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AAAA critical view on the significance of HAZ toughnesscritical view on the significance of HAZ toughnesscritical view on the significance of HAZ toughnesscritical view on the significance of HAZ toughnesstestingtestingtestingtesting

Andreas Liessem Europipe GmbH, Mlheim, GermanyMarion Erdelen-Peppler Salzgitter Mannesmann Forschung, Duisburg, Germany

International Pipeline Conference IPC 2004October 4 - 8, 2004 Calgary, Alberta, Canada

TP63

EUROPIPE. The world trusts us.


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1 Copyright 2004 by ASME

Proceedings of IPC 2004International Pipeline Conference

October 4 - 8, 2004 Calgary, Alberta, Canada

IPC04-0315

A CRITICAL VIEW ON THE SIGNIFICANCE OF HAZ TOUGHNESS TESTING

Andreas LiessemTechnical ServiceEuropipe GmbH

Formerstrasse 4945470 Ratingen, Germany

[email protected]

Marion Erdelen-PepplerSalzgitter Mannesmann Forschung

Ehinger Strae 20047259 Duisburg, Germany

[email protected]

ABSTRACTWithin the heat affected zone (HAZ) along the weld seam

of LSAW linepipes discrete microstructural regions of reducedtoughness can not be avoided and are commonly designatedwith the term Local Brittle Zones. The nature of these LBZ has

been intensively investigated and the gathered knowledge isexploited in todays steel technology, plate processing and pipemanufacturing. The HAZ toughness has been improved ingeneral by reducing M-A constituents and by austenite grainrefinement. Nevertheless local areas of low toughness withinthe CGHAZ can not be avoided completely. They are

statistically distributed in every pipe.Furthermore it seems to be widely accepted that the

structural reliability of LSAW linepipe produced and inspectedwith sta te-of-the-art technology is not influenced as these areasof low toughness have a limited size and distribution. This has

been demonstrated by numerous investigations including smallscale (CVN, CTOD), wide plate and burst tests. The essence ofthese investigations is that the failure behaviour of linepipecontaining part wall defects in the HAZ is toughnessindependent. So far researchers world is clear and in goodshape. Nevertheless many linepipe specifications tend tostipulate stringent test requirements with regard to acceptancecriteria for the HAZ. In the occurrences of test failures a re-test

procedure for test lot acceptance is carried out. As a matter offact the LBZ are present along the weld seam over limited areasin each pipe. Therefore such a re-test procedure is regarded to

be inappropriate in terms of quality inspection as it randomlysorts out pipes just by the statistical chance. With regard toHAZ toughness the pipes failed by this test do not differ fromthose pipes accepted and released for dispatch.

As a final conclusion it can be stated that the existing test procedures for the HAZ toughness testing of the main standardsand specifications do not reflect the current developments withregard to improved HAZ toughness achieved by thedevelopment of optimised steel composition and with regard tothe enhanced defect detection probability along with modern

NDT inspection methods. An amendment of the current test

procedures in this direction is proposed. Therefore proposalsare made as start for a common discussion.

Keywords: HAZ toughness, submerged arc welding, defect probability, CVN test, CTOD

INTRODUCTIONWithin this paper the main influencing factors for the

toughness in the heat affected zones within the production ofLSAW pipes will be reviewed. Statistical evaluation of previouslarge scale production will be presented and reviewed withregard to the requirements of major standards and

specifications. Beside the material properties the occurrence probabili ty for defects and imperfections plays the secondimportant role within the assessment of structural integrity. Theuse of most modern, automated welding and inspectiontechnologies therefore represents a prerequisite to assure highquality with low defect probability and need to be considered indefining toughness requirements for CVN and CTOD testing.Fixed fracture toughness requirements as 0,20mm CTOD aremore frequently specified, but do not adequately consider theintended application and service conditions and should bereplaced by minimum values specific for the intendedcombination of applied stress, crack dimensions and materialfracture toughness.

NOMENCLATURELBZ: Local brittle zoneHAZ: Heat affected zoneCTOD: Crack Tip Opening Displacement

HAZ TOUGHNESS IN LONGITUDINAL SUBMERGEDARC WELDED LINEPIPES

High strength large diameter linepipes used for longdistance or deep water pipeline are manufactured in the mosteconomic way by submerged arc welding (SAW) in two passes.This high performance welding process is characterised by a


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high heat input with cooling conditions that impacts on thetoughness properties in the zone adjacent to the weld. Themicrostructure and herewith the toughness of this heat affectedzone (HAZ) is mainly influenced by the cooling time t 8/ 5 from800C to 500C, the maximum reheating temperature and thechemical composition. The HAZ is characterised by a widerange of different microstructures, depending on the distancefrom the fusion line and the cooling conditions (Figure 1).

Intercritically heated zone

Fine Grain HAZ

Grain coarsened HAZ

Intercriticallyreheated graincoarsened HAZ

Intercritically reheatedfine grain HAZ

FL CVnotch positions

Figure 1: HAZ microstructural regions within two pass weldwith typical Fusion Line CVN position (acc. [1])

Generally the lowest toughness values are expected in thegrain-coarsened heat affected zone (CGHAZ), as the toughnessdecreases with increased heat input and an increase in grainsize. With increasing cooling time the microstructure in theHAZ is transformed from martensite to upper bainite as shownin Figure 2. This transformation to upper bainite is shifted tolonger cooling times by significant additions of nickel whichleads to lower FATT even at higher carbon equivalent.

-200

-180

-160

-140

-120-100

-80

-60

-40

-20

0

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60

80

100

1 10 100 1000 10000Cooling time from 800 to 500C (s)

F A T T ( C )

100% martensite (M)Upper bainite (UB) (with MA constituent)martensite + lower bainite

Range fo r two-pass SAW

CE IIW=0.55

(1% Ni)

CE IIW=0.37

CE IIW=0.42

(0.11% C)

CE IIW=0.73

(5% Ni)

CE IIW=0.72

(3.3% Ni)

Rangefor pipeline

girth welding

Figure 2: Correlation between FATT and the cooling timet8/5 for different steel composit ions and microstructuresoccurring in the CGHAZ (acc. [2])

The upper bainite is coarse grained and exhibits fractions

of M-A constituents. Both microstructural features typicallyreveal low toughness properties. As the cooling conditions intwo pass SAW welding remain virtually constant irrespective ofwall thickness the grain growth and the formation of M-Aconstituents has to be limited by appropriate chemicalcomposition. The main measures currently known to improveHAZ toughness by optimised chemical composition aresummarised in Table 1. Reduction of both carbon and carbonequivalent play a predominant role in avoiding the formation ofM-A constituents. Furthermore the controlled use of limitedadditi ons o f microalloying elements is reported to inhibit thegrain coarsening by formation of finely dispersed nitridesand/or oxides [3[5]. These favourable effects have beenconfirmed by weld cycle simulation tests for a variety of steelcompositions. [2-[5].

with positiveinfluence onMeasure

FATT Upper

shelfenergy

Remarks

Reductionof C and

CE

l l Decreases hardenability andlimits formation of M-A

constituents

Limitationof Nb

l Decreases hardenability andlimits formation of M-A

constituents

Controlleduse of Ti

l l Restricts austenite graincoarsening

Additionof Ni

l Retards the formation of upper bainite to longer cooling t imes

Low S, P,O

l Reduces precipitations andsegregations

Table 1: Improvement of HAZ toughness by adapted steelcomposition

The review of the published results shows that there is afair level of understanding and that this knowledge iscontinually exploited in the steel chemistry and processingroutes for linepipes, by which the overall HAZ toughness has

been considerably improved and the statistical probability ofhaving low toughness values in the HAZ has been reduced. Nevertheless values below the minimum requirement are stillfound due to the presence of LBZs when the notch isfavourably positioned in the CGHAZ. LBZs are discretemicrostructural regions of low toughness within the CGHAZ,

but surrounded by microstructures with higher toughness. Asthe toughness of the CGHAZ depends mainly on the chemicalconditions and the welding procedure (heat input, coolingtime), which are technically constant due to strict control withinnarrow ranges for a specific linepipe production, it becomesobvious that LBZ are existing in each pipe of production.


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HAZ TOUGHNESS TEST REQUIREMENTS ANDPRODUCTION RESULTS

In Table 2 the test and acceptance criteria of some majorlinepipe standards and specifications are compared.Differences can be found mainly with regard to notch positionin the HAZ, the amount of tests and the acceptance criteria. TheHAZ is generally tested in case of offshore application, whereasHAZ testing is not mandatory for most onshore standards. Inmost cases the specimen with notch position Fusion Line(FL) is defined to sample 50% weld metal and 50% HAZ.Other specification requires a position where the root of thenotch is in the CGHAZ to the maximum extent possible.

Standard Applica-tion

Test Acceptance criteria

DNV OS-F101

Offshore Charpy-V 38/45J at FL, FL+2, FL+5(outside sub-surface and inaddition mid thickness fort>20mm)

CTOD Not required for HAZ0.20mm min. for basematerial and weld (MPQTonly)

ISO 3183-3 Offshore Charpy-V 38/45J at the notch locationwith the lowest min. averageduring MPQT

CTOD for information only from base material, HAZ, weld asan option during MPQT

Statoil R-SP-230

Offshore Charpy-V 34/45J at FL, FL+2, FL+5(outside subsurface)

CTOD for information only from base material, HAZ and weldduring MPQT

ISO 3183-2 Onshore Charpy-VCTOD

Both tests not required inHAZ

CSAZ245.1-02

On-/Offshore

Charpy-VCTOD

Both tests not required inHAZ

DEP31.40.20.30

On-/Offshore

Charpy-V 34/45J at FL, FL+2, FL+5 atmid thickness

CTOD Not required

Table 2: Requirements of important technical standards withregard to HAZ toughness testing for grade X65 or equivalent

The test and acceptance criteria according DNV OS-F101turn out to be the most stringent ones. In total , 6 sets ofspecimen have to be positioned at different areas within theHAZ. In Figure 3 the notch positions of these 6 sets are markedin a macrograph of a weld with 34,9mm wall thickness.Compared to the STATOIL specification R-SP-230 and otherstandards for offshore application the amount of testing isdoubled and consequently the probability of Charpy-V testfailure. The current ISO 3183-3 requests only the notchlocations no. 1 to 3 of Figure 3 to be performed duringmanufacturing procedure qualification test (MPQT). The notch

location giving the lowest minimum average is set to be takenfor production control tests .

Figure 3: Macrograph of weld cross section (34,9mmwall thickness) with possible HAZ notch locations acc. DNVOS-F101

With this macrograph it can be demonstrated that the weldseam geometry, which in practise is not perfectly symmetric,

plays an important role with regard to HAZ Charpy-V notchtesting. As the heat input increases with increasing wallthickness the width of the HAZ increases. Therefore it is the

purpose of the linepipe manufacturer to limit this unavoidableenergy increase as much as possible. The most effectivemeasure to lower the heat input is increasing the welding speed.The welding lines of Europipe will therefore by equipped withnew power sources allowing the use of up to 4 wires for insidewelding, respectively up to 5 wires for outside welding. Herebya heat input reduction of approximately 10% is enabled.Alternatively the heat input can be limited by reducing the weldseam cross section. Such a narrow weld cross section willhave on the other hand a much steeper fusion line profile. Anotch positioned at the fusion line boundary (50% weld/50%HAZ) of such a steep line lies with a higher percentage of itslength in the CGHAZ as in case of a less steep fusion line asindicated in Figure 3 by notch position 4a. Of course the failure

probability for a notch along such a steep fusion line isincreased despite the lowered heat input.

In Figure 4 the production results (SAWL 450, 34.9mmWT) for Charpy-V notch tests with notch location FL andFL+2mm according DNV OS-F101 are presented. It is evidentthat the great majority of values for FL are at a test temperatureof 20C significant above the minimum requirement of 36J.But some tests reveal values which are in the range of thislower limit with one value that failed. The distribution forspecimens with notch location FL+2mm is shifted in general tohigher values as the CGHAZ is sampled to a smaller amount.Failures are not reported for this notch location. Thisdemonstrates the effectiveness of the measures with regard tooptimised steel composition for HAZ toughness. As the resultsof FL+5mm are almost at the level of base material they are notshown here.

3 2 1

6 5 4 4a 5a 6a


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T= - 20C

0

5

10

15

20

25

30

35

20 45 70 95 120 145 170 195 220 245 270 295 320 345

CVN toughness, individual Value [J]

P e r c e n

t a g e [ %

]

FL (root) FL(root)+2mm

Figure 4: Influence of notch position on HAZ toughness

How the toughness values of HAZ depend on the heatinput is revealed in Figure 5 and Figure 6 for different pipe

productions of X65/X70 and 30C test temperature. The testand acceptance criteria followed the requirements of the Statoilspecification RSP-230. As the heat input increases withincreasing wall thickness the minimum and average values fordifferent HAZ notch locations are evaluated depending on wallthickness. As the width of the HAZ is growing with increasedheat input/wall thickness the average toughness for the threedifferent notch locations is reduced. For wall thickness below25mm there is a more distinctive increase of the valuesreducing the risk of failures to virtually zero. With increasingwall thickness the probability for test failures increases to a

maximum rate of 3% for FL notch position as the averagevalues decreases. For FL+5mm test failures even for thethickest wall thickness have never been reported.

0

50

100

150

200

250

300

20 24 28 32 36 40

Wall thickness [mm]

A v e r a g e

C V N t o u g

h n e s s

[ J ]

FL FL+2mm FL+5mm

T= -30CGrade X65/X70

45J min. average

Figure 5: Influence of wall thickness on average CVNtoughness for different notch locations in the HAZ

0

20

40

60

80

100

120

20 24 28 32 36 40

Wall thickness [mm]

M i n

. I n d i v i d u a

l C V N t o u g

h n e s s

[ J ]

FL FL+2mm FL+5mm

T= - 30CGrade X65/X70

34J min.

Figure 6: Influence of wall thickness on minimum individualCVN toughness for different notch locations in the HAZ

The current procedure applied in case of a test failure is toreject the initially failed pipe and to retest two other pipes from

the same test lot. In case that all retests from the two pipes passthe minimum and the average requirement the lot is releasedexcept the initially failed pipes. Such a retes t procedure is incontrast to the nature of LBZs in the CGHAZ along the weldseam. The LBZs are characterised by a limited size of amicrostructural magnitude but occur with a statisticaldistribution in each pipe length. This has been proven by theresults of additional tests from failed pipes. In all cases withinEuropipes experience this second test passed successfully theminimum requirements. As the statistical risk for failures isgenerally low the chance of a second failure of the same pipe isnearly zero. Concluding the above it can be stated that the pipesinitially failed and consequently rejected do not have different

properties as those pipes accepted and released. To increase the

number of notch locations or a higher test frequency willtherefore not lead to higher quality of the pipe delivery, butonly to higher reject rates and costs.

LIKELIHOOD OF HAZ RELATED DEFECTS ANDTHEIR PROBABILITY OF DETECTION

The likelihood of the different known welding defects isdepending on the type of welding process as the main welding

parameters like heat input significantly differ. As describedabove the SAW process is characterised by a high heat inputwith the discussed detrimental effect on the HAZ toughness,

but the likelihood for having any planar defects like lack offusion is very limited. A weld macrograph with markedlocations for possible defects in the HAZ is shown in Figure 7.


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Figure 7: HAZ related defects and detectability by inspectionmethods of Europipe

It is obvious that the number of possible defects is smalland their detectability is high if modern NDT techniques areused. The non-destructive inspection of the HAZ starts alreadyin the plate mill where a 100% UT-coverage of the plate bodycan be offered by the plate mills associated with Europipe. Thequality standard of Europipe includes visual, automated UT andX-Ray inspection [6]. As the pipes are cold expanded theseinspection are performed before expansion according to asignificant higher sensitivity than required by specification.Hereafter the pipe is plastically deformed by the expander.

Although this process virtually constitutes only the last formingstep, it fulfils also a function as quality control. It should benoted that the residual stress level of the pipe is herewithsignificantly reduced. The pipe is then loaded in the millhydrotester, normally in the range of 95% yield stress. Thesecond automatic UT inspection of the weld seam is performedafter hydrotesting followed by X-Ray inspection of the pipeends and possible weld repairs. By this extensive inspection

procedure the probabili ty for undetected defects is very low. Ina risk analysis performed by DNV on behalf of Europipe, the

probability of a linepipe delivered by Europipe with anundetected failure has been estimated to be 8.5 10 -8 [7].

FRACTURE MECHANIC EVALUATION OF HAZ

TOUGHNESSA major application of the assessment routines is the

investigation of the structural integrity of welds. Welds arethought to be generally defective, thereby increasing the risk offailure in comparison to non-welded components. An importantdifference between the two that has to be accounted for is theinhomogeneity in terms of mechanical properties of the weld.As shown before, low toughness is commonly associated withwelds in pipes and structural steels. Although the alloying andwelding techniques have been optimized to reduce the amountof such low toughness areas, commonly referred to as Local

brittle zones (LBZ), they cannot be completely avoided. Theimportant question that has been subject of discussion within

the last years is the significance of such LBZ to the structuralintegrity of welded components.

A considerable amount of research work was directedtowards fracture toughness tests, wide plate tests and thedifferent behaviour of those tests when encountering areas oflow toughness in the HAZ. The influence of the size of LBZs[8[9], the sampling methods [10], notch location, weldgeometry mismatch and loading mode [11[12] wereinvestigated. It was shown that the CTOD test is very sensitiveof boundary conditions, therefore it was even consideredquestionable it the critical CTOD is a material constant [13].Wide plate tests were conducted to correlate the results offracture toughness tests to the structural behaviour of weldedcomponents [14-[18], the discussions were controversial.Whereas some authors did not see the safety of the structuresendangered [16[18], others found low CTOD values to coincidewith fractures in wide plate tests [14,[15]. The followingexplanations for the discrepancy between fracture toughnesstests, wide place tests and actual structures in service werefound:

Fracture Toughness tests like CTOD are very criticalin terms of placing pre-cracked defects so that theysample a large quantity of low toughnessmicrostructure

Constraint is much higher in the deep notched SENBspecimens than in the actual structures , thereforeinstability predictions based on CTOD results may beoverly conservative and not realistic [11]

The probability for a combination of large defects, lowtoughness and high loads is very low in the realstructure

In a large structure, the areas of low toughness aresurrounded and supported by tougher material, CTODtests aim at excluding this effect

If the above named mechanism does appear in aCTOD test in form of a pop-in, the assessment is verystringent although the appearance of pop-ins is alsoattributed towards the test itself (high constraint-highload)

It should be noted, that the research work summarizedabove was mainly conducted on welds on structural plates foroffshore industry. A relatively small amount of experimentalwork has been conducted on welded pipes exposed to internal

pressure. In all cases, the pipes failed at stress levels above theyield strength, regardless of the low toughness values found inthe HAZ. In [19], CTOD values down to 0.022 mm wereobtained at -10C. Ring expansion and burst tests wereconducted where the test pipes contained defects in a range ofdepths up to 50% of the wall thickness and lengths up to 450mm. The failures initiated from defects successfully positionedinto the CGHAZ, yet the failure pressure level was well abovethe levels predicted by fracture mechanics assessmentaccording to PD6493 [20], in fact, the test results suggested thatthe failure behaviour was toughness -independent. In [21], thedefects were exposed to fatigue loading to sharpen the notch tipradius, the defect depth equalled 50% of the wall thickness.Regardless of the sharpened notch tip which is in this respectcomparable to the notch tip in fracture mechanics tests, the

pipes failed at stress levels above yield strength. The study wasaimed at evaluating fracture mechanics concepts, it isnoteworthy in this context that the Battelle concept [22] yielded

Type of defect Defect position Detectability Planar surface breaking defect 1 Safe detect ion (Visual, expanding, AUT weld

seam) Undercut Safe detection (Visual, AUT weld seam)

Minor undercuts can be tolerated Slag inclusion Planar defects, safe detection Lack of fusion 2 Virtually not existing for SAW and perfectly

detectable by AUT Lack of overlapping/

interpene ration 3 or 4 Safe detection (AUT weld seam, X - Ray,macro at pipe ends)

1 2

3 4


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the most accurate predictions. A recent study [23] on seam welddefect tolerance applied a constraint based fracture mechanicsanalysis on a surface breaking defect. Despite the lowtoughness values that were found in the CTOD tests, the ringtension tests which were conducted exhibited plastic collapsefailure mode. Standard fracture mechanics assessment routinesimplied the defective ring to be unsafe, whereas constraint

based fracture mechanics assessments were able to predict the plastic collapse failure mode in a conservative manner.

The question whether or not a brittle failure may take placeis always related to the triangle of load-defect-toughness, thefocus in the past being clearly held on the estimation of thetoughness. Especially the estimation of a realistic defect size isof large importance when assessing the impact of lowtoughness. Modern NDT systems allow a safe detectionespecially of the more detrimental planar defects, a fact thatshould be considered in an ECA. A calculation based on BS7919 [24] utilizing the computer program Crackwise [25] wasconducted to demonstrate these effects. The load was assumedto be 70% of SMYS of grade X80 which is realistic for aninternal pressure load in a pipeline. A calculation of the criticalflaw height for a surface flaw and an embedded flaw positionedin different ligament heights varying the assumed toughnessfrom CTOD = 0.01 mm to 0.1 mm was conducted. CTODvalues below 0.1mm represent the lower bound HAZ results incritical situations, e.g. heavy wall pipes, as can be seen inFigure 8.

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have therefore already demonstrated the resistanceagainst fracture at a stress level higher than yieldstrength

DISCUSSION

It can be concluded that the manufacturing potential forimprovement of the HA Z toughness has been widely applied.Alloying techniques are continuously being modified toenhance the HAZ toughness and to restrict the formation ofLBZs. The research on welding technology has focused onimproving the, i.e. lowering, the heat input in the SAW processwhile maintaining the benefit of high deposition rates andwelding speed and thus having a very economical process whilemaintaining acceptable toughness properties. This has led to the

point where the reject rates are low but under certaincircumstances, e.g. high wall thickness, cannot be totallyneglected.

At the same time as the focus was set on the alloyingtechniques and processing routes to optimize the HAZtoughness, progress has also been made in other areas ofresearch. The welding technique itself has been improved tolower the rate of defects and very importantly, the NDTcapability has been developed. Nowadays, modern NDTtechniques are utilized to inspect the complete longitudinalweld of each individual pipe.

The status quo for longitudinal welded pipes may besummarized as follows. There is a relatively low probability ofdefects, especially in comparison to the multi-pass welding

process used in offshore welding. At the same time, again incontrast to offshore structural welding, each weld is inspectedwith NDT suitable for a safe detection of the most critical, i.e.

planar, flaws. And on top of these measures, there is the coldexpansion process subjecting the pipes to approximately 1%deformation and the hydrostatic test conducted in the mill priorto delivery at a load level close to the yield strength of the pipe.These production steps would lead to failures if there was acombination of low toughness and large flaws present in thetested pipe. Assuming each of these pipes d oes contain areas oflow toughness in the HAZ, a proof test of the completestructure has thereby been conducted.

On the other hand, the important question is: How has thetesting philosophy in terms of small scale toughness testschanged in the course of the time to take into account thesefindings? How is the difference to typical offshore structuralwelding, which was basis for research on LBZ for a long time

period, accounted for?

It seems that the testing was intensified in numberconducting the tests in a more stringent manner with the aim tofind LBZs. In this context, it is important to realize that

published research results (details in the previous chapter) onthe significance of low toughness to the structural integritysupported the thesis that due to the supporting effort of areas ofhigher toughness, the LBZs do not lead to premature fracture.These findings are not accounted for in current requirements.

Also, the statistical nature of the localized brittle zones isnot accounted for in the retesting procedures, instead pipes arerejected which are, by the nature of the production processwhich is stringently controlled in modern pipe manufacture, notdifferent to the rest of the batch which is released.

It seems that at this stage, the most important issue thatneeds to be resolved is: Under which circumstances can LBZ ina longitudinal weld of a pipe endanger the structural integrity?

These boundary conditions need to be excluded with themeans of appropriate testing and the stipulation of adequaterequirements. Looking at the triangle of load, toughness anddefect interacting necessary to cause failures in structures, onlythe latter can be influenced in pipe production and are thereforeaddressed in this context. Solutions to the problems could be:

Determination of the extension of the LBZs. Anindication for the width of the zone is given by theresults of the FL+2mm and the FL+5mm tests,where the latter are in the range of the basematerial and the former can be somewhat in

between the FL and base material. Therequirements for the FL values could be relaxed ifit can be proven that the extension is not critical.

Specifying a fracture toughness test that leads to a better correlation with full scale test results.Research is ongoing in this area and aims attesting with a lower constraint which is morerepresentative of the flawed pipe subjected tointernal pressure

Incorporating the probability of occurrence ofdefects and the maximum defect geometry whichcan survive the expansion and mill hydraulic

pressure test to set up acceptance criteria fortoughness levels tolerable within localized zones.

Retesting the pipe that was rejected and releasingit if the acceptance levels are then met. This

procedure does not solve the unansweredquestions, yet it is a pragmatic way of makingallowance for the statistical nature of LBZs.

CONCLUSIONS

Since more than two decades the existence of lowtoughness areas in the HAZ of submerged arc welded linepipeis known and investigated in comprehensive studies with regardto their metallurgical nature and their significance on structuralintegrity. The alloying techniques and steel making practiceshave been successfully developed to improve HAZ toughnessand are nowadays technical standard in modern manufactures.

Nevertheless low toughness values in Charpy-V and CTODtests are still found as frequency and criticality of testing has

been increased in current specifications. As low toughness HAZareas are statistically distributed along the weld seam of each

single pipe length, an increase in test frequency will not lead tohigher quality of the pipe delivery, but to higher reject rates andcosts due to the higher probability of encountering these areasof limited size. Furthermore recent investigations on structuralsignificance of low toughness HAZ showed no implications onthe integrity.

It is therefore recommended that the existing proceduresfor defining and testing HAZ toughness are re -considered underconsideration of the actual loads, the defect probability and,incorporating the size of the LBZ, also their significance on thestructural integrity.


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REFERENCES

[1] Niederhoff, K and Grf, M.K., 1990, ToughnessBehaviour of the Heat -Affected Zone (HAZ) in DoubleSubmerged-arc Welded Large Diameter Pipe, Proc. Pipeline

Technology Conference, Oostende, Belgium, Vol. B, pp. 13.1 -13.9

[2] Grf, M.K. and Niederhoff, K., 2000, Properties ofHAZ in Two Pass Submerged-arc Welded Large DiameterPipe, Proc. Pipeline Technology, Volume II, R. Denys, Ed.,Elsevier Science BV

[3] Kanazawa, S. and Nakashima, K., et al., 1976,Improvement of Weld Fusion Zone Toughness by fine Ti/N,Trans. ISIJ, 1976, 16, pp. 486-495

[4] Suzuki, M., Tsukada, K. and Watanabe, I., 1982,Newly Developped Arctic Grade High Al/Low N/Micro TiType Offshore Structural Steel, Proc. Int. ConferenceOffshore Welded Structures, The Welding Institute, London,

paper 16

[5] Chijiiwa, R., Tamehiro, H. et al., 1988, Extra HighToughness Titanium Oxide Steel Plates for Offshore Structuresand Line Pipe, Proc. 7 th Int. Conf. OMAE Houston, USA,1988, ASME, pp. 165-172

[6] Liessem, A., Grimpe, F. and Oesterlein, L., 2002,State-of-the-Art Quality Control during the Production ofSAW Linepipes, Proc. 4 th Int. Pipeline Conference, Calgary,Alberta, ASME, paper IPC2002-27141

[7] DNV report 2003-1138, 2003, Risk Analysis-LinePipe Fabrication

[8] Haze, T. and Aihara, S., Influence of toughness andsize of local brittle zones on HAZ toughness of HSLA steels,7th International Conference on Offshore Mechanics and ArcticEngineering, Houston, 1988

[9] Devillers, L., Ka plan, D. and Maas, E., The influenceof local brittle zones on plate toughness application to weldedstructures, International Conference on weld failures, London,1988

[10] Toyoda, M., Minami, Y., Yamaguchi, Y. and

Kaneshima, Y., Influence of extraction method of CTODspecimen on HAZ toughness measurement, 10 th InternationalConference on Offshore Mechanics and Arctic Engineering,Stavanger, 1991

[11] Kocak, M., Yao, S. and Chen, L., Fracture behaviourof local brittle zones of offshore weldments, p.365-379

[12] Kocak, M. et al., Effects of notch position and weldmetal matching on CTOD of HAZ, International Conference onWeld Failures, London, 1988

[13] Satoh, K. and Toyoda, M., Evaluation of LBZ: HAZfracture toughness testing and utilisation of toughness data tostructural integrity, 7 th International Conference on OffshoreMechanics and Arctic Engineering, Houston, 1988

[14] Webster, S. E. and Walker, E.F., The significance of local brittle zones to the integrity of large welded structures, 7 th International Conference on Offshore Mechanics and ArcticEngineering, Houston, 1988

[15] Webster, S.E. and Bateson, P.H., Significance of local brittle zones to structural behaviour, Materials Science andTechnology, January 1993, Volume 9, page 83-90

[16] De Koning, A.C. et al, Feeling Free despite LBZ, 7 th International Conference on Offshore Mechanics and ArcticEngineering, Houston, 1988

[17] Hubo, R., Krebs, H. and Dahl, W., Influence of LocalBrittle Zones on the failure behaviour of a submerged arc

multilayer welded joint of a thermomechanically treated FE E460 steel, 7 th International Conference on Offshore Mechanicsand Arctic Engineering, Houston, 1988

[18] Pussegoda, L.N. et al., Significance of local brittlezones in weld heat-affected zones wide plate tests, 11 th International Conference on Offshore Mechanics and ArcticEngineering, Calgary, 1992

[19] Fu, B., Guttormsen, S., Vu, D.Q., Chauhan, V. and Nokleebye, A., Significance of low toughness in the seam weldHAZ of a 42-inch Diameter Grade X70 DSAW Line pipe Experimental studies, 13 th Biennial PRCI/EPRG JointTechnical Meeting, New Orleans, 2001

[20] PD6493:1991, Guidance on methods for assessing theacceptability of flaws in fusion welded structures, PublishedDocument of the British Standards Institute, BSI 1991

[21] Dren, C., Junker, G. und Korkhaus, J., Bewertungvon bruchmechanischen Konzepten anhand von Bauteil- undLaborversuchen an lngsnahtgeschweiten Grorohren,3RInternational, 1987

[22] Kiefner, J.F. et al., Failure stress levels of flaws in pressurised cylinders, ASTM STP 536, 1973

[23] Martin, J.T., Koers, R.W.J. and Ohm, R.K., Seamweld defect tolerance in ultra high strength steel pipelines,Pipeline Technology, Volume ?, pp. 521-530, Elsevier ScienceB.V. 2000

[24] BS7910:1999, Guide on methods for assessing theacceptability of flaws in metallic structures, British StandardsInstitute BSI 10-2000

[25] Crackwise 3, BS7910:1999 fracture/fatigueassessment procedures , version 3.14


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