Evaluation of Hydrogen EmbrittlementMechanisms C. F. BARTH AND E. A. STEIGERWALD The i n c u b a t i o n t i m e which p r e c e d e s the i n i t i a t i o n of slow c r a c k growth in the delayed f a i l u r e of h i g h - s t r e n g t h s t e e l c o n t a i n i n g h y d r o g e n was r e v e r s i b l e with r e s p e c t to the applied s t r e s s . The k i n e t i c s of the r e v e r s i b i l i t y p r o c e s s i n d i c a t e d that it was c o n t r o l l e d by the diffusion of h y d r o g e n and had an a c t i v a t i o n e n e r g y of a p p r o x i m a t e l y 9000 cal p e r m o l e . R e v e r s i b l e h y d r o gen e m b r i t t l e m e n t s t u d i e s were also c o n d u c t e d at liquid n i t r o g e n t e m p e r a t u r e s where d i f f u s i o n a l p r o c e s s e s s h o u l d not o c c u r . The p r e v i o u s l y r e p o r t e d low t e m p e r a t u r e e m b r i t t l e m e n t b e h a v i o r was c o n f i r m e d i n d i c a t i n g a b a s i c i n t e r a c t i o n between h y d r o g e n and the l a t t i c e . The e x p e r i m e n t a l r e s u l t s could be s a t i s f a c t o r i l y e x p l a i n e d b y the l a t t i c e e m b r i t t l e m e n t t h e o r y p r o p o s e d by T r o i a n o . h LTHOUGH the p r o b l e m of h y d r o g e n e m b r i t t l e m e n t in m e t a l s has b e e n e x t e n s i v e l y studied, c o n t r o v e r s y s t i l l e x i s t s c o n c e r n i n g the s p e c i f i c m e c h a n i s m i n volved. In addition, f r o m a p r a c t i c a l viewpoint f r e quent s e r v i c e f a i l u r e s can s t i l l be d i r e c t l y a t t r i b u t e d to the d a m a g i n g effects of h y d r o g e n when a p p r o p r i a t e d e s i g n and p r o c e s s i n g p r e c a u t i o n s a r e not c o n s i d e r e d . The g e n e r i c t e r m h y d r o g e n e m b r i t t l e m e n t h a s b e e n used to d e s c r i b e at l e a s t four types of d i s t i n c t e m b r i t tlement phenomena: 1) Hydrogen attack (e.g., H20 f o r m a t i o n in c o p p e r ; m e t h a n e r e a c t i o n in s t e e l ) . 2) Hydride f o r m a t i o n (e.g., h y d r i d e p r e c i p i t a t i o n i n n i o b i u m , t i t a n i u m , z i r c o n i u m , and so forth). 3) I r r e v e r s i b l e c r a c k i n g (e.g., b l i s t e r i n g of s t e e l , c r a c k i n g of S i - F e ) . 4) R e v e r s i b l e e m b r i t t l e m e n t (e.g., d e l a y e d f a i l u r e in h i g h - s t r e n g t h s t e e l s ) . C o n s i d e r i n g t h e s e e m b r i t t l e m e n t types, the r e v e r s ible c a t e g o r y has r e c e i v e d the m o s t a t t e n t i o n b e c a u s e of its c o m m e r c i a l i m p o r t a n c e and t h e o r e t i c a l c h a l l e n g e , z In m a n y r e s p e c t s h y d r o g e n is unique in its d a m a g i n g effects s i n c e the r e v e r s i b l e e m b r i t t l e m e n t is a c c e n t u a t e d by slow s t r a i n r a t e s and is o b s e r v e d p r e d o m i n a n t l y in a specific t e m p e r a t u r e r a n g e . ~'2 The i n f l u e n c e of s t r a i n r a t e and t e m p e r a t u r e is e x p l a i n e d on the b a s i s that a c e r t a i n a m o u n t of t i m e is r e q u i r e d d u r i n g a t e s t to develop a s u f f i c i e n t h y d r o g e n c o n c e n t r a t i o n at an i n t e r n a l defect to p r o d u c e e m b r i t t l e m e n t . F a s t s t r a i n r a t e s or low t e m p e r a t u r e s m i n i m i z e the o b s e r v e d e m b r i t t l e m e n t s i n c e they e i t h e r d e c r e a s e the t e s t t i m e for hydrogen c o n c e n t r a t i o n to o c c u r o r the m o b i l i t y of the h y d r o g e n a t o m . On this b a s i s a d e l a y e d f a i l u r e t e s t which e m p l o y s a s t a t i c load r e p r e s e n t s the m o s t s e n s i t i v e method for studying h y d r o g e n e m b r i t t l e m e n t s i n c e it i n c o r p o r a t e s the m a x i m u m t e s t time. ~ Studies p e r f o r m e d on h y d r o g e n - i n d u c e d delayed f a i l u r e of s h a r p l y - n o t c h e d h i g h - s t r e n g t h s t e e l s p e c i m e n s i n d i c a t e that an i n c u b a t i o n t i m e p r e c e d e s c r a c k i n i t i a tion. C r a c k i n g then p r o c e e d s d i s c o n t i n u o u s l y until the c r i t i c a l length is attained, a n d r a p i d f a i l u r e o c c u r s . 3 A l o w e r c r i t i c a l s t r e s s also e x i s t s below which d e l a y e d f a i l u r e i s not o b s e r v e d . The e s s e n t i a l c h a r a c t e r i s t i c s of the d e l a y e d f a i l u r e b e h a v i o r a r e shown s c h e m a t i c a l l y in Fig. 1. C. F. BARTH and E. A. STE1GERWALDare with MaterialsTechnology Laboratory, TRW Equipment Group, Cleveland,Ohio, Manuscript submitted July 17, 1970. METALLURGICALTRANSACTIONS

The p u r p o s e of this i n v e s t i g a t i o n was to p e r f o r m a s e r i e s of e x p e r i m e n t s a i m e d at d i s t i n g u i s h i n g between the s p e c i f i c m o d e l s that could a c c o u n t f o r r e v e r s i b l e hydrogen embrittlement. Basically three mechanisms have b e e n p r o p o s e d to e x p l a i n the o b s e r v e d r e s u l t s . The f i r s t , i n i t i a l l y p r e s e n t e d by Zapffe 4 and modified by T e t e l m a n , 5 d e s c r i b e s the role of h y d r o g e n a s one w h e r e the a t o m i n s o l u t i o n " p r e c i p i t a t e s " a s a gas i n a void. The p r e s s u r e developed in the void is added to the applied s t r e s s to d e c r e a s e the load c a r r y i n g a b i l i t y . The m o d e l d e s c r i b e d by T r o i a n o z'8 f r o m r e s u l t s on h i g h - s t r e n g t h s t e e l s i n d i c a t e s t h a t h y d r o g e n i n the l a t t i c e i s the d a m a g i n g s p e c i e a n d the p r e s e n c e of hydrogen c a n a c t u a l l y r e d u c e the c o h e s i v e s t r e n g t h of the b a s e m e t a l bond. He has f u r t h e r s u g g e s t e d a m o d e l w h e r e b y the h y d r o g e n atom i n t e r a c t s with the 3d e l e c t r o n s h e l l to l o w e r the b i n d i n g e n e r g y . The t h i r d m e c h a n i s m , o r i g i n a l l y p r o p o s e d by Perch, i n volves a l o w e r i n g of the f r a c t u r e e n e r g y by a d s o r p t i o n of the h y d r o g e n . 7 W i l l i a m s and Nelson have applied the s u r f a c e a d s o r p t i o n m o d e l to d e l a y e d f a i l u r e of h i g h - s t r e n g t h s t e e l in an e n v i r o n m e n t of h y d r o g e n gas and r e l a t e d the k i n e t i c s of a d s o r p t i o n to the k i n e t i c s of c r a c k growth. 8 In c a s e s where the s u r f a c e a d s o r p tion or s t r e s s s o r p t i o n m o d e l involves i n t e r a c t i o n of the h y d r o g e n with the 3d e l e c t r o n s h e l l ( c h e m i s o r p tion) at the tip of a c r a c k o r an e m b r y o n i c c r a c k , it is d i r e c t l y c o m p a r a b l e to m a n y a s p e c t s of the T r o i a n o model. The e x p e r i m e n t a l a p p r o a c h i n v o l v e d : 1) An e x a m i n a t i o n of the r e v e r s i b i l i t y of the i n c u b a tion t i m e for c r a c k i n g in h y d r o g e n a t e d h i g h - s t r e n g t h s t e e l notched s p e c i m e n s s u b j e c t e d to s t a t i c - l o a d i n g . 2) A study of e m b r i t t l e m e n t at - 321~ of c a t h o d i c a l l y c h a r g e d s p e c i m e n s w h e r e the c o n t r i b u t i o n of diff u s i o n a l p r o c e s s e s should be n e g l i g i b l e . Each of t h e s e t e s t s was d e s i g n e d to p r o v i d e data which would allow s e p a r a t i o n of the v a r i o u s e m b r i t t l e ment mechanisms. u~pEn cmTic~t. ~NeO~ON

rut

FRACTURETIMs

V)

g i

.......

0.1

~ (STATIC FATIGUELiMiT)

I

I

I

I

I0 100 FRACTURE T i M E ~ HOURS

I000

Fig. I--Schematic representation of delayed failure characteristics of a hydrogenated high-strength steel. VOLUME 1, DECEMBER 1970-3451

NOTCH R,%DJUS-O.DO]"

Fig. 2--Specimen geometries. (a) 60 deg V notch tensile specimen. (b) Unnotched tensile specimen.

(a) 1-1/2"

(b)

Table I. Mechanical Properties of 4340 Steel

Smooth

Room Temperature Ultimatetensile strength, ksi

Notched

223

Reductionin area, pct

Room Temperature -321~

-321~ 288

48.8

275

38.0

275

-

-

MATERIALS AND PROCEDURE A c o m m e r c i a l heat of a i r m e l t e d SAE-AISI 4340 s t e e l with the following c o m p o s i t i o n was used in the study. Compositionof 4340 Steel,wt pct C

Mn

Si

Ni

Cr

Mo

0.39

0.85

0.86

1.75

0.87

0.27

P

0.012

S

Fe

0.022

Bal.

The r e v e r s i b i l i t y e x p e r i m e n t s w e r e p e r f o r m e d on notched t e n s i l e s p e c i m e n s h a v i n g the c o n f i g u r a t i o n shown in Fig. 2(a) while the low t e m p e r a t u r e s t u d i e s w e r e conducted on the s m o o t h s p e c i m e n s shown in Fig. 2(b). All the s p e c i m e n s w e r e heat t r e a t e d p r i o r to f i n i s h m a c h i n i n g a c c o r d i n g to the following s e quence : 1) N o r m a l i z e 15 rain, s a l t bath at 1700~ a i r cool. 2) A u s t e n i t i z e 30 m i n , s a l t bath at 1550~ oil q u e n c h . 3) Double t e m p e r , 1 h r + 1 h r at 700~ a i r cool. The m e c h a n i c a l p r o p e r t i e s p r o d u c e d by this heat t r e a t m e n t a r e s u m m a r i z e d in Table I. H y d r o g e n was i n t r o d u c e d e l e c t r o l y t i c a l l y into the s p e c i m e n s and two types of c h a r g i n g conditions w e r e e m p l o y e d . F o r the delayed f a i l u r e s t u d i e s , notched s p e c i m e n s were d e g r e a s e d in a c e t o n e and then c a t h o d i c a l l y c h a r g e d for 30 rain in a 4 pct s u l f u r i c acid s o l u tion at a c u r r e n t d e n s i t y of 0.07 amp p e r sq in. F o l l o w ing c h a r g i n g , the s p e c i m e n s w e r e r i n s e d in w a t e r and c a d m i u m p l a t e d in a s o d i u m c y a n i d e - c a d m i u m oxide bath c o n t a i n i n g o r g a n i c b r i g h t e n e r s for 15 rain at a c u r r e n t d e n s i t y of 0.14 a m p p e r sq in. In o r d e r to h o m o g e n i z e the r e s u l t i n g h y d r o g e n d i s t r i b u t i o n , the s p e c i m e n s were baked for 30 m i n at 300~ in a i r . P r e v i o u s work s had indicated that this t r e a t m e n t e f f e c t i v e l y e l i m i n a t e s the h y d r o g e n g r a d i e n t p r o d u c e d by c h a r g i n g . The delayed f a i l u r e t e s t s were conducted on c o n s t a n t load r a c k s u s i n g u n i v e r s a l g r i p s and 0.005 in. thick lead w a s h e r s u n d e r the s p e c i m e n button heads 3452-VOLUME 1,DECEMBER 1970

to m i n i m i z e load e c c e n t r i c i t y . A c o m p l i a n c e gage was employed to m e a s u r e i n c u b a t i o n t i m e s and s u b s e q u e n t c r a c k e x t e n s i o n s . Aging t e s t s were conducted at e l e vated t e m p e r a t u r e s using a s t r e a m of s i l i c o n e oil pumped under p r e s s u r e from a thermostatically cont r o l l e d r e s e r v o i r . The 32~ aging t r e a t m e n t s were p e r f o r m e d in an i c e bath while the r o o m t e m p e r a t u r e s t u d i e s were conducted in a i r . A thin c o a t i n g of s i l i cone g r e a s e was u s e d on the s p e c i m e n s i m m e r s e d in the w a t e r bath to p r e v e n t r e a c t i o n with the s p e c i m e n o v e r the t e s t d u r a t i o n . Lag t i m e s b e t w e e n t e m p e r a t u r e changes w e r e m i n i m i z e d by u s i n g s t r e a m s of p r e s s u r i z e d oil o r a i r i m p i n g i n g on the t e s t a r e a of the s p e c i m e n . A hollow s p e c i m e n was u s e d c o n t a i n i n g a t h e r m o c o u p l e at the notch a r e a to v e r i f y that the p r o g r a r n e d t e m p e r a t u r e p r o f i l e s w e r e o b t a i n e d for each aging s e q u e n c e . A m a x i m u m lag t i m e of 17 sec was noted for the g r e a t e s t t e m p e r a t u r e e x c u r s i o n examined. The low t e m p e r a t u r e t e s t s were p e r f o r m e d on s p e c i m e n s which w e r e c h a r g e d in an a q u e o u s s o l u t i o n of 4 pct s u l f u r i c a c i d plus a poison. The p o i s o n which was a s o l u t i o n of 2 g of yellow p h o s p h o r u s in 40 cu cm of c a r b o n d i s u l f i d e was added in the r a t i o of 10 ca c m to e v e r y 900 cu c m of e l e c t r o l y t e . The s p e c i m e n s were c h a r g e d for 24 h r to obtain a u n i f o r m h y d r o g e n content without baking. The c h a r g i n g c u r r e n t d e n s i t y which d e t e r m i n e s the h y d r o g e n c o n t e n t 9 was v a r i e d b e t w e e n 0.0001 and 0.30 amp p e r sq in. In the r a n g e where r e v e r s i b l e e m b r i t t l e m e n t o c c u r s the h y d r o g e n content u n d e r the specific c h a r g i n g conditions employed has b e e n found to be l i n e a r l y r e l a t e d to the l o g a r i t h m of the c h a r g i n g c u r r e n t d e n s i t y a c c o r d i n g to the relation: ~~ H2 = 1 3 . 3 -

2.7 log

I

where H~ = h y d r o g e n content, ppm I = c u r r e n t d e n s i t y , amp p e r sq in. T e n s i l e t e s t s w e r e p e r f o r m e d in liquid n i t r o g e n i m m e d i a t e l y a f t e r c h a r g i n g . During t r a n s f e r f r o m the c h a r g i n g a r e a to the t e n s i l e t e s t f a c i l i t y , the s p e c i m e n s were i m m e r s e d in liquid n i t r o g e n . Reduction in a r e a was used a s the index of e m b r i t t l e m e n t . To i n s u r e that the e m b r i t t l e m e n t was r e v e r s i b l e duplicate s p e c i m e n s w e r e a l s o c h a r g e d and baked for 1 h r at 300~ to r e m o v e the hydrogen and then t e s t e d under the i d e n t i c a l c o n d i t i o n s as the h y d r o g e n a t e d m a t e r i a l . R E S U L T S AND DISCUSSION In a delayed f a i l u r e t e s t on a h i g h - s t r e n g t h s t e e l c o n t a i n i n g hydrogen the p r e s s u r e t h e o r y e x p l a i n s the i n c u bation p e r i o d as the t i m e r e q u i r e d for the h y d r o g e n to build up s u f f i c i e n t g a s e o u s p r e s s u r e in a void to develop a detectable c r a c k , In the a d s o r p t i o n t h e o r y this p r e s s u r e d i r e c t l y c o n t r o l s the r a t e of a d s o r p t i o n so that p r e s s u r e is the c r i t i c a l p a r a m e t e r i n v o l v e d with both m o d e l s , In the l a t t i c e e m b r i t t l e m e n t e x p l a n a t i o n the incubation p e r i o d is the time r e q u i r e d to develop a c r i t i c a l c o n c e n t r a t i o n by s t r e s s - i n d u c e d diffusion at a c r a c k e m b r y o . If the s t r e s s is r e m o v e d p r i o r to the i n i t i a t i o n of c r a c k i n g then the i n c u b a t i o n t i m e should be r e v e r s i b l e s i n c e the s t r e s s which is the d r i v i n g METALLURGICAL TRANSACTIONS

f o r c e for the hydrogen c o n c e n t r a t i o n is no l o n g e r p r e s ent. In the p r e s s u r e theory or the s t r e s s sorption t h e o r y the g a s e o u s hydrogen which is f o r m e d in a void would have to d i s s o c i a t e and d i f f u s e back into the l a t t i c e . Each of t h e s e p r o c e s s e s i s t h e r m a l l y a c t i v a t e d with the following activation e n e r g i e s :

Embrit flement Mechanism

Activation Energy kcal/mole

Reversibility Process

Pressure

H2 -~

2H

H 2 ~

2H

103

rAILU~" ace

7oe

~oo =

soo

Ref. 11 12

5.2

(clean iron surface)

20O

Lattice interaction

Lattice diffusion

9.0

13-16

Adsorption

H2 ~ 2H (clean iron surface)

5.2

12

INCUBATION

IO0

~o

eo

I=o

18o

TIM(, m~TES

F i g . 4 - - C r a c k g r o w t h c a r v e for d e l a y e d f a i l u r e of h y d r o g e nated h i g h - s t r e n g t h s t e e l l o a d e d at 240 k s i .

The notch s p e c i m e n s used in the d e l a y e d f a i l u r e t e s t s had a room t e m p e r a t u r e incubation t i m e of 30 =L5 m i n at an applied s t r e s s of 240 k s i . * A d e l a y e d

i

I

i

i

i

'

I~0

i

J

i

*Stress is defined as the applied load divided by the cross-sectional area at the base of the circumferential notch.

f a i l u r e curve was p r e p a r e d for the hydrogenated s p e c i m e n s to define the m a t e r i a l r e s p o n s e for the aging s t u d i e s , p a r t i c u l a r l y in the range of 240 k s i . The data are p r e s e n t e d in Fig. 3. The k i n e t i c s of the r e v e r s i b i l i t y p r o c e s s w e r e studied by: a) s t r e s s i n g the s p e c i m e n s at 240 ksi for 24 min to the point of i n c i p i e n t c r a c k initiation, b) r e m o v i n g the load to p e r m i t aging the s p e c i m e n for v a r i o u s t i m e s at four t e m p e r a t u r e l e v e l s , and c) reapplying the load at room t e m p e r a ture to again monitor the t i m e for crack initiation (incubation t i m e ) . Aging for s u f f i c i e n t t i m e s at any t e m p e r a t u r e r e s u l t e d in full r e s t o r a t i o n of the 30 m i n incubation t i m e . Short aging t i m e s produced c o m p a r a bly s h o r t e r incubation t i m e s . T y p i c a l crack growth c u r v e s obtained in the d e l a y e d f a i l u r e t e s t s are shown in F i g s . 4 and 5. Once i n i t i a tion o c c u r r e d , the crack a l w a y s grew in a d i s c o n t i n u ous m a n n e r . These data w e r e c o n s i s t e n t with p r e v i o u s l y r e p o r t e d r e s u l t s which indicated that the growth c u r v e was r e a l l y a s e r i e s of incubation p e r i o d s f o l -

~

boo

STRESS R EkiOV EO SP(CINEN AGEO

STRESSAPPLt ED AT ?~OF, 200

o AT 75 F

INITI TION

CRACK

I

~

20

40

60

80

'

100

120

'

I

io

,8',

'

zoo

TIME -KIPIUTES

F i g . 5 - - C r a c k g r o w t h c u r v e for d e l a y e d f a i l u r e of h y d r o g e nated h i g h - s t r e n g t h s t e e l l o a d e d at 240 k s i , s t r e s s r e m o v e d d u r i n g incubation p e r i o d .

/ ~

o

E 18

FAILURE

a

25O

\

! CRACK

i.i-

I

..,r,,,,O.~" I

I I

~o

I

I 10

i co A~I~ TI,E.

=o.ooo

100.0oo

StC0.0S

F i g . 6 - - R e c o v e r y of incubation t i m e as a function of aging at various temperatures.

I I00

I I000

TI~E, HOURS

F i g . 3 - - D e l a y e d f a i l u r e r e s u l t s for h y d r o g e n a t e d and p l a t e d notch t e n s i l e s p e c i m e n s , 4 3 4 0 s t e e l , 223 ksi s t r e n g t h l e v e l . METALLURGICAl,. TRANSACTIONS

ioo

lowed by i n s t a n t a n e o u s crack e x t e n s i o n s , xs The r e m o v a l of the load and the i n s e r t i o n of the aging p e r i o d had no effect on the b a s i c c h a r a c t e r i s t i c s of the s l o w , d i s c o n t i n u o u s c r a c k growth once the c r a c k was i n i t i ated. The r e s u l t s of the r e c o v e r y s t u d i e s are p r e s e n t e d in Fig. 6 for aging t e m p e r a t u r e s of 252 ~ 174 ~ 75 ~ VOLUME 1, DECEMBER 1 9 7 0 - 3 4 5 3

i

Table II. Hydrogen Diffusion Coefficients Used in Calculation of Effective Radius (r0),18 Temperature, ~

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O

. Y O A O ~ t ~ T t O ONLY

LE~E~

Diffusivity, cm~/sec

32 75 174 252

0.79 X 2.2 X 16.1 X 46.9 X

10-7 10-7 10-7 10-7

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~

4 ~

O00001

q - 9ooo

2.~

2.8

3.= I T x i o 3 o~-I

I.~

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Fig. 7--Activation energy for the recovery of the incubation time.

and 32~ The r e v e r s i b i l i t y of the i n c u b a t i o n p e r i o d for d e l a y e d f a i l u r e in h y d r o g e n a t e d s t e e l s is c l e a r l y e v i d e n t and the r e c o v e r y p r o c e s s follows c l a s s i c a l d i f fusion c o n t r o l l e d b e h a v i o r . The solid c u r v e s in Fig. 6 were c a l c u l a t e d using a m o d e l of hydrogen diffusing f r o m a c y l i n d e r of r a d i u s ro w h e r e the i n i t i a l c o n c e n t r a t i o n was u n i f o r m throughout the c y l i n d e r while the c o n c e n t r a t i o n at the s u r f a c e was m a i n t a i n e d at Co. The g r a p h i c a l solution to these b o u n d a r y conditions p r e s e n t e d by Crank ~ was e m p l o y e d to define the r e c o v e r y k i n e t i c s . Using the hydrogen diffusivity coefficients shown in Table II the effective r a d i u s r o o v e r which the h y d r o g e n was r e m o v e d to p r o d u c e complete r e c o v e r y of the incubation p e r i o d was c a l c u l a t e d as 0.014 in. (0.056 cm). Although the use of a c y l i n d r i c a l m o d e l and the a s s u m e d b o u n d a r y conditions m a y not e x a c t l y apply to the case of the incubation p e r i o d r e c o v e r y p r o c e s s , the r e s u l t s do p r o v i d e an i n d i c a t i o n that the effective d i s t a n c e o v e r which hydrogen m o v e m e n t o c c u r s in the delayed f a i l u r e t e s t on notched s p e c i m e n s a p p r o x i m a t e s the p l a s t i c zone size. The a c t i v a t i o n e n e r g y for the r e c o v e r y p r o c e s s was d e t e r m i n e d by plotting the l o g a r i t h m of the aging t i m e to r e a c h an i n c u b a t i o n time of 15 m i n as a function of the r e c i p r o c a l of the a b s o l u t e t e m p e r a t u r e . The r e s u l t s given in Fig. 7 indicate that the r e v e r s i b i l i t y of the i n c u b a t i o n p e r i o d could be c h a r a c t e r i z e d by a t h e r m a l l y a c t i v a t e d p r o c e s s with a n a c t i v a t i o n e n e r g y of a p p r o x i m a t e l y 9000 cal p e r m o l e . For c o m p a r i s o n p u r p o s e s the 5200 cal p e r m o l e which would c o r r e spond to the a c t i v a t i o n e n e r g y for solution of h y d r o g e n 3454-VOLUME 1, DECEMBER 1970

L 0.01

L 0 I

Fig. 8--Embrittlement of hydrogenated 4340 steel at liquid nitrogen temperature ( - 321~

o

2.0

I O.~r

f r o m a c l e a n i r o n s u r f a c e is also p r e s e n t e d in the figu r e . This lower a c t i v a t i o n e n e r g y would c h a r a c t e r i z e the r e v e r s i b i l i t y involved with a d e c r e a s e in void p r e s s u r e where h y d r o g e n would p a s s f r o m a c l e a n void s u r face back into the l a t t i c e . The data i n d i c a t e that the r e v e r s i b i l i t y of the incubation t i m e is c o n t r o l l e d by the s a m e k i n e t i c s that g o v e r n the i n i t i a l f o r m a t i o n of the c r a c k , i.e., the diffusion of hydrogen in the lattice. ~ On this b a s i s the r e s u l t s lend a d d i t i o n a l s u p p o r t to the l a t t i c e e m b r i t t l e m e n t theory. If a b a s i c i n t e r a c t i o n between the l a t t i c e and h y d r o gen e x i s t s then it should be p o s s i b l e to obtain e m b r i t t l e m e n t at low t e m p e r a t u r e s where d i f f u s i o n a l p r o c e s s e s a r e n e g l i g i b l e . P r e v i o u s work z~ h a s indicated that e m b r i t t l e m e n t at liquid n i t r o g e n t e m p e r a t u r e s does o c c u r and a d d i t i o n a l e x p e r i m e n t s w e r e p e r f o r m e d to s u b s t a n t i a t e this c r i t i c a l point. The s p e c i m e n s were c h a r g e d at r o o m t e m p e r a t u r e and t e s t e d at - 321~ Under t h e s e conditions the h y d r o g e n p r e s s u r e in the voids would d e c r e a s e as the t e m p e r a t u r e were l o w e r e d and a c c o r d i n g to the p r e s s u r e theory no e m b r i t t l e m e n t should o c c u r , s In addition, any p l a s t i c d e f o r m a t i o n should e n l a r g e the voids and f u r t h e r d e c r e a s e the p r e s s u r e . The e x p e r i m e n t a l r e s u l t s , shown in Fig. 8, i n d i c a t e that at a c u r r e n t d e n s i t y in the r a n g e of 0.001 to 0.003 amp p e r sq in. (~ 5 p p m of hydrogen) the r e d u c t i o n in a r e a dropped f r o m 37 p c t to a p p r o x i m a t e l y 5 pct. S i m i l a r s p e c i m e n s that w e r e charged and baked exhibited no c o m p a r a b l e d e c r e a s e in d u c t i l ity indicating that the low t e m p e r a t u r e e m b r i t t l e m e n t p r o d u c e d by the hydrogen was t r u l y r e v e r s i b l e . At c h a r g i n g c u r r e n t d e n s i t i e s above 0.10 a m p p e r sq in. (~ 8 ppm of hydrogen) c r a c k i n g o c c u r r e d d u r i n g c h a r g ing and the e m b r i t t l e m e n t was i r r e v e r s i b l e , i.e., not r e c o v e r e d by the baking o p e r a t i o n . The p r e s e n c e of low t e m p e r a t u r e e m b r i t t l e m e n t is i n c o n s i s t e n t with the p r e s s u r e t h e o r y of hydrogen e m b r i t t l e m e n t . 5 In addition, it is also difficult to explain the fact that the s p e c i m e n s exhibited a p p r o x i m a t e l y 5 pct r e d u c t i o n in a r e a p r i o r to f r a c t u r e using the a d s o r p t i o n model. A ductility of this m a g n i t u d e indicates that c o n s i d e r a b l e void f o r m a t i o n and c o a l e s c e n c e o c c u r r e d p r i o r to f r a c t u r e . The d e c r e a s i n g void p r e s s u r e and the n e g l i g i b l e r a t e of s u r f a c e m i g r a t i o n 8 at the low t e m p e r a t u r e should e l i m i n a t e the r e d i s t r i b u tion of hydrogen along the newly c r e a t e d void s u r f a c e which is n e c e s s a r y for the a d s o r p t i o n model. In the l a t t i c e e m b r i t t l e m e n t theory the void f o r m a t i o n and c o a l e s c e n c e could p r o d u c e the n e c e s s a r y s t r e s s c o n c e n t r a t i o n which would be r e q u i r e d with the specific hydrogen c o n c e n t r a t i o n to cause e m b r i t t l e m e n t . METALLURGICALTRANSACTIONS

S U M M A R Y AND C O N C L U S I O N S Studies were conducted to define the kinetics inv o l v e d w i t h t h e s t r e s s d e p e n d e n t r e v e r s i b i l i t y of t h e incubation period which occurs in the delayed failure of h y d r o g e n a t e d h i g h - s t r e n g t h s t e e l . S p e c i m e n s w e r e s t r e s s e d f o r a t i m e w h i c h w a s 0 , 8 of t h e o r i g i n a l p e riod required for crack initiation. The stress was then removed and the specimens aged for various times and temperatures. When the stress was reapplied the incubation period varied directly with the a g i n g v a r i a b l e s . A g i n g t i m e s of 150 r a i n a t 7 5 ~ o r 6 r a i n a t 2 5 2 ~ p r o d u c e d f u l l r e c o v e r y of t h e i n c u b a tion period indicating that this parameter was truly reversible with respect to the applied stress. The k i n e t i c s i n v o l v e d i n t h e r e c o v e r y of t h e i n c u b a t i o n t i m e c o u l d b e d i r e c t l y r e l a t e d to t h e d i f f u s i o n of h y drogen in the lattice. The activation energy for the r e c o v e r y p r o c e s s w a s a p p r o x i m a t e l y 9000 c a l p e r mole which is equal to the activation energy for the original crack initiation process and the diffusivity of h y d r o g e n i n t h e l a t t i c e . U s i n g a s i m p l e c y l i n d r i c a l model for outgassing the effective diameter over which the stress-induced hydrogen movement occurred was calculated as approximately 0.014 in. Reversible hydrogen embrittlement at liquid nitrogen temperatures w a s a l s o o b s e r v e d in c a t h o d i c a l l y charged high-strength steel specimens. Embrittlem e r i t , a s m e a s u r e d b y a d e c r e a s e i n r e d u c t i o n of a r e a , was present at hydrogen concentrations in the range of 5 t o 8 p p m . H o w e v e r , e v e n i n e m b r i t t l e d s p e c i m e n s the reduction in area was approximately 5 pct indicating that considerable void coalescence was required prior to fracture. T h e r e s u l t s of b o t h t h e r e v e r s i b i l i t y a n d l o w t e m perature experiments supported the lattice embrittlement theory proposed by Troiano. 1 Neither the pres-

Corrections

sure theory nor the adsorption theory could properly account for the kinetics involved in the reversibility of t h e i n c u b a t i o n p e r i o d o r t h e p r e s e n c e of l o w t e m perature embrittlement where diffusional processes should be negligible. ACKNOWLEDGMENT The work described in this report was performed u n d e r s p o n s o r s h i p of t h e O f f i c e of N a v a l R e s e a r c h , Contract N0014-69-C-0286 with Dr. P. Clarkin acting as program manager for the Navy. RE FERENCES 1. A. R. Troiano: Trans.ASM, 1960, voL 52, p. 54. 2. T. Toh and W. M. Baldwin: Stress Corrosion Crackingand Embrittlement, pp. 175-86, John Wiley & Sons, New York, 1956. 3. H. H. Johnson, J. G. Morlet, and A. R. Troiano: Trans. TMS-AIME, 1958, vol. 2t2, pp. 528-36. 4. C. Zapffe: Trans. ASM, 1947, vol. 39, p. 191. 5. A. S. Tetleman: Fracture of Solids, p. 671, John Wdey & Sons, New York, 1962. 6. J. G. Morlet, H. H. Johnson, and A. R. Troiano: J. Iron SteelInst., 1958, vol. 189, p. 37. 7. N. J. Petch: Phil Mag., 1956, vol. 1, pp. 331-35. 8. D. P. Williamsand H. G. Nelson: Met. Trans., 1970, vol. 1, pp. 63-68. 9. F. de Kazinezy:dernkontoretsAn~, 1955, vol. I39, pp. 466-80. 10. E. A. Steigerwald, F. W. Schaller, and A. R. Troiano: Trans. TMS-AIME, 1960, voI. 218, pp. 832-40. 11. C. F. Prutton and S. H. Maron: PhysicalChemistry, p. 723, The MacMillanCo., New York, 1949. 12. C. M. Sturges and A. P. Miodownik:Acta Met., 1969, vol. 17, p. 1206. 13. J. D. Hobson: s 1ton SteelInst., 1958, vol. 189, p. 315. 14. R. C. Frank: s Appl Phys 1958, vol. 29, p. 1262. 15. E. A. Steigerwald, F. W. Schaller, and A. R. Troiano: Tran~ TMS-AIME, 1959,vol. 215, p. 1048. 16. R. Gibala: Trans. TMS-AIME, 1967, vol. 239, p. 1574. 17. J. Crank: The Mathematics of Diffusion, p. 67, Clarendon Press, 1956. 18. R. C. Frank: Internal Stresses and Fatigue in Metals, p. 417, ElsevierPublishingCo., 1959.

to M e t . T r a n s . , 1 9 7 0 , v o l . 1.

The O r i g i n o f F r e c k l e s i n U n i d i r e c t i o n a l l y S o l i d i f i e d C a s t i n g s , b y S. M , C o p l e y , A. F. G i a m e i , S. M. J o h n s o n , a n d M. F . H o r n b e c k e r , p p . 2 1 9 3 - 2 2 0 4 .

Page 2204 References

28 t h r o u g h 32 s h o u l d r e a d :

28. c. Wagner:Z Metals, 1954, vol. 6, pp. 154-60. 29. S. Chandrasekhar:Hydrodynamic and Hydromagnetic Instability, Oxford University Press, London, 1961. 30. G. Veronis: Astrophys. J., 1963,vol. 137, pp. 641-63.

METALLURGICAL TRANSACTIONS

3 I. R. S. Cremisio: International Transactions Vacuum Metallurgy Conference, E. L. Foster, ed., American Vacuum Society, New York, 1968. 32. R. J. McDonaldand J. D. Hunt: Trans. TMS-AIME, 1969, vol. 245, pp. 199397.

VOLUME I,DECEMBER 1970-3455