Spun bushing annealed hardened steel shaft?

[BradF]

Hi,

I've got a steel shaft where we lost an oil feed and subsequently overheated a Vespel bushing. Not only is the bushing trashed, but the steel shaft got so hot that it appears it could have gone through some phase transformation, possibly annealed.

Based on the colors we can see on the shaft, and a basic heat treat/annealing guide (

http://www.threeplanes.net/toolsteel.html

- I am not a Materials guy) it looks as though the shaft could have gotten up to 500-600 deg F. The shaft is 4340 Alloy steel, heat treated to about 50-60 Rockwell C.

As I said, we expect that we lost the forced oil feed through the bushing, so the shaft would have simply cooled through room temperature air. Could we have annealed the steel in these temperature ranges? If so, would that mean that we've lost some surface hardness? What temperature would we expect to have problems with 4340 that had been heat treated to 50-60 Rc?

Thanks for your expertise!
Brad

 

[swall]

After the Vespel bushing "went away", what was the steel shaft running in?

 

[BradF]

The Vespel didn't actually "go away", it seems to have melted a bit on the inside, and lost its finish, but remained in contact with the shaft, and in fact bound up on the shaft once it started to melt (high temp vespel, good to 600F). That said, at some point while the shaft was getting hot, there seems to have been some contact between the shaft and a 440C Stainless steel housing, probably due to run out. The problem with assuming the heat is from this, however, is that there were actually three shafts (out of four) that the overheating happened to, and only two of them contacted the metal. The third looks like it never contacted any metal, but still has the same discoloration as the other two that did contact the metal. This leads me to believe that it is not that the metal-metal contact that created the heat in the first place, but rather the bushing, which to reiterate did not deteriorate completely.

 

[swall]

If any of the shafts actually reached 500C, the hardness would have dropped to about Rc 35.

 

[EdStainless]

clean them up and take some hardness readings. You will know pretty quickly.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[GRoberts]

Swall,
The original post was in F, not C.

Brad,
Without a hardness check, you will never really know since the oxide colors change based on length of exposure and the environment. They can provide a ROM guess at best.

 

[swall]

Yes, I see. In that case, 600F would drop the shaft to about Rc 50