Effect of hydrogen on the dislocation density distribution in 1090 steel

Scripta

METALLURGICA

V o l . 18, pp. Printed in

539-541, 1984 the U.S.A.

Pergamon P r e s s Ltd. All rights reserved

EFFECT OF HYDROGEN ON THE DISLOCATION DENSITY DISTRIBUTION IN 1090 STEEL I. R. Kramer* and J. P. Hirth+ *Metallurgical Materials Laboratory, University of Maryland, College Park, Maryland 20742 USA +Metallurgical Engineering Department, The Ohio State University, Columbus, Ohio 43210 USA

(Received

February

16,

1984)

In a series of investigations, the influence of hydrogen on the plastic flow behavior of a spheroidized 1090 steel was studied from the viewpoint of determining strain localization and its influence on the ductility [1-4]. From these investigations it was concluded that in plane strain tensile and notch-bend tests, hydrogen promotes strain localization and decreases the critical strain for the onset of surface shear instability ultimately leading to mixed-mode fracture along shear bands. The cracks always were initiated at free surfaces rather than in the interior where both the normal stress and the triaxial stress are a maximum. In the present investigation the dislocation density-depth profile was determined on some of the same tensile specimens used in Ref. [4]. In brief, the specimens were held at 750°C for 2 h, furnace cooled and then spheroidized for 20 h at 704°C. The average grain size was 15 ~m and the carbide size ranged from 0.5 to 4 ~m with an average of 1.2 ~m. The specimens were 22 mm wide with a reduced cross-section 6.2 mm. The gage section was metallographically polished to a 2 final finish with 1 ~m diamond paste. The specimens were cathodically charged for 2 h at 80 A/m in IN H2SO 4 with 1 g/l of thiourea, conditions which do not produce mechanical damage in the absence of external stress [ 5 ] . Further details may be found in Refs. [1-4]. The X-ray data, using a cobalt target with an Fe203 filter, were obtained in a conventional X-ray apparatus equipped with a position sensitive detector with resolution of 1 channel equal to 0.005°(@). X-ray studies were performed on the middle of the wide face of the specimens. The X-ray line profiles of the (ii0) and (220) lines were analyzed by the Warren-Averbach method [6] for the root-mean-square strain <£>. Instrumental broadening corrections were made according to the recommendations of Stokes [7] and the K~I - K~9 separation was carried out by the Rachinger method. In order to obtain the dislocat~on-de~th profile the specimens were electrolytically polished in an acetic-perchloric acid solution to remove incremental amounts of metal. To prevent heating the metal was removed at a rate of 50 ~m per h at 270OK. Results The Warren-Averbach analysis gave values for for a value of their length parameter L = 5.0nm.The values of were converted to dislocation density p by the procedure of Williamson and Smallman [8]. p = 12 2/b2F

(i)

The calculations were made by assuming that the dislocation distribution parameter F = i, that is an essentially random, uniform distribution of dislocations. Other distributions would change the constant in Eq. (i), but all distributions would retain the proportionality of P and <£>2. Since in this investigation it is the relative change in D that is of interest, the choice of F is not critical. The validity of the assumption F = 1 may be obtained by a comparison of the ratio P~/PD where PC is calculated from Eq. 1 assuming F = i and PD is the dislocation density calcuIat~d from th~ particle size, D:

1

(2)

PD = D-2-

539 0036-9748/84 $ 3 . 0 0 + .00 Copyright (c) 1 9 8 4 P e r g a m o n Press

Ltd.

540

HYDROGEN

& DISLOCATION

DENSITY

DISTRIBUTION

IN

STEEL

Vol.

The values of P£ and p_ for the unchanged and hydrogen charged specimens strained amounts as well as Pe ~nd Pn values obtained from measurements at the surface and are presented in Fig. i. Since all of the data points fall on the 45 ° lin~, PE = assumption F = 1 is valid. Further since the curve is linear it is appare t that

18,

No.

5

various the interior p- and the t ~ dislocation

Values of P for several cases are presented in Fig. 2. The data for specimens strained 3 and 18 percent after being charged with hydrogen (3H, 18H) and for uncharged specimens strained the comparable amounts of 3 and 21 percent (3A, 21A). The data show that the dislocation density is greater in hydrogen-charged specimens than in uncharged specimens. The factor of increase in the bulk is about a factor of two, with a somewhat smaller factor near the surface. There is also a general trend for all specimens for an increase in p at the surface. Discussion Prior work [1-4] on similar material showed that hydrogen enhanced plastic instability as manifested by surface rumpling and by bulk shear bands emanating from the surface and leading to void sheets and ultimately fracture along the trace of the shear bands. The effect of hydrogen was to lower the critical strain for the onset of these instability related phenomena by a factor of about two. The present results are consistent with the earlier findings in that the local strain events near the surface have progressed further in hydrogen charged specimens at about the same nominal overall strain. This would be consonant with more localized shear in the hydrogencharged specimens, leading to enhanced multiple-slip induced by incompatibilities where the shear bands are blocked by carbide particles. Of considerable interest is the result that hydrogen effects appear at a strain of 3 percent, well below the lowest critical strain, 13 percent, for the instability events studied earlier. This result provides further support for the hypothesis that hydrogen directly influences the onset of plastic instability at the surface rather than doing so directly by affecting void nucleation and growth in these plane strain tests [1-4]. Of importance in this connection are the recent observations made by Tabata and Birnbaum [9] on specimens strained in situ in a transmission electron microscope showing that hydrogen increases both the velocity and the multiplication rate of dislocations in pure iron. The finding of an increased value of p near the surface is suggestive, the gradient in p could be a factor additional to other microstructural effects in influencing the propagation of shear instability bands from the surface into the interior of the specimen. Alternatively, a change in dislocation structure to a more coplanar, pileup-type arrangement, as has been observed to be the effect of hydrogen on the deformation of pure iron [10], would lead to a larger value of F to accord with the present results. The pileup configurations, equivalent in the continuummechanics analog to mode II crack nuclei, could indirectly enhance the initiation of shear instabilities. Direct observations of the near-surface dislocation structure would be valuable in deciding among these possibilities. Acknowledgements The authors are grateful to S. C. Chang for supplying the strained tensile specimens and for the support of this research by the National Science Foundation under Grant DMR 7815735 (JPH) and by the National Science Foundation under Grant number DMD-81-08422. References !. 2. 3. 4. 5. 6. 7. 8. 9. 10.

T. D. Lee, T. Goldenberg and J. P. Hirth, Met. Trans. AI__~0, 199 (1979). O. A. Onyewuenyi and J. P. Hirth, Met. Trans. AI3, 2209 (1982). O. A. Onyewuenyi and J. P. Hirth, Met. Trans. AI4, 259 (1983). S. C. Chang, Ph.D. Thesis, Ohio State University, Columbus, Ohio, 1982. S. X. Xie and J. P. Hirth, Corrosion, 38, 486 (1982). B. E. Warren and B. L. Averbach, J. Appl. Phy., 23, 497 (1952). A. R. Stokes, Proc. Phy. Soc., 25, 254 (1948). G. K. Williamson and R. E. Smallman, Phil. Mag., i, 34 (1956). T. Tabata and H. K. Birnbaum, Scripta Met., 17, 947 (1983). H. Matsui, A. Kimura and H. Kimura, in Strength of Metals and Alloys, P. Haasem, V. Gerold and G. Kostorz, eds., Vol. 2, Pergamon, Oxford, 1979, p. 77.

Vol.

18, No.

5

H Y D R O G E N & D I S L O C A T I O N DENSITY D I S T R I B U T I O N

S I D I A •

H2 AIR

4-

04

'E

3-

U

/

_o o

o/

p, ,2 <,>2 =

Fb2 PD" "-~'2

2

I

I I

I 2

I 3

I 4

Pe (10il cm-2) Fig.

1

C o r r e l a t i o n of D i s l o c a t i o n Density C a l c u l a t e d from M i c r o s t r a i n and Particle Size D.

3.0

I

oE 2.5 O

0 18H 2.0 i

N

1.5

a

~

i 1.0~" 0.5

0.0

:.

21A

~ 3H

-

T 3A

I

I

I

I

I

I

20 40 60 80 100 120 140 160 DISTANCE FROM SURFACE, p.m

Fig.

2

Influence of H y d r o g e n C h a r g i n g on the D i s l o c t i o n D e n s i t y - D e p t h Dist r i b u t i o n of S t r a i n e d Specimens of 1090 Steel.

IN STEEL

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