Why do stainless steel fasteners have a tendency to gall?

[A930Rocket]

I installed my cat bypass yesterday, and several times, new stainless steel bolts, and type C nuts, galled before I could tighten them down. I also used anti-seize on them. what the heck?

Don’t look at my booger welds. I will work on those later.

[Zeke]

Maybe it's your source.

[LWJ]

I believe some metals are simply more prone to galling? Or, maybe the mill source metalurgy is a little off spec? As Zeke says, perhaps the source? I can't think of any SS fasteners galling. Is it 308?

[herr_oberst]

Would copper anti-seize help? (The cost of good SS fasteners is galling enough.)

(Also, when I was doing what you're now doing with my 911; IE removing and replacing the muffler and "test pipe" fairly often, I found that serrated flange lock nuts rather than split lock washers made the process a WHOLE lot easier.)

[aschen]

Its a characteristic of the metal itself more than physical properties (hardness, toughness, yield, etc). Some stainless alloys will be better than others. SS are generally stainless because the oxide outer layer is protective and sticks to the surface. If it gets knocked off by scraping or high pressure on make up, the metal can self weld.

Never had a huge problem with it on fasteners personally. At least those bolts are easy to cut off if needed. I think plain steel would work just as well there though.

[A930Rocket]

I had to cut two off before they were all the way down, and I couldn’t back them off.

I’m tempted to replace them with yellow zinc fasteners now.

I bought these from Bel-metric.

Stainless Steel A2-70
DIN 931 / 933

Stainless Steel
Base DIN 934 Pinch

[unclebilly]

Nickel alloys are all prone to galling.

You can liquid nitride them which helps a lot but otherwise not a bunch you can do.

Avoid fine threads.

[ben parrish]

Use copper nuts with the stainless steel fasteners. The copper is more "slippery" and will not gall on the stainless steel bolts.

[Scott Douglas]

ben you beat me to it.

+1 on the copper nuts.

[908/930]

I am surprised when they don't gall. Use steel grade 8 SHCS with copper nuts. I avoid stainless from years of having to cut them off to remove, really avoid threading stainless into aluminium. If you really need stainless try 17-4 PH fasteners.

[A930Rocket]

So much for thinking I was making it easier for myself down the road. 🤪

Next time I work on it, I’ll replace them and use the copper nuts.

On another note, it’s way too quiet. 🤬

[p911dad]

I can recall having copper nuts on the exhaust on my G series Carrera.

[rfuerst911sc]

Surprised they galled with anti seize but not unheard of . I agree with others steel bolts with copper nuts . Also if you have a bottle of Milk of Magnesia a thin coat on the threads is helpful . A Marine helicopter mechanic gave me that tip .

[John Rogers]

When I was in the US Navy nuclear field, we had both stainless or Alloy 600 or Inconel and we were supposed to use powdered graphic in iso-alcohol which was terrible. When no one was looking we would use a graphic based, hi temp anti-seize. that was the same the gunners and missile folks used on their rails or other moving parts. The heat from a missile launch would not burn it off. I bought ours at Summitt Racing as used on nitro dragsters/funny cars.

We also found that tolerances between male/female threads was way closer than regular steel nuts and bolts so we would do a test fit and at times dress the male threads slightly to provide some clearance.
John Rogers the oldracer

[vash]

corrosion from dissimilar metals.?

[fanaudical]

Aschen nailed it: Stainless steel parts rubbing together under high pressure scrape the protective oxide layer off and then the ductile metal under the oxide layer welds itself together. Dissimilar metals (like the copper nut suggestion above) help prevent galling. I've used Nitronic 60 nuts on 18-8 and 316/316L fasteners to prevent galling when alloy steel or copper wasn't an option.

https://www.nickel-systems.com/materials/nitronic-60/

[A930Rocket]

I used what I called type C nuts, which have three small triangle shaped impressions on the top of the nut, to keep it from backing off. Sounds like they were scraping off the material you guys are talking about, and they jammed.

[Porchdog]

Aschen is correct - the Oxide layer is very thin and the bare alloy under pressure welds easily.

We are forced to use stainless nuts and bolts in a lot of situations (food and Pharma). Using 18-8 nuts can help. Stainless grade anti seize is a must. If they are acceptable I always spec. copper alloy nuts.

For auto applications I use high strength non stainless fasteners. These have enough alloy in them to minimize corrosion and they don't gall.

Deformed stainless lock nuts are one time use at best.

[Pazuzu]

Use either high temp copper antisieze, or they make some $$$ stuff specifically for stainless fittings (we used lots of it when building a very complex hydrogen reactor once, HUNDREDS upon HUNDREDS of stainless bolts, nuts, and Swagelok fittings.

Swagelok Silver Goop (oh geeze, I didn't know it was THAT much money when we were using it!!!) One ounce, $200...

But, I'll tell ya, stainless to stainless and stainless to aluminum was painless, and never galled.

[sc_rufctr]

According to AI

"AI Overview

Stainless steel fasteners gall because their protective oxide layer scrapes away under friction and pressure. Without this barrier, the exposed, relatively soft base metals cold-weld together on a microscopic level. The threads seize and tear as they slide against each other."