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Characterisation of the Cementation Layer by the Barkhausen Noise B.Moulti, , M. Zergoug, A.Benchaala, A.Haddad, S.Mebtouch, T.Larsaoui, Laboratoire d'électronique et d'électrotechnique Centre de Recherche Scientifique et Technique en Soudage et Contrôle Route de Dely Ibrahim BP 64, Cheraga . Alger. Algérie Contact

Abstracts

The measure of Barkhausen noise phenomena has application in research, development and industrial environments. The potential of this method is used to characterise the cementation layer up to 1.1mm thickness.
We discuss The source mechanisms of the acoustic Barkhausen (AB) and magnetic Barkhausen (MB) signals which are affected by many process as nucleation, growth and annihilation of domains.
The effect of the hardness and cementation layer on the aforementioned signals is developed.
The use of Barkhausen inspection in comparison with the conventional method namely the micrography and the measure of the Vickers hardness, shows its efficiency in the non destructive testing.

Introduction

Microstructure evaluation induced a change in the mechanical properties of steels , its therefore necessary to develop a non destructive evaluation technique for explaining the different microstructure change .
It has found that a magnetic NDE shows a great advantage over other NDE methods , because of its high sensibility to the change of metallurgical state .
In 1917, Barkhausen [7] identified the magnetic domains which are separated by the domain walls, he postulated that the specimen (ferromagnetic) is formed of innumerable small magnetic domain , and each one is deemed to act as a small magnet postured in a randomly oriented position within the specimen.

When ferromagnetic material are subjected to the variation of the applied magnetic field , many processes such as the creation and the annihilation of domain and the domain wall motion's are observed. Each domain is strained along its direction of magnetisation , the magnetostriction phenomena occurs , if the domain wall motion's are sufficiently rapid , the abrupt changes in local strain give rise to the generation of elastic waves .
The magneto acoustic emission MAE is so the measure of elastic waves emitted by the sudden change in stress field during the dynamic interaction of domain walls with the defects (segregation ,dislocation , non magnetic inclusion..)

The MAE is know to be sensitive to the microstructural condition of ferromagnetic materials ; for example: the ferrite structure generates a strong MAE signal , while martensite gives a little or no MAE at all [3]
The MAE methods exhibits characteristics well suited to the evaluation of the surface condition where the magnetic properties are different .
The present paper is concerned in the main with the use of the MAE signal as a non destructive testing to evaluate the case layer thickness, in the second part , we discuss two models which are related to the source mechanism of the MAE signal and a theoretical evaluation of the case layer.

Experimental techniques

Specimen and microstructure
Five cylindrical specimens of 12NC6 of 80 mm diameter and 10mm length are used , this kind of steel is employed in the national society of industrial cars (SNVI)
After a normalisation heat treatment , they were case hardened with a range of depth from 0.4 mm to 1.5 mm using the cementation process [6]
Heat treatment times ranged from 7 to 24 hours.




The process follow the Fick law [6]:



MAE measurements
The experimental technique was reported by many papers [2,3,4,5].
The technique consist in the first part, to saturate magnetically the material using the U core electromagnet. The current driven magnet is capable of applying a field strength sufficient to nearly saturate the specimen. Fore each measurement, the magnetic field is cycled at 1Hz to ensure a standard magnetic history for the material.
The magnetic field H is swept linearly from its saturation positive value to a negative value over a different time depending on the excitation frequency f_E, which determines via a skin depth relation the interaction depth.
By the choice of the f_E , the measurement can be restricted to the surface alone (for 5mm , f_E =50mhz). The experience shows that f_E must be low enough to extract out the information coming from different surface and near surface layers.
The MAE signal was detected by a 230 Khz damped piezoelectric attached at the end surface of the specimen.
The need for amplification of the small signal magnitude was for a long time a draw back for the instrument realisations, with the modern electronic equipment, this has been overcome .
The band pass frequency filter ranging between 20Khz to 250Khz , with the gain of 40 db
The characteristics of the MAE signals were plotted using a digital memory oscilloscope recorded to the computer.
For each specimen , a range of five modulation frequencies were used in succession from 0.04hz to2.23hz

Fig 1: Experimental arrangement for measurement of MAE signal

fA analysing frequency H magnetic frequency fE exciting frequency of hysteresis loop

Results and discussion

In order to understand the effect of the different modulation frequency on the MAE signal , The signal has been plotted as a function of the time .
The changes in the observed MAE signal with increasing modulation frequency are controlled by two major factors :

• The increase in the rate of change of the applied field H will increase the MAE generated from unit volume of sample
• The volume of material sampled interrogated is reduced.
So, the MAE signal may decrease or increase depending upon many parameters such as the specimen geometry, material and frequency range .
In the Buttle and Dalzell studies[3 ] , they observed that the MAE first increase with increasing frequency and then decreases for all the case depth shown, no such variation was observed in the present study.
The figure 3 is an example of the MAE unprocessed signal from the transducer, we note that it is extremely difficult to analyse it, thus we found more advantages to study the Final peak of the observed MAE signal with depending on the frequency range .

Fig 3: The unprocessed signal of the MAE signal

From the figure 4 , we observed that the large case depths result in the smaller MAE signal

Fig 4: MAE signal as a function of the modulation frequency with different case depths

Theoretical Models

The MAE is a phenomena where acoustic noise is generated due to the motion of the non-180° magnetic domain in a ferromagnetic material with non zero magnetotriction constants.
The MAE signal occurs during the magnetisation process, the number of acoustic pulses change depending upon the micromagnetic and the microstructure processes.[2]
The source of the MAE is the emission of elastic waves when domain wall pinned at defects are released , this rapid motion causes a change in the localised magnetostriction strain, Kamed and all assumed that the source of the MAE signal is due to the acceleration of the domains walls released from pinning sites , the MAE signal intensity V_MAE (H) can be given by:


C: numerical constant depending on the specimen volume and geometry , the elastic module and the characteristics of instrument
f_{g :} the volume fraction of defects
ds/dt : average accelerating velocity of domain walls released from the pinning sites
x _{m :} the local magnetostriction strain change
F_d (H) : domain wall density
P_g (H) : reflects the macroscopic drag effect

The MAE measurement can reflect the jumping frequency of domain walls pinned at defects on a microscopic scale..
The model given by Buttle and Dalzell [3]explain the change observed on the MAE signal from one specimen to another where only the case layer thickness is variable


m _f ,m _c : permeability
a _c ,a _f : sensitivities
d : case depth
s: electrical conductivity
n : modulation frequency

The theoretical and experimental data show relatively the same behaviour [3]

Conclusion

In this study , the MAE signal has been used to investigate the thickness of case hardened cylindrical 12NC 6 steels.
The microstructure of the hardened steel were non-destructively evaluated by the measuring of the MAE signals induced during the magnetisation process, we have observed a dependence between the case layer thickness and the frequency modulation.
The MAE technique can be used in this case, to apply this method it would be necessary that the ferromagnetic properties of the hardened zone must be different from the bulk material properties enough that they can be separated during the signal process.
However, to describe correctly the MAE signal obtained , we must consider interfering effects such as residual stress, defects and the variation of the micromagnetic parameters

Reference

1. W.A.Theiner and all, Materials evaluation/46/Nov 1988. Pp 1570-1580
2. J.Kamed and R Ranjan. Acta metall.Vol35, No7, pp 1515-1528
3. Buttle and Dalzell .AEA Industrial Report . July 1991
4. D.Atherthon and Ajay Dhar . J.App.Phys.72(2). 15 July 1992
5. M.Namking and all.Proc IEEE Ultrasonics Suposium .Vol2 (1992).pp867
6. Le Precis de mettalurgie . Edition 1981
7. H.Barkhausen , physik. Z.20,401(1919).

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