I have that same 50 amp plug/receptacle on all my machines. Some of my machines are on a circuit that has a 70 amp breaker even though the machines will draw more than that at full power. I've just been too cheap to upgrade the wireing and breaker. I've tripped the breaker a few times with no visable damage to the plugs or receptacles. I believe with machines that draw over 50 amps the manufacturers intend for the machine to be hard wired to the building.
If you tend to plug in and unplug your welder very often I would recommend checking the wire connections on both ends every so often. I seem to be always switching machines around and once in a while the wires will work loose causing arcing, sparking....and sometimes scaring the crap outta me