The green wire is chasis ground. In the welder outlet the white and black wires are the 230 volt feed, 230 volts between white and black wires, and 115 volts from white to ground or black to ground.

With regard to the oven, You really have 4 wires to contend with. They are red black, white and green. When hooked to a 3 wire plug, the white neutral wire is hooked to the green chasis ground wire. The red and black wire are the 230 volt feed, each one will have 115 volts to either the green or white wire. The unit uses 115 volts from the red or black to the white for control circutry.

If this sounds confusing, remember that the common white wire and the green ground wire are both hooked to the same place, the middle of the secondairy winding, out on the power transformer.

It would be a lot easier to understand if Your welder plug had a black, red and ground wire, but when You have 2 conductors and a ground they use black & white plus a ground. This wire can be used for 115 volt or 230 volt service.

Wire with 3 conductors plus ground has a red, black & white wire and can be properly used to feed 115 and 230 volts to the same or different appliances at the same time. In this case the white wire is common and bare or green is ground.

You should not generally use the chasis ground wire as a neutral for 115 volt loads, but it has been commom practice in the past on electric ranges and cloths dryers. Some places code now requires a 4 wire cord for these appliances.

Clear as mud?