Just bought older welder. Instead of plug it has two black wires and a post for the ground. Welder requires 230v 60hz single phase. I have a standard 220 outlet

They both are. Welders may not have a neutral connection.

You have 2 hots and a ground. Each hot is 180 deg out of phase and will measure 120 V with respect to ground/neutral. They will measure 240 with respect to each other.