What Goes Into a Signature Burns Stainless Weld?
"Gas tungsten arc welding (GTAW) is the process most often used to meet high aesthetic and quality standards. It is the most complex of welding processes and takes considerable practice to master."Marcia Sommer- thefabricator.com
What steps go into a signature Burns Stainless weld? What steps should our customers take in assembling our products into their build? Although not exhaustive, here is an overview of the "art of TIG welding."
The Birth of Tungsten Inert Gas Welding
Historically, the first welding technique was performed in the middle ages by blacksmiths. Known as forge welding (Figure 1), two pieces of metal were heated until glowing and then hammered together to form a bond. This technique was employed by skilled craftsman through the Renaissance and after. By the late 19th century, electrical arc welding was introduced using carbon electrodes. Over the next many decades, arc welding was perfected by the development of technologies such as coated electrodes and alternating current transformers, Gas Tungsten Arc Welding (GTAW), aka TIG welding, was perfected in 1941 and played a major role in the success of the American war machine. TIG welding has also played an immensely important role in motorsports. TIG (Figure 2) is an arc welding technique that utilizes a non-consumable electrode, usually made from tungsten, a separate filler material and an inert gas (argon) shield. It is a manual process that requires tremendous skill, and patience! TIG can be employed with all weldable metals but is the most useful for the welding of alloys such as stainless steel and for thin materials. For this reason, Burns Stainless uses TIG welding.
TIG Welding Equipment Components
The major components of a TIG welder are the transformer, foot-controller, inert gas supply, torch and electrode. The transformer provides the high voltage current needed to heat the base and filler metal. The electric current is controlled by the operator with a foot pedal. The inert gas is supplied by a compressed gas cylinder and controlled by a pressure regulator and flow valve. Argon is most often the preferred gas. The argon gas provides a reactant free zone around the weld and heat-affected zone (HAZ) reducing the formation of oxides that can lead to a weakened weld. Alloys such as stainless steel are prone to the formation of these oxides due to the low thermal conductivity of the base metal that causes the metal to remain hot and reactive for a longer period of time. The argon not only provides an inert atmosphere, but it also aides in cooling the weld joint. More on oxides later.
The welding electrode gets very hot and must be water cooled to maintain its integrity. Though the electrode is ideally not consumed, in reality, small amounts of the electrode can deposit on the weld causing contamination. In order to alleviate this, the tungsten is alloyed with various elements, most popularly with radioactive thorium, to increase the operating temperature of the electrode. The thorium can pose an inhalation risk during grinding. Care should be taken to minimize this risk. Newer electrodes are available that do not contain thorium, and should be considered, though many welders prefer the welding characteristics of thoriated electrodes.
The welding torch (Figure 3) is a relatively complex device incorporating the electrode leads, the electrode, water cooling and a gas shield. An important component of the torch is the "gas lens" which focuses the inert gas on the weld. There are various configurations of lenses the selection of which depends on the type of welding to be done. A great deal of skill is needed to be able to accurately control the torch by hand during welding. The welder must also add filler metal to the weld with a TIG rod with his other hand and control the welding current with his foot. To "put-down" a great bead as shown in Figure 4 requires a great deal of coordination, and is why the TIG welder is truly an artisian.
MIG Welding - Used by Others. but not Burns Stainless
MIG welding should be mentioned as it is a popular welding method due to its ease of use. In fact, it has been referred to as a "hot-glue-gun" by minimally trained production welders. Similar to TIG, MIG (Figure 5) utilizes an inert gas shield. The electrode/filler are combined in an automatically fed filler wire. MIG welding lends itself to automation and is often used to fabricate production exhaust systems (Figure 6). But MIG should not be used to join the thin sections of tubing used for racing exhausts, sometimes as thin as 0.028". We have seen poor-quality MIG-welded collectors with welding wire protruding into the critical flow path forming "mazes."
TIG Welding Application
TIG welding should be employed when welding thin sections of exhaust tubing, especially when using specialized alloys such as stainless steel or Inconel®. The header in Figure 7 is an Inconel® Pro-Stock header fabricated by Jack Burns from 0.028" wall tubing. The considerable control of the welding process by the operator and the inert shielding gas help assure proper weld strength for these alloys that has a propensity to form unwanted oxides during heating. This allows for proper welding of thin walled tubing commonly used in motorsports exhaust applications.
Prep for a Quality TIG Weld
Material Prep is Key
When welding thin sections of stainless steel tubing as used in racing exhaust headers, care must be taken to properly prepare the joints. This is a crucial step, and trying to rush through is simply not acceptable. First of all, the tubing must be clean and free of any oily residue. When tubing is mandrel bent, thick lube oils are used to help slide the mandrels through the tubes. Over the past few years, environmental rules have minimized the use of strong organic solvents, so sometimes the tubes will contain residual amounts of lube in the bend area. The residue must be removed from the tube prior to welding. Any residual lube will contaminate your welds. A squirt bottle of acetone and rag usually does the trick.
No Gaps Allowed
The next step is to properly fit the tubes. When two sections of tubing are joined, there should be no gaps – and we mean none (Figure 8). Welders who are used to working with mild steel know how easy it is to fill gaps with the torch and filler rod. This is not acceptable when welding stainless steel as residual stresses can be imparted into the weld. Also, the resultant larger heat affected zone (HAZ) can cause the formation of the aforementioned oxides, both of which can lead to premature failure. Excessive gaps will also cause your assembly to distort during welding. So after welding, when you install that header tube that you spent hours fitting together and it now hits up against the shock tower you know you should have spent a little more time on fitting. When fitting tubes, you should be able to hold them up to a light and not see any light shining through. Another good test is to hold the tubes together by hand and try "rocking" the tubes together. You should feel no movement.
Tack Weld First
After properly fitting the tubes, it is time to tack the parts together (Figure 9). This will be an excellent test of the joint preparation, as any gaps will be "found-out" when you get a "blow-through" when striking the arc. And believe me, blowing through 0.028" wall tubing is not hard to do. Good witness lines are a must when fitting tubes. A fine tipped "Sharpie" is great for this purpose. It should also be noted that there is a good tool on the market to help with the fitment process. It is a tack-welding clamp from icengineworks.com. The clamp is designed to hold the tubes together before tacking. It is an excellent tool for the novice header builder, though experienced "fabbies" may find them cumbersome. Two tacks per tubing joint are sufficient. You will find that you will need to break tacks from time to time as your header design evolves, so as few tacks as needed is best.
Shielding The Root
When heated to welding temperatures and not shielded from oxygen present in the air, alloys such as stainless steel and Inconel® are prone to the formation of oxides. Technically known as "sugaring" or "noogies", these oxides will result in a poor weldment. They are characterized by a black, crusty appearance of the weld metal on the inside (root) of the pipe and an irregular root cross-section. Weld defects include lack of fusion to the base metal, root-pass cracking and incomplete penetration. Since it is most often impossible to see the weld root in an exhaust application, many poor fabricators may think "out of sight, out of mind" is acceptable - not for Burns Stainless.
TIG welding provides a gas shield around the welding electrode to prevent the formation of these oxides on the front of the weld. In the same manner, a shield must be provided on the root, or backside of the weld to prevent "sugaring." There are several techniques available including shielding the weld root: Weld-backing tape (Figure 10), Solarflux (Figure 11), and gas back-purging (Figure 12).
Tape does not lend itself well to tubing, as it would not be possible to apply and/or remove the tape except possibly in the case of short 4" or 5" tubes. Solarflux is a powder that is mixed with alcohol (methanol, ethanol or isopropyl) to form a paste. The paste can be applied with an acid brush onto the backside (inside) of the tubes prior to tacking. When the base metal is heated by the welding torch, the Solarflux paste crystallizes and forms a shield preventing oxygen from contaminating the weld. Solarflux B is an excellent option for stainless steel welding. Solarflux 1 is also available for high nickel alloys such as Inconel.
At Burns Stainless, gas back-purge is the preferred method of weld root shielding. Almost all stainless steel or Inconel® welding done at Burns Stainless is back-purged with argon gas. Reasons include minimized tube preparation, easy availability of argon gas, and the fact that it provides strong and consistent weld quality. It would be difficult to duplicate the signature Burns Stainless weld using any other technique. At Burns Stainless, Solarflux is used for welding when back-purging is not feasible such as welding flat plates such as flanges or for some repair work.
Back-purging requires an inert gas supply, a metering system and a gas dam system. The gas supply is typically the gas cylinder used for TIG shielding gas, typically argon. A back-purge metering system (Figure 13) is "tee'd" off the regulator and a gas line run to the gas dam. For welding exhaust tubing, rubber stoppers with holes make excellent dams.
For example, when welding a primary pipe, the stoppers would be placed at each end of the tacked together tube. The gas line is attached to one of the stoppers (Figure 14), and the other stopper left open (Figure 15). The shielding gas is metered into the tube until all the air in the tube is displaced with the inert gas. The gas should flow through the tube continuously during welding.
Weld the tube as usual. Before welding the last joint, it is a good idea to stop the purge-gas flow. The combination of heating the gas and eliminating one of the "leaks" can cause the stopper to blow-off the tube. Loud bangs are best avoided when under the welding hood!
When welding stainless steel and other exotic alloys, shielding the root of the weld is extremely important to insure the integrity and strength of the weldment. For welding exhaust tubing, inert gas-purging and Solarflux B are excellent choices for shielding.
Proper Treatment of the Flange/Exhaust Port Interface
So we are now finally ready to weld. Well, no, not quite. Above we have discussed various welding techniques, tube fitment and backpurging. As you can see, the key to a proper header build is attention to the details. Here we will discuss the proper treatment of the flange/exhaust port interface as this is a critical point and can make big differences in header performance. One of the most critical areas of concern in an exhaust header is the treatment of the exhaust port/header interface (Figure 17).
While working with a customer, Burns Stainless performed dyno testing on a Ferrari 3.0l, V-8 street engine. During the tuning process we noted that the stock exhaust header inside diameter was slightly smaller than the exhaust port of the engine. With no other changes, except simply “porting” the exhaust header using a conventional die grinder, dyno tests showed approximately a 4 hp gain in peak power from 231hp to 235 hp. The smaller pipe formed a “dam” to exhaust flow resulting in flow-loss and reduced power. This test proved to us how critical this area is to the performance of a header.
So, when fitting the header pipe at the head, it is critical that the exhaust pipe be either “port-fit,” or slightly larger (0.030” or less) than the exhaust port for proper flow. The top of the port is the most critical section as this is typically the place of highest gas velocity during blowdown. It is best that the header tube be oriented parallel to the top of the port. Also, we prefer that the transition from the port to the primary tube be as smooth as possible over at least the top 90 degrees. Some header builders prefer the “step” at the port reasoning that the step will minimize back-flow or reversion. If this method is employed, the step should only be used at the bottom of the port.
In some cases we find that the exhaust port is larger than needed (usually over-zealous head-porters). Calvin from Elston Headers maintains that an area reduction of as much as 12% (sometimes more) can be had at the port in order to improve header scavenging. It is imperative that the transition from the port to the smaller tube is gentle, tapering over at least 1” length, and that there is no “wall” at the port. Also, you must be sure that the port is in-fact too large. Burns Stainless then uses the Burns Stainless proprietary parametric exhaust design service, X-Design as a guide for the expected size of the port.
The technique used to attach the primary tube to the head flange is important from a mechanical standpoint, as this is a likely area for metal failure. Stress-risers should be avoided at all costs as they are a sure lead to pre-mature failures. A common mistake made by header builders, especially with thin-gage tubes, is when welding on the backside of a flange, too much penetration is used. The penetration results in a discontinuity in the tube that can lead to a stress fracture.
Burns Stainless prefers to use a head flange that has a through-hole (Figure 18) that matches the general shape of the port and slightly larger than the port (about 1-tubewall thickness around). Using the flange as a guide, we then shape the header tube to the shape of flange. With the tube pushed all the way through the flange, we weld the tube to the flange from the head-side of the flange. There is little danger of too much weld penetration so a strong weld can be made. After the tube is welded, the entry into the header tube can be “ported” to match the port as discussed above.
On the back-side of the flange, Burns Stainless prefers to weld/braze the joint utilizing a silicone-bronze filler rod (Figure 19).
This technique has been successfully used in NASCAR Cup headers for many years. The lower welding temperature of this process makes it less likely to over penetrate. In addition, the softer bronze metal gives the weld joint some resilience helping absorb the vibrational energy transmitted from the engine and chassis. And finally, the smooth golden appearance of the silicone bronze gives the header a beautifully detailed look.
Time to Weld!
We have our TIG welder ready to go. We have our header tubes tacked together with perfect fitment. We have properly fit the pipes to the flange and have our back purge system setup. So you may ask, are we ready to weld? Yes, we are finally ready to weld up the header. It is worth repeating that a well fabricated header is the result of many hours of diligent planning and preparation. Burns Stainless often get calls from customers who want to fabricate headers who tell us "I can weld stainless." Not to take away from the skills of a good welder, but the welding is only one step of a multi-step process. So here we go.
It is best to weld up each header tube independent of each other. This way you can get good torch access all around the tube. We prefer to hold the tube in a vise. It is usually not possible to weld a tacked joint in one pass. A small moveable bench vise works well as you can move the vice around by hand. If not, you can reposition the tube in a stationary vice. Next, install your back purging dams or stoppers to the tube and make sure that they fit snuggly. You do not want the stopper to come out in the middle of the process.
Next, you want to set the TIG to the proper settings. I get many calls from welders who want to know why their welds do not look like our welds (Figure 20). Many of them are properly back purging and are experienced welding stainless, but they complain that the welds are too dark. In almost every case, I find that they are trying to weld too fast and with too much heat. Slow down, and back-off on the welder amperage! Welding stainless with a TIG torch can actually provide you with a "spiritual" moment, so enjoy it. Your welds will be evidence of your success.
As mentioned above, a 2%-thoriated tungsten electrode is preferred. When welding thin walled stainless, Burns Stainless uses a 3/32" or 1/16" diameter electrode. A sharp electrode is a must (Figure 21), so sharpen the electrode using a grinding wheel making sure you do not inhale the metal dust as it is radioactive. As for filler rod, the rule of thumb is to use an equal or higher grade of stainless than the material you are welding. Our recommendation is to use 308 rod when welding 304SS. If you are joining a stainless tube to a mild steel flange, use a 309 rod, specially alloyed for dissimilar metals. Welds on 321 require a 347 filler rod. Note that by our rule of thumb, 347 welding rod could also be used to weld 304 or 304 to 321. We recommend 0.030" to 0.035" diameter TIG rod for 18 and 16 gage tubing.
Welding stainless requires use of DC current with straight polarity (electrode negative). The parameters for TIG welding are dependent upon tube wall thickness. For our Miller Sychrowave 250 welder, a setting of 35-40 amps works well for 16g or 18g stainless tubing. For 20 g, you will want to go down to 30-35 amps. You will want to fine tune the setting for your welder and technique. It is also a good idea to use a gas-diffuser (Figure 21) for the torch to provide a good distribution of shielding gas during welding.
Since the header tube needs to be as smooth as possible on the inside surface, we generally do not want 100% weld penetration as is customary in most welding practices. To compensate, weld beads need to be slightly convex to add strength to the weld. It should be noted that very thin-wall tubes (i.e. 20g or thinner) are often fusion welded. These projects should be left to the "ubër-welder."
When you are ready to strike the torch, turn on the shielding gas, both for the torch and the back-purge system. Burns Stainless uses a flow-rate of approximately 5 SCFH for the shield and 10 SCFH for the purge. Use good quality welding gloves and welding hood with automatic darkening lens. There are some pretty interesting helmets available (Figure 21).
Once the arc is started and the weld pool is formed, carefully add filler rod while keeping an oscillating motion with the torch (Figure 22). For most cases, it is best to keep the torch at approximately 10 degrees off vertical. Add filler rod to the pool with your free hand while modulating the welding current with the foot pedal. If you can chew gum as well, go for it!
It is important to keep a constant speed to assure a good weld. When you come to the end of the weld, it is important to hold the torch on the weld after you have stopped the welding current. Your TIG shielding gas should continue to flow for a few seconds after releasing the foot pedal. This will help cool the weld in a controlled fashion. You will notice that the weld will be dark and dull and more apt to "crater" at the end if you skip this step. Cratering should be eliminated as it will be a stress riser and may lead to cracking. If you find that there is a crater in the weld, it is a good idea to do another pass near the crater. It is best to do this right-away while the metal is still hot. Continue welding all the joints in similar fashion. Remember, when you come to the last weld on a tube, you will want to stop the purge gas before welding. The combination of heating the gas and closing up the last "leak" could cause the stopper to blow-off, sometimes with a bang.
After you have welded all the primaries, it is time to fit them to the flange and weld. You may find when fitting the tubes into their jig that they may not fit the same way as they did when they were tacked together. This is because during the weld process, the tubes "moved." If you did a good job with your fitment, a little minor "encouragement" is all that is needed. If not, you will have to do a bit of "snake-charming." This is why it is so imporatant to fit up the tubes and collector, tack them together, and then fit them to the car, or a jig. It will be a lot easier to correct any misalignments at this point before everything is completely welded.
Above we discussed the proper welding technique at the flange. Again, the thing to remember is not too much heat, especially when using stainless flanges. It is easy to warp a stainless flange by overheating. You will find that the tubes will "walk" due to flange warpage again requiring your "snake-charming" skills. Mild steel is much more forgiving in this respect, and many racing fabricators prefer to use a mild steel flange. Only the best welders get away without any warpage, so it is a good idea to "face" the flange on a disc grinder after welding to assure it is true.
While a long article, it is our desire that you now know what makes up a quality Burns Stainless signature weld. The staff at Burns Stainless is here to assist you with all your exhaust fabrications needs. Please feel free to contact us if you have any questions on one of your projects.