Galling Information? 1

[tc7]

I need to learn about galling particularly of threaded fasteners. I have seen most of what has been written in Eng-Tips.Com in regard to galling of stainless and aluminum, but how can I predict the circumstances when galling will occur with alloy steel fasteners? For any given alloy, is there a torque value or tooth stress beyond which galling can be expected? What causes galling??? Is galling ever a danger with press fit assemblies? All advice or referrals welcome.

Thanks in advance
tc7

 

[Maui]

A general definition of galling is this: when two metal surfaces are in intimate physical contact and move relative to one another, sticking or welding may occur between them resulting in surface roughening and increased wear rates. Since no surface is completely flat, roughness in the form of asperities often produce an interaction between the two mating surfaces, which in many cases results in very high localized stresses. For example, this is what happens at the thread contact area between the alloy steel fasteners that you asked about and the parts they are used in. These high localized stresses produce high frictional values. And welding followed by shearing can take place between the parts under the right circumstances. If the weld is weaker than either metal, then relatively little wear takes place. This is the case when a tin based alloy bearing wears against steel. If the junction is stronger than one of the metals, then shearing will take place in the weaker metal. If you have a contact area between steel and lead, for example, the lead will shear. If the junction is stronger than both metals, then tearing occurs in both metals and the wear rate increases rapidily. Lubricants do a good job in reducing galling because they reduce the fricional contact between the parts. And if the frictional forces are decreased, then the possibility of sticking or welding is decreased. I hope that this helps you.


Maui

 

[tc7]

Thanks Maui,
That helps me get an idea of the phenomenon, but when you mention "welding" occurs, I can't relate to this happening without extremely high (melting point) tempertures occurring. Could you explain the type of welding that is taking place? Thanks for your time.
tc7

 

[kenvlach]

Maui’s explanation is correct but incomplete. An explanation of galling must include the role of the oxide film which covers all non-precious metals. This oxide film permits the widespread use of threaded fasteners by limiting the amount of metal to metal contact. Galling (cold welding) occurs when torqueing a threaded fastener results in a) shearing away the intervening oxide layers, and b) direct contact of the bare planes of metal atoms under mechanical pressure. Galling is worst (occurs at a stress lower than the desired working value of the fastener) when the oxide film is very thin and the 2 metal surfaces share the same atomic configuration. Thus, it frequently occurs for 2 pieces of 300 series SS having the same fcc lattice structure. It can also occur for Al, where the softer substrate gives less support to the thin surface oxide when loaded.

Anyhow, cold welding can also be done by hammering gold (no surface oxide) or lead (the interface gets broken up, and the oxide gets dispersed into the matrix). This also happens with some explosive and ultrasonic bonding processes. The trick with most metals is in getting rid of the tenacious oxide film.

Re press fits: Galling is not a problem if the assembly is meant to be permanent!

Hope this helps,
Ken