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DAVID N . FRENCH , INC. , METALLURGISTSSUMMER 1989 ON E LANCASTER ROAD VOL. V I , NO

N O R T H B O R O U G H . MA SSA C H U SET T S 0 1 5 3 2( 5 0 8 ) 393-3635

A VIEW FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOILERSCREEP AND REMAINING LIFE

The high-temperature components, superheaternd reheater tubes, f a i l by a creep or st res s-ure mechanism, a t th e end of t h e i rrv ice l i f e . Fuel-ash corrosion may haveeduced th e wall thick ness so th at the onsetny event, a determination of th e use fulmaining l i f e or material-conditioni s made t o as su re replacement i n afashion. The question is , "What are

it ia ti o n of creep damage well befo re1Creep i s defined as the time-dependantmation a t ele vate d temperature (see Vol.

No. 4 of t h is new slette r). The ASMElowable s t re sses for a l l boi ler mater ials

in th e creep range. One of the c r i t e r i aed i n the dete-&ination of th ese allowableess values i s 1% reep expansion ori n 100,000 hours of ser vi ce . Thussome creep deformati.on w i l l occur.

Fuel-ash corrosio n or oxidation may reducee wall thickness and i ncre ase th e hoopre ss . Steam reac ts with st e el , forms ironde, and c w e r s the ID surface of a l lSince the thermal conductivityi s much less than steel, theseas an insulatinga r r ie r t o heat flow (VOL I , No. 1 ) . The neti s to raise tube-metal temperatures.he combination of increased metalomotes ea rl y tube fa il ur es . The lo ca l heatas the

i s a function of heat flux.is a wide variation in thel condition of a superheater orA tube cl os e to the roof seldoma i l s by creep and often con tains no askosion wastage; th e same tube a t the

u ally f a i l s f i r s t and shows the most seriouswastage.

In any material-co ndition assessment, thmost distressed tubes and highest heat-fluregion should be examined f i r s t . Any si ntube can show wide differences inmetallurgical features. There ar e sub tlechanges within the microstructure that ocduring the creep deformation process.However, th e f i r s t si gn of c reep damage ioften the appearance of longitudinal cracwith in th e steam-side sc al e, se e Fig. 1.

Figure 1Since the duct i l i ty of the s teel i s greatthan the d u c ti li t y of th e steam-sidemagnetite layer, longitudinal cracks withthis steam-side scale form as the oxidecannot expand with the steel.Once a longitudinal crack forms in theoxide, th e path between steam and s t e e l ishortened. With continued tube expansiont o creep deformation, th e ID sca le reformthe crack. Locally a cusp forms a t th escale/metal interfac e, see Fig. 2.

Figure 2

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as a stress raiser andens ifi es the hoop str es s a t the cuspThe stress i s also slightly higheris s l igh t ly l e sst the cusp. Depending on the circumstances

ay appear i n the fi re si de sc ale . These ODre more noticeable i n gas-firedas fuel-ash corrosion may obscure

Often the f i r s t s ign s of creep damage withinte el microstructure are creep voids a t, see Fig. 3.

Figure 3this damage,

As the creep deformation continues,OD and ID cracks and creep damage fanward each other, se e Fig. 4. Creeplur es may then i ni ti at e a t the ID as the

s in tensif ica t ion a t the cusp t i p i s aate r fa ct or than th e higher temperature a te OD surface.

Ultimately the effective cross section thickness of the tube w a l l i s reduced t o point where the str eng th of the materialinadequate to contain the internal steampressure and f ai lu re occurs. The timebetween the appearance of t he f i r s tlongitudinal cracks within the I D sc al e af ina l f a i lu re i s , of course, variable. Idepends on the tube-metal operatingtemperature and whether there is any OD tuwastage from ash corrosion or fly-asherosion. The time i s quite sensi t ive toactual operating temperature. Heat flux wbe variable along the length of an individtube. Thus one por tion of the tube may shwell-advanced creep damage; while 20' awath e same tube, the microstruc ture shows nevidence of creep damage, not evenlongitudinal cracks in the I D scale.Thus the es timation of remaining l i f e anthe determination of creep damage i ssensitive to the location within the boilSamples for evaluation should be removed fthe highest heat-flux regions of asuperheater or reheater. If fa i lures havoccurred, samples from si mi la r locat ion s

known t o be l1hotl1should be included i n thsamples se lected . Thermocouple measuremeof steam temperature in individual tubeshelpful i n evaluating the "hot" ci rc ui ts .Steam-side scale-thickness measurements bultra-sonic techniques tha t do not req uirth e removal of tube samples may a lso beuseful. However, t o accur ately assess thoverall metallurgical condition, it isimperative t o remove tube samples f or ca rmetal lurgical evaluation. Only bymet al lo gra pb can these stean-i-side scalecracks, cusps, and creep damage be accuraevaluated. Depending on th e re su lt s, p a rreplacement of a few seriously degraded turather than complete replacement may be thbest course of action.

Figure 4 The be st number for meta llu rgical advicOF OTHEFS WHO MIGHT BE Imm IN R E C E m OUR WS L E I T E R S :

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