induction heat treatment technology

8/9/2019 Induction Heat Treatment Technology

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HEAT TREATING - PROCESS OVERVIEW

Heat treatment is the controlled heating and cooling of 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. Typicalheat t reating operations include:■ Anneal ing—used to sof ten met als t o improve formab il it y and

machinability.■ Hardening—used to increase the strength and deformation resis-

tance of metals (such as steels).■ Tempering—used to increase the toughness of hardened metals

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

A glossary at t he end o f thi s TechCommentary precisely definesthese and other important heat treating terms.

There are two broad categories of heat treating processes—thoseinvolving indirect heating and those using direct heating. Withindirect methods, heat is pro duced in a furnace by burning a fuel orby converting electrical energy into heat by passing a currentthrough resistance heating elements. This energy is then transferredto the workpiece by radiation, convection, or conduction. By contrast,direct heating methods, including induction, direct r esistance, andflame heating, supply heat directly to the workpiece. An in-depthdiscussion of how induc tion heating works is included in TechCom- mentary Vol. 2, No. 1.

Using Induction HeatTreatment to ObtainSpecial Properties CostEffectively

Heat treatment is often one of the

most important stages of metalprocessing because it determinesthe final properties that enablecomponents to perform under suchdemanding service conditions ashigh load, high temperature, andadverse environment. This Tech-Commentary illustrates how you canuse the special features of nductionheat treatment, such as its

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 with

induction? What are some typicalparts and materials that are induc-tion heat treated? What propertiescan 1 obtain with induction heattreatment?

The differences between inductionand conventional furnace-basedheat treating processes and someadvantages of induction heating arediscussed in TechCommentary

Vol. 2, No. 1. Advantages specific toinduction heat treating are:Speed —In heat treatment, thehigher heating rates play a centralrole in designing rapid, high-temperature heat treating processes.

Induction heat treating of steel maytake as little as 10 percent or less ofthe time required for furnacetreatment. Short heating times alsolead to less scaling for materialssuch as steels that oxidize readily athigh heat treating temperatures.Selective Heating — Controlling theheating pattern by selecting the rightinduction equipment allows surfaceand selective heat treatments thatyield an attractive blend of properties(e.g., high strength and toughness).Such treatments are not feasible withfurnace processes, which are slow

and heat the entire workpiece.Energy Savings —In addition toeliminating dwell periods, inductionheat treating techniques put energyonly where needed, improvingenergy efficiency.incr eased Product ion Rates —Rapid heating often increasesproduction and reduces labor.

Types Of Induction HeatTreatments

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

Heat Treatment With Induction Heating

DaWei Induction Heating Machine Co.,Ltd

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Figure 1 Hardness Of As-Quenched Martensite As A FunctionOf Its Carbon Cont ent

those heat treated in furnaces.In many applications, however,surface hardened and temperedsteel parts surpass their furnacetreated counterparts. This isbecause surface induction heatingleads to:■ A hard case and a soft core,

which provide a good blend ofstrength and toughness not attain-able with furnace through heating.Further, because the hardness ofas-quenched steels depends onlyon carbon content (Figure 1), thiscombination of properties can beobtained in inexpensive carbonsteels. Induction heat-treatedcarbon steel parts can be used inmany applications that requirealloy steels with good toughness,which are through heat treated infurnaces.

■ Compressive residual stresses atthe surface, which are importantin combination with the surfacehardness. These residual stressesarise primarily from the densitydifference between the hardmartensite layer and the softerinterior layer of pearlite or bainitein surface hardened parts.

The combination of a hard surface,compressive residual stresses, anda soft core results in excellent wearand fatigue resistance. Improvementin bending fatigue when comparedto furnace treatments is shown inFigure 2 for axle shafts. The attri-butes of induction treatment areparticularly attractive in bendingfatigue in which high levels of

tensile stress are generated at thesurface, and no stresses areimposed at the center. Here, the

property distribution in the surfacehardened part is well matched with thedemands placed on it in service.

Anothe r e xample is the wear resis-tance afforded gear teeth by -selective surface hardening. Proper-ties of induction surface hardenedparts are discussed further in Tech-Commentary (Vol. 2, No. 3).

Heat Treating ProcessesThat Compete WithInduction

Considerations of speed, selectiveheat treating requirements, and

process economics are often sufficientto establish whether you should useinduction or conventionalfurnace-based methods for throughheat treatment. On the other hand, ifyou've determined that surfacehardening is necessary, there are anumber of other processes to consider.These include conventional carburizing and nitri ding, ion-nitriding, laserhardening and electron beamhardening:

Carburizing and nit riding arewell-established technologies in whichsurfaces are alloyed with carbon ornitrogen by placing parts in a gaseous orliquid environment. The alloying resultsin surface hardening. Cost data fromcommercial heat treaters indicates that

the overall cost for induction hardeningis much less than for carburizing, saltbath nitriding, or gas nitriding. (The costratio for the 4 processes in the orderlisted is 0.11:2.5:1.75:8.) Thus, if yourpart geometry and production volumeallow the use of induction, it is thepreferred surface hardening methodfrom a cost standpoint.

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

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Figure 2 Bending Fatig ue Response Of Medium-Carbon SteelTractor Axl es Which Were Either Furnace Hardened OrInduction Hardened

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Figure 3Relation Between Diameter OfRound Steel Bars And MinimumGenerator Frequency For Effici ent

Au st en i ti zi ng Using Indu ct ionHeating

Coil Design —Solenoidal coils areeasily designed for round parts. For morecomplex parts, design procedures aredescribed in induction heating textbooksor can be provided by equipmentmanufacturers.

Generator Frequency — This isdetermined by the material, part size,and the need for through or surfaceheating. Low frequencies, which providelarge "penetration" depths of the inducededdy currents, are used for through heattreating. High frequencies are used forsurface heat treating. The minimumfrequencies for efficient throughaustenitizing of steel bars are shown inFigure 3. Frequencies for surfacehardening of steels are chosen to ensurepenetration depths of about twice therequired hardened depth. Othervariables, such as power density, areimportant in selecting frequency in theseinstances as well and are discussed in TechCommen- tary, Vol. 2, No. 3.

used to obtain shallow case- hardeneddepths (0.02 in. or less), lon-nitriding issimilar to other nitriding processesexcept that a glow-discharge method isemployed.

Laser and EB techniques both useextremely high-energy input rates thataustenitize a very thin surface layer in afraction of a second. The bulk of thesubstrate remains cool and provides anadquate heat sink for "self-quenching".

Adva nta ges inc lude:■ Minimal workpiece distortion■ Ability to sel ectively har den

portions of a surface and tocontrol the process in general

■ Ability t o harden areas inac ces-sible to conventional inductiontechniques

■ Repeatability • High speed. Accord ing to a recent American

Society for Metals (ASM) survey, ion-nitriding offers the greatest challenge toinduction heating for purposes ofsurface hardening. Laser and EBprocesses are also expected to satisfysome of the applications now handled byinduction, particularly those for whichinduction coils are difficult to design.Nevertheless, in situations in which casedepths of 0.02 to 0.04 in. are required,induction systems that produce very

high-power inputs per unit of surfacearea (i.e., "high intensity" inductionsetups) can compete effectivelybecause of lower equipment cost, higherproductivity, less maintenance, andlower floor space requirements. Forexample, in typical i ndustrialapplications, a 2 to 3 kilowatt laser,costing approximately $250,000 isrequired. This is about 3 times the costof comparable induction equipment.Electron-beam hardening also has someimportant limitations, such as the needfor a vacuum atmosphere.

Design Of Induction HeatTreatment Processes

To make induction heat treatmentwork for you, carefully select equipmentand understand the metallurgicalvariables that control heat treatingresponse. The most importantequipment parameters are coil design,generator frequency, and applied power density.

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Power Densit y (or power per unit ofsurface area) — The same power inputcan lead to a low heating rate for a largepart but a high heating rate for a smallpart. As with frequency, power density

level is selected based on the need forthrough or surface heat treating. Theschematic diagram in Figure 4illustrates the typical temperatureversus time behavior for an inductionheated part and is useful in explainingpower density effects. Because themagnitude of the eddy currents aregreatest at the surface and least at thecenter, the surface heats more rapidly.

After an ini tial transien t, the dif fer encein temperature remains fixed. Thisdifference represents an equilibriumbetween heat input via induction andheat transfer between the surface andcenter by conduction. As such, it is afunction of the applied power densityand workpiece electrical and thermalproperties. For through heating andheat treating, lower power densities of0.1 to 0.5 kW/in? are recommended toensure a more or less uniform heatingpattern. High power densities(approximately 10 to 20 kW/in.z) areneeded for surface heating and heattreatments of steel.

Using data on coil design and powersupply requirements, you can figure outwhere the heating power is going to belocated in your heat treating process.To ensure success, however, it isimportant to understand themetallurgical properties of theworkpiece and how these interact withyour heating cycle. Of utmostimportance is the design of thenecessary time-temperature cycles,which are typically quite different fromthose for furnace heat treatments.

The data in Table 1 reveal that highertemperatures are employed for

induction austenitizing of steels. Youneed these to compensate for short orzero soak times. Similarly, short-timetempering treatments involving short orzero soak time can be designed aroundinduction heating processes by usingthe so- called tempering parameter,

T(14.44 + log 10 t ), where T is the tempering temperature indegrees Rankine and t is the temperingtime in seconds. An inductiontempering treatment



Glossary Of Heat Treating Terms*

Alloy Steel —St eel con taining signif i-cant quantities of alloying elements(other than carbon and the commonly

accepted amounts of manganese,silicon, sulfur, and phosphorous)added to effect changes in themechanical or physical properties.

Anneal in g —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 annealingprocesses include:Recrystallization annealing —Anneal-

ing cold or hot worked metal toproduce a new grain structurewithout phase change.Spheroidization anneating — Heatingand 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 tominimize the development of newresidual stresses.

Austeni ti zing — Forming of the face-centered-cubic austenite phase insteel by heating above the trans-formation temperature range.Process forms the basis of hardeningof steels.Bainite —A 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. Itsappearance is feathery if formed inthe upper part of the temperaturerange; acicular, resembling temperedmartensite, if formed in the lowerpart.Carbon Steel —Iron alloy containingcarbon up to about 2 percent andonly residual quantities of otherelements 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 iro n andcarbon found frequently in steelsknown chemically as iron carbide andhaving the approximate chemicalformula Fe.C.

Ferrite — A solid solution of 1 or moreelements in body-centered cubic iron;the solute is generally assumed to becarbon unless designated otherwise.Hardenability — In steels, the propertythat determines the depth anddistribution of hardness induced byaustenitizing and quenching.Hardenability is a function of alloycomposition and quenching medium.

Hardening — Increasing the hardness bysuitable treatment, usually involvingheating and cooling. For steels, thistypically consists of austenitizingfollowed by cooling to form pearlite,bainite, or martensite, or a combinationof these constituents.

Martensite —A metastable phase ofsteel formed by a transformation ofaustenite below the M s temperatureand composed of a body-centeredtetragonal lattice. Its microstructure ischaracterized by an acicular, orneedle-like, pattern.

Microstructure —The structure ofpolished and etched metals as revealedby a microscope at a magnificationgreater than 10 diameters.Normalizing — Heating a ferrous alloy toa suitable temperature above thetransformation range and then cooling in

air to a temperature substantially belowthe transformation range.

Pearlite —A lamellar aggregate of ferriteand cementite, often occurring in steeland cast iron.Quench Hardening — Hardening aferrous alloy, such as a steel, byaustenitizing and then cooling rapidlyenough so that some or all of theaustenite transforms to martensite.

Tempering — Reheating a normalized orquench-hardened ferrous alloy such as asteel to a temperature below thetransformation range and then cooling at

any rate desired.Transformation Temperature —Thetemperature at which a change in phaseoccurs. The term is sometimes used todenote the limiting temperature of atransformation range.....

Time

Figure 4 "Schematic Illust ration Of The Surface

An d Cen te r Temperatur e Hi s tor ies Of ABar Heated By Ind ucti on (Note that,following an initial transient, thesurface-to- center temperaturedifference is con stant during theheating cycl e.)

whose tempering parameter is equivalentto that for a longer-time,lower-temperature treatment will give apart with equivalent properties. Theapplication of this concept is discussedfurther in TechCommentary, Vol. 2, No.4.

A fi nal f actor t hat warran ts consider ati onin process design is the starting condition ofthe workpiece. This is particularlyimportant for steels that are to be hardened.

In these cases, austenitizing behavior willbe affected greatly by the startingmicrostructure. An example of thismicrostructure effect is seen in Figure 5. Asshown here, when using the same inductionheating parameters, the case hardeneddepth for a 1070 steel bar increases as thestarting structure becomes finer: quenchand tempered = fine martensite structure,normalized = 'ine pearlite with thin lamellarcarbides, and annealed = structure withcoarse, difficult-to-dissolve spheroidalcarbides. Such variations in induction heattreatment results can be overcome Source: Metals Handbook. Vol. 1, Eighth

Edition,American Society For Metals,1961.

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Figure 5 Effect Of Starting Microst ructu re In 1070 Steel Bars On SurfaceHardening Response Using A 450 kHz Induct ion GeneratorOperated At A Power Density Of 2.5 Kilowatts Per SquareCentimeter (15.9 kilowatt s per s quare in.)

by determining (on a trial-and-errorbasis) modified peak temperatures orheating rates for your particular alloyand expected variations in startingcondition.

In SummaryInduction heat treatment is appli-

cable to hardening, tempering,normalizing, and annealing a widerange of parts, particularly in ferrous

alloys — medium-and high-carbon steels,alloy and stainless steels, and toolsteels. It is also finding increasingapplication in the nonferrous metalsindustry. Advantages include highheating rates, selective heating

capability, improved production rates,and energy savings. Induction heattreatment can produce surface hardenedparts with soft cores that exhibitexcellent wear and fatigue resistance.However, satisfactory heat treatmentrequires careful selection of equipmentand good understanding of themetallurgical variables involved. Forsurface hardening applications, thereare a number of competing processes toconsider. □

This TechCommentary provides anoverview. It is intended to familiarize youwith the important applications ofinduction heat treatment. If you areinterested in a more detailed background,please refer to subsequent issues of TechCommentary on "Surface andSelective Heat Treatment" (Vol. 2, No. 3),"Induction Temperin g" (Vol. 2, No. 4), andthe sources listed below.

The Center's mission is to assist industry inimplementing cost- and energy-efficientelectric-based technologies in the metalsfabrication and related fields. TechCommentary is one communicationvehicle that the Center uses to transfertechnology to industry. The Center alsoconducts research in metal heating, metalremoval and finishing, and fabrication.This issue of TechCommentary wasmade possible through the cooperation ofBattelle staff members Lee Semiatin, author;Jack Mortland, editor; Laura Cahill,publications coordinator. Sources used inthis issue of TechCommentary were:Induction Hardening and Tempering,

American Society for Metals, 1964, T.H.Spencer, et al.Induction Heat Treatment of Steel, AmericanSociety for Metals, 1985, S.L. Semiatin andD.E. Stutz.

Metals Technology, Vol. 9, No. 12, pp.493-498, The Metals Society, 1982,"Transverse Flux Induction Heating of

Aluminum-Alloy Strip, " R. W aggott , 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 of

ASST, Vol. 12, p. 871, 1927, W.P. Sykes andZ. Jeffries. High-Frequency InductionHeating, McGraw- Hill Book Company, 1950,F.W. Curtis.Basics of Induction Heating, John F. Rider,1960, C.A. Tudbury. Induction Hardeningand Tempering, American Society for Metals,1964, T.H. Spencer, et al.

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