Ok, and I understand how hardness is achieved, but I am not worried about the welded area that will be recieving the brunt of the heat. I also won't comment.on that as I don't know what type of welder he is using, his amperage, speed, etc, but I will say that something that small, being notched, should be a quick weld without much heat transfer, and most of that heat should stay in the welded area.
But I digress.
The area I would be worried most about is the threading. Now I may be wrong to worry at all, but putting that much added leverage onto the threading could possibly cause a weak point on a standard bolt. Diy spindles I have seen use hardened bolts, and for good reason. Threads are easily stripped on stabdard bolts, easily stretched, etc. On hardened bolts it is harder to do this. Using a hardened bolt the threads should stay hardened.
The amount of welding sjould be minimal, and amperage low enough that the heat will generally stay near the weld, the area near the threads should stay hardened for the most part to help prevent failure of said threads.
I wouldnt worry about the weld because of the way he shows it being notched, he is onviously going for maximum penetration. Using a standard bolt though, just I myself wouldn't do it. I've seen nuts literally rip off of standard threads at high stress applications where a hardened bolt should have been used.
Just my .02€ my friend...