SELECTION RESISTANCE

OF

CONDITIONS

TO

FOR

HYDROGEN

T. K. Sergeeva, G. and A. G. Zakurdaev

TESTING

EMBRITTLEMENT G.

UDC 620.171.3:669.788

Irzhov,

A b s o r p t i o n of hydrogen into m e t a l s in s e r v i c e is often the r e a s o n for p r e m a t u r e failure of the equipment through b r i t t l e f r a c t u r e . Steels a r e t h e r e f o r e widely t e s t e d for r e s i s t a n c e to hydrogen e m b r i t t l e m e n t . The t e s t s include static tensile testing of s a m p l e s p r e v i o u s l y hydrogen c h a r g e d or c h a r g e d during s t r e s s . L o s s in ductility or s t r e n g t h is d e t e r m i n e d with r e s p e c t to a s - r e c e i v e d s a m p l e s . The loss is d e t e r m i n e d b y the amount of d e f o r m a t i o n and a b s o r b e d hydrogen. The l a t t e r depends on the h y d r o g e n - c h a r g i n g conditions, i.e., its duration, cathodic c u r r e n t density, s u r f a c e r o u g h n e s s , etc. With so m a n y f a c t o r s , choosing t e s t conditions f o r a specific type of s t e e l b y sequentially varying one f a c t o r keeping the o t h e r s constant r e q u i r e s a l a r g e n u m b e r of e x p e r i m e n t s and is often a r b i t r a r y . The choice would b e b e s t a c c o m p l i s h e d through s t a t i s t i c a l design of e x p e r i m e n t s [1, 2]. In this w o r k we used the s t e e p a s c e n t m e t h o d to s e l e c t o p t i m u m conditions for testin_g s t e e l s intended f o r o p e r a t i o n in m e d i a containing hydrogen sulfide. Contributions of e a c h of the f a c t o r s w e r e c o n s i d e r e d of i n t e r e s t . We studied 09G2S s t e e l (0.10~ C; 0.70 Si; 1.5 Mn; 0.10 Cr; 0.040 S; 0.035 P). We used c y l i n d r i c a l s a m ples with a d i a m e t e r of 6 m m and a working section length of 40 r a m . The anode w a s platinum and the e l e c troly-te was 0.1 N sulfuric acid containing 1.5 g / l i t e r of thiourea. The s a m p l e s w e r e pulled in a i r . The amount of hydrogen e m b r i t t l e m e n t was d e t e r m i n e d b y the change in c r o s s - s e c t i o n a l c o n s t r i c t i o n ACH , c a l c u l a t e d a c c o r d i n g to the equation: ,aq,. = (.~o - 4,~)~.o. lOO o,6,

(1)

w h e r e r is the c r o s s - s e c t i o n a l c o n s t r i c t i o n of the a s - r e c e i v e d s a m p l e (not c h a r g e d with hydrogen} and CH is the c r o s s - s e c t i o n a l c o n s t r i c t i o n of the hydrogen c h a r g e d s a m p l e . This c h a r a c t e r i s t i c was chosen as the optimization p a r a m e t e r .

F r o m h e r e on it will be d e s i g n a t e d by y.

The following f a c t o r s w e r e studied: cathodic c u r r e n t density, hydrogen charging time, d e f o r m a t i o n r a t e during test, s t r e s s during hydrogen charging, and heat t r e a t m e n t p r i o r to t e s t (to r e m o v e m e c h a n i c a l w o r k hardening and to keep the original s t r u c t u r e of the s t e e l [3]). Main levels of these f a c t o r s and r a n g e s of v a r i a t i o n for the f i r s t set of e x p e r i m e n t s w e r e s e l e c t e d on the b a s i s of an a n a l y s i s of l i t e r a t u r e data as well as a 25-2 f r a c t i o n a l replication design shown in Table 1. Each e x p e r i m e n t was r u n in duplicate to i n c r e a s e a c c u r a c y in checking d i s p e r s i o n u n i f o r m i t y and d e t e r m i n i n g r e p r o d u c i b i l i t y e r r o r s . Sequence of the e x p e r i m e n t s was r a n d o m i z e d with r e s p e c t to t i m e . R e s u l t s of the f i r s t s e r i e s of t e s t s and t h e i r s t a t i s t i c a l t r e a t m e n t a r e p r e s e n t e d in Table 2. H e r e yIi and yIIi a r e the r e s u l t s of p a r a l l e l e x p e r i m e n t s ; the s u b s c r i p t i is the e x p e r i m e n t n u m b e r ; Yi is the a r i t h m e t i c a l a v e r a g e for the r e s u l t s of two p a r a l l e l e x p e r i m e n t s ; Yi is the calculated optimization p a r a m e t e r ; and Ay = The following equation was obtained as a r e s u l t of the f i r s t s e t of e x p e r i m e n t s : y = 68.8 -- 0,225 X, ~ 4.400 X~ -- 1,650X 3 -I- 9.275X~ -- 1.525X5.

(2)

C o c h r a n ' s t e s t showed the hypothesis of d i s p e r s i o n uniformity to be valid.

I. P. Bardin Central S c i e n t i f i c - R e s e a r c h Institute of F e r r o u s Metallurgy, Moscow. T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a Mekhanika Materialov, Vol. 14, No. 3, pp. 29-33, May-Jtme, 1978. Original a r t i c l e s u b m i t t e d August 28, 1975.

0038-5565/78/1403- 0249$07.50 9 1978 Plenum Publishing C o r p o r a t i o n

249

T A B L E 1. Cathodic [Hydrogen Defonna~Stregs dur- fRemoval i current ]charging rate s, ing hydro- Iof work gen cnarg-lhardening density i, ttime r, ~ c mg ~, 2 t(byvac -~ mA/cm ~ ih kgf/cm /uum heating)

Factor~ studied

Code designations of the variables Main leveZ Upper level

Xa

X4

5,0

10-3

30

70

2,0

10-4

0

1

15 §

15 §

-

+

85 100

Lower level Variation interval d

2

=

3

"~

4 5 6

o ~2

3,5

-

--

& T e.mpering in a vacuum Without tempering

15

-1--

+

+

+

+

T

-

+

§

§

7 8

+ +

N o t e . M a i n l e v e l s and v a r i a t i o n i n t e r v a l s f o r X 3 and X 5 h a v e no s i g n i f i c a n c e a s o n l y two l e v e l s w e r e o f i n t e r e s t . Dispersion reproducibility is represented as: 8

Srepro = 2 E (Yll - - Y,)2 8 = 5.90, Srepro = 2.43.

(3)

1

R e g r e s s i o n c o e f f i c i e n t s i g n i f i c a n c e w a s found b y e s t i m a t i n g t h e d i s p e r s i o n of the c o e f f i c i e n t s : S~j = Sr2epro~16 = 5,90q6 = 0.37. E x p e r i m e n t a l a n d t a b u l a r v a l u e s f o r t h e S t u d e n t t t e s t a t a 5% l e v e l of s i g n i f i c a n c e f o r 8 d e g r e e s of f r e e d o m a r e the f o l l o w i n g : t~ = 0.37, t 2 = 7.24, t 3 = 2.72, t 4 = 15.27, t 5 = 2.51, and t t a b l = 2.306. It i s e v i d e n t f r o m t h e s e d a t a t h a t a l l the c o e f f i c i e n t s o f Eq. (2) a r e s i g n i f i c a n t e x c e p t t h a t f o r X 1. We a p p l i e d the F i s h e r t e s t to c h e c k the a d e q u a c y of the m a t h e m a t i c a l m o d e l o b t a i n e d f o r h y d r o g e n e m brittlement: 2

2

~

9

2

F = Sg4/Srepr o -- n.~ (Ay)-fSrepro,

(4)

w h e r e n i s the n u m b e r o f p a r a l l e l t e s t s , f = N - (k + 1) r e p r e s e n t s t h e d e g r e e s o f f r e e d o m , N i s t h e n u m b e r of e x p e r i m e n t s , and k + 1 i s the n u m b e r of m o d e l c o e f f i c i e n t s i n c l u d i n g the f r e e t e r m . E x p e r i m e n t a l and t a b u l a r v a l u e s f o r the F i s h e r t e s t w e r e f o u n d t o b e 8.56 and 19.3, r e s p e c t i v e l y , i . e . , F < F t a b l . Thus, the h y p o t h e s i s on a d e q u a c y of t h e l i n e a r m o d e l i s n o t r e j e c t e d . The m a t h e m a t i c a l m o d e l 0.05

o b t a i n e d c o r r e s p o n d s to known p h y s i c o c h e m i c a l t h e o r i e s on h y d r o g e n e m b r i t t l e m e n t . I n d e e d , h y d r o g e n e m b r i t t l e m e n t i s now t h o u g h t to d e p e n d on h y d r o g e n c o n c e n t r a t i o n in t e r m s of h y d r o g e n c h a r g i n g c o n d i t i o n s . An i n c r e a s e in t h e d u r a t i o n of h y d r o g e n c h a r g i n g a s w e l l a s the p r e s e n c e of e l a s t i c s t r e s s e s d u r i n g c h a r g i n g p r o d u c e s a h i g h h y d r o g e n c o n c e n t r a t i o n in t h e s t e e l and c o n s e q u e n t l y a g r e a t e r d e g r e e of e m b r i t t l e m e n t . T h i s c o r r e s p o n d s to p o s i t i v e c o e f f i c i e n t s o f X 2 a n d X 4. An i n c r e a s e in d e f o r m a t i o n r a t e o f t h e h y d r o g e n c h a r g e d s a m p l e d u r i n g s t a t i c t e n s i o n s l o w s down d i f f u s i o n due to r e l a x a t i o n t h r o u g h m i c r o p l a s t i c d e f o r m a t i o n , c a u s i n g a r e d u c t i o n in h y d r o g e n e m b r i t t l e m e n t . T h i s c o n d i t i o n i s r e f l e c t e d b y a n e g a t i v e c o e f f i c i e n t o f X 3. T h e n e g a t i v e e f f e c t of f a c t o r X 5 ( 2 - h t e m p e r i n g in a v a c u u m to r e m o v e w o r k h a r d e n i n g ) i s a p p a r e n t l y r e l a t e d to s o m e s u r f a c e o x i d a t i o n due to an i n s u f f i c i e n t l y h i g h v a c u u m . The low c o e f f i c i e n t of X 1 i n d i c a t e s t h a t h y d r o g e n c h a r g i n g i s w e a k l y r e l a t e d to c a t h o d i c c u r r e n t d e n s i t y w i t h i n t h e r a n g e s t u d i e d , a g r e e i n g w i t h n u m e r o u s d a t a in [4, 5].

250

TABLE

2.

~d] yIl

yll/

1 2 3 4 5 6 7 8

T A B L E 3. _

I

Yi (YIi-Yi)-"

69,0! 64,0 66,5 , 80,0 85,0 82,5 75,5 79,5 77,5 i 64,0 68,5166,25! 71,0 73,5i72,25i 53,0 53,0 53,0, 53,0 515 52,25 80,0i 80,0 80,0

6,25 6,25 4,00 5,05 1,56 0 0,55 0

.~

ly

tya

nationDesig" [yl

163,676 '. @2,925 ' 8,556 .85,425 --2,925,8,556 i79,525 ~--2,025 '~4,101 164,275 ~ 1,975' 3.901 j70,275. 1975'3901 155,025 --2 0~5 i 4 I01 155,2251 --.,9~o, 9' ~-' 8,851 ' :77,075 i 2,97518,556

ylI

Yi Yli--Yi (YIi--Yi)'

+ J - 800f817 8235 235 5,52 d 2I + 2_ t71:0177:7174:35]=3:35 11,22 3 - + 8o,o 18o,o 18o,o o 0 5,06 - - t,5,5:1,01,3,251+2,251

ua i

To d e m o n s t r a t e the p o s s i b i l i t y of n o n l i n e a r s u r f a c e r e s p o n s e we c a r r i e d out a c o n t r o l e x p e r i m e n t using the following v a l u e s f o r the f a c t o r s studied: X t = 85 m A / c m 2, X2 = 3.5 h, X 3 = 10 -3 s e c -1, X 4 = 30 k g f / m m 2, and X 5 is an u n t e m p e r e d s a m p l e . E x p e r i m e n t a l r e s u l t s showed t h a t y = 78% and c a l c u l a t i o n s that y = 77.9%, w h i c h is a r a t h e r good a p p r o x i m a t i o n of s u r f a c e r e s p o n s e t h r o u g h a l i n e a r equation within the f a c t o r r a n g e s studied and c o n f i r m s o u r c o n c l u s i o n a s to the a d e q u a c y of the m o d e l s e l e c t e d . A f t e r c a r r y i n g out the f i r s t s e t of e x p e r i m e n t s we d e c i d e d to k e e p the following f a c t o r s c o n s t a n t : X 3 = 10 -3 s e c -1, X 4 = 30 k g f / m m 2, and X 5 is an u n t e m p e r e d s a m p l e . The value f o r X 3 was taken at the l o w e r level s i n c e it g r e a t l y s p e e d s up the t e s t and its e f f e c t on the o p t i m i z a t i o n p a r a m e t e r is s m a l l . In a s e c o n d s e t of e x p e r i m e n t s (Table 3) we studied the effect of f a c t o r s f o r X 1 (main level of 70, u p p e r of 100, and l o w e r of 40 m A / c m 2) and X 2 ( m a i n level of 5, u p p e r of 6.5, and l o w e r of 3.5 h). We obtained the following e q u a t i o n as a r e s u l t of the s e c o n d s e t of e x p e r i m e n t s

A y = 77,48-]--0,86X l + 3.68 X2 + 0.31 XlX~.

(5)

A m o r e e x a c t value f o r d i s p e r s i o n r e p r o d u c i b i l i t y is S~epro = 10,9, Srepr o = 3-17. Only the c o e f f i c i e n t s f o r X 2 w e r e found s i g n i f i c a n t (t 1 = 0.94, t 2 = 4.04, and t12 = 0.34). In this connection, it w a s d e c i d e d to b r o a d e n the v a r i a t i o n r a n g e s s u b s t a n t i a l l y in the t h i r d s e t of e x p e r i m e n t s (Table 4): m a i n level f o r Xt of 150, u p p e r of 200, and l o w e r of 100 m A / c m 2 ; m a i n level f o r X 2 of 11.5, u p p e r of 16.5, and l o w e r of 6.5h. The e q u a t i o n o b t a i n e d had the f o r m :

A

y = 80.63 -- 1.71 X 1 @ 1,71 X2 + 1.71 X1X~.

(6)

All the r e g r e s s i o n c o e f f i c i e n t s w e r e found insignificant. A n a l y s i s of E q s . (2), (5), and (6) indicated that c h a n g e s in c u r r e n t d e n s i t y within the r a n g e studied (40200 m A / c m 2) had no s i g n i f i c a n t e f f e c t on l o s s of ductility. Of all the f a c t o r s studied the g r e a t e s t e f f e c t s w e r e p r o d u c e d b y loading the s a m p l e d u r i n g p o l a r i z a t i o n and d u r a t i o n of h y d r o g e n c h a r g i n g . A m a x i m u m ductility l o s s of 87% w a s a t t a i n e d b y h y d r o g e n c h a r g i n g f o r 6.5 h; i n c r e a s i n g the c h a r g i n g t i m e to 16.5 h had no f u r t h e r effect. To p r o v i d e sufficient r e s o l u t i o n we c h o s e t e s t conditions s u c h that the d u c t i l i t y l o s s of the " r e f e r e n c e " 09G2S s t e e l w a s not e x c e s s i v e , i.e., 40-50%. T h i s l o s s w a s a c h i e v e d at the l o w e r level of the f a c t o r s studied, i.e., upon h y d r o g e n c h a r g i n g the u n s t r e s s e d s a m p l e f o r 2 h followed b y s t a t i c t e n s i o n at a r a t e of 10 -3 s e c -1. P o l a r i z i n g c u r r e n t d e n s i t y c a n be s e l e c t e d a r b i t r a r i l y b e t w e e n 40 and 200 m A / c m 2. T A B L E 4. Designation

y,] y~. yli--y--i(Yli--.~-i)

[y, 77,7

1

!

1 -~-@ '87,0:82,35--4,65 21,62 ~_+_+ +4,50 20,2~ 80.0171,0175,5 0,:7.80,08 ,35:§ 5,52 -

r..r..1

*

1

i8t,7182,35! --2,35 ,

]

i

5,52

*The fourth e x p e r i m e n t w a s included in the s e c o n d s e r i e s . 251

In conclusion it should b e noted that the m a t h e m a t i c a l model of hydrogen e m b r i t t l e m e n t given in this w o r k has other advantages in addition to a r a t i o n a l selection of t e s t conditions; it can be useful in developing alloying p r i n c i p l e s to produce s t r u c t u r a l s t e e l s r e s i s t a n t to hydrogen e m b r i t t l e m e n t . To a c c o m p l i s h this we m u s t i n t e r p r e t the r e g r e s s i o n coefficients as a function of a s t e e l ' s s t r u c t u r a l state and composition. LITERATURE 1.

2. 3. 4. 5.

CITED

V. V. Nalimov and N. A. Chernova, Statistical Methods for Designing E x t r e m a l E x p e r i m e n t s [in Russian], Nauka, Moscow (1976). Yu. P. Adler, E. V. Markova, and Yu. V. Granovskii, The Design of E x p e r i m e n t s to Find Optimal Conditions [in Russian], Nauka, Moscow (1976). A. P. Gulyaev, P h y s i c a l M e t a l l u r g y [in Russian], Metallurgiya, Moscow (1977). G. V. Karpenko and R. I. Kripyakevich, Effect of Hydrogen on the P r o p e r t i e s of Steel [in Russian], Metallurgizdat, Moscow (1962). M. Smialkwski, Hydrogen in Steel, P e r g a m o n , London (1962).

CHANGE

IN

STEEL

THE

12MKh

HYDROGEN Yu. and

STRENGTH UNDER

AT

A

AND

THE

INFLUENCE

PRESSURE

I. Archakov, V. M. Sedov

B.

DUCTILITY

OF M.

i000

OF OF

kgf/em

2

UDC

Teslya,

Little r e s e a r c h has b e e n done on the b e h a v i o r of m e t a l s and alloys in hydrogen under p r e s s u r e s above 800 k g f / c m 2 and the few data in [1-7] a r e c o n t r a d i c t o r y . The p u r p o s e of this w o r k was to c l a r i f y the effect of hydrogen at a p r e s s u r e of 1000 k g f / c m 2 on the m e c h a n i c a l p r o p e r t i e s and c o r r o s i o n r e s i s t a n c e of steel 121~IKh at e l e v a t e d t e m p e r a t u r e s . The c o m m e r c i a l h e a t of this steel, which is widely used in v a r i o u s b r a n c h e s of industry [8, 9], had the following composition: 0.149o C, 0.59~ Mn, 0.509c Cr, 0.50~ Mo, 0.15/Q Ni. The steel was n o r m a l i z e d at 920~ and t e m p e r e d at 680 ~. The phase c o m p o s i t i o n was d e t e r m i n e d by m e a n s of e l e c t r o l y t i c s e p a r a t i o n with x - r a y and differential c a r b i d e a n a l y s i s . Alloyed c e m e n t i t e was found in s t e e l 12MKh. Tensile t e s t s a m p l e s (3 m m in d i a m e t e r , 15 r a m long in the reduced section) w e r e subjected to gaseous hydrogen under p r e s s u r e in s p e c i a l a u t o c l a v e s . The beginning of hydrogen c o r r o s i o n was d e t e r m i n e d f r o m c b , n g e s in the c a r b o n content and the p h a s e c o m p o s i t i o n of the s t e e l and also f r o m the a n a l y s i s of the s t r u c t u r e and the m e c h a n i c a l p r o p e r t i e s b e f o r e and a f t e r the t e s t s . The m e c h a n i c a l p r o p e r t i e s w e r e t e s t e d at r o o m temperature. T e s t s lasting f r o m 40 rain to 1500 h w e r e m a d e at t e m p e r a t u r e s of 600, 550, 500, and 450 ~ F i g u r e 1 shows the data c h a r a c t e r i z i n g the kinetics of the changes in the p r o p e r t i e s of the steel in the p r o c e s s of holding under a hydrogen p r e s s u r e of 1000 k g f / e m 2 at different t e m p e r a t u r e s . As can be seen, changes in the c a r b o n content and the reduction of the strength o c c u r a l m o s t s i m u l t a n e o u s l y . Beginning at 550 ~ the ductility d e c r e a s e s s o o n e r than the s t r e n g t h c h a r a c t e r i s t i c s . At lower t e m p e r a t u r e s the r a t e of the d e c a r b u r i z a t i o n p r o c e s s d e c r e a s e s s h a r p l y . Samples 7 m m in d i a m e t e r w e r e d e c a r b u r i z e d in 20 h at 600 ~ but d e c a r b u r i z e d only 409c a f t e r 1500 h at 450 ~ At t e m p e r a t u r e s of 600 and 550 ~ t h e r e a r e two s t a g e s in the change of the ductility, and at 500-450 ~ t h e r e axe t h r e e stages, differing in the r a t e of change.

All-Union S c i e n t i f i c - R e s e a r c h Institute of P e t r o c h e m i c a l P r o c e s s e s , Leningrad. T r a n s l a t e d f r o m F i z i k o - K h i m i c h e s k a y a Mekhanika Matertalov, Vol. 14, No. 3, pp. 33-36, May-June, 1978. Original a r t i c l e s u b m t t t e d S e p t e m b e r 28, 1976.

252

0038-5565/78/1403-0252S07.50

9

Plenum Publishing C o r p o r a t i o n