Comparison of ultrasonically and metallographically indicated sizes of steelmaking defects in a large 3·5Ni-Cr-Mo-V rotor forging

Abstract

Fracture-mechanics acceptance procedures for steelmaking defects in components require an accurate non-destructive determination of the defect size. In large components this is only possible using ultrasonics and the equivalent reflector (AVG) system has become accepted for this purpose in cases, such as steam turbine and generator rotors, where the critical defect size is significantly smaller than the ultrasonic beam diameter. To assist in evaluating the AVG procedure, detailed metallographic and ultrasonic examinations of eight non-metallic inclusions in a low-pressure steam turbine rotor forging have been carried out. Simple inclusions were accurately sized by the AVG system, but more complex groupings of inclusions were underestimated in size by factors of up to 5 in equivalent diameter if assumed circular. Infact, the defects examined were elliptical with a typical aspect ratio of 1:3, comparable to that normally assumed for fracture-mechanics calculations.