thoughts and opinions on annealing an HK

Ok guys here is my problem. A friend of mine works for a metal coating industry. He owes me a few favors so I was considering letting him put a black titanium coating on my HK91. this coating is supposed to be virtually impervious to rust. The problem is, the process requires a annealing stage of 3 hours in an oven at 375 degrees to prep the gun.... I am very skeptical about doing that to my HK. what do you guys think?

somewhat tough question.

My first reaction was hell no, dont anneal, because that will distroy the temper and weaken the steel. Potentialy creating a dangerous situation in the breach.

However, 375F is not hot enough to anneal steel. Its the kind of temperature range you use to draw the temper after quenching, and then you dont do much tempering. Tempering temperatures in the 500 to 1000F range are not that uncommon. I would doubt it would draw the temper, but I would be reluctant to test fire it after the process without a lot more knowledge of the specific metalurgy of the gun.

Sounds like a marginal idea. It might work nicely, it might distroy your gun.

Not nitpicking...........

Heat tempers the steel, it doesn't destroy the temper :-D

Correct, Heat tempers parts, but not all tempers are the same.
In general, higher temperatures and longer times result in softer, weaker parts in the same alloy.
The tough question is were critical parts that will be exposed to 375 F in this process intended to be tempered at a higher temperature or for a shorter time at 375 F.

The 375 F range is approprate for tempering some steels to specific hardness and strength ranges. This would typicaly be hard, high strength applications.

So, is 375 F for 3 hours safe. Dont know without knowing the alloys in the gun and what the current hardness and strengths are.
Could be the parts were drawn at 900F for an hour and exposure to 375F wont be a problem. Could be they were drawn at a much lower temperature.
Most of the tempering charts I have seen have values below 200F on them. This is typicaly for very hard parts (punch dies, end mills, etc).
I also recall that the charts had a standard drawing times of about an hour.

(any stainless, aluminum or other alloys that are not hardened by martensitic transformations and the problem becomes even more complicated)

Yes, there is a lot of old info out there about home shop tempering. The rules of thumb are ok for parts that dont fail in manners that harm the operators.
A lot is based on stuff written by blacksmiths useing simple carbon iron steels.
Chrome, Ni, Mo, and other elements all change how easily an alloy tempers. These elements are all common in steels used in firearms manufacturer.
Going by color or flashing in a pool of oil to judge the temper may be fine for hammer springs, but not for locking lugs on a breach.

No good way to know without being a lot more farmilear with the gun.

375 C? I would be very cautious and find out more about the whole process and metallurgical effects!

Also you need to be aware that doing this may act as a stress relief, and when the stress is relieved your receiver may no longer be straight, square, and or true :-).