Improvement in Efficiency of Ultrasonic Tests for the Macroscopic Inclusions Evaluation

Abstract

Rolling-contact-fatigue life is a key characteristic of bearings. Reliable bearings are expected to have long L10 life as a bearing performance index, as well as long commercial life without unexpected premature failure on any pieces of bearings in use. It has long been known that cleanliness is an important material factor to determine the fatigue life. The aforementioned early failure could be caused by macroscopic inclusions, which have small density of presence in recent clean steel and, thus, detection for those inclusions requires evaluation on a large volume of steel. Although higher-frequency ultrasonic tests have advantages for non-metallic inclusion detections, relatively low frequency of 10- to 20-MHz ultrasonic tests are useful for both evaluation efficiency of large volume and for detection performance of macroscopic inclusions. The authors apply a focused-beam-type 15-MHz ultrasonic test to detect 100-μm or larger macroscopic inclusions, where the efficient evaluation is needed because of the assessment for a large volume of steel. In this paper, the basic characteristics of several procedures of ultrasonic tests will be discussed from the viewpoint of the evaluation efficiency for macroscopic inclusions.