·Table of Contents
·Materials Characterization and testing
Assessment of Steels ageing by Rayleigh wave velocity measurementsC. Pecorari, X. Gros, B. Acosta, L. Debarberis, M. Beers and G. Manna
Commission of the European Communities, Joint Research Centre
Institute for Advanced Materials
PO BOX 2, 1775 ZG Petten, The Netherlands
Tel.: (+31) 224 565435, Fax: (+31) 224 561432
The phase velocity of Rayleigh waves propagating of the stress-released surface of thermally aged steel samples is measured by means of an interferometric technique. Results are reported which show that the Rayleigh wave velocity is sensitive to changes of the material structures brought about by the thermal treatment. Two mechanisms are offered as plausible explanations for the phase velocity change. The acoustic measurements are correlated with results obtained by other techniques.
Prior to cracking, and possibly consequent catastrophic failure, materials usually undergo transformations that result in the degradation of their physical properties. The generation of defects such as vacancies and dislocations, or the migration of metals with low melting temperature towards grain boundaries are examples of possible mechanisms eventually leading to loss of structural integrity of engineering materials and components. Understandably, issues of this nature are of great relevance to the operation of nuclear power plants insofar as their safety depends on the assessment the ageing process occurring to the plant's structural components.
The AMES (Ageing of Materials Evaluation and Studies) program supported by the DG XII of the European Commission provides the framework to investigate, among other issues on the above-mentioned subject, the suitability of several nondestructive techniques to assess material ageing induced by both thermal and irradiation treatment . In this work, measurements of Rayleigh wave phase velocity have been carried out to assess the sensitivity of this technique to the aging process induced by thermal treatment in JRQ steel samples. Two additional samples, one untreated (fresh) and treated (heat-treated) that had been especially introduced in the project to provide reference results were also tested. An attempt to provide a physical explanation of behavior of the Rayleigh phase velocity is offered. Finally, the measured values of the Rayleigh velocity are correlated to measurements of other physical properties of the same materials obtained by other techniques.
A schematic diagram shown in Fig. 1 illustrates the experimental set-up utilized in this investigation. Two surface acoustic wave transducers, operating at nominal frequencies of 2.25 MHz and 10 MHz, respectively, were used to excite Rayleigh waves in the samples of interest. The acoustic signals consisted of a train of sinusoidal oscillations containing about 15 cycles. The temporal length of the train was determined by the need to produce a wave that was as monochromatic as possible in order to avoid undesired
|Fig 1: Experimental set-up, and time-domain signals.|
The material used in this works is JRQ steel cut in small bars with dimension of 10 x 10 x 55 mm3. Prior to annealing for 1 hour at temperatures ranging between 400 oC and 750 oC, a series of samples was heat treated at 900 oC and subsequently water quenched. Of the two samples used as reference, the aged one was heat treated at 700 oC and then water quenched.
Figure 2 reports the values of Rayleigh phase velocity on the JRQ steel samples measured according to the procedure described in the preceding section. The results show an increase of the phase velocity with annealing temperature for the whole sequence with the exception of the last sample that was annealed at a temperature of 750o C. Similarly, Figure 3 illustrating the results obtained on the samples named fresh and thermally treated, shows a clear difference in the Rayleigh phase velocity measured in the two samples. In both figures the vertical bars indicate the maximum variation of the measured values.
|Fig 2: Rayleigh phase velocity versus annealing temperature for the JRQ samples.||Fig 3: Rayleigh wave velocity in the fresh and heat-treated reference JRQ samples.|
|Fig 4: Correlation between Rayleigh velocity values and STEAM measurements.||Fig 5: Correlation between Rayleigh velocity values and sample's hardness.|
Measurements of Rayleigh wave phase velocity have been carried out on thermally treated JRQ steel samples. The values obtained in this work indicate that the phase velocity is sensitive to changes of the physical properties caused by the treatment. The most likely physical mechanisms responsible for the increase of the wave velocity with annealing temperature are the segregations at the grain boundaries and the decrease of the average grain size. The velocity measurements presented here correlate well with those obtained by means of other techniques on the same samples.
This work has been carried out as part of the AMES-NDT project sponsored by the DGXII (Science, Research and Development) of the European Commission.
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