Table of Contents ECNDT '98 Session: Materials Characterization

Investigation of the Austenitic Structures by Ultrasonic Spectral Analysis Petre Petculescu, Ovidius University Constanta Razvan Ciocan, Institute for Nuclear Research Pitesti Vasilios Bokas, T.B.A. S.R.L. Corresponding Author Contact: Petre Petculescu Ovidius University, Mamaia Ave. 124, 8700 Constanta, Romania, 04041-550064, 04041-672488 Email: petculescu@ovidius.ct.ro

TABLE OF CONTENTS

Introduction The description of the experimental system Results Conclusions References

Introduction

Ultrasonic spectroscopy denotes the methods used to analyze the frequency components of ultrasonic signals that are wideband pulses. When an ultrasonic signal traverses a medium, the frequency components associated with the input signal are altered. To produce the frequency domain information a discrete Fourier transform can be used. It is possible to study the effect of material properties on the alteration of the input signal subjecting to frequency-spectrum analysis the echo or transmitted pulse for different materials. A given input impulse will emerge having different frequency spectra after traversing dissimilar materials. Spectrum analysis essentially gives an ultrasonic"signature" that is generic to the particular microstructure type concerned.

An important application of the frequency-spectrum analysis involves the differential analysis of successive echoes. Each successive time domain echo will be more attenuated than its predecessor. In the frequency domain the ratio of the amplitudes for a series of frequency components represents the basis for determining a functional relation between the attenuation coefficient and frequency, (f).

The description of the experimental system

We have performed an ultrasonic system for the investigation of the material structure (USIS). The ultrasonic data are acquisited from US instrument via RS-232 interface. The program performs the saving of the radiofrequency ( RF) signals and the storing of all the investigation parameters.

The USIS program performs the following operations:

• Shifts echoes E₁, E₂ and E₃ to overlap by applying a coincidence criteria.
• Computes the time delay corresponding to the shift interval corresponding to overlap.
• Computes the propagation velocity.
• Computes the reflection coefficient
• Performs the Fourier transforms of the E₁, E₂ and E₃ echoes.
• Computes the spectrum amplitude ratios, ( f ) = A_i / A_i+1, over a specified frequency range
• Computes the diffraction correction required by the chosen configuration [3].
• Compute the attenuation coefficient
• Fit the experimental dependence ( f ) through a theoretical dependence chosen adequately
• Display the results obtained

The output of the USIS program consists of the following information:

• The sample thickness
• The attenuation determined at peak frequency
• The propagation velocity
• The calculated polynomial coefficients of the proposed theoretical dependence ( f ) ( obtained by the fitting of the experimental results ).
• The values of the attenuation obtained from the echo amplitude ratio
• The computed diffraction corrections
• The attenuation values in the considered frequency range We present the front panel of the USIS program in figure 1.

Results

In order to evaluate the performances of the USIS program we have investigated the samples from steels of three kinds:

• W.1.4541 ( X10CrNiTi18.9) with grain size 5-6;
• W.1.6903 ( X10CrNiTi18.10) with grain size 6-7;
• HP50, a stainless steel having the following chemical composition: Cr 24-28%, Ni 33-37%, C 0.35%-0.45% and stabilized with Niobium. The metalografical analysis of this sample has shown a structure with austenitic dendrite and niobium carbides.

We have compared the grain sizes evaluated of each sample from methalographical images (ASTM E112) and those evaluated using the data given by USIS program. Each sample was investigated in different positions. The velocities determined on each sample are showed in figure 2.

Fig. 2: The velocity values obtained at the investigation of the three samples in different points

The grain size evaluation using the ultrasonic data is based on the Lifshits and Parkhomovstkii theory [1]. The attenuation coefficient for cubic crystallites in Rayleigh scattering range can be written in the following form:

sL(mm -1) = sL,L + sL,T = (8 2/375) (T f 4 A2)/ ( o2 vL3) ( 2/ vL5 + 3 / vT5) =S D3 f 4

(1)

where we have used the following notations:

f - the frequency;
A- the anisotropy factor , A= c₁₁-c₁₂-2c₄₄ , (c₁₁, c₁₂ , c₄₄ are the elastic moduli for cubic materials );
v_L , v_T are the longitudinal and transverse wave velocities
D - the average grain size;
T- the average grain volume ;
S- proportionality factor.

The relation (1) was written taking into account the mode conversion at scattering process too, for an incident longitudinal wave. The grain size was obtained by fitting the experimental data with the following dependence [2]:

( f ) = c1f + c4 f4

(2)

In this relation we have considered supplementary and the elastic hysteresis loss - c₁ f. The coefficients c₁ and c₄ have been determined by a least squares fit algorithm implemented in the USIS program. The grain size was determined by equating the coefficient of f⁴ from equations (1) and (2). The S factor has been calculated assuming that the elastic coefficients of the three considered kinds of steel have the values equal with that for 316 steel [3]. The 316 steel has been studied intensively and therefore its elastic coefficients have been available. We have presented in figure 3 the values of attenuation obtained for each sample. The values are calculated including also diffraction corrections calculated as presented in [4]

Fig. 3: The attenuation values obtained at the investigation of the three samples in different points

Types of Steel	Grain size obtained by methalography	Grain Size obtained by ultrasonic spectroscopy(µm)
W.1.4541	5-6 (45mm-65mm)	31.20655
W.1.6903	6-7(32mm-45mm)	27.50247
HP 50	-	93.48207

Table 1: The values determined by metalography (ASTM E112) of the
grain size compared with those obtained by ultrasonic spectroscopy

Conclusion

We performed a new system based on ultrasonic spectroscopy to investigate the material structure ( USIS). The experimental results obtained with USIS in grain size evaluation have been compared with those obtained by methalographical analysis (see table I) . The good agreement between them prove that USIS can be used in nondestructive evaluation of grain size for austenitic materials . It is necessary to point out that this evaluation can be performed automatically too. We can also solve with USIS the reverse problem: using the determined grain size we can evaluate the elastic coefficients.

References

1. Pappadakis E.P., Scattering in polycrystalline media in Methods of Expeimental hysics,Vol. 19
2. Goebels K., Structure Analysis by Scattered Ultrasonic Radiation in Research in NonDestructive Testing , Vol. 3
3. M. G. Silk, Ultrasonic Techniques for Inspecting Austenitic Welds in Research in NonDestructive Testing , Vol. 3
4. J. Krautkramer, H. Krautkramer, Ultrasonic Testing of Materials Springer-Verlag, 1990