Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation

 

 

Material Properties - Case Study No. 117: Loss of Ball Temper from High Heat

 

To say that bearings can lose their strength at 3500 F. sounds preposterous when considering the fact that iron doesn't become molten until it reaches 2,8000 F. However, let's take a closer look.

Inner Race of Double Row Ball Bearing - Severe Deformation

One very reputable bearing manufacturer (of a bearing different than the one in this picture) actually derates their bearing at temperatures starting at 3400 F. At that temperature the design radial load rating decreases by 5%. At 3900 F. the radial load rating drops by 15%. By the time the bearing operating temperature reaches 5700 F. the design radial load has decreased by a whopping 40%! There you see, you can run the bearing at higher temperatures if you know you're going to run that hot in the first place. On the other hand, if you don't, then the bearing is going to fail because it is too soft to handle the added load.

Why does it get softer? It has to do with a microstructure know as martensite; the more martensite there is within the metal, the harder the material. Manufacturers get the martensite by rapidly quenching the molten metal to a temperature under about 7000 F. The benefit of martensite is that it is very hard (typical hardness 324ksi); the disadvantage is that it is quite unstable, as well as very brittle. It's a microstructure that really doesn't like being there and would rather be something else. As a matter of fact, if encouraged with a little heat, it will transform into other microstructures. The best temperature range where martensite will transform into other microstructures is between 300 and 3900 F. Manufacturers will purposely soften, or "temper," the martensite in this temperature range to get a compromise between strength and toughness. Some of the other microstructures that are formed, such as cementite, or bainite (typical hardness 247ksi), are softer and more desirable. Take a closer look at the bearing color on the left side raceway, toward the middle between the top and bottom of the raceway. Click on it to see a close up picture. That dark blue color on the bearing raceway was at a temperature of 6000 F., the martensite at the surface of the raceway transformed into something softer and lost about 10 to 20 points on the Rockwell C hardness scale.

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