It depends on the amperage, the material and the type of weld joint. A 150-amp welder can weld steel, using a V and root gap of 6 mm. If you were welding a butt weld, this drops to 4 mm. It’s not uncommon for 150-amp welders to fuse 0.25-inch steel.
Step up the amperage to 200, and you could increase that thickness to 0.50 inches.