Thread galling 2

[vennardd]

I need some help on some galling issues. I desing instruments for medical devices, so the material is usually stainless steel. When I have thread mating instruments (both of same material), I have been told that differing the heat treat between the two parts will reduce the likelyhood of galling. For example, Part 1 will be 17-4PH (H900) and Part 2 will be 17-4PH (H1150). So these two parts are supposedly more gall resistent than 2 mating threads of both 17-4PH (H900).

A material science friend showed me this threshold galling data (below) that gives threshold galling values for the pairing of materials with the same heat treat, but this still leaves the question of mixing and matching heat treats unanwsered.

does anyone have some insight on this subject?

fyi, adding a lubricant isnt an option.

-----------
1st material
17-4PH heat treat to H900 (HRC-45)

2nd material
17-4PH heat treat to H900 (HRC-45)

Threshold Galling Stress (MPa)
69
------------
1st material
17-4PH heat treat to H1150 (HRC-34)

2nd material
17-4PH heat treat to H1150 (HRC-34)

Threshold Galling Stress (MPa)
34
--------------
thanks
dan.

 

[Robertmet]

Do the two parts have to be the same material ? Do you have a choice of stainless steels ? Have you thought of Nitronic 60 as an excellent gall resistant alloy ? Other considerations such as corrosion resistance ?

 

[Goahead]

You may wish to check the following:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700018937_1970018937.pdf

that reports:
"The nuts a r e usually s i l v e r plated t o overcome galling. Silver does not readily sublimate and acts. as. a lubricant and. antigalling agent"

Regards,

http://www.welding-advisers.com/

 

[bcd]

For anti-galling, it is best to maximize the difference in hardness between the materials. 20 points Rc is a common recommendation in my Machine Design textbooks. Also, the difference in hardness between the various conditions is not much. You cannot positively confirm which condition you have by hardness alone as their ranges overlap so much. If 17-4PH, the best you can do is H900 into H1150M, which gives you a typical hardnesses of 44Rc to 27Rc. Enough of a difference to help, but definately not enough to prevent any dings or burrs on the threads from galling. This stuff easily work hardens so just machining can eradicate any hardness difference.

bcd

 

[OTHGWayne]

Besides the relative material hardnesses, Galling tends to be related to both the load that the threads see and the number of times that the joint is made and broke. If the loads are high, you will not eliminate the galling process. You can only postpone the occurance. To do this you need to make one of the surfaces very hard sush as using secondary processes on the treads. Typically a titanium nitride (or similar) coating helps a bunch. There are also other anti-galling processes available (Google them). Ultimately, you need to determine MTBF and swap out parts before failure occurs, cause it WILL happen if the parts are torqued and re-torqued often.

 

[NickE]

How about a PVD/CVD/DLF coating?

(There must be one of them that is certified for medical devices.)

Nick
I love materials science!

 

[plasgears]

In jet engine work we used milk of magnesia to counter galling with stainless steel fasteners. It worked, but don't ask me why.

 

[coatingengineer]

Hi.

Yes, I agree with the others. Creating a hardness gap between metals by 20 (Rc) is very good. So is adding a coating for 'dry' lubrication.

Is this for an implantable device? Maybe it's short-term, direct tissue contact, re-usable? Or perhaps it's short-term, direct tissue contact, disposable? Your answers are most important in choosing a coating.

Like Nick says, PVD/PACVD/CVD may serve you well.

For more help, see:
<a href="

http://www.industrialcoatingsworld....-Deposition-Low-Friction-Coatings.html">Vapor

Deposition Low Friction Coatings</a>

Good luck!




William Gunnar

http://www.IndustrialCoatingsWorld.com

 

[plasgears]

Try milk of magnesia, USP, air dryed on mating parts. It's not a lube, but it sure combats galling. We used it on jet engine afterburning fasteners.

 

[SMF1964]

Milk of magnesia (magnesium hydroxide) probably works as a dry lubricant because it has a hardness of around 2.5 Mohs scale, which puts it on par with gold and considerably less than the typical hardness ranges for your afterburner fasteners. But that's just a guess.

 

[vc66]

You should be able to easily coat one of the 17-4 pieces with AlTiN. That will prevent galling.

V

 

[andyfox1]

TiN (PVD) is FDA approved and used in all sorts of medical applications such as knee implants, medical reamers etc. It is also used by racing teams as an anti-galling barrier for wheel nuts etc.

As it is a basic coating provided by pretty much any coatings provider, you will find it cheap in comparison to AlTiN.

You could also try out CVD Diamond-like Carbon (DLC), as it has a much lower coatings-steel coefficient of friction, but it will be a more expensive application and if the torque on the threads is high it may crack the coating due to its high hardness.

Finally, you could try tungsten disulphide (WS2); it may be more difficult to locate a vendor in the US than TiN coaters, but it works similarly to MoS2 but binds to the surface better, so you get a longer life. Also, this will cope with any potential substrate deformation better than any coating, as it is a very thin application and can migrate between surfaces.

Andy

 

[coatingengineer]

Hi, again.

Material choices with lower ductility are less likely to experience galling. This is because under higher loads, surface asperities are more prone to fracture-dislocation during interlock.

Conversely, high ductile materials are much more prone to plastic deformation. Therefore, galling is more likely to occur.

Assuming you've taken into account contact stress and finish, consider surface treatments, like nitriding, carburizing, hard face coatings and ion implantation. They can serve you well, assuming your thread configuration is compliant.

For more information, just see:
<a href="

http://www.industrialcoatingsworld.com/Wear-Resistant-Coatings/index.html"/Wear

Resistant Coatings</a>

Generally speaking, for austenitic stainless steels, consider Cr-Mn-N over Cr-N alloys. And for the heat treat hardenable stainless steels, higher threshold galling stress materials like PH martensite, under decreased aging temperature, are the way to go.

Preventing galling through a large difference in hardness is a misconception. Actually, alloys with good adhesive wear, like 440C (57 HRC) and S21800 (92 HRB) are excellent choices, regardless of the hardness of the other alloy.

Good luck!


William Gunnar

http://www.IndustrialCoatingsWorld.com

 

[DKirkham]

I don't know much about medical requirements, so my thoughts may not be applicable. We just work on cars.

In our world, we use silver plated nuts and bolts on the exhaust on our really expensive cars. They also work really well on axle CV's which are abused without end on a race car. The silver plated stuff never galls--problem solved.

Also, we have found that lousy machining leaves mirco-burrs which will gall in a heart-beat. Nicely machining your parts goes a long way to prevent galling. Perhaps you could try electro-polishing or cleaning up your threads somehow. Look at them under a microscope--you may not like what you see.

As for making parts with different hardnesses, I am not sure about that one. The hubs on an F1 and the wheel nuts are made from the same material--Steel (can't divulge the alloy) hardened to 55 Rockwell C. Talk about loading, slamming, banging, taking off an putting back on--there may be no equal. They avoid galling by truly beautiful machining practices.

David

http://www.kirkhammotorsports.com

 

[belowzero]

We had galling problems on shafts made from H1150 and H1025, we found the surface finish of the component played a major part in the 'pickup' of material when looked at under electron microscope.
B

 

[growjohf]

Thread rolling (external) or thread forming (internal) will produce much stronger thread and superior surface finish because the thread is extruded rather than cut. Shot peening is another method of cold working thread if you are working with alloys that cannot be heat treated such as austenitic stainless steels.

Depending on the thread type and size, you may consider thread inserts such as Heli-coil.

 

[israelkk]

If you can not use silver coating on the threadm the only reliable choise to my opinion is to try ally such as Nitronic 60 or Gall tough from Carpenter. They are atainless and especially developed to avoid/minimize galling.