I'm sorry but what compelled you to do that? Cutting into an empty propane tank with an angle grinder? You like being in close proximity to explosions or something?
Ahhh, a buddy and I were making a forge for knife making. I can’t defend my choices.
I mean, it's just metal. As long as you make sure it is empty this is perfectly safe. Once a propane tank has reached the end of it's life as a propane tank it is completely reasonable to re-purpose them. You just have to be safe. Welders do this all the time. The scent they put in is actually somewhat flammable too. You take the valve out, leave it inverted, clean clean it out with soapy water, etc. It is a non-trivial process, but propane tanks are good steel.
I’ve seen it done to make barbecue grills. I’ve always used hot water heater tanks for that purpose.