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Ê.ò.í. Cîøêî Â.À.

Õåðñîíñêèé íàöèîíàëüíûé òåõíè÷åñêèé óíèâåðñèòåò

 

Hydrogen permeation into iron and steel during plastic deformation

         

 

          Recently there has been increasing interest in the study of hydrogen permeation into metals preliminarily subjected to plastic deformation [1]. Most work in this area has concerned the investigation of hydrogen diffusion into iron and steels plastically deformed right: before diffusion experiments. The peculiarity of such diffusion process is specific dependence of diffusion coefficient on plastic deformation degree [2]. The mechanism of the process is usually treated in terms of creation of hydrogen traps (dislocations, vacancies, grain boundaries etc.) in metal crystalline lattices, for bcc metals or alloys binding energy of hydrogen atom to trap being much more in comparison with activation energy of diffusion [3].

          By now hydrogen diffusion into metal directly during its plastic deformation (in situ) is not widely studied due to experimental difficulties [4]. At the same time this problem is extremely interesting from both theoretical and applied points of view, covering wide range of phenomena from thermodynamically nonequilibrial  mechano-chemical processes to the effect of  hydrogen containing media on the wear of machine elements during friction, cutting fluids efficiency in metal working etc.

          The object of this paper is to present the preliminary results of investigation of hydrogen permeation into iron and steel during plastic deformation in hydrogen containing media: water, hydrocarbons etc.

 

Methods of hydrogen detection

 

          Several methods of hydrogen detection in metals are described in literature:

          1. Permeation of hydrogen into metals, which are capable of crystal hydrides formation (titan, nickel etc.)  may be detected directly by crystallographic      methods [1,5].

          2. Kinetic or steady state measurements of hydrogen permeation through thin metal membranes [6,7].

          3. Nuclear physics methods, including tritium irradiation measurements and recently developed approach based on reaction of helium-3 atoms preliminary implanted into metal with deuterium atoms diffused through metal and trapped by defects created around helium nuclei [8,9].

          4. The most widely spread method uses simple measurement of gas evolution from the sample at heating. It gives usually only the total amount of hydrogen in the

sample but not its spatial distribution or energetic characteristics [10,11].

          Method of gas evolution was modified in our work by applying the idea of thermo-desorption spectroscopy (TDS) usually used in surface science investigations [11]. The sample was heated in vacuum at a rate of 0.5 K/sec. with continuous mass-spectral analyses of the gas phase. Experimental curves present the dependence of mass-spectral signal on the selected mass (proportional in our case to the rate of hydrogen or deuterium evolution) on temperature of the sample. Usually the curve (TDS-spectrum) has one or several peaks with maximal according to temperature of evolution of hydrogen of different "types". The positions of maximal on the curve as well as the peak shapes reflect the complex process proceeding in the case of linear temperature raises, the main components of which are the diffusion of hydrogen atoms through the metal, their exit and recombination on the surface leading to molecular hydrogen evolution. It is clear that the distribution of hydrogen along the sample before heating as well as the typical size of the sample determine form and positions of the peaks.

          Two kinds of plastic deformation - compression and cutting were studied.

 

Plastic deformation during compression

 

The fact of hydrogen permeation into metal was established in the experiment of compression the iron sample in D2O. The cubic sample (1=3mm) was placed between parallel platens inside D2O drop. After load of 20 tons the sample was compressed to thickness of 0.5 mm (relative deformation of compression 80%). Then the surface layer of 50-mu thickness was grind from both sides of the sample in order to remove possible surface compounds containing deuterium. Using of marked water enables to avoid detection of "outside" hydrogen already present in the initial sample or introduced from atmospheric water during grinding. Peak of deuterium evolution with maximum at 540 K was found in the TPD spectra of compressed and grind sample, thus proving its permeation into iron (see Fig. 1).

 

Fig.1

 

Plastic deformation during cutting

 

          Main parts of the experiments were made with cutting (drilling), which is accompanied by high-speed plastic deformation of the treated material. Cutting was made in different medias listed in the table. Cutting of the sample presaturated with hydrogen by electrochemical method was also made. The same amount of chips (50 mg) was taken for each TPD experiment after cutting.

    Hydrogen peaks position and corresponding time of sample exposure at room temperature (texp) before TPD beginning are tabulated in the Table.

 

Hydrogen peaks position in TPD spectra.

 

NO              Media                                                texp                        T max (oÊ)

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1                 air                                                      0.5                        480

2                 dry nitrogen                                        0.5                          -

3                 H2O                                                   0.5                        425

4                  H2O                                                   2                           425

5                 H2O                                                   23                         493

6                 H2O                                                   300                         -

7                 H2O                                                   1 (=70 C)              490

8                 D2O                                                   0.5                        425

9                 D2O (large fraction)                            7                           480

10                D2O (small fraction)                            7                           490

11                D2O (large fraction)                            50                         505

12                D2O (small fraction)                            50                         505

13                electrolysis                                         0.5                        475

14                electrolysis                                         29                         480

15                ethanol                                               0.5                        480

16                ethanol + water                                   0.5                        443

17                n-heptane                                           0.5                        455, 505

18                vaseline oil                                         0.5                        455, 505

19                oleic acid                                           0.5                        500

20                vaseline oil + water                    0.5                        425, 480

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Note: 0.5 hour – minimal time for preparing the TPD experiment.                                                                                                                                     

 

1. Cutting in water

 

          TPD spectra received 0.5, 2, 23 and 300 hours respectively after cutting in water are shown in the Fig. 2. Maximum of the TPD peak of fresh chips is located at lowest temperature and has largest integral intensity. The total amount of hydrogen penetrated into chips is estimated to be of the order of 3-10 ppm.

Fig.2

 

          With increase of Lime the peak maximum shifts to higher temperature with simultaneous lowering of intensity reaching zero after several days. If the chips are kept at higher temperature before TPD experiment then hydrogen redistribution accelerates and the peak shift to higher temperature need much less time.

          To exclude the possibility of registration the "outside" hydrogen in TPD    spectra cutting in D2O was also made. The results were the same (see table).

          In order to estimate the influence of chips size on the peak position the TPD experiment for two different chips fractions (sieve size - 0.3mm) was made. It is seen from the table that the particle size does not influence noticeably the peak position for time of exposure of 7 and 50 hours.

          The dependence of peak maximal position in TPD curve after cutting in water on time of exposure is shown in Fig. 3. The dependence is asymptotic; the temperature of peak maximum reaches its limit at approximately 6 hours.

Fig.3

 

          These results could be easily interpreted if we assume that, like in the case of compression, plastic deformation of metal during cutting is accompanied by hydrogen transport into bulk of metal. Hydrogen at first concentrated in the outer part of chips redistributes with time by (a) diffusion into the bulk of material which leads to smoothing of concentration and (b) hydrogen transport to surface, recombination and evolving into the gas phase.

          Redistribution of hydrogen in the bulk along with lowering of its total content in the sample leads to the observed high temperature peak shift in the TPD spectra and diminishing of integral intensity with time.

          Briefly the mechanism of hydrogen transfer seems to be the following. During plastic deformation both in the case of compression and cutting fresh metal surface is produced. The hydrogen containing molecules of media like water, saturated or unsaturated hydrocarbons react with this highly chemically active surface producing adsorbed hydrogen atoms after partial dehydrogenation. We must also notice that adsorbed hydrogen atom has to overcome high activation barrier to penetrate into bulk, the transfer being endothermic. Thus the rate of hydrogen permeation is noticeable only at high temperature and considerable hydrogen concentration (pressure). Temperature of chips in the moment of shearing usually does not exceed 600-700 C. The rates of diffusion as well as hydrogen concentration in the near surface region are too small for hydrogen to penetrate deep enough into the outermost layers of metal.

          The depth of permeation (R) could be estimated by formulae

R2=Dt

where D - coefficient of diffusion, t= l/v, v - cutting speed, 1 and t,  respectively,  typical shearing zone and  contact  time, during which plastic deformation occur. Usually l = 1mm, v = 0.1m/sec, then contact time is of the order of 10-2 sec, and for relevant diffusion coefficient the depth of hydrogen permeation, even overestimated, could not exceed 10-3 cm. However, as hydrogen permeation does take place, it is worth assuming that high rate of hydrogen transport into the bulk is caused by very high instantaneous "heating" of the crystal lattice freedom degrees responsible for hydrogen transfer.

 

Cutting in ethanol

 

          TPD spectrum after cutting in ethanol is shown in Fig. 4. The position of peak maximum is noticeably shifted to higher temperature compared with water. As concerning peak intensities, though they are of' the same order, however, the total amount of hydrogen desorbed is smaller for ethanol than for water. TPD spectrum of chips after cutting in ethanol-water solution (1:1) in comparison with pure water is shown in Fig. 4. Peak maximum for solution is in the intermediate position. We must point out that cutting in organic media always leads to high temperature maximum shift in comparison with water.

Fig.4

 

          This fact can be explained by taking into account special physical properties of water, which lead to higher race or cooling or, in other words to hardening and concentrating of hydrogen only in the thin outermost layer of the sample, while cutting in water. After cutting in air (containing water vapor) the peak is positioned at higher temperature, although hydrogen-containing compound is the same in both cases, cooling conditions are different and hydrogen can diffuse deeper into bulk.  Ethanol-water solution has an intermediate thermocondactivity property, which lead to intermediate peak position.

 

Cutting in Vaseline oil-water emulsion

 

          Water emulsions or suspensions of polymers are known to be among the most effective metal working fluids. A hypothesis was suggested that hydrogen plays an important role in facilitation of plastic deformation of the treated material. TPD experiment data for chips after cutting in Vaseline oil – H2O (D2O) emulsion in comparison with water is shown in Fig. 5. It is seen that for two observable hydrogen peaks in the TPD spectrum of emulsion the low temperature peak is represented by hydrogen absorbed mainly from water (consists of D2 for D2O emulsion) and the high temperature peak by hydrogen mainly from organic compound (consists of H). The appearance of two additive peaks in the TPD spectrum points out that there exist two different contact regions, where cutting proceeds either in water or in hydrocarbon. But the most interesting thing is that addition of an emulator leads to high increase in intensity of the low temperature peak, i.e. promotes the permeation of hydrogen from water into treated material. Maybe this fact is connected directly with nigh efficiency of emulsions as cutting fluids in metalworking.

Fig.5

 

Conclusions

 

          l. Plastic deformation during compression as well as cutting of metal is followed by hydrogen transport into bulk of metal.

          2. After plastic deformation in water hydrogen is concentrated in thin outermost layer while in the case of organic media its spatial distribution is wider.

          3. Hydrogen transport during plastic deformation could not be described in terms of equilibrium thermodynamics of diffusion process and some other theoretical approaches.

          4. Adding small amounts of organic emulators causes high increase of hydrogen transfer into metal, water being the main source of this hydrogen.        

 

List of used literature.

 

1.     Ñîøêî À.È.,  ñá.: Ïîëèìåðû â òåõíîëîãè÷åñêèõ ïðîöåññàõ îáðàáîòêè  ìåòàëëîâ. Êèåâ: Íàóê. Äóìêà, 1977, ñ. 7-15.

2.      Øâåä Ì.Ì., Ñëàáêîâñêèé È.Ñ., Ñûíèêî Â.Ñ., Ôèçèêî-õèìè÷åñêàÿ ìåõàíèêà ìàòåðèàëîâ, 1970, ¹3, ñ.11-112.

3.      Øìåëåâ Á.À. Â êí.: Ìåòîäû îïðåäåëåíèÿ è èññëåäîâàíèÿ ñîñòîÿíèÿ ãàçîâ â ìåòàëëàõ. Ì.: Íàóêà, 1968, ñ.45-48.

4.      Morris M.A., Bowker M., King D.A., Chem.Kinetics, 1948, v.19, ð.1-179.

5.      Êóäðÿâöåâ Â.Í., Âàëàêèí Þ.Ï., Ëÿõîâ Á.Ô., çàâîä. ëàá., 1968, 24, ¹2, ñ.196-199.

6.      Ôàñò Äæ.Ä. Âçàèìîäåéñòâèå ìåòàëëîâ ñ ãàçàìè. Ïåð. ñ àíãë. Ì.: Ìåòàëëóðãèÿ, 1975, 352ñ.

7.      Benziger J.B., Madix R.J., J. Catalysis, 74, ¹1, ð.55-66.

8.      Ëîñåâ Â.Âë., ×åð÷åëü Î.Â., Êóïàíåíêî È.Â., Ýíòåëèñ Ñ.Ã., Êèíåòèêà è êàòàëèç, 1988, 29, ¹2, ñ.860-865.

9.      Øâåä Ì.Ì. Èçìåíåíèå ýêñïëóàòàöèîííûõ ñâîéñòâ æåëåçà è ñòàëè ïîä âëèÿíèåì âîäîðîäà. Êèåâ: Íàóê. Äóìêà, 1985, 119 ñ.

10.  Konig W., Vits R. Lubrication in metal woking, 3rd Int. Coll., 1982, v.2, ð.62.

11.  Áîêøòåéí Á.Ñ. Äèôôóçèÿ â ìåòàëëàõ. Ì.: Ìåòàëëóðãèÿ, 1978, 247ñ.