A lot of knifemakers say to buy new steel of a certain type so you know what you have and how to heat treat it. BUT
I like a lot of others are cheap and like to use what ever comes my way.
I would agree if you are making a high dollar knife for sale new stee is the way to go but if you are experimenting or trying to learn the trade and are using the knives yourself or giving them to friends try different steels.
As for heat treating to see what you have pound out a thing piece.
Heat it about 100 degrees above nonmagnetic and quench in oil
wearing safety glasses and other safety equipment place this quenched piece in a vise with just a little sticking out.
With a hammer tap the piece sticking out to see if it breaks
If not hit it harder until it breaks or bends over.
If it bends and not breaks heat another piece and quench in water then do steps 2 and up.
Hardest quench is salt water
Good luck and remember safety glasses as un tempered pieces can shatter sending very sharp shrapnel everywere.