Volume 3, Issue 7 (July 2006)
Statistical Analysis of Nonmetallic Inclusions for the Estimation of Rolling Contact Fatigue Range and Quality Control of Bearing Steel
The cleanliness of bearing steels is a critical factor that influences directly the rolling contact fatigue performance and other mechanical properties of bearings. Hence, the evaluation of nonmetallic inclusions is a very important issue for steelmakers, bearing producers, and users. The material qualification procedure includes classical inclusion rating and rolling contact fatigue tests as acceptance tests for evaluating steel suppliers and ensuring the required bearing performance. The evolution of steel cleanliness had led to an increase in fatigue life so that running time of rolling contact fatigue tests have been considerably extended, until they became too time and cost consuming for a realistic qualification. In this paper, quantitative metallographic evaluation of the inclusion cleanliness used to collect data on the size, type, and expected number of inclusions in small samples is presented. The data were analyzed using Statistics of Extreme Values (SEV) and the results were used to extrapolate in larger samples representative of actual volumes of material involved in rolling contact fatigue tests and were related to the fatigue life of bearing steels from different suppliers. This method gives useful and reliable statistical data on inclusions for evaluating the performance level under rolling contact fatigue and defines the required cleanliness characteristics for bearing steels qualification. Other information of this work is the ability of the statistical inclusion rating to serve as a substitute for rolling contact fatigue tests to forecast the performance level of bearing steels.