it all depends.... as to aluminum vs steel..
Aluminum is more susceptible to heat than steel GENERALLY speaking...there are a few exceptions with steels that are really temp sensative, but in a general sense, aluminum alloys are more temp sensative than almost any steels...
However, any metal which is heat treated or cold worked, forged, recrystalised, etc. will suffer some effects from heat, but some effects could be better, worse, almost so close to no change, that we dont care...
in the case of aluminum, and specifically certain types of aluminum alloys (for instance those with a significant amount of magnesium content)....
Im going to give the elementary school description of the general way aluminum is hardened:
SO for an aluminum alloy to be heat treatable, the constituent phase elements (magnesium et al) must have significant solubility in aluminum, specifically, we want that solubility to get significantly HIGHER as temperatures go UP...
This is all determined from phase diagrams.... something i basically spend my life with my nose in..
Think of this similar to salt in water, or sugar in water... at room temperature, you can put x amount of salt in it, but if you heat the water up, you can put a LOT more salt in solution...
similar.... notion
(my materials engineering prof's would be crying at this explination)
Ok, now, this high saturation occurs at about 420 C..
So what they will do, for heat treating is this...
Take a hunk of aluminum alloy that has been made... this aluminum will be a mish mash of different compounds inside the aluminum, and generally have a much lower strength than treated aluminum..
SO they stick it in the oven at 420 C and leave it there for a bit...
The idea is to get all the constituent phases to homogenise back into a big solid solution... That is, they want the aluminum to get up to the point where its solubility for all the extra bits that make the alloy is really high again, so all the compunds disapear back INTO the aluminum again...
This would be the same idea as heating up the water, and stiring back in to solution, the salt particles that have settled out on to the bottom of your glass...
So now we have hot aluminum, BUT its all solutionised, so there are no extraneous compounds, this is GREAT...
So we quench this down to room temperature... depending on the alloy we are dealing with, this could be quenching in water, oil, molten salt baths, leaving it on a counter top with a fan blowing accross it, or just turning off the oven and letting it cool...
All alloys have a different cooling curve which dictates the speed which it MUST be cooled to maintain the fully solutionised aluminum...
Ironicly, the alloying elements put into the aluminum have a HUGE impact on this
So now that we have aluminum in single phase at low temperature.... We need to harden it now...
SO NOW it is reheated to about 135-175 c (different temperatures accomplish different things, and at different RATES)
What this causes to happen is some of those constitutent elements start precipitating out into the aluminum... basically they look like littly black dots and are intermetallic compounds, which will fall onto the grain boundaries and all around of the aluminum...
This USUALLY is done over the course of 1hr to 1 day.. depending on
temperature, desired temper, desired properties, etc..
Believe it or not, there are SOME aluminums, which will do this at room temperature, so they must keep the metal in a freezer ....
Certain aircraft parts are this way... They put them in place in the airplane and then they basically turn ultra hard after they have been put in place... its VERY cool!
So, basically, any time the metal is brought back up to this temperature range, its causing these little particles to start growing again...
I tried to find a hardening curve for various temperatures online, but this is the best i could find... and its not specific to aluminum, its a different alloy all together (some magnesium alloy)...
BUT the important trend is there... The y axis is hardness, and the x axis is time... The temperature is held constant, and each line is a different alloy...
The important thing to note, is for any alloy, when the alloy is left in the oven too long, the hardness starts falling off FAST...
and the higher the temperature the FASTER this occurs... which is why most heat treatments are done at fairly low temperatures...
For an idea of how fast this can happen in aluminum if you go back to my CHAZZO, tests.... i was able to destroy the heat treatment in that aluminum, at 325 C in under an hour.... an extreme example to be sure, but the idea remains the same... lower temps take longer to accomplish the same idea..
SO, in regards to the sprocket... the big point is, how hot and how long...
If these are kept in check and reasonable, no worries... You may end up weakening the sprocket a little bit, but i doubt it will be to a point that it matters...
It really depends on the treatment of the sprocket itself, and determining where the company that made it has the sprocket in the hardening curves...
If they take it to the raged edge, it has no where to go but down when heat is applied....
Would i ever do a powder coat to an aluminum sprocket and put it on a turbo bike.... EHHHH probally not the chances of stress cracks developing would be too high....
a stock bike, i think you could get away with it no problems.. as long as things are kept REASONABLE...
Also, in regards to steel.... toss all this information out the window... its an entirely different process all together... and
MOST steels will be fine under a standard low temp powdercoating condition.. or for that matter cast irons will have almost no change (why ductile/nodular irons are great in these temperature ranges)...
Also, this is why aluminum engine blocks are made from TOTALY different aluminums... they are casting alloy's like c355.... otherwise you would end up with engine blocks failing all day long..
Lastly, i skipped over a LOT of information specifics, and critical aspects to the process trying to keep it understandable.... This is far and away NOT the whole story... there is a LOT more going on here...
Some cases i used analogies that get a basic concept across, but fail to model other very important ideas...
So just realise that this is not the whole story, but more like the cliff's cliff's notes...
disclaimer: hehehe like a lawyer, this is not a professional opininion, and all this advice should be taken as nothing more than internet hear say, and not a suggestion of methodologies, or processes for alloying, treating, or hill billy garage working materials.... i am not responsible for you doing something stupid or playing metalworker at home!
I said in Inglis por favor 
lease: 
