Galling on Stainless Steel Joints

You need to use an anti-seize compound.

Look here.

Stainless steel comes in the different alloys.

Depending on what you need it for, they offer different strength, corrosion or chemical durability.

All the different kinds have their own hardness.

Stainless Steel seizes easily when used in bolt- nut combinations.

Lubrication, even grease, or specific seizing paste or compounds may be used, together with a threated washer.

It is also important when using a torque wrench, to torque them right to tension from the first application.

Friction damage can give you a wrong torque.

This seizing and galling problem can be solved at least in mildly corrosive environments where 18-8 stainless steels are acceptable by fasteners made of nitrogen strengthened stainless steels such as Nitronic 60.

Unfortunately (as far as I know) Nitronic 60 bolts and nuts are not produced in sufficient volumes to be competitive with 316 in price. I'm not sure if any of the custom suppliers are thread rolling and/or cold heading their Nitronic 60 products. The higher strength does increase the difficulty and tooling wear for cost saving cold working processes like thread rolling and cold heading. Nitronic 60 round bar stock is readily available; so custom machined fasteners are practical. But they would be priced according to the manufacturing process.

Some 10 years ago the semiconductor equipment industry talked the Helicoil people into producing helicoil thread inserts in small machine screw sizes up to around 3/8-16 to help with the nagging problem of seized fasteners and the associated particle contamination issues during assembly and service activities. I'm not sure how far in the size range the Helicoil folks have extended the sizes of their nitronic 60 product line since then (and my retirement). Also it's important to note that their suitability in large process chemical equipment may be limited where there is limited experience with them and considerable concern with the reliability of pressure vessel fasteners.

Austenitic stainless steels do not have a specified tensile/yield strength above the annealed condition because hardness can only be raised by cold working; so is variable based on that factor. Important to note is that the annealed strength of the nitrogen strengthened alloys starts from a higher baseline. This strength is closely related to their galling resistance.

Ed Weldon

also note that ASME pressure vessel code does not allow the use of B8M where there may be elevated temps that could cause the bolt to anneal.

Consider Xylan coated fasteners for this.

Hello

i'e found these article from BSSA on Galling of stainless steel:

Category: Galling & Wear

http://www.bssa.org.uk/topics.php?sub_category=63

Galling and Galling Resistance of Stainless Steels

http://www.bssa.org.uk/cms/File/SSAS5.60-Galling%20and%20Galling%20Resistance%20of%20Stainless%20Steels.pdf

hope this help

Vitt

corrosion & rust prevention

We have a similar problem with small fasteners.

Our customer specifies 4-40 UNC stainless locking Helicoil inserts in an aluminium housing & we use stainless screws to hold this housing in various process and test set-ups.

Several times we have had to scrap a finished product because a screw has snapped off in the housing at one of the final stages. Our customer has vetoed the use of lubricants or hardened screws in case they reduce the effect of the locking action of the insert. I have been looking for Teflon coated screws but not found any yet.

The Nitronic sounds like it might be a solution but, again, our customer might be resistant to this.

Hey Nigh, I sounds like your client wants the cake and eat it too. I mean dang, looks like they would get over it and at least try one of your suggestions to solve a problem. I get so tired of people asking or requiring things to be made in one way for no other reason than they want it and they are the check writer even when it don't come close to making sense.

http://texasbolt.com/

Try these guys for teflon coated bolts , I have bought their Teflon coated bolts, studs and nuts in many colors for some of the piping skids I make and they are awesome to say the least and very reasonable in price as well. The Teflon will keep everything going smooth and so far I never got one of these to gauld.

You're right about our customer, they cry that they would have to repeat their qualification testing if anything changes but some of our suggestions make no change to the finished product. As you say, they pay the bills & have the last word.

I'll check out Texas Bolt to see if they can help.

You may all laugh at this solution, but try it and you will probably be amazed. Purchase a block of beeswax from McMaster Carr. Rub the face of the nut on this block and scrape the threads of the bolt once across this block before you assemble. You will get a true torque reading. I have used beeswax for years in high pressure drilling, tapping and assembly of aluminum and stainless parts where all other recommended lubricants failed. Good luck.

"You will get a true torque reading"

Engr, torque, as a method of determining fastener pre-load is heavily dependant on knowing the friction losses of the threads themselves, if you change the lubricity of the threads you change the resultant axial pre-load by a HUGE amount. I've seen bolts torqued clean off that were torqued to "factory specs" but had been lubricated whereas the factory specs assumed dry threads. Adding beeswax, or Xylan, or Molybdenum Disulfide, or even grease, will change the pre-load, therefore the applied torque should be recalculated with this lubricity in mind. But even then, torque is a very unreliable method of determining pre-load. For anything critical, torque should be the LAST way of determining pre-load you should rely on.

Which brings me to the stainless #4-40 fastener in the stainless helicoil issue noted above. It is very easy to twist the heads off of 4-40 screws as it is without throwing galling into the mix. Use of torque screwdrivers, especially powered ones, is hit and miss at such a low torque value (+/- 20%!). In a former life we were making ultra-sensitive magnetic reed switches for a downhole oil tool and all of the fasteners had to be non-magnetic to prevent the slight magnetic field of the screw from tripping the switch. Conventional 18-8 stainless screws were too magnetic due to the cold work they had undergone. These screws were #2-56 screwed into a non-magnetic nickel alloy. We had tried brass but we were unable to find a consistent torque value that wouldn't pop the heads off. We ended up settling for beta titanium to solve the problem.

I would also add thread locking compounds such as Loctite to your list of lubricants, people forget that they reduce the installation torque whilst still liquid.

With regard to the 4-40 screws, we have reverted to hand tightening rather than torquing but our breakages are usually when removing the screws rather than installing. They will come out part way then lock up.

If you are only using the 4-40 screws for assembly purposes, could you go to nylon screws? That way if broken, they could be removed with no damage to the completed assembly.

Unfortunately not, the screws are used for various tasks including holding the unit for vibration testing. We did try nylon screws to plug holes when the unit is filled with potting rubber but the screws don't like the locking action of the Helicoil & tend to break or damage the driver slot when they reach the lock portion.

I don't know if you've used locking inserts but they work by having one coil deformed into a slight hexagon shape half way down the coil.

Nigh, I would strongly suggest the use of Nitronic locking helicoils if you can convince the customer to allow you to. This was the very application they were invented for. Screw hardness will not affect the locking action because you are not deforming the screw, you are deforming the helicoil. Unless your customer PLANS for the two to gall, which would definitely lock the screw. Which brings me to your point about vibration testing, the fretting action from the vibration table could be accentuating your galling issue.

You could be right about the fretting.

I've made enquiries with the Helicoil supplier about the Nitronic inserts, we'll see how it goes.

One always gets a "true torque reading" if the torque wrench has been calibrated .

However, the problem is that the same "torque" applied to two identical fasteners (even with a calibrated torque wrench!) may result in one of those fasteners being significantly under-tightened while the other could be overtightened.

Too many people still don't understand that "torque" isn't a measurement of a fastener's "tightness". It's merely a measurement of the resistance felt when trying to turn a nut (or bolt head).

Consider the OP's particular application. If his fasteners galled after the head made contact with the washer/flange etc and torque alone was used as a control mechanism, the Assembler would have stopped torquing upon reaching the "correct" torque. Unfortunately, this would have been at a point well before the required clamp load had been applied. As far as he'd be concerned, he had done his job properly (it should be clear to anybody who understands the concept that he hadn't). Compounding this dangerous false sense of security would be the QA guy signing-off on the job because the proper procedures were followed. Alas, the assembly is sure to fail in service.

Consider your own critical bolted joints. Scary when you really sit back and think about it, huh?

Link: Torque Misconception

Be sure to use the nickle type anti-seize on stainless steel ,the copper type anti seize isn't compatible with stainless and it can actually cause galling on some types of stainless. I've never used the silver type anti seize though, anyone know what material it is to be used on?

I have another process to try. I'm getting some stainless screws coated with tungsten disulphide (WS2), this is a 0.5µm low friction coating which was developed by NASA to prevent galling on stainless components. I'm told that it will not transfer to the Helicoil inserts so should leave the assembly we send to our customer unchanged.

How much will that add to the cost of the bolts? What size were the bolts again?

I had used a spray from Dow that was Molybdium disulphide based. It stunk like hell, but was a good dry lube for ball valves. It was supposed to set up after a time period. I think it was sold as Dri-Moly, but I don't have the can here. Would that work for you?

I don't have costs yet, I'm just getting samples but when we break a screw as we have been doing it essentially writes off a unit that would sell for about £7000. They can be recovered with a fair amount of rework but cost is not our driving force here.

We offered a moly lubricant, it's something we use in other areas but our customer will not allow any lubricant that might transfer to the Helicoils.

Nigh -- FWIW I'll relate a solution to a problem that I solved 25 years ago when I worked for Memorex at the Santa Clara CA disk drive operation. The 3680 14" disc drives had a 7075 formed aluminum clamp for the disk stack that was held in tension by a something like eight 5/16 or 3/8 dia 24 TPI A286 stainless socket head screws. Try as I might I cannot remember what material the mating female threads were; but the problem was that the screws were seizing and breaking when torqued at assembly (where I was the manufacturing engineer). Class 100 clean requirements prohibited any kind of thread lubricants and it was still another 5 or so years before the use of silver plating for stainless screws in semiconductor equipment became widely known to the Silicon valley engineering community.

Sometime around 1979 Machine Design magazine published a short article on the new Nitronic 50 and 60 stainless steels and their anti-galling properties. What struck me was that this stuff was real. The article reported that one of the first successes was in propellor shafts for commercial fishing boats that operated in the Pacific Northwest. (Talk about rough service!) I dug this old article out of my files and used it to persuade the design engineer to try making threaded inserts from Nitronic 60 for the 3680 disk clamps. It worked.

When I went to work for Applied Materials I promoted the idea of Nitronic 60 thread inserts. I like to think I helped influence the development of the Nitronic 60 helicoils that became popular and are readily available from the local fastener supplier, Olander.

Not to be outdone, helicoil competitor Fairchild Fasteners has made their Keenserts available in Nitronic 60. Keenserts are a more robust design (and more expensive); but are preferred by some engineers for critical applications where weight is not an issue. A range of sizes shows in the Olander online catalog. I cannot speak to the availability of these inserts from stock.

Also Olander seems structured primarily to serve customers in the Western USA. Their website makes no mention of international sales. So inquiry direct to the manufacturers, Helicoil or Fairchild Fasteners as the case might be, will likely be necessary should you choose to pursue this course.

There is always the possibility of machining inserts direct from bar stock with whatever external mounting features, threads or something else, are appropriate. this is what was done at Memorex 25 years ago. Nitronic 60 round bar stock is readily available in the USA and something similar should be obtainable in the UK or EU. It even shows up in the ebay aftermarket.

Ed Weldon

I picked up your mention of Nitronic 60 in post #4 & I've enquired about inserts from our local supplier, no reply yet.

Even if this is a solution our customer may not allow it. As an example, I expect you know what a pain it can be remove the driver tangs from small Helicoils in blind holes. Our customer insists that the tangs are completely removed but would not allow us to change to the tang-less version of the same insert.

Well, I tried, Nigh. There seem to be technical answers to your problem. But the non technical side appears to be mounting impossible roadblocks. It's as if your customer has your company in an Intel style "copy exact" stranglehold. Or maybe it's some blind adherence to a "we've always done it that way" cultural artifact.

I went back and checked the original posting and realized that we have two topics going here. This is about the question posed in reply #7.

I think the one additional point I could make is that 4-40 helicoil is a tiny part. All your company might ever need would likely fit in a one liter jar.

Here are the Olander part numbers for various lengths:

/1185-04EN112 4-40 F/R HELI-TRONIC INSERT LOT#____________ NITRONIC Add to basket
/1185-04EN168 4-40 F/R HELI-TRONIC INSERT LOT#____________ NITRONIC Add to basket
/1185-04EN224 4-40 F/R HELI-TRONIC INSERT LOT#____________ NITRONIC Add to basket
/1185-04EN280 4-40 F/R HELI-TRONIC INSERT LOT#____________ NITRONIC Add to basket
/1185-04EN336 4-40 F/R HELI-TRONIC INSERT LOT#____________ NITRONIC

The Olander Company Inc. is located at 144 Commercial Street, Sunnyvale, CA 94086Call Toll Free (800) 538-1500 Fax: (408) 735-6515 E-mail: customercare@olander.com

I'd like to respectfully suggest that someone in your company, best a purchasing agent, lift himself off his dead posterior and contact these folks. If they can't do business internationally I'm sure they can find a US agent who can act as a middleman. Olander is well acquainted with several such agents in California who make a good living finding fastener sources for corporate customers that lack the specialized knowledge to source such requirements on their own.

All you need to do is provide the engineering selection of the proper length of helicoil.

Waiting for your local fastener supplier to get back to you appears to be like waiting for Stonehenge to rebuild itself. All the above information on Olander came from a 15 minute Google search. Surely someone over there measures the passage of time in minutes rather than centuries!! Meantime the rest of this competitive connected world is measuring time in seconds.

BTW, Nitronic 60 helicoils and the process Applied Materials uses to remove the tangs (Simple good workmanship, the correct installation tools and a little SPC) are good enough for their customers who number most all of the major semiconductor manufacturers in the world including the "dreaded" Intel. Perhaps your Sales team could remind your customer of the profitability of these major corporations.

Good luck, Nigh.......... Ed Weldon

As in your 1st comments, there are several viable technical solutions but our problem is finding one that our customer approves of. I'm sure we could get the nitronic inserts even if we had to buy from the USA but I'm not sure if we could persuade them to change. Their mantra is always that it would invalidate their qualification testing.

I have high hopes for the WS2 coated screws, I should have some of them early next week.

I have used heat treated c/s nuts on stainless (B8)studs and bolts before because we were having a very bad galling problem . T first after we investigated this problem it was clear that this was caused by over tightening the bolts/studs. Next we started checking every nut with a torque wrench to prevent the over tightening the studs and hopefully stop the galling problem.

What we found next was that the flanges would leak until we over-tightened the studs and then we were back in the same boat we started with because this caused the stud/nuts to gall the only thing we could do to keep the flanges tight and not gall the s/s studs. Is there one element of the s/s alloy materials that causes this galling when the bolts are over tightened or is it a combination of more than one eleminate that causes it?

The problem with using CS nuts on Stainless studs is that you may have traded one problem for another. Yes, you've solved the galling issue, but now you run the risk of pitting corrosion on the threads of the studs because you've embedded CS into the surface and interrupted the protective Chrome Oxide layer.