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D A VID N. FR EN C H . IN C . , META LLU R GISTSNTER 1 9 8 8 ONE LANCASTER ROAD VOL. I V , NO. 4

NORTHBOROUGH, MASSACHUSETTS 01532(617)3 9 3 - 3 6 3 5

A VIEW FROM THE PENTH OUSE: USEFUL INFOR MAT ION FOR THE WORLD OF B OI L ER S

Metals, l i k e e v e r y t h i n g e l s e , a r e m a d eo f a to m s . F o r o u r p u r p o s e s , a t o m s w i t h -n a metal l ic c r y s t a l a re r e g u l a r l y a r r a n g e dg r e a t d i s t a n c e s , d i s t a n c e s t h a t a re h u g een c o m p ar ed w i t h a t o m i c d i m e n s i o n s . T h e r es , t h e n , l o ng - ra n g e a t o m i c o r d e r w i t h i nc r y s t a l s . W here a d j a c e n t c r y s t a l s

i s a c r y s t a l b o u n d a r y , a z o n e o f s h o r t -d i s o r d e r . T h e s e c r y s t a l b o u n d a r i e sm i n e i n n o small way t h e u s e f u l p r o p e r -i e s o f e n g i n e e r i n g m a t e r i a l s .A l l b o i l e r a l l o y s a re made up o f many

s t a l s o f v a r i o u s o r i e n t a t i o n s . T he sev i d u a l c r y s t a l s a re c a l l e d " g r a i n s . " I ne g r a i n a l l a t o m s a r e a r r a n g e d w i t h o n eu l a r o r i e n t a t i o n a n d o n e p a r t i c u l a rT he j u n c t u r e b e tw e en a d j a c e n t g r a i n s11s c a l l e d a " g r a i n b o u n d a r y . T h e g r a i n

i s a t r a n s i t i o n r e g i o n i n w h i c h t h ea re n o t e x a c t l y a l i g n e d w i th e i t h e r

G r a in s i z e c a n v a r y g r e a t l y d e p e n d i n gl l o y a nd h e a t t r e a t m e n t . F o r r e f e r -a t y p i c a l g r a i n s i z e i s a b o u t 1 m i lT h u s , t h e r e may b e a b i l l i o n ( 1 09 )s p e r c u b i c i n c h o f a l l o y . The ASTM

i s o n e s t a n d a r d f o r d e t e r -i n g t h e a v e r a g e g r a i n s i z e . t h e ASTM"N" i s d e f i n e d b y :

i s t h e num ber o f g r a i n s p e r s q u a r ec h w he n v i e w e d a t 1 0 0 x . T h e u s u a l r a n g es f r o m 1 t o 9 ; n o t e t h a t a s t h e g r a i n s g e tt h e g r a i n - s i z e num ber g e t s l a r g e r .e u s u a l way o f r e p o r t i n g g r a i n s i z e i s t ov e t h e a v e r a g e an d t h e r a n g e ; s a y , ASTM 6

a s l a r g e a s 43 a n d a s small a s 7 .e s h a p e o f a g r a i n i s g o v e r n e d b y i t so r s . A l th o u g h n o g r a i n i s e v e r ab u t a n i r r e g u l a r p o l y h e dr on t h a t whenc k ed t o g e t h e r w i t h o t h e r s c o m p l e t e l y f i l l si t s c h a r a c t e r i s t i c l e n g t h i s r e f e r r e do a s a " d i a m e t e r . " A t e q u i l i b r i u m t h e s h ap es t o m in i m i ze t h e g r ai n - b o u nd a ry s u r f a c e

a g i v e n v ol um e . F o r e x a m p le , t h i s

m i n i m i z a t i o n o f t h e s u r f a c e t o vo lu me ( S /V )r a t i o i s t h e d r i v i n g f o r c e f o r t h e s p h e r o i dz a t i o n o f i r o n c a r b i d e i n p e a r l i t i c s t e e l s .G r a i n s a re s a i d t o b e " eq u ia x ed " when t h ec h a r a c t e r i s t i c d im e n s i o n s a r e t h e same i n ad i r e c t i o n s . G r a i n s a r e s a i d t o b e " e l o n g a t ewhen t h e c h a r a c t e r i s t i c d i m e n s io n s a r e n o tt h e sam e b u t o n e d i r e c t i o n i s m u c h l o n g e rt h a n t h e o t h e r s .G r a i n b o u n d a r i e s a r e u s u a l l y c o n s i d e r etw o d i m e n s i o n a l b u t a r e a c t u a l l y o f f i n i t et h i c k n e s s , 2-10 a t o m i c d i s t a n c e s . T h e m i s -m a tc h o f t h e o r i e n t a t i o n o f n e i g h b o r i n g g r al e a d s t o a l e s s e f f i c i e n t a t o m i c p a c k i n g w ii n t h e g r a i n bo un d a ry . H en c e t h e a t o m s i n ab o u n d a r y h a v e a l e s s o r d e r e d s t r u c t u r e a nd ah i g h e r i n t e r n a l e n e r g y . T h e d i s o r d e r e d a t o mi c a r r a ng e m e n t an d h i g h e r e n e r g y c an e x p l a i ns e v e r a l f e a t u r e s a s s o c i a t e d w i t h mater ia lsd e g r a d a t i o n f ou n d i n b o i l e r s . T he n e x ts e v e r a l e x a mp le s w i l l d e s c r i b e t h e r o l e t h ag r a i n b o u n d a r i e s p l a y .ETCHING: I n o r d e r t o o b s e r v e t h e m i c r o s t r u ct u r e , a p i e c e o f t h e m e t a l i s s m o o t h l y p o l i se d t o a p l a n e , m i r r o r - l i k e f i n i s h . T h e p r e -p a re d s u r f a c e i s c h e m i c a l l y a t t a c k e d w i t hd i l u t e a c i d , a p r o c e s s c a l l e d " e t c h i n g , " f oa s h o r t p e r i o d . T h e g r a i n - b o u n d a r y a t o m s a rm ore e a s i l y a n d r a p i d l y d i s s o l v e d t h a n a to mw i t h i n t h e g r a i n s . A small g r o o v e i s l e f t at h e g r a i n b o u n d a r i e s . S i n c e a g r o o v e w i l ln o t r e f l e c t l i g h t a s d o t h e f l a t , p o l i s h e dg r a i n s , t h e g r a i n b o u n d a r i e s a p p e a r as b l a c kl i n e s a n d t h e s t r u c t u r a l d e t a i l s a r e v i s i b l eDIFFUSION: G r a in b o u n d a r i e s a r e r e g i o n s o fa t o m i c m i s m a tc h a n d l e s s d e n s e a t o m i c p ac k-i n g . L e s s d e n s i t y o n a n a t o m i c scale i m p l i eb i g g e r , a t om - s iz e h o l e s t h r o u g h w hi ch a t om sc a n m o re e a s i l y m ove. S u c h a t o m i c m o b i l i t yi s c a l l e d " d i f f u s i o n . " T h us g r a i n b o u n d a r i ew i l l o x i d i z e o r c o r r o d e m or e r a p i d l y , u s u a l lr e f e r r e d t o as " g r a i n - b o u n d a r y p e n e t r a t i o n . "O xygen d i f f u s e s a l o n g t h e g r a i n b o u n d a r i e s ,reac t s w i t h t h e s t e e l a n d f o r m s i r o n o x i d e it h e g r a i n b o un d ar y . S i n c e t h e v olu me o f o x ii s g r e a t e r t h a n t h e vo lu me o f m e t a l f r o m w hi t f o r m s , a w ed g in g a c t i o n r e s u l t s . T he t i po f t h e g ra i n - b o un d a r y o x i d e w ed ge i s i n t e n -s i o n , f o r c i n g op en t h e mater ia l a t t h e c r a c k

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ip. The high stress enhances the oxidationor corrosion which also drives the grain-oundary penetration.

re 1. Oxide penetration along grainreheater tube sampleyears operation. Oxygen diffusionh easier thanl lattice of a grain. Hencethe grain boundary.rbide particles at the ferrite

nd ferritic stainlessls, diffusion of an atom along the grainary leads to the formation of chromium

As these carbides farm, they de-boundary of chromium. As the chromium

"sensitization." Oncensitized, they areintergranular-corrosion attackn as "IGA. tDAMAGE: Hydrogen damage, see Vol.No. 4 of this newsletter, is a circum-rosion, hydrogenide to form methane.collects at grain boundaries; andpressure build-Gp is large enough,will form. The strength of the steely the destruction of the grain

At elevated temperatures the strengthgrain is greater than the strength of aboundary. At room temperature thef a grain boundary is greater than

Figure 2. Microstructure-of 347H stainlessteel after nearly 26 years of operation ia secondary superheater. Note grain boundies are clearly etched. Along grain boundies are carbide particles, thus the structis said to be "sensitized." 500xthe strength of a grain. Where the grain-boundary strength equals the grain strengtis known as the "equi-cohesive temperatureElevated-temperature failures follow grainboundaries and are referred to as "intergrular" (between the grains) failures. Roomtemperature failures are usually "transgralar" (across the grains) failures. Elevattemperature deformation occurs by one graimoving past its neighbor, a process known "creep." Voids first form where severalgrains join, where grain-boundary sliding limited. Individual voids then link up toform grain-boundary cracks, often referredas "grain-boundary separation I'

Figure 3. Creep voids along ferrite grainboundaries and at triple points where thregrains come together. Note that the ferrigrain boundaries contain carbides and thestructure is fully spheroidized. The micrstructure is from a T-22 superheater whichfailed after 37 years. 500x

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