quote:

Originally posted by enfieldspares:
I am a bit confused by your questions, so I maight be giving a reply to which you already know the answer.

Colour CASE HARDENING is exactly that. It is case hardening of the part. That a lovely blue-brown-grey finish results is a feature of the process that some turn into a benefit...that it imparts a nice "colour" to the action.

However the true point of the process is to case harden the part to give increased wear resistance and etc.

Rust blue is a process designed to, by creating CONTROLLED rusting on the part, give it a resistance to uncontrolled rusting. It does not alter the hardness of the metal in any way and can be done on hardened or unhardened parts.

Their resistance to wear remains unchanged.

Between the two is another process. That is heating the item and quenching in oil. Usually old sump oil. This does both give a degree of change to the wear resistance of the metal part and, at the same time, colours it black. The burn carbon in the oil in fact.

I would say that colour case hardening gives better corrosion resistance to the metal than does rust blue. However this is pretty much the same as would be achieved by highly polishing the metal and leaving it "in the white".

Sometimes the heat processes in colour case hardening will cause movement in the metal and the parts will need to be "rectified" afterwards to get back to a correct fit.

Heat treatig won't affect the application of a rust blue except, if they are highly polished and hardened to make it harder to "take".

Colour case hardening will effectively "turn back the clock" if the parts have been previously heat treated.