FOURCRACK – ADVANCED COAL-

FIRED POWER PLANT STEELS

Avoidance of Premature WeldFailure by ‘Type IV’ cracking

PROJECT SUMMARY 304

OBJECTIVES

• To generate and assess cross-weld creep rupturedata on welded joints failing by the Type IV Heat-Affected Zone (HAZ) cracking mechanism insteels for high temperature application inadvanced PF plant.

• To assess the long term risks of welded plantcomponent failure.

• To optimise steel selection for thick section highalloy ferritic steel components in advanced coal-fired power plant.

• To provide a sound basis for determiningappropriate limiting plant design temperature andstress conditions.

SUMMARY

Type IV cracking in the weld Heat-Affected Zone(HAZ) is likely to be the critical problem which willlimit design conditions for satisfactory operation ofadvanced PF plant. The FOURCRACK project carriedout high temperature creep testing of welds inadvanced high alloy steels with a range ofspecifications, supplemented by specialised testing,optical and electron metallography, weld simulationand data assessment. Further work outsideFOURCRACK will extend testing to longer durations.

E.ON UK led the project and undertook metallurgicalinvestigation and assessment. Mitsui Babcockcarried out weld manufacture and creep rupturetesting. RWE npower investigated and characteriseda special weak material. In parallel work,Loughborough University carried out electronmetallography and weld simulation. Five externalorganisations also provided test materials and/orweldments.

CLEANER FOSSIL FUELS PROGRAMME– CLEANER COAL R&D PROGRAMME

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However, most of the longterm creep test datacurrently available appliesonly to the parent steels.

If we choose which steel touse on the basis of parentdata alone, we may notmake the best choice forsatisfactory long term weldperformance. If we designcomponents without takingaccount of weld properties,we risk incurring major weldinspection, repair andreplacement costs duringthe lifetime of the plant. Ifwe instead designconservatively on the basisof inadequate information,we run the opposite risk ofan over-conservative andhence uncompetitive plantdesign. ProjectFOURCRACK aimed toovercome theseuncertainties, establish theeffective limits on powerplant design conditions andoperating temperature, andthereby improve the marketprospects for cleaner coaltechnology in the UK andworldwide.

The earlier DTI supportedPIPPE project, undertakenby Mitsui Babcock EnergyLimited (MBEL), BritishSteel (now Corus) andPowergen (now E.ON UK),established the newEuropean E911 steel as aviable high temperatureplant material in competitionwith Japanesedevelopments. PIPPEshowed that E911 has goodcreep properties and

Figure 1. Type IV weld HAZ cracking in a FOURCRACK P122 weld creeptest specimen. (Courtesy of E.ON UK)

The results showed that stronger parent materials oftenhave stronger HAZs, but there is no direct correlation.Future development should therefore focus on optimisingweld HAZ as well as parent material properties. Theresults confirmed the conclusions of the earlier DTI CleanCoal PIPPE project, and showed that they apply toJapanese and US as well as European steels. Long termweld HAZ creep strength typically falls to no more thanabout 60% of that of parent material. Conservativedesign will therefore be required for welded componentsif recurrent plant maintenance and repair problems inservice are to be avoided.

BACKGROUND

Advanced coal-fired power plant, operating at highertemperatures and pressures, can achieve substantialimprovements in thermal efficiency and hence reductionsin carbon dioxide emissions. However, theseopportunities may be lost if plant reliability, availability,maintainability and operation cannot be guaranteed.Because of the perceived technical risks anduncertainties with advanced plant, established subcriticalpower plant designs are still frequently successful on the international market, despite their relatively poorefficiencies. Lack of confidence in the long termperformance of welds in advanced ferritic steels is one ofthe main concerns. It is becoming increasingly clear thatType IV cracking in the weld heat-affected zone may be the life-limiting failure mechanism in advanced hightemperature components such as superheater andreheater headers and steam pipework, chests and valves.

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oxidation resistance and does not sufferundue long term degradation in service.However, the relatively short term creeptests carried out on welded joints were lessencouraging. Extrapolation of the datasuggested that long term weld strengthmight, because of Type IV cracking, fall to aslittle as 50% of the parent material rupturestrength. This would put all welds - not justthose under substantial system loading - atrisk of early failure. However, the severeextrapolation was subject to uncertainty.Also, little comparative data assessment hadbeen carried out, and it was not clear towhat extent alternative high alloy steelswould be at comparable risk.

THE FOURCRACK PROJECT

Outline

The project was designed to produce cross-weld creep data on a wide range ofalternative advanced high alloy ferritic steels,manufactured with different processingroutes, heat treatment conditions, andchemical compositions. The European E911steel was compared with the leadingJapanese alternatives P92 and P122 andwith the older established steel P91. Thewelded materials tested included forging,casting, plate, and pipe components. Aspecial cast of Grade 91 steel with known

poor properties due to its adversealuminium / nitrogen ratio was alsoinvestigated. Separate test series werecarried out after welding onto this material inthe as-manufactured, renormalised andtempered (strengthened), and preaged(weakened) conditions.

A novel test matrix was devised toinvestigate variations in both stress andtemperature with four tests per weldment.This made it possible to test as many as tenweldments and obtain sufficient data to ranktheir relative performance over a range oftest conditions. The programme wassupplemented by additional testing with aspecial waisted specimen geometry,comprehensive metadata compilation, weldthermal simulation, a creep dataassessment project on E911 data suppliedby the European Creep CollaborativeCommittee (ECCC), and optical and electronmetallographic investigation.

Results and Discussion

The assessment of the ECCC E911 databasewas carried out at an early stage, incollaboration with Dr C. Servetto of theItalian Welding Institute. A larger data setthan that of PIPPE and an improved PD6605based assessment procedure indicated thatweld creep rupture strength falls toward afloor value of about 60% of the parentstrength in the longer term. Whilst this is

Figure 2. Newly installed P91 steel headers in660MW Changshu spiral wound supercritical coal-fired power plant.(Courtesy of Mitsui Babcock Energy Ltd.)

Figure 3. Type IV crack on the header side toe of alarge P91 branch weld on a coal-fired power plant. (Copyright and used by permission of RWE npower)

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slightly less pessimistic than the PIPPEprediction, it confirms the general trendsand the scale of the problem. Theassessment also provided reference lifepredictions for mean E911 weldperformance as a function of testconditions. These provided a baseline forcomparison of the alternative weldedmaterials investigated in FOURCRACK.

The test results on the stronger P92 andP122 parent materials included many shortterm failures in the weld metal. Hence,there is some concern that for thesestronger alloys, the creep properties ofmatching weld metals cannot keep pacewith those of the parent materials. However,there were clear trends toward HAZ asopposed to weld metal failure as the testtemperature, and/or test stress, wasreduced to a lower value, closer to theconditions which would apply to realcomponents. As both temperature andstress acceleration promote weld metalfailure, it can be difficult to find testconditions which produce Type IV failure in a short term test. However, it can beconcluded that it is HAZ Type IV failurewhich remains the greater long term riskunder advanced plant operating conditions.

Those weldments which did fail in the HAZshowed significant differences in relativecreep rupture strength, and these proved to be largely independent of the testconditions. The HAZ failure data couldtherefore be used to determine a broadlyconsistent ranking order of the relative creeprupture strengths of the different weldments.Hence, in those cases where weld metalfailure did not intervene, short termaccelerated test HAZ failure data appeared to be a reasonable guide to relative creeprupture strength in the longer term.

An overall assessment of the data showeda broad correlation between parent material

and cross-weld HAZ creep performance.Stronger parent materials tend to have

stronger HAZs. However, the correlation is far from perfect. Hence, future alloydevelopment should address theoptimisation of cross-weld, as well asparent, material creep properties.The main reason why parent and HAZproperties are not always perfectlycorrelated was identified as the effect ofvariations in prior heat treatment. These cansubstantially affect parent materialproperties, but may have only a minorinfluence on the properties of a subsequentweld HAZ. Hence, thick sectioncomponents, with relatively poor parentcreep properties associated with long heattreatment times, may tend to have relativelygood HAZ properties. The work on Grade 91material provided the most strikingdemonstration of the complex influence ofheat treatment. Renormalising andtempering, while greatly strengthening theweak parent material employed, actually hada marginally adverse effect on theperformance of the subsequentlymanufactured weldment.

In most cases, it was apparent that changesin alloy chemistry which strengthen theparent material also tend to strengthen theHAZ. Thus, P92 and P122 generallyoutperformed E911 both as parent materialsand as weldments, although the weakercasts of these Japanese materials whentested in FOURCRACK were not greatlysuperior to the mean E911 weld data.Conversely, the older Grade 91 (P91) materialis generally weaker than E911, both in testson the parent material and on the weldment.However, the special high aluminium weakGrade 91 material tested, which substantiallyunderperforms mean P91 parent creep data,showed a lesser shortfall in its cross-weldperformance. It could therefore be that highaluminium, which is known to combine withnitrogen and suppress the formation of keycreep strengthening carbonitrideprecipitates, has a more substantiallyharmful effect on parent steel propertiesthan on those of the weld HAZ.

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In summary, the results confirmed the broadconclusions of PIPPE, and showed that theyapply generally to Japanese and US steelsas well as to European steel E911. Thecross-weld creep strength falls to a level atwhich weld HAZ cracking must generally beexpected to occur far in advance of parentmaterial life exhaustion. Conservative plantdesign may therefore be essential to avoidexpensive long term maintenance and repaircosts.

CONCLUSIONS

• Type IV cracking in the weld heat-affectedzone is likely to be the life-limiting failuremechanism in advanced high temperatureplant components.

• Weld HAZ creep performance shows abroad, but far from perfect, correlationwith parent material performance.

• Generally, weldments in the establishedsteel P91 are weakest, with E911intermediate, and advanced materials P92and P122 strongest. However, there issubstantial cast-to-cast variation.

• The weaker of two parent material castscan thus, in some cases, have thestronger HAZ. The main reason for this is

the effect of prior heat treatment onparent material strength.

• Typically, weldment creep rupturestrength appears to fall toward a floorvalue of the order of 60% of the parentstrength in the longer term.

• However, different material casts showsignificant differences in their weldment/parent material creep strength ratios.Hence, future alloy development shouldfocus on optimisation of cross-weld, aswell as parent, material creep properties.

• Conservative design of advancedsupercritical PF plant may be essential toavoid the risks of costly long termmaintenance and weld repair problems,and to gain the confidence of new plantbuyers and operators.

POTENTIAL FOR FUTUREDEVELOPMENT

At present, the prospects of developing newhigh alloy ferritic steels or weldingprocesses which would eliminate the TypeIV cracking problem appear fairly slim.However, it is much more likely thatmaterials selection to optimise weldperformance will achieve usefulimprovements in achievable plant designlimits and operational reliability. New alloys,for example those now being developed forimproved oxidation resistance under thenew COST 536 programme supported byDTI, thus need to be critically evaluatedagainst established competitors both interms of cross-weld and parent materialcreep performance.

FOURCRACK successfully obtained weldcreep rupture data to durations beyond10,000 hours. However, truly long termcreep data, out to over 30,000 hours andbeyond, are normally required for designpurposes. Commonly, only parent materialdata to these durations are available, andplant is designed using simplified safety

Figure 4. FEGSEM electron micrograph showinggrain boundary microcracking close to the Type IVzone fracture face in a FOURCRACK P92 weld testspecimen.(Courtesy of Loughborough University)

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Further information on the Cleaner Fossil Fuels Programme, and

copies of publications, can be obtained from:

Cleaner Fossil Fuels Programme Helpline, Building 329, Harwell International Business Centre, Didcot, Oxfordshire OX11 0QJTel: +44 (0)870 190 6343 Fax: +44 (0)870 190 6713E-mail: helpline@cleanercoal.org.ukWeb: www.dti.gov.uk/cct/

© Crown copyright. First printed July 2005.Printed on paper containing a minimum of 75% post-consumer waste. DTI/Pub FES 05/954

COST

The total cost of this projectis £304,000, with theDepartment of Trade andIndustry (DTI) contributing£152,000, and E.ON UK,Mitsui Babcock, and RWEnpower contributing thebalance.

DURATION

48 months – January 2001to December 2004.

CONTRACTOR

E.ON UK plcPower TechnologyRatcliffe-on-SoarNottinghamNottinghamshireNG11 0EETel: +44(0)115 9362497Fax: +44(0)115 9362416Email: david.allen@eon-uk.com

COLLABORATORS

Mitsui Babcock EnergyLimitedRWE npower plcLoughborough University

factors to allow for weld performance. For advanced highalloy ferritic steels, therefore, there is a need forgenuinely long term cross-weld data. This will enable thedevelopment of improved design philosophies whichrecognise the critical importance of welds, and therebyimprove confidence in the future reliability and hencemarket prospects for advanced coal-fired power plant.

The UK High Temperature Power Plant Forum (HTPPF)has recognised these critical needs. A HTPPF testprogramme which will use specimens from theFOURCRACK Grade 91 weldments to extend the testdata out to 50,000 hours has just commenced, while asecond test programme on the FOURCRACK advancedsteel weldments is in preparation.

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