It takes approximately 60-70 amps to weld 1/4 steel. The exact amount of current needed will depend on the thickness of the metal and the type of welder being used.
A higher amp welding machine will be able to weld thicker metal than a lower amp machine.
For example, a 220-amp welder can weld metal that is up to 1/4 inch thick a 140-amp welder can weld metal that is up to 3/16 inches thick a 100-amp welder can weld metal that is up to 1/8 inch thick.