304 electropolishing pitting? 2

[adock]

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Hello!

We have 304SS rolled thread parts that screwed together like butter prior to electropolishing. Now they've become very sticky, almost unusable. Wondering what the thoughts of the forum are? I'd love to add some lubricant but the parts will be cleaned 1-7x per week with rubbing alcohol

 

[TugboatEng]

You need to silver plate one side of the thread.

 

[EdStainless]

Roll the threads after EP.
Your EP is taking off the work hardened layer from the rolling.
Long term you will need a lube.
Either a synthetic grease that you reapply each time or silver plate as TBE suggests.
Even that won't last forever.
One of the parts needs to have a different hardness.
Maybe use Nitronic 50, or 17-4PH (H1150), or even a higher alloy grade such as AL-6XN.
In reality if you want long term performance use a stub of AL-6XN with the female threads and use silver plated 304 plugs.
These will be a maintenance item in that as they wear, they will be discarded.
One of these alloy options along with silver plate should get you some reasonable service.
But if this is a hygienic system, why do you even have threads in the first place?
This should be a blank flange.


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P.E. Metallurgy, consulting work welcomed

 

[adock]

Thank you TBE and Ed,

I'm not using the best definition of roll formed threads. Think the threads you would see on a consumer stainless water bottle. Same concept, 0.039" stainless with ~ 1"-4 thread size

The rolled formed section is welded to other pieces after it's formed so EP after forming isn't an option. Would the harder alloy with silver plate still be an improvement if EP was the last step?

 

[TugboatEng]

EP would remove silver plating. Silver plating needs to be the final step. If you have any stainless steel Swagelok fittings in your facility they use silver plating on the female thread. Have a look to see what we are talking about.

The Nitronic alloy recommended by Ed is going to be the most durable solution. From what I have read about it I would argue that Nitronic 60 would be superior but Ed has real experience with the stuff.

Silver plating is more convenient as you don't have to change anything about the process you already have provided your threads have sufficient clearance to accommodate the plating.

 

[EdStainless]

If these are the more open 'wave' threads then you might be able to buy some thin silver foil (this is a regular craft item) and apply it by simply wrapping the plug with foil and running it in and out a few times.
The silver is very soft and will literally pressure weld to the clean SS surfaces.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[adock]

Cool! Thanks again TBE and Ed!

 

[adock]

Just a follow up...
We looked into silver coating but it was cost prohibitive
We may experiment with moly-sil dry lubricant coating, having trouble predicting the life span of it, however
Current plan is masking the male threads from electropolishing. that seems ok so far
Seems like the silver foil is fixing some of the 100% electropolished pcs that have really sticky threads. Silver foil was tricky to find but there's a specialty shop online that has exotic metal foils for reasonable prices