Stainless steel bolt kits.

300 series stainless doesn't harden through heat-treating. It work hardens, meaning as you bend or deform the material it will harden up. 304 is not very hard. 316 is harder.
300 series isn't 'brittle', but as it work hardens from deformation it will fail in a brittle manner.
The other thing with stainless is that it can pit and erode when in an oxygen free environment. Think threads, or mating surfaces of two parts bolted together. Having parts pre-passivated (acid treated to pre-oxidize the surface) can help, but as threads engage they can scratch and remove the 'white layer' that provides the corrosion resistance and then galvanic action can attack the threads. Most hardware comes passivated.

Really hard to beat tempered alloy steel bolts for strength (most blackened hardware, including socket head cap screws).