Defects diagnosis in forged rotors by means of ultrasonic imaging methods
As turbine diagnosis aims at localizing and differentiating the size and distribution of pointlike inclusions with maximum possible lateral resolution, all relevant ultrasonic echoes diffracted or back-scattered by the inclusion have to be acquired by an automatic data acquisition unit. Owing to the fact that SAFT (Synthetic Aperture Focussing Technique) neither uses an amplitude threshold nor rectifies the A-scans, this imaging method has been used for the investigation of a forgin with 780 mm diameter. Nonmetallic inclusions of circular or oval cross section located very close to each other were detected in the disc. The shape and size of the nonmetallic inclusions, consisting of a mixture of Al2O3 and CaO, are clearly visible in the two cross-sectional images. The individual indications are of longitudinal shape, measuring up to some 10 mm in length and between 0,2 and 0,3 mm in thickness. They are located at a distance of 1-3 mm from one another.