Internal stress in superficial layers on carbon steels.

Barhalescu, Mihaela ; Oanta, Emil ; Sabau, Adrian 等

1. INTRODUCTION

Processing by sparks is a simple and cheap process, in comparison with other methods to process the surfaces and it has wide applicability in industry. The method of processing using electric sparks of the surfaces of materials is based on the phenomenon of electric-erosion and the polar transfer of the anode material (the electrode) to the cathode (the metallic piece) during the electric discharges in impulses between the anode and the cathode. That discharge takes place in a gas environment. Unlike the classic processing through electric-erosion, at the electric sparking is used a power that is recovered pulsate with reversed polarity. In this case the processing through electric spark has the air as a gas environment and the electrode execute a vibrating movement.

Non-stationary processes of heating and cooling of the electrode materials by pulse in the discharges area is the main cause of appearance of the internal stresses in the layers obtained by electrical sparking.

In the sparking process, during the interaction of the anode with the cathode material, new phases are created, phenomenon which leads to the appearance of the structural stress.

Quantitative determinations of internal stress are based on Hook's law which links the strain and the stress, which means that the measurements do not cause stress but they cause strains. Therefore, each type of stress is related to a certain structural change.

Tracking the evolution of second rank stress allows us to optimize the sparking process in order to determine the parameters of the device, leading to a layer structure with superior mechanical--physical properties.

Pursue further research analysis superficial layers obtained by the electric discharge, with electrodes of hard carbides, in terms of hardness, wear and fatigue resistance.

2. EXPERIMENTAL RESEARCH

The tests are dedicated to the superficial processing with electric discharges in impulses using hard sinter carbide electrode (Ti15Co6, WCo8) on the OLC 45 carbon steel samples. Sparking was done on the clean and degreased plane surfaces that belong to parallelepiped samples (20x20x10) made of from carbon steel. The processing with electric discharge was done in manual working conditions using an electrode bent at 60[degrees] with respect to the treated plane surface. The experiments were dedicated to the superficial treatment using impulse electrical discharges, made with the ELITRON 22A equipment.

In table 1 are presented the recommended values for the electrode cross section with respect to the work regime of the ELITRON--22A equipment and the current value at every standard working state (*** 1991).

The electrodes that can be used, on ELITRON equipment, must have circular or rectangular section (for the rectangular section the report between sides should not be greater than 3 or 1 square) and initial length should not be less than 20 mm.

For the experimental research of the structure of the processed surfaces it was used X-ray diffraction method (Chatterjee, 2008). Another methods such as mechanical method (dissection techniques) and non linear elastic method (ultrasonic and magnetic techniques) for spatial and depth resolution have orders of magnitude less than the X-ray diffraction, therefore it was used the X-ray diffraction method (Ladd, 1985).

Microscopic stress or micro-stress can be determined separately from the diffraction-peak position and breadth (Pauleau, 1994).

The structural investigations were done on X-ray diffract-meter DRON-3, which has a radiation tube with Molybdenum anode, in the following working conditions:

* Radiation MoK[alpha], with [lambda]Mo = 0.7107 [Angstrom]

* The acceleration tension on the tube: 40KV

* Cathode current supply: 15 mA

* Proof rotation speed: [omega]1=4o/min

* Inscriptions band speed: 1800 mm/h

* Working slit: 1mm and 0.5 mm.

The structural transformations produced in layer and samples are correlated with internal stresses induced by the technological processes parameters. According to the spreading model of the stresses in polycrystalline body volume, the stresses may be included in one of the following classes:

* the first rank stress ([[sigma].sup.I])--they influence the microscopic distances;

* the second rank stress ([[sigma].sup.II])--this kind of stress appears in every crystal grain and its range is from the level of a few elementary cells up to an entire crystal grain;

* the third rank stress ([[sigma].sup.III])--are correlated with the regular distribute modification of some reduced atoms groups in every crystal grain.

According to the diffraction method applied for the sample, it is supposed to identify two different maximums of the diffraction, for the same phase (Ladd, 1985). One is noticed at a small angle and it is related to the plane with (h, k, l) indexes. Another maximum is noticed for a larger angle and it is related to a plane with (n x h n x k, n x l) indexes. The breadth line experimentally measured for the examined sample is compared with the experimental breadth for the standard line having the same index. For the experimental case studies presented in the paper it was used Merk's standard iron powder, treated 10 hours in vacuum at 850[degrees]C.

Because not all the substructure elements have a scientific and technical practical interest, are determined only the second rank stress and the mosaic blocks size made of the carbon steel basic samples sparked with Ti15Co6 and WCo8 electrodes.

During the experimental research, it was used OLC 45 steel samples, sparked with the same electrode, at different working conditions, and OLC 45 steel samples sparked with the same electrode at different working time of the sparking process.

From the whole set of diffraction-graphics are selected for examination only two diffraction lines, for both standard and for the studied samples, a small diffraction angle, 2 x [[theta.sub.1] and another at a large diffraction angle, 2 x [[theta].sub.2] (Ladd, 1985).

It is not recommended to select these lines from crystalline planes belonging to the same family of planes. This means that one line will be selected from the plane with (h, k, l) indexes and the second line from the plane with (n x h, n x k, n x l) indexes.

In the first stage it was determined the experimental width, in radians, of the diffraction lines previously selected (Gheorghies, 1990). Next step consisted in determining the [delta] distance between[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] doublets, using the equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

In the next stage it was determined the real physical breadth [[beta].sub.i], the influence factors "m" and "n" from the mosaic blocks dimensions and respectively the internal stress and mosaic blocks dimensions D.

The internal stress values, for these samples, are presented in table 2. The values from parenthesis in column 1 represent the specific processing time minutes/square centimetre.

Real physical breadth [beta], of the diffraction line can be computed using a specific equation:

[beta] = [(m + 2 x n).sup.2]/ m + 4 x n (2)

where "m" is the diffraction line width at half intensity line and "n" is the diffraction line width determined by micro stress. This equation is used for carbon iron alloys.

If real physical breadth of the diffraction line is determined only by the dispersion of mosaic blocks, their size "D" is determined by the equation:

[D.sub.hkl] = k x [lambda]/[[beta].sub.1] x cos([[theta].sub.1]) (3)

where k is shape factor of crystals, [lambda] is wavelength of the x-ray used.

3. CONCLUSION

The internal stress at second rank is superficial layers which obtaining with electrical discharge method had demonsrated that: while the increase the working conditions, increase the values of internal layers stress too.

The material electrode, has no influence on the stresses in the layer, but an overall value of the stresses can be noted in the superficial layers obtained with the wolfram electrode.

The specific processing time influence leads to an increase of internal stress values, just in the first stage. When the processing time increases, for the same electrical discharge conditions, the level of the stresses will decrease as result of the material heating.

4. ACKNOWLEDGEMENTS

Some of the results presented in the paper are inspired from the accomplishments of the "Computer Aided Advanced Studies in Applied Elasticity from an Interdisciplinary Perspective" ID1223 scientific research project, under the supervision of the National University Research Council (CNCSIS), Romania, (Oanta et al., 2007).

5. REFERENCES

Chatterjee, S.K. (2008). Crystallography and the World of Symmetry, Springer, ISBN: 978-3-540-69898-2, Berlin

Ladd. M.F.C. & Palmer. R.A. (1985). Structure determination by X ray Crystallography, Plenum Press, ISBN: 0306368444, New York

Oanta, E.; Panait, C.; Nicolescu, B.; Dinu, S.; Hnatiuc, M.; Pescaru, A.; Nita, A. & Gavrila, G. (2007-2010). Computer Aided Advanced Studies in Applied Elasticity from an Interdisciplinary Perspective, ID1223 Scientific Research Project, under the supervision of the National University Research Council (CNCSIS), Romania;

Pauleau, Y. (1994). Materials and Processes for Surface and Interface Engineering, Kluwer Academic Publ., ISBN: 9780-7923-3458-3, London

*** (1991) Ustanovska ELITRON 22, Academia Nauk, Chisinau

Tab. 1. The values for the electrode cross section

Electric work state Electrode cross Work current
 ELITRON--22A section value [mm] [A]

 1 4 0,5
 2 5 0,8
 3 4 / 6 1,3
 4 5 / 6 1,8
 5 6 / 9 2,3

Tab. 2. Values of the internal stresses

Sample--Electrode function parameter D [[sigma].sup.II]
 [Angstrom] [MPa]

OLC 45 (0.5)--WCo8--regime 4 968 16.401
OLC 45 (0.5)--WCo8--regime 3 954 14.805
OLC 45 (0.5)--WCo8--regime 2 920 14.322
OLC 45 (1)--Ti15Co6--regime3 1049 13.818
OLC 45 (2)--Ti15Co6--regime3 1126 14.742
OLC 45 (3)--Ti15Co6--regime3 1068 15.078
OLC 45 (4)--Ti15Co6--regime3 1042 14.616
OLC 45 (5)--Ti15Co6--regime3 998 13.671