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A Fresh Look at Induction Heating of Tubular Products

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A Fresh Look at Induction Heating of Tubular Products htprof.qxd 5/2/04 1:22 PM Page 1 he extensive use of metal tubing A fresh look at in thousands of products de- Tube T mands a wide range of process concepts. For example, in automotive induction heating manufacturing alone, new applica- Solid cylinder tions for tubing are being advanced at High coil efficiency for of tubular products: an expanding rate. These typically solid cylinder small- and medium-size tubular parts Coil efficiency Part 1 include stabilizer bars, intrusion High coil efficiency for beams, structural rails, steering hollow cylinder columns, axles, and shock absorbers. F F F The air conditioning and refrigeration 1 2 Frequency 3 industries and oil- and gas-transmis- Fig. 1 — Conditions for maximizing the elec- sion lines have high-pressure require- trical efficiency of the induction coil are different ments where larger tubular prod- for tubes and solid cylinders, as shown by these ucts are used. In all of these plots of coil efficiency vs. frequency. (Ref. 1) PROFESSOR applications, induction heat- ing has proven effective. the optimal frequency, which corre- INDUCTION Although there are many sponds to maximum coil electrical ef- Valery I. Rudnev • Inductoheat Group similarities, there are several process ficiency, is shifted toward lower fre- features and physical phenomena quencies (frequencies between F1 and Professor Induction wel- that distinguish induction heating of F2 for tubes instead of between F2 and 1 comes comments, ques- tubular products from induction F3 for solid cylinders). The optimal tions, and suggestions for heating of solid cylinders. The condi- frequency for heating tubes (hollow future columns. Since tions for maximizing the electrical ef- cylinders) typically provides a current 1993, Dr. Valery Rudnev ficiency of the induction coil is one of penetration depth, d, greater than the has been on the staff of In- them.1 The coil efficiency of an induc- tube wall thickness (except for heating ductoheat Group, where he currently serves as group tion tube/pipe heater is a complex small-diameter tubes). This can result director – science and tech- function of several design parameters, in a noticeable increase in coil efficiency. nology. In the past, he was an including “coil ID-to-tube OD” air In some applications, it is possible to associate professor at several gap, metal electromagnetic properties, gain an improvement in electrical effi- universities, where he taught coil length, tube wall thickness, and ciency of 10 to 16%; in others, the in- graduate and postgraduate the very-critical frequency. crease in coil efficiency is less pro- courses. His expertise is in ma- nounced and may not be noticeable. terials science, heat treating, ap- Solid cylinders and frequency The total gain in electrical efficiency plied electromagnetics, com- When induction heating a solid when applying a lower frequency is puter modeling, and process cylinder (Fig. 1), there will be high coil not only derived from improvements development. He has 28 years efficiency when the applied frequency in coil efficiency, but is also the result of experience in induction 1 heating. Credits include 15 F > F2. F2 corresponds to a ratio of of lower bus bar losses and a short patents and 118 scientific and cylinder outside diameter, OD, to cur- heat time, as well as reductions in engineering publications. Con- rent penetration depth, d, greater than transformer and capacitor losses. tact Dr. Rudnev at Inductoheat three (OD/d > 3). The use of a fre- Another advantage of using lower Group, 32251 North Avis quency which results in a ratio of frequencies is improved overall Drive, Madison Heights, MI OD/d > 6 will only slightly increase system cost-effectiveness, since, typi- 48071; tel: 248/585-9393; fax: the coil efficiency. At the same time, cally, the cost of lower frequency 248/589-1062; e-mail: rudnev@ the use of very high frequencies (>F3 power supplies is less than that of inductoheat.com; Web: www. higher frequency units. inductoheat.com. in Fig. 1) tends to decrease the total ef- ficiency due to higher transmission Numerical computation using In- losses and high heat losses, since a ductoheat’s ADVANCE software en- long heating time will be needed to ables selection of the optimum fre- provide the required “surface-to-core” quency for a particular application.1 temperature uniformity. If the chosen To obtain a quick, but rough estimate frequency results in a ratio OD/d < 3 of the appropriate frequency, several (frequency <F2), the coil efficiency will simplified formulas are used in the 1 dramatically decrease.1 This is due to tube/pipe industry. For example, in the cancellation of induced eddy cur- the case of “electromagnetically long” rents circulating in the opposite sides solenoid-type inductors, a suitable fre- of the solid cylinder. quency, Foptimal, in hertz, can be calcu- lated using the simplified formula rec- Tubular parts and frequency ommended in Ref. 2: Figure 1 also shows that there is a r 5 different optimal frequency for in- Foptimal = 8.65 ( • 10 /Am • h), where duction heating of tubular products, compared with that for solid cylinders. r = electrical resistivity of the heated mW In induction tube and pipe heating, metal, in • m, and Am = average HEAT TREATING PROGRESS • MAY/JUNE 2004 17 htprof.qxd 5/2/04 1:22 PM Page 2 PROFESSOR INDUCTION, continued tube diameter (OD – wall thickness, Coil 1 Coil 2 Coil 3 Quenching h), in meters. In cases where induction heaters must be considered “electromagneti- 1000 cally short,” rather than long, the value (1832) of the optimal frequency will be higher than that determined when using this T formula. OD 800 Note, too, that audible noise can be (1472) a dominant factor in tube/pipe heating, and one that greatly affects frequency selection. (Certain tubes/ pipes exposed to certain frequencies 600 (1112) at high power density emit resonant TID sound waves that exceed the audible limit.1) 400 In-line tube/pipe heat treating (752) Tavg Continuously-fed, multicoil induc- Temperature, T, °C (°F) tion heaters similar to the in-line sys- tems used in bar/rod/wire heating applications are popular for through- 200 heating of long tubular products. (392) Hardening and tempering are among the suitable processes. An example is shown in Fig. 2: a time vs. tempera- ture plot for carbon steel tubes hard- 20 40 60 80 100 120 ened by an in-line system. The induc- Time, s tion through-heating system consists of three in-line coils and a water-spray Fig 2 — Temperature vs. time plot for the in-line induction hardening of carbon steel tubing, 127 3 mm (5 in.) OD, 12.7 mm (0.5 in.) wall, at a frequency of 3 kHz. Each of the three induction coils is quench chamber. Tubes having an 40 cm (1.31 ft) long. Distance between coils: 20 cm (7.9 in.). (Ref. 1) OD of 127 mm (5 in.) and a wall thick- ness of 12.7 mm (0.5 in.) are hardened Success of induction bright an- ing zone to await full grain recrystal- at a rate of 3 metric tons/h (3.3 nealing of stainless steel tubes with a lization. The tube then enters the tons/h). Frequency: 3 kHz. gas quench also is greatly affected by quench station where it is cooled Annealing stainless: In-line “whole- the quality of the gas. As the dew rapidly to normal handling tempera- body” induction annealing is suitable point rises, the gas contains more ture. A typical induction “basket-to- for carbon and stainless steels, as well moisture and the corresponding basket” system for annealing thin-wall as nonferrous metals such as titanium volume of hydrogen has to be in- copper tubing is shown in Fig. 3. It and copper. creased; otherwise, the process will was manufactured by Inductoheat Gas-quench bright annealing of not provide the required quality and Australia, Seaford, Victoria, in collab- stainless steel tubes is a good example surface appearance. The culprit is ox- oration with Mitsui Engineering & of the in-line system design approach.3 idation: the chromium in the stainless Shipbuilding Co. Ltd., Tokyo. Its stan- In this application, tubing is induction steel reacts with moisture to form dard line-speed is 200 to 500 m/min heated to about 1150°C (2100°F) and Cr2O3. (660 to 1640 ft/min). than fed into a hydrogen-nitrogen gas- Nonferrous metals: Induction quench tunnel. heating also can be used to anneal Tube-coating applications During the quench, nitrogen is used nonferrous metals; for example, the Deposition of metallic and non- to purge the system of oxygen, while, copper tubes used in plumbing and metallic coatings on steel tubular prod- simultaneously, hydrogen is fed into in air conditioning and refrigeration ucts also may demand in-line induc- the system. This eliminates the possi- (ACR) applications. Requirements for tion heating.1 An example of the bility of an explosion that might occur thin-wall ACR copper tubing in- former is galvanizing, in which a fine should an excessive hydrogen and clude:1,4 zinc or Zn-Al alloy layer is deposited oxygen leak develop. • Small wall-thickness eccentricity on the OD surface of a steel tube. A Also, an annealing system’s quench • Accurate dimensional tolerances metallurgical bond is formed.1 tunnel should be sufficiently long to • Maximum heat transfer area Examples of nonmetallic coatings ensure that the temperature of the • Proper grain size and properties that benefit from in-line induction stainless steel tube as it exits the of the annealed tube to facilitate heating are solvent-based paints chamber is below the oxidation point. forming (primers and top coats), epoxies and This will prevent tarnishing of the tube • Clean inside wall other polymers, and heat-cured pow- surface when it reacts with the oxygen After the tube is heated to the an- ders.
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