A weld purge monitor verifies no oxygen during argon back purge for TIG welding or argon CO2 blend for MIG welding.
A portable trace oxygen analyzer verifies that 100% of the oxygen has been purged from the back side of TIG welded stainless steel, titanium, nickel or zirconium alloys. If oxygen gas is present at the weld point an inferior weld can occur. Trace oxygen analyzers are most often used to verify low oxygen levels when welding pipe or a hollow metal design.
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.
Why is weld purging important?
Weld purging removes oxygen, water vapor and any other gases that might be harmful to a welding joint as it is being welded. The oxygen, water vapor or other gases are replaced with inert purge gases like argon or carbon dioxide from a welding joint as it is being welded. Stainless steel, titanium, nickel and zirconium alloys are sensitive to the presence oxygen and water vapor and will combine with the hot metal as it is being joined. This results in a sub-standard weld.
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.
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.
What is back purging in welding?
During welding, inert argon or CO2 gas flows from a tank to the tip of the welding rod. This shielding gas is concentrated in a small cup that surrounds the tip of the rod. Because of the gas pressure from the tank, no oxygen is present at the welding tip.
Purging is the process used to surround the backside of the weld inside the pipe with the same inert gas. Welders use inflatable plugs, stoppers or purge dams to seal both ends of the pipe, then fill the pipe with the shield gas. This displaces the oxygen inside the pipe. While it is common to turn on the gas and “wait a while” before welding, best practices are to verify the shield gas has completely displaced the oxygen before starting the weld.
Verified back purging of welds is critical to insure there are no leaks or weak points in the weld. In cases like welding in nuclear plants, submarines or when welding food-grade pipes, weld purge gas levels must be verified and recorded as part of the welding quality control process.
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.
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.
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.
There are several factors that impact the dimension of the gas plume. 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 by TecSense. This oxygen sensor provides accurate measurements down to 0% oxygen for back purging of oxygen during welding.
TIG Argon Gas Weld Purge Monitor
To verify there is no oxygen at the weld point the TecPen Weld Purge Monitor provides accurate measurements down to 0% oxygen for back purging of argon during welding.
As a trace oxygen analyzer, the TecPen Weld Purge Monitor is most commonly used to verify argon flooding during welding of stainless steel or titanium pipe in the aerospace, automotive, nuclear, medical and food-grade industries where x-ray verification of welds is required or the backside of welds cannot be inspected.
The TecPen features measurements in less than three seconds to ensure you can get measurements while working quickly, which is imperative in TIG welding. The large display is easy to read during the welding process, and the TecWeld comes pre-calibrated from the factory. As an added benefit, the TecPen records oxygen levels in its build-in memory which is important when needing to form a paper trail for inspection or verification.
The TecPen Weld Purge Monitor also features a built-in 10,000-hour gas sampling micro-pump and is an ideal choice for TIG welding of stainless steel or titanium.
MIG Argon Carbon Dioxide Weld Purge Monitor
The TecPen Oxygen and CO2 Weld Purge Monitor combines all the features of the TecPen Weld purge monitor with a carbon dioxide sensor for MIG welding. It provides accurate measurements down to 0% oxygen and up to 100% carbon dioxide when back purging with an argon and CO2 gas mix during welding. It is most commonly used for verifying C25 or C100 shielding gas during MIG welding.
Like the TecPen Oxygen Weld Purge Monitor, this monitor uses needle purge testing of oxygen and carbon dioxide levels in the back purge gas. In the case of C100 shielding gas, the TecPen can verify up to 100% CO2 levels before welding. If C25 shielding gas is used, the TecPen can also verify 0% oxygen and 25% CO2 if a C25 combination Argon and CO2 shielding gas is used.
Inside the TecPen, a micro pump continuously samples oxygen and carbon dioxide until the optimum gas level is achieved. Then welding can begin. Pressing the Save Data button records the O2 and CO2 levels to the TecPen’s built-in memory. The logged data can later be downloaded to a PC as a spreadsheet for inspection and quality control documentation.
References:
https://www.thebalance.com/what-is-the-definition-of-oxidized-metal-2340018