COAL-FIREDPOWER MATERIALSMajor advances in materialstechnologyover the last decade haveenabled building coal powerplants with much higherthan the current generation.

Vis Viswanathun"Electric Power Research Institute

California

Energy of OhioRobert

PatriciaNational Energy Technology

ery powerplants capableof operatingat steam

and much hgher thanpossible today are under development to

provide relatively low-cost power with much lesspollution.

Projectssponsored by the U.S. Departmentof En-ergy and the Coal Development Officehaveset psi as the goal for what might betermed "Advanced (AUSC)coal-fired power plants. have carried outsubstantialresearchalong the same under theAD 700 program. It is estimated that efficiencyunder these conditionswill improve from tonearly a increase. results not only inreduced fuel costs, but also in reduced plant costs,due to lesspulverizing, transportation, waste emission controls, andconsumption. It can also lead to a decrease in allwaste products and emissions.

The major enabling technology for these AUSCplants is the availabilityof materials with hgh creepstrength,corrosion resistance, andarticle reviews someof thesealloys,with special ref-erence to recent activitiesof a consortium of com-panies Power,Foster Wheeler, Riley Power, Electric Power Re-search Institute, Oak Ridge National Lab, and En-ergy Industries of Ohio. The consortium is underthe sponsorshipof the National Energy TechnologyLaboratory of the Department of Energy and theOhioCoal DevelopmentOffice.

Boiler materials The function of a boiler is to convert water into

superheated steam, which is then delivered to a steam turbine.Fuel with preheated air isburned

of ASM International

Fig. - of a typical arc pipeseither

header) or header.) courtesy Dr. E

in the furnace, is constructed with a wall ofwelded tube panels known as the waterwall. Thecombustion gases flow through the furnace andevaporate the water into steam inside the furnacewaterwall tubes. The steam is then further super-heated in the superheater section, and delivered tothe turbine via main steam pipes. The low-pressuresteam exhausted from the high-pressure turbine isagain reheated in a reheater section, and is deliv-ered to the low-pressure turbines via the reheatpipes.

The fluidsin the system include combustion gases on the outside and on the inside oftubes and pipes.

The key components of a boiler carryingtemperature, high-pressure steam can therefore bebroadly into two configurations: tubes andtluck-wallpipes.The pipes are generally known asheaders or steam pipes, while the tubes are catego-rized into furnace-wall tubes and superheater/ reheater tubes.

Headers:An example of a header is shown inFig. It is a pipe penetrated by a numberof tubes. Theseheavy-sectioncomponents have tomeet creep strength and fatigue strength require-ments based on thermal fatigue.

Femtic and martensitic steels are preferred forthisapplication because of their lower coefficientofthermal expansion and thermal conductivity compared to austenitic steels. Many of the earlyproblems in the AUSC plants in thick-sectionaustenitic steel componentswere traceable to theirsusceptibilityto thermal fatigue.

Therefore, research during the last decade has cused on cost-effective, ferritic steels

ADVANCED MATERIALS 2008 47

48

that could help avoid austenitic stainlesssteels in heavy-section components.Thishas resulted in ferritic steels capable ofoperating at metal temperatures up to620°C with good andfracture toughness. Nickel-base alloysare chosen for higher temperatures, thusbypassing the need for austenitic stain-less steel components.

Tubes: For tubing applications,thermal fatigueisnot a key issue becauseof the smaller sectionsize.

Austenitic steels are suitable up toabout 1250°F(680°C).However,base alloysare needed at higher temper-atures,because they provide more creepresistance. Developments are also un-derway to extend the limiting tempera-turefor austeniticsteels as a cost-effectivemeans of replacing the nickel tubing. Asample list of boiler materials for plantsoperatingwith various steamconditionsisprovided in Table 1.Thebasic composi-tions of the alloysare shown in Table 2.

In addition to creep strength, resistanceis needed tohot corrosion from the fire-side, and to oxidationcorrosionfrom thesteam-sideof tubes.Fireside canlead to premature failuresdue to the in-crease in stresses by reduced crosssection.

Steam-sideoxidationpromotes acceler-ated creep due to reduction of cross sec-tion, progressive increase in tube metaltemperature, and blockage of tubes byspalled oxides. In addition, it causes ero-sion of turbine steam path components

exfoliated oxide particles.To minimize

ADVANCED MATERIALS 2008

corrosion while keeping costs down, austeniticsteel tubes serve inthe portions of the boiler.

Property requirements forwall tubes are similar, with the caveat that the fireside corrosion mechanisms are somewhat dif-ferent.Since the temperatures are lower than for superheaters,ferriticsteels with claddings or coatings possess adequatecreep strength and corrosion resistance.

Advanced alloys forIn the high-temperature and

stress environment envisaged forthe AUSC steam boiler, the limiting mechanical material property islong-term creep strength.A generalcriterion of an average stress of 100

(14.5ksi) to produce rupturein 100,000 hours can serve as aguideline to set the temperature orstress limits. Based on the creepstrengthcriterion, the program se-lected six advanced alloys as can-didatematerials for the target plantof 1400°F psi (35

steam conditions. Basic compositionsof the candi-

date alloys areincluded in Table2. Nickel-base alloy Haynes 230, 740, and CCA 617were selectedfor heavy section, headers, pipes, and tubing.

The austenitic steels HR 6W and Super 304HSteelwere considered for tubing;and the ferritic steel SAVE 12was selectedas a can-didate for both applications at lower temperatures.Figure 2 includes the stress ranges for the alloyclasses and shows that for temperatures above-700°C age hardenable alloys 740shown) and Haynes 230 will be necessary to meetthe 100 criteria.

In support of the U.S.program, extensive long-term creep-rupture testing (now beyond 38,000hours) is being conductedby Oak Ridge NationalLaboratoryto understandhow isaffected by microstructural changes, heat-to-heatmaterial variability, fabrication processes, and welding.

Utilizing the findings of these studies, the con-sortium is gaining confidence in the performanceof new materials, and is providing the groundworkfor the development of stress allowables, developing improved fabrication rules, and deter-mining the applicability of weld materials/ processes. Microstructuralcharacterization, material modeling, and computational thermodynamics are key tools in the evaluation process.

In addition to the creep strengthof thebase metal, strength of the welds is also a major determiningfactor.Weld-strengthreduction factor (WSF) is de-fined as the ratio of the stress-to-rupture of thewelded joint, to the stress-to-rupture of the base metal for a given time and temperature. For a

number of candidateAUSC nickel-base alloys (in-cluding Haynes the weld metal is the weakest

and it fails at a shorter time interval or lower stress level than the base metal.

In the case of Haynes230, the weld strengthfactorhas not been found to be a strong function of testingtime, so a generalWSF of 0.8 is appropriate,whichshould not be a hindrance to current boiler designof component welds. However, for other AUSCmaterials, data indicate lower WSF. Therefore,re-search is ongoing to improve weld performancethrough changes to weld processes, filler metalchemistry,and post-weld heat treatment.

Another consideration for boiler materials is thatcold strain due to fabricationprocesses (bending,swaging, etc.), may cause a degradation in creepstrength. To understand this phenomenon, full-scale pressurized creep tests are being conducted on cold-bent boiler tubes (twoinchesO.D.,wall thickness) in the as-bent condition and in the heat-treated condition.

After one year of testing on a nickel-base alloy,metallographic examination revealed extensive creep damage (cavitation)and some

in a highly strained tube bend. However, a second bend with slightly less cold strain had nocreep damage even at 1/3 life. These data serveasthe basis to set rational cold-work limits for the

in the Code.In addition to creep strengthconsiderations,the

project consortium has also extensively evaluatedthe steamsideoxidation, fireside corrosion,and fab-ricability of the alloys selected for AUSC plantboilers. These studieswill be discussed in Part of

paper next month. Review of turbine materialswill be included in a future issue.

VisViswanathan,Electric Power

94304; tel:

nww.epri.com.

Other projectcontributorsinclude

Bob Brown,and

RobertSwindeman,John

Jeff Sarver, Ian

Sanders,Mike

ADVANCED MATERIALS 2008 49