p91 welding technology6

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the Conduit A quarterly publication frM&M Engineering Associates, IVol. 10, NoBy Ronald E. Munson, P.E.Corporate Engineer

The energy crisis of the mid-1970sled indirectly to the accelerateddevelopment of a class of Grade 91ferritic/ martensitic alloy steels.These steels were developed toreplace the chromium-molybdenumalloy steels such as P22/T22 (2 Cr-1 Mo) or P11/T11 (1 Cr- Mo)used extensively for hightemperature service in power plants.The new generation of Grade 91steels would be suitable in advancedgeneration power plants with highersteam temperatures and pressures .The ultimate goal was higherthermal efficiency. Higher thermalefficiency also translates intoenvironmental benefits as less fuel isburned to produce equivalentamounts of thermal energy.

The new generation materials weresteels with ostensibly 9% or 12%Chromium with other elements suchas tungsten, molybdenum,columbium, vanadium and coppernickel added to achieve optimalmicrostructure and properties. Themicrostructures of these steels weremartensitic as compared to themore typical bainitic structures forthe P11/T11 or P22/T22 low alloysteels. The martensiticmicrostructure is developed byprecise heat treatment to developboth superior high temperaturecreep resistance and tensilestrength.

The development of the Grade 91(P91/T91) family of steels has movedsteadily forward since the mid-1970swith increasing use in new powerplants and some older plant retrofitapplications. The Grade 91 type

materials are well-suited for the nhigher temperature boilers and HeRecovery Steam Generators (HRSbeing constructed. Because theGrade 91 steels have higher streng-to-weight ratios the componentthickness (and therefore, thermalmass) can be considerably less thawith the Cr-Mo steels. With thepressure components thinner andless massive the units can be starteand shut down more quickly withless thermal fatigue damage. Thischaracteristic is well suited for thedual-cycle combustion turbine HRplants being constructed today.

So if Grade 91 is so good why notuse it more extensively? There arseveral reasons. Grade 91 alloy isinherently more difficult to heattreat and is less tolerant to over-temperature abuse in service thanP11/T11 or P22/T22 ancestors.Lets discuss some issues.

Mechanical Fabrication

Because the strength of the P91 isdependent upon propermicrostructure any mechanicalfabrication of the steel must respe

and preserve the structure. Forexample, hot bending of tube orpipe will remove the martensiticmicrostructure and result in poor -formed properties. Cold-bendingdifficult and also results in theformation of detrimental residualstresses. P91 must be re-normalizand tempered after bendingoperations.

(Continued on pag

EPRI Publication Available ................. 3

The M&M Engineering Laboratory .. 4

Events and News ................................. 6

In this Issue

P91 Steel - No Good Deed Goes Unpunished

Figure 1. T91/T22 tube stub showing a crack adjacent to the P91weld fusion line (arrow).


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Welding

The welding of P91/T91 isparticularly troublesome if one doesnot understand or follow a rigorousprocedure. First of all, the weldingconsumables must be closelymonitored and chosen. Current

practice is to require a certifiedmaterials test report (CMTR) foreach batch of welding wire. Thecomposition must be kept withinspecific limits that are much morerigid than the current AWS orASME guidelines. For example, ifthe sum of the nickel and manganesein the weld consumable is too highthe post weld heat treatment cyclescan actually exceed the lower criticaltemperature for the alloy. If that

happens then the componentmicrostructure is completelycompromised.

Welding Group 91 and the otherCreep Strength Enhanced Ferritics(CSEF) require more care and

control than T22/P22 alloys. Theyall require a high pre-heat, aminimum cooling hold point and apost weld tempering heat treatment.If low hydrogen consumable andwelding procedures are notmeticulously followed the weldingmay also require a post weldhydrogen bake (immediately afterwelding). These requirements areindependent of thickness. Even asimple field repair on a tube will

require a full post weld temper heattreatment at 1400F.Figure 1 and Figure 2 show what canhappen when welding proceduresare not followed precisely. In thiscase, safe-ends were fabricated to

join a new P91 header to existing

T22 tubes. Safe ends are made bywelding short sections of T22 tubingto T91 tubing using a low alloy steelweld. After welding, there was aconsiderable delay before post weldheat treatment resulting in aneventual field failure. The strong butbrittle post weld microstructureallowed environmental cracking tooccur in the welds high residualstress area before the toughening ofthe post weld tempering was done.

The Grade 91 alloys also offer an in-service challenge. Because thesealloys loss their strength if exposedto temperatures of more that 1400F they are less tolerant to boilerupsets such as dry-firing or lowwater conditions. Whilecomponents manufactured of P22 orP11 have been shown to be verytolerant to short durationexposures during these events, P91

alloys have no tolerance for theseconditions. Even short exposures tothese conditions will requirereplacement. Normal fabricationpractices, such as local torch heatingduring fit-up can significantly damageP91/T91 parts.

In summary, as the saying goes nogood deed goes unpunished. TheP91 family of alloys has a vastimprovement in their properties, but

at the cost of being more demandingto fabricate and weld. They are lessforgiving than their T22/P22 or P11/T11 ancestors. As technologyadvances, the knowledge of how toproperly utilize the technology mustincrease. Be sure your fabricatorsand contractors respect thedemands of this new technology.

Figure 2. Metallographic section showing environmentally-assisted cracking in a T22/P91 tube safe endweld.


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By Andi Cragen,Mechanical Engineer

In July of 2010, The FeedwaterHeater Tube Failure Manual(Report 1017622) was madeavailable for purchase anddownload by the ElectricResearch Institute (EPRI). M&MEngineering Associates Inc.conducted the primary researchand documentation for thismanual. The report will allowplant engineers, metallurgists,chemists, and laboratory failureanalysis personnel to identifyfailure mechanisms in feedwaterheaters to provide a basis forroot cause analysis and guidanceon corrective action.

This guide is a valuable tool forunderstanding failure

mechanisms observed in bothold and new feedwater heaters

and assists in developingcorrective actions to mitigate oreliminate damage. The manualincludes comprehensiveinformation and guidance on thematerials of construction, anadvanced understanding ofdamage mechanisms, and thetechnical criteria for mitigationor elimination of damage infeedwater heaters.The primary tube failuremechanisms discussed in themanual include: the following:

Wet Steam ImpingementErosion

Tube Vibration Flow Accelerated Corrosion

(FAC) General Corrosion Stress Corrosion Cracking

(SCC) Cavitation-Type ErosionTo find more informationregarding the manual visitwww.epri.com.

Contact the Authors

Ron Munson, P.E., [email protected]

Andi Cragen, 512-407-3760

[email protected]

Spencer Rex, 512-407-3755

[email protected]

Feedwater HeaterTube Failure Manual

Failures in Steam/Power Generation

Staying Out of Trouble!!!

M&M Engineering Associates, Inc. will behosting a Two-Day Conference that will focus

on the issues most common in steamgeneration systems.

The Conference is scheduled forMarch 9-10, 2011, in Austin, Texas.

Check our website and the Winter issue oftheConduit for a list of topics that will be covered.
http://www.epri.com/http://www.epri.com/


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By Spencer Rex,Metallurgical Engineer

A consulting engineering firmthat has a laboratory in-househas many advantages in themarketplace. The ability toobserve and direct thelaboratory process directly isinvaluable to the lead engineer.M&M Engineering Associates,Inc. has a world classmetallurgical laboratory wherelaboratory analyses areperformed according to ASTM,ASM, NACE and other industry

standards. All testing equipmentin the laboratory is calibratedand maintained to ensure thatall measurements anddocumentation meet the highestindustry standards fortraceability and accuracy. Alllaboratory personnel have beenthoroughly trained in the testingthat they perform while atrained Laboratory SafetyOfficer ensures that safety is thetop priority. The following is a

brief description of some of thecapabilities of the M&MEngineering Associates, Inc.Laboratory in Austin, Texas.

The first step in anyinvestigation is a thorough visualexamination and documentationof the sample as it is received.Often, a trained eye knowswhat to look for and will seefeatures normally overlooked.Photography is done in ourPhotography Room (Figure 1)

using a variety of different digitalcameras and lighting options.Also available is aStereomicroscope (Figure 2)which is used to examine anddocument surface features atmagnifications up to 60X.

Non destructive testing (NDT)capabilities include magneticparticle and dye penetrant

testing for detection of cracks,defects and flaws, and ultrasonictesting for thickness

measurements. NDT is helpfulin a laboratory setting to locateotherwise invisible features thatwill help guide the investigation.The location of cracks and flawstypically dictates where asubmitted sample will besectioned. For weld inspection,M&M Engineering has aCertified Welding Inspector onstaff.

The M&M EngineeringLaboratory has a wide array ofequipment used for sectioning(Figure 3). Our precisioncutting is done on a diamondsaw with a blade kerf of lessthan .035-inches. Our largestcuts are made using a HEM sawwhich has a throat depth of 18.5-inches and height of 28-inches.The various saws may be fittedwith different blades allowingsectioning of a wide variety ofmaterials. To move and liftheavy samples onto the saws,

the M&M EngineeringLaboratory has an array ofcranes and forklifts, the largestof which is rated to handle5,000 lbs.

Metallography is the techniquemost commonly used whenstudying the microstructuralfeatures of a material.Microstructural features indicatemanufacturing, as well as inservice conditions of the item

being evaluated. This processbegins by deciding how thesample should be mounted andin what media. The M&MEngineering Laboratory has bothhot and cold mountingcapabilities and can mount in avariety of conductive, non-conductive, hard or soft mediadepending on the material andthe examination to be

The M&M Engineering Associates, Inc.Laboratory

Figure 3. Machine Shop

Figure 1. Photography Room

Figure 2. Stereomicroscope Room


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performed. Mounts are thenground, polished and etched,a process that variesdepending on the material.Mounted samples can thenbe viewed on a metallographwith up to 1,000X

magnification. Themetallograph is equippedwith a digital camera fordocumentation of theobserved microstructures,cracks, flaws, etc. (Figure 4).

In cases where very smallsurface features areimportant to an investigation,M&M Engineering possessesan AMRAY 1830 scanningelectron microscope (SEM)which can magnify an image

up to 50,000X (Figure 5).The magnification and depthof field capabilities of theSEM are important whenexamining and documentingfracture features. The imageoutput from the SEM can beviewed in the SEM Room ona 46-inch color monitor,where seating is available forup to ten observers. TheSEM is fitted with an EDAXenergy dispersive X-ray

spectrometer (EDS), which iscontrolled by an advanced 4-pi imaging system. The EDSsystem identifies andquantifies elements presentat a specific location. Thisimaging system is able tocolorize the location ofvarious elements, providing amap of their locations. Thisis especially useful ininvestigations involvingunknown corrosive agents.

The M&M EngineeringLaboratory has varioushardness testing capabilities.Hardness testing is importantwhen the properties of amaterial are suspected incausing or contributing to afailure. Bulk hardness isdone with a WilsonHardness Tester wheremeasurements can be taken

in Rockwell A, B or C,superficial hardness, andVickers hardness scales.Microhardnessmeasurements are doneusing a Struers Duramin A-300, Automated

Microhardness Tester thatoutputs the hardness datainto a statistical programwhere it can be analyzed andpresented in a variety ofdifferent ways (Figure 6).

Metallography, NDT,ultrasonic thickness testingand hardness testing can allbe done in the field duringplant outages or onequipment too large to shipto our Laboratory. All

equipment is easily packed infly-away kits for fastresponse. The ability toprovide quick and accuratefailure analysis, diagnostic andusable life analysis whileminimizing customerdowntime is the core of theM&M Engineering Associates,Inc. business model.

The team of mechanical,metallurgical and materialsengineers and scientistsinterpreting the laboratorydata has a combined 502years of experience. Theunique combination of ametallurgical laboratory in-house and extensiveexperience allows M&MEngineering to quickly andefficiently interpret and applylaboratory results to findapplicable solutions andrecommendations for ourclients.

Editorial Note: After graduating

from the University of Utah in

Metallurgical Engineering in

June of 2009, the author

worked in the M&M

Engineering Associates, Inc.

Laboratory before joining our

Engineering Staff earlier this

year.

Figure 4. Metallography Room

Figure 5. SEM Room

Figure 6. Micro Hardness Machine


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Mark Tanner will be attending the Wind Turbine Conference

to be held October 26-27, 2010, at the Renaissance AustinHotel in Austin, Texas.

Dave Daniels will be attending the Nalco Conference Power Plant ChemistrySeminar to be held October 12-13, 2010, at the Northern Illinois UniversityConference Center in Naperville, Illinois.

Ron Munson will be presenting at the Gas TurbineUsers Symposium to be held October 4-7, 2010, atthe George R. Brown Convention Center inHouston, Texas. His presentation is entitledFailure Investigation Principles for Combustion

TurbinesCombining Science and Art .

Dave Daniels will be presenting at the InternationalWater Conference to be held October 24-28, 2010, atthe Crowne Plaza Hotel Riverwalk in San Antonio,Texas. His presentation is entitled HRSG and HighPressure Boiler Water Treatment and Operation.

Dave Daniels will be attending the Second International ConferenceInstrumentation for Power Plant Chemistry to be held November 15-18, 2010,at the Movenpick Hotel Stuttgart Airport in Stuttgart, Germany. Furtherinformation can be found at http://www.ppchem.net/news/Instrumentation%

202010.pdf The conference will consist of both invited and contributedtechnical papers and the official language will be English. There will be a specialvendor exhibition at the Conference.

Spencer Rex will be attending the Battery Power 2010 Conference and Expoto be held October 19-20, 2010, at the Gaylord Texan Resort andConvention Center in Grapevine, Texas. This international conference andexpo will be highlighting the latest developments and technologies in thebattery industry with more than thirty presentations on portable, stationaryand automotive battery technology.

Seminars and Workshops

Upcoming Events

2010 International Water Conference:October 24-28, 2010, The Crowne Plaza Hotel Riverwalk,San Antonio, Texas

Spencer Rex joined the M&M Engineering Associates, Inc.Staff in May of 2009 as an entry-level engineer. Before

joining M&M Engineering, Spencer graduated with adegree in Metallurgical Engineering from the University ofUtah in May of 2009. On Thursday, September 2, 2010,Spencer gave a presentation about Failure Analysis at

M&M Engineering to the Metallurgical Engineering undergraduate students at the University of Utah, givingthe students an idea of the transition from college to the working world.
http://www.ppchem.net/news/Instrumentation%202010.pdfhttp://www.ppchem.net/news/Instrumentation%202010.pdfhttp://www.ppchem.net/news/Instrumentation%202010.pdfhttp://www.ppchem.net/news/Instrumentation%202010.pdfhttp://www.ppchem.net/news/Instrumentation%202010.pdfhttp://www.ppchem.net/news/Instrumentation%202010.pdfhttp://www.ppchem.net/news/Instrumentation%202010.pdf


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the Conduit is distributedfree of charge by M&MEngineering Associates, Inc..We welcome your comments,questions, and suggestions, andwe encourage you to submitarticles for publication.

We grant limited permissionto photocopy all or part of thispublication for nonprofit useand distribution.

For technical information,please contact:

David Daniels(512) [email protected]

Ron Munson(512) [email protected]

Karen Fuentes

(512) [email protected]

Texas Illinois Oklahoma

www.mmengineering.com

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the ConduitM&M Engineering Associates, Inc.4616 W. Howard Lane, Bldg. 2, # 500Austin, TX 78728-6302

p91 welding technology6

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