The effect of shot peening on hydrogen absorption by and hydrogen permeation through AISI 4130 steels

Scripta METALLURGICA et MATERIALIA

Vol. 26, pp. 627-652, 1992 Printed in the U.S.A.

Pergamon Press plc All rights reserved

THE EFFECT OF SHOT PEENING ON HYDROGEN ABSORPTION BY AND HYDROGEN PERMEATION THROUGH AISI 4130 STEELS B.E. Wilde* and I. Chattoraj**. * Fontana Corrosion Center, Dept of Materials Science & Eng; The Ohio State University, Columbus, OH 43210 * National Metallurgical Laboratory, Jamshedpur, India, 831007. ( R e c e i v e d May 22, 1991) ( R e v i s e d D e c e m b e r 13, 1991) Introduction AISI 4130 steels are used in spring and axle fasteners which are often exposed to water containing road salts and as a consequence have been observed to fail prematurely by hydrogen embrittlement (HE). For a steel with a given microstructure and strength, the most important factor influencing its mechanical behavior is the amount of hydrogen ingress into the material. A quantitative estimate of the amount of hydrogen entering the material can be obtained from the relatively simple but quite accurate method of electrochemical charging using double cells devised by Devanathan and Stachurski (1). These authors subjected membrane specimens to galvanostatic (or potentiostatic) hydrogen charging on one surface which resulted in concentration gradients across the membranes and hydrogen diffusion resulting in a permeation flux. The apparent diffusivity of hydrogen can be calculated from the shape of the permeation transient and the steady state value of the flux is a good indicator of the susceptibility of the material to hydrogen embrittlement as well as the efficacy of the various modification treatments used to counter HE. Several surface modification approaches (2-6) have been described to reduce susceptibility to HE by reducing the hydrogen ingress into the steel. Shot peening is one such surface modification technique that is a relatively cheap and industrially feasible to accomplish. Wilde and Shimada (5) have shown that peening decreases hydrogen absorption by plain carbon steels and markedly improved the HE resistance of AISI 4130 low alloy steel although the precise mechanism of this effect is as yet not known. Since shot peening is known to result in the introduction of surface compressive stresses and also severe surface cold work, this investigation was conducted to identify the relative contribution of these latter two variables of peening on hydrogen entry into AISI 4130 steels. Variations in the depths of the residual compressive stress layer was accomplished by varying the intensity of peening. Variations in the surface cold work was accomplished by changing the duration of shot impingement (or surface coverage) at a given intensity of peening. Comparison of the various results obtained from the permeation transients helped in establishing correlations between the intensity of peening and hydrogen ingress and is discussed in this paper.

Materials and Experimental Procedures Square blanks (5cm X 5cm X 0.25cm) were used for the permeation experimentsand were fabricated from AISI 4130 steel that was austenitized at 870°C for one hour in an inert atmosphere, quenched in water and tempered at 425°C for one half hour. This heat treatment produced a tempered martensitic structure having a yield strength of about 1250 MPa. After heat treatment, the specimens were ground to a 600 grit finish, ultrasonically cleaned and were shot peened. Following peening, the specimens were thoroughly washed with acetone. Residual stress profile X-ray measurements following peening were not conducted. Instead, the degree of peening was quantified according to the method devised by Almen. In this method, the arcing or bending of the flat specimens caused by the peening induced plastic deformation was measured; the arc height was measured using an Almen

627 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc

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intensity gauge. The intensity of peening tends to reach a saturation value with increasing duration of peening so that on further peening there is no appreciable change in the intensity unit but the amount of surface cold work continues to increase. Specimens peened for time period corresponding to the attainment of the saturation intensity are referred to here as 100% peened; those peened for twice the time period are referred to as 200% peened and so on. The purpose of increasing the time of peening was to change the amount of cold work in the surface layer while the peening intensity was invariant. Permeation experiments were conducted in a Devanathan type cell in which the surface of the specimen was instantaneously cathodically polarized with a known galvanostatic charging currenL The opposite surface of the membrane was palladium coated and was maintained at a constant anodic potential of +100 mV SCE to facilitate the oxidation of diffusing hydrogen. Reagent grade sodium hydroxide was used as the electrolyte used on both sides of the membrane,(0.1N or IN made with double distilled, demineralized water) depending on the experiment. Nitrogen gas was bubbled through the electrolyte to eliminate or reduce oxide formation on the specimen surfaces. In this study, permeation experiments in which the peened surfaces were made the cathodic or hydrogen charging surfaces, will be referred to as forward charging experiments. Those where the peened surfaces were subjected to the constant anodie overpotential while the unpeened sides were exposed to charging solutions, will be called reverse charging experiments. Peening introduces a residual compressive stress on the unpeened side due to the very process of bending of the specimens. The magnitude and depth of this compressive stress is however, smaller than those produced on the peened surface. The purpose of the reverse charging experiments was to study the effect on the kinetics of hydrogen ingress of these induced compressive stresses in the absence of any cold work. Results and Discussion Hydrogen permeation transients at a constant charging current density of 2 A/m2 for specimens peened at the same intensity but for different durations are shown in Fig. 1. It is evident that shot peening significantly reduced the absorption rate of hydrogen into the steel; however, increase in the surface cold work due to increase in the peening duration was not a significant factor in influencing hydrogen ingress. The variation of the steady state hydrogen permeation flux (J) with charging current density also showed a decrease in hydrogen permeation for all the peened specimens when compared to untreated base steel (Fig. 2). The influence of cold work could be expected to manifest itself in a change in the apparent hydrogen diffusivity through the material,thus resulting in change in the recorded permeation transient. Apparent hydrogen diffusivity calculations from the permeation rise transients are listed in Table 1, and indicate that there is no influence of increasing surface cold work on the kinetics of hydrogen absorption. It should be mentioned here that the depth of the cold worked layer was very small compared to the overall thickness of the specimens so that even if the hydrogen diffusivity was affected the apparent diffusivity calculations would not be expected to reflect any changes. TABLE 1 Permeation Parameters for Specimens Peened for Different Times Condition Diffusivity cm2/s Steady state flux (Aim2)

Base steel

100%SP

200%SP

4.07x10 7

3.5x10-7

5

5.5x10 -3

2.6x103

3 x 10.3

X 10 `7

Specimen Thickness = 0.25cm, IN NaOH

300%SP

400%SP

3.6x10 "7

3.9X 10.7

2.3x10 3

3.1x 10.3

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The effect of changing the intensity of peening on the steady state flux is presented in Table 2. The steady state flux decreased with increasing intensity of peening or increasing depth of the residual compressive stress layer, thereby demonstrating the ability of peening to reduce hydrogen absorption into AISI 4130 steels. Diffusivity (D) calculations were made from both the permeation rise and decay transients. The similarity of the D values for a given permeation transient calculated from the decay and rise part of the transient clearly indicate that the permeation process was not significantly affected by the presence of oxide layers nor by trapping -detrapping phenomena. The invariance of D with different peening intensity indicates an independence of hydrogen diffusivity from residual stress variations, and is in accordance with the predictions and findings of Bockris et al.(7) and Beck et al.(8). Further substantiation that residual compressive stress is the most important factor in affecting hydrogen permeation following peening is obtained from reverse charging experiments. Figure 3 shows a consistent decrease in the steady state hydrogen flux at all charging currents compared to the base steel on reverse charging a specimen peened at low intensity (7A). For specimens peened at a high intensity (22A) the decrease in flux is more noticeable (Fig.4). TABLE 2. Effect of Shot Peening Intensity on Hydrogen Permeation at an I~ of 2A/m2. Intensity

Diffusivit¥ rise,cm2s-~

Diffusivity decay,cm2.s1

Flux, J. A/m2

zero 7 12" 12"* 15 22

4.10x10 -7 3.50x10 -7 5.13x10 -7 3.62x10 -7 3.87x10 "7 7.60x10 -7

4.30x10 7 4.38x10 "7 3.83x10 "7 3.80x10 "7 3.61xl0 "7

10.05xl0-3 7.17x10 "3 5.0x10 -3 6.7x10 -3 6.7x10 "3 4.25x10 3

0.1 N NaOH Specimen thickness = 0.19 cm * Peening bead diameter = 0.(343 cm ** Peening bead diameter= 0.083 cm The specimen surface subjected to hydrogen charging during a reverse charging experiment had an induced residual stress whose magnitude and depth were less than the corresponding peened surface on the oxidation side of the membrane which also increase with increasing peening intensity. The results illustrated in Figs.3 and 4 show that hydrogen ingress is reduced due to the presence of a compressive stress at the surface even in the absence of any cold work, and further, there is direct correlation between the depth of the residual stress layer and the amount of hydrogen absorbed by the material. The effect of elastic stress on the thermodynamic solubility of hydrogen (7-9) cannot account for the entire reduction in flux. Experiments were conducted to study the effect of compressive elastic stresses on hydrogen permeability through AISI 4130 steels. It was found that neither the diffusivity nor the steady state permeation flux was significantly affected by pure compression. In contrast, the shot peened specimens had a stress profile across the surface shown schematically in Fig. 5. The effect of shot peening we propose, is a result of the presence of this profile which causes a redistribution of the electrons in response to the stress and consequently influences the hydrogen entry kinetics at the steel surface. This effect is discussed elsewhere (10,11).

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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

M.A.V. Devanathan and Z. Stachurski, Proc. Roy. Soc., A270, 90, (1962). B.E. Wilde, I. Chattoraj and T.A. Mozhi, Scripta Met., 21, 1369, (1987). P.J.Grobner, D.L. Sponseller and D.E. Diesburg, Corrosion, 35, 240, (1979). J.B. Greer, Materials Performance, 14, no. 3, 181, (1975). B.E. Wilde and T. Shimada, Scripta Met. 22, 551, (1988). M. Zamanzadeh, A. Allam, H.W. Picketing and G.K. Hubler, Journal of Electrochem. soc., 127, 16887 (1980). J.O'M Bocktis, W. Beck, M.A. Genshaw, P.K. Subramanyan and F.S. Williams, Acta Met., 19, 1209, (1971). W. Beck, J.O'M Bockris, J. McBreen and L. Nanis, Proc. Roy. Soc., A290, 220, (1965). R.A. Oriani, Trans. Amer. Inst. Min. Engr., 236, 1368, (1966). I. Chattoraj, Ph.D Thesis, The Ohio State University, 1991. 11. I. Chattoraj and B.E. Wilde, to be published.

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