Evaluation of induced defetcs in AISI 304 a steel test pieces using conventional and non conventional ultrasonic inspection methods

The use of patterns of reference in the evaluation of pieces by non destructive methods, particularly with ultrasonic techniques, is indispensable because the features of this pattern's defects can be correlated with those found in pieces that are in service. In the industry, stainless steels, particularly Type 340 austenitic steels, are widely used in some systems for their excellent mechanical properties in corrosive mediums. But these same pieces may have discontinuities and defects in the welded regions, that occur mainly during production and installation and are also due to subsequent operating conditions. Volumetric inspection techniques reveal the integrity of a material in its thickness and detect internal discontinuities that are not visible on the surface of the piece. Conventional and non conventional ultrasonic techniques are used most often now and among the non conventional techniques is Ultrasonic Spectroscopy [1-2]. Ultrasonic spectroscopy aims to determine the dependence of the properties and characteristics of the material under study with the frequency. These may be geometric in origin (thickness of layers, size, shape and direction of the discontinuity) or inherent (attenuation, dispersion, absorption). The dependence of the frequency is usually connected to some microscopic geometric properties, such as the grain size in polycrystalline materials. In the characterization and detection of discontinuities, the frequency spectrum of an ultrasonic pulse contains information about the shape, size and direction of a discontinuity [1-3]. Therefore, the ultrasonic spectral analysis is very important for the characterization of materials and contributes to the study and evaluation of the ultrasonic signals [4-6]. This study evaluates the behavior of the ultrasonic signals obtained from patterns of welded pieces of AISI 304 stainless steel by using conventional and non conventional methods of ultrasonic inspection. Welded test pieces of AISI 304 stainless steel were produced measuring about (180x150x12.5) mm and (150x120x10) mm and defects occurring from TIG welding processes were generated. The contact pulse-echo method was used for the ultrasonic inspection of the pieces, with transmitter receptor paddles each measuring 0.250 inches, with an incidence of 45o and frequencies of 5 and 7 MHz . The signals were digitized with a 60 MHz Tektronix Oscilloscope model 1002 and recording length of 2,500 points per channel and the signals obtained were processed with MATLAB 6.5. With the tools contained in this software, the spectral analysis for each of the signals was carried out, including the determination of the amplitude spectrums, the power spectral density, total power, band width as well as the frequency range. Different characteristics were defined and established quantitatively, that became the basis for the characterization, evaluation and detection of the discontinuities in each of the test pieces evaluated. The ultrasonic signals were analyzed as well as the spectrums from each one of the defects and the behavior of each of the signals was evaluated. The quantitative evaluation of the frequency spectrums established the effect that size, shape and direction have on the defect in the behavior and shape of each of the spectrums. The amplitude and power spectrums display very specific behaviors in these materials and they vary according to the frequency of the paddle used, enabling the frequencies recommended in the detection and location of each kind of defect in each one of the test pieces to be established (CW)