- Email: [email protected]
Effect of hydrogen on yield of iron
Scripta M E T A L L U R G I C A
Vol. ii, pp. 849-852, 1977 P r i n t e d in the United States
P e r g a m o n Press,
Inc
E F F E C T OF H Y D R O G E N ON Y I E L D OF IRON
Y. Tobe and W.R. Tyson Physical Metallurgy Research Laboratories CANMET, EMR 568 B o o t h St. Ottawa, Ont. KIA 0GI (Received July 20, 1977) Introduction W h i l e the d e t r i m e n t a l e f f e c t s of h y d r o g e n on the fracture of steel are w e l l known, the i n f l u e n c e of h y d r o g e n on y i e l d and flow has b e e n a subject of c o n s i d e r a b l e debate. Early w o r k p r o d u c e d a c o n f u s e d p i c t u r e of effects, w i t h y i e l d points e i t h e r eliminated(l) or created(2) and strain h a r d e n i n g either increased(3) or d e c r e a s e d ( 4 ) . In his r e v i e w of 1965, Farrell(5) c o n c l u d e d that the effects seen to that time were p r o b a b l y a r e s u l t of p r e - s t r a i n r a t h e r than i n t e r a c t i o n s b e t w e e n h y d r o g e n and d i s l o c a t i o n s . However, in r e c e n t years there has b e e n an a c c u m u l a t i o n of e v i d e n c e that h y d r o g e n can be b o u n d to d i s l o c a t i o n s (6,7), t r a n s p o r t e d by t h e m (8,9), and i n f l u e n c e their m o b i l i t y ( 1 0 , 1 1 , 1 2 ) . By a n a l o g y w i t h o t h e r i n t e r s t i t i a l solutes in iron, it w o u l d be e x p e c t e d that h y d r o g e n should cause the c l a s s i c f o r m a t i o n of atmospheres, y i e l d points, and flow stress elevation. The w o r k d e s c r i b e d b e l o w was done to look for some of these effects. Experimental Procedure Two g r a d e s of m a t e r i a l w e r e used, a v a c u u m m e l t e d e l e c t r o l y t i c iron (i0 ppm C, 80 ppm Si+Mn+S+P, 650 ppm Ni) and a c o m m e r c i a l Q&T steel (.135C, 1.39Mn, .385Si, .0012B) of 690 M P a (I00 ksi) y i e l d strength. The iron was c o l d r o l l e d 70% after casting and m a c h i n e d into flat tensile specimens of gauge s e c t i o n ii x 5 x 2 mm; the Q&T steel was m a c h i n e d from asr e c e i v e d p l a t e into s p e c i m e n s of c y l i n d r i c a l gauge section 19 x 3.2 mm. The t e n s i l e axis was always p a r a l l e l to the rolling direction. Tensile s p e c i m e n s w e r e c h a r g e d at e l e v a t e d t e m p e r a t u r e in 1 atm. of H 2 to ensure a h o m o g e n e o u s d i s t r i b u t i o n of hydrogen, the iron for 1 hr at 850°C (to ~2 ppm H) and the Q&T steel for 3 hr at 650°C (to ~i ppm H), followed by a w a t e r q u e n c h in l i q u i d N2. "Uncharged" samples w e r e g i v e n an identical thermal t r e a t m e n t in 1 a t m . of Ar instead of H2. The o p t i c a l m i c r o s t r u c t u r e s w e r e e q u i a x e d grains of d i a m e t e r ~30 ~m for the iron, and t e m p e r e d m a r t e n s i t e for the steel. T e n s i l e tests w e r e p e r f o r m e d using an Instron m a c h i n e e q u i p p e d w i t h a zero s u p p r e s s i o n device, using liquid baths of N,, m e t h y l b u t a n e , alcohol, and w a t e r for t e m p e r a t u r e control and a strain rate of ~ ~6 x 10 -4 sec -I. To i n v e s t i g a t e s t r a i n aging, p r e - s t r a i n e d specimens w e r e aged under a small load ( a p p r o x i m a t e l y 10% of yield) at either the test t e m p e r a t u r e or elev a t e d temperature. E x p e r i m e n t a l Results T y p i c a l s t r e s s - s t r a i n curves are shown in Fig. i. It is e v i d e n t that h y d r o g e n p r o d u c e s a n o t i c e a b l e i n c r e a s e in the flow stress. This is further i l l u s t r a t e d in Fig. 2, w h e r e the i n c r e a s e d y i e l d stresses of c h a r g e d samples are shown clearly. Some of the Q & T steel samples w e r e Cd p l a t e d after c h a r g i n g to impede h y d r o g e n loss; no s i g n i f i c a n t d i f f e r e n c e s w e r e o b s e r v e d b e t w e e n t e n s i l e p r o p e r t i e s i n c l u d i n g r e d u c t i o n in area(16) of p l a t e d and u n p l a t e d samples even at the h i g h e s t t e m p e r a t u r e used (100°C).
849
850
EFFECT OF HYDROGEN ON YIELD OF IRON
#00
1200 . liT
~
V01.
,
,
l
ii, No. I0
,
STEEL ,-IOO" C
80( ,./!"/"
iooo~-\\
k
800
" "' "\
"tO(
o
g.
|
[
"
°".. ~ ~
z
r
:
")
',,~,~
4oo~ 20(
200:i
I0(
°-~oo NATURAL STRAIN, %
FIG.
-~-o ;
1
FIG.
T y p i c a l true s t r e s s - n a t u r a l (plastic) s t r a i n curves at strain rates of ~6x10 -4 sec -I . D u r i n g the test i n t e r r u p t i o n s shown as vertical lines, s p e c i m e n s w e r e aged for 20 min at -20°C (Q&T steel) and 0°C (iron).
~, ,oo
TEMPERATURE, °C
2
E f f e c t of h y d r o g e n on y i e l d stress. Points are solid for h y d r o g e n c h a r g e d specimens; crosses i d e n t i f y c a d m i u m - p l a t e d specimens, o y t a k e n at u p p e r y i e l d for Q&T steel, at 0.2% o f f s e t for e l e c t r o l y t i c iron; strain rate ~6x10-4 sec-l.
To reveal d i f f e r e n c e s in the strain aging r e s p o n s e of c h a r g e d and u n c h a r g e d samples, the stress i n c r e a s e after aging was m e a s u r e d (Fig. 3). The w e l l - k n o w n u n l o a d i n g y i e l d p o i n t is r e f l e c t e d in the stress i n c r e a s e for the u n c h a r g e d samples. It is e v i d e n t that the p r e s e n c e of d i s s o l v e d h y d r o g e n cont r i b u t e s a f u r t h e r i n c r e m e n t to 4o at low strains, the size of this i n c r e m e n t at a g i v e n test t e m p e r a t u r e i n c r e a s i n g w i t h aging time and temperature. 20
I
I
I
i
IRON
i
i
i
TESTED O ' C AGED O "C
IO
~o--::'~___o..~..__ ~
40
I
!
I
!
I
o 20
I
Q6T STEEL
3O
TESTED-IOO*C
I AGING TIMEI
J,o 20 rain. -
AoE0 20"c J e .
3 0 rain. I~A 4 0 min.
g.
I
'~
g. Io 6 o; 2oj •
~
IOI •
i
i 8
TESTED - 8 0 " C -SO'C
.
-,eL ~.~_
AGED _.op,__
O'C
_ o_..
I 12
NATURAL PLAST~ STRAIN,%
NATURAL PLASTIC STRAIN, %
(a)
,~
TESTED - 80 eC
--4-~,~...~,~.
I0 I 4
.
z ~
o,
"'~-.,~AGED
,
FIG.
3
(b)
S t r a i n aging r e s p o n s e of c h a r g e d (solid curves, filled points) and u n c h a r g e d (dashed curves, open points) specimens of (a) Q&T steel and (b) iron. Key for aging time as shown in Fi . 3(a). ~ ~ is the i n c r e a s e in stress at 0.3% p l a s t i c strain and ~ ~ 6 x 10 -~ sec -I after the aging c o n d i t i o n s specified.
Vol.
II, No.
I0
E F F E C T OF H Y D R O G E N ON YIELD OF IRON
851
Discussion These results are c o n s i s t e n t w i t h a h y d r o g e n - d i s l o c a t i o n i n t e r a c t i o n of the c l a s s i c type, a solute a t m o s p h e r e b e i n g formed and m o v e d w i t h the dislocations. The high initial y i e l d p o i n t is c o n s i s t e n t w i t h a t m o s p h e r e formation on d i s l o c a t i o n s in the a s - q u e n c h e d m a t e r i a l . P e r s i s t e n c e of the stress increase after y i e l d indicates that the h y d r o g e n causes either a v i s c o u s drag as an atmosphere, or s o l u t i o n hardening. The lattice s o l u b i l i t y of h y d r o g e n is very low at the test t e m p e r a t u r e s , m o s t of the h y d r o g e n b e i n g t r a p p e d in l o w - e n e r g y sites (including d i s l o c a t i o n s ) ; hence, s o l u t i o n h a r d e n i n g w o u l d be very small and it m u s t be c o n c l u d e d that the h y d r o g e n is d r a g g e d as a solute atmosphere. In this case, the force F that m u s t be e x p e c t e d by a d i s l o c a t i o n on an atom of the a t m o s p h e r e to drag it by a v e l o c i t y v is(13)
F=
(kT/D') -~
(i)
w h e r e T is the t e m p e r a t u r e and D is the d i f f u s i o n coefficient. The equilib r i u m c o n c e n t r a t i o n C 1 of solute atoms around a d i s l o c a t i o n can be e s t i m a t e d from(17)
C~
:
Coexp I ÷ Co e x p ( u / k T )
w h e r e C O is the lattice c o n c e n t r a t i o n s o l u t e - d i s l o c a t i o n b i n d i n g energy. U and is g e n e r a l l y taken to be of o r d e r Cottrell(13) has shown that the stress m a t e l y by
(2)
far from the d i s l o c a t i o n and U is the is a m a x i m u m at the d i s l o c a t i o n core 6 kcal/mol(18). For a dilute atmosphere, to drag the a t m o s p h e r e is g i v e n approxi-
w h i c h g i v e s a ~3 MPa f o r C o ~ l ppm ( = 5 . 5 x 10 - 5 a t c o n c . ) a n d b ~3 x 10 - 1 0 m. For more concentrated atmospheres, however, the stress c a n b e much l a r g e r than this. For a row of solute atoms condensed along a dislocation line at a
spacing ~b, each w i l l exert a force of o r d e r U/b at the c r i t i c a l drag v e l o c i t y v c. We then have
u/b w h i c h i s ~ 1500 MPa using the values g i v e n above; for v e l o c i t i e s less than Vc, s w i l l be p r o p o r t i o n a l l y smaller. The actual n u m b e r of solute atoms in the a t m o s p h e r e w i l l vary w i t h t e m p e r a t u r e in a complex w a y as i n d i c a t e d by Eq. (2). However, o b s e r v e d values of the y i e l d stress i n c r e m e n t are ~20 to 100 MPa (Fig. 2), w i t h i n the a d m i t t e d l y b r o a d limits e s t i m a t e d above, and d e c r e a s e w i t h i n c r e a s i n g T in a g r e e m e n t w i t h the model. F l o w stress i n c r e m e n t s due to h y d r o g e n of the order of those found here have also b e e n o b s e r v e d r e c e n t l y by A s a n o and Otsuka(12) and Y a n c h i s h i n et al(ll) d u r i n g c o n c u r r e n t h y d r o g e n charging. Using a similar technique, Bernstein(10) o b s e r v e d b o t h i n c r e a s e s and d e c r e a s e s in y i e l d stress; the o r i g i n of the d i f f e r e n t b e h a v i o u r is not yet clear. As shown in Fig. 3, the c o n t r i b u t i o n of h y d r o g e n to A~ after strain aging b e c o m e s v e r y small after about 5% p l a s t i c strain. This is m o s t likely due to the i n c r e a s e d d i s l o c a t i o n d e n s i t y p; in these experiments, the a m o u n t of h y d r o g e n is c o n s t a n t and so above some value of p there m u s t be more disl o c a t i o n trap sites than h y d r o g e n atoms to fill them. As in o t h e r i n t e r s t i t i a l alloy systems such as C in Fe(14), there should be three regions showing d i f f e r e n t e f f e c t s on flow c h a r a c t e r i s t i c s . At the lowest t e m p e r a t u r e s there s h o u l d be y i e l d drops (discontinuous yield-
852
EFFECT OF HYDROGEN ON YIELD OF IRON
Vol.
II, No.10
~ng) after initial yield, at intermediate temperatures dynamic pinning (the Portevin-le Chatelier effect) should appear, and at the highest temperatures there should be a viscous drag of solute atmospheres. The conditions for dynamic pinning at which dislocations are just able to break away from their atmospheres can be estimated from Eq. i, taking F~ U/b for solutes ~ear the dislocation core and ~ ~ e/(pb) with ~ ~ 10 -4 sec -I and p ~ 1014 m- ; this gives D/T=(k/F)~=(k/U) (~/p)~3xlO -22 m 2 sec -I K -I. For hydrogen in iron, taking D=0.78x10 -7 exp (-1900/RT) m 2 sec -I (15) this occurs at T ~32K. By analogy, for C in iron for which U is about three times as large, we expect dynamic pinning at D/T ~10 -22 m 2 sec -I K -I. Taking D=2.2x10 -4 exp (-29,300/RT) m 2 sec -I we find T ~410 K, in good agreement with the observed temperature range of serrated yielding of 330 to 480 K(14). Serrated yielding was not observed in any of our experiments, even at the lowest temperatures. This indicates that for the present experiments, hydrogen atoms were sufficiently mobile that they produced a viscous drag on dislocations rather than sharp yield drops or serrated yielding. Conclusions i.
Hydrogen dissolved in iron and steel can increase the flow stress and yield stress after strain aging, as shown in the present work using electrolytic iron and a quenched and tempered steel.
2.
The effects observed in the present work using electrolytic iron and a quenched and tempered steel are consistent with a hydrogen-dislocation interaction of the classic type, with hydrogen atoms being dragged by dislocations over the temperature range 123 to 473 K (-150 to I00°C) at a strain rate e ~ 6x10 - 4 s e c -I. Acknowledgements: We wish to thank NRC and EMR of Canada for award of a fellowship and the Japan Defense Agency for leave (Y.T.), Canadian Heat Treaters Ltd. for generous provision of Q&T steel, and the technical staff at PMRL, EMR for prompt and capable support. References 1 2 3 4 5 6 7. 8. 9.
I0. II. 12. 13. 14. 15. 16. 17. 18.
A. Cracknell and N.J. Petch, Acta Met. 3, 200 (1955). A.M. Adair and R.E. Hook, Acta Met. 10,-741 (1962). K. Farrell, J.I.S.I.203) 457 (1965). - P. Bastien and P. Azou, Proc. Ist World Met. Congress ASM (1951), p. 535. K. Farrell, J.I.S.I. 203, 71 (1965). R. Gibala, Trans. Met. Soc. AIME 239, 1574 (1967). A.P. Miodownik and B.S. Achar, in "L'Hydrog~ne dans les M~taux '), Paris, May 1972, P. Bastien (chairman), p. 106. J.A. Donovan, Met. Trans. 7A, 1677 (1976). R Broudeur, J.-P Fidelle~-and H. Auch~re, in "L'Hydrog~ne dans les M~taux", Paris, May 1972, P. Bastien (chairman), p. 106. I.M. Bernstein, Scripta Met. 8, 343 (1974). F.P. Yanchishin, N.Ya Yaremch~nko, and M.M. Shred, Fiz.-Khim.Mekh. Mat. i0 (3), 98 (1974); Engl. trans. Soy, Mat. Sci. S. Asano and R. Otsuka, Scripta Met. i0, 1015 (1976). A.H. Cottrell, "Dislocations and Plast-Tc Flow in Crystals", O.U.P. (1953), pp. 134-147. A.S. Keh, Y. Nakada, and W.C. Leslie, in "Dislocation Dynamics", A.R. Rosenfield et al (eds.), McGraw-Hill, NY (1968), p. 381. R.A. Oriani, Acta Met. 18, 147 (1970). Y. Tobe and W.R. Tyson, "Hydrogen Embrittlement of a Boron Steel", 2nd International Congress on Hydrogen in Metals, Paris, June 1977. J.C.M. Li and Y.T. Chou, Trans. Met. Soc. AIME 245, 1606 (1969). R. Gibala, "Hydrogen-Defect Interactions in Iron-Base Alloys" Proc. Int. Conf. on Stress Corrosion Cracking and Hydrogen Embrittlem~nt'in Iron-Base Alloys, Unieux-Firminy, France, June 1973.
Vol. ii, pp. 849-852, 1977 P r i n t e d in the United States
P e r g a m o n Press,
Inc
E F F E C T OF H Y D R O G E N ON Y I E L D OF IRON
Y. Tobe and W.R. Tyson Physical Metallurgy Research Laboratories CANMET, EMR 568 B o o t h St. Ottawa, Ont. KIA 0GI (Received July 20, 1977) Introduction W h i l e the d e t r i m e n t a l e f f e c t s of h y d r o g e n on the fracture of steel are w e l l known, the i n f l u e n c e of h y d r o g e n on y i e l d and flow has b e e n a subject of c o n s i d e r a b l e debate. Early w o r k p r o d u c e d a c o n f u s e d p i c t u r e of effects, w i t h y i e l d points e i t h e r eliminated(l) or created(2) and strain h a r d e n i n g either increased(3) or d e c r e a s e d ( 4 ) . In his r e v i e w of 1965, Farrell(5) c o n c l u d e d that the effects seen to that time were p r o b a b l y a r e s u l t of p r e - s t r a i n r a t h e r than i n t e r a c t i o n s b e t w e e n h y d r o g e n and d i s l o c a t i o n s . However, in r e c e n t years there has b e e n an a c c u m u l a t i o n of e v i d e n c e that h y d r o g e n can be b o u n d to d i s l o c a t i o n s (6,7), t r a n s p o r t e d by t h e m (8,9), and i n f l u e n c e their m o b i l i t y ( 1 0 , 1 1 , 1 2 ) . By a n a l o g y w i t h o t h e r i n t e r s t i t i a l solutes in iron, it w o u l d be e x p e c t e d that h y d r o g e n should cause the c l a s s i c f o r m a t i o n of atmospheres, y i e l d points, and flow stress elevation. The w o r k d e s c r i b e d b e l o w was done to look for some of these effects. Experimental Procedure Two g r a d e s of m a t e r i a l w e r e used, a v a c u u m m e l t e d e l e c t r o l y t i c iron (i0 ppm C, 80 ppm Si+Mn+S+P, 650 ppm Ni) and a c o m m e r c i a l Q&T steel (.135C, 1.39Mn, .385Si, .0012B) of 690 M P a (I00 ksi) y i e l d strength. The iron was c o l d r o l l e d 70% after casting and m a c h i n e d into flat tensile specimens of gauge s e c t i o n ii x 5 x 2 mm; the Q&T steel was m a c h i n e d from asr e c e i v e d p l a t e into s p e c i m e n s of c y l i n d r i c a l gauge section 19 x 3.2 mm. The t e n s i l e axis was always p a r a l l e l to the rolling direction. Tensile s p e c i m e n s w e r e c h a r g e d at e l e v a t e d t e m p e r a t u r e in 1 atm. of H 2 to ensure a h o m o g e n e o u s d i s t r i b u t i o n of hydrogen, the iron for 1 hr at 850°C (to ~2 ppm H) and the Q&T steel for 3 hr at 650°C (to ~i ppm H), followed by a w a t e r q u e n c h in l i q u i d N2. "Uncharged" samples w e r e g i v e n an identical thermal t r e a t m e n t in 1 a t m . of Ar instead of H2. The o p t i c a l m i c r o s t r u c t u r e s w e r e e q u i a x e d grains of d i a m e t e r ~30 ~m for the iron, and t e m p e r e d m a r t e n s i t e for the steel. T e n s i l e tests w e r e p e r f o r m e d using an Instron m a c h i n e e q u i p p e d w i t h a zero s u p p r e s s i o n device, using liquid baths of N,, m e t h y l b u t a n e , alcohol, and w a t e r for t e m p e r a t u r e control and a strain rate of ~ ~6 x 10 -4 sec -I. To i n v e s t i g a t e s t r a i n aging, p r e - s t r a i n e d specimens w e r e aged under a small load ( a p p r o x i m a t e l y 10% of yield) at either the test t e m p e r a t u r e or elev a t e d temperature. E x p e r i m e n t a l Results T y p i c a l s t r e s s - s t r a i n curves are shown in Fig. i. It is e v i d e n t that h y d r o g e n p r o d u c e s a n o t i c e a b l e i n c r e a s e in the flow stress. This is further i l l u s t r a t e d in Fig. 2, w h e r e the i n c r e a s e d y i e l d stresses of c h a r g e d samples are shown clearly. Some of the Q & T steel samples w e r e Cd p l a t e d after c h a r g i n g to impede h y d r o g e n loss; no s i g n i f i c a n t d i f f e r e n c e s w e r e o b s e r v e d b e t w e e n t e n s i l e p r o p e r t i e s i n c l u d i n g r e d u c t i o n in area(16) of p l a t e d and u n p l a t e d samples even at the h i g h e s t t e m p e r a t u r e used (100°C).
849
850
EFFECT OF HYDROGEN ON YIELD OF IRON
#00
1200 . liT
~
V01.
,
,
l
ii, No. I0
,
STEEL ,-IOO" C
80( ,./!"/"
iooo~-\\
k
800
" "' "\
"tO(
o
g.
|
[
"
°".. ~ ~
z
r
:
")
',,~,~
4oo~ 20(
200:i
I0(
°-~oo NATURAL STRAIN, %
FIG.
-~-o ;
1
FIG.
T y p i c a l true s t r e s s - n a t u r a l (plastic) s t r a i n curves at strain rates of ~6x10 -4 sec -I . D u r i n g the test i n t e r r u p t i o n s shown as vertical lines, s p e c i m e n s w e r e aged for 20 min at -20°C (Q&T steel) and 0°C (iron).
~, ,oo
TEMPERATURE, °C
2
E f f e c t of h y d r o g e n on y i e l d stress. Points are solid for h y d r o g e n c h a r g e d specimens; crosses i d e n t i f y c a d m i u m - p l a t e d specimens, o y t a k e n at u p p e r y i e l d for Q&T steel, at 0.2% o f f s e t for e l e c t r o l y t i c iron; strain rate ~6x10-4 sec-l.
To reveal d i f f e r e n c e s in the strain aging r e s p o n s e of c h a r g e d and u n c h a r g e d samples, the stress i n c r e a s e after aging was m e a s u r e d (Fig. 3). The w e l l - k n o w n u n l o a d i n g y i e l d p o i n t is r e f l e c t e d in the stress i n c r e a s e for the u n c h a r g e d samples. It is e v i d e n t that the p r e s e n c e of d i s s o l v e d h y d r o g e n cont r i b u t e s a f u r t h e r i n c r e m e n t to 4o at low strains, the size of this i n c r e m e n t at a g i v e n test t e m p e r a t u r e i n c r e a s i n g w i t h aging time and temperature. 20
I
I
I
i
IRON
i
i
i
TESTED O ' C AGED O "C
IO
~o--::'~___o..~..__ ~
40
I
!
I
!
I
o 20
I
Q6T STEEL
3O
TESTED-IOO*C
I AGING TIMEI
J,o 20 rain. -
AoE0 20"c J e .
3 0 rain. I~A 4 0 min.
g.
I
'~
g. Io 6 o; 2oj •
~
IOI •
i
i 8
TESTED - 8 0 " C -SO'C
.
-,eL ~.~_
AGED _.op,__
O'C
_ o_..
I 12
NATURAL PLAST~ STRAIN,%
NATURAL PLASTIC STRAIN, %
(a)
,~
TESTED - 80 eC
--4-~,~...~,~.
I0 I 4
.
z ~
o,
"'~-.,~AGED
,
FIG.
3
(b)
S t r a i n aging r e s p o n s e of c h a r g e d (solid curves, filled points) and u n c h a r g e d (dashed curves, open points) specimens of (a) Q&T steel and (b) iron. Key for aging time as shown in Fi . 3(a). ~ ~ is the i n c r e a s e in stress at 0.3% p l a s t i c strain and ~ ~ 6 x 10 -~ sec -I after the aging c o n d i t i o n s specified.
Vol.
II, No.
I0
E F F E C T OF H Y D R O G E N ON YIELD OF IRON
851
Discussion These results are c o n s i s t e n t w i t h a h y d r o g e n - d i s l o c a t i o n i n t e r a c t i o n of the c l a s s i c type, a solute a t m o s p h e r e b e i n g formed and m o v e d w i t h the dislocations. The high initial y i e l d p o i n t is c o n s i s t e n t w i t h a t m o s p h e r e formation on d i s l o c a t i o n s in the a s - q u e n c h e d m a t e r i a l . P e r s i s t e n c e of the stress increase after y i e l d indicates that the h y d r o g e n causes either a v i s c o u s drag as an atmosphere, or s o l u t i o n hardening. The lattice s o l u b i l i t y of h y d r o g e n is very low at the test t e m p e r a t u r e s , m o s t of the h y d r o g e n b e i n g t r a p p e d in l o w - e n e r g y sites (including d i s l o c a t i o n s ) ; hence, s o l u t i o n h a r d e n i n g w o u l d be very small and it m u s t be c o n c l u d e d that the h y d r o g e n is d r a g g e d as a solute atmosphere. In this case, the force F that m u s t be e x p e c t e d by a d i s l o c a t i o n on an atom of the a t m o s p h e r e to drag it by a v e l o c i t y v is(13)
F=
(kT/D') -~
(i)
w h e r e T is the t e m p e r a t u r e and D is the d i f f u s i o n coefficient. The equilib r i u m c o n c e n t r a t i o n C 1 of solute atoms around a d i s l o c a t i o n can be e s t i m a t e d from(17)
C~
:
Coexp I ÷ Co e x p ( u / k T )
w h e r e C O is the lattice c o n c e n t r a t i o n s o l u t e - d i s l o c a t i o n b i n d i n g energy. U and is g e n e r a l l y taken to be of o r d e r Cottrell(13) has shown that the stress m a t e l y by
(2)
far from the d i s l o c a t i o n and U is the is a m a x i m u m at the d i s l o c a t i o n core 6 kcal/mol(18). For a dilute atmosphere, to drag the a t m o s p h e r e is g i v e n approxi-
w h i c h g i v e s a ~3 MPa f o r C o ~ l ppm ( = 5 . 5 x 10 - 5 a t c o n c . ) a n d b ~3 x 10 - 1 0 m. For more concentrated atmospheres, however, the stress c a n b e much l a r g e r than this. For a row of solute atoms condensed along a dislocation line at a
spacing ~b, each w i l l exert a force of o r d e r U/b at the c r i t i c a l drag v e l o c i t y v c. We then have
u/b w h i c h i s ~ 1500 MPa using the values g i v e n above; for v e l o c i t i e s less than Vc, s w i l l be p r o p o r t i o n a l l y smaller. The actual n u m b e r of solute atoms in the a t m o s p h e r e w i l l vary w i t h t e m p e r a t u r e in a complex w a y as i n d i c a t e d by Eq. (2). However, o b s e r v e d values of the y i e l d stress i n c r e m e n t are ~20 to 100 MPa (Fig. 2), w i t h i n the a d m i t t e d l y b r o a d limits e s t i m a t e d above, and d e c r e a s e w i t h i n c r e a s i n g T in a g r e e m e n t w i t h the model. F l o w stress i n c r e m e n t s due to h y d r o g e n of the order of those found here have also b e e n o b s e r v e d r e c e n t l y by A s a n o and Otsuka(12) and Y a n c h i s h i n et al(ll) d u r i n g c o n c u r r e n t h y d r o g e n charging. Using a similar technique, Bernstein(10) o b s e r v e d b o t h i n c r e a s e s and d e c r e a s e s in y i e l d stress; the o r i g i n of the d i f f e r e n t b e h a v i o u r is not yet clear. As shown in Fig. 3, the c o n t r i b u t i o n of h y d r o g e n to A~ after strain aging b e c o m e s v e r y small after about 5% p l a s t i c strain. This is m o s t likely due to the i n c r e a s e d d i s l o c a t i o n d e n s i t y p; in these experiments, the a m o u n t of h y d r o g e n is c o n s t a n t and so above some value of p there m u s t be more disl o c a t i o n trap sites than h y d r o g e n atoms to fill them. As in o t h e r i n t e r s t i t i a l alloy systems such as C in Fe(14), there should be three regions showing d i f f e r e n t e f f e c t s on flow c h a r a c t e r i s t i c s . At the lowest t e m p e r a t u r e s there s h o u l d be y i e l d drops (discontinuous yield-
852
EFFECT OF HYDROGEN ON YIELD OF IRON
Vol.
II, No.10
~ng) after initial yield, at intermediate temperatures dynamic pinning (the Portevin-le Chatelier effect) should appear, and at the highest temperatures there should be a viscous drag of solute atmospheres. The conditions for dynamic pinning at which dislocations are just able to break away from their atmospheres can be estimated from Eq. i, taking F~ U/b for solutes ~ear the dislocation core and ~ ~ e/(pb) with ~ ~ 10 -4 sec -I and p ~ 1014 m- ; this gives D/T=(k/F)~=(k/U) (~/p)~3xlO -22 m 2 sec -I K -I. For hydrogen in iron, taking D=0.78x10 -7 exp (-1900/RT) m 2 sec -I (15) this occurs at T ~32K. By analogy, for C in iron for which U is about three times as large, we expect dynamic pinning at D/T ~10 -22 m 2 sec -I K -I. Taking D=2.2x10 -4 exp (-29,300/RT) m 2 sec -I we find T ~410 K, in good agreement with the observed temperature range of serrated yielding of 330 to 480 K(14). Serrated yielding was not observed in any of our experiments, even at the lowest temperatures. This indicates that for the present experiments, hydrogen atoms were sufficiently mobile that they produced a viscous drag on dislocations rather than sharp yield drops or serrated yielding. Conclusions i.
Hydrogen dissolved in iron and steel can increase the flow stress and yield stress after strain aging, as shown in the present work using electrolytic iron and a quenched and tempered steel.
2.
The effects observed in the present work using electrolytic iron and a quenched and tempered steel are consistent with a hydrogen-dislocation interaction of the classic type, with hydrogen atoms being dragged by dislocations over the temperature range 123 to 473 K (-150 to I00°C) at a strain rate e ~ 6x10 - 4 s e c -I. Acknowledgements: We wish to thank NRC and EMR of Canada for award of a fellowship and the Japan Defense Agency for leave (Y.T.), Canadian Heat Treaters Ltd. for generous provision of Q&T steel, and the technical staff at PMRL, EMR for prompt and capable support. References 1 2 3 4 5 6 7. 8. 9.
I0. II. 12. 13. 14. 15. 16. 17. 18.
A. Cracknell and N.J. Petch, Acta Met. 3, 200 (1955). A.M. Adair and R.E. Hook, Acta Met. 10,-741 (1962). K. Farrell, J.I.S.I.203) 457 (1965). - P. Bastien and P. Azou, Proc. Ist World Met. Congress ASM (1951), p. 535. K. Farrell, J.I.S.I. 203, 71 (1965). R. Gibala, Trans. Met. Soc. AIME 239, 1574 (1967). A.P. Miodownik and B.S. Achar, in "L'Hydrog~ne dans les M~taux '), Paris, May 1972, P. Bastien (chairman), p. 106. J.A. Donovan, Met. Trans. 7A, 1677 (1976). R Broudeur, J.-P Fidelle~-and H. Auch~re, in "L'Hydrog~ne dans les M~taux", Paris, May 1972, P. Bastien (chairman), p. 106. I.M. Bernstein, Scripta Met. 8, 343 (1974). F.P. Yanchishin, N.Ya Yaremch~nko, and M.M. Shred, Fiz.-Khim.Mekh. Mat. i0 (3), 98 (1974); Engl. trans. Soy, Mat. Sci. S. Asano and R. Otsuka, Scripta Met. i0, 1015 (1976). A.H. Cottrell, "Dislocations and Plast-Tc Flow in Crystals", O.U.P. (1953), pp. 134-147. A.S. Keh, Y. Nakada, and W.C. Leslie, in "Dislocation Dynamics", A.R. Rosenfield et al (eds.), McGraw-Hill, NY (1968), p. 381. R.A. Oriani, Acta Met. 18, 147 (1970). Y. Tobe and W.R. Tyson, "Hydrogen Embrittlement of a Boron Steel", 2nd International Congress on Hydrogen in Metals, Paris, June 1977. J.C.M. Li and Y.T. Chou, Trans. Met. Soc. AIME 245, 1606 (1969). R. Gibala, "Hydrogen-Defect Interactions in Iron-Base Alloys" Proc. Int. Conf. on Stress Corrosion Cracking and Hydrogen Embrittlem~nt'in Iron-Base Alloys, Unieux-Firminy, France, June 1973.