I am going to get my OEM rims repainted as they have a couple of scratches. I was going to get them powder coated the original silver but I was told the heat ~ 400 degrees for 30 minutes can make the rims "brittle" as the heat can temper them. Another engineer said no, the heat for the time in the powder coater, 30 minutes @ 400 won't have any effect on them at all if anything it will actually make them stronger.
Has anyone here had the original OEM rims powder coated?

I had SL550 wheels done (same exact style as our stocks, but wider and different offset) and no issues. I've never heard of anyone having issues due to the heat.

I've never heard anything about that in my 20 years of being on automotive forums. I have OEM W210 E55 wheels that have been powder coated on my W210 wagon and have had no issues. They were restored by one of the most reputable shops in the area as well.

Someone is blowing smoke up your ***.

i powder coated two sets of rims for some small Fords i owned in the past with no issues. ran the rims on the racetrack too. i had a similar set spray painted but the powder coat was more durable.

I work in a steel foundry and have managed both the heat treat and melting departments so I can give some input. While rims are usually not steel, at least not the kind we are using on AMG's, the heat treating principals are similar. The rims we use are made out of an alloy of aluminum and magnesium, both have a melting point of around 1,200°. Heating an alloy rim to 400° might soften the metal but I would think it would be negligible due to the fact that 400° is not close enough to the upper critical temperature of aluminum and 30 minutes isn't long enough to bring the inner temperature to 400°. Aluminum is different than steel in that it doesn't go through the martensitic transformation. This means a low temperature (400°) could potentially harden the metal, this is known as age hardening, BUT the aluminum would need to go through a solution anneal cycle prior to the age hardening. The solution anneal cycle is a higher temp (650°+) and would soften the metal before the age hardening. The place that is powder coating your rim is not going to do this. They are subjecting the rim to the 400° simply to cure the power coating.

People have been powder coating rims for years. I'm sure you are fine.

good info, but I was puzzled by this? aluminium is used as heat sinks for electrical equipment due to it's ability to transfer heat. i would have thought this would allow the inner temp to rise to 400° quite easily?

Aluminum is highly conductive when it comes to heat but the thickness needs to be taken into consideration. Using rough math, and some guestimates on how many watts the oven is producing, it should take 60-80 minutes to get the inner temp of a 25 lbs alloy wheel to 400°