Examining Bearing Steel Microstructures
Bearings are quite essential to the performance of rotating machinery across a wide variety of industries. The Timken Company designs and manufactures bearings known for high reliability in demanding applications, like wind energy, mobile and process industries. We strive to continually deepen our understanding of how our products perform in these growing markets.
For example, in the last decade, some wind turbine bearings have experienced premature damage due to white etching cracks (WECs) formation in the bearing races. WECs are a kind of damage mode in bearings, named due to their white appearance of cracks under an optical microscope. WECs often precede unexpected flaking and spalling of raceways. However, Timken bearings have been known to endure this kind of damage and offer longer lives due to their material characteristics. We know our steels and associated metallurgical processes result in better product reliability, but why and what are those underlying mechanisms?
Metallurgy is a core competency at Timken. So, we decided to dig deeper and determine the mechanisms for this superior material performance. In this regard, we have life tested different materials, heat treatments and microstructures in relevant laboratory scale tests and found that the performance is related to our heat treat parameters —specifically how much carbon is forced into the steel during the heat treat.
To further understand the mechanisms on atomic level, the steels are studied “in-situ” using high energy X-Rays during heat treatment and during tensile loading. I worked with researchers in the UK to examine how the atomic arrangement changes in a bearing steel during heat treatment and also during the loading and unloading.
The in-situ studies during heat treatment provided insights on how strengthening phases (martensite or bainite) form in the bearing steel and how those phases respond to elevated temperatures. In addition, we focused our study on comparing load responses of bainite and martensite microstructures (with equal amounts of retained austenite) using in-situ synchrotron x-ray diffraction.
Our findings suggest that atomic lattice characteristics of major strengthening phases (martensite /bainite) determine the stability of microstructure under mechanical load. It is further realized that the amount of carbon entrapped in the atomic lattice of the strengthening phase and associated lattice distortion seem to be playing key role in bearing steel reliability. Our internal bearing life test results also support these correlations.
Understanding these mechanisms reveals how and why different steel microstructures perform differently in a given application, deepening our product knowledge. With this information, we can further optimize our products for better life and cost as per the application requirements.
Read the full papers here: