The extension of hydrogen blister-crack array in linepipe steels
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1. INTRODUCTION
HYDROGENinduced fracture in linepipe steels
is characterized by the formation of internal blisters caused by hydrogen precipitation at an inclusion-matrix interface, followedby the formation of blistercrack array by cracking the region connecting the b l i s t e r s through the action of i n t e r n a l hydrogen p r e s s u r e , p, and e x t e r n a l s t r e s s , a ~r (pipe hoop s t r e s s ) , I which is p o s s i b l e even in the a b s e n c e of the e x t e r n a l stress.2 When a p a r a l l e l - t o - p l a t e c r a c k - l i k e cavity is long c o m p a r e d with the wall t h i c k n e s s o r is located a suff i c i e n t l y s m a l l d i s t a n c e below s t e e l s u r f a c e , s h e a r s t r e s s at the cavity tips is r e l a x e d by p l a s t i c deform a t i o n in the r e g i o n between cavity tips and f r e e s u r face to produce a s u r f a c e b l i s t e r i n g (Fig. 1). When n o n c o p l a n a r p a r a l l e l i n t e r n a l c r a c k - l i k e cavities a r e configured as shown in Fig. 2, s h e a r s t r e s s at the cavity tips can also be r e l a x e d by p l a s t i c def o r m a t i o n induced b e t w e e n the cavity t i p s . Shear s t r e s s at the cavity tip p r o d u c e d by the i n t e r n a l p r e s s u r e , p, shown in Fig. 3 is abated by s h e a r s t r e s s induced by a r176as one u n d e r s t a n d s that the a ~ induced s h e a r s t r e s s a c t s in the opposite s e n c e of the s h e a r s t r e s s produced by p on an a r b i t r a r y s i g n i ficant line at the cavity tip d r a w n in Fig. 9 below. H y d r o s t a t i c s t r e s s a r o u n d the cavity i n c r e a s e s l i n e a r l y with a ~. In other words a s t r e s s state in the v i c i n i t y of cavity tip is s e n s i t i v e l y influenced by ext e r n a l s t r e s s , a ~ . It is t h e r e f o r e expected that the m a n n e r of e x t e n s i o n of the h y d r o g e n induced f r a c t u r e of this type, and c o n s e q u e n t l y the r e s u l t of e v a l u a t i o n of h y d r o g e n f r a c t u r e r e s i s t a n c e of m a t e r i a l s should be influenced by a ~ . S t a r t i n g f r o m the above m e n t i o n e d p r i m a r y p r e e s t i m a t i o n c o n s i d e r a t i o n will be given to the influence
of an e x t e r n a l s t r e s s on the e x t e n s i o n of the hydrogen induced f r a c t u r e of linepipe s t e e l s . 2. E X P E R I M E N T A L PROCEDURES F o r o b s e r v a t i o n of the a p p e a r a n c e of hydrogen i n duced f r a c t u r e developed in linepipe s t e e l s l a b o r a t o r y t e s t s to be d e s c r i b e d below were m a d e . T h e test s,eo,,~,,~e \ \
\ P
Fig. 1--Pressure relaxation by plastic deformation of a surface blister; p is the pressure of hydrogen precipitated within the blister. /
METALLURGICALTRANSACTIONSA
P
/,
/ P
Fig. 2--Pressure relaxation by plastic deformation of internal blisters; p is the pressure of hydrogen precipitated within the blister. through-thickness direction
rolling dire
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