cold work effects on sulfide stress cracking

7/30/2019 Cold Work Effects on Sulfide Stress Cracking

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62 H. HUANGand W. J. D. SHAWTABLE 1. CHEMICALCOMPOSITIONOF AISI 1020 STEEL(wt%)

C Si Mn P S Ni Cr Mo Cu AI Fe0.19 0.21 0.93 0.01 0.04 0.17 0.13 0.01 0.14 0.03 Balance

hydrog en evolu t ion when steel is exposed to acid environm ents . 26'27 Hy dr og enatoms adso rbed on the e lectrodic surface can e i ther react with one anot her to formhydro gen gas, or diffuse into the s teel and pro mote embri t t leme nt . Highly local ized,fine scale plastic deformation has been directly associated with hydrogen inducedbritt le fracture .28 The prese nce of i nter nal hydr ogen leads to strain locali zation in theform of dense dislocation structures which in tur n en han ce britt le fracture of steel. 29Cold work is expected to enhan ce hy drog en-in duced fa ilures by faci l ita t ing thepart ic le matrix separat ion, void ini t ia t ion and growth, local s tress co ncentr a t ioncaused by plastic incom patib ili ty and particle cracking. 24 Simult aneo us tensilestra ining and charging with various fugaci ties of hydro gen were found to enh ancevoid nu cle ati on in sph ero idi zed 104521'22 and 109523 steels. So me d ef or ma ti on mod eshave be en f oun d to exert beneficial effects on the resist ance of steel to hyd rog enembri t t lem ent . For example , mo derat e cold work (up to 15% reduct ion of thicknessfor a 0.16% C steel and 7% tensile straining for a 0.15% C low Cr -N i- Co steel 24 werefound to decrease the suscept ibi l i ty to hydro gen ind uced degrad at ion, while largerdeformation causes void formation and growth. Increased resis tance to hydrogenemb ri tt l eme nt was re por ted 24 for a low alloy steel with cold work of 50% and 90%.This beneficial effect was attrib uted to a decrease in corrosio n and hyd rog enpermeation rates as a result of compressive stresses in the subsurface layer of thesteel.

The mai n purpo se of this study is to investigate the effects of cold work on SSCbehavior of pipel ine s teel in a sour gas environment . An at tempt is made todetermine the re la t ionship of anodic SCC and hydr ogen embr i t t l ement of coldworked steel . Addit i onal ly this work provides some quan t i ta t ive informati on thatwil l help in obta i ning a bet ter unde rsta ndin g during the design process.

EXPERIMENTAL METHODThe material used in this study was a commercial ferritic/pearlitic AISI 1020 steel in the hot rolled

condition and is referred to as 0% cold worked material. The composition of the steel is listed in Table 1.The grain size of the hot rolled material was measured to be ASTM No. 8.38 according to ASTM E 112.3oThe cold rolled conditions were produced by reducing various thickness of the hot rolled material to afinished thickness of 6.35 mm in percentages from 10% to 70% occurring in steps of 10%.The tensile specimens were subsized being 200 mm in overall length with threaded ends and a reducedsection of 3.2 mm in diameter. The gage length was 27 mm. The elongation data were adjusted to astandard gage length of 50 mm according to Barba's law.31 Compact fracture toughness specimens25.4 mm in width and 6.35 mm in thickness were taken from the L-T orientation. All compact fracturetoughness specimens were fatigue precracked below Kma = 30 MPa mI/2 in ambient air, according toASTM E-399.32 The configurations of the tensile specimen and the compact fracture toughness specimenare shown in Fig. 1.The aqueous and gaseous environment used for this study was based upon analysis of some problemwells in Alberta. 33 The aqueous components (in mg 1-1 ) are as follows: 48,500 Na , 14,250 Ca2 , 1045Mg2, 91,500 CI , 180 HCO 3 and 150 SO4 -. The gaseous components consist of 34% (vol) hydrogensulfide, 10% (vol) carbon dioxide with the balance being methane at 1 atm pressure. This simulated saltcontent and gaseous components are somewhat simpler than that found in the field but the majorconstituents affecting the material have been duplicated. This environment has been used for investigating


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S u l f id e s t r e s s c r a c k i n g o f p i p e l i n e s t e e l 6 3c o r r o s i o n m e c h a n i s m s i n p r e v i o u s s t u d i e s 33 a n d i s r e f e r r e d t o a s t h e s o u r g a s e n v i r o n m e n t i n t h i s p a p e r .T h e t e s ts w e r e p e r f o r m e d i n t h e b r i n e e n v i r o n m e n t s a t u r a t e d w i t h t h e H 2 S / C O 2 / C H 4 m i x t u r e , w h i c h w a sb u b b l e d t h r o u g h a t a f l o w r a te o f 0 . 4 1 m i n - 1, a t 1 a t m p r e s s u r e a n d 2 5 C t e m p e r a t u r e .

A w i d e l y u s e d l a b o r a t o r y e n v i r o n m e n t f o r d e t e r m i n i n g m a t e r ia l r e s i s ta n c e to S S C i s th a t r e c o m -m e n d e d b y N A C E , 34 b e i n g 5 % N a C I , 0 . 5 % a c e ti c a ci d a n d s a t u r a t e d w i t h h y d r o g e n s u l fi d e. C o m p a r i s o n so f S S C r e su l ts a f t e r te s t in g in b o t h t h e s o u r e n v i r o n m e n t a n d t h e N A C E s o l u ti o n a r e m a d e .

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t o u g h n e s s s p e c i m e n .


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68 H. HUANG and W. J. D. SHAWS l o w S t r a i n R a t e T e n s i l e T e s t s

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F=G. 5. The re lationsh ip of SSR vs ductility of 0% cold work ed steel.

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C o l d W o r k , %FnG. 6. The e nvi ronmental effects on the ductility of cold worked steel al a strain nate of

1 10-Ss u


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Sulfide s t ress c racking of p ipe l in e s tee l 73in a n i n c r e a s e in e m b r i t t l e m e n t . T h i s e x p l a n a t i o n i s s u p p o r t e d b y S S R t e s t s a f t e r 72 hu n s t r e s s e d p r i o r i m m e r s i o n a l lo w i n g a s t e a d y s t a t e h y d r o g e n l e v el to o c c u r i n 2 0 %c o l d w o r k e d s t e e l. T h e s e t e s t s s h o w e d t h a t a n i n c r e a s e i n h y d r o g e n a s a r e s u l t o f 7 2 hp r i o r i m m e r s i o n c a u se s t he h y d r o g e n e m b r i t t le m e n t m e c h a n i s m f o r S S C o f 2 0 % c o ldw o r k e d s te e l u n d e r S S R t e s ti n g t o b e c o m e d o m i n a n t . A n o t h e r p o s si b le r e a s o n fo rt h e S S R e f f e c t i s t h e s e n s i ti v i ty o f t h e d e f o r m e d s t r u c t u r e t o h y d r o g e n e m b r i t t l e -m e n t , w h i c h w i ll b e d i s c u s s e d i n th e s e c t i o n o n e f f e c t s o f d e f o r m e d s t r u c t u r e .

C O 2 a n d b r i n e e n v i r o n m e n t e f f e ct sS e v e r a l c as e s o f c a r b o n a t e t y p e c r a c k in g h a v e r e c e n t l y b e e n r e p o r t e d i n

e q u i p m e n t . 43'44 A n u m b e r o f in v e s t ig a t o r s h a v e s h o w n a n o d i c S C C i n l o w c a r b o na n d l o w s t r e n g t h s te e l i n t h e p r e s e n c e o f c a r b o n a t e / b i c a r b o n a t e o r a m m o n i u mc a r b o n a t e s o l u t i o n . 43-46

F i g u r e 8 i n d i c a t e s t h a t C O 2 / C H 4 b r i n e s o l u t i o n w i t h t h e a b s e n c e o f H z S r e s u l t s i na m o d e r a t e a m o u n t o f e m b r i t t le m e n t o n t h e 0 % c o ld w o r k e d s te e l a n d t h e lo w c ol dw o r k e d s t ee l u n d e r S S R l o a d i n g c o n di t io n s . H u d g i n s et a l . 46 h a v e s h o w n t h a t a n o d i cS C C o f c a r b o n s t ee l c a n o c c u r u n d e r a h ig h C O 2 p r e s s u r e e n v i r o n m e n t . T h e a n o d i cs t r es s c o r r o s i o n c r a c k i n g is n o r m a l l y a s s o c i a t e d w i t h t h e p r e s e n c e o f c h l o r i d e a n do x y g e n a n d i s f a v o r e d b y a c i d i c c o n d i t i o n s . T h e b r i n e s o l u t i o n i n th i s s t u d y c o n t a i n s av e r y h i g h le v e l o f c h l o r i d e . T h e S S R t e c h n i q u e is b e l i e v e d t o b e o n e o f t h e m o s ts e v e r e t e s t i n g m e t h o d s f o r s t r e s s c o r r o s i o n c r a c k i n g . T h i s m a y b e w h y t h e a n o d i cS C C o f t h e 0 % c o l d w o r k s t e e l a n d t h e l o w c o l d w o r k e d s t e e l is f o u n d t o o c c u r in t h eC O 2 / C H 4 b r i n e s o l ut i on .

M u l t i p l e c r a c k i n i t i a ti o n o n t h e s i d e s u r f a c e o f s p e c i m e n s i s b e l i e v e d t o b e r e l a t e dt o t h e a n o d i c S C C m e c h a n i s m o f s u lf id e s tr e s s c r a c k i n g o f l o w s t r e n g t h s t e e l d u e t ot h e c o m b i n e d c o n t r i b u t io n o f C O 2 c o m p o n e n t s a n d b r i n e s o l ut i on i n t h e s o u r ga se n v i r o n m e n t . 47 M u l t i p l e c r a c k s i n g e n e r a l h a v e b e e n f o u n d t o b e a s s o c i a t e d w i t h p it sa n d t r e n c h e s , s'1 4 P i t s a n d t r e n c h e s r e s u l t f r o m t h e i n t e r a c t i o n o f a p p l i e d s t r e s s a n dc o r r o s i o n d i s s o l u t i o n o n t h e s i d e su r f a c e o f t e n s il e s p e c i m e n s . T h e s e f in d i n g s h a v es h o w n t h a t i n t h e a n o d i c a ll y p o l a r iz e d s p e c i m e n , m a n y s h a l l o w p it s a n d d e e pt r e n c h e s f o r m w h i c h s u b s e q u e n t l y a c t a s i n i t ia t i o n s i t es fo r m u l t i p l e c r a c k s .

H y d r o g e n a t o m s p r o d u c e d f r o m t h e c o r r o s io n r e a c t i o n o n t h e s u rf a c e t e n d t op r e f e r t o d i f f u s e t o t h e c e n t e r o f te n s i le s p e c i m e n s d u e t o t h e p r e s e n c e o f t r ia x i a ls t r e ss u n d e r S S R l o a d i n g c o n d i t i o n . A s a re s u l t o f t h is , c r a c k i n i ti a t io n o c c u r s a t t h ec e n t e r o f t h e t e n si le s p e c i m e n w h e n t h e h y d r o g e n e m b r i t t l e m e n t e f f ec t is d o m i n a n t .T h e s e a r g u m e n t s a r e i n a g r e e m e n t w i t h t h e o b s e r v a t i o n f o u n d h e r e . T h e r e w a s a na b s e n c e o f m u l t i p l e c r a c k s o n th e s i d e s u r fa c e o f t h e s p e c i m e n s t e s t e d i n 1 0 0 % H z Sb r i n e s o lu t i o n a n d t h e N A C E s o l u ti o n u n d e r t h e s a m e l o a d in g c o n d it io n s . T h i s w a s ad i r e ct r e s u lt o f an i n c r e a se i n th e h y d r o g e n e m b r i t t l e m e n t e f fe c t. T h e o c c u r r e n c e o fm u l t i p l e c r a c k s o n t h e s i de s u r f a c e o f s p e c i m e n s o f t h e 0 % c o ld w o r k e d s t ee l a n d l o wc o l d w o r k e d s t e e l a t a s t r a in r a t e o f 1 1 0 - 5 s - t a n d t h e c o m b i n e d e f f e c t o f t h e C O 2c o m p o n e n t a n d a h i g h le v e l o f c h l o r id e i n t h e s o u r e n v i r o n m e n t s u g g es t s t h a t t h ea n o d i c S C C f a il u r e m e c h a n i s m is p r e s e n t . I t c a n b e c o n c l u d e d t h a t C O 2 a n d a h ig hl e ve l o f ch l o r id e i n th e s o u r g a s e n v i r o n m e n t c a u s e s th e 0 % c o l d w o r k e d s t ee l a n d l o wc o l d w o r k e d s t e e l t o b e s e n s i ti v e t o a n o d i c S C C , w h i c h is i n a d d i t i o n t o t h e h y d r o g e ne m b r i t t l e m e n t e f fe c t . H e a v i l y c o ld w o r k e d s t ee l is m o r e s e n si ti v e to h y d r o g e ne m b r i t t l e m e n t .


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FIG. 10.F ig . 11 .

F r a c t o g r a p h o f S S C o f 2 0% c o l d w o r k e d s t e el .F r a c t o g r a p h o f S S C o f 7 0 % c o l d w o r k e d s t ee l .

75


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78 H. HUANG and W. J. D. SHAW37. G. I. OGUNDELE and W. E. WHITE,C o r r o s i o n 42, 398 (1986).38. J. R. SCULLYand P. J. MORAN,C o r r o s i o n 44, 176 (1988).39. H. H. BIRNBAUM,Hydrogen related failure mechanisms in metals, in E n v i r o n m e n t - s e n s i t i v e F r a c t u re

o f E n g i n e e r i n g M a t e r i a ls (ed. Z. A. FOROULIS), p. 326. AIME, War rendale, PA (1979).40. K. N. AKHURSTand T. J. BAKER,Metal l . Trans . 12A, 1059 (1981).41. G.T. MURRAY,H. H. HONEGGER and T. MOUSEL,C o r r o s i o n 40, 146 (1984).42. S. X. XIE and J. P. HIRTH,C o r r o s i o n 38,486 (1982).43. E. MIRABAL S. BHAYI'ACHARJEEand N. PAzos, M a t e r . P e r f o r m . 30, 41 (1991).44. R. N. PARKINS,A. ALEXANDRIDOUand P. MAJUMDAR,M a t e r . P e r f o r m . 25, 20 (1986).45. J. M. SUTCLIFFE, R. R. FESSLER, W. K. BOYD and R. N. PARKINS,C o r r o s i o n 28,313 (1972).46. C. M. HUDGINS, R. L. MCGLASSON, P. MEHDIZADEH and W. M. ROSBOROUGH,C o r r o s i o n 22,238

(1966).47. H. HUANG and W. J. D. SHAW,Micros t ruc t . Sc i . in press (1992).48. P. GAY, P. B. HIRSCH and A. KELLY,Acta Crys ta l l . 7, 41 (1954).49. M. W. JOOSTEN, J. J. MURALI and J. L. HESS, Interactive effects of cold work, yield st rength , and

temperature on sulfide stress cracking, in E n v i r o n m e n t a l l y A s s i s t e d C r a c k i n g (eds T. W. CROOKERandB. N. LEIS), p. 136. ASTM, Balt imore, MD (1990).

cold work effects on sulfide stress cracking

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