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D AV I D N . F R E N C H , I N C . . M E TA L L U RG I S T S

FALL 1987 O N E L A N C A S T E R R O A D VOL. I V , NO.N O R T H B O R O U G H. M A S S A C H U S E T T S 0 1 5 3 2

( 6 17 ) 3 9 3 - 3 6 3 5

A V I E W FROM THE PENTHOUSE: USEFUL INFORMATION FOR THE WORLD OF BOI LERS

ERUMPENT FAILURES

The dynamic balance between heat genera-i o n a n d f l u i d f l o w when i n p r o pe r e q u i l i b r i u me a d s t o s a f e m e ta l t e mp e ra t ur e s t h a t a s s u r eo ng b o il e r- t ub e l i f e . When th e f l u i d f l ows i n t e r r u p t e d by a p a r t i a l o r t o t a l b lockageu be -m et a l t e m p e r a t u r e s c an r i s e t o 16 00 F

o r s o i n a fe w m i nu t e s. A t t h i s t e m p e r a t u r ef a i l u r e s f o l lo w i n v er y s h o r t o r d er . F a i l u r e so f t h i s t yp e a r e u s ua l l y r e f e r r e d t o a s

1 s h o r t - t e r m o v e r h e a t i n g . "

T he se f a i l u r e s a r e c h a r a c t e r i z e d by aw id e- op en b u r s t w i t h f r a c t u r e e d g e s d ra wn t o

n e a r k ni f e- e dg e c o n d i t i o n . Therei s

c o n s i d e r -a b l e d u c t i l i t y t o t h e r u p t u r e , and t h e a p pe ar-a n c e i s r e f e r r e d t o s o me ti me s a s a " fi s hm ou thf a i l u r e . " ( Se e F i g u re 1 )

(T-11) an d 2$r-1Mo (T-22) m a t e r i a l s commonlyu se d i n b o i l e r s .

TABLE I . ELEVATED TEMPERATURE STRENGTHS

TEMP, OF TENSILE STRENGTHS, KSI

SA-210 T-11 T-22

T h e s e c o n d i m p o r t a n t f e a t u r e i s t h e a l l e -t r o p i c t r a n s f o r m a t i o n i n h e r e n t t o l ow -a ll oya n d c a r b o n s t e e l s . A t room t emp era tu re t hec r y s t a l l og r a p hi c s t r u c t u r e of f e r r i t i c s t e e l

- -

i s body-centered c ub ic (BCC). The in di vi du ala t o ms of i r o n a r r a n g e t h e m s e l v e s on a c u b i cl a t t i c e w i t h an atom a t t h e c o r n e r of e ac hc ub e a nd on e a t t h e c e n t e r o f t h e cu be . TheBCC form of i r o n i s c a l l e d " f e r r i t e . " A th i gh t e m p e ra t u r e s t h e c r y s t a l l o g r a p h i c a r -rangement i s f ace -c en te r ed cub ic (FCC). Int h i s c a s e , i n d i v i d u a l at om s o f i r o n a r r an g et h e m s e l v e s on t h e c o r n e r of a c u b e w i t h a d d i -

t i o n a l a t om s i n t h e mi dd le o f t h e 6 f a c e s o ft h e cub e. The FCC form of i r o n i s c a l l e d

F i g u r e 1. A f i s h m o u t h f a i l u r e

I n o r d e r t o f u l l y e x p l a i n t h e s e e rumpentf a i l u r e s , two f e a t u r e s t o c a rb o n and l o w- al lo ys t e e l s need t o be d i sc u ss e d. F i r s t , a s tem-p e r a t u r e i n c r ea s e s , t h e t e n s i l e s t r e n g t hd e c r e a s e s . When t h e h oo p s t r e s s f ro m t h es t e a m p r e s s u r e i s a p pr o xi m at e l y e q u a l t o t h eh ig h- te mp er at ur e t e n s i l e s t r e n g t h , f a i l u r ef o l l o w s . E q u a t i o n 1 g i v e s t h e h oop s t r e s s .

where S = hoop s t r e s s , p s iP = s te a m p r e s s u r e , p s iD = t u b e d i a m e t e r , i n c hW = t u b e w a l l , i n c h

Ta b l e I l i s t s t h e e l e v a t e d t e m p e r a t u r et r e n g t h s o f c a rb o n s t e e l (SA-210), l a ~ r - + ~ o

11a u s t e n i t e . "

F u r t h e r , t h e s o l u b i l i t y o f c ar bo n i n t h e s etwo forms i s d i f f e r e n t . A t h i g h t e m p e r a t u r e ,t h e FCC l a t t i c e w i l l d i s s o l v e u p t o 2 % c a r b o n ,w h i l e t h e BCC l a t t i c e w i l l d i s s ol v e l e s s t h an0 .0 2% c a r b o n . D u ri n g s t e e l m ak in g a n d f a b r i -c a t i o n of t h e s e m a t e r i a l s , t h e c o ol i n g r a t efr om t h e h i gh - te m pe r a tu r e c r y s t a l l o g r a p h i cform o f FCC t o t he room t e mper a tu re c r ys t a l -lo gr a ph ic form of BCC i s s l o w en ou gh t o g i v et h e c a r b o n a t o m s a d e q u a t e t i m e t o un-mix.S i n c e t h e BCC l a t t i c e w i l l accomodate only0.02ZC, a n o t h e r c o n s t i t u e n t f o rm s o f h i g h e rc a r b on c o n t e n t . T h i s o t h e r p h as e i s i r o nca rb id e , Fe3C, ca l l e d "cemen t i t e . " The no rma la p pe a ra n c e o f m i x tu r e s o f f e r r i t e and i r o nc a r b i d e i s l a m e l l a r p e a r l i t e , s e e F i g u re 2.

The amount of p e a r l i t e i n t h e m i c r o s t r u c t u r edepends on th e %C. High-carbon s t e e l s havem or e p e a r l i t e t h a n l ow -c ar bo n s t e e l s . A sc o o l i n g r a t e s from a u s t e n i t e t o f e r r i t e and

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Figure 2. Lamellar pearlite and ferrite

pearlite are increased, there is inadequateime for carbon un-mixing. Thus the micro-

structure at room temperature will be aquenched or rapidly cooled st ructure, eitherbainite or martensite.

The temperature at which the BCC latticeransforms to the FCC lattice is known as the

1 lower-critical transformation temperature"

(LCTT). This particular temperature is afunction of the alloy content of the steel,as shown in Table 11.

TABLE 11. LOWER CRITICAL TRANSFORMATION

TEMPERATURE, LCTT

MATERIAL

Carbon steeC -s ~ M O ...T-11, T-12.T-22, T-3..T-5 ........T-7........T-9........

TEMP., OF

I........................................................................................................

When failures occur at temperatures abovehe LCTT, the microstructure will reflecthis peak temperature. When the tube finallyfails the eruption of steam or water throughhe failure rapidly cools the steel to theemperatureof the steam or water. When theemperature is above LCTT, the microstructure

will be rapidly cooled from the FCC lattice

configuration to bainite as shown in Figure 3. - -WOULD YOU LIKE A SEMINAR TO LEARN MORE ABOUTFAILURES???

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Figure 3. Bainite and ferri te

Thus there are a couple of ways to estimatethe peak metal temperature at the moment offailure. A) The hoop str ess is about equalto the strength. Thus a calculation of stressfrom EQ 1 i s equated with the tensile strengthgiven in Table 11. B) For temperatures abovethe LCTT the mixtures of ferrite and bainitewill indicate that the temperature was aboveLCTT

The causes of erumpent failures are rela-tively simple to enumerate. Any circumstancethat totally or partially blocks the flow offluid will lead to a rapid increase in metaltemperatures. During start-up, condensate ina pendant superheater or reheate r, for example ,will allow little or no steam flow through thatparticular tube. Since there are many tubeswithin a superheater or reheate r, steam willflow through all but the tube with condensate,That tube will be rapidly heated toward theflue-gas temperature and an erumpent failure

will follow. For waterwall tubes, flameimpingement will lead to an excessive heat-fluxwhich will cause the formation of steam blan-ket s on the water side. The steam blanket isan effective insulator and the tube tempera-ture will rise to fai lure in a short period oftime. Corrosion wastage will lead to a thin-wall tube which will be able to tolerate lessof a temperature rise. During start-up, forexample erumpent failures will follow becausethe wall thickness has been reduced substan-tially. (See VOL. 11, No. 3 of this newslet-ter.) On rare occations there may be a defect

in the electric resis tance weld that would leadto reduced effective wall thickness and awide-open burst would follow. Final ly, foreignmaterial may be left in the tube that effect-ively blocks the steam flow and leads to thesehigh-temperature, erumpent failures,

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