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-aged samples, the number of tests
withvaluesoutofgeneralcondition requirementsare12and2% for4.76and
8.00mmdiam. respectively.
Thedatafrommechanicalpropertytestingshowsthatthesoftestconditioninthe
steel is usually developed by treating diam. 8 mm tubes (same induction
heating parameters for both of the diameters).This behaviour can be
rationalized by considering the starting structure of steel and residual stressesafter cold working:
Diam. 8 mm is less work-hardened than diam. 4.76 mm. Induction annealing is
moreeffectiveonlargerdiameters.
To achieve the optimum combination of properties (low strength, high total
elongation,lowstrainagingindex);theidealcarboncontentisbetween0.015
and0.02%.SomecastingsofFeP04 fall in thisrangeofcomposition,and the
bestresultsofinductiontreatmentareexplained.
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M
Figure3:PlantforinductionheattreatmentofRSt342tubes.Weldedtube10.3
x0.68mm.reducedto8.0x0.7mm.bya1-stepcoldrollingreduction.
TABLE5:MECHANICALPROPERTIESBEFOREINDUCTIONHEAT
TREATMENT(WORK-HARDENED STEEL)
tube,(mm)
Re(N/mm
2)
(*)
Rm(N/mm
2)
(*)
A (%),(*)
HardnessHV0.2, weld
area
HardnessHV0.2, base
metal
4.76 714 724 0.7 261215 201177
8.00 - - - 170144 152130
(*) Data from the customer
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Figure5:4.76mm.Microstructure,originalmagnificationx1000.Work
hardenedsteel:theoriginalferritegrainsareheavilydeformedduetoreduction.
Figure6:8mm.Microstructure,x1000.Tubeafterweldingandreduction.
Ferritegrainswithsomecementite.
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TABLE6:MECHANICALPROPERTIESOFRST34-2INDUCTIONHEAT
TREATEDTUBESAT900930C, 4.76MM
Speed
(m/min)
Blower,
power(%)
Re
(N/mm2
)
(*)
Rm
(N/mm2
)
(*)
A
(%),0
days
(*)
Hardness
HV0.2,weld
area,(**)
Hardness
HV0.2,base
metal,(**)
42 20 408 486 39 177156 163152
52 56 431 486 38 181158 188158
62.5 60 420 486 39 184
160 175
154(*)Datafromthecustomer,meanvaluesat0days.(**)Microhardnesstestinthe
transversedirection,frominsidetooutside;datafromTermomacchine.
Generalconditionsforannealedtubes(DINEN10002):310
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TABLE8:MECHANICALPROPERTIESOFRST34-2INDUCTIONHEATTREATEDTUBES,DIFFERENTDIAMETERS:
Temperature
(C)
Diameter
Mm
Re(N/mm
2)
Rm(N/mm
2)
A
(%)
925 4.76 x 0.7 420 486 39
925 8.00 x 0.7 255 402 44
925 10.00 x 0.7 239 392 44
Theweldareahasaheataffectedzone in which the microstructure isdifferentfrom that of the basemetal.Annealingreducesstructuralandhardnessdifferencesaround the cross-section.
Figure7:Cross-section,mountedspecimenformicrohardnesstest
Soaking is tooshort tobeeffectiveonmechanicalpropertiesofannealedsteel.
Considering diam. 4.76, data obtained at different speed are very close. The
critical parameter iscooling: ifthe linear tube speed is increased,alsocooling
(blower) must bestrongertoremoveheatandavoidoxidationofthe tubeleaving
theline.
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Figure8:blower
Figure9:Detailofcoolingsection
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Toofastcoolingdeterioratesthe mechanicalproperties.
Ifsteelisrapidlycooledfromtheintercriticaltemperature(fromabout780/800Cto680C),tensilestrengthishighandductilityis unacceptable.
Data related to diam. 8 and 10 (mm) give the best resultsobtained by induction
annealing. Bending tests on diam. 8 show a high ductility; tensile strength Rm is
closeto400N/mm2
(300N/mm2
isobtainedonlybyfurnaceannealing).
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PARAMETERS THAT AFFECT THE EFFECTIVENESS OF
ON-LINE ANNEALING
TEMPERATURE:therearenoreferencesofrecrystallizationannealingat
temperaturesabove850C.ConsideringtheinductionheattreatmentofRSt34
2,themaximumprocesstemperatureof930-950C(aspecificationprovides
for heating at 950 C to have the product certified) may balance high heating
rate and possibly is sufficient to obtain a good amount of recrystallization.
However, the soaking shouldbe rather long toensureasufficientslowcooling
rate(
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STARTINGSTRUCTURE:referring to thesameplantandcomparableprocess
parameters, diam. 8 (mm) annealed tube has acceptable mechanical properties
while diam. 4.76(mm)does not.The degree of work-hardeningofsteelprior
to annealing is fundamental for predicting the effectiveness of heat treatment.
The solutionshouldbe foundbeforeinductionannealing:
- themaxweldeddiametershouldbe10.35x0.7mm
- reductionbyfewsteps(possibly1)coldrolling
COOLING:
1)Exittemperatureis420Cwhenthetubeisonlinezinccoatedafterannealing.
It ispossible toheat the tubeat750Cmax insteadof930C,obtaininggood
results due to soaking step (with controlled temperature in the critical range,
fromannealingtemperatureto680C).
Figure11:Soaking
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Figure12:4.76mm:ferritegrainstructure(originalmagnificationx100)of
inductionannealedspecimen(test),with15850Csoaking.Slowcoolingto about
650C.Grainsize=ASTM8.Hardness=130145HV0.2
2)Exit temperature of less than 100C, with control of oxidation, is required by
other producers. With a fixed length of the plant, for ex. 100m, a higher exit
temperatureofthetubewouldbepreferredbecausegascoolingfrom400to 100
C requires - 2/3 of the tunnel length, so there is no room left for soaking
section.Thisinstallationallows,however,reliableresultswith8(mm): tensile
strengthofannealedtubeisclosetotheoreticaldatafromfurnace annealing.
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REFERENCES
InductionHeatTreatmentofSteel,byS.L.Semiatin,D.E.Stutz,andI.L.Harry:
AmericanSocietyforMetals,MetalsPark,Ohio,1986
MetalsHandbook,Vol.4,HeatTreating:AmericanSociety forMetals,Metals
Park,Ohio,1991
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