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Using Induction Heat speed and selective heating capabil- Vol. 2, No. 1. Advantages specific to Treatment to Obtain ity, to produce quality parts cost induction heat treating are: Special Properties effectively. It will answer such ques- Speed - In heat treatment, the Cost Effectively tions as: What are the advantages higher heating rates play a central of induction heat treatment? What role in designing rapid, high- Heat treatment is often one of the heat treatments can I conduct with temperature heat treating most important stages of metal induction? What are some typical processes. Induction heat treating of processing because it determines parts and materials that are induc- steel may take as little as the final properties that enable tion heat treated? What properties 10 percent or less of the time cqmponents to perform under such can I obtain with induction heat required for furnace treatment. demanding service conditions as treatment? Short heating times also lead to less hiah load, hioh temperature, and The differences between induction scaling for materials such as steels aiverse environment. This Tech- and conventional furnace-based that oxidize readily at high heat Commentary illustrates how you heat treating processes and some treating temperatures. can use the special features of advantages of induction heating are Selective Heating-Controlling the nduction heat treatment, such as its discussed in TechCommentary heating pattern by selecting the right induction equipment allows surface and selectwe heat treatments that \ yield an attractive blend of proper- &. HEAT TREATING - PROCESS OVERVIEW ties (e.g., high strength and tough- Heat treatment is the controlled heating and coolingof a solid ness). Such treatments are not metal or alloy to obtain desired properties. Depending on the material feasible with furnace processes, and its intended use, heat treatment can improve such characteris- which are slow and heat the entire tics as formability, machinability, and service performance. Typical workpiece. heat treating operations include: Energy Savings - In addition to Annealing-used to soften metals to improve formability and eliminating dwell periods, induction machinability. heat treating techniques put energy only where needed, improving m Hardening-used to increase the strength and deformation resis- energy efficiency. tance of metals (such as steels). increased ProductionRates m Tempering-used to increase the toughness of hardened metals - and thereby improve their resistance to brittle, catastrophic failure Rapid heating often increases in high-stress. high-integrity applications. production and reduces labor. A glossary at the end of this TechCommentary precisely defines Types Of Induction these and other important heat treating terms. There are two broad categories of heat treating processes-those Heat Treatments involving indirect heating and those using direct heating. With You can utilize the speed and indirect methods, heat is produced in a furnace by burning a fuel or selective heating characteristics of by converting electrical energy into heat by passing a current induction processes for: through resistance heating elements. This energy is then transferred , Hardening of Steels-Steels are to the workpiece by radiation, convection, or conduction.By contrast, hardened by heating to austenitizing direct heating methods, including induction, direct resistance, and temperatures and then quenching. flame heating, supply heat directly to the workpiece. An indepth The speed of induction heating and discussion of how induction heating worksis included in TechCom- minimal soak time mandate higher mentary Vol. 2, No. 1. . austenitizing temperatures than those used with furnace processes (Table 1). Depth of austenitization is Furnace Heating, Induction Heating, alloys are induction heat treaied F (C) F (C).. less often than steels. Pending the development of an information base 1650 to 1700 (900 to 925) 1550 to 1600 (845 to 870) on process economics, heat treat- 1525 to 1575 (830 to 855) 1650 (900) ments for titanlum and nickel-base 1525 to 1575 (830 to 855) 1600 to 1650 (870 to 900) alloys, whch are readily heated by 1475 io 1550 (800 to 845) 1600 to 1650 (870 to 900) induction, could become a reality in 1475 to 1550 (800to 845) 1600 (870) the future. Technical And Economic Considerations Determining whether induction heat treatment is suited to your needs requlres evaluation of such easily controlled so that parts can ment for a wide range of parts. The technical factors as: be surface or through hardened. brief list of common applications Workpiece Size and Shape- Tempering of Steels-Tempering that follows is only representative of These influence the shape of the of hardened steels is a function of its potential use. induction coil to be used. It is easiest to design coils for symmetrlc both time and temperature. Induc- Surface Hardening and Tempering parts. However, induction heat treat- tion tempering uses shorter heating of Steels: times (usually only seconds) and ing is readily applied to irregularly higher temperature to produce Transportation-crankshafts, shaped components by using results equivalent to furnace temper- camshafts, axle shafts, transmis- special coil designs. sion shafts, spllned shafts, univer- ing treatments that often require Subsequent Processing- Should .hours. sal joints, gears, valce seats, wheel spindles, bearings. permit easy integration of induction Normalizing -Since normalizing Machine tools-lathe beds, heat treating. This includes, for requires heating to a high tempera- transmission gears, shafts. example, the design of quench ture followed by air cooling, it is an stations following steel hardening ideal application of induction Metalworking, hand tools- rolling operations. The high speed of induc- heating. Induction normalizing mill rolls, pliers, hammers. tion heating may accommodate treatments are often used for Through-Hardening and machining operations on the same products that can be processed on Tempering of Steels: processing line. a continuous basis, such as pipe. m Oil-country tubular products, struc- Metallurgical Condition- Must the Weld Seam Annealing-The speed tural members, spring steel, chain part meet special service require- and selectivity of induction heating links. ments that induction heating can can be called on for local stress Annealing: provide, such as a blend of high relief annealing of welds in sheet Longitudinally welded steel tubes, strength and toughness? Some of metal, tubular, and other kinds of nonferrous sheet metals, ferrous the properties and characteristics of products. and nonferrous wire, steel pipes, induction heat treated parts are Recrystallization Annealing- tanks. discussed below. Annealing treatments for sheet Induction heat treating is also useful In addition to evaluating technical metal have been carried out for a wide range of materials, but feasibility, you must also determine successfully using a new process steel parts comprise the largest if induction heating is cost effective called “transverse flux” (TFX) induc- portion of induction heat-treated ,Nhen compared to other techniques. tion heating, in which sheet is fed components. Some typical induction Important economic factors in your continuously through a specially heat-treated steels are decision should include production lot size, equipment and energy designed coil. Total heating time is Medium-carbon steels, such as only a few seconds, compared to costs, labor requirements, scale and 1030 and 1045, used for welded scrap losses, and floor space many hours for conventional furnace tubing, automotive shafts, and or batch annealing of coils of sheet requirements. The way each of gears metal. Moreover, the fine tempera- these elements figure in an overall ture control with induction heating High-carbon steels, such as 1070, cost analysis is discussed in detail permits partial as well as full used for hand tools in TechCommentary, Vol. 2, No. 1. recrystallization annealing, depend- Alloy and stainless steels used for ing on the annealing temperature bearings, automotive valves, and Properties Of Induction chosen. Thus, a range of properties, machine tool components Heat Treated Parts not obtainable with batch annealing, Tool steels, such as M2 and D2, m The largest amount of property used for cutting tools and metal- is possible. information on induction heating is working dies. Induction Heat Treating for hardened and tempered steel The induction annealing of parts. Such data indicate that induc- Applications And Materials aluminum and its alloys is certainly tion through heat-treated parts have You can use induction heat treat- feasible, but, at present, nonferrous mechanical properties similar to 70 process economics are often suffi- cient to establish whether you $ 60 should use induction or conventional 0) C furnace-based methods for through E 50 Q heat treatment. On the other hand, if X you've determined that surface 0 40 - hardening is necessary, there are a al 3 30 number of other processes to 0 consider. These include conven- 20 tional carburizing and nitriding, ion-nitriding, laser hardening and 10 electron beam hardening. 0 0.20 0.40 0.60 0.80 1.oo 1.20 1.60 1.40 Composition (pct. carbon) Carburizing and nitriding are welLestablished technologies in Figure- 1 Hardness Of As-Quenched Martensite As A Function which surfaces are alloyed with Of Its Carbon Content carbon or nitrogen by placing parts in a gaseous or liquid environment. The alloying results in surface hardening. Cost data from commer- those heat treated in furnaces. property distribution in the surface cial heat treaters indicates that the In many applications, however, hardened part is well matched with overall cost for induction hardening surface hardened and tempered the demands placed on it in service. is much less than for carburizing, steel parts surpass their furnace Another example is the wear resis- salt bath nitriding, or gas nitriding. treated counterparts. This is tance afforded gear teeth by,'- (The cost ratio for the 4 because shrface induction heating selective surface hardening. Proper- processes in the order listed is leads to: ties of induction surface hardened 0.1 1 :2.5:1.75:8.) Thus, if your part parts are discussed further in Tech- A hard case and a soft core, geometry and production volume which provide a good blend of Commentary (Vol. 2, No. 3). allow the use of induction, it is the preferred surface hardening method strength and toughness not attain- Heat Treating Processes able with furnace through heating. from a cost standpoint. That Compete With Further, because the hardness of Ion-nitriding, laser hardening, as-quenched steels depends only Induction and electron-beam (EB) hardening on carbon content (Figure l), this Considerations of speed, selective are emerging technologies that are combination of properties can be heat treating requirements, and obtained in inexpensive carbon steels. Induction heat-treated 70 I carbon steel parts can be used in Medium-carbon steel many applications that require axle shafts alloy steels with good toughness, I which are through heat treated in 60 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 . interiorlayer of pearliteor bainite 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 10 I t I particularly attractive in bending 0.01 0.1 1 10 fatigue in which high levels of Millions of Cycles to Failure . tensile stress are generated at the Figure 2 Bending Fatigue Response Of Medium-Carbon Steel surface, and no stresses are Tractor Axles Which Were Either Furnace Hardened Or imposed at the center. Here, the Induction Hardened I .. used to obtain shallow case- Power Density (or power per unit hardened depths (0.02 in. or less). of surface area)-The same power lon-nitriding is similar to other input can lead to a low heating rate nitriding processes except that a for a large part but a high heating glow-discharge method is employed. rate for a small part. As with Laser and EB techniques both use frequency, power density level is extremely high-energy input rates selected based on the need for that austenitize a very thin surface through or surface heat treating. layer in a fraction of a second. The The schematic diagram in Figure 4 bulk of the substrate remains cool illustrates the typical temperature and provides an adquate heat sink versus time behavior for an induc- for “self-quenching”. Advantages tion heated part and is useful in include: explaining power density effects. Minimal workpiece distortion Because the magnitude of the eddy currknts are greatest at the surface m Ability to selectively harden and least at the center, the surface portions of a surface and to heats more rapidly. After an initial control the process in general transient, the difference in tempera- m Ability to harden areas inacces- ture remains fixed. This difference sible to conventional induction represents an equilibrium between techniques heat input via induction and heat Repeatability transfer between the surface and High speed. center by conduction. As such, it is a function of the applied power According to a recent American density and workpiece electrical and Society for Metals (ASM) survey, ion- thermal properties. For through nitriding offers the greatest .1 .2 1 2 heating and heat treating, lower challenge to induction heating for 0 .05 .5 Diameter, (in.) power densities of 0.1 to 0.5 kWlin? purposes of surface hardening. are recommended to ensure a more Laser and EB processes are also or less uniform heating pattern. High expected to satisfy some of the Figure 3 power densities (approximately 10 to applications now handled by induc- Relation Between Diameter of 20 kWlin.2) are needed for surface tion, particularly those for which Round Steel Bars And Minimum heating and heat treatments of induction coils are difficult to design. Generator Frequency For Efficient steel. Nevertheless, in situations in which AustenitizingUsing Induction ~ Using data on coil design and case depths of 0.02 to 0.04 in. are Heating power supply requirements, you can required, induction systems that figure out where the heating power produce very high-power inputs per Coil Design -Solenoidal coils is going to be located in your heat unit of surface area (i.e., “high are easily designed for round parts. treating process. To ensure intensity” induction setups) can For more complex parts, design success, however, it is important to compete effectively because of procedures are described in induc- understand the metallurgical proper- lower equipment cost, higher tion heating textbooks or can be ties of the workpiece and how these productivity, less maintenance, and provided by equipment interact with your Heating cycle. Of lower floor space requirements. For manufacturers. utmost importance is the design of example, in typical industrial Generator Frequency -This is the necessary time-temperature applications, a 2 to 3 kilowatt laser, cycles, which are typically quite costing approximately $250,000 is determined by the material, part size, and the need for through or different from those for furnace heat required. This is about 3 times the treatments. cost of comparable induction equip- surface heating. Low frequencies, which provide large “penetration” The data in Table 1 reveal that , ment.Electron-beam hardening also depths of the induced eddy currents, higher temperatures are employed has some important limitations, such for induction austenitizing of steels. as the need for a vacuum are used for through heat treating. High frequencies are used for You need these to compensate for atmosphere. surface heat treating. The minimum short or zero soak times. Similarly, frequencies for efficient through short-time tempering treatments Design Of Induction Heat austenitizing of steel bars are shown involving short or zero soak time Treatment Processes in Figure 3. Frequencies for surface can be designed around induction To make induction heat treatment hardening of steels are chosen to heating processes by using the so- work for you, carefully select equip- ensure penetration depths of about called tempering parameter, ment and understand the metallurgi- twice the required hardened depth. T(14.44 + log,&, cal variables that control heat treat- Other variables, such as power ing response. The most important density, are important in selecting where T is the tempering tempera- equipment parameters are coil frequency in these instances as well ture in degrees Rankine and t is the design, generator frequency, and and are discussed in TechCommen- tempering time in seconds. An applied power density. tary, Vol. 2, No. 3. induction tempering treatment Glossary Of Heat Treating Terms*

8 Alloy Steel -Steel containing signifi- Ferrite-A solid solution of 1 or more cant quantities of alloying elements elements in body-centered cubic iron, (other than carbon and the commonly the solute is generally assumed to be accepted amounts of manganese, carbon unless designated otherwlse. silicon, sulfur, and phosphorous) Hardenability- In steels, the added to effect changes in the property that determines the depth mechanical or physical properties. !?! and distribution of hardness induced Annealing -Heating to and holding by austenitizing and quenching. L m L at a suitable temperature and then Hardenability is a function of alloy a, n cooling at a suitable rate for such composition and quenching medium. 5 purposes as reducing hardness, I- Hardening - Increasing the improving machinability, facilitating hardness by suitable treatment, cold working, producing a desired usually involving heating and cooling microstructure, or obtaining desired For steels, this typically consists of mechanical, physical, or other proper- austenitizing followed by cooling to ties. Specific types of annealing form pearlite, bainite, or martenslte, orocesses include: or a combination of these Recrystallization annealing "Anneal- constituents. ing cold or hot worked metal to produce a new grain structure Martensite -A metastable phase of without phase change. steel formed by a transformation of austenite below the M, temperature Spheroidization anneating- Heating and composed of a body-centered and cooling to produce a spheroidal Figure 4 ietragonal lattice. Its microstructure or globular form of carbide in steel. is characterized by an acicular, or Schematic Illustration Of The Stress relief annealing-Heating to a needle-like, pattern. Surface And Center Temperature suitable temperature, holding long Histories Of A Bar Heated By enough to reduce residual stresses, Microstructure-The structure of and then cooling slowly enough to polished and etched metals as Induction (Note that, following revealed by a microscope at a an initial transient, the surface-to- minimize the development of new residual stresses. magnification greater than 10 center temperature difference is diameters. constant during the heating cycle.) Austenitizing - Forming of the face- centered-cubic austenite phase in Normalizing - Heating a fe:rous steel by heating above the trans- alloy to a suitable temperature above formation temperature range. the transformation range and then whose tempering parameter is Process forms the basis of hardening cooling in air to a temperature equivalent to that for a longer-time, of steels. substantially below the transformatior lower-temperature treatment will Bainite - A decomposition product of range. give a part with equivalent proper- austenite consisting of an aggregate Pearlite-A lamellar aggregate of ties. The application of this concept of ferrite and carbide. In general, it ferrite and cementite, often occurring is discussed further in TechCom- forms at temperatures lower than in steel and cast iron. mentary, Vol. 2, No. 4. those where very fine pearlite forms Quench Hardening - Hardening a A final factor that warrants and higher than that where marten- ferrous alloy, such as a steel, by consideration in process design is site begins to form on cooling. Its austenitizing and then cooling rapidly the starting condition of the work- appearance is feathery if formed in enough so that some or all of the piece. This is particularly important the upper part of the temperature austenite transforms to martensite. range; acicular, resembling tempered for steels that are to be hardened. In martensite, if formed in the- lower Tempering -Reheating a normalizec these cases, austenitizing behavior part. or quench-hardened ferrous alloy will be affected greatly by the start- such as a steel to a temperature . ing microstructure. An example of Carbon Steel - Iron alloy containing below the transformation range and carbon up to about 2 percent and this microstructure effect in then cooling at any rate desired. is seen only residual quantities of other Figure 5. As shown here, when elements except those added for Transformation Temperature-The using the same induction heating deoxidation. with silicon Usually temperature at which a change in parameters, the case hardened limited to 0.60 percent and manga- phase occurs. The term is sometime: depth for a 1070 steel bar increases nese to 1.65 percent. Also termed used to denote the limiting tempera- as the starting structure becomes plain-carbon steel. ture of a transformation range. , finer: quench and tempered = fine Cementite -A compound of iron and martensite structure, normalized = carbon found frequently in steels 'ine pearlite with thin lamellar known chemically as iron carbide and carbides, and annealed = structure having the approximate chemical -Source: Melals Handbook. Vol. 1. Eighth with coarse, difficult-todissolve formula Fe,C. Edition.American SocietyFor Melals.196 spheroidal carbides. Such variations in induction heat treatment results can be overcome 70 alloys-medium- and high-carbon steels, alloy and stainless steels, 60 and tool steels. It is also finding increasing application in the nonfer- rous metals industry. Advantages 50 include high heating rates, selective v) heating capability, improved produc- (0 m tion rates, and energy savings. C Induction heat treatment can $ 40 X produce surface hardened parts -0 with soft cores that exhibit excellent $ 30 wear and fatigue resistance. How- x ever, satisfactory heat treatment 0 0 z requires careful selection of equip- ment and good understanding of the 20 metallurgical variables involved. For surface hardening applications, there are a number of competing 10L I I I S 0.02 0.04 0.06 0.08 0.1 0 Drocesses to consider. 0 Distance Below Surface (in.)

Figure 5 Effect Of Starting Microstructure In 1070 Steel Bars On This TechCornrnentary provides Surface Hardening Response Using A 450 kHz an overview. It is intended to famil- Induction Generator Operated At A Power Density Of iarize you with the important 2.5 Kilowatts Per Square Centimeter (15.9 kilowatts per applications of induction heat treat- square in.) ment. If you are interested in a more detailed background, please refer to by determining (on a trial-and-error In Summary subsequent issues of TechCorn- basis)modified peak temperatures Induction heat treatment isappli- rnentary on “Surface and Selective or heatingrates for your particular cable to hardening,tempering, Heat Treatment” (Vol. 2, No. 3), alloyand expected variations in normalizing, and annealing awide “Induction Tempering” (Vol. 2, No. startingcondition.parts,particularlyferrous rangeof in 4), and the sources listed below.

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

The Center’s mission is to assist Metals Technology, Vol. 9, No. 12, pp. Applicable SIC Codes industry in implementing cost- and 493-498, The Metals Society, 1982, 33- 12, 15, 16, 17, 21, 34, 41, 51, 53, energy-efficient electric-based “Transverse Flux Induction Heating of 54, 55, 56, 98, 99 technologies in the metals fabrication Aluminum-Alloy Strip,” R. Waggott, et al. 34- 11,12, 21, 23, 25, 29, 41, 43, 49, and related fields. TechCommentary is Sources for tables and figures were: 52, 62, 63, 71.-,79, 83, 84, 89, 93, one communication vehicle that the 95, 99 Center uses to transfer technology to Transactions of ASM, Vol. 26, pp. 1-36, American Society for Metals, 1938, 35- 23, 24, 31, 32, 33, 36, 41, 42, 44, industry. The Center also conducts “Quantitative Hardenability,” J.L. Burns, 45, 46, 47, 62, 66, 67, 68, 92, 99 research in metal heating, metal T.L. Moore, and R.S. Archer. 36-21 removal and finishing, and fabrication. 37- 11, 14, 21, 24, 28, 43, 51, 61, 64, Transactions of ASST, Vol. 12, p. 871, 69, 95, 99 This issue of TechCommentary was 1927, W.P. Sykes and Z. Jeffries. made possible through the cooperation High-Frequency Induction Heating, For further information on Center of Battelle staff members Lee Semiatin, programs, write or call author; Jack Mortland, editor; Laura McGraw-Hill Book Company, 1950, F.W. Cahill, publications coordinator. Curtis. Basics Induction Heating, John F. Sources used in this issue of of Rider, 1960, C.A. Tudbury. TechCommentary were: Induction Hardening and Tempering, Induction Hardening and Tempering, American Society for Metals, 1964, T.H. American Society for Metals, 1964, T.H. Spencer, et al. Spencer, et al. CENTER FOR MATERIALS FABRICATION Induction Heat Treatment of Steel, An EPRl RBD Apphcallons Center American Society for Metals, 1985, S.L. 505 King Avenue - Columbus, Ohio 43201-2693 Semiatin and D.E. Stutz. (614) 424-7737 Copyright 1985 Battelle Memorial Institute Columbus, Ohio Reprinted 3905