Welders talk about purging for TIG welds like it is some great mystery. Sure, it sounds dark and dangerous. It may conjure images from creepy movies or other dark secrets. In truth, purging is a simple process that improves the quality of a weld.
What is purging, and how is it done? Purging is the process of removing the oxygen from the inside of an enclosed part prior to TIG welding. Removing oxygen from inside the part protects the back of the weld from oxidation due to the high heat of TIG welding.
Before we discuss how to purge, let’s explore why purging is important.
Why Purging Is Needed
TIG welding generates intense heat that causes the metal to react with oxygen in the air. The resulting oxidation makes the weld look bad, and it also makes the weld weaker than it should be. To protect the weld at its hottest, TIG welders stream an inert gas (argon, helium, or a mix of the two) over the weld. The inert gas doesn’t react with the weld, and it pushes oxygen out of the way of the weld.
Unshielded TIG welds look terrible, with lots of pitting and spatter. There will also be a lot of black crud on top of the bead from oxidation. Unshielded TIG welds are weaker than they should be, and subject to cracking and (eventually) breaking. The shield gas is a vital part of TIG welding.
This process works great for steel, aluminum, copper, and many other metals. It doesn’t work as well for stainless steel. With stainless, the chromium mixed into the alloy can burn off on the back side of the weld as well as the front side. If this happens, the back of the weld joint will be black and covered with rough specks. Welders call this effect “sugaring.”
Why “Sugaring” Needs to be Prevented
To protect stainless steel from sugaring, you must protect the back of the weld from oxidation as well as the front. There are several ways to protect the back of a flat weld from oxidation. All of them involve some sort of coverage on the back of the weld joint to keep air out. If you are welding pipes or other enclosed vessels, none of the methods for protecting a flat weld will work.
This is a problem with stainless-steel because it is commonly used for piping in food production and pharmaceutical manufacturing. These pipes for food and pharma need to be smooth on the inside, with no rough patches or gouges that can harbor bacteria. These pipes can’t be screwed into fittings like most pipes; they must be welded. Since the joint has to be smooth, there can be no sugaring on the inside.
Stainless pipe is also used for engine exhaust system manifolds, which require strong welds to stand up to the vibration of being bolted to an engine. Sugaring on the back of these welds isn’t a matter of smoothness as much as strength. Sugared welds will eventually crack and break, requiring repair or replacement of the manifold.
Any welded stainless-steel pipe application requires shielding the back of the weld as well as the front when TIG welding. Purging the weld is the best way to protect the inside of pipes from oxidation. There are six basic steps to purging a TIG weld:
- Prepare the pipe ends for welding
- Seal the inside of the area to be welded with caps or dams
- Fit pipe ends together for welding
- Tack weld the pieces together for stability
- Purge the weld zone of oxygen
- Weld the joint together
Prepare the Pipe
Just like other TIG welding applications, the metal must be spotlessly clean before TIG welding. Wipe the ends of the joint to be welded with acetone, methyl ethyl ketone, or paint thinner to remove grease and oil. Degrease both the outside and inside of the pipe. These products also remove paint, but that is not usually an issue with stainless steel. Since it resists corrosion, it is almost never painted.
Once the metal has been degreased, rinse it with distilled water to remove all traces of the cleaner from the joint. If you are TIG welding most steels, you also have to grind the metal along the joint to remove rust and mill scale. However, that usually isn’t a problem with stainless. Double-check it for rust and corrosion, just in case. Dry-fit the pipes to make sure everything will align and is the right size.
If you are welding pipe 1/8 thick or less, you are ready to proceed. If the pipe is 3/16 inch thick or more, you need to bevel the edges. Grind the edges back at a 60° angle on both pieces of pipe. This gives the bead more surface area to work on and keeps the weld from being as tall above the surface of the pipe.
Some welders prefer to bevel the outside of one pipe and the inside of the other. This lets one slide just a little into the other pipe, giving a tighter seal when the pipes are dry-fitted together. Check the plans or specifications for your job before doing this. If they call for a specific bevel, match that. If the plans don’t say anything one way or the other, go for it.
Seal It Up
The pipe must be sealed before purging to keep oxygen from coming back in. For small pieces, sealing is accomplished by capping or plugging one end with a solid cover and the other end with a fitting that allows you to attach a gas hose. These fittings will hold the inert gas in place during welding to protect the weld from oxygenation.
Bigger systems usually use removable dams or water-soluble seals for purging. This lets you seal up just the section around the weld joint instead of filling the entire system with expensive purge gasses. If you are welding a system with lots of pipe, the cost of the gas needed to purge the entire system can be excessive. It’s much cheaper to just seal up around the welds.
In some cases, the manufacturing specifications call for purging the entire system instead of just damming sections, so review your specs before damming. If you have to purge the entire system, plug the ends and purge just like working on a small piece. Make sure to allow enough time for the entire system to purge before welding
Isolate Smaller Areas
For sealing bits of a larger system, removable dams can be purchased or fabricated. The dams may be solid or inflatable. They are sold in pairs that are connected together by a line or strap. That way, when you pull the near dam out, the far dam follows. Install the dams on either side of the weld joint. Leave enough space between the weld and the dam to protect the dam from weld heat.
If you can’t easily access the dams after welding is complete, water-soluble adhesives or water-soluble dams are used. Adhesives are used to glue a film like plastic wrap inside the pipe at a safe distance from the weld. Soluble dams are installed like removable dams but left in place after welding.
Whether you use water-soluble adhesive or soluble dams, either the glue or the dams themselves will disintegrate when exposed to water. When the system is flushed or pressure-tested, the water will break down the adhesive or the dam and remove the obstruction.
The seals used for purging pipe aren’t perfect seals. One must have a fitting or a hole to allow the purge gas in; the other should have a pinhole or two to allow air to escape. Be careful when introducing the purge gas. If you flood the gas into the pipe at high pressure without thinking, it will mix with air and leave some oxygen behind in the pipe.
Pour the Gas, Don’t Spray
Imagine what it looks like to spray water from a hose into a pool or bucket. The spray makes tiny bubbles that take a while to disperse. Purge gasses work in the same way, but purge gas dispersed into air takes much longer to dissipate. You want to make the entry of the purge gas as smooth as you can to prevent mixing with air.
The arrangement of the holes should laid out based on what purge gas you are using. Argon is heavier than air. It purges best if you feed the gas in from the bottom and have the escape hole at the top. This lets the argon flow smoothly in while the air floats up and out the pinhole. Picture filling a tub with water – you would get much less splashing if you run water up from the bottom instead of from the top.
Helium should have the opposite arrangement because it is lighter than air. Feed in the helium from above and let the air drain out the bottom of the pipe. Arranging the holes to take advantage of the properties of the purge gas is the best way to keep the gas pure in the pipe. It’s like pushing air into a bucket instead of bubbling air up from the bottom.
For small purges, you can just feed the gas straight in from a hose. However, for big projects with higher flow rates, it is best to use a gas diffuser for smooth flow and no gas / air mixing. The diffuser spreads the gas out over a larger area and slows it down. It’s the difference in a hard spray of water versus a gentle pour. The gentle feed doesn’t disturb the air and keeps it from mixing with the purge gas.
Fit the Pipes Together
Once your caps or dams are in place, fit the pieces of pipe together. Some welders leave a tiny gap between segments, less than 1/8 of an inch. This makes for a strong weld. Others butt them tightly together to help seal the weld joint tighter. Some go so far as to make opposite bevels that let one pipe fit into the other for the tightest seal.
However you fit them up, make sure the pipes are perfectly aligned all the way around and run straight. Check the alignment with a straight edge to guarantee the alignment is correct. Clamp the pipes in place to hold them still. It is important to get this right because it’s your last chance to change things. After you fit them up, you will tack weld, and the relationship will be permanent.
Tack Weld for Stability
Once the pipes are in place, you want to keep them aligned. Put three or four little tack welds around the circumference to hold them together properly. Don’t overdo the tack welds – since there is no purge gas inside yet, they will sugar. A tiny bit of sugaring on tack welds isn’t a problem, but you don’t want much. Just a few little tacks to keep everything nice and stable for the real weld.
All you need is a tiny bit of connection to hold the pieces in place. Since the backs of these welds are unprotected, you need to minimize how much you do. Short welds will shorten the amount of sugaring on the back. Quick welds will also minimize sugaring because the steel won’t heat up as much.
To make the tack welds, start by cracking an arc. Let a weld puddle form, then dab the rod into the puddle. Then stop. If you are welding pipe, make another tack 1/3 of the way around, then a final tack midway between the others. If you are welding really big pipe, you might want to put four tacks instead of three. For flat stock, put a tack about every foot.
Purge the Weld Zone of Oxygen
Once the pipes are stable, it’s time to purge the oxygen. You can use argon or helium, or a mix of the two as a purge gas. Argon is preferred because it’s less expensive than helium. On the other hand, helium carries arc heat better. Helium lets the weld penetrate better for thick materials. It also lets you work faster because of the higher heat.
Don’t use carbon dioxide for welding stainless – the carbon can react with stainless steel and cause problems with the weld. You will wind up with carbon steel around the joint instead of stainless. This is a worse defect than sugaring, so avoid CO2.
Some purge mixes contain a little bit (five to ten percent) of hydrogen. The hydrogen reacts with any residual oxygen in the chamber to form water before the oxygen can react with the stainless steel. 200 and 300 grade stainless benefits from purge gas with hydrogen are the most common types of stainless, especially for food-grade applications.
If you are just doing a little bit of purging, you can purge with the same gas you use for shielding. Install a Y-fitting on the hose from the gas bottle and use the second line as your purge line. You will need to increase the gas flow rate coming out of the bottle to compensate for the second line.
Hook your purge line up to the inflow fitting on your purge dam and start the gas flow. The standard flow rate for purge gas is forty cubic feet per hour (CFH). Give the purge some time to work. If you want to be absolutely sure it has taken effect, oxygen meters are available that will tell you when the oxygen level has dropped enough to weld.
How Long to Purge?
You can calculate how long it will take to replace the air with purge gas.
- The volume of a cylinder is π R2H, or 3.14 X radius (half the diameter) X radius X length.
- If you measure in inches, divide the result by 1,728 to find cubic feet.
- At a flow rate of 40 CFH, it takes 1.5 minutes to move one cubic foot of gas.
- Add 50% to the time needed just to be on the safe side.
Since purging takes time, you can use this opportunity to practice the weld you are about to make. This is especially important if the pipes are being welded in place in a larger system. Sometimes it can be hard to reach the back of the weld, leading to awkward torch angles. Poor torch control leads to bad welds. You have time now to think through the weld and make sure you can keep the torch running right.
Once you have given the purge gas time to fill the chamber, you can start welding. Keep the purge gas flowing while you weld so that air can’t get back into the purge zone. Once the weld is done, you can cut off the gas and remove the seals or dams on the pipe.
Welding is the easiest part of this process. Crack an arc and lay a bead in the joint. The weld works just like any other TIG weld. Let a puddle form, dab the rod in, and move the torch. Keep dabbing and moving until you have gone all the way around the pipe, and the weld is complete.
Protecting Flat Welds
If you are welding flat material, you may still need to protect the back of the weld from sugaring. You can’t seal up a flat weld like a pipe, but there are other ways to protect the back of the weld.
Flux is a combination of chemicals that protect metal from corrosion during welding. Stick welders and some wire for MIG welders include flux in the rod. Blacksmiths use powdered flux in forge welds. You can apply powdered flux to the back of a stainless-steel joint to protect the weld from sugaring. There are fluxes made specifically for welding stainless.
Stainless steel fluxes may be liquid, gel, or powder. After cleaning and preparing the joint, apply the flux as directed on the packaging. Be careful with the flux – some contain acid or other corrosive materials, so you don’t want to get it on bare skin.
Once you have completed the weld, you will need to remove the flux from the back of the weld. An angle grinder is the easiest way to do this. You can also use a wire brush to scrub it off, but this can take a while.
If you are working with flat stock, flux removal isn’t a problem. If you are welding pipe that must be sanitary (for food or pharma manufacturing), you won’t be able to clean out all the flux from inside the pipe. Purging is the way to go for sanitary pipe. Flux is fine for exhaust manifolds and other pipes that don’t have to be sanitary inside.
Fiberglass tape is also used to protect welds from sugaring. The tape contains a strip of adhesive down the outside thirds of the tape, with a strip of fiberglass down the middle third. This tape protects the back of the weld from oxygen and captures the shield gas from the welding torch in the weld.
You can’t use just any tape for this. The fiberglass strip is heat-resistant so that it will stay in place even with the heat of welding. It also keeps the adhesive away from the joint because the adhesive itself can become a contaminant. If you want to protect your weld with tape, only use welding tape.
To use welding tape, fit and clamp the pieces to be welded. Flip them over so that you can access the back of the weld joint. Lay a strip of tape along the joint with the fiberglass centered along the joint edge. Turn the piece back over, and you are ready to weld.
A backing bar is a plate that sits behind the joint while welding. Backing bars that have capacity for gas flow are available. These have a groove down the length of the plate with holes connected to a shield gas supply. The groove becomes a little purge chamber that protects the back of the joint, just like the shield gas flowing from the torch protects the front.
A traditional protector for the back of flat welds is big pieces of copper. The copper protects the weld in two ways. First, it reduces the oxygen exposure and traps shield gas. It does this just by being physically in the way of air flow to the joint. Second, copper absorbs heat very well. It keeps the back of the weld joint cooler, which also reduces oxidation.
The key to making copper backing work is mass. You need sheets long and wide enough to block air from the whole weld. You also need enough weight to absorb heat from the weld and move it away from the joint. Thin sheets won’t cut it here. These pieces should have some weight.
Flux-Core TIG Rods
A new development in TIG welding is the flux-core rod. Like flux-core wire and arc rods, these rods have flux incorporated into the rod. You can TIG weld with these just like any other rod and be certain that the back of your weld won’t get sugar, even if it’s not otherwise shielded. Like standalone flux, flux core rods shouldn’t be used for sanitary pipe.
Sometimes you need to weld something with an odd shape that can’t be purged or protected in the traditional ways. For those items, you need a purge chamber. Purge chambers are clear plastic bubbles that can be filled with purge gas. The bubbles have built-in gloves that are sealed at the elbow so that you can handle objects inside the chamber without letting gas out.
To use a purge chamber, you clamp up the pieces to be welded and insert them into the chamber along with the rod and torch. The chamber is inflated with purge gas, and then you can weld the part with everything shielded.
Purge Gas Safety
Purge gasses are clear and odorless. They (by definition) contain no oxygen. While that is good for stainless steel, it is bad for you. Breathing pure argon or helium is deadly. Helium collects at the top of sealed chambers, while argon collects at the bottom. You can’t see them, but the gasses pool up just like water in a bowl.
Never put your head into a purge chamber while the gas is flowing or immediately after welding. Give the gas time to disperse before you put your head in. Let the weld cool and the gas disperse before looking at it. You can’t smell pure helium or pure argon, and you can’t see them either. Breathing either will remove oxygen from your blood quickly. The first symptom of breathing purge gas is passing out.
If you are working with purge gasses, do not work alone. If you breathe 100% argon or helium, you will pass out without ever being aware of the danger. The gasses don’t look, feel, or smell unusual. One second you are breathing, the next you are out. The good news is that you will be fine – if you are pulled out of the purge gas immediately. The bad news is that you can’t pull yourself out.
Never use purge gas alone, and never put your head into a purge chamber. Breathing purge gasses won’t hurt you – it will kill.
As you can see, purging is neither dangerous nor difficult, but it’s important it be done correctly.