Time of flight diffraction testing of austenitic stainless steel weldments at elevated temperatures

Abstract

Ultrasonic testing is a mandatory requirement during inservice inspection of the welds in the main and safety vessels of a prototype fast breeder reactor. Presently, conventional ultrasonic techniques have been proposed for use. Two of the main challenges likely to be encountered during ultrasonic testing are the high temperature of the vessels (around 423-473 K during shutdown) and the background radiation. Ultrasonic time of flight diffraction is now well established as a technique for NDT of thick (greater than 12.5 mm) weldments. The main advantage of using time of flight diffraction is its increased accuracy for discontinuity detection and fast scanning times. Conventionally, time of flight diffraction has been applied only for carbon steel weldments and at ambient temperatures. This paper highlights the successful application of time of flight diffraction for testing of stainless steel weldments at high temperature (423 K). Experimental studies reveal that the diffracted signal amplitude decreases and, hence, the time of flight diffraction image contrast decreases with increasing temperature. Additional gain was necessary to compensate for the decreased signal amplitude. Analysis of the experimental data indicated that the additional gain necessary to compensate for the lower signal amplitude varied nonlinearly (quadratically) with temperature. The errors in the discontinuity dimensional measurement were observed to be less than 5% compared to measurements at ambient temperature.