What is a weld purge monitor?
A weld purge monitor verifies that all of the oxygen has been purged from the back side weld. It is a type of trace oxygen analyzer specifically designed for industrial use.
Weld purge monitors are typically used during TIG welding of stainless steel, titanium, nickel or zirconium alloys. If oxygen gas is present at the weld point an inferior weld can occur.
Weld purge monitors are most often used to verify low oxygen levels when welding pipe or a hollow metal design where precise welds are critical. For example, they are used in the construction of food processing equipment, power plants, aircraft submarines, or anywhere a defective weld could have catastrophic or deadly results.
What is weld purging?
Weld purging is the displacement of oxygen from the backside of a weld. Stainless steel and some other metals are sensitive to the presence oxygen during TIG welding. The oxygen chemically combines with the hot metal as it is being joined and weakens the bond. Purging solves this problem by displacing the oxygen with an inert gas. It is most commonly used when welding pipe or other structural metal objects.
What are weld purging benefits?
- Saves time. No need to rely on a rough estimation of time to purge pipe. Welding can begin the moment purging is complete.
- Saves money. Verifying when purging is complete reduces purge gas consumption to only the amount needed.
- Saves product. Welding is never started before purging is complete. This reduces scrap by producing a top-quality weld every time.
Why is weld purging important?
TIG welders are often asked to fix a pipe crack or leak from an improperly purged weld. The problem began when the original welder used a welding purge plug on one end of a pipe and aluminum tape on the other end to seal the interior for an argon gas purge. The original welder purged the pipe “for a while” until they guessed it was full of argon, and then started welding. Because argon is a colorless, odorless inert gas they have no way of verifying if all the oxygen is purged from the backside of the weld.
In addition, weld purging is even more vital due to its ability to prevent oxidation and avoid discoloration and weld under bead discontinuities.
What is the process of weld purging?
The most common way to remove gas from the weld zone is to flush it away with inert gas. The weld zone can be contained to prevent fresh gas from entering once the contained volume has been purged.
Once the weld is complete, the only verification that the purge successfully removed the oxygen from the backside of the weld is to visually examine it for carbide precipitation or “sugaring. If the backside of the weld cannot be seen, they may assume that because they used argon gas to purge the weld it is “good.”
In other cases, when welding a long stretch of stainless steel pipe the welder may start welding before the purge is complete. Depending on the length of the pipe, it may take hours to completely fill a pipe with argon gas. Even if the welder uses weld purge paper or inflatable stoppers to create a purge dam (not always allowed) they still cannot guarantee the oxygen purge was complete.
The issue of improperly back-purged welds becomes compounded when a TIG welding job is sub-contracted. Unless the contractor requires x-ray inspection of every weld, only a trace oxygen analyzer with data logging can verify the complete purging of oxygen before welding is started.
Are weld purge monitors used for TIG or MIG welding?
Weld purging is most often used in Metal Inert Gas (MIG) welding and Tungsten Inert Gas (TIG) welding, also known as Gas Tungsten Arc welding (GTAW).
While both MIG and TIG welding use an electric arc to create the weld, MIG is more common because it can be used on steel, stainless steel and aluminum of almost any thickness. TIG welding can be used on more kinds of metals or can be used to weld 2 dissimilar metals. However, both MIG and TIG depend on shield gases to create good welds.
What is gas plume measurement?
Another factor to keep in mind during welding is the weld plume. The dimension of the plumes are important. The plume needs to be large enough to cover the entire area of the weld, and be at or very near the eutectic point where the metal loses its hardness and becomes receptive to oxidation and other undesirable processes.
The probe nature of the TecWeld allows the dimension of the plume to be measured. Too much flow will be indicated by an oversize plume, which can in turn cause inappropriate cooling at the weld point and cause the weld to become brittle. Brittleness can occur when the heavy element of the filler metal and the base alloy are joined and cooled at different times. This is sometimes referred to as the passivation of the weld, because it does not allow the stratification of the weld component from uncontrolled atmospheric or thermal conditions.
Oxidation in welding
Oxidation is a form of metal corrosion that forms during the welding process. Oxidation prevents a good weld from forming because too much oxygen enters the area where the weld is being created. Electrons move from the metal that is being welded to the oxygen molecules during the process. Too much oxygen in the welding environment leads to corrosion, which then in turns causes a weak and unreliable weld. Corrosion can be dangerous if it affects an integral inner structure such as buildings and bridges, pipes, and ships. A weakened infrastructure can put lives at risk, so reduce oxidation in welding is essential for safety.
An example of oxidation in welding is carbide precipitation, also called sugaring, which occurs on stainless steel. Sugaring is the result of the back side of the weld being exposed to oxygen in the atmosphere during the welding process. Even though the weld looks good from the front, it is weak at the backside and over time can display stress cracks. The best way to avoid sugaring is to purge the back of the weld with a shielding gas such as argon. This is especially important when welding pipe where you cannot view the backside of your weld.
How does a trace oxygen analyzer verify weld purging?
A portable trace oxygen analyzer measures the oxygen level near the backside of a weld down to the parts-per-million (ppm) level. A hollow stainless steel needle is inserted near the back side of a weld where a gas sample is taken. Purge dams are used to close off the area to be welded and the purge gas is applied. A micro-pump pulls the gas sample across a high-speed optochemical oxygen sensor and displays the oxygen level on the screen. Once the oxygen level measures 0% or less than 2 ppm the pipe can be considered “purged” and welding can be started.
An example of a trace oxygen analyzer for welding is the TecWeld TS-300 Weld Purge Monitor. This oxygen sensor provides accurate measurements down to 0% oxygen for back purging of oxygen during welding. Learn more about the TecWeld Portable Weld Purge Monitor here.