Multiple Pass versus Single Pass? Part 3

When welding a ¼” in a single pass, you have to consider joint design. A quarter inch, single pass weld is certainly possible, but the heat carrying capability of the ¼” plate isn’t as much as the 2 inch wall pipe.

So overheating becomes a real concern, not only for the weld material, but for the torch itself, because it’s operating in a hotter environment, typically. A single pass ¼” weld on a ¼” inch plate isn’t a quality weld, any more than a ½” thick weld is on a ½” plate, because the plate heats up faster, and you have a larger heat affected zone.

But even on a thicker piece of metal, you won’t always find it practical to lay a thick, rope of a weld.  You will have more warping, and disfigurement of the part being welded.  In a single pass thinner gauge weld as it is cooling there is greater stress placed on the joint in a single, full penetration weld. The weld will typically vacate the shielding gas more quickly than a deep joint would as well and then leave you a contaminated weld because the weld cooled without adequate shielding.

Trying to weld thick metal in a single pass allows the metal to cool to quickly, even after being overheated and doesn’t allow enough time for contaminates to be floated out of the thicker weld.   You’ll also see issues with porosity, penetration, and contamination if you are overheating the metal while you weld it.

Higher gas flow rates will likely be required, and larger tungsten (if TIG) and filler metal. Quite simply, you should match your weld pass thickness to the diameter of the filler metal (except MIG).