Basic questions

[salmon2]

Experts,

I have two basic questions:

1) what chemistry is susceptible to galling? I know maintaining hardness differential is good, reduce contact stress, lubrication, etc. But other than those what chemical element makes the metal volnerable to galling? I got a feeling it is Cr. If it is true, then Monel K500 is better than Inconel 718 since k500 does not have nominal Cr.

2) what chemistry is susceptible to SCC? Seems like CS is immune to SCC, but CRA is not. Is it true?

Thanks a lot.

 

[metengr]

salmon2;
For your questions above there will be no one answer where "one size fits all" type of generalization.

Exclusive of specialized surface treatments to improve galling resistance, galling behavior is related to the mechanical/chemical properties of an alloy itself and how it is coupled in service. Microstructure and hardness play direct roles regarding galling behavior, in lieu of any one alloying element.

SCC is produced by exposure to tensile stress, having a susceptible metal and exposure to a an environment that is conducive to SCC. All three of these factors must be present simultaneously to have SCC. Most important, you need to know the environment and then evaluate alloy systems for use based on strength level and applied service stresses. Your example of SCC and CS, in caustic solution CS will exhibit SCC at a specific concentration and applied tensile stress.

So, the bottom line is this, try not to generalize galling and SCC resistance to any one controlling parameter. There is plenty of information in the public domain regarding galling of metals and SCC of metals.

 

[Compositepro]

Galling is somewhat related to low hardness. Surface oxides prevent cold welding between contacting surfaces. Low hardness allows surface deformation which breaks the oxide layer and allows galling to occur. Nickel, copper, and aluminum are prone to galling.

 

[swall]

To add to what the others have said--as Compositepro noted, galling (adhesive wear) is cold welding on a microscopic scale between asparities of mating surfaces. To mitigate the micro welding, either provide a film, such as an oxide or lube or use metal couples than tend to not weld, such as aluminum and steel. Alloying also has an effect, if an alloy addition provides a phase that helps prevent the welding. One example would be resulfurized steels, as the sulfides help prevent the welding. Another example would be the graphite flakes in cast iron.

 

[EdStainless]

Galling is a wear phenomena, so better wear resistance usually goes with better galling resistance. The hardness miss-match help because the softer material wears and the harder one is undamaged.
Soft alloys that work harden tend to do better.


There is an environment that will cause stress corrosion cracking in every known alloy. Some may be uncommon, but they exist. Given a specific alloy, the right (or wrong) environment, tensile stress (applied or residual), and temperature (it takes 'enough') the material will crack.
This is driven by corrosion resistance and microstructure. There are entire books on the subject.

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