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Why Does Stainless Gall

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  • Why Does Stainless Gall

    I would like to know what causes stainless to gall like this. I've never had this issue personally. I have always used 303 for my U-bolt projects and what ever the nuts are 316?
    This u-bolt was from a friends project. I ended up having to cut it. So I took the piece home just out of curiosity to see what it looked like inside..... like I didn't know what to expect.
    Anyway, this is what I found.
    This was only out in the weather for a year or so and was not over tightened. He started to loosen it and it became stuck withing one turn.

    Both the nut and stud are non magnetic so I guess that makes it a 300 series ??. Is there such thing as low quality garbage non magnetic SS??
    Could it be that the nut or the stud are too soft? Too much play between the threads of teh bolt and the nut causing one of the crests to start to dig in?

    I know that stainless against stainless under pressure tends to stick. But I've had SS hardware out side for years and never had this happen.

    JL..................

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  • #2
    Many believe that a breakdown of the oxide surface layer during assembly or disassembly causes galling.
    Fasteners and assemblies of metals other then stainless are also prone, aluminum being one example.

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    • #3
      Others with more knowledge will chime in here I'm sure. As I've been told, stainless is not particularly strong- which I take to mean it's a bit soft. That will make it want to deform under pressure, which in turn makes the parts almost weld together. Anti-seize would almost be mandatory.

      I used to think of stainless as being- well, stainless. No rusting, no change in the surface with exposure to the elements, etc. That's not really true- even if you don't consider salt water exposure. My basement sink is rusting- if I scrub it well enough to remove all traces of rust, I'm left with pits. If the stainless is not that inert, then it doesn't surprise me that it can gall easily.

      I used to install satellite dishes. Many of them were assembled with stainless hardware, which often used stainless nylock nuts. There was never any assembly lube used, and for the few that I was commissioned to take apart and remove, I don't recall ever having the hardware jam. Of course it was never assembled all that tightly because you were bolting thin tubing together, which meant that metal to metal forces weren't that high. I think that's another aspect of the hardware galling- if you get above a certain metal to metal pressure, you'd automatically hit that point. Regular steel hardware would take more pressure before that happened.

      That's just my take on it.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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      • #4
        It does not develop a strong oxide layer. Neither does aluminum. The thin layer wears off, and then it is metal to metal. With no oxide between, the metal can stick together just because there is nothing preventing that, and the metal is compatible.

        Nuts and bolts are bad for it because the action of turning them rubs off oxide. It also can develop local heating, in case that is of assistance. Not likely to be welding heat, but it need only be in tiny peaks as they rub together, so it is potentially possible.

        I have found that most types of "Never-Seez" or similar products, will act to prevent galling.

        BTW, pressure is not needed at all. I have had clean stainless lock up when I was spinning a nut on by hand. No wrench, and well short of snugging up the nut, I had a quarter inch to go, nothing in contact yet.
        1601

        Keep eye on ball.
        Hashim Khan

        Comment


        • #5
          Isnt it because the lack of surface oxides means the metal can cold weld to itself?

          Never looked to far into it myself either, i always just assumed that the lack of surface oxides meant that there was a high affinity between the metal and itself, so when close contact was made it just tries to weld itself together, leading to chunks being pulled off in the form of galling. Be interested to see if thats actually the case though

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          • #6
            Yeah, it some sort of welding and it can just happen at random. Ive seen stainless, aluminum, aluminum on stainless, and copper gall. Im not sure if I have seen steel gall outside of some sort of external damage, contamination, or corrosion.

            Vacuum and gas hardware has the threads silver coated to stop galling.

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            • #7
              I don't know why it does only that it's a pain in the butt. I assembled some parts using stainless hardware just loose finger tight and of course one of the damn nuts galled. When I was working, on some of the projects we used stainless bolts with carbon steel nuts. That seemed to help. Other projects using all SS hardware we coated the threads with anti-seize at final assembly and that also helped.

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              • #8
                I've read about, and watched vids about friction welding. What happens when you friction weld stainless? Almost seems like something you could do at home with a drill press.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

                Comment


                • #9
                  I have had stainless gall on several occasions when I have been too impatient to use any lubrication.

                  Recently I was using some M3 bolts and Nyloc nuts on an aluminium antenna installation, and I found out the expensive way that I had to spin the nuts onto the bolts very slowly to avoid their galling.

                  The mechanism seems to be that the inevitable rough spots on the threads shear minutely against each other, generating enough significant localised heat to cause instant welding.

                  Darryl, stainless bolts are indeed much weaker than regular high-tensile bolts. Laymen sometimes confuse hardness with strength—stainless has that hard surface layer, but the internal metal is little if any stronger than mild steel. The only virtue of stainless fastenings is that they stay nice and shiny on the bits you see. But they will corrode badly if they are deprived of the oxygen that keeps the transparent surface layer of chromium oxide intact.

                  This is especially likely to happen in the marine environment. I have had 1/4" bolts rot completely away within their deck fitting because of water getting in to the exclusion of air. This took only two years to occur.

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                  • #10
                    Used to work designing chemical analysis equipment. Wetted parts were 316SS. The worst for galling and seizing. Commercial fittings were usually coated to prevent problems. If you don't need the properties of 316, use 303/304.

                    Mike

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                    • #11
                      We used to use a lot of 304 at work and I had to deal with this at least weekly. Even brand new parts will gall together, just lock right up. I don't know the mechanism of why that happens, but I do know that its an expensive PITA.

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                      • #12
                        Originally posted by Mike Burch View Post
                        .........

                        Darryl, stainless bolts are indeed much weaker than regular high-tensile bolts. Laymen sometimes confuse hardness with strength—stainless has that hard surface layer, but the internal metal is little if any stronger than mild steel.

                        .........
                        The hardenable (thru) stainless is stronger, but also not truly stainless, more "rust-resistant", not rostfrei.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          Originally posted by J Tiers View Post
                          It does not develop a strong oxide layer. Neither does aluminum. The thin layer wears off, and then it is metal to metal. With no oxide between, the metal can stick together just because there is nothing preventing that, and the metal is compatible.

                          Nuts and bolts are bad for it because the action of turning them rubs off oxide. It also can develop local heating, in case that is of assistance. Not likely to be welding heat, but it need only be in tiny peaks as they rub together, so it is potentially possible.

                          I have found that most types of "Never-Seez" or similar products, will act to prevent galling.

                          BTW, pressure is not needed at all. I have had clean stainless lock up when I was spinning a nut on by hand. No wrench, and well short of snugging up the nut, I had a quarter inch to go, nothing in contact yet.
                          I find it hard to believe that could even happen just by spinning a nut on by hand with no force be placed against it.
                          Are you sure there wasn't a small chip or something, burr etc. hidden in the root of a thread where you may have not seen it, or even in the threads of the nut and it got caught up in between the threads??

                          I've found very small metal chips in nuts before that went unnoticed until I tried screwing them on a bolt. I haven't found it to be a common thing but the nut threads are cut and then they are probably washed before packaging and sometimes. Stuff happens!

                          JL...............

                          Comment


                          • #14
                            Originally posted by MikeL46 View Post
                            Used to work designing chemical analysis equipment. Wetted parts were 316SS. The worst for galling and seizing. Commercial fittings were usually coated to prevent problems. If you don't need the properties of 316, use 303/304.

                            Mike
                            I make all my U-bolts out of 303. I have good luck with that series, but I just use regular off the shelf SS nuts, I think those are 316 ?
                            Never thought of ordering them in a different alloy. Probably not easy to find.



                            JL..............

                            Comment


                            • #15
                              Originally posted by barracudajoe View Post
                              I don't know why it does only that it's a pain in the butt. I assembled some parts using stainless hardware just loose finger tight and of course one of the damn nuts galled. When I was working, on some of the projects we used stainless bolts with carbon steel nuts. That seemed to help. Other projects using all SS hardware we coated the threads with anti-seize at final assembly and that also helped.
                              That helps but the ant seize over time will dry out and all that is left is the powder which will eventually hold moisture.
                              As a rule now...... before I remove any nuts from SS hardware that has been out side I give it a shot of WD, that seeps into the threads pretty fast and give some instant lubrication as soon as the nut starts to turn. Hopefully before something sticks.

                              JL..................

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