Austenitic Stainless Steel-To-Ferritic Steel Transition Joint Welding For Elevated Temperature Service.pdf

7/28/2019 Austenitic Stainless Steel-To-Ferritic Steel Transition Joint Welding For Elevated Temperature Service.pdf

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the high" stresses created by the- a -cyclic temperature service. It was

also found that a sharp increase inhardness occurred in a narrow bandin the weld metal adjacent to the

T!T|_fusion line. The formation of car-UN^ bi de precipitates in this region

caused the hardness increase and in-dicates carbon diffusion from the.ferritic steel to the weld metal.

A as An exact failure mechanism could notbe determined from this examination,but it was reaffirmed that transi-tion joint design is very important.The basic concerns that have beenrecognized for many years are stillapplicable, including: (1) Select filler metals that\ ' minimize migration of carbon from the ferritic

T W T ' steel;MARGIN- (2) Minimize the differences

: L__iR.jpoefficients .of .thermal! expansion between the1 metals in the joint; and

(3) Minimize stresses at thej weld interfaces.

3. TRANSITION JOINT DESIGNA study^ ' was conducted to estab-lish the design of piping transitionjoints for a liquid metal fast breederreactor plant. The basic materialsto be joined are 2 1/4 Cr-1 Mo fer-fitic steel and type 316 austeniticstainless steel. The mismatch inthermal expansion properties ofthese two materials is the majorcause of high stresses at the ferriticsteel-austenitic steel weld jointduring cyclic temperature service.

The least severe stress situationoccurs when the joint is maintainedat a constant temperature, since thestress level will decrease by relax-ation, which in turn lowers thecreep rate and the joint reachesequilibrium. When large temperature N

..." "fluctuations"cannotHbe avoided, asin this case, the effect of the differ-ence in thermal expansion must bereduced. A practical way of doingthis is to separate the dissimilar -C,INmetal pair with a material or seriesof materials whose coefficient of : .thermal expansion (CTE) are betweenthose of the two to be joined. Theeffect of this is to distribute thedifference in CTE over several inter-faces so that the mismatch at anyone interface is acceptably low.The coefficients of thermal expan-sion were examined for a number of :-.materials considered for. use in the MA,joint between 2 1/4 Cr-1 Mo and type316 stainless steel (Fig. 3 ) . Anattractive candidate for an inter-mediate material is alloy 800H.Other materials were ruled out be- :cause they were not approved for usein this application by ASME Code Case1592 which limits materials to 2 1/4Cr-1 Mo steel, alloy 800H, and types304 or 316 stainless steel. ' It wasalso desirable to select weldingfiller metals which have a CTE be-tween the base materials to bejoined in each joint.A welding filler metal meeting ANSspecification ERNiCr-3 (commonlyknown as Inconel 82) has a CTE


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g r e a t e r t h a n t h e 2 1 /4 C r - 1M o s t e e l . T h i s f il l e r m e t a l , c o n -v e n t i o n a l l y u s e d i n a u s t e n i t i c /f e r r i t i c d i s s i m i l a r w e l d j o i n t s , w a ss e l e ct e d f o r t h e 2 1 /4 C r - 1 M o s t e e l

T ! 7 L $ O a l l o y 8 0 0 H j o i n t . F i l l er m e t a l s- '^-"with C T E f a l l i n g b e t w e e n o r n e a r t h e

a l l o y 8 0 0 H a n d t y p e 3 1 6 s t a i n l e s ss t e e l a r e t h e i r o n b a s e a u s t e n i t i c s .I t w a s c o n c l ud e d a f t e r e v a l u a t i o n

A 8 S - p f A s e v e r a l f i l l e r m e t a l s o f t h i s .M A R c l a s s t h a t t y p e 1 6 - 8 -2 ( 1 6 % C r - 8 %

N i 2 % M o ) w a s t h e b e s t c h o i c e f o rt h i s a p p l i c a t i o n . T h e s e c o n s i d e r a -t i o n s p r o d u c e d a t r a n s i t i o n j o i n ts p o o l p i e c e d e s i g n w i t h a g r a d u a lc h a n g e i n e x p a n s i o n p r o p e r t i e sj (Fi g. 4 ) .

4 . D E S I G N A N A L Y S I S; i

T o d e t e r m i n e i f s i gn i f i ca n t i m p r o v e -m e n t c o u l d b e o b t a i n e d f r o m t h i st h r e e - m e t a l t r a n s i t i o n j o i n t , a d e -" tS il ^a ^ sT rf e ss ' a n a l y s i s w a s ~ p e r f o r m e dt o c o m p a r e i t w i t h a c o n v e n t i o n a ld i r e c t j o i n t b e t w e e n 2 1/ 4 C r - 1 M os t e e l a n d t y p e 3 1 6 s t a i n l e s s s t e e l .A f i n i t e e l e m e n t c o m p u t e r m o d e l w a su s e d t o d e t e r m i n e t h e e f f e c t o f b o t ht h e j o i nt m a t e r i a l s a n d j o i n t g e o m e t r yo n s t r e s s e s a t t h e d i s s i m i l a r m e t a lw e l d . A n e l a s t i c a n a l y s i s w a s p e r -f o r m e d o n a 6 0 0 - m m ( 2 4 - i n . ) O D , 1 3 - m m-w a l l ( 0 . 5 - i n . - w a l l) t h i c k n e s s p i p eo p e r a t i n g a t 5 1 9 C ( 9 6 5 F ) w i t h a ni n t e r n al p r e s s u r e o f 1 .1 0 4 M P a . T h et h e r m a l t r a n s i e n t u s e d w a s a d ro p i nt e m p e r a t u r e f r o m 5 1 9 C ( 9 6 S F) t o3 4 3 C ( 6 5 0 F) i n 1 0 0 0 s e c . T h e c o n -t r i b u t i o n t o t h e t o t a l s t r e s s w a sf r o m t h r e e s o u r c e s : ( 1 ) t h e r m a l

e x p a n s i o n m i s m a t c h , ( 2 ) t e m p e r a - :t u r e g r a d i e n t s t h r o u g h t h e w a l l t h i c k -n e s s , a n d ( 3) i n t e r n al p r e s s u r e .S t r e s s e s w e r e c a l c u l a t e d f o r a d i r e c tw e l d b e t w e e n 2 1 /4 C r - 1 M o a n d t y p e3 1 6 w i t h a 7 5 i n c lu d e d a n g l e u s i n g "ITL

'_iNE. ER N iC r- 3. _f il le r. m e t a l . A s e c o n d .c a l c u l a t i on w a s m a d e w i t h a l l o y 8 0 0 Hs u b s t i t u te d f o r t y p e 3 1 6 . T h e m a x i -m u m s t r e s s e s w e r e o b s e r v e d t o o c c u ra t t h e s a m e l o c a t i o n f o r b o t h o f ' J S T S A C Tt h e s e w e l d s ( F ig s . 5 a n d 6 ) . M a x i - "m u m t e n s i l e s t r e s s e s o c c u r a t t h ei n s i de d i a m e t e r i n t h e t y p e 3 1 6 o ra l l o y 8 0 0 H a d j a c e n t t o t h e w e l d . T h em a x i m u m . c o m p r e s s i v e s t r e s s e s o c c u ra t t h e s a m e l o c a t i o n i n t h e 2 1 / 4C r - 1 M o s t e e l . C o m p a r i s o n o f t h e s et w o j o in t t y p e s r e v e a l e d t h e u s e o fa l l o y 8 0 0 H i n t h e t r a n s i t i o n j o i n tp r o v i d e d a 3 7 % d e c r e a s e i n p e a k h o o ps t r e s s i n t h e 2 1 /4 C r - 1 M o s t e e l , r c rH o o p s t r e s s e s r e p r e s e n t t h e - l a r g e s t . ;..;.-,c o m p o n e n t o f t o t a l s t r e s s .T h e e f f e c t o f w e l d j o i n t g r o o v e a n g l ew a s i n v e s t i g a t e d a f t e r t h e b e n e f i c i a le f f e c t o f a l l o y 8 0 0 H w a s d e m o n s t r a t e d .A n a l y s e s i d e n t i c a l t o t h e o n e j u s t

Austenitic Stainless Steel-To-Ferritic Steel Transition Joint Welding For Elevated Temperature Service.pdf

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