4 Eng Induction Welded Small Diameter Tube

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    Outside

    Diameter

    Wall

    Thickness

    mm mm

    4.76 0.7

    6.35 0.7

    7.92 0.9

    8.0 0.7

    9.5 0.7

    10.0 0.7

    1.0

    12.0 0.71.01.5

    INDUCTIONWELDEDSMALLDIAMETERTUBEPRODUCTIONLINE

    Target

    Production of welded single - wall tubing manufactured to ASTM A

    513 and zinc-coated as per UNI 5741-66, suitable for many lowpressure and structural applications such as:

    Refrigeration tubes

    Evaporation condensers

    Compressor lines

    Fuel lines

    Power steering lines

    Engine oil cooling lines

    Structural support components

    Giorgio Mazzola

    Massimo Mosca

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    COMPOSITION OF THE LINE

    1.1 DoubleStripDecoiler

    1.2 StripButtWeldingMachine

    1.3 PinchRoll

    1.4 HorizontalStripAccumulator

    1.5 StripContinuousCleaningUnit

    1.6 FormingMachine

    1.7 H.F.InductionWelder

    1.8 NTDEddyCurrentFlow

    1-9 TubeCooling1.10 TubeTensionCompensation

    1.11 ColdDrawingBench

    1.12 TubeContinuousCleaningUnit

    1.13 H.F.InductionAnnealingUnit

    1.14 ProtectiveAtmosphereTubeCoolingSection

    1.15 TurnWheel

    1.16 MaintainingZone

    1.17 HotDipZincCoatingUnit1.18 TubeCooling

    1.19 NDTEddyCurrentFlow

    1.20 WoundUpReel

    1.21 TubeAirPressureWaterTest

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    TECHNOLOGICAL CYCLE

    1.1 Thestripcoilcomingfromthecoilstorageiswound onthedoubledecoiler

    1.2 Coilsarejointedheadtoheadusingthestripbuttweldingmachine

    1.31.4

    Stripisaccumulatedinahorizontalstripaccumulatorwhichavoidsstoppingthelineforchangingstripcoil

    1.5 Thestripiscleanedusingcompressedwater

    1.6 Thetubeisformedintheformingmachine

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    1.7 Tubeisweldedwiththe100KWHighFrequencyweldingunit

    1.8 TheNDTEddycurrentflowdetectorcontrolstheweldbeam

    1.9 Theweldedtubeiscooled

    1.10 Thetubetensioncompensatorunitmaintainsthetubestraight

    1.11 Inthecolddrawingbenchthediameteroftheweldedtubeisreducedtotherequiredone

    1.12 Thetubeiscleaned

    1.13 ThetubeisannealedbymeansoftheHighFrequencyunit

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    1.14 Runningintheprotectiveatmospheretubecoolingsectionthetubeiscooledwithoutoxidation. If Zinccoatingisrequired,thetubeiscooledtoonly 420C

    1.15

    1.15 In order to avoid having an excessively long line, after the first

    cooling sectionthetubereturns onthetopofthelinebymeansofaturn wheelwhichoperates under theprotective atmosphere

    1.16 Thetubeismaintainedunder a protectiveatmosphereuptotheZinccoating unit

    1.17 HotdipZinccoatingunit:ifrequired,thetubeisZinccoated1.181.19

    ThetubeiscooledandcontrolledagainbyNDTEddycurrentflow

    detector

    1.20 Tubeiscoiledonawoundupreel

    1.21 Tubeistestedwithpressurized water

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    WHY DOES THE TUBE HAVE TO BE ANNEALED?

    The object of the heat treatment of welded tubes is the stress relief and

    recrystallization of steel. There are three processes that may be used to treat low

    carbon steel products: furnace annealing, continuous annealing (with resistance or

    gasheating),and inductionannealing.Performances thatcanbeobtainedfrom the

    traditional furnaces are definitely correct; production costs, however, are high if

    comparedwithon lineprocesses.Continuousfurnaceannealingrequires largeand

    expensive installations, it is not applicable to the tube manufacturing. The

    induction annealing equipment allows the producers to increase the line speed of

    theircurrentlinewhilestillproducingacceptablequalitytube.

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    THEORY OF ANNEALING

    The FURNACE (or batch) ANNEALING process for coils of low-carbon steel

    requires a long heating period (on the order of several hours - one day)

    because of the large mass. Annealingper se is done near the lower critical

    temperature, A1, around 690 to 730 C (12751350 F). During this soak,

    recrystallizationandsomegraingrowthoccur.Inaddition,carbides,which have

    beenformedduringcoolingofsteelthatwashotrolledpriortocold rolling, may

    redissolve.However,slow cooling (20C/h, furnacecooling)ofthecoilsaftersoakingpromotesreprecipitationof thesecarbides.Asannealed the

    low-carbonsteelisrelativelysoftandhasagrainsizeofASTM6to8.

    Becausetheannealedsteelhasbeencooledfromtheannealingtemperatureand

    because all carbon has come out of solution, one of the major age-hardening

    problems in low-carbon steels, quench age-hardening (or quench aging) is

    avoided. Quench aging leads to gradual hardening and decreased ductility with time at

    room temperature due to carbide precipitation. The problem is well known in

    the practice of CONTINUOUS ANNEALING of strips. During continuous

    annealing,uncoiledsteelsheet ispassed througha two-stage furnace.The first

    stage heats the steel (750850C) and accomplishes re crystallization. The

    maximum temperature is maintained for401.The second stageheatsatalower temperatures (350 400 C) to overage the steel and to remove carbon

    from solution effectively. Without this step, the thin sheet would cool too

    rapidlyand retain carbon in solution; this carbon would eventually cause strain

    or quench agingandreducesheetformability.

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    Incontrast tofurnaceannealing,INDUCTION ANNEALINGof low-carbon

    steel for purposes of recrystallization usually consists of fast heating to temperature,little or no soaking at the maximum temperature, and cooling to room

    temperature. Some studies have shown that soak times between 1 and 20

    areneededforcompleterecrystallizationattemperaturesjustabovethe A1 point.

    Others have shown that no soak time is necessary provided that a high

    enoughtemperatureisreachedattheendoftheheatingcycle.

    If the steel is cooled rapidly from the intercritical temperature (A1

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    RESULTS OBTAINED FROM INDUCTION ANNEALING

    Thisreportcontainstherelevantmechanicalpropertiesofsteelstripandtube whichis

    usedbysomeleadingmanufacturingcompanies.

    Tubes are manufactured using Fe P04 (St 14, SAE 1008) and RSt 34, strip

    welded,andon-lineannealedbyTermomacchineshighfrequency inductionmachinery.

    Thecorrespondingmechanicalpropertiesoftheannealedtube arereportedin orderto

    evaluatetheeffectivenessofinductionheattreatment.

    TABLE1:DESIGNATIONOFSTEELS

    StandardNo. SymbolDIN C(%) Si(%) Mn(%) P(%) S(%)

    1.0034(*) RSt342 0.17 - - 0.045 0.045

    1.0338 St14 0.08 (**) (**) 0.045 0.045

    (*)SAEcarbonsteel1013

    (**) Fe P04 in Italy is classified as extra low carbon steel, considered by UNI

    specification No. 5866: Cold rolled flat finished products of unalloyed quality

    steel. Cold drawing, cold bending of sheets and coils. Quality, requirements and

    tests.MechanicalpropertiesofFeP04areguaranteedfor8daysfromdelivery.

    Thechemicalcompositionisnot specified,exceptC%maxandimpurities(P,S).

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    TABLE2:MECHANICALPROPERTIESOFSTEELSTRIPS

    Designation ofsteel

    Re(N/mm2) Rm(N/mm2) A (%) Hardness (HR 30T)

    St 14 ( Fe P04) 38 38 50

    RSt 34 2>300 > 28

    TABLE3:MECHANICALPROPERTIESOFFEP04STEELSTRIPS

    FROMDIFFERENTSUPPLIERS

    SupplierRe (N/mm

    2) Rm (N/mm

    2) A(%) Hardness(HR30T)

    A 168

    152192

    292

    275334

    38

    3543

    45

    4148

    B 163

    129213

    302

    263371

    39

    3446

    44

    3851

    C 168

    131196

    299

    270354

    40

    3443

    42

    4048

    D 184

    160254

    293

    280306

    37

    3143

    44

    4047

    General

    conditio

    ns

    38 3850

    Note: thedataindicatedinthe abovetable(andonthefollowingone)are

    themeanvaluesandtherangesinwhichthesevalueswerefound.

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    Figure2:plantforinductionheattreatmentofFePO4tubes.

    Weldedtube10.3x0.68mmreducedto4.76x0.7mmbya3-stepcoldrolling

    reduction.

    TABLE4:MECHANICALPROPERTIESOFFEP04INDUCTIONHEATTREATEDTUBESAT740C

    (mm) Re(N/mm

    2)

    Rm(N/mm

    2)

    A (%), 0

    daysA (%),

    30 daysHardness

    (HR 30T),

    0 days

    Hardness

    (HR 30T),

    30 days

    4.76 225

    178/283

    341

    298/411

    33

    26/38

    26

    20/36

    51(*)

    46/56

    56 (**)

    51/62

    8.00 194158/250

    321273/379

    3930/44

    3222/36

    4944/55

    5449/62

    General

    conditions> 173 > 290 > 25 > 25 < 55 < 55

    (*)90100HV0.2;(**)100120HV0.2(approximateconversiontoVickers)

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    CastingsofFeP04have,probably,abroad rangeofcompositions referring to

    C%:thestripsmanufacturedbydifferentsuppliersd o nothavecomparable

    mechanicalproperties, thus affectingtheresultsofinductionheattreatment.

    Minimumhardnessvaluesofinductiontreatedtubesareratherhigherthan thoseof

    originalcoil.

    The material is subject to a process of room temperature aging, with a

    consequent loss of ductility; referring to un-