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The Steel Grain Magnification Influence on the Ultrasound wave attenuation Z. CHERROUF, N. OUALI, S. OUALLAM, G. KAMEL, Laboratory of Metallurgy Scientific Research centre and Technique in Welding and Control Road of Dely Ibrahim, BP 64, Cheraga. Algiers. Algeria. Contact

SUMMARY

The experience showed that steels or joints welded ultrasonic inspection can be optimised if the metallurgic phase and the grain middle size are known. This work lands in a first time the grain measurements effect on the propagation of the elastic wave, to different frequencies.
The experimental results are gotten from tests achieved on four steel samples types.

I. INTRODUCTION:

In the mechanical industry, one is often brought to control production pieces to insure that they fill the function correctly for which it is destined there.
Among these controls one distinguishes the shortcomings research in materials by no destructive control methods as the ultrasonic technique.
In general, materials presenting a strong anisotropy are with difficulty controllable to ultrasonic as austenitic steels of which the structure refinement is very difficult to get.
Nevertheless this factor is not the only criteria to take in consideration, another material characteristic is major to know the grain average size.
Indeed when the grain size reached a certain size this translates himself by a strong report signal-murmurs reduction at the time of the ultrasonic signal reflection on a defect so that this material control is put back in question.
So this work lands in a first time the grain measurements effect on the ultrasonic wave attenuation.
As this work first left concerns the treaty steels samples metallurgic characterization that is: Z6 CNDT1712, XC 48, 45 SCD6 and the E 24.

II. TENTATIVE PROCEDURE:

The weld ability factor is bound to the definite chemical composition by the equivalent carbon and the equivalent chrome/nickel notion.
The used steels are weakly the XC 48, 45 SCD6 and the E 24 allied steels and the Z6 CNDT1712 greatly allied steel.
The samples chemical composition according to by the norm NF is given by the table 1.

		% in element
Steel		C	S	P	Si	Mn	Cu	Mo	Ni
	E24	£0,17	£0,045	£0,045
	XC48	0,45-0,51	£0,035	£0,035	0,10-0,40	0,50-0,80
	45 SCD6	0,42-0,50	£0,025	£0,030	1,30-1,70	0,50-0,80	0,50-0,75	0,15-0,30
	Z 6CNDT 17 - 12	£0,08	£0,030	£0,040	£1,00	£2,00		2,00-2,50	10,5-13,0
Table 1: Samples chemical composition

These steels have sudden of the overheats thermal salary to high temperature during an important maintenance how presented on the picture 2 .
Overheats it is characterized by a grain magnification exaggerated.
This one is gotten to the extensively superior temperature to the Ac3 temperature criticizes and spreads until the one of limit solidus / liquidus.
This magnification is also accentuated by the increase in the maintenance time.
This thermal treatment choice is relative to difficulties that present the grain size steel when to the scattering and the acoustic wave absorption to grains joints (greatly anisotropy steel).
This problem is especially accentuated for the austenitic type rustproof steels.
The grain average diameter is: where m is the grains number by surface unit.
The grain magnification treatments, the gotten by the relation m = 8 x 2 ^G are given by the picture 2.

		initial grain		thermal treatment element			final grain
Steel		G	d (mm)	Maintenance temperature	Maintenance	Cooling nature	G	d (mm)
	E24	11	0,0078	1150°	4h	furnace	6	0,0442
	XC 48	8	0,0221	1150°	4h	furnace	5	0,0625
	45SCD6	10	0,0110	1150°	1 h 30 min.	air	6	0,0442
	Z 6 CNDT17-12	6	0,0442	1200°	4h	air	4	0,0884
Table 2: Thermal treatment parameters, middle diameters and numbers grains

II-A- Micrographs
To discern the best treatments results undergone, micrographs have been achieved in order to put in evidence the grains structural magnification.

Fig 1: XC48 (X200) a- no treated G =8 ; b-treated G =5		Fig 2: 45SCD6 (X200) a- no treated G=10 ; b- treated G=6

Fig 3: E24 (X200) a- no treated G=11 ; b- treated G=6		Fig 4: Z6CNDT 17-12 (X200) a- no treated G=6 ; b- treated G=4

The metallurgic view point meadow-treatment analyses are:
• uniform structures single phased , ferrite and austenitic essentially constituted (E24 and Z6CNDT1712),
• ferrite - perlite structures (XC48 et45SCD6).
After thermal treatment observations are:
• dark pearliest spindles for the E24,
• twined austenitic grains and carbides for the Z6CNDT1712.

III. ULTRASONIC MEASURES:

Ultrasonic measures have been achieved in a immersion vat under normal impact in order to raise signals amplitudes, necessary for the attenuation determination by the multiple echo method.
The hover wave weakening is proportional to its amplitude and the course browsed.
While reasoning on the acoustic pressure, the attenuation coefficient writes :
a = (20/x). log(P₀/P) [dB/m] (with P₀ the pressure to the abscissa point x0 = 0)

III-A - The attenuation
Grains have a disorganized orientation, ultrasonic wave undergoes a diffusion more or less pronounced anisotropy degrees function and the grain middle size. brWhile considering the case of structures single phased, one distinguishes three domains for which diffusion coefficients are given:

1. domain of Rayleigh, for which the diffusion coefficient writes :
   for d <l /3,ad=Cm.F.ad³.f⁴;
2. high frequency domain, for which the diffusion law writes :
   for d>3l ,ad=Cm.Fa./d;
3. intermediate domain, characterized by the relation,:
   for l /3 <d <3l ,ad=Cm.Fa.d²; .

For the three domains Cm is the characteristic material constant, Fa the factor determining the anisotropy, d the grain average diameter, f and l the frequency and the length of wave.
For the steels Ivens proposed the empiric law as :
ad=100.(d/l)³.dB/m for a perlitic structure and ad=35.(d/l)³.dB/m
for a bainitic structure.
Concerning this law, the wave diffusion remains negligible for a grains size understood between l /1000 et l /100 and increases quickly between l /100 and l .
The diffusion effect is boring, it reduces shortcomings echoes amplitudes and the bottom in their submerging addition in a bottom noise; that can not be adjusted by a simple signals amplification as in the absorption case.

III-B- Results
In practice, the attenuation coefficient is determined while measuring corresponding successive signal amplitudes to the wave number go - return in the piece.
These signals are identified while measuring the time between every two consecutive signals top.
Curves (Fig. 5) representing the relative amplitude decrease according to the browsed course are illustrated for three frequencies: 5, 10 and 15 MHz.
The table gives all calculated essential values from the achieved measures.
It is place to note, for most cases, the average attenuation and the longitudinal speed wave increase for the treated steel.

		Average attenuations (dB/m )						longitudinal velocity (m/s)		Wave lengths (mm)
steel		5 MHz		10 MHz		15 MHz				5 MHz		10 MHz		15 MHz
		No treaty	treaty	No treaty	treaty	No treaty	treaty	No treaty	treaty	No treaty	treaty	No treaty	treaty	No treaty	treaty
	E24	55,76	61,36	74,34	86,04	91,09	129,70	6169	6341	1,23	1,27	0,62	0,63	0,41	0,42
	XC48	65,18	84,04	129,50	152,74	137,84	143,27	6077	6558	1,22	1,31	0,61	0,66	0,41	0,44
	45 SCD6	53,81	58,92	91,09	98,06	89,68	120,96	6564	6991	1,31	1,40	0,66	0,70	0,44	0,47
	Z 6CNDT 17 - 12	68,02	78,13	120,76	136,15	156,16	173,57	6059	5937	1,21	1,19	0,61	0,59	0,40	0,40
Table 4: Essentiel ultrasonic parameters comparative table

Fig 6: Treaty and no treaty materials attenuations Gap according to the frequency

With regard to the attenuation theory presented above, the application domain is the Rayleigh one being given the value of d (diameter of the grain) compared to the wave length.
These curves show the attenuation gap increase between steels treaties and no treaties thermally according to the frequency.
For it, histograms representing these gaps are represented by the figure 6 that shows that:

• outside the steel XC48 for which the very small attenuation incomprehensible gap to 15 MHz remainder, the three other steels present the same gap variation attenuation between the steel treaty and no treaty according to the frequency.
• Concerning steel Z6CNDT17-12, the three frequencies attenuations gap varies very little seen that the grain indication variation is weakest and bound to the presence of the titanium that pulls up magnification.

IV - CONCLUSION

The metallurgic factors play an important role in no destructive testing in general and in ultrasonic in particular.
In ultrasonic control it is essentially the grains dimension more that their orientation that creates difficulties in the alloys single phased soldering case as austenitic steel.
Some particular precautions must be taken, notably while doing to vary the frequency. [6]
Even though work only undertook present a theoretical study confirmation in ultrasound waves attenuation matter in materials, the tentative concerning its application to thick grains steels opens some interesting investigating perspectives and more especially in the no destructive testing domain.
In no destructive testing matter by ultrasound, it would be discriminating to study the grain magnification influence on the shortcoming detection sensitivity and resolution according to the frequency.

Bibliographic credentials:

1. metals and Alloys. H.DE LEIRIS Masson 1971
2. metallurgy and thermal treatments of metals. I. LAKHTINE. Mir 1971.
3. elastic wave in the strong. E. DIEULESAINT, D ROYER.
4. Ultrasonic testing of materials. J. KRAUTKRAMER, H. KRAUTKRAMER. 1983.
5. the control no destructive by ultrasonic sounds. J. PERDIJON. Hermès.
6. metallurgic bases of the H.GRANJON weldering.

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