Published by the EPRl Center for Materials Fabrication Vol. 2, No. 1 Revised 1993

A Cost-Effective And Energy-Efficient Method For Process Heating

Induction heating is a relatively recent process heating method, introduced commercially about 50 years ago. Prior to its develop- ment, gas- and oil-fired furnaces provided the prime means of heating metals. Induction heating offers a number of advantages over furnace techniques that make it appropriate for a variety of applications. The most significant advantages are: rn Quick Heating - Developing heat

within the workpiece by induction provides much faster heating rates than the convection and radiation processes that occur in furnaces (Figure 2).

rn Less Scaling Loss - Rapid heating significantly reduces material loss due to scaling (e.g., for steels) relative to slow gas- fired furnace processes.

rn Fast Startup - Furnaces contain large amounts of refractory mate- rials that must be heated during startup, resulting in large thermal inertia. The internal heating of the induction process eliminates this problem and allows much quicker startup.

rn Energy Savings -When not in use, the induction power supply can be turned off because restart- ing is so quick. With furnaces, energy must be supplied continu- ously to maintain temperature during delays in processing and avoid long startups.

rn Higher Production Rates - Because heating times are short, induction heating often allows increased production and reduced labor costs.

Continued on page 2

. PROCESS DESCRIPTION Electromagnetic inducrion, or simplyci'nduction-~;is.a way to'heat

electrically conductive materlals, such as metals:;lr.is _commonly used in process heating prior to metalworkjng,'. . . and$heat :treating, '-. welding, and melting': . . .

induced internally in the wcrkpiece materia(. These"s0fcalled eddy . . . currents dissipate energy and bring ab.out-heating:l.The .basic. cpmpo- .nents of an induction heating system are.an'i~auction.coilI an .

alternating current (ac) power supply, and..the Cvorkpiece itsdf: '(Characteristics of coils and power supplies aie'discussed on . - . pages 5 and 6.) A simple setup involving-a'solenoidal coil and,rcund workpiece is shown in Figure 1, The CoIl; which may take differerit shapes depending on the required heating pattern; is. connected io the power supply. The flow of 6c current throughthe coii.generates an alterpating magnetic iield, which cuts. through-the,'workpiece. This. , alternating magnetic field indcces eddy currents and the heating of the workpiece. Moreover, because the magnitud.e:of.the. eddy currents decreases with disiance from the':workpiece surface.: surface hesting and heat treatirig are possible. In..cont(ast, by allowing sufficient time for heat conductiork.-relatiieTy:uniform~ heating patterns can be obtained far purposes of.:through. heat treaiing, heating prior to metalworking, and so forth', :CarefuKattention,to coil design and selection of the power supply frequencyand rating.. ensures close control of the heating rates-nd pattern..,,.. . " . .-.

There are several differenczs between induction and traditional heating techniques. Most significant is that induction heat is. .. generated within the workpiece. In furnaces, on the.other. hand, heet produced by a burning fuel is transpoded throu-gh,the furnace atmosphere via convettion and radiation processes. Because heat is

enclosure or a large working area. . . j:.. -'. . .. . . ' .. . -!:'

. . . ' * ~. . .. , ":.. , . . _ _ . . . . ...L .- ., ,- .;:. ;, , . . . . . . - . . As implied, induction .heating relies,on electrical:currents.t~~t are .

. . . .. . -

. .

. generated internally, induction processes d o not require a furnace .. ..- . ' .. *

!

Additional advantages that induction systems offer are:

Ease of automation and control rn Reduced floor space requirements

Quiet, safe, and clean working

Low maintenance requirements.

Applications Primary induction heating applica-

tions in the metals industry fall into four major categories: heating prior to metalworking, heat treating, welding, and metal melting. While these are the most common uses, a variety of other operations (such as drying of paint on metal surfaces and brazing) are also amenable to induction heating methods. Some typical products for the major application areas are listed in Table 1 .. Preheating - Induction heating prior to metalworking is well accepted in the forging and extru- sion industries. It is readily adapted to through preheating of steels, aluminum alloys,and specialty metals, such as titanium and nickel- base alloys. Frequently, the work- piece in these types of applications consists of round, square, or round- cornered square bar stock. For steels, the high heating rates of induction processes minimize scale and hence material losses. The rapid heating boosts production

conditions

rates. Induction heating is also useful for selectively preheating bah stock for forming operations such as heading. Heat Treating - Induction heating is used in surface and through hardening, tempering, and anneal- ing. A primary advantage is the ability to control the area that is heat treated (Figure 3). The most common induction heat treating operation, hardening, improves the strength, wear, and fatigue proper- ties of steels. Steel tubing products, for example, lend themselves quite readily to hardening via induction in continuous line operations. Steel tempering by induction, though not as common as induction hardening of steels, restores ductility and improves fracture resistance. Recent work at the Centerfor Metals Fabrication has firmly established the viability of induction tempering. While also less commonly applied, induction anneal- ing restores softness and ductility, important properties for forming of steels, aluminum alloys, and other metals. Welding and Melting-The use of induction heating for welding and metal melting is well accepted. High, uniform heating and welding rates are obtainable using induction methods. High-frequency induction welding offers substantial energy savings because heat is localized at

Bar Diameter (cm.)

Figure 2 Comparison Of Heating Time Via Induction And Gas-Fired Furnace Techniques As A Function Of Bar Diameter

the weld joint. The most common application of induction welding is welded tube or pipe products that lend themselves to high-speed, high- production automated processing (Figure 4).

frequently used to melt high quality steels and nonferrous alloys (e.g., aluminum and copper alloys). In addition to the advantages of induc- tion heating already given, induction melting offers added benefits over other melting processes. These include a natural stirring action (giving a more uniform melt) and long crucible life.

Induction processes are

Melting

Air Melting of Steels Ingots Billets . . ' Castings

. Vacuum Induction Melting :- Ingots ' .

. Billets ,.; . . ~ : Castings 1:. . .

"Clean"-steels. .. Nickel-base superalloys

Titanium alloys

Figure 3 A Camshaft Section Showing Selectively Hardened Areas

Technical Considerations When determining whether induc-

tion heating is suited to your needs, technical 'considerations include the workpiece material and size. Some materials, such as steels (magnetic and nonmagnetic, carbon, and stain- less steels), titanium alloys, and nickel alloys, are readily heated using induction methods. Low- resistivity metals, such as aluminum and copper alloys, although frequently heated by induction, are more difficult to heat at high system efficiencies. Regarding workpiece size, products that can be processed on a.continuous basis, such as tubing, are prime candi- dates for induction. Processing of

discrete parts is also technically (and economically) feasible within certain size ranges.

Part shape also plays a role in determining whether induction heating is technically feasible. The part shape and desired heating pattern determine in large measure the necessary induction coil design.

Finally, induction processes are almost always used for certain types of heating, such as that for surface and selective heat treating. This is because conventional furnace-based heating methods are effective only for through heating.

Economic Factors And Economic Analysis

feasibility of induction heating for a specific application, you need to determine if it will also be cost effective. Important economic factors in your decision should include equipment and energy costs, production lot size, scale and scrap losses, and labor requirements. Let's consider each of these with regard to induction versus gas-fired furnace heating methods. Equipment Costs - Induction power supplies, the major cost item in an induction system, typically . cost 2.5 or 3 times as much as gas-fired furnaces of equal capacity. Gas furnaces with recuperators (used to preheat combustion air) have higher efficien-

Once you've established technical

Figure 4 High-Frequency Induction Butt Welding Of Tubular Products

cies than conventional furnaces, and cost about one-half as much as comparable induction equipment. Both induction power supplies and furnaces have very long service lives. Because of this, amortization of both types of equipment is readily spread over 5 to 10 years or even longer. Thus, induction heating equipment will cost more, but the cost can be offset by increases in productivity and energy savings. Production Lot Size and Ease of Automation - Because a given induction coil can often be used only for a given part and heating applica- tion, induction heating is almost always selected for large production runs of a given product for which coil changes are not necessary. Such processes are best carried out on an automated basis, which can lead to substantial economic benefits. (See following discussion of labor costs.)

Energy Costs - Energy costs are determined by the base fuel cost and the overall heating system effi- ciency. On the average, electricity costs about 3.5 times as much as natural gas. However, heating system efficiencies can mitigate this difference in fuel cost. Efficiency is the ratio of energy used for actual heating to the energy supplied to the system. For induction heating, system efficiencies of 55 to 85 percent are typical. For gas furnaces, in which heat losses through doors, walls, etc., are common, typical efficiencies are 15 to 25 percent; with special insulation methods, which, of course, increase the furnace cost, it is reported that these efficiencies can be increased to perhaps 40 to 50 percent. In any case, it is obvious that the higher efficiencies of induction units ca'n often offset the cost of electricity. Scale and Scrap Losses-Scale and scrap losses are of particular importance in an economic analysis because a large part ot production cost is tied up in the workpiece material itself. Often, the material cost constitutes as much as one-half or more of the net cost of the finished product. Because of this, material loss, as scale or scrap, is a prime factor in any economic analysis.

Scaling occurs while heating.. . steels prior to forming or during heat

Economic Comparison Of Induction And Gas-Fired Table 2 Furnace Heating For Forging Billets'

Induction Furnace

$600,000 60 % $720,000

1 I 4 % $1 50,000 1 Operator 114 Maintenance $60,000 $930,000

1 f 2 '/Q

Gas-Fired Furnace

$200,000 1 5 % $540,000 2 % 1 % $600,000 2 Operators 112 Maintenance $120,000 $1,230,000

'Electrtclty charges may vary widely by region and customer usage. Case descriptlon assumes steel hot-forgmg operation, 3Q000 tonslyr throughput, 4,000 hrs/yr operation Raw materlal value: $500/ton. product scrap value: $l,OOO/ton. Labor cost: $12/hr. Energy cost: electrctty. $.OG/kWh; gas $4.00 MBtu.

r - .

treating processes. When heating formation is somewhat less,, around steel billets to hot working tempera- 1 to 2 percent for furnacesiyn tures (2200 F) in air using gas-fired contrast, the shorter heating times furnaces, scaling often amounts to 2 characteristic of induction lead to to 4 percent of the total part weight. considerably less scale, usually rt3

more than one-fourth of the amount formed during furnace heating

At hardening temperatures ( - 1650 F), the amount of scale

Table 3

processes. Furthermore, scrap losses due to improper heating are usually higher in gas-fired-furnace operations. In furnace-based processes, thermal cracking or distortion are the usual causes of part rejection. Such defects cause scrap losses of 1 to 2 percent for steels. Scrap losses associated with induction heating are typically around onequarter to one-half percent. Thus, considering both scale and scrap losses, induction heating could result in materials savings of as much as 3 to 4 peicent. Labor Costs - Induction systems require less labor for both operation and maintenance than do compara- ble furnace-based heat-treating systems. This is because the wide- spread use of reliable solid state power generators and the fast response and controllability of induc- tion heaters make these systems very amenable to automation. On the other hand, furnace heat-treating processes normally require a considerable amount of labor for loading, unloading, and handling of

Heating Processes That Compete With Induction

Heat Treatment

Gas furnaces - used for through heat treatment of ferrous and nonferrous alloys

Electric furnaces - used for through heat treat- ment of ferrous and nonferrous alloys

Laser hardening, electron beam harden- ing, ion nitriding - used for very shallow case depths (i0.020 in.) on steel where hardness pattern, part design, or lot size preciude use of induction

High-frequency resis- tance hardening -used for shallow (-0.003 in.), irregularly-shaped hardened cases on steels

Welding

Conventional welding techniques - used for all applications except for mass production of seam welded products (see TechCommentary, Vol. 1, No. 1)

~~ ~~ ~ ~ ~ ~ ~ ~~~~~

Melting

Conventional gas furnaces and electric furnaces - used for large heats of metals for which property specifications are not stringent

Vacuum arc melting, electron beam melting -used for reactive and refractory metals of high purity for high perfor- mance applications. Also used interchangeably with vacuum induction melting for clean steels and nickel-base superalloys

the steel to be processed. In addition to requiring fewer opera- tors, automation also allows the use of less-skilled operators with a minimal amount of training. The use of automatic as opposed to manual operation in a single-shot induction application has reduced labor requirements as much as 50 to 80 percent in some instances. In addition to the direct labor, mainte- nance costs for furnaces (which need to be relined intermittently) tend to be higher. Taking all these factors into account, a general rule of thumb for cost analysis purposes is that induction heat treating usually requires one-half the labor needed for furnace processes with an equivalent production rate.

factors is used to estimate the cost of induction versus gas furnace- based heating processes. An example qf such an analysis for heating of steel forging billets is shown in Table 2. For the operation

Data derived from analyzing these

described in this example, the higher cost of induction equipment is offset by lower operating expenses in less than 2 years.

Competitive Processes

heating can be readily justified as the method to use. In some instances, other heating techniques offer viable alternatives. A list of these alternatives is given in Table 3. An estimate of the usage of induction heating for several heat treating processes is shown in Figure 5.

Components of Induction Heating Systems

As mentioned above, the basic components of an induction heating system are the workpiece (material to be heated, welded, or-melted), the induction coil, and the induction power supply. Induction Coil -This is designed to

For many applications, induction

provide the desired heating pattern with as great a heating efficiency as possible. This involves determining the basic coil shape and the number of turns or loops in the coil. The proper number of turns will ensure a good impedance match to the power supply and tuning of the pertinent electric circuit so that adequate power can be transferred to the workpiece. Besides the simple solenoidal coil (Figure l ) , other designs include channel and nonen- circling coils (Figure 6), and pancake, internal, and other symme- tric and irregularly shaped coils (Figure 7). Uses of various coil designs are discussed in TechCom- mentary, Vol. 2, No. 3. In almost all instances, induction coils are con- structed of copper tubing. Copper is selected because of its low electri- cal resistance. Tubes are used to allow water cooling to prevent over- heating during operation. Induction Power Supply - Line frequency current is converted to

Competition Between Induction And Other Heat Treating Processes Figure 5 In Terms Of Estimated Usage

Heat Treatment

1: Surface hardening

2. Annealing Tempering Recrystallization

Part

- Small parts - TOOIS - Rolls (for rolling mills and paper - Combustion engine components:

machines)

Crankshafts =.camshafts

steering parts

Heavy duty Standard

- Axles, racks, automotive - Gears:

- Pinions - Bearings (rollers, locomotives) - Bars, rods, rounds - Chains - Bearings - Longitudinally welded tubes - Chains - Pipes and tanks - Case-hardened parts (local annealing) - Induction hardened parts

(tempering at 150-300 C) -Wire tempering (400-450 C),

recrystallization (500-600 C)

Position of Induction Heating

0 Pct. Induction 100 Pct. induction 3

3

9 h

+ + Case-hardening +

+ Salt bath

iw * *

+ Furnace 9

b

b Furnace

i +

HenriHighlight

Figure 6 .Typical Design Of (A) Channel (Slot) Induction Coil And (e) Single-Shot, Non-Encircling Coils

alternating current of a higher frequency using the power supply. The major characteristics of the power supply are its output frequency in hertz (cycles per second) and power in kilowatts. The frequency determines the penetra- tion depth of the eddy currents induced in the workpiece. The higher the frequency, the thinner the so-called skin-depth, i.e., the depth at which current magnitude drops to 37 percent of its value at the work- piece surface. For efficient heating, skin depth should not exceed a certain fraction of the workpiece size (one-fourth the diameter for rounds, one-half the thickness for pieces of rectangular cross section). Based on this concept, tables of minimum power supply frequencies for heating metals with different electrical properties have been formulated. One such listing, which you will find useful, is shown in Table 5.

the power supply is its power rating. For through heating and melting applications, the required power is determined by the overall efficiency of the induction heating system and the desired heating time. Table 4 gives the average power requirements for induction heating in typical metalwork- ing processes.

The second major characteristic of

Average Energy Requirements, kWhIlb, For Induction Table 4 Heat Processing Of Common Metals

Curing of Coatings 0.025 0.025 0.0375 0.055

* Based on In-line continuous process

Figure 7 Multiturn Coils F_or Various Induction Heating Applications

Round Rectangular

Formed Pancake

Spiral-helical Internal

HenriHighlight

HenriHighlight

Approximate Smallest Diameter (Inches) That Can Be Heated Efficiently By The Equipment Indicated To

Table 5 The Temperature Shown

qu6:nHczyy, IT Freque i:; 1 0.781 0.45

0.88 0.52

4;: 1 0.76 1 0.44 0.63 0.37 0.81 0.47

3.6 I 0.87 I 0.50

In through and surface heating applications, the power density is also an important design feature that must be taken into account. The power density is the power supplied to the workpiece divided by the surface area surrounded by the induction coil. With high power densities, the surface will be heated very rapidly. At lower power densities, time for heat conduction into the interior of the workpiece enhances temperature uniformity. Typical power densities for through heating of steels for acceptable temperature uniformity are 0.1 to 0.5 kilowatt per square inch. Low values pertain to heating to low temperatures (11 000 F) and higher values to high temperatures (21500 F).

Power densities for surface harden- ing of steel tend to be considerably larger than those for through heating,

Solid State Systems, ~i~~ F ~ ~ - Motor Generators

vacuum Tube

!ncy kHz Oscillators, - "

-

-

0.23 0.10 0.035 0.28 0.12 0.041 0.32 0.14 0.047

0.24 0.1 1 0.036 0.20 0.09 0.029 0.26 1.2 0 039

0.52 0.23 ' 0.078 0.56 0.25 0384

0.11 0.05 0.017 0.85 0.38 0.125 0.88 0.39 0.130 0.90 0.40 0.133

0.28 I I 0.12 0.041

through hardening, and melting of ferrous and nonferrous metals, usually ranging from 10 to 30 kilowatts per square inch. Furthermore, because a range of frequencies can be employed to obtain the same hardened depth, it is often easiest to refer to nomographs when specifying frequency, power, and power density requirements.

In. Summary Induction heating is a well

accepted technology used in the metal forming, heat treating, welding, and melting fields. In many cases, it provides faster heating times than conventional furnace-based pro- cesses, fast startups, high production rates, ease of automation and control, and good working conditions.0

The information provided in this issue of Techcommentary is an overview. It is intended to familiarize you with the important applications of induction heating, as well as the technical and cost factors you should be aware of when selecting such process heating equipment. If you are interested in more-detailed background information, please refer to Techcommentary Vol. 2, Nos. 2, 3, and 4. The sources listed on the back page also contain useful information.

The Center for Materials Fabrication (CMF) is operated by Battelle In Columbus, Ohlo. Fundmg is provided by The Electric Power Research Institute (EPRI), a nonprofit organization whose mission IS to dlscover, develop, and deliver advances in science and technology for the benefit of member utilities, their customers, and society

Techcommentary is one of the ways EPRl assists the industry in implementing cost- and energy-efficient, electric-based technologies in the fabrication of metals, plastics, ceramics, composites, and wood. Members are welcome to quote verbatim from this publication provided CMF is credited. CMF approval is required for editorial incorporation, as pertaining to U.S. copyright laws. For ordering information, call EPRls AMP Program at

This issue of Techcommentary was made possible through the cooperation of Battelle staff members Lee Semiatin, author; Jack Mortland, editor; and Laura Cahill, publications coordinator. Techni- cal review was provided by Tom Clark, Franklin Steel Treating; and Dick Haimbaugh, Induction Heat Treating Corporation.

1 -800-4320-AMP.

Sources used in this issue of Techcommentary were: Basics of Induction Heating, John F. Rider, 1960, C.A. Tudbury. Induction Heating Handbook, McGraw- Hill Book Company, 1979, J. Davies and P. Simpson. Induction Heat Treatment of Steel, American Society for Metals, 1985, S.L. Semiatin and D.E. Stutz. Elements of Induction Heating: Design, Control, and Applications, ASM Interna- tional, 1988, S. Zinn and S.L. Semiatin.

Sources for tables, photographs, and figures were:

High-Frequency Induction Heating, McGraw-Hill Book Company, 1950, F.W. Curtis.

Proceedings of the EnergySeminar/ Workshop on New Concepts in Energy Conservation and Productivity Improve- ment for Industrial Heat Processing Equipment, 1982, Innovative Induction Systems for the Steel Industry.

Proceedings of the International Confer- ence on Induction Heating and Melting, 1978, The Philosophy of Induction Heat Treatment. Tocco Product Brochure DB-2034. Unpublished Battelle research.

Applicable SIC Codes 33-12, 15, 16, 17, 21, 34, 41, 51, 53, 54,

5 5 , ~ 98,99

62,63,71, 79,83,84,89, 93,95,99 34- 1 1, 12, 21, 23, 25, 29, 41, 43, 49, 52,

35-23, 24, 31, 32, 33, 36, 41, 42, 44, 45, 46, 47, 62, 66, 67, 68, 92, 99

36-21

37-11, 14, 21, 24, 28, 43, 51, 61, 64, 69, 95,99

For technical questions and for further information on Center programs, contact

CENTER FOR MATERIALS FABRICATION An EPRl RBD Appllcatlons Center 505 King Avenue Columbus, Ohio 43201-2693 (614) 424-7742

Copyright 0 1993 Electrlc Power Research Institute. Palo Alto. CA

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