135

STRENGTH PROPERTIES

H Y D R O G E N EMBRITTLEMENT OF STEEL Prof. L. S. MOROZ

and Eng. T. E. MINGIN

The p r e s e n t - d a y t h e o r i e s on hydrogen e m b r i t t l e m e n t a r e b a s e d on c e r t a i n p e c u l i a r i t i e s of the effect of hydrogen on the m e c h a n i c a l p r o p e r t i e s of steel. P a r t i c u l a r i m p o r t a n c e i s attributed to the s e n s i t i v i t y of hydrogen e m b r i t i l e m e n t to the s t r a i n rate. The g r e a t e s t b r i t t l e n e s s o c c u r s in hydrogenabso rb ed s p e c i m e n s of p l a s t i c s t e e l with low s t r a i n r a t e s . As the s t r a i n r a t e i s i n c r e a s e d , the b r i t t l e n e s s decline s , and at r a t e s of 1 - 4 m / s e e v a n i s h e s a l l together. F i g u r e 1 shows the p l a s t i c i t y of h y d r o g e n - a b s o r b e d s p e c i m e n s of ST. 3 s t e e l as a function of the s t r a i n rate.

acting m e c h a n i s m s i n h y d r o g e n e m b r i t i l e m e n t . The f i r s t does not depend on the s t r a i n rate, i s not connected with the diffusion of the hydrogen and cannot t h e r e f o r e be s u p p r e s s e d by any s t r a i n rate, no m a t t e r how high. The b r i t t l e n e s s cansed by t h i s m e c h a n i s m i s t e r m e d " i r r e v e r s i b l e " ; t hi s i s a conditional t e r m , s i nc e i t i s p o s s i b l e i n a n u m b e r of c a s e s to r e c o v e r the i n i t i a l p r o p e r t i e s of the m e t a l by re movi ng the hydrogen from the s p e c i m e n by heating.

A p a r t i c u l a r l y h a z a r d o u s form of hydrogen e m b r i t t l e ment is s o - c a l l e d s t a t i c fatigue or delayed b r i t t l e f a i l u r e of h i g h - s t r e n g t h h y d r o g e n - a b s o r b e d s t e e l s at r e l a t i v e l y low applied s t r e s s e s . In seek ing to e xplain t h e s e phenomena, all i n v e s t i g a t o r s a g r e e that hydrogen c a u s e s b r i t t l e n e s s whenever it concent r a t e s in m i c r o s c o p i c r e g i o n s ; it i s h e r e that i n i t i a l c r a c k s begin. P l a s t i c as well as e l a s t i c deformation speeds up diffusion of the hydrogen and d i r e c t s it toward the points at which the t e n s i l e s t r e s s e s concentrate and w h e r e the r e s i d u a l defo rmations a r e localized. It has been proved e x p e r i m e n t a l l y that e l a s t i c and p l a s t i c deformation have t h i s effect on the diffusion of hydrogen. It i s a l s o usual to c o n s i d e r that at high s t r a i n r a t e s the diffusion of hydrogen toward concentration points i s not completed in t i m e and no b r i t t l e n e s s o c c u r s . The t e s t s c a r r i e d out by the authors to v e r i f y the d uctility of h y d r o g e n - a b s o r b e d s t e e l s p e c i m e n s did not conf i r m t h i s point. At a f a i r l y high d e g r e e of s a t u r a t i o n by hydrogen, the ductility of the s p e c i m e n s of s u p e r s t r e n g t h s t e e l was not r e c o v e r e d at all as the s t r a i n r a t e i n c r e a s e d . The tab le shows the r e s u l t s of s t a t i c and dynamic t e n s i l e t e s t s on Cr-Ni-Mo s t e e l s p e c i m e n s before and aft e r 2-hour cathode t r e a t m e n t in 10% aqueous solution of H2SO 4 with the addition of AS203. It follows f r o m the table that the ductility of the hydrogen - abso rb ed s t e e l during dynamic t e n s i o n does i n c r e a s e , although it i s not fully r e c o v e r e d , as during the t e s t s on i n d u s t r i a l i r o n (see Fig. 1). In o r d e r to check w h e t h e r or not b r i t t l e n e s s under i m p a c t t e n s i o n i s a r e s u l t of an insufficient r a t e of elongation, we c a r r i e d out e x p e r i m e n t s in which the s t r a i n r a t e exceeded the i m p a c t t e n s i o n r a t e in the t e s t e r by two o r d e r s of m a g nitude. Even in this case, however, the b r i t t l e n e s s was not comp letely elimin ated. These facts s u g g e s t that t h e r e a r e two s i m u l t a n e o u s l y

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Strain r a t e Fig. 1. Effect of s t r a i n ra t e on ductility of indust r i a l i ron s p e c i m e n s s a t u r a t e d with hydrogen: 1 -- e l e c t r o l y t i c hydrogen absorption; 2 -- hight e m p e r a t u r e hydrogen a bs orpt i on The second m e c h a n i s m has an incubation pe ri od during which the hydrogen diffuses and c onc e nt ra t e s in m i c r o s c o p i c re gi ons until t h e r e i s enough for e m b r i t t l e m e n t . The d u r a tion of the incubation period i n c r e a s e s as the hydrogen conc e n t r a t i o n in the l a t t i c e c r y s t a l of the s t e e l d e c r e a s e s . Consequently, the o c c u r r e n c e of t h i s kind of b r i t t l e n e s s i s a function of the s t r a i n ra t e and can be s u p p r e s s e d at high r a t e s at which the de forma t i on t i m e i s l e s s than the incubation period. This type of b r i t t l e n e s s can t h e r e f o r e be conditionally called "reversfDle. '~

r Specimen

Nct saturatedwith hydrogen .... Saturated with hydrogen ....

Note.

vI = 4 m m / m i n

%at v2 = 4 m / s e c

62

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Ultimate s t r e n g t h of s t e e l 92 k g / m m 2.

136

In this way, reversible and i r r e v e r s i b l e brittleness can be separated in practice by applying different s t r a i n rates to the specimens. The dependence of the plasticity of Cr-Ni-Mo steel on the time for which it is aged after hydrogen absorption is shown in Fig. 2. The ductility of the metal was determined during slow tension (v1 = 4 m m / m i n ) and dynamic tension (v2 = 4 m / s e e ) .

The fact that Curves 1 and 2 do not match

confirms that the variation in ductility during slow and impact tension is due to different factors, i . e . , it confirms the existence of two mechanisms of hydrogen embrittlement.

70 .

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3 7

be a function of the strain rate, although the initial ductility is certainly net yet attained. This is because the hydrogen concentration dissolved in the crystal is considerably s m a l l e r a day later. Under conditions of this kind we usually find i r r e v e r s i b l e brittleness due to the molecular hydrogen in the pores. As microscopic investigation shows, the pores causing i r r e v e r s i b l e brittleness are most often located along the grain boundaries. The i r r e v e r s i b l e brittleness of specimens at room temperature is not completely eliminated since the molecular hydrogen remains in the pores for a long time after a reduction in p r e s s u r e .

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Aging time of specimens after hydrogen absorption Fig. 2. Effect of aging time after hydrogen a b s o r p tion on plasticity of Cr-Ni-Mo steel (0.17% C; gb = "85 kg/m2): 1 -- strain rate 4 m / s e e ; 2 -- s t r a i n rate 4 m / m i n ; 3 -- impact t e s t s on nonhydrogen absorbed specimens (each point is the mean r e s u l t of five tests) The atomic hydrogen penetrating into the metal during cathode treatment f o r m s a supersaturated interstitial solid solution. This is shown b y X - r a y analysis which revealed an increase in the lattice p a r a m e t e r for iron and steel when absorbing hydrogen. Since the solubility of hydrogen in solid steel is extremely small, when the specimens are soaked after hydrogen absorption the concentration of dissolved atomic hydrogen is continually reduced through desorption into the surrounding space and into the internal pores always present in actual metals. The atomic hydrogen generated in the p o r e s reeombined into molecular hydrogen creating very high p r e s s u r e s under certain circumstances. When hydrogen-absorbed specimens are allowed to age, the concentration of hydrogen dissolved in the crystal lattice is constantly reduced through desorption into the surroundlag atmosphere and partial t r a n s f e r to the pores. The hydrogen concentration in the pores immediately after hydrogen absorption, in accordance with the equilibrium conditions, is at f i r s t higher, but then declines when the solid solution has been depleted. The qualitative nature of the redistribution of hydrogen in steel is shown in Fig. 3. Comparison of Figs. 2 and 3 shows that a reduction in the reduction of a r e a of a metal when aged (during dynamic tension) may be explained by r e distribution of hydrogen due to an i n c r e a s e in its concentration in the p o r e s . A constant increase in the reduction of a r e a during slow tension may be put down to removal of the atomic hydrogen from the crystal lattice. In this way, molecular hydrogen entering the pores before the mechanical t e s t s are begun causes the i r r e v e r s i b l e brittleness. The reversible brittleness only occurs when there is atomic hydrogen present capable of diffusing toward the p o r e s during strain. This fact can be used to explain certain features in the diagram in Fig. 2. F o r example, a day after hydrogen absorption, the ductility of the specimens almost ceases to

Time of hydrogen Agingtime after hydrogen absorption absorption

Fig. 3. Variation in hydrogen content during hydrogen absorption and subsequent desorption from specimen at 20 ~ and atmospheric p r e s s u r e (diagramatic): 1 -- in crystal lattice; 2 -- in pores Thus, three months after hydrogen absorption specimens tested in dynamic tension showed a reduction of area of 50%, i . e . , considerably l e s s than without hydrogen (see Fig. 2). A r i s e in the dynamic ductility curve (Fig. 2) during the f i r s t few hours after hydrogen absorption is explained by the fact that the reversible brittleness is not completely supp r e s s e d at very high hydrogen concentration and the impact tension r a t e s used, but nevertheless gradually declines as the hydrogen concentration in the crystal lattice decreases. Figure 4 shows the dependence of the ductility of industrial iron on the aging time after hydrogen absorption on the basis of impact tension test data Curve 1); in this case very little i r r e v e r s i b l e brittleness occurs. W

Affln~tzrlae

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h ydro~eri after'hydrogen absorptfon absorption Fig. 4. Variation in ductility of industrial iron under tension at rate of 4 m / s e e as function of time specimens are kept after hydrogen absorption (each point r e p r e s e n t s the mean result of five tests): 1 -- hydrogen absorption of specimen without work hardening; 2 -hydrogen absorption of specimen after work hardening by 8 - 10% stretching

137

As a result of 8 - 10% plastic deformation of the specimens by tension just before hydrogen absorption, t h e n u m b e r of p o r e s is increased and the brittleness after the absorption is increased (Curve 2). This confirms the fact that the i r r e versible brittleness is due to the hydrogen p r e s e n t in the pores.

The s t r e s s e d state created by the p r e s s u r e promotes plastic deformation in the surface layer of the pore. According to the s y s t e m shown, there must be an octahedral tangential s t r e s s in an a r e a equally inclined toward all three axes in the section BB 1 equal to the following

Let us consider the s t r e s s e d state of the metal in the surface layer of a pore subjected to internal p r e s s u r e by hydrogen. The surface layer of the pore experiences comp r e s s i v e s t r e s s in a radial direction, and half as much tensile s t r e s s in a tangential direction (Fig. 5). A P !

P/2

p, ~

E

P/2

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B,

The addition of the tangential s t r e s s due to an external s t r e s s , the tangential s t r e s s due to the p r e s s u r e of the :hydrogen creates a tangential s t r e s s concentration over the octahedral a r e a when the specimen is stretched. This brings about p r e m a t u r e plastic deformation in the elementary volume located on the surface of the pore in the section BB t. In the elementary volume situated on the surface of the pore in the section AA1 there is increased plastin deformation resistance because of the hydrogen. The action of the s t r e s s e d state in the Sections AA1 and BB 1 must lead to an

Fig. 5. Diagram showing s t r e s s e d state in surface layer of pore subjected to internal p r e s s u r e by hydrogen It follows f r o m this diagram that the normal tensile s t r e s s due to an internal force (when stretching the specimen in the direction AA1) is added to the n o r m a l tensile s t r e s s in the surface layer of the pore occurring in the section BB1 through internal hydrogen p r e s s u r e . This leads to the p r e m a t u r e formation of a crack, since the given s t r e s s concentration, just as the internal hydrogen p r e s s u r e , is retained throughout the plastic deformation of the specimen. Cases are known in which cracks occur along the grain boundaries solely through the p r e s s u r e of the hydrogen in the pores and without the application of muy external force.

RELAXATION

increase in the nonuniformity of the plastic deformation and localization of it, which also helps to cause p r e m a t u r e cracks. The total resistance to plastic deformation in the specimen due to the effect of the s t r e s s e d state (at medium hydrogen concentrations) virtually remains unchanged. This conclusion is born out by experimental data~ The embrittling effect of hydrogen is intensified in cases in which the pores are not spherical in shape, but like cracks. The formation of crack-like pores should be expected during the plastic deformation of metals. On account of this, the reversible hydrogen embrittlement occurring during slow deformation of the steel {at low hydrogen concentrations) is m o r e strongly manifested than the i r r e v e r s i b l e brittleness during dynamic tension (see Fig. 2).

STRENGTH OF E1612 A N D 2 0 K h I M I F I D U R I N G T E N S I O N A N D FAILURE

STEELS

Cand. Tech. Sci. L. YA. LIBERMAN (Central Polzunov Boiler-Turbine Research Institute) Fastening parts used for prolonged periods at high t e m p e r a t u r e s fail on account of s t r e s s concentration predominately in the f i r s t t u r n of the thread. The notch sensitivity depends on the ductility of the steel.

The specimens were stretched until they failed, which occurred during the relaxation or creep. This method makes it possible to ascertain the features of the relaxstinn in the specimen just before it fails.

We studied the relaxation of s t r e s s e s in cylindrical specimens 12 m m in diameter in 155 m m in gage length when stretched to the point of failure.

2) cyclic relaxation until failure by repeated loading every 100 - 120 hours up to the initial s t r e s s ;

Figure 1 shows data on the relaxation of s t r e s s e s in EI612 steel at initial s t r e s s e s of 25 and 30.5 k ~ / m m 2. A gradual decline in s t r e s s is observed every 1000 hours when the initial s t r e s s is 30.5 kg/mm2. In both cases repeated loading during t e s t s lasting 300 - 900 hours brought about no great change, compared with the f i r s t loading. It may be a s s u m e d that the decline in s t r e s s e s in the second rels~ation period during the second loading is slower, and that the residual s t r e s s e s are higher than the corresponding vaiues during the first loading.

3) alternating relaxation cycles for different periods and creep until a p r e s e t degree of plastic deformation was attained.

In o r d e r to study the effect of multipte loading we carried out cyclic relaxation t e s t s .

EI612 and 20KhlM1F1 steels were tested at: 1) a set t e m p e r a t u r e and initial s t r e s s for 700 - 1000 hours, after which they were loaded successively until the initial s t r e s s was reached;