I have a part (it's a gear) and I'm uncertain if it is heat treated.
Since I don't know, is there any harm in having it heat treated ?

If it is NOT previously heat treated, then I'm safe.

If it IS in fact already heat treated, then it would be " doubly" heat treated. Is this bad ? Is this even a concern ?

See my question ?

Maybe
No
Probably not
But the guy at the heat treat place can answer all of that. It will be a lot easier for him because it will be in his hand.

Gears are usually case hardened, a heat treat facility will have a hardness tester and can advise you. A simple "file test" will usually identify a case-hardened part...if an ordinary file will remove material from the gear, it's not case hardened.

,,
hi

dependes also on what you want to gain

how about cyrogenic treatment.

i am currently waiting on a trsns i sent out to be treated cyrogenic

Who did you have do the work? I have a T-10 that I'm seriously considering doing this to the gearing.

sometimes, re-heat treating a part will ruin it.
at work we heat treated a few parts we made in house, and if not done correctly, trying to redo them usually ended up causing the parts to be scrapped because they became very brittle.
as i was not personally involved in the heat treat process, other than machining the parts, i don't know if it was the process used, the guy doing it, or other factors.
just my [very limited] personal experience. your mileage will vary.

The shop doing thew heat treating should be able to guide you what needs to be done if anything.

What John said WRT to case hardening (carburizing) is most often correct. However, you really need to know what steel grade you have if it needs further heat treatment.

OK, I took a file to the gear, fairly sharp file. It kind of "marred" the gear surface, maybe took off a VERY slight amount
of surface metal, but I was using pretty light pressure.
But it's definitely harder than angle iron.

hi

should be able to get it done in any decent size city near you .

i found a guy in york pa , 15 miles from me . 717 309 0639

search the net for cyrogenic treatment near you !

Thanks for the info. I'm in ND, there is no "decent size city" near me. (of course I guess it depends on what you consider "near", definitely not 15 miles away)

A thorough answerer can be better provided with more info. What type of gear is it, what's it indented use etc.?
Heat treating is not "Just" hardening of metal. I suggest you read this HEAT TREATMENT