The effect of hydrogen on microhardness of spheroidized AISI 1090 steel

Scripta METALLURGICA

Vol. 15, pp. 113-118, 1981 Printed in the U.S.A.

THE EFFECT OF HYDROGEN ON MICROHARDNESS SPHEROIDIZED AISI 1090 STEEL

Pergamon Press Ltd. All rights reserved

OF

Oliver A. Onyewuenyi and J. P. Hirth Metallurgical Engineering Department The Ohio State University, Columbus OH 43210 (Received November

i0, 1980)

INTRODUCTION Dissolved hydrogen (H) degrades the mechanical properties of iron and steel as reviewed elsewhere. TI,2] While there is good agreement on H~enhanced loss of toughness, the effect of H on plastic flow remains unresolve~ in view of conflicting experimental results. Studies indicate that H increases the flow stress [3-6] and decreases the extent of plastic deformation [3] in steel. It also hardens zone refined iron [47], except that of very high purity [7]. However there is also increasing evidence that H softens the same materials, possibly by inducing localized softening [8] or by promoting inhomogeneous deformation. For example, H assists the plastic flow of ferrite [8-10], lowers the stress-strain curve in mild steel[ll] andincreases theplastic zone size in both spheroidized 1090 steel [9] and in high strength steels [12]. Other manifestations of Hinduced softening include softening of very high purity zone refined iron TT], accelerated creep in 99.95% pure iron [13], changes in dislocation structure [7, 14], and promotion of the onset of plastic instability in spheroidized 1090 steel [ 9 ] . Thus, the basic question of whether an interstitial H atom will harden or soften iron and steel remains a perplexing issue. It has long been suggested that the change in hardness caused by H at slow indentation rates [6] would be relevant to the effect of hydrogen-on plastic flow. Microhardness testing measures the resistance to plastic flow [15, 16] and the presence of inhomogeneities in strain distribution [15]. It is also sensitive to the local plastic behaviour in the surface layers of the test piece [14, iS], where hydrogen effects are known to be greatest [9,17,18]. Furthermore, the test can be designed to simulate creep at slow strain rates by a careful choice of load and loading periods [6,16]. Among the early attempts to study the change of hardness in steel caused by H was the work of Vaughn and Morton [6] who first showed that H increased the hardness of mild steel, and interpreted their result as agreeing with an increase in tensile workhardening rate being characteristic of mild steel charged with hydrogen. More recently, Gerasimova et al [19] observed that (i) cathodically charged H lowers the microhardness of steels of equivalent compositions as AISI 1020 and-1050, in both pearlitic and tempered martensitic conditions, (ii) the loss of microhardness was recoverable with room temperature aging, and (iii), the degree of loss of microhardness caused by cathodically charged H correlated with increased susceptibility to hydrogen embrittlement as measured-by time to failure in sustained load tests. However, no attempt was made to extend these observations over a series of charging current densities and charging times, or to examine the case of spheroidized structures containing a ferritic matrix, for which much of the H-induced softening/hardening effects have been observed [7,9,10]. Slezhki~ and Sergeev [20] varied the charging current density for the case of spring steels (0.6C, 0.8-1.1 Mn), and noted an increase in microhardness only near the surface (<15 ~m) of the charged specimens, with no effect on bulk hardness. The results of Slezhkin and Sergeev [20] are particularly interesting 113 0036-9748/81/010113-06502.00/0 Copyright (c) 1981 Pergamon Press Ltd.

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EFFECT OF H ON MICROHARDNESS

since softening attributable other studies [12].

to surface

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layer damage has been observed

in many

1

The present work is a preliminary part of a program designed to study the effects of H on plastic deformation and fracture of spheroidized 1090 steel. The effect of H on the microhardness of this steel was studied under different charging conditions, and with particular emphasis on surface preparation and charging conditions. EXPERIMENTAL PROCEDURE The reliability of microhardness in depicting the true character and intensity of stress/strain configurations in a metal depends on the successful removal of the surface cold worked layer [15,16]. Proper choice of a chemical etchant was therefore considered critical. A modified etchant developed by Kelly and Nutting [21] was used with a composition of: 1 g FeCI 3 + 0.25 g CuCI 2 + 4 ml HCI + i00 ml HoO. Five mls of this solution were diluted with 20 mls of doubly distilled water and warmed to = 35°C. Etching was done by immersion. The 1090 steel stock was spheroidized as described elsewhere [9]. A method described by Glazov et al. [16] was used to obtain the microhardness vs. etching time calibration curve shown in Fig. I. Attainment of the plateau region of this curve assured a successful removal of the surface cold worked layer by chemical etching [16]. From Fig. i, 3-4 minutes was selected for the etching time for this steel. Two adjacent faces of each specimen were fine polished through 0.0S ~m alumina powder, immersion etched for 3.5 minutes and buffed lightly with 0.0S ~m alumina power for 40 to 60 s to remove the etching film. Steel wires were spotwelded on two opposite unpolished faces. The sample was then coated with nonconducting acid resistant paint (microstop) except for one of the unpolished faces. The electrolyte - IN H2SO 4 containing 1 g/l Thiourea poison was flushed with purified N 2 for 3 h. N 2 was purified by passing it through acidified chromium solution in contact with Zn amalgam, and humidified by passing it through a gas washing bottle containing the electrolyte. The purification of N2, and deoxidation of the test solution were found to be required in earlier permeation experiments in which a trace of 02 at = ~.02% in solution was shown to cause a large background reduction current, an an anomalous drift and fluctuation in the permeation current [22]. Hydrogen was cathodically charged by galvanostatic control into the uncoated face of the sample at a desired current density and time. The charged samples were ultrasonically cleaned and microhardness measurements taken on the charged face. However, when surface blistering occurred because of charging at high current densities, microhardness indentations were made on the uncharged polished face, thus avoiding errors associated with any surface damage on the charged face. All microhardness measurements were made with a Knoop diamond indenter using a Shimadzu microhardness tester equipped with variable time controls for varying the period under load. All indentations were made at 100gm load with a long loading period of 3Ss. This combination of low constant load and long loading period was found to be necessary for insuring a consistent trend for any H effects. Such test conditions led to a stable creep tendency and low strKin rate for the test piece [6, 16], allowing enough time under load for H to interact effectively within the process zone of the indentation. The-tested samples were stored in an evacuated dessicator for desired periods of time, after which some samples were lightly buffed with 0.0S ~m alumina powder and retested. Experiments were performed at current densities from 20 to 250 A/m 2. On the basis of ancillary permeability experiments [ ~ ] , these correspond to ~ntrapped lattice concentrations of hydrogen of 8.85XI0 ~v to 3.$4XI0 ~ atomH-m -~. RESULTS Fig. 2 shows the combined effects of charging current density and charging time on microhardness, while Fig. 3 elucidates the dotted region of Fig. 2 by showing the effect of charging current on microhardness for 2 hrs of charging time. Each data point represents the average for two samples. The average reading for each sample was taken from IS selected readings. The readings were selected on

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EFFECT OF H ON MICROHARDNESS

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the basis that the indentations correspond approximately to an "ideal" indentation (a pyramid indenter shape reproduced almost exactly in the test piece) and with no more than three carbide particles intersecting the major diagonal of the indentation when viewed at 100X. Figs. 2 and 3 indicate a reduction in microhardness as a result of cathodically charged H in agreement with the result of Gerasimova et al. [19]. The amount of microhardness relaxation increased more rapidly at longer charging times for a given current density. Also, increasing the charging current tended to induce more softening than increasing the charging time. For specimens charged at 30 A/m 2 and i00 A/m 2 for periods of 21ess than 2.5 h no significant softening occurred. Actually, for the 30 A/m case, some slight hardening is indicated as shown more clearly in Fig. 3. However, the scatter for such light charging conditions makes the establishment of such h ~ d ~ g effects questionable. Samples charged at 150 A/m for 20 h showed degree of surface blistering as well as void formation at the interfaces of large carbides and manganese sulfide~ within the matrix. No such defects were observed for the 30 A/m and i00 A/m ~ cases at all charging periods up to i0 h Fig. 3 further indicates that for 2 hours of charging, a minimum current density of 12 A/m is required for any observable H effects. While the existence of a critical current density was not observed ~y Gerasimova et al [19], it is quite consistent with most other observations in which H effects were studied over a range of input fugacities [3, 23], particularly in those studies in which traps influence or control the H effects [23]. Table I depicts the~reversibility of microhardness losses caused by H. Samples charged at I00 A/m" for 0.5 to i0 h essentially recovered their loss in microhardness after i00 h of room temperatur~ aging, while those charged for 15 h did not. Samples charged at 150 A/m for more than i0 h showed evidence of permanent damage--voids, fissures, and irrecoverable loss of microhardness. DISCUSSION It would appear that the two regions of behavior in Fig. 3 can be rationalized in terms of trapping and damage. In the short time, shallow-slope stage, no visible damage is observed in the microstructure and the microhardness is recoverable. This reversible softening is interpreted to be associated with localization of the hydrogen to the near surface region because of trapping and enhanced injection of dislocations at the surface, together with enhanced motion of screw dislocations because of the lowering of the kink formation energy as postulated to explain lower flow stresses in iron [2,7], and steel [2,24]. The second stage in Fig. 3 with steeper slope is associated with deeper penetration of hydrogen leading to irreversible damage as reflected by the lack of c~mplete recovery of microhardness with aging. At the current density of 150 A/m , the damage consists of voids and microfissures formed at the interfaces of large ~ carbide particles and sulfide inclusions. Such voids would produce geometric softening as suggested by Oriani and Josephic [3] and as demonstrated qualitatively by Hutchinson [26], who also showed that such voids would tend to promote plastic instability, another softening effect produced by H charging of steels [9]. For the lower charging current densities, no voids are formed, and the damage is instead postulated to be in the form of dislocations emitted into the ferrite matrix from the particle interface [2]. Incidentally, contrary to the hypothesis in [3], this work shows that the charging conditions which produced plastic instability in the work of Lee et al. [9] were moderate in the sense of not producing irreversible damage. The larger effect on softening of increased charging current than of increased charging time can be interpreted in the same manner as the results of load relaxation studies on pearlitic 1045 steel by Oriani and Josephic [3], and as suggested in general as a mechanism for softening by Sethi and Gibala .[25]. That is, the initiation of increased H charging creates an increased surface gradient of H, and hence strain gradient. The resulting surface stresses should enhance dislocation injection, leading to more damage than continued charging into an already saturated specimen.

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EFFECT OF H ON MICROHARDNESS

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Thus the present findings indicate that H causes reversible softening manifested fis microhardness relaxation in spheroidi~ed 1090 steel, in agreement with other observations of H-induced softening in iron [7,10] and steel [9,11,12]. Also, hardening is suggested at very low input fugacities, but with the effect complicated by low initial H concentration and cold work hardening within the surface layers of the specimen. -It is plausible that surface work hardening is responsible for the apparent surface hardening observed by Slezhkin and Sergeev [20] in H-charged spring steels. 1.

CONCLUSIONS Cathodically charged H decreased the microhardness of spheroidized 1090 steel at intermediate-and large input fugacities.

2.

For low and moderate charging conditions the microhardness was recoverable after room temperature aging. Severely charged samples showed irrecoverable loss of microhardness.

3.

The H-induced microhardness relaxation is interpreted to be associated with: (a) Geometric softening caused by voids and fissures at severe charging conditions (high fugacities). (b) Dislocation injection from second phase particles at intermediate Hinput fugacities, and (c) Reversible softening caused by H-enhanced screw dislocation mobility and dislocation injection at surfaces at low fugacities. ACKNOWLEDGMENT

This research was supported by the National Science Foundation under grant DMR 7815735. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. ii. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

REFERENCES J.P. Hirth and H.H. Johnson, Corrosion 32, 3 (1976). J.P. Hirth, Met. Trans. llA, 861 ( 1 9 8 0 ) 7 R.A. Oriani and P.H. Josep~ic, Acta Met. 27, 997 (1979). A.M. Adair, Trans. TMS-AIME 236, 1613 (19b~-6). S. Asano and R. Otsuka, Scripta Met. 10, 1015 (1976) V.G. Vaughan and M.E. DeMorton, Nature, No. 4522, 1225 (1956). H. Matsui, H. Kimura, and S. Moriya, Mater. Sci. Eng. 40, 207 (1979) S. Moriya, H. Matsui and H. Kimura, ibid, p. 227. C.D. Beachem, Met. Trans. 3, 437 (1972). T.D. Lee, T. Goldenberg an~ J.P. Hirth, Fracture 2, 243 (1977), R. Taplin ed., Pergamon, Oxford; Met. Trans. 10A, 439 (1979). I.M. Bernstein, Scripta Met. 8, 343 (1974). N.J. Grant and J.L. Lundsford? Iron Age 175, 92 (1955). W.Y. Chu, C.M. Hsiao and S.Q. Li, Scripta---~et. 13, 1063 (1979). J. Nagakawa, K.S. Shin, C.G. Park and M. Meshii~--2nd Jap. Inst. of Metals Symp. on H in Metals, Tokyo, Nov. (1979). F. Alex and J. G. Byrne, Mat. Res. Bull. 14, 821 (1979). ASM: The Science of Hardness Testing and--Irts Research Applications, ed. J. Westbrook and H. Conrad, Metals Park, Ohio (1973). V.M. Glazov and V.N. Vigderovich, Microhardness of Metals and Semiconductors Translation: Consultant Bureau, New York (1971). T.S. Sudarshan, M.R. Louthan, Jr., and R.P. McNitt, Scripta Met 12, 799(1978) M. Smialowski, Scripta Met 13, 393 (1979). V.V. Gerasimova, L.V. Dranis-~nikova, L.V. Kiseleva and T.D. Shtcherbakova, Korroz, Zashch. 4, 6 (1975). V.A. Slezhkin an~ M.I. Sergeev, Korroz, Zashch. 4, 129 (1978). P.M. Kelly and J. Nutting, J. Iron and Steel Inst. 192, 246 (1959) P. Wilson and O. Onyewuenyi, unpublished research. T-~ Ohio State University Columbus, Ohio, July, 1979. G.M. Pressouyre, Acta Met. 28, 895 (1980). R.A. Oriani, Ann. Rev. Mater. Sci. 8, 327 (1978). V.K. Sethi and R. Gibala, Acta Met. 25, 321 (1977). J.W. Hutchinson, research in progress--~ Harvard University, Cambridge, Mass. Nov. 1980.

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EFFECT OF H ON M I C R O H A R D N E S S

TABLE

ll7

1 AGING TIME IN AIR

8 I C (A/m 2 )

i00

150

Tc (h)

Initial Microhardnes

24

(h)

100

Final M i c r o h a r d n e s s

(KHN)

(KHN)

0

174

173

175

174

0.5

174

174

174

175

5

168

172

176

174

i0

157

168

170

175

15

143

166

168

170

0.5

170

164

164

173

5

158

166

170

170

i0

153

158

157

166

20

106

122

124

154

Effect of room temperature aging in air on microhardness of spheroidized 1090 steel c a t h o d i c a l l y charged with Hydrogen.

2i0

I

I

I

I

I

I

2OO

FIG. 1 E f f e c t o f e t c h i n g t i m e on m i c r o h a r d n e s s of spheroidized 1090 s t e e l .

IOO

.~ 170 ~E 160

i 50

i

I

I

2

L

I

3

4

Etching Time,

i

I

5

6

minutes

7

118

EFFECT OF H ON MICROHARDNESS

Vol. iS, No. 1

180

170

- -

FIG.

A/cm z)

2

Effect of charging time and charging current density on microhardness of spheroidized 1090 steel.

¢~ 140

~,~, '~100 I10

Ioo

I

I

I

5

I0

15

1 t % 5O

I

20

Charging Time (Tc)(Hours)at Charging Current Density ic (A/m I)

I

I

I

I

I

I

190

180

FIG.

3

Effect of charging current density on microhardness of spheroidized 1090 steel cathodically charged with hydrogen for 2 hours.

~,7o ~,eo

~'~ 140

130

120

i

I

I

I

I

I

50

I00

150

200

250

500

Charging Current Density (A/mZ)for 2hrs.