially dangerous. In these instances,helium leak checking should be performed.After leak checking, the induction coil

should be connected to water lines. With a spe-cific pressure applied, the flow rate of the waterthrough the induction coil should be measured.For every induction coil, a minimum flow rateshould be defined. If the flow rate does not meetthis minimum level, it means that the coolingchannel is obstructed. The obstruction must beremoved before the induction coil can beshipped; therefore, the induction coil shouldbe sent back to the coil builder.

Silver Plating

Silver plating of the induction coil contactstypically is done after cleaning. The main roleof silver plating is to act as a corrosion inhibitorfor the electrical contacts. Corrosion increasesthe contact resistance and can lead to prematureinduction coil failure.Silver plating materials can come in liquid,

paste, or powder form. Liquid silver plating solu-tion from HM Products Inc. is most commonlyused in the trade. The solution is either brushedor rubbed onto the contact area. After application,the contact area is flushed with water. If the sur-face is not flushedwithwater shortly after applica-tion, the silver carrier corrodes the contacts, and agreen oxide formation occurs. Care should betaken while using the silver plating materials,because most of them contain sodium cyanide,which is a toxic material.

Electrical Parameter Measurement

On some induction coils, such as cast orpotted inductors, it is not possible to physicallysee or mechanically inspect all of the criticaldimensions of the induction coil. Also, on induc-tion coils with magnetic flux controllers, it is notpractical to count the number of laminations, orit may be difficult to determine if the correct softmagnetic composite material grade was used.To ensure that the induction coil perfor-

mance is consistent with other induction coilsbuilt to that print, coil electrical parameter mea-surements should be made prior to shipping.The best inspection method involves measur-ing the inductance or impedance and the resis-tance of the induction coil with the componentit is designed to heat at the frequency of theheating. If this is not possible, valuable informa-tion can be obtained about the coil consistencyfrom a measurement of the inductance withouta part.Devices capable of measuring coil inductance

or impedance and resistance (LCR meters) at aselectable frequency typically cost several thou-sand dollars and are available from a numberof manufacturers. For inductance measurementonly, less-expensive devices are available. Atthe time of this writing, one such device thatworks well, the L/C Meter IIB, is manufacturedby Almost All Digital Electronics and costsunder $200 (Ref 24). Over time, new deviceswill come to the market with even better charac-teristics for induction coil quality control.

REFERENCES

1. A. Muhlbauer, History of Induction Heat-ing and Melting, Vulcan Verlag GmbH,Huyssenallee, Essen, Germany, 2008

2. H.B. Osborne, Jr., Induction Heating andHardening, Part A, American Society forMetals, Metals Park, OH, 1977

3. V.P. Vologdin and B.N. Romanov, Devicefor Hardening Crankshafts Using HF Cur-rents, USSR Patent 48416, filed Dec 16,1935, issued 1936

4. V.P. Vologdin, Induction Surface Harden-ing, Oborongiz, Moscow, 1947

5. R.E. Haimbaugh, Practical Induction HeatTreating, ASM International, MaterialsPark, OH, 2001

6. C.A. Tudbury, Basics of Induction Heating,Vol 1, John F. Rider, New York, NY, 1960

7. S.L. Semiatin and D.E. Stutz, InductionHeat Treatment of Steel, American Societyfor Metals, Metals Park, OH, 1986

8. M.G. Lozinskiy, Industrial Applicationsof Induction Heating, Pergamon Press,Oxford, London, New York, 1969

9. V.Rudnev et al.,Handbook of InductionHeat-ing, Marcel Dekker, New York, NY, 2003

10. R.C. Goldstein et al., “Virtual Prototypingof Induction Heat Treating,” 25th ASMHeat Treating Society Conference, Sept14–17, 2009 (Indianapolis, IN)

0

20

200 400

Temperature, K

600 800

15

10

Co

effic

ien

t o

f th

erm

al e

xp

an

sio

n

Thermal expansion of copper data

5

0

Fig. 33 Temperature dependence of the coefficient of thermal expansion of copper. Source: Ref 22


11. J. Cai et al., Integration of Induction HeatTreat Simulation into Manufacturing Cycle,J. Heat Treat. Prog., Vol 3 (No. 2), 2003

12. V.S. Nemkov, Modeling of InductionHardening Processes, Handbook of Ther-mal Process Modeling of Steel, C.H. Gurand J. Pan, Ed., CRC Press, 2009

13. W.I. Stuehr and D. Lynch, “How toImprove Inductor Life,” 23rd ASM HeatTreating Society Conference, Sept 25–28,2005 (Pittsburgh, PA)

14. W.I. Stuehr and D. Lynch, “How toImprove Inductor Life, Part II,” 24th ASMHeat Treating Society Conference, Sept17–19, 2007 (Detroit, MI)

15. Ajax Tocco Magnethermic, http://www.ajaxtocco.com/default.asp?ID=134

16. V.S. Nemkov and R.C. Goldstein, DesignPrinciples for Induction Heating andHardening, Chap. 15, Handbook of Metal-lurgical Process Design, G. Totten, K.Funatani, and L. Xie, Ed., Marcel Dekker,New York, NY, 2004

17. V.S. Nemkov, Resource Guide for InductionHeating, CD-R, Fluxtrol Inc., 2006

18. V.I. Rudnev, Systematic Analysis of Induc-tion Coil Failure, Parts 1–11, Heat Treat.Prog. Mag., Aug 2005–Sept/Oct 2007

19. H. Svendsen and S.T. Hagen, “Thermo-Mechanical Fatigue Life Estimation ofInduction Coils,” International ScientificColloquium onModeling of ElectromagneticProcessing, Oct 27–29, 2008 (Hannover,Germany)

20. R.C. Goldstein and V.S. Nemkov, “Influ-ence of Cooling Conditions on InductionCoil Temperatures,” International Sympo-sium on Heating by Internal Sources(Padua, Italy), 2007

21. H. Svendsen and S.T. Hagen, “TemperatureDistribution in Selected Cross-Sections ofInduction Heating Coils,” InternationalSymposium on Heating by Internal Sources(Padua, Italy), 2007

22. Case Studies in Process Modeling, Engi-neering Statistics Handbook, NIST/SEMATECH, http://www.itl.nist.gov/div898/handbook/pmd/section6/pmd6.htm

23. Fluxtrol, Inc. internal documents24. L/C Meter IIB, Almost All Digital Elec-

tronics, http://aade.com/lcmeter.htm

606 / Equipment

design and fabrication of inductors for induction heat treating

Engineering

induction coils

induction coildesigns

induction coil designprocedure

preferred induction

induction toolingmicah

successful heat

firstinduction heat

coil optimization wasmade