Talk:Galling

From Wikipedia, the free encyclopedia

	It is requested that a photograph or photographs be included in this article to improve its quality.
	The Free Image Search Tool (FIST) may be able to locate suitable images on Flickr and other web sites.

This looks more like a dictionary entry. Should it be here, or in wiktionary? Motor 03:01, 16 Mar 2004 (UTC)

I think it just needs expanding and/or merged with some more appropriate article. --144.131.67.249 13:41, 10 October 2006 (UTC)

[edit] Original research

I plead guilty to original research in this case. I just removed the following paragraph, and replaced it with my own explanation of galling as a cold welding phenomenon:

Galling can occur when metal parts, such as the threads of nuts and bolts, are forced together and rubbing generates friction between surface asperities. The friction causes heat, which is mainly isolated to these asperities. The asperities weld together but further displacement causes these tiny welds to break, which makes the surface even rougher, creating more opportunity for friction. Galling should not be confused with cold welding.

The friction welding explanation does not match my own observations, see the main article for more details. Unfortunately, I have never found a credible source discussing galling, so I have no references to offer.--Yannick 01:06, 20 May 2007 (UTC)

I myself am an active researcher within the area of tribology, with particular focus on wear in sheet metal forming, which is often related to galling. Very often in text books, galling is used as a term describing cold welding, scuffing or severe adhesive wear for metals with no emphazises on that the metals should be a certain Fe-alloys or a pure element such as Al, as the previous author described. Generally, these materials materials are more suspectible to galling, but indeed other metals suffer from galling as well. The statement that carbon-steels do not suffer from galling is incorrect, which has been shown by several researchers including myself. If this was the case, than the automobile industry, as an example, should have no need for the use of press-lubricants during forming of carbon-steel components. But this is of course not the case.

However, the definition of galling as a single event, or wear mechanism, is very rarely used. Rather, most authors, including myself, use the expression to describe a wear process, composed of several regimes and wear mechanisms. A short description is the following: Initially when two surfaces are brought into contact and slid relative each other, a regime of mild wear is initiated, meaning that transfer of material from one surface to the other will occur. Generally material transfer occurs from the softer surface to the harder surface. This regime will continue for some sliding distance, depending on the loading, temperature, materials etc. The continuous pick-up of material on one surface localizes the contact, leading to a concentration of the contact pressure which subsequently localizes the areas of where material transfer occurs. Therefore, lumps of transferred material will grow in certain areas, which at a certain lump size initiates the entering into the second regime, where scratching of the counter-surface by the lump initiated. Generally, the lump hardness is increase by strain hardening and oxidation phenomena, which makes scratching possible. In this stage, the metal forming industry often starts to notice the phenomena of galling, since scratches begin to be visible on the formed parts. If the sliding is allowed to continue, at least in un-lubricated conditions, scratching will transfer into severe metallic adhesive wear, cold welding or scuffing etc, which is related with very high friction and possible seizure of the tool and work material. /Anders

If I understand you correctly, you're saying that the former friction-welding definition is closer to the truth than my cold-welding definition currently in the article. And yet this doesn't match my practical experience, which is that galling happens exclusively to corrosion resistant metals, and preferentially to similar mating materials. For example, even a 304 / 17-4 pair will gall less than either of those materials gall with themselves. I've found a weak web reference [1] which might make us both right. According to this, the mild wear regime which you describe will only occur after the oxide surface film has broken down, which of course is thinner on corrosion-resistant metals. Once that happens, material similarity would accelerate asperity accretion since it essentially becomes localized cold-welding. That will rapidly take us to the second regime where all the visible scratching occurs.

In other words, the mechanism you explained is universal, (and may apply to carbon steels in unusual situations which I haven't seen,) but it is greatly magnified by cold-welding in corrosion-resistant metals. Is this a good technical basis for a better definition of galling?--Yannick 04:37, 31 August 2007 (UTC)

[edit] Electrolytical explanation of galling

Could it not be that because when the oxide layer is removed and there is a electrolyt present that ther is a galvanic exchange in metal and that is why the bounding occurs? Willems.stijn (talk) 04:37, 18 January 2008 (UTC)