·Home ·Table of Contents ·Materials Characterization and testing

Characterisation of Grains by Barkhausen Noise and Eddy Current Testing M. ZERGOUG, N. BOUCHERROU, N. BENNACEUR, A. HADDAD, S. MEBREK, A. HAMMOUDA, F. SELLIDJ, B. MOULTI, A. BENCHAALA Laboratoire d'Electronique et d'Electrotechnique Centre de soudage et de contrôle, Route de Dely Ibrahim, B.P:64, Chéraga, Tipaza Tel : (213)(02) 36 18 54 agrave; 55, Télex : 71 179, Fax: (213)(02) 36 18 50 Contact

ABSTRACT

Controlling of some industrial components require the development of new and particular non destructive testing techniques.To characterise a steel, it would be necessary to know its chemical composition, physico-chemical constitution, metallurgical state (annealed, hammered) and other parameters (superficial and chemical processing, ...). The microstructure of metallic alloy depend on the chemical composition, elaboration processing, and thermal processing.
The testing method using Barkhausen noise (B.N.) is a particular method ,which can be applied on ferromagnetic materials .It is a magnetic non destructive evaluation (NDE) method and can provide very important information about the material structure .The eddy current methods can also be applied
The aim of our work is to study the material structure using these techniques ,to characterize the region submitted to thermal processing .Sample of steel have been heated at the temperature d 1100 °C with variable maintain time. These methods permits to see the influence of the degree of cold deformation on grain size ,microstructure types ,microstructural changes, hardness changes after thermal treatment.

Introduction

To characterize a steel, it would be necessary to know its chemical composition, physico-chemical constitution, metallurgical state (annealed, hammered) and other parameters (superficial and chemical processing, ...). The microstructure of metallic alloy is function of chemical composition, elaboration processing, and thermal processing [1], ...etc.
The application of eddy currents in non destructive testing was very developed during the recent years. Adding to the defects characterization, actual studies deal with the metallurgical evaluation of materials. Surface processing allow to increase the material endurance and consequently its life duration.
During the control by Eddy Current [4-7], we exploit the information provided from a distance by impedance variation of a solenoid to the vicinity of a conductor material.
It is the indication of a transfer of energy from a distance between the solenoid and the conductor material. The cause of the variation of impedance observed is the modification of the magnetic flow through the solenoid. The flow varies in amplitude and in phase.
The electromagnetic properties of steels depend on their composition, microstructures and applied stresses. One of the NDE methods is the magnetic method based on the Barkhausen effect which occur during magnetization of ferromagnetic material and following to movements of magnetic domains under the influence of a variable magnetic field .This method permits to see the influence of the degree of cold deformation on grain size ,microstructure types ,microstructural changes, hardness changes after thermal treatment ....
The objective of our work is to be able to determine the metallurgic features has a steel weakly alloy of nuance 20NC6 having sudden a thermal treatment of homogenization and to characterize by the classic methods as the métallography, microdureté ,toughness and by methods no destructive

THEORY

The structure of domains corresponds to a minimum of internal energy of the sample . This minimum can be obtained by reducing internal energy of each grain, considered as isolated of others. For each grain ,It is evident that magnetostatic energy depends strongly of the environment where it is.The structure of domains is influenced by the presence of punctual defects, such as cavities, inclusions of not magnetic matter or precipitate.
Because the laws governing the structural state of a material (state of constraint and microstructure ) phenomena of magnetization are identical, we can understands easily that techniques such BN can become a tool of non destructive testing for ferromagnetic materials
The physical principle of this method based on the microstructural defect(dislocations, grain boundaries..)[2-3], which are responsible of mechanical properties like hardness ,are responsible for the magnetization ,shape of the hysteresis loop ,coercive force ,remanence magnetization . The defects are prohibitive against the move of Bloch walls and the reorientation of magnetic domains under external magnetic forces.
The Magnetization therefore is not obtained continuously but by steps and these changes of magnetization are accompanied by electrical ,magnetic and acoustic signals that can be amplified and evaluated.
Thanks to model of four domains, it is possible to describe qualitatively the magnetization to a more microscopic scale using the Bloch walls.

The austhenitisation has this temperature permits the stake in solution of chromium carbides and the cémentite. The composition of the austhénite can be thus different of the one of steel.
The presence of fine scattering of carbides gives back more difficult the migration of grain joints (magnification of grains). A longer maintenance permits a better diffusion of the carbon and a stake in more important carbide solution.
The middle volume of grains (size of grain) can be characterized by different conventional sizes deducted of the observation of a cut hovers steel prepared to make appear contours of crystal sections by plan of the cut.

Samples
The welding causes enormously problems to physicists, by the creation of assigned affected thermally zone (ZAT).This zone is fragile, characterized by a new structuring, that depends on operative conditions to the preliminary, following the gradient of temperature assigned.
so as to simulate the ZAT , we have submitted samples to close temperature to those of preceding regions
One appropriated on a rod of cylindrical shape of 80mm of diameter five samples of nuance 20NC6, 10mm of thickness each. Sample of steel has been warmed at the temperature 1100 °C that are located therefore in domain austhenitic with variable maintain time(1hour30 at 6 hours)

EXPERIMENTAL PROCEDURE

1-Barkhausen Noise(BN) control
During the measurement at 0.7 Hz magnetic field H using a triangular wave form of high amplitude (field about 200 A/cm.) is applied ,the coercitive and remanence field where measured from the hysterisis loop obtained by integrating induced voltage signal across a pair of search coils encircling the samples.
Barkhausen signals in steels are obtained in the frequency range of 20-200 kHz.
Instrumentation for measurement BN has been developed .The experimental test permits the excitation, and detection which are composed of two single yoke. The yokes arrangement restricts the mutual magnetic influence and make easier the flux wave form control. The magnetizing Voltage is passes through a power amplifier ,and the pick-up induced voltage is filtered , acquired for processing.
Software permit to draw the envelop ,to found the number ,the height of pick...

2 Eddy current control
The probe is put in a mechanical system that allows using step by step three motors .The movement following the three axes :X, Y and Z .A software allows the acquisition of measure parameters of the impedance bridge by the means of an IEEE 488 interface and the mechanical system control by the means of the RS 232 interface
The testing data and conditions are directly provided to the software .The results appear under the from of a color representing the impedance variation .
This visualization is realized in comparison with the display parameter in respect to that of reference previously taken for each one of the sweeping points.
Besides the color chart is affected by a fixed sensibility that corresponds to a color set.
This one is function of the minimal required variation .
In order to optimize the testing it is necessary to consider certain parameters:

I - Presentation and interpretation of results
Results gotten by eddy current and barkhausen noise non destructive testing, the calculate grains size, the determination of phases and the measure hardness, allows:
To facilitate the interpretation of results of submissive samples to the thermal processing, and to explain the different results, we have taken account of sample micrography obtained by optical microscope.

eddy current testing evaluation has show that impedance have a height sensitivity to the steel treatment (maintain time) due to the phenomenon of diffusion and the obtain structure.
All metallurgic modification of the structure to train simultaneous variation of the conductivity and the permeability .this is observed by impedance variation obtained in eddy current testing (Fig:3)

Image 2D of treatment sample

Fig 3: Impedance diagram


the second technique used is Barkhausen noise, our objective is to value the structure of materials ferromagnetic having modifications during the thermal treatments. Software allows the processing of results of the acquisition of the BN by the FFT and the spectral density. This computer processing facilitates the interpretation of results.
The spectral density of the signal captured by Barkhausen noise gives excellent results for non destructive evaluation of these treatments. As the shows the value of picks increases with the time of maintenance. (Fig:1 and 2)

Micrography
BN
FFT

Spectra Density
Fig 1: BN

Fig 2: Pic of FFT and density spectral with maintain time
Fig 4: Impedance with maintain time

To facilitate the interpretation of results of submissive samples to the thermal processing, and to explain the different results, we have taken account of sample micrography obtained by optical microscope.(Fig:1)
The microhardness allows the size grain measure. This evolution is the same between the size grain ,the first pic in the spectral density obtained by BN ,and the inverse impedance eddy current testing with the sample treatment (Fig:2 , 6 and 7).

Fig 6: admitance with maintain time

Fig 6: grain size and hardness with maintain time


The hardness of samples treatments has the same evolution of the impedance variation (Fig:5and7)

Fig 5: Impedance variation with maintain time

The reconstitution of 2D controlled samples,. This visualization is realized in comparison with the display parameter in respect to that of reference previously taken for each sweeping point. The results in a graphical color map show the variation of sample treatment thermal with the variation of the metallurgic characteristic (Fig:3)

The global different structure analysis revealed that the dependence of the grain size with the temperature follows a linear behavior, however, one notices a local magnification no uniform of grains, it is owed to the presence of the nickel, the manganese, and especially foulness whose main effect is to prevent the magnification of the grain and the total decomposition of the perlite.

CONCLUSION

The welding causes enormously problems to physicists, by the creation of assigned affected thermally zone (ZAT).This zone is fragile, characterized by a new structuring, that depends on operative conditions to the preliminary, following the gradient of temperature assigned.
The evaluation of the welds is extremely important for the performance of materials, hence the quality and integrity of these joints must be controlled, to ensure that no problem exist in weld
The aim of this work is to use the BN and eddy current testing for the evaluation of ferromagnetic structure having sudden modifications .Steels chosen are commonly used in industrial field and often submitted to thermal processing such as operations of welding .
We showed that the impedance variation of low frequency probes is influenced by the treatment, the increase of size grain decrease the permeability and the electrical conductivity.

The spectral density gotten from the Barkhausen noise and the measure of the grain size follows the same evolution ,it would also permit to determine the different phases without resorting to the destructive means. Elsewhere results obtained on Elsewhere results obtained on samples heated to height temperature for different times, permit to conclude about the metallurgic specimen aspect, specially the increase of grains in the allocated zone. Finer structure or smaller grain sizes increase the density of available pinning sites ,reducing domain wall motion and therefore result in decreased BN . Finer structure or smaller grain sizes increase the density of available pinning sites ,reducing domain wall motion and therefore result in decreased BN .
We showed that the impedance variation of low frequency probes is influenced by the samples heated to height temperature for different times. Consequently, the increase time treatment decrease the permeability and the electrical conductivity.

The sensitivity of BN technique and eddy current shows that these methods can be used in industry to detect defective components during manufacturing and permit to replace a destructive testing

REFERENCES

1. J.Chicois, O.Saquet, D. Tapuleasa, Traitement thermique des aciers,La technique Barkhausen appliquée aux contrôles non destructifs ,Traitement thermique pp18-23 N°300 juin-juillet 1997.
2. M. Birsan, J.A Szpunar, T.W. Krause and D.L. Atherton, Magnetic Barkhausen Noise study of Domain Wall Dynamics in Grain Oriented %3 SI-FE , IEEE Transactions on magnetics, Vol 32, No 2 , March 1996 pp527-534
3. R.Ranjan, D.C. Jiles, O. Buck and R.B. Thompson , Grain Size Measurement Using Magnetic and Acoustic Barkhausen Noise, Ames Laboratory, Iowa State, University, Ames, Iowa 50011 , J.appl Phys 61 (8), 15 April 1987
4. M. ZERGOUG, F. SELLIDJ, F. H. MANSOURI, N. CHEIKH, L. GACI, N. HANED, Automatisation d'une chaîne de contrôle. 2ème Conférence maghrébine sur l'automatique, L'électrotechnique et l'électronique industrielle, Décembre 1996, Tlemcen, Algérie.
5. M. ZERGOUG, F. SELLIDJ, A. HAMMOUDA, S. MEBREK, élaboration d 'un modèle physique par courant de Foucault et son application dans la caractérisation des défauts longs dans les produits cylindriques creux non ferromagnétiques. 3ème congrès de mécanique, Avril 1997, Tetouan, Maroc.
6. M. ZERGOUG, F. SELLIDJ, A. HAMMOUDA, Caractérisation non destructive par courant de foucault de couches de rechargement et de revêtement dans les matériaux conducteurs. First Arab mechanics congress CAM 97, June 1997, Damascus, Syria.
7. M. ZERGOUG, F. SELLIDJ, A. HAMMOUDA, S. MEBREK, Réalisation des sondes et caractérisation non destructive par courant de Foucault de ouches de rechargement et de revêtement dans les matériaux conducteurs. Conférence maghrébine sur le controcirc;le non destructif, Juin 1997, Alger, Algérie.

© AIPnD , created by NDT.net

|Home|    |Top|