The Timken Company, a manufacturer of alloy steel and bearings, has developed a 15 MHz ultrasonic inspection method that correlates steel cleanness to bearing fatigue performance. It is used to qualify worldwide bearing steel suppliers for cleanness requirements, to monitor their compliance and qualify process changes. This method has permitted the appropriate steel cleanness to be selected for bearing applications. Through Continuous Improvement (CI) methodology, steelmaking productivity advancements have occurred along with advancement in steel cleanness. These efforts have led to 4 orders of magnitude steel cleanness improvement, and nearly 20 times bearing performance improvement over the past 15 years.
Comparative life tests of tapered roller bearings in mineral oils and synthetic fluids, Mil L-7808-F and Mil L-23699, demonstrate a strong influence of lubrication upon bearing life and failure characteristics. The inclusion origin mode of failure propagates very rapidly and takes on the macroscopic appearance of the point surface origin mode when fatigue cracks interact with a lubricant having low viscosity at the operating temperature. Observations indicate that bruises from debris in the lubricant are stress raisers of severity equivalent to nonmetallic inclusions in bearing materials for nucleation of contact fatigue cracking. Surface fatigue by “peeling” or superficial pitting can also act as the nucleus of fatigue cracks which propagate in the same manner as the point surface origin mode. In general, any surface stress concentration in combination with low lubricant film thickness promotes surface origin modes of contact fatigue and low lubricant viscosity at the operating temperature promotes rapid propagation by a hydraulic pressure mechanism.
The internal quality of steel is one of the critical factors in the performance of bearings. A multi- national company using steel from many sources around the world must have a detailed and comprehensive program for evaluating steel suppliers to assure ongoing, consistent bearing performance. There are many techniques that have been developed and used over the years such as blue fracture test, microscopic evaluation, oxygen analysis, sophisticated ultrasonic techniques and longer range tests such as fatigue life testing. The control of the process steps in making steel is a critical part of clean steel technology. Timken has developed an ultrasonic technique as well as specific sample preparation requirements to assure the quality of steel will meet the stringent requirements of bearing performance. Recent data from evaluation of many world steel suppliers will be summarized to demonstrate the importance of assuring clean steel. Ultrasonic data from several configurations of samples as well as reduction ratios will demonstrate the importance of a tightly controlled evaluation program.
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