Recent Evaluation Procedures of Nonmetallic Inclusions in Bearing Steels (Statistics of Extreme Value Method and Development of Higher Frequency Ultrasonic Testing Method)

Abstract

Large microscopic inclusions from 20 to 100 μm in diameter have recently been paid attention from a viewpoint of fatigue strength of steel products. As the inclusion detection capability, or probability, is practically limited by a conventional optical microscopy, however, the evaluation of the distribution of these inclusions used to be feasible only by extraction method such as acid-solution procedure. It has been found that ultrasonic testing with 50 to 12S MHz frequency, focus-type transducers is suitable to evaluate the inclusions where the echo intensity is proportional to inclusion diameter. This paper will describe recent inclusion evaluation procedures and, as a main body, the results of the application of the new ultrasonic testing system. Included are: The relationship between oxygen content in steel and maximum inclusion diameter in steel predicted by statistics of extreme value with the aid of microscopic image analysis, and The inclusion diameter distribution measured directly by the ultrasonic testing