TECH- ;OWEN1;9RY 0 DIRECT A!!: L

RESISTANCE HEATING """"

Published by the Center for Metals Fabrication

Putting the Heal Where It's Needed

Electric resistance heating offers manufacturers precise control and '?

dmcted heat for applications suchy+." ating billets for forging,

unique hardening metals, selectively , . '

rging dies and maintaining -s at constant temperature. lbclbet resistance heating works only for electrically conductive workpieces; while any material, either solid or liquid can be heated with an encased resistance heater.

This issue of TechCommentary describes how these two heating techniques are used and the factors to consider in deciding whether one of them is appropriate for your application.

Direct Resistance Heating By generating heat within the

workpiece rather than in a furnace, direct resistance offers a number of benefits over fuel-fired furnaces including: rn Rapid heating rn Fast start up

Energy savings r rn Higher production rates

Ease of automation Material saving as a result of

rn Improved working environment rn Reduced floor space requirements rn Low maintenance costs.

reduced scale

Applications The major metalworking

applications of direct resistance heating are heating prior to forming, heat treating, and seam welding. Glass melting is the major nonmetals

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application. Other, less common, uses include heating concrete to accelerate setting, producing silicon carbide, and remelting metals in electroslag.

Hoatlng prior to hot working- Direct resistance is used to preheat round or square metal bar stock prior to operations such as forging, stamping, extrusion, bending (for chains),, and upsetting.

Workpiece material and shape are both important in determining the success of direct resistance heating. Materials with fairly high electrical resistivity, such as carbon and low- alloy steels and nickel alloys, are readily heated by direct resistance. With low resistivity materials like copper and aluminum, the process is

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often not cost-eftecthre. For most efficient heating the

workpiece should have the following geometry: rn Diameter (or one side for square

stock) less than 1.5 in. rn Length-to-diameter ratio at least

6:l. For smaller ratios, line and contact losses reduce efficiency (Figure 1). Generally, to ensure even heating,

the cross section of the workpiece should be uniform. Otherwise, the heating rate will be high where the cross section is small and low where it is large. However, workpieces with variations in their cross section, such as rivets, can sometimes be uniformly heated by pulsing the power during the heating cycle.

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TechComrnentaryNd. 31No. 8 1

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working operations. - Selective heating of ends 01

I specific sections of bars by appropriate electrode placement. Usually, direct resistance and

induction are complimentary rather than competing heating techniques. However, for those workpieces of I 1 the right shape and size, direct

Figure 7. Heating efficiency as a function of billet length and diameter.

resistance heating is more economical than induction* Equipment is less expensive and more energy efficient, so operating costs are also lower.

more versatlle. Heating modules can

s over gas furnace

are inefficient at the temperatures required for preheating.

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transtertothewwk@kceisby condcIctloll rawr th8n axmdim.

resl8tanceheatingIsqnsxoslknt method tot mikcthe wrface hardening carbon and low-alloy steels and cast iron. Such hardening improves the wear resistance of products like automobile universal

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Flgun, 2. Parts hardened by direct l : _ j

resistance heating.

joints, engine piston rings, wrenches, leaf springs, and hedge clippers. -

For hardening. two power contact$ are attached to the wwkpkce just beyond the ends of the area to be hardened. 'A water. i t ' + * c h

"proximity conductor" is 'ti;' placed close to the workpiece *I yh surface, and connected between the electrodes. When high frequency current (usually radio frequency [RF],- 400 kHz) is passed between the I power c o n t a c t s , the narrow strip of Wo-e surface under the pmximity connector heats up. rn Hserting depth is about 0.03 in. The saip reaches hardening temperature (rbout 1m F in steel) in about 0.5 sac, so the surrounding workpiece iFL i!llgE.81zE material remains cool. Thus, the ',,

process is self-quen ng, so there is aknost no part distogn. The hardening pattern produced mirrors the proximity conductor shape, and, with appropriately-designed conductors, many different, wel l -

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This fuel oil pipe must be kept a 130 F even when the outside temperature is 0 F and there is a 20 mph wind. The company chose constant-wattage heating cable, laid in parallel lengths along the pipe, even though steam was available as a plant byproduct. i The cable is insulated with 2 in. of calcium silicate and covered by an ~

aluminum jacket so that it retains its shape even when walked on. This system was easy to install, operates on their available 277 V supply, and I reauires little maintenance.

Flgun, 3. A an encased resistance element. in an enormous range of “packages” and have a

defined hardening patterns, on either flat or tubular workpieces, are possible. Some of these patterns are

,.,., shown in Figure 2. ‘,.I{- , 8 , ; 7 , . ,,/, Direct resistance through heating vi , and surface hardening differ in their

m r supply requirements. A line

Ir economical only in high volume applications.

Other methods used for selective hardening include induction heating and laser and electron beam hardening. (Convection furnaces do not work because of the difficulty of controlling the hardening pattern.) All of these methods are fast and produce little heat distortion. However, laser and electron beam hardening are limited to case depths

Joining- High-freque6y resistance heating is widely used for sqhm welding steel tubing, and . occasionally copper and aluminum tubing. The tube is moved past two stationary electrodes that pass RF current through its edges. Heat generated by the current causes the edges to melt and fuse together. The process is always automated, making direct resistance welding fast and economical. Unlike induction welding, which is energy efficient only for small diameters, any size tube can be resistance welded.

Resistance weldina is also used

to patterns for which a coil can be designed. However, direc and induction hardening u same power supply, so yo produce almost any patt selecting either a coil or a contact/proximity-conductor, assembly.

of less than 0.02 in. In addition, for many special joiing applications, equipment is expensive and of both similar and dissimilar metals, complex, both to operate and to which would be impossible with maintain. induction. These include structural

Induction hardening can produce shapes such as I- and H-beams and greater case depths but is restricted T-sections, and welding fins to cubes.

Steam heating lines I

this diversity is an’advantage in itself since a heater can be found for practically any application. Other advantages include: rn Low cost. Heaters are fairly

inexpensive, reliable, durable, and require little maintenance.

rn Energy efficiency. Close to 100% of the applied energy is converted to useful heat.

Thermostats ensure wellcontrolled heating. An encased resistance heater

consists of an electric wire or ribbon resistance element Wrounded by an electrical insulator enclosed in an outer envelope. The outer covering provides mechanical and chemical protection (Figure 3). The heater is placed on or in the solid or liquid to be heated. Heat generated by current flow through the resistance element is transferred to the workpiece by conduction. These heaters almost always use line frequency, and they can be designed to operate at whatever voltage a plant has available.

classified by shape, maximum operating temperature, type of insulation, and application. Commonly used types include: rn Heating cartridges rn Immersion heaters rn Heating collars rn Heating tapes.

rn Precise temperature control.

Encased resistance heaters are

g#ne of the characteristics and a few common applications of each type are summarized in Table 1.

Te-cRniicrrI Considorations While applicatioTdZleWiri5s

which particular heater is most appropriate, in all situations the heater power density and outer casing material must be carefully considered.

material from which the actual resistance element is made, the maximum temperature the case can withstand, and the shape of the heater and its cross section. While it is generally true that a higher power density heats faster, this is not necessarily desirable. Suppose, for 01117P(e, that a liquid is to be held at

Power density is a function of the

a surface area sufficient

W r a t u r e difference between the M r surface and the fluid.

Similarly, if a high-powered heater is used with a poorly conducting material, heat will not be removed from the heater fast enough, resulting in overheating and premature failure of the heater element. Equipment suppliers are experienced in determining the right heating element, and they will help you make the best choice.

Case material must be able to withstand the environment in which it will be used, both thermally and chemically. Plastic casing materials are only useful up to about 300 F, steels to around 950 F, nickel alloys to 1500 F, and some ceramics up to about 1800 F. The liquid in which they are immersed also influences the choice of outer sheath material.

solution to heating problems than are the competing processes.

In Summary There are a number of non-

furnace”heatingtechniqTZiiwfiEt7’ use resistanceheating. Two of the most important are direct resistance .+& and encased resistance heating. Direct resistance heating may well be the simplest and most economical method for through heating or heat treating workpieces of the appropriate material and geometry. Encased resistance heaters, available in a wide range shapes, sizes, and electrical rating are generally used when reliable, long-term heating at low to mediu temoeratures is reauired. ~~ ~~

For example, copper or stainless The information in this issue of steels work well for water heaters but FTFE-coated steel gr nickel

echCommqntary is an overview and is intended only to familiarize you

alloys may be required fw. highly corrosive nquids. . .c forms of resistance heating. For

There is no average cost f&. more information please contact encased resistance heaters because CMF or one of the suppliers of of the wide range of models resistance heating equipment. available. However, they are frequently a more cost-effective

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.> . with the basic aspects of special

The Center for Metals Fabrication (CM9 is‘ operated by Battelle’s Columbus Division. Basic funding is provided by the Electric h w e r Research Institute, a nonprofit institute that conducts research and development on behalf of the United States electric utility industry. The Center’s mission is to assist industry Hallowell, Industry Applications Engineer, Surface Hardening.” Fabricator. April in implementing cost- and energy- Laura Cahill, Manager, Marketing and 1984. efficient, electricdased technologies in Communications, and Denise Sheppard, Inactoheat Product Literature, Madi metals fabrication and related fields. Publications Coordinator; and Anne Heights, MI. Techcommentary is one communication Moffat and Dorothy Tonjes of Prowrite. orfeuil, Electric process Heating.. vehicle that the Center uses to transfer Technical review was provided by John technology to industry. The Center also Meek, Chromalox, Paul Miller, conducts research in metal heating, Inductoheat, and Humfrey Udall, metal removal and finishing, and Thermatool. fabrication. Sources used in this issue of Chromalox (Figure on &e 2)

Techcommentary wece: This issue of Techcommentary was Chromalox Product Literature, Pittsburgh, 34-23, 29, 52, 62, 82, 83, 89 mebe po8eible through the cooperation PA. 35-23, 24, 31-33, 44, 45, 68 of Battelle staff members Lee Semiatin, Hubbard, C. “High Frequency Heating 37-1 1, 13-1 5,21, 24,28,43, 51, 61, Senior Research Scientist .Inh$a Offers ,More Versatility to Selective 95, 99

Technology/Equipmn~~/ica/Applications, Battelle Press (in press). Photographs courtesy Fabricator (Figure 2), New England fwer and

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Contor for M.1.l~ F.kicrtion 505 King Avenue Columbus, Ohio 43201 -2693 (61 4) 424-7737

Copyright 01986 Battelle Memorial Institute Columbus, Ohio