Fretting Fatigue 15-5
Fretting Fatigue 15-5
(OP)
Hey all,
Been lurking for a few years and can usually find what I'm looking for by searching. But I'm stumped on this one and would really appreciate some input from the experts.
I've got a valve body (15-5 Rc= 36-38) and valve stem (17-4 Rc= 36-38)used for pumping fairly clean high pressure process water (40k). During the valve travel, the stem seems to be fretting the pressurized body bore. The main bore of the body is ballised post machining. We have recently experienced several failures with the fatigue failure with the crack origins in the fretted main bore. Cavitation is not likely as the system would have indications elsewhere. I've attached a picture of the bore in the area of interest. The material is Chinese (unfortunately) and the MTR's seem ok, and of course the 3rd party micrographs don't tell much of a story either.
We have made quite a few of these units over the years and have few failures. Suddenly we are having a rash.
I've looked into the ferrite stringers and banding, seems that what I've read 15-5 is less prone to this, but I could be wrong. The application and use of this valve has not been a problem with moderate fretting. I seem to be stuck on the fact that the fretting takes off the Cr2 03 and since its in water the oxide layer can't reform. There is some discoloring in the fretted area (right side of image), and I am not sure if it is material transfer from the stem to the bore, or corrosion taking place due to the absence of the protective layer.
So my question(s) are:
Is there a significant mechanical effect of high pressure water on this newly unprotected surface? Or could this type of failure linked more closely to the residual stress present due to the ballising and acting like a SCC problem?
Thanks for reading.





RE: Fretting Fatigue 15-5
RE: Fretting Fatigue 15-5
RE: Fretting Fatigue 15-5
Aaron Tanzer
www.lehightesting.com
RE: Fretting Fatigue 15-5
RE: Fretting Fatigue 15-5
I questioned calling it galling or fretting it but it does seem to me like either way its mechanical damage with slip stick behavior lowering the fatigue strength. The indications are mostly parallel with the bore but some do run circumfrencially (bottom of image right side). There is plenty of radial clearance between the parts and when the unit is operating it is possible for this valve to experience thousands of vibratory cycles without actuation.
I apologize for the picture quality the flash/focus weren't cooperating with me. I've attached some more images I captured with a USB microscope of another valve of a similar failure mode. It looks like mechanical damage initiated a spot for washout (jetting) to occur. Direction of flow is top to bottom in the picture of the sectioned valve.
The valve in my first post did not experience the jetting and consequential material removal. It did however share the same contact appearance and longitudinal running crack that appears to originate in the transition between the damaged and undamaged portion of the bore. Transition zone is out of the frame towards the bottom of the image. (more pictures to follow).
Mr. Failure is correct there is some red/copper/gold transfer coloring in the severly abraided regions. Do you see the material removal/smearing in the linear direction as well as some indications in the circumferential direction?
metengr,
The inlay idea sounds like its worth looking into, have you had any success with it?
Thanks again.
RE: Fretting Fatigue 15-5
Before jumping off the cliff to re-engineer the current problem I re-reviewed your OP and found this statement to be significant…This statement above indicates a process change has occurred resulting in the rash of failures. A root cause analysis approach needs to be implemented to determine what process change(s) have occurred.
I would go back and review each manufacturing step to determine what has changed from past manufacturing practice or supply vendors. Review tolerances, clearances, heat treatment and most certainly the ballising process. I believe you will find a process change has taken place and resulted in the sudden increase in failures.
Longer term you could investigate the use of a Nitronic 60 split sleeve as an insert or Stellite weld inlay for the valve body. However, this would add cost but possibly could further improve your valve performance. You would need to develop a mock-up and test performance in service.
RE: Fretting Fatigue 15-5
Aaron Tanzer
www.lehightesting.com
RE: Fretting Fatigue 15-5
I suspect that the result is an inhomogeneity of the material in the bore.
This could be chemistry, or heat treatment, or finishing.
My fist thought is that the boring and ballising are not not being done correctly.
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Plymouth Tube
RE: Fretting Fatigue 15-5
Completely agree on the process verification, I'm leaning towards that as a root cause with fretting fatigue being the resulting failure mechanism. The ballising/cold work hasn't been well documented, so here's the opportunity to do it.
Cheers!
RE: Fretting Fatigue 15-5
After talking to the AM operator, he tells me he verifies the process "by feel" (feel = .0007). Since we've recently expanded the night shift with new guys, I'm guessing the night operator might not have "the feel".