B411 Shaft Pitting

dLj

.
Mar 23, 2017
3,370
Belliure 41 Sailing back to the Chesapeake
This indicates cold working does in fact cause austenite (non magnetic) to transform into ferrite (magnetic) as it is plastically deformed.
Sorry, hoping not to be pedantic, but the austenite transforms to martensite, not ferrite. Martensite is also magnetic.

Bet you a Buffalo Nickel that small magnet, is Rare Earth Magnet.
Neodymium magnet - Wikipedia
If you are taking the position that it is NOT a Rare earth, I'll bite, and if it is a Rare Earth Magnet you send me a Buffalo Nickel, I take the bet!!!! LOL ( I'm thinking that's not your position???)

3. I am familiar with cold working and SS magnetic properties. My shaft is strongly magnetic indicating NOT 304 or 316. Could be 17-4 or a similar alloy which are strongly magnetic.
So let me see, do you have knowledge of the magnetic properties of this shaft prior to this happening? I'm guessing the answer is no. You also probably don't have any documentation indicating exactly what the alloy of our shaft is made from. At least I've never seen sailboats (other than mine) that may have this information. So in fact you have no base line. You are attempting to determine what the alloy is through the use of a magnet. I am going to repeat (with the same caveat the I'm not trying to be pedantic) - you cannot know the alloy from the application of a magnet. What a magnet can tell you is if it is not magnetic, it can't be iron, among other things. However, if it is magnetic, you need to do other testing to know what the alloy is. A magnet can rule things out, but it can't identify what you've got. Just because it's magnetic, only tells you - it's magnetic. End of story.

dj
 
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  • Helpful
Likes: JamesG161
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
2. I looked back at my Fall notes and i had noted that the zinc, while still there, had degraded more than a previous year.
Could indicate external current.

3. I am familiar with cold working and SS magnetic properties. My shaft is strongly magnetic indicating NOT 304 or 316. Could be 17-4 or a similar alloy which are strongly magnetic.
Really getting technical here. 17-4 is martensitic. A prop shaft is usually machined for the final fit into the x-mission coupling. If it were martensitic before machining, it would be a challenge to machine. Any chance of running a file across it to get an idea of how hard it is ?

4. Could the very mild rotating contact between the shaft and the cutlass bearing be affecting the stainless steel anti corrosion layer? This with a weak zinc created the pits.
The chromium oxide is always rebuilding in an oxygenated environment. A single zinc, off in the distance isn't going to help.
 

RitSim

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Jan 29, 2018
406
Beneteau 411 Branford
17-4 can be carbide tool machined in the hardened state without issue.

Good to know the layer rebuilds. I think it was a zinc issue. The shaft threads had some scale on them - perhaps diminishing the effectiveness of the zinc. Going to do a mid season dive this year to change tohe zinc if necessary. Thanks to all who commented.
 
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
Sorry, hoping not to be pedantic, but the austenite transforms to martensite, not ferrite. Martensite is also magnetic.
Pedantic ? Here ? NEVER !

Agreed, martensitic as per ASSDA. Lousy glasses.

The Cliff Notes on this whole discussion IMHO is that @RitSim 's shaft, whatever it is, is not of the best material. Anything that permits pitting (due to a lack of O2) under the cutalass does not win first prize.

Speaking of pitting under the cutlass, this has got me thinking about several posting from way back when, in which people are griping about frequently replacing their bearings because they become sloppy after logging a few years of usage. Usually attributed to poor alignment. Makes on wonder if pitting may be a contributing factor ? ? ? ?
 

dLj

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Mar 23, 2017
3,370
Belliure 41 Sailing back to the Chesapeake
17-4 can be carbide tool machined in the hardened state without issue.

Good to know the layer rebuilds. I think it was a zinc issue. The shaft threads had some scale on them - perhaps diminishing the effectiveness of the zinc. Going to do a mid season dive this year to change the zinc if necessary. Thanks to all who commented.
Completely agree with the above.

If your shaft is 17-4 PH, it should have been heat treated to a minimum of H1000 for this application. H1000 is not a common heat treatment. H1025 however is. The most common heat treatment used in industry for this alloy is H900, and that would not be recommended for a marine, especially submerged, application.

For your information, here's the common heat treatments for this alloy and the corresponding minimum hardness values:

H900 - HRC 40 min
H925 - HRC 38 min
Neither of the above are appropriate for this shaft. The ones below are.
H1025 - HRC 35 min
H1075 - HRC 32 min
H1100 - HRC 31 min
H1150 - HRC 28 min.

There are more, but these are the ones more likely to be used and the above 4 are recommended for this shaft if it is 17-4 PH. That being said, I do not think any of the above 4 heat treatments were used on your shaft if indeed it is 17-4 PH or you would never have had the pitting you've observed.

fwiw,

dj
 
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