Post-Flow Functions in a Welding Machine

What It Does

• The post-flow function in an arc welder keeps the shielding gas flowing over the weld for several seconds after you've completed the weld and cut off the welding arc. The weld metal takes several seconds to cool from its molten stage to the solid state. If shielding gas stopped flowing the moment the arc was cut off, atmospheric oxygen would contact the molten weld before it had fully solidified and immediately form metallic oxides. These oxides are far weaker than the solid metal and would leave a weak spot in the welded joint that could lead to a joint failure.

Shielding Gas

• Gas-shielded welders have a either a foot pedal or a hand trigger on the welding torch to start and stop the arc and regulate the amperage. The arc control is tied to the shielding gas flow control, so that the machine starts the shielding gas flowing before sending electricity to the torch. There is also a programmable post-flow timer for stopping the gas flow after the arc cuts off. A ceramic cup on the torch directs the shielding gas onto the workpiece.

Using Post Flow

• Typically, you set your shielding gas pressure at around 20 pounds per square inch, and the flow control to deliver somewhere between 8 and 40 cubic feet per hour of shielding gas. The gas-flow setting will depend on the type and thickness of the metal you're welding. The post-flow control can be programmed to give you anywhere from one to 25 seconds of shielding gas flow after you cut off the electric arc.

Rule of Thumb

• The post-flow time you select depends on the type and thickness of the metal being welded. A rule of thumb is to set the post-flow timer at 4 times the millimeter diameter of the electrode you use. For instance, if you're welding with a 2.5 mm electrode, you set the post-flow timer for 10 seconds. This post-welding gas flow helps cool the weld -- while keeping out oxygen contamination until the weld is completely safe and solid.