Austenite/Austenitization... 1

[AshleyK]

I'm currently researching a succession of bearing failures as part of a work-related project and I was told that the current theory is based around austenitization (austenitisation, for the UK?) of the bearing material. However, after some Googling and looking through a few of my engineering handbooks I haven't really found any evidence to back the theory.

Our alarm system warns us when the thermocouple located around 2 inches from the bearing's outer race reads above 100C (I've seen some on the alarm system reading 130+ before we've managed to stop the system). Obviously the bearing itself would be warmer than that, and having seen the discoloured and compacted grease I can believe they are getting too hot to retain their clearances, but from what I've read about austenitization the temperatures needed are 900C+, which is surely way too high? If this was occurring, somehow, is it true that the bearing never returns to its original dimensions once austenitized, and will eventually keep getting larger every time this occurs?

Has anyone got any experience of austenitization? An explanation in layman's terms would be very welcome.

Many thanks in advance,
Ashley.

 

[lukerobert]

Austenite is a crystal that starts to form when the metal surpasses 723C. For bearings, I believe they would fly apart at this temp. I don't really see a correlation between austenite formation and bearing failure. IMO, anytime a bearing gets that hot, it is toast.

 

[metengr]

Just a clarification to the above, austenite is a phase which forms when a certain temperature is reached during heating.

http://en.wikipedia.org/wiki/Austenite

 

[NickE]

You're not reaching austenitization temperatures, well it's highly unlikely. Usually by that point the steel has much less physical strength and stiffness.

What could happen is the bearing steel, (52100?) is tempered around 150C, if your service temperature exceedes that you see tempering of the steel and softening, with consequent reductions in yeild and tensile strength. Once that happens the failure mode of the bearing depends more on the service stresses. I could see the races stretching due to the hoop stress, but the steel doesn't actually "grow".


Nick

 

[Metalguy]

How are the bearings failing? Ball, roller, needle, or what?

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[bearingsteel]

Austenitization is the first part of the heat treatment called quenching(followed by tempering), when steel is heated in furnace at around 945 degrees Celsius for 1, 1,5 hours.
After this, it is quenched in oil, and then it's tempered at around 170-200 degrees Celsius.
When quenched, the microstructure transforms almost instant from austenite into martensite, thus hardening.
Depending on the thickness of the ring, bar, etc. , not all austenite transforms, the remaining austenite is called retained austenite.
Because of retained austenite, in time, the probe increases in size.

 

[jwhit]

If I remember the transitions correctly, over time and depending on temperature, any retained austenite will transform to untempered martensite. Untempered martensite is a relatively brittle phase and could cause catastrophic failure of the bearing. If you are seeing retained austenite after tempering, an additional tempering cycle may transform the retained austenite to tempered martensite. The tempered martensite is much tougher and more ductile, thus less prone to catastrophic failure.

 

[dbooker630]

You can have localized austenitization that is associated with grinding burn on the race or ball during manufacturing. Such conditions can cause spalling of the bearing which could lead to overheating.

 

[AshleyK]

Thanks very much for all the replies so far.

I can't believe our bearings have ever got to anything above 200C, let lane the 700+ needed for austenitization. I'm fairly sure this theory has come from our technology department though, where I'm assuming a removed bearing has been analysed. Is it possible for a manufacturing fault to leave this crystal structure in the bearing material? The only other thing which could cause it is possibly when we heat the race up to remove a failed bearing, but even then we don't get them anywhere near hot enough to start seeing clear colour changes in the race material. All in all, this is looking more and more like a red herring.


Our bearings are 6-series (6335? maybe) roller bearings. I'm not really sure that they physically fail, to be honest, we just react to the high temperature alarms by either replacing the bearing or by, at the very least, cleaning out all of the discoloured grease and replacing it if it looks like we've caught the overheating before too much damage has been done.

Any further help would be much appreciated. I'm not really sure where to go with this investigation now.

Thanks again for all the help so far.

 

[metengr]

I think for you to move forward with this investigation you need to have any metallurgical lab information sent to you so that you can determine the mechanism of failure, not the root cause of failure.

This part of the investigation deals with review of the lab results followed by review of operating data and maintenance history. I can't tell how many times I have been involved with failure investigations and the personnel in the metallurgical lab think they know the root cause of failure. Unfortunately, conducting a proper root cause of failure comes with experience and knowing that lab results is only one part of the overall investigation.

 

[Guest102023]

With any failure involving lubrication, have the oil/grease analyzed, as well as any hard deposits found. Discolouration could be caused by several factors including oxidation. Based on the limited information provided I am doubtful the bearing reached the austenitization temperature range.

Better yet, establish an oil monitoring program; it is cheap and easy and the lab will track all trends related to performance and which could point to failure.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"