craigd
I've had success with brass brazing bolsters, then tempering O-1. Brass melts close to O-1's critical temp so with a little care, it doesn't run all over the place. If you heat treat with a torch, the is no reason to get the bolster area all that hot.

I have switch to fitted bolsters - the tang is stepped (ie the blade has shoulders) and I make a cap with a slot for the tang. The other end of the cap fits the handle like a socket. I use a steel hammer for driving these chisels.