Stainless steel is the preferred material in many applications because of its resistance to corrosion. When preparing to weld a stainless steel pipe, purging is an important step to ensure that the weld is successful and that it remains intact for the long term. While our demonstration involves a vehicle exhaust tube, purging stainless steel is important when welding pipe for food storage and other applications where impurities must be avoided. For example, failure to purge a pipe that transports food or beverage materials could lead to particles becoming trapped in hard-to-clean grooves, resulting in contamination. On the outside, a weld on purged and non-purged stainless steel pipe looks identical. On the inside, however, is where the important difference occurs. A purged weld is flat and tied in on the inside where the weld metal comes together. There are no valleys that could potentially threaten the integrity of the weld. Both portions of the tube look exactly alike. A non-purged tube displays a “sugaring effect” resulting from the presence of chromium oxide. The presence of oxygen in the weld seam results in oxidation. A noticeable line forms where the weld metal comes together because it doesn’t fuse well. Non-purged stainless steel could crack, especially in high-vibration conditions. Caustic materials could become wedged in the line and eat away at the weld. Purging removes oxygen and nitrogen and replaces those gases with a 100% argon gas atmosphere. So to ensure the integrity of the weld, purging is an important part of the process when connecting (or repairing) stainless steel pipe.
An interesting thing to note is than some codes of construction do not address root oxidation as a discontinuity that must be evaluated. The requirements for this are often addressed in project specifications .
