David, I am not an metallurgy heat treatment expert, but I do know that heat-hardening alloys, at some stage of their existence, had to get into their "softer" state somehow, and getting them there again is a matter of following the heat treat recipe.
Like any stuff, there is a temperature it will lose that property, still short of melting. I expect if you know what is in the bar, you can find the annealing temperature recipe profile. Even not knowing, there is very likely a heat regime that will anneal a wide range of alloys. Ultimately, absolutely all can be made to melt. Carefully though. Some will chemically convert, e.g Titanium, unless heated in vaccum or inert surrounds..
Have a little experiment.
If you can grind off a small piece, perhaps using a Dremel-type little diamond disc cutter. It need only be a tiny chunk, and then try a few experiments. Try squeezing it between the sides of a couple of lathe cutter tools in a vise, and see which gets marked. Then put it in a few grams of silica (fine sand) and heat it up with torch to any crazy point you please, then throw on some more sand, heat it, and have it cool slow. Then try the squeeze again.
Pretty much any material can be processed back to any state. The question of whether it is a worthwhile thing to do, is different.
