CHARACTERIZATION OF THE CEMENTATION LAYER BY THE ULTRASONIC PULSE ECHO METHOD
B.Moulti, N.Tala Ighil
Laboratoire de Caracterisation et d’Instrumentation
Centre de Recherche Scientifique et Technique en Soudage et Contrôle BP 64. Route de Dely Ibrahim, Cheraga. Alger. Algérie
moultib@yahoo.com
ABSTRACT
The cementation layer measurement is an important parameter in process control. In this case, it is necessary to measure this layer with a sufficient precision.
In this study, we use the ultrasonic pulse echo method as a non destructive testing in the cementation layer characterisation of 12 NC6 steels with a rang of 0.6 mm to 1.5 mm for the cementation thickness.
The ultrasonic measurement are based on the reflection of signal at the interface of two materials having different impedance and on the determination of the longitudinal elastic waves velocity as well as the sound attenuation coefficients based on the Rayleigh model.
The paper discuss also the microstructure influence on the ultrasonic measurement parameters and the precision obtained.
We conclude our work by analysing the ability of the ultrasonic pulse echo for the cementation layer measurement.
Key words
Cementation process, ultrasonic attenuation, reflection coefficient, pulse –echo method.
I. INTRODUCTION
Cementation process is used in the manufacturing industry for different application. Measuring the case layer thickness is vital to quality control of machining work piece, especially when we use a non destructive technique compared to the destructive one, which requires interruption of the machine process and may make miro-scratches on surface.
Frequent mechanical and metallurgical testing is needed to ensure that the required quality is maintained, and that specification are met. However, a truly
representation of real cementation process is obtained by the non destructive testing.
Some works have been reported studying non contact techniques for the assessment of this process using the Eddy current method and Rayleigh waves [1].
The advantage of using the ultrasonic technique compared to the eddy current is that this method is independent of substrate material and can measure the individual layer [2]. It is necessary to develop an NDT method that could identify the matrix structures for quality control and reliability purposes.
The Purpose of this paper is to experimentally study the correlation
between bother the acoustic attenuation and reflection coefficients of longitudinal Ultrasonic waves propagated in samples having different case layer thickness.
II. THEORETICAL BACKGROUND The measurement of ultrasonic velocities depends upon generating a dynamic pressure wave into a material of known thickness and measuring the transit time of emerging acoustic pressure wave.
The generation and detection of an acoustic wave is usually accomplished by a piezoelectric transducer. The shape of the pulse, generated by the electronic pulsar, has a major influence on the pressure wave induced in the material.
II.1. Reflection of the ultrasonic wave Reflection of the ultrasonic wave occurs at the interface of two different acoustic impedance. The acoustic impedance is defined for bulk materials where the sound propagation is normal to the transducer to specimen interface as ρV1, where ρ is the mass density and V1 is the longitudinal velocity. The general equation for the sound pressure Reflection coefficient is given by:
R= ((Z2/Z1) – 1) / (( Z2/Z1) + 1) (1) Where r = Z2/Z1 is the acoustic impedance ratio for pressure waves
Using the echo relative amplitude
R= [( A2 A0) / (A12 + A0 A2)]1/2 (2) AN is the echo number
II.2. Attenuation of the propagating waves
Sound pressure p is expressed as:
p(x) = p0 exp( -α x) (3)
where x, p0 and α denote propagation distance of ultrasonic wave, initial sound pressure and attenuation constant,
respectively. The attenuation constant α is proportional to the power of frequency w.
With increase in frequency, sound pressure becomes smaller. Because of surface roughness and porosity of cementation material, higher frequency components of ultrasonic wave attenuate.
There are three basic processes that account for stress pulse energy loss, namely, beam spreading, absorption and scattering [3]. The first, the second and the third interface echo A0 , A1, A2 are equal to:
A0 = R
A1 = (1-R2 ) e-2αL A2 = R’(1-R2 ) e-4αL
R’ is the phase reversal on the second reflection: numerically the same as R but reversed polarity.
L is the length of the sample.
α = ln [(A2 A0 /A12 + A0 A2)1/2 A1/A2]/2 L The previous equations are used in the calculations of the ultrasonic parameters due to the cementation process, keeping other variables unchanged.
III. EXPERIMENTAL TECHNIQUES III.1. 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 [4]. Heat treatment times ranged from 7 to 24 hours. the process follow the Fick law :
2 C O CO2+ C
CH4 2 H2 +C
CS – C / CS – C0 = erf (x/2 (Dt) (Fick law) erf(u) = 2/(x)12∫ exp (-u2) du
III.2. Ultrasonic measurement
Pulse Echo technique with A- Scan presentation was used in the measurement by the ultrasonic immersion system.The longitudinal ultrasonic waves were generated and applied by Krautkramer straight beam probes with the frequencies of 2.25Mhz, 5Mhz and 10 Mhz respectively. Much consideration went into the choosing of the probe as well, due to the changing sensitivity of the ultrasound frequencies to different layers. For smaller layer, the higher frequency of the ultrasound is required to detect it.
Unfortunately, if the frequency is too higher, the rate of signal dampening in a material became important. To reduce the dampening of ultrasound within the sample, lower frequency probes are preferred, however, the performance of the probe is compromised.
The main testing equipment used is the described in the standard procedure of
“Normal beam pulse –echo” procedure [5].
The integrated data acquisition and waveform analysis system consist of transmitter/ receiver (T/R) ultrasonic instrument, a digital storage oscilloscope read out a software package. The resulting data is then represented in various forms with the use of software developed in the laboratory.
IV. EXPERIMENTAL RESULT AND DISCUSSION
The numerical values for surface cementation are obtained from the ultrasonic waves are then used, as independent variable to illustrate the main important parameter to characterise this process. The cementation layer responses of ultrasonic signals of the A-Scan are obtained for this experiment in figure 1.
The reflected signals show a little energy loss pulse attenuation due to the tested materials property, but it can not identify
material properties variation directly on the reflected signals.
1) – Reference sample
2) - 0.5-0.7 mm cementation layer
3) – 1.1-1.3 mm cementation layer Fig.1: The pulse–Echo of the ultrasonic waveforms For different cementation depth.
0,00000 0,00002 0,00004 0,00006 0,00008 0,00010
-4 -2 0 2 4 6
f = 2,25Mhz
Amplitude (V)
T im e of flight (s)
0,00000 0,00003 0,00006 0,00009 0,00012 0,00015 0,00018 -6
-4 -2 0 2 4 6
f = 5Mhzf = 10Mhz Amplitude (V) 0,00000 0,00002 0,00004 0,00006 0,00008 0,00010 0,00012 0,00014
-6 -4 -2 0 2 4 6
Amplitude (V)
0,0 0 0 00 0,0 0 0 02 0,0 0 0 04 0,0 0 0 06 0,0 0 0 08 0,0 0 0 10
-4 -2 0 2 4 6
f = 2,25Mhz
Amplitude (V)
Tim e of flight(s)
0,0 0 0 00 0,0 0 0 03 0,0 0 0 06 0,0 0 0 09 0,0 0 0 12 0,0 0 0 15 0,0 0 0 18 -6
-4 -2 0 2 4 6
f = 5Mhzf = 10Mhz
Amplitude (V) 0,0 0 0 00 0,0 0 0 02 0,0 0 0 04 0,0 0 0 06 0,0 0 0 08 0,0 0 0 10 0,0 0 0 12 0,0 0 0 14
-6 -4 -2 0 2 4 6
Amplitude (V)
0,00000 0,00002 0,00004 0,00006 0,00008 0,00010
-4 -2 0 2 4 6
f=2,25Mhz
Amplitude (V)
T im e of flight (s)
0,00000 0,00003 0,00006 0,00009 0,00012 0,00015 0,00018
-6 -4 -2 0 2 4 6
f=5Mhzf=10Mhz
Amplitude (V) 0,00000 0,00002 0,00004 0,00006 0,00008 0,00010 0,00012 0,00014
-6 -4 -2 0 2 4 6
Amplitude (V)
In the frequency domain, the amplitude ratio for a series of frequency components forms the bases for the deduction functional relation between the attenuation coefficients and the frequency.
After obtaining ranges A0, A1 and A2
corresponding to the selected echoes, the results are graphically displayed indicating the relationship between the attenuation coefficient and frequency is shown in figure 2.
It is evidence from this graph that increasing in the frequency leads to an appreciate increase in attenuation of the ultrasound waves.
A model was developed to estimate the attenuation coefficient as a function of frequency, this model have the form :
α (f) = a0 +a1 f +a2 f2 (5) The model’s coefficients which are experimentally determined are given in table 1
Cement.
Depth d (mm)
a0 a1 a2
0 16.35038 0.38269 0.00545
0.6 17.01575 0.48437 0.0069 0.8 16.61792 0.71362 0.01016 1.0 20.27796 1.51301 0.05812 1.2 24.12747 0.1106 0.08123
Table 1 : model coefficients
Fig.2: The frequency effect on the attenuation of ultrasonic waves The visualisation from figure 3 presents the determined attenuation coefficients of the ultrasonic waves and the cementation depth. The results of this relation show, that as the value of the cementation layer increase, the attenuation coefficient increase. This behaviour can be attribute to the important loss in stress wave energy upon in transmission across the case hardened layer.
We propose a quadratic model to describe the experimental results, giving the expression of the attenuation coefficients α (d).
α (d) = b0 +b1 d+ b2 d2 (6) Frequency
(MHz) b0 b1 b2
2.25 17.22483 -4.80278 9.59169 5 18.29749 -3.01646 8.98891 10 20.6947 -4.63807 12.8530
Table 2 : Experimentally coefficients
2,5 5,0 7,5 10,0
15 20 25 30 35
α (db/m)
f (Mhz)
0 mm 0,5-0,7mm 0,7-0,9mm 0,9-1,1mm 1,1-1,3mm)
Fig.3: Cementation layer effects on attenuation of ultrasonic waves.
The following figure 4 summarised the different results obtained from the ultrasonic waves propagation for each test specimen.
Fig.4 :Ultrasonic wave attenuation as a function of frequency and cementation layer
V.CONCLUSION
We presented in this paper, the result of ultrasonic studies of cementation layer and we introduced a developed analysis models. From the experimental results obtained through A-Scan method, we come to conclusion that the effects of cementation process in relation with some ultrasonic parameters such as the attenuation coefficient and the frequency dependence can be evaluated. It was found, that in one hand, cementation process is greatly degraded the reflected echoes of tested samples. On the other hand, the ultrasonic attenuation coefficients are proportional to the cementation surface depth and the work frequency.
The ultrasonic technique control is based on a stable physical foundation. With the aid of this technique many practical problems concerning the coating and surface treatment could be solved [6].
REFERENCES
[1] F. LELEUX AND C.FLAMBARD.
CETIM Inf. Avril 1970.
[2] J.BUCHLE, T.PAGEL. Roma 2000, 15th WCNDT.www.ndt.net.
[3] E.P.PAPADAKIS, Phys.ac.
W.P.Mason, P.277, 1968.
[4] PRECIS DE MATALLURGIE. Edition 1981.
[5] J. DORING, J.BARTUSCH, J.
MCHUGH, W. STARK.Contribution to Ultrasound Cure Control for Composite Manufacturing. Roma 2000, 15th
WCNDT.www.ndt.net.
[6] M.ABDELHAY, I.M.MUBRK
www.ndt.net. April 2004. Vol.9 No 04
0,0 0,2 0,4 0,6 0,8 1,0 1,2
15 20 25 30 35
f = 2,25 Mhz
d (mm)
α (db/m)
15 20 25 30 35
f = 5 Mhz
α (db/m)
15 20 25 30 35
α (db/m) f = 10 Mhz
0,0
2,5 5,0 7,5
10,0 12,5 15
20 25 30 35
0,0 0,2 0,4 0,60,81,01,21,4
α (db/m)
d (mm) f (Mhz)
