Induction PWHT

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. . .. . .. . .-Publishedyhe PRl Center for Materialsabrication Vol. 2, No . 2, 1985ReprintedMarch, 1990

Using Induction HeatTreatment to ObtainSpecial PropertiesCost Effectively

Heat treatment is often one of themost important stages of metalprocessing because it determinesthe final properties that enablecqmponents to perform under such

demanding service conditions ashiah oad, hioh temperature, and

speed and selective heating capabil-ity, to produce quality parts costeffectively. It will answer such ques-tions as: What are the advantagesof induction heat treatment? Whatheat treatments can I conduct withinduction? What are some typicalparts and materials that are induc-tion heat treated? What propertiescan I obtain with nduction heat

treatment?The differences between induction

aiverse environment. This Tech- and conventional furnace-based

Commentary illustrates how you heat treating processes and some

can use the special features of advantages of induction heating are

nduction heat treatment, such as its discussed in TechCommentary

& . HEAT TREATING- PROCESS OVERVIEW

Heat treatment is the controlled heating and cooling f a solidmetal or alloy to obtain desired properties. Depending on the materialand its intended use, heat treatment can improve such characteris-tics as formability, machinability, and service performance. Typical

heat treating operations include:

Annealing-used to soften metals to improve formability and

m Hardening-used to increase the strength and deformation resis-

m Tempering-used to increase the toughness of hardened metals

machinability.

tance of metals (such as steels).

and thereby improve their resistance to brittle, catastrophic failurein high-stress. high-integrity applications.

A glossary at the end of this TechCommentary precisely definesthese and other important heat treating terms.

There are two broad categories of heat treating processes-thoseinvolving ind irect heating and those using direct heating. Withindirect methods, heat is produced in a furnace by burning a fuel or

by converting electrical energy into heat by passing a current

through resistance heating elements. This energy is then transferredto the workpiece by radiation, convection, or conduction.By contrast,

direct heating methods, including induction, direct resistance, andflame heating, supply heat directly to the workpiece. An indepthdiscussion of how induction heating workss included in TechCom-

mentary Vol. 2, No. 1. .

Vol. 2, N o. 1. Advantages specific toinduction heat treating are:

Speed- n heat treatment, thehigher heating rates play a centralrole n designing rapid, high-temperature heat treatingprocesses. Induction heat treating ofsteel may take as little as10 percent or less of the timerequired for furnace treatment.

Short heating times also lead to lessscaling for materials such as steelsthat oxidize readily at high heattreating temperatures.

Selective Heating-Controlling theheating pattern by selecting the rightinduction equipment allows surface

and selectwe heat treatments thatyield an attractive blend of proper-ties (e.g., high strength and tough-ness). Such treatments are notfeasible with furnace processes,which are slow and heat the entireworkpiece.

Energy Savings- n addition toeliminating dwell periods, nductionheat treating techniques put energyonly where needed, mproving

energy efficiency.

increased ProductionRates-Rapid heating often ncreasesproduction and reduces labor.

Types Of InductionHeat Treatments

You can utilize the speed andselective heating characteristics ofinduction processes for:

, Hardening of Steels-Steels arehardened by heating to austenitizingtemperatures and then quenching.The speed of induction heating andminimal soak time mandate higheraustenitizing temperatures thanthose used with furnace processes(Table 1). Depth of austenitization is



70

$ 60

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3 30

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0 0.20 0.40 0.60 0.80 1 oo 1.20 1.40.60

Com po s i t i on (pc t . c a rbon)

Figure 1 Hardness Of As-Quenched Martensite A s A FunctionOf Its Carbon Con tent

those heat treated n furnaces.

In many applications, however,

surface hardened and tempered

steel parts surpass their furnace

treated counterparts. This isbecause shrface nduction heating

leads to:

A hard case and a soft core,

which provide a good blend of

strength and toughness not attain-

able with furnace through heating.

Further, because the hardness of

as-quenched steels depends only

on carbon content (Figure l ) , his

combination of properties can be

obtained n nexpensive carbon

steels. Induction heat-treated

carbon steel parts can be used in

many applications that require

alloy steels with good toughness,

which are through heat treated in

furnaces.

m Compressive residual stresses at

the surface, which are important

in combination with the surface

hardness. These residual stresses

arise primarily from the density

difference between the hard

martensite layer and the softer

interiorayer of pearlite r ainite

in surface hardened parts.

The combination of a hard surface,compressive residual stresses, and

a soft core results in excellent wear

and fatigue resistance. Improvement

in bending fatigue when compared

to furnace treatments is shown in

Figure 2 for axle shafts. The attri-

butes of induction treatment are

particularly attractive in bending

fatigue in which high levels of

tensile stress are generated at the

surface, and no stresses are

imposed at the center. Here, the. .

property distribution in the surface

hardened part s well matched with

the demands placed on it in service.

Another example is the wear resis-

tance afforded gear teeth by,'-selective surface hardening. Proper-

ties of induction surface hardened

parts are discussed further in Tech-Commentary (Vol. 2, No. 3) .

Heat Treating Processes

That Compete With

Induction

heat treating requirements, and

Considerations of speed, selective

process economics are often suffi-

cient to establish whether you

should use nduction or conventional

furnace-based methods for through

heat treatment. On the other hand, if

you've determined that surface

hardening s necessary, there are a

number of other processes toconsider. These include conven-

tional carburizing and nitriding,ion-nitrid ing, laser hardening and

electron beam hardening.

Carburizing and nitridi ng are

welLestablished technologies n

which surfaces are alloyed with

carbon or nitrogen by placing parts

in a gaseous or liquid environment.

The alloying results n surface

hardening. Cost data from commer-

cial heat treaters ndicates that the

overall cost for nduction hardening

is much ess than for carburizing,

salt bath nitriding, or gas nitriding.

(The cost ratio for the 4processes in the order listed is

0.1 1 :2.5:1.75:8.) Thus, if your part

geometry and production volume

allow the use of induction, it s the

preferred surface hardening method

from a cost standpoint.

Ion-nitriding, laser hardening,and electron-beam (EB) hardeningare emerging technologies that are

70 IMedium-carbon s teel

axle shaf ts

60I

10 I t I0.01 0.1 1 10

M i l l i o n s of Cy c les to Failure .

Figure 2 Bending Fatigue Response Of Medium-Carbon SteelTractor Axles Which Were Either Furnace Hardened OrInduct ion Hardened



used to obtain shallow case-

hardened depths (0.02 n. or less).

lon-nitriding is similar to other

nitriding processes except that a

glow-discharge method s employed.

Laser and EB techniques both use

extremely high-energy nput rates

that austenitize a very thin surface

layer in a fraction of a second. The

bulk of the substrate remains cooland provides an adquate heat sink

for “self-quenching”. Advantages

include:

Minimal workpiece distortion

m Ability to selectively harden

portions of a surface and tocontrol the process in general

m Ability to harden areas inacces-

sible to conventional nduction

techniques

Repeatability

High speed.

According to a recent American

Society for Metals (ASM) survey, on-

nitriding offers the greatest

challenge to induction heating for

purposes of surface hardening.

Laser and EB processes are also

expected to satisfy some of the

applications now handled by induc-

tion, particularly those for which

induction coils are difficult to design.

Nevertheless, in situations in which

case depths of 0.02 to 0.04 in. are

required, induction systems that

produce very high-power inputs perunit of surface area (i.e., “high

intensity” nduction setups) can

compete effectively because of

lower equipment cost, higher

productivity, ess maintenance, and

lower floor space requirements. For

example, n typical ndustrial

applications, a 2 to 3 kilowatt aser,

costing approximately $250,000 is

required. This is about 3 times the

cost of comparable induction equip-

ment.lectron-beamardeninglsohas some important limitations, such

as the need for a vacuumatmosphere.

Design Of Induction Heat

Treatment Processes

To make induction heat treatment

work for you, carefully select equip-

ment and understand the metallurgi-

cal variables that control heat treat-

ing response. The most mportant

equipment parameters are coil

design, generator frequency, and

applied power density.

0 .05 .1 .2 .5 1 2

Diameter , (in.)

Figure 3Relation Between Diameter o fRound Steel Bars And Minimum

Generator Frequency For Efficient

Austenitizingsingnduction ~

Heating

Coil Design -Solenoidal coilsare easily designed for round parts.

For more complex parts, design

procedures are described n nduc-

tion heating textbooks or can be

provided by equipment

manufacturers.

determined by the material, part

size, and the need for through or

surface heating. Low frequencies,

which provide arge “penetrat ion”

depths of the induced eddy currents,

are used for through heat treating.

High frequencies are used for

surface heat treating. The minimum

frequencies for efficient through

austenitizing of steel bars are shown

in Figure 3. Frequencies for surface

hardening of steels are chosen toensure penetration depths of about

twice the required hardened depth.

Other variables, such as power

density, are mportant n selecting

frequency in these instances as well

and are discussed in TechCommen-

tary, Vol. 2, No. 3.

Generator Frequency -This is

Power Densit y (or power per unit

of surface area)-The same power

input can ead to a ow heating rate

fo r a arge part but a high heating

rate for a small part. As with

frequency, power density level is

selected based on the need for

through or surface heat treating.

The schematic diagram in Figure 4

illustrates the typical temperatureversus time behavior for an nduc-

tion heated part and s useful n

explaining power density effects.

Because the magnitude of the eddy

currknts are greatest at the surface

and east at the center, the surface

heats more rapidly. After an nitial

transient, the difference n tempera-

ture remains fixed. This difference

represents an equilibrium between

heat nput via nduction and heat

transfer between the surface and

center by conduction. As such, it is

a function of the applied powerdensity and workpiece electrical and

thermal properties. For through

heating and heat treating, ower

power densities of 0.1 to 0.5 kWlin?

are recommended to ensure a more

or less uniform heating pattern. High

power densities (approximately 10 to20 kWlin.2) are needed for surface

heating and heat treatments of

steel.

Using data on coil design and

power supply requirements, you can

figure out where the heating power

is going to be located in your heattreating process. To ensure

success, however, it s important to

understand the metallurgical proper-

ties of the workpiece and how these

interact with your Heating cycle. Of

utmost importance is the design of

the necessary time-temperature

cycles, which are typically quite

different from those for furnace heat

treatments.

The data n Table 1 reveal that

higher temperatures are employed

for induction austenitizing of steels.

You need these to compensate forshort or zero soak times. Similarly,

short-time tempering treatments

involving short or zero soak time

can be designed around induction

heating processes by using the so-called tempering parameter,

T(14.44+ og,&,

where T is the tempering tempera-

ture in degrees Rankine and t is the

tempering time in seconds. An

induction tempering treatment



La,

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8lossary Of Heat Treating Terms*

Figure 4

Schematic Illus tration Of TheSurface And Center TemperatureHistories Of A Bar Heated ByInduction (Note that, followingan ini tial transient, the surf ace-to-center temperature difference isconstant during the heating cycle.)

whose tempering parameter isequivalent to that for a longer-time,

lower-temperature treatment willgive a part with equivalent proper-ties. The application of this concept

is discussed further in TechCom-mentary, Vol. 2, No. 4.

consideration in process design isthe starting condition of the work-

piece. This is particularly important

for steels that are to be hardened. In

these cases, austenitizing behavior

will be affected greatly by the start-

ing microstructure. An example ofthis microstructure effect is seen in

Figure 5. As shown here, whenusing the same induction heating

parameters, the case hardeneddepth for a 1070 steel bar increasesas the starting structure becomesfiner: quench and tempered = finemartensite structure, normalized ='ine pearlite with thin lamellarcarbides, and annealed = structure

with coarse, difficult-todissolve

spheroidal carbides.

A final factor that warrants

Alloy Steel -Steel containing signifi-cant quantities of alloying elements(other than carbon and the commonlyaccepted amounts of manganese,silicon, sulfur, and phosphorous)added to effect changes in themechanical or physical properties.

Annealing -Heating to and holdingat a suitable temperature and thencooling at a suitable rate for suchpurposes as reducing hardness,improving machinability, facilitatingcold working, producing a desiredmicrostructure, or obtaining desiredmechanical, physical, or other proper-ties. Specific types of annealingorocesses include:

Recrystallization annealing "Anneal-ing cold or hot worked metal to

produce a new grain structurewithout phase change.

Spheroidization anneating- eating

and cooling to produce a spheroidalor globular form of carbide in steel.

Stress relief annealing-Heating to asuitable temperature, holding longenough to reduce residual stresses,and then cooling slowly enough to

minimize the development of new

residual stresses.

Austenitizing- orming of the face-centered-cubic austenite phase insteel by heating above the trans-formation temperature range.Process forms the basis of hardeningof steels.

Bainite- decomposition product ofaustenite consisting of an aggregateof ferrite and carbide. In general, itforms at temperatures lower thanthose where very fine pearlite formsand higher than that where marten-site begins to form on cooling. It sappearance is feathery if formed inthe upper part of the temperaturerange; acicular, resembling temperedmartensite, f formed in the- lowerpart.

Carbon Steel- ron alloy containingcarbon up to about 2 percent andonly residual quantities of other

elements except those added fordeoxidation. with silicon Usuallylimited to 0.60 percent and manga-nese to 1.65 percent. Also termedplain-carbon steel.

Cementite - A compound of iron andcarbon found frequently in steelsknown chemically as iron carbide andhaving the approximate chemicalformula Fe,C.

Ferrite-A solid solution of 1 or moreelements n body-centered cubic ron,the solute is generally assumed to becarbon unless designated otherwlse.

Hardenabi lity- In steels, theproperty that determines the depthand distribution of hardness nduced

by austenitizing and quenching.Hardenability is a function of alloycomposition and quenching medium.

Hardening- ncreasing thehardness by suitable treatment,usually involving heating and coolingFor steels, this typically consists ofaustenitizing followed by cooling to

form pearlite, bainite, or martenslte,or a combination of theseconstituents.

Martensite A metastable phase of

steel formed by a transformation ofaustenite below the M, temperature

and composed of a body-centeredietragonal attice. It s microstructureis characterized by an acicular, orneedle-like, pattern.

Microstructure-The structure ofpolished and etched metals asrevealed by a microscope at amagnification greater than 10

diameters.

Normalizing- eating a fe:rousalloy to a suitable temperature abovethe transformation range and thencooling in air to a temperaturesubstantially below the transformatior

range.Pearlite-A lamellar aggregate offerrite and cementite, often occurringin steel and cast iron.

Quench Hardening- ardening aferrous alloy, such as a steel, byaustenitizing and then cooling rapidlyenough so that some or all of theaustenite transforms to martensite.

Tempering -Reheating a normalizecor quench-hardened ferrous alloysuch as a steel to a temperaturebelow the transformation range andthen cooling at any rate desired.

Transformation Temperature-Thetemperature at which a change inphase occurs. The term is sometime:used to denote the limiting tempera-ture of a transformation range. ,

-Source: Melals Handbook. Vol. 1. EighthEdition.American SocietyFo r Melals.196

Such variations in induction heat

treatment results can be overcome



70

60

50

v)

(0

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$ 40

X0

$ 30

-

x0

z

20

1 0 L I I

S 0.02 0.04 0.06 0.08 0.1 0

Distance Below Surface (in.)

Figure 5 Effect Of Starting Microstructur e In 1070 Steel Bars OnSurface Hardening Response Using A 450 kHzInduct ion Generator Operated At A Power Density Of2.5 Kilo watts Per Square Centimeter (15.9 kilowatts persquare n.)

by determining (on a trial-and-error

basis)modifiedeakemperaturesnductioneatreatment isppli-

or heating ates oryourparticularcable to hardening, empering,

alloy nd xpected ariationsn ormalizing, nd nnealingwide

startingondition.angefarts,articularlynerrous

In Summary

alloys-medium- and high-carbon

steels, alloy and stainless steels,and tool steels. I t is also inding

increasing application n the nonfer-

rous metals ndustry. Advantages

include high heating rates, selective

heating capability, mproved produc-

tion rates, and energy savings.

Induction heat treatment can

produce surface hardened partswith soft cores that exhibit excellent

wear and fatigue resistance. How-

ever, satisfactory heat treatment

requires careful selection of equip-

ment and good understanding of the

metallurgical variables nvolved. Fo

surface hardening applications,

there are a number of competing

Drocesses to consider.0

This TechCornrnentary provides

an overview. It is intended to famil-

iarize you with the importantapplications of induction heat treat

ment. If you are nterested n a more

detailed background, please refer tsubsequent issues of TechCorn-rnentary on “Surface and Selective

Heat Treatment” (Vol. 2, No. 3),“Induction Tempering” (Vol. 2, No.4), and he sources isted below.

The Center for Materials Fabrication (CMF) is operated by Battelle’s Columbus Division. Bast funding is provided by the ElectricPower Research Institute, a nonprofit institute that conducts research and development on behalf of the United States electricutili!y industry

The Center’s mission is to assistindustry in implementing cost- andenergy-efficient electric-basedtechnologies in the metals fabricationand related fields. TechCommentary isone communication vehicle that theCenter uses to transfer technology toindustry. The Center also conductsresearch in metal heating, metalremoval and finishing, and fabrication.

This issue of TechCommentary wasmade possible through the cooperationof Battelle staff members Lee Semiatin,author; Jack Mortland, editor; Laura

Cahill, publications coordinator.Sources used in this issue ofTechCommentary were:

Induction Hardening and Tempering,American Society for Metals, 1964, T.H.Spencer, et al.

Induction Heat Treatment of Steel,American Society for Metals, 1985, S.L.Semiatin and D.E. Stutz.

Metals Technology, Vol. 9, No . 12, pp.493-498, The Metals Society, 1982,“Transverse Flux Induction Heating ofAluminum-Alloy Strip,” R. Waggott, et al.

Sources for tables and figures were:

Transactions of ASM, Vol. 26, pp. 1-36,American Society for Metals, 1938,“Quantitative Hardenability,” J.L. Burns,T.L. Moore, and R.S. Archer.

Transactions o f ASST, Vol. 12, p. 871,1927, W.P. Sykes and Z. Jeffries.

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

Curtis.Basics of Induction Heating, John F.Rider, 1960, C.A. Tudbury.

Induction Hardening and Tempering,American Society for Metals, 1964, T.H.Spencer, et al.

Appli cable SIC Codes

33- 12, 15, 16, 17, 21, 34, 41, 51, 53,54, 55, 56, 98, 99

52, 62, 63, 71.-,79, 83, 84, 89, 9395, 99

45, 46, 47, 62, 66, 67, 68, 92, 99

34- 11,12, 21, 23, 25, 29, 41, 43, 49,

35- 23, 24, 31, 32, 33, 36, 41, 42, 44,

36-2137- 11, 14, 21, 24, 28, 43, 51, 61, 64

69, 95, 99

For further information on Centerprograms, write or call

CENTER FOR MATERIALS FABRICATIO

An EPRl RBD Apphcallons Center

505 King Avenue - Columbus, Ohio 43201-2693

(614) 424-7737

Copyright 1985

Battelle Memorial InstituteColumbus,Ohio Reprinted39

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