Effect of hydrogen on the Young's modulus of commercial purity iron

Scripta METALLURGICA

Vol. 207 pp. 503~507, 1986 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

EFFECT OF HYDROGEN O~ THE YOUNG'S MODULUS OF COMMERCIAL PURITY IRON Tong-Yi Zhang, Fang-Xing Jiang,* Wu-Yang Chu, Chi-Mei Hsiao. Dept. of Metal Physics, *Dept. of Physics Beijing University of Iron and Steel Technology, CHINA

(Received October 21, 1985) (Revised January 23, 1986) Our analysis (I) showed that the s~lution of the flexural vibration equation of a beam under stress was E=C(~)~ m and that if the resonance frequencies of the first and the third tone were measured at about the same time, the internal stress ~ and the Young's modulus E could be calculated. The change of the apparent Young's modulus after charging is defined as &Eao.=AEI(H)+AE 2, where only AEI(H) relates to the change of the interatomic cohesive force i.e. aEI(H) is the intrinsic modulus change caused by hydrogen, aE2 is induced by the changes of defect number and/or structure, i.e. defect relaxation, which can produce the change of elastic strain as well as internal stress for a substained displacement specimen and has nothing to do with hydrogen. The result on high-purity iron showed that during either aging after charging or artificial partial stress relaxation, the changes of the apparent Young's modulus were the same, i.e. AEo~ =LSE~(1). Thus, the ~ E I ( H ) associated with the interatomic cohesive f o r c ~ o e s ~ n o t evidently change during aging with escaped hydrogen of 7 to 8 wppm from the specimen at room temperature. In the present work, the effect of hydrogen on the Young's modulus of commerica~ purity iron was investigated using the same experimental method described in Ref.(1). The commerical purity iron used had a composition in weight percent of O.02Cr, O.02Mn, 0.01S, 0.008Si, O.O03Cu, O.O02Ni, 0.001~o, 0.001Ge, 0.035C. It was shown in Ref.(1) that the dimension of the specimen which was fixed on a special loading system might change after charging or artificial partial stress relaxation, resulting in a great error. Therefore, in the present work, the specimen, i.e., the vibrating part, and the loading system was made as a whole, as shown in Figure I. The dimensions of the vibrating part were 0.37 mm thick by 5 mm wide and 45.6 mm long. Hydrogen was introduced by cathodic charging at room temperature in IN H2SO4+250mg/1 As203 with i = 100 mA/cm 2 for 2 hrs. Result and Discussion The periods of the clamped-clamped specimens were measured immediately after charging with hydrogen. It was evident that the period of the first tone T I after charging was much higher than that before charging, i.e. the frequency decreased after charging. During aging at constant room temperature after charging, the hydrogen was evolving continuously from the specimens and the period of the first tone was gradually decreasing, as shown in Fig.2. The TI/T 3 ratio after charging was larger than that before charging and was decreasing gradually during aging, as shown in Fig.2. The change of TI/T 3 indicated that the elastic strain ~id not remain constant, i.e. defect relaxation

503 0036-9748/86 $3.00 + .00 Copyright (c) 1986 Pergamon Press Ltd.

504

}IYDROGEN IN IRON

Vol. 20, No. 4

occurred, during aging after charging. The relative changes of the apparent Young's modulus E and elastic strain during aging after charging were calculated and listed in Table I. Where 8, E and ml stand for the elastic strain, the apparent Young's modulus and the resonance frequency of the first tone measured immediately after charging, respectively, 8~, E ~ and m1~ for corresponding values measured after charging and then aging at room temperature for 72 hrs. Table I.

The relative changes of E and ~ during aging after charging, (percent).

No.

3

4

5

6 9.4

(S~ - 8)/8

14.1

18.9

2.2

(E~-

0.21

0.02

0.13

3.83

4.57

0.92

E)/E

( mloo-ml )/m I

7

average

20.1

12.9

0.20

-0.02

0.11

2.59

5.61

3.50

Table I indicated that during aging the relative change of the resonance frequency was 0.92% to 5.61% and the average value for 5 specimens was 3.50%. If E=A~ L and A were a constant, the increase of frequency by 3.5% would make the Young's modulus increase by 7%. But Table I shows that in average the Young's modulus increased only by 0.11%. This shows that the change of the frequency is induced by the change of elastic strain i.e. the defect relaxation. Table I shows that the greater the increase in frequency, the greater is the increase in elastic strain. However, the increase of apparent Young's modulus during aging after charging was still by 0.11% in average. In order to clarify whether the increase of apparent Young's modulus during aging is caused by the change of the interatomic cohesive force or by the defect relaxation with which the elastic strain change is accompanied, an artificial partial stress relaxation test was completed. The result of simulated test indicated that the period of the first tone and the TI/T 3 ratio evidently increased after partial stress relaxation. During aging both of the period T I and TI/T5 were gradually restored and final reached a steady state, as shown in Fig.3. A comparison Fig.2 with Fig.3 indicates that the changes of T I and TI/T 3 produced by charging with hydrogen were completely similar to those produced by artificial partial stress relaxation (for details see Ref. 1). The relative changes of Young's modulus and elastic strain during aging after artificial partial stress relaxation are listed in Table 2, where 8, E and ml stand for values measured immediately after stress relaxation and ~ , E ~ and m1~ for steady state values after 72 hours aging at room temperature. Table 2.

No.

The relative changes of E and e during aging after stress relaxation, (percent). I

2

average

C~- s)/~.

2.1

2.7

2.4

(E~-

0.08

0.10

0.09

0.72

0.93

0.83

E)/E

(w Ioo-mI )/ml

Vol. 20, No,

4

tiYDROGEN IN IRON

S05

Table 2 shows that the resonance frequency of the first tone and elastic strain increased by 0.83% and 2.4% in average over 2 specimens, respectively, during aging after partial stress relaxation and the changes were less than those after charging. This occurs because the amount of stress relaxation is less in artificial partial stress relaxation than in charging. However, the experiment shows that for both the charged and the stress relaxation specimen the apparent Young's modulus was gradually increasing and reached a steady state value during aging. In particular the apparent Young's modulus change for the charged specimen has almost the same average value as that for partial stress relaxed specimen, i.e. AEan.=AEI(H)+~E 2 is nearly equal to &E2, comparing Table I with Table 2. Dislocations and other defects, i.e. hydrogen damage,were introduced by t1~ severe charging, which was observed by metallographic examination. Hydrogen damage could relax the internal stress. Besides, the changes of defect number and/or structure caused by charging, even during charging with low fugacity, could also make the internal stress relax , which was found in the previous work (I). Thus the apparent modulus change after charging or during aging after charging may be due to hydrogen or/and the change of defects. The apparent modulus change resulting from the change of defects, i.e. hE2 could be simulated and measured as we did in Ref. 1 as well as in the present work. Since the dimension of the specimen may be changed during the artificial partial stress relaxation, which could introduce a large error in E/E*, we put attention on aging after charging and after artificial stress relaxation. The ~ E 2 during aging after artificial stress relaxation was near identical with that during aging after charging, therefore, the effect of hydrogen on the intrinsic Young's modulus, i.e. AEI(H) could be investigated through charging and artificial stress relaxation tests. Since AEap.=&EI(H)+~E2=&E 2 (within the accuracy of experiment, see Ref. I), it is very evident that ~EI(H)=O during aging with the evolution of hydrogen from the charged specimen. That is to say, the increase in the apparent Young's modulus is a result not of the evolution of hydrogen, but of the restoration of partial stress relaxation. Since only ~ E I ( H ) has relation with the change of the perfect crystal interatomic cohesive force, ~ E 2 is caused by the change of defects which make the elastic strain change and there is no effect of evolving hydrogen, even after severe charging, on the Young's modulus association with the interatomic cohesive force, i.e., ~ E I ( H ) = 0 : the hydrogen atoms evidently do not decrease the interatomic cohesive force of commercial purity iron. Thus, the result of the present work for commercial purity iron is identical with that for high-pure iron (I). Recently, we investigated the effect of hydrogen on the Young's modulus of pure nickel (2). It was found that the Young's modulus of pure nickel decreased by 2.87% during aging after charging. This is caused by the resolution of the unstable hydride produced during charging. This means that hydrogen atoms increase the Young's modulus by forming a hydride. Buck et al. (3) measured the changes of the full set of second and third order anisotropic elastic constants of Nb, induced by hydrogen. There is an intrinsic and a volumetric component of the total modulus change. The intrinsic component is increased by hydrogen but the volumetric one is decreased (3). Summar~ I. The apparent Young's modulus and elastic strain of commercial purity iron increased by 0.11% and 12.9%, respectively, during aging after severe charging. 2. The increase of apparent Young's modulus of commercial purity iron during aging after charging was nearly equal to that after artificial partial stress relaxation. That is to say, hydrogen does not significantly change the Young's modulus associated with the interatomic cohesive force of commercial purity iron, even during aging after severe charging.

506

HYDROGEN

IN IRON

Vol.

20, No. 4

~eferences I. T.Y.Zhang., p. 1655

F.X.Jiang,

W.Y.Chu and C.M.Hsiao;

2. T.Y.Zhang, F.X.Jiang, W.Y.Chu and C.M.Hsiao; young's Modulus of Nickel, to be published. 3. O.Buck, L.A.Ahlbeag, L.J.Graham, G.A.Alers, Stat. Sol. (a), 1979, vol.55, P.223.

Met. Trans. A, !985, vol.15A, Effect of Hydrogen on the C.A.Went and K.C.Hsien:

I

1.

2

2. solidifying part

vibrating part 1(a). specimen

1

2

3

I. specimen 2. electrode 3. tighting screw 4. loading nut I (b). gripper Fig. I. Specimrn and gripper

4

Phys.

Vol.

20,

No. 4

~(~)

hYDROGEN IN IRON

507

TI:49~.91 ~

'

1- 3

1 4.200

T1/T3 3"3612

,ooo

V\\_

I

:4.120

880.0

0

10

20

30

40

50

60

70

time,(hr.) Pig. 2. The variations of T I and T1/T 3 with time during aging after charging in No 3 specimen. 71o.o I

3.920

TI (~s) ~

before relaxation

~I =488.03~B

TI/T3

T I/T3=3.4339

7o5.o

3.910

L O

,

700.0 o

I 10

,

J

,

20

i 30

,

t~, Fig. 3. The variations of T I ~

! 40

,

, I 5o

,

' 60

,

I

3.900

?o

(h~.)

TI/T 3 with time during aging after artificial

partial stress relaxation in No I specimen.