
2/22

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


3/22

TECHNOLOGICAL CYCLE

1.1 Thestripcoilcomingfromthecoilstorageiswound onthedoubledecoiler

1.2 Coilsarejointedheadtoheadusingthestripbuttweldingmachine

1.31.4

Stripisaccumulatedinahorizontalstripaccumulatorwhichavoidsstoppingthelineforchangingstripcoil

1.5 Thestripiscleanedusingcompressedwater

1.6 Thetubeisformedintheformingmachine


4/22

1.7 Tubeisweldedwiththe100KWHighFrequencyweldingunit

1.8 TheNDTEddycurrentflowdetectorcontrolstheweldbeam

1.9 Theweldedtubeiscooled

1.10 Thetubetensioncompensatorunitmaintainsthetubestraight

1.11 Inthecolddrawingbenchthediameteroftheweldedtubeisreducedtotherequiredone

1.12 Thetubeiscleaned

1.13 ThetubeisannealedbymeansoftheHighFrequencyunit


5/22

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


6/22

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.


7/22

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.


8/22

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


9/22

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).


10/22

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.


11/22

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)


12/22

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.


13/22

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


14/22

Figure5:4.76mm.Microstructure,originalmagnificationx1000.Work

hardenedsteel:theoriginalferritegrainsareheavilydeformedduetoreduction.

Figure6:8mm.Microstructure,x1000.Tubeafterweldingandreduction.

Ferritegrainswithsomecementite.


15/22

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


16/22

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.


17/22

Figure8:blower

Figure9:Detailofcoolingsection


18/22

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).


19/22

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(


20/22

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


21/22

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.


22/22

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

Download - 4 Eng Induction Welded Small Diameter Tube