Effect of nitrogen on internal friction of hydrogen charged stable austenitic stainless steels

Scripta METALLURGICA et MATERIALIA

Vol. 29, pp. 177-182, 1993 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

E F F E C T OF NITROGEN ON INTERNAL FRICTION OF HYDROGEN CHARGED STABLE AUSTENITIC STAINLESS STEELS

V.G. Gavriljuk 1, H. Hinninen 2, A.V. Tarasenko I and K. Ullakko 2

1 Institute of Metal Physics, Academy of Sciences of Ukraine, Kiev, Ukraine 2 Helsinki University of Technology, Laboratory of Engineering Materials, Espoo, Finland (Received August 6, 1992) (Revised April 27, 1993)

1. Introduction Hydrogen charging of iron-based austenites causes relaxation phenomena at temperatures below RT. Internal friction peaks were observed in stainless steels [1-4], Fe-Ni and Fe-Ni-Mn alloys [5-7]. A single peak located at about 300 K for frequencies of about 500 Hz occurs in austenitic stainless steels according to [3,8,9]. Cold work after hydrogen charging causes two additional peaks at 230 and 330 K in the same range of frequencies [9]. The latter peak was also found at 270-280 K for frequency of about 1 Hz [4]. In studies of Fe-Ni invar alloys a single peak was observed at temperatures 230-250 K at the frequency range of 200-1600 Hz [5,7]. There is a strong controversy concerning the nature of above mentioned peaks. The main peak was attributed by Asano [3,7] to reorientation of H-H pairs, while Zielinski [4,10] attributed this peak to dislocations and hydrides induced by cathodic charging. The high temperature satellite was interpreted to be the hydrogen cold work peak [9] or to be associated with the surface cracks [4], while the low temperature satellite was attributed to hydrogen atoms bound by tight traps [9]. As it was shown in [11], the hypothesis of H-H pairs is not consistent with the linear dependence of the area under the main peak on hydrogen concentration, which occurs for experimental data obtained with different charging techniques (cathodic and gaseous). The aim of the experiments described below was to obtain data assisting understanding of mechanisms of the hydrogen caused relaxation in austenitic stainless steels. The effect of composition was studied in order to examine the possible contributions of interaction between solute atoms and hydrogen into relaxation.

2. Experimental Steels were produced by argon-arc melting followed by plasma-arc remelting into ingots of about 50 kg and nitrogen was introduced during remelting (see Table I). After hot forging and rolling the sheets were mechanically ground to about 1 mm of thickness, solution treated for 30 min in a protecting atmosphere at 1200 oC for CrNi and 1150 oC for CrNiMn steels and water quenched. Specimens of dimensions 40 x 1.0 x 0.5 mm 3 were cut out from the sheets and then etched in order to remove the surface layer. A platinum anode and 1 N H2SO 4 solution containing 250 mg/1 NaAsO 2 were used for cathodic charging at the current density of 50 mA/cm 2. After termination of charging samples were stored in liquid nitrogen. Installation of the samples in the pendulum and cooling to 80 K took less than 120 s. The internal friction spectra were measured using a torsional pendulum at a frequency of about 1 Hz in the temperature range of 80-450 K with the heating rate of 1.5 K/min. The equipment is described in detail elsewhere [12]. For fitting of spectra the Pearson line shape was used

177 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.

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A = Ao/[4.106( 1/T- 1/Tmax)2/oto2 + 1]ct where: Ao - height of the peak; Tma x - temperature of the intensity maximum, K; ct - the Pearson parameter; 0 - the half-width of the peak. This distribution provided more precise fitting of the experimental data as compared to the Iognormal distribution [13]. TABLE I Composition of steels, mass %

Steel

C

Cr

Ni

Mn

Si

S

P

N

CrlSNil 5(1) Cr18Ni15(2) Cr18Ni16Mn10(1) CrlSNi16Mn10(2) CrlSNil6MnlO(3)

0.03

18.23

15.10

0.33

0.27

0.015

0.019

0.07

18.48

16.13

9.64

0.45

0.004

0.008

0.08 0.21 0.07 0.22 0.56

3. Results Figure 1 shows the internal friction spectra and square of frequency, which is proportional to the shear modulus, for Crl8Nil5(1) and (2) steels after hydrogen charging. In contrast to the previous studies [2-4,8-10] the spectrum consists of 5 peaks at temperatures of 195, 220, 295, 330 and 390 K (at a frequency of 0.4 Hz), which is evidence of higher resolution of the equipment used in this study. For convenience we denote them as HI...H 5 in order of increasing temperature. The peaks H i at 195 K and H 2 at 220 K obviously correspond to the main peak discussed in the above mentioned studies. The marked decrease of the shear modulus accompanies them and also the peak H 3 (Fig. lb). Three peaks H3, H4, H 5 have not been observed before in annealed stainless steels exept in [11]. Addition of nitrogen to CrNi austenite markedly decreased the intensity of all peaks (see Fig. 1, curve 2). Alloying of CrNi steels with manganese also results in a decrease of damping caused by hydrogen (see Fig. 2). Increase of nitrogen content decreases the intensity of peaks H l , H3...H 5 but does not markedly affect peak H 2. Experiments with changing frequency in the range of 0.2...2 Hz were performed on Crl8Nil6Mnl0 steel, and a frequency shift was observed for peaks H1, H 2 and Hy The temperature positions of peaks H 4 and H 5 remained unchanged. Outgassing of hydrogen from the samples during heating changes intensities and positions of the peaks (see Fig. 3). Internal friction spectra were run after successive heatings of the same sample to different temperatures followed by cooling to 80 K The first heating to 200 K did not affect the spectrum, which is clearly observed during the 2 nd run. The second heating to 200 K resulted in an increase of the intensities of peaks H 1 and H 2 and shifted them to higher temperatures (3 rd run). After the third heating to 300 K the temperature shift of peaks H 1 and H 2 becomes more distinct with a slight decrease of their intensities while peak H 4 exhibits its common intensity and position (4 th run). The fourth heating to 350 K led to disappearance of peaks H 1 and H4, a significant decrease of peak H 2 and slight decrease of peak H 5 (5 th run). 4. Discussion The complicated internal friction spectra and their evolution due to outgassing of hydrogen give evidence of a variety of hydrogen states in cathodically charged austenitic steels. The available data is not sufficient for precise identification of operating mechanisms and we shall discuss only general features of the possible contributions of hydrogen into internal friction on the basis of the results obtained.

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N IN S T A I N L E S S

STEELS

179

It is well known that cathodic charging of stable austenitic stainless steels causes formation of two phases: eH and YI-I (see, for example, [14-16]). The ell-phase is considered as a solid solution of hydrogen in e-martensite, while the nature of YH is a controversial issue. According to Narita et al. [15] spinodal decomposition occurs in cathodically charged austenitie steels. In addition to the X-ray reflections of hydrogen enriched primary austenite, the authors in [ 15] distinguish reflections of the YI-I-phase with the lattice parameter of about 5.2 % (not less than 4.6 %) larger than that of initial `/-phase. In contrast, Mathias et al. [16] observed a continuous spectrum of dilatation of the y-crystal lattice produced by dissolved hydrogen and concluded, that %q-Iis expanded `/. Kamachi [14] distinguished reflections of two hydrogen-induced phases as a result of fitting, although reflections in [14] really represent a continuous spectrum in the angle range of Yrl. The X-ray study of Crl8Nil6Mnl0 steels alloyed with nitrogen was performed in [17] after cathodic charging at the current density of 50 mA/cm2, which is much less severe as compared to 500 mA/cm2 used in [15]. The same crystal lattice dilatation of about 5.2 % was observed without any distinct division between ~ and hydrogen enriched `/. Thus, we can conclude that hydrogen in charged stable austenitic stainless steel is distributed between at least two solid solutions of e- and "/-phases. On the basis of the above discussion it would be natural to attribute peaks H 1 and H 2 to the relaxation phenomena caused by hydrogen in e- and ~/-crystal lattices, respectively. Such a suggestion is supported by a fact revealed in [17], that nitrogen prevents the ell-phase formation in the Crl8Nil6Mnl0 steel. As shown in Fig. 2, increase of nitrogen content significantly decreases the height of peak H1 in this steel. Nitrogen does not change the H 2 peak in the Cr18Ni16Mn10 steel and decreases all the peaks in the CrlSNil5 steel (Figs. 1 and 2). For understanding the results obtained it is necessary to study effects of nitrogen on absorption of hydrogen and its distribution in cathodically charged CrNi and CrNiMn stable austenites. As was mentioned above, peaks H 4 and H 5 are not affected by a change of frequency, i.e. they have a hysteretic nature and are not associated with any relaxation process. Probably they are caused by outgassing of hydrogen from the samples. If this suggestion is valid, peaks H 4 and H 5 can be interpreted in terms of two main kinds of hydrogen state in solid solution characterized by different binding energies and different stability against heating, respectively. Nitrogen alloying decreases intensity of both peaks, which can be explained by trapping of hydrogen atoms with nitrogen. Peak H 3 has more complicated nature. It is accompanied with a decrease of the shear modulus, which is the sign for a relaxation process. Successive measurements of internal friction after preceding heatings of the same hydrogen charged specimen to different temperatures revealed high stability of peaks H l and H 2 up to 300 K. Some redistribution of hydrogen occurs at temperatures between 200 and 300 K, increasing the binding of hydrogen atoms in the lattice, which results in a shift of relaxation peaks H 1 and H 2 to higher temperatures (see Fig. 3). Heating to 350 K leads to almost complete disappearance of peak H4, leaving a small unsplit peak instead of H l and H 2 and the whole peak, H 5. Thus, peak H 5 is really associated with hydrogen atoms bound more tightly in solid solution as compared to H 4. The experimental data does not contradict the suggestion that reorientation of atomic pairs involving hydrogen and solute atoms can cause peaks H 1 and H 2. However a discrepancy of two order of magnitude exists between the pre-exponential factor re, measured by Asano [8] and that calculated from the value of the diffusion coefficient D o for hydrogen in austenitic steels (see discussion by Zielinski [18]). This discrepancy is the basis for questioning the proposal that a single elementary jump of the hydrogen atom is the mechanism of relaxation.

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5. Conclusion 1. The internal friction spectrum of the cathodically charged stable austenitic stainless steels consists of 5 peaks for CrNi and CrNiMn steels: H l at 195 K, H2 at 220 K, H 3 at 295 K, H4 at 330 K and H 5 at 390 K (for frequency of 0.4 Hz). 2. Nitrogen alloying suppresses peak H 1, does not affect peak H2 and decreases the intensity of the peaks H4 and H 5. 3. Peaks H l and H 2 are suggested to be associated with relaxation phenomena caused by hydrogen in 8Hand ~,H-Phases, correspondingly. 4. Peaks H4 and H 5 are suggested to be caused by outgassing of hydrogen atoms bound with different energy in solid solution.

The financial support of the Technology Development Centre of Finland is gratefully acknowledged. This study was performed in the framework of Finnish-Ukrainian cooperation organized through the Academy of Finland and State Committee on Science and Technology and Academy of Sciences of Ukraine. The authors express their appreciation to Mr. A. Tereshchenko for assistance in the firing of internal friction spectra.

References 1. J.A. Peterson, R. Gibala and A.R. Trojano, J. Iron and Steel Inst., 207, 86 (1969). 2. S. Asano, M. Goto and R. Otsuka, J. Japan Inst. Metals, 39, 1318 (1975). 3. S. Asano, Scripta Met., 19, 1081 (1985). 4. A. Zielinski and E. Lunarska, J. de Physique, C-10, 46, 131 (1985). 5. S. Asano and H. Seki, Scripta Metali., 18, 177 (1984). 6. Y.Nishino, T. Kato, S. Tamaoka and S. Asano, Scripta Metail., 21, 1235 (1987). 7. S. Asano, Y. Nishino and K. Fu.jiyoshi, Mater. Trans., JIM, 31, 995 (1990). 8. S. Asano, M. Shibata and R. Tsunoda, Scripta Met., 14, 377 (1980). 9. S. Asano and K. Oshima, Trans. Japan Inst. Metals, 23, 530 (1982). 10. A. Zielinski, Scripta Met., 19, 173 (1985). l l. V.G. Gavriljuk, H. H~nninen, Ju.N. Jagodzinski, A.V. Tarasenko, S. Tiihtinen and K. Ullakko, to be published. 12. K. Ullakko, Doctoral Thesis, Report No. 3, Laboratory of Engineering Materials, Helsinki University of Technology, Espoo, Finland, 1992. 13. A.S. Novick and B.S. Berry, Anelastic Relaxation in Crystalline Solids, p. 678, Academic Press, New York and London (1972). 14. K. Kamachi, Trans. ISIJ, 18, 485 0978). 15. N. Narita, C.J. Altstetter and H.K. Bimbaum, Metall. Trans., 13A, 1355 (1982). 16. H.Mathias, Y. Katz and S. Nadir, Metal Science, 12, 129 (1978). 17. V.G. Gavriljuk, H. H~nninen, A.S. Tereshchenko and K. Ullakko, to be published. 18. A. Zielinsld, Acta Metall. Mater., 38, 2573 (1990).

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2

N IN STAINLESS

400

500 30

300

STEELS

Temperature, K 250 200 H2 I

1) (N)--0.08 mass % 2) (N)=0.21 mass %

25

H4

HI I

(a) Crl8Nil5(N) H-50mA/cm2-72h

/~!

o

Hs

15o

~

"" 20 x o15

181

H3

J ~

i'it

~~_/2:/` I

I

lI

\

0 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

.

6.0

6.5

7.0

1000/T Temperature, 500 0.3 0.28

400

300

250

K 200

150 '(b)

Cr18Ni15(N)

0.26 ¢q

N

0.24

¢q

o 0.22

o

0.2

1/

0.18

1) (N)~.O8 mass% 2) (N)~.21 mass%

j 0.16 0.14 2.0

' . . . . . . . 4.5 5.0 5.5 6.0 6.5 7.0 1000/T FIG. 1. The internal friction (a) and square of frequency (b) of the hydrogen charged steels Crl8Nil5(1)

and (2).

. 2.5

.

3.0

.

. 3.5

.

4.0

182

N IN S T A I N L E S S

500 5

400

300

STEELS

Temperature, K 250

200

1) (N)=0.07 mass % 4 2) (N)=0.22 mass % ~~.~o~3 3) (N)=0.56~5 a4H3mass%

2

3

Vol.

29, No.

150 Crl8Nil6MnI0(N) .....~-~nmA/cm -.,v 2-79h

~'

4

5

6

7

1000/T

FIG. 2. Internal frictionin the hydrogenchargedCrl8Nil6Mnl0 steelalloyedwith differentcontentsof nitrogen. Fittingis performedfor the steelcontaining0.07 % of N. 400 300 7 Crl8Nil6Mnl0(1) 6 H-50mA/cm2"72h

Temperature, K 250 '

200 ~run

170

o N3

~2

0

~4'hrun s',," \"-.._

/ /I

t

1

i

i

3

4

5

l ~ \ \ \\.,

6

1000/T

FIG. 3. Influenceof outgassingduringsuccessivebeatingson internalfrictionof the Cr18Ni16Mn10(1) steel.

2