the point i was tring to make is that the filler has no effect on what you are welding its the heat you need to put it in that causes the problem hence the braze on old racers.
heat treating is a remidy to the effect of the heat on an already hard metal.
but there is more than one way to skin a cat
there are meny diffrent fillers (rods,wire whatever) in the welding world trust me i know i had to sell them over std mild stuff
we dont need anything other than a good old mild steel tube for a wishbone and making it without a welded seem makes it better