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OilfieldTechnology
February
2014
``
RefinedgrainHAZ (FGHAZ): is thebasemetal that hasbeen
heatedaboveAe3, but not toa temperaturehighenoughas to
produceexcessivegraingrowth.
``
Inter‑criticalHAZ (ICHAZ): is thearea that hasbeenheated
toa temperaturebetweenAe3and the lower transformation
temperature (Ae1).
``
Sub‑criticalHAZ (SCHAZ): is the region that reacheda
temperatureclose tobutbelowAe1. Some softening,without
important changes ingrain refinement,maybeexpected in
comparison to thebasematerial.
``
Basematerial: unaffectedby theweldingprocess.
The resultingHAZmicrostructuresare the resultof thesteel
response toheatingandcooling. Thecooling rate is ingeneral
limitedbynatural heat transfermechanisms, i.e., byconduction
through thesteel stripandconvection to theatmosphere;
but itmaybeenhancedwithcoolingmethodssuchaschill
blocks, forcedairorwater spray. Figure3presentsexamples
ofmicrostructuresofCT‑110steel stripanddifferent regionsof
thebiasweld. Thesemicrographscorrespond to theas‑welded
condition, beforeapplyinganyPWHT.
Thesteel strip (Figure3a) is typicallycomposedof a
dispersionof carbides (cementite) ina ferriticmatrixof fine
elongatedgrains, resulting from theTMCPhot rollingprocess. In
the ICHAZ (Figure3b), asignificant fractionof retainedaustenite
blockscanbe found.Certainsteel alloyingelements, suchasMn,
Ni andCu, helpstabilise theaustenite.
2
Finally, upperbainite
withelongatedcarbide forms inboth theCGHAZ (Figure3c)and
FZ (Figure3d). In these last regionspriorausteniticgrainscan
beas largeas500
μ
m. Theas‑weldedmicrostructurehasseveral
features that canbe identifiedaspotentialweakpoints for tube
performance:
``
Retainedaustenite in the ICHAZ thatmay transform into
martensiteunder applied stresses.
3
``
Coarsegrainupperbainite in theCGHAZandFZ.
``
Small patchesofmartensite in theFZandcentral segregation
band.
All theseconstituentsmayprovidecracknucleationsitesand
easypaths for fracturepropagation leading to reduced low‑cycle
fatigue resistance. InFigure4, acrackproducedby fatigue
damageduringservicecanbeobserved (CT‑110). In thiscase the
crack isassociatedwithcoarsegrainmicrostructures in the fusion
zoneof thebiasweld.
ThecurrentbiasweldPWHT reduces,butdoesnoteliminate,
thebrittleconstituents that reduce fatigueperformance.
Continuouscooling transformation (CCT)curvesweredeveloped
forcoiled tubingsteelsusingaGleeble thermomechanical
simulator.Phase transformationanalysisshows thesteel strip
microstructureproducedbyTMCPcannotbe reproducedby
continuouscooling.Modifications to thecurrentPWHTcan
eliminatebrittleconstituents,but lack thegrain refinement to
produce the requiredstrength.Thecurrentmanufacturingprocess
has reached its limit toproducehighstrength,highperformance
coiled tubing.
Tenarishasdevelopedanewprocessandmaterial to
manufacturecoiled tubes thaterases thedamage resulting from
formingandweldingby transforming the
entire lengthof thecoiled tube toasingle
homogeneousphase.Themicrostructures
obtainedwith thenewprocessaresimilar
inbase tube,biasweld (fusionzone)and
ERW line,asshown inFigure5.Theadvantage
of thenewprocess is that thismicrostructural
homogeneityallows theproductionofhigher
gradeswhile improving fatigue lifeand
SSC resistance.
Figure6 shows the results of tensile
tests ofmany samples of coiled tubes
Figure4.
Crackformedduringservice inthebiasweldFZ(growingfromthe
internaltubeface).Thecrack isrunning inthedirectionofthe largeupperbainite
laths.
Figure3.
Microstructuresfound indifferentzonesofaCT-110biaswelds.(a)base
tube,(b) ICHAZ,(c)CGHAZ,and(d)FZ.
Figure5.
Microstructuresobtainedwiththenewprocessatdifferentregions.(a)basetube,(b)FZof
thebiasweld,and(c)ERW line.
