i'll second that blowtorch method,mainly 'cos i didn't have the luxury of an oven.I have brazed tipped lathe tools which is a very similar operation. The way I did it was to cut a brass shim, place it on the cleaned shank, sprinkle with borax lightly, then with the carbide tip sitting on it, place it in a heat treatment oven and raise to temperature. I have also done it with a torch but it tends to blow the borax, (and sometimes the carbide and shim as well!) away so I prefer the oven approach.