A fresh look at induction heating of tubular products by thu16555


                                                  he extensive use of metal tubing
    A fresh look at                               in thousands of products de-
                                                  mands a wide range of process

  induction heating                         concepts. For example, in automotive

                                                                                           Coil efficiency
                                                                                                                          Solid cylinder
                                            manufacturing alone, new applica-

of tubular products:                        tions for tubing are being advanced at
                                            an expanding rate. These typically
                                                                                                                   High coil efficiency for
                                                                                                                       solid cylinder
                                            small- and medium-size tubular parts
             Part 1                         include stabilizer bars, intrusion
                                            beams, structural rails, steering
                                                                                                                   High coil efficiency for
                                                                                                                      hollow cylinder
                                            columns, axles, and shock absorbers.
                                                                                                             F1   F2   Frequency              F3
                                            The air conditioning and refrigeration
                                            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-
                                                                                          the optimal frequency, which corre-
      INDUCTION                                      ing has proven effective.
                                                      Although there are many             sponds to maximum coil electrical ef-
                                                                                          ficiency, is shifted toward lower fre-
       Valery I. Rudnev • Inductoheat Group similarities, there are several process
                                              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
            comes comments, ques-           tubular products from induction               F3 for solid cylinders).1 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                                                        not only derived from improvements
                                             cylinder (Fig. 1), there will be high coil
       development. He has 28 years
       of experience in induction
                                             efficiency when the applied frequency        in coil efficiency, but is also the result
       heating. Credits include 15           F > F2.1 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
                                             dramatically decrease.1 This is due to       tube/pipe industry.1 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
                                             different optimal frequency for in-          Foptimal = 8.65 (r • 105/Am • h), where
                                             duction heating of tubular products,
                                             compared with that for solid cylinders.      r = electrical resistivity of the heated
                                             In induction tube and pipe heating,          metal, in mW • m, and Am = average
HEAT TREATING PROGRESS • MAY/JUNE 2004                                                                                                             17
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                                              TOD
formula.                                                                    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

                                               Temperature, T, °C (°F)
at high power density emit resonant                                      (1112)                                 TID
sound waves that exceed the audible
In-line tube/pipe heat treating                                           (752)                          Tavg
   Continuously-fed, multicoil induc-
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
                                               Fig 2 — Temperature vs. time plot for the in-line induction hardening of carbon steel tubing, 127
of three in-line coils and a water-spray    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.3 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. Clean, oxide-free tube surfaces
in the air.                                 nealing temperature, it enters a hold-                                    are required to meet demands for uni-
18                                                                                              HEAT TREATING PROGRESS • MAY/JUNE 2004
                                                            Power supply
   Pay-off basket     Dancer roll    Degreaser wiper                                     Holding zone Quench chamber


            Pay-off table


                                       Guide roll            Straightener       Automatic tensioner      Lubricator       Casting roll   Receiving table

                                    4.5 m (15 ft)                                 11.05 m (36 ft)                      3.1 m (10 ft)

                                                                       18.65 m (61 ft)

   Fig. 3 — Photo and typical layout of a “basket-to-basket” induction
system for annealing ACR (air conditioning and refrigeration) copper
tubing. Courtesy Inductoheat Australia.
form heating and a constant applica-                   soft, which allows solvents
tion temperature.                                      to evaporate from the coating
   In a typical pipe coating system, the               much more rapidly than if cured
outside of the pipe is first induction                 in a furnace by convection or radi-
heated to about 210°C (410°F). Note                    ation, which heats from the out-
that surface radiation and the cold                    side in. In furnace heating, the
“sink” effect of a thick-wall pipe are                 surface of the coating cures and
important considerations in deter-                     hardens first, trapping solvents be-
mining the total mass of metal to be                   tween the substrate and this “skin.”
heated.1                                               Much more time is needed for sol-
   As the pipe emerges from the in-                    vents to evaporate, and pinholes in
duction heater, heat is lost by radia-                 the coating also may be produced.
tion and convection over a large sur-                  Pinholing is not a problem with in-
face area. These losses may be                         duction curing.                  HTP
relatively insignificant, compared with
the heat loss from the surface due to
the sink effect of the inside wall. If the             References
system is designed to through-heat the                 1. Handbook of Induction Heating, by V.
pipe, then thermal conduction from                     Rudnev, D. Loveless, R. Cook, and M.
OD to ID is eliminated. This is a sound                Black: Marcel Dekker Inc., New York, 2003,
                                                       800 p.
engineering design that minimizes the
                                                       2. Induction Heating Equipment, by A.
possibility of rejects. The downside of                Slukhotskii, et al.: St. Petersburg, Russia,
through-wall heating is increased en-                  1981 (in Russian).
ergy consumption, which adds to total                  3. “In-Line Gas Quench System Bright An-
operating costs. In general, however,                  neals Korean Tube,” by B. Vinton: Heat
through-wall heating should be spec-                   Treating, June 1988.
ified unless the wall is very thick,                   4. “Induction Heating of Pipes Before
and/or the coating station can be lo-                  Coating,” by J. Powell: Tube & Pipe Equip-
cated very close to the exit end of the                ment, September 1996.
induction coil.
   Furnace heating: There is a funda-                    Correction noted: In Dr. Rudnev’s
mental difference between induction                      HTP March/April 2004 Professor In-
heating and heating in furnaces that                     duction column, the label “Heating
rely on thermal convection and/or ra-                    time, 1.5 s” in Figure 5 on page 19 is
diation. Induction can heat the metal                    incorrect. It should be “Quenching
                                                         time, 1.5 s.” We apologize for the error.
substrate beneath the coating — from
                                                                                  — The Editors
the inside out — leaving the surface
HEAT TREATING PROGRESS • MAY/JUNE 2004                                                                                                                     19

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