Among the "jobs" that the slag does is protecting the hot bead from the atmosphere, and to a lesser degree, slowing the cooling of the bead. Alowing the weld to cool before removing the slag is the way to go.
Aww, they're just having a little fun after a long hard week is all.
I don't know a specific place to find an answer to your question but it has been a "rule of thumb" that you should let the weld cool below red before trying to remove slag from a weld. That is mostly to prevent nitrogen and oxygen from reacting with the hot metal.
Now I won't say there is a any sound basis to that - it seems to make sense but it may be just a myth for all I know.
Hopefully someone will jump in with a good answer for you.
The slag insulates the weld to reduce the cooling rate and it protects the hot metal from the atmospheric gases. The color of the weld, be it silver, yellow, blue, purple, etc., is indicative of the temperature of the weld when the slag was removed. Since the cooling rate influences the hardness of the weld bead and HAZ, it is best to leave it intact as long as practical. As for a magic number, I believe I would opt for 300 degrees F since all of the constituents are solidified and cooled to the point where the bulk of the retained Austenite should have decomposed into Ferrite, Pearlite, or Martensite by the time 300 degrees has been reached. That is not to say all the retained Austenite has decomposed by that temperature. Martensite can continue to form from Austenite as long as the temperature drops, but most of the decomposition (maybe 98%) into Ferrite, Pearlite, or Martensite has occurred by the time the temperature is at or below 300 degrees for low alloys of steel.
Best regards - Al
KK,
I had to hesitate after your last post. Answers given in jest are still answers. Chet is right, the guys, all of us, like to have fun, and brag about ourselves, on occasion. But, come on, sure, I can teach any monkey to pull the trigger on a GMAW, and maybe, strike an arc with a SMAW, but run a bead where the slag will actually peel up. Don't slam good men with pure nonsense, unless you're talking about 7024 on a flat plate, then, who needs a monkey? I can just lay it down and it will weld a beautiful bead and the slag will start peeling up seconds later, WHILE IT IS COOLING.
See, actually, up-ten & sw, along with the other posters, all went basically the same direction. Once the slag cools to the proper temperature it starts cracking, popping off, and even peeling up if all conditions are right. At this point you can be sure it is 'safe' to start chipping the rest of the slag.
As to your question though, if you want an answer from a 'code', I can't find one. Our common answer comes from experience and common sense. Then there was DaveBoyer's information that touched on the metallurgical aspect. Without a book handy to help me through this, there probably is a 'more proper' temperature range at which to chip slag because it does have a purpose in covering and protecting the weld while it is solidifying. But I don't know what that range is. There are many elements which need to be kept away from the molten metal and it aids in temperature stability as cooling takes place. Slower cooling is much better. That's why pre-heat and post-heat are often recommended and at times required (per 'code'). Hopefully one of our more science minded contributors will show up to better explain the things that I have so poorly stated.
Hope this is of help to answer your question.
Have a Great Day, Brent
Ummmm, 
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