Scarfing is to remove the inner welded seam of a piece of tubing so the mandrel will fit inside. When tubing is created, it comes from a flat piece of material on a long coil. Then it is forced into rollers over a long table until the ends touch. Then a laser welder welds the two edges together. After the weld is created, the exterior "flashing" or ridge is removed by a cutting blade. In normal tubing, that is the finished product. But when Mandrel Bending is done, the material must have this flashing removed otherwise the mandrel will not fit, even though the ridge is only .010 high!
All tubing is not created equal. No matter what others tell you on the internet. We have been bending for over 10 years and the owner is an ASE Master Technician with over 35 years experience in the automotive industry. We hear a lot about claims on the internet that just aren't true....
Mild steel tubing is another name for plain steel tubing. It is easy to bend on a mandrel bender, but highly difficult to bend without kinking on a JD bender or exhaust shop bender. It welds easy with a MIG welder, but DON'T use a flux welder. Poor metal penetration occurs and your welds will break. When cutting, a simple sawzall or band saw will work great. To prevent the material from rusting when done making your product, it is best to have it powder coated or Ceramic-coated. Otherwise, if you just paint it, the inside will rust. Powder coating coats both the inside and outside of the tube. A "rattle can" paint job is temporary. Just like buying a set of painted headers from some well known header companies. It's only used as a rust inhibitor during shelf life. Once you put them on a vehicle without having them chrome plates or ceramic coated, the paint will burn off and rust really fast. Life expectancy un-coated, 6 months...coated, 10 years +
Aluminized tubing is not known to many who have never done exhaust. Back in the 80's, aluminized tubing replaced plain steel tubing for exhaust. It is steel tubing, dipped into molted aluminum for a protective barrier inside and out. People think this is aluminum tubing or galvanized tubing. It is not aluminum tubing, and unlike galvanized, aluminized tubing is not toxic when welding. When you weld galvanized tubing, caustic and deadly gasses are released. You can usually tell galvanized welding because of the intense smoke and blue/green colors coming from the surface during welding. DO NOT BREATHE THE VAPORS WHEN WELDING GALVANIZED MATERIAL! Also, galvanized material is usually PIPE and not tube. The aluminized tubing is easy to bend with a mandrel bender and it's just "OK" when bending with a crush exhaust type bender. (you get the 40% crushed bent area) A JD or this type of bender may work, but just like steel tubing, it needs a extra large radius to keep from kinking. It will weld really easy to steel with a gas shield MIG welder. Do not use a flux core welder, they just don't work. It also cuts easy like steel. Don't be afraid of this tubing, it's made to be on your vehicle. Life expectancy is 10+ years except the welds are not coated inside and out and this makes the welds rust out over time.
409 Stainless is a softer form of 304. It IS magnetic, but still retains some anti-rusting materials. Normally used in after market exhaust systems from the "BIG GUYS", 409 stainless is the best for exhaust. It bends just as easy as steel and aluminized with the same "springback" as those materials. It looks great, welds great and is highly under rated in the aftermarket "builds" you see on youtube and other websites. 409 can be welded with a MIG welder or TIG welded for a better look. 409 DOES NOT RETURN TO IT'S ORIGINAL SHAPE WHEN HEATED. In other words, if you make an exhaust system with this material, it will retain it's shape when heated. The bends just stay where they are bent. Kind of hard to find, but is best on exhaust. Not very polish-able as the exterior has to be sanded to be super smooth prior to polishing. Will discolor over time. Doesn't "rust out" like steel. Life expectancy is 20+ years if you use 16 gauge. If you use 18 gauge (thinner material) I would assume 15 years+
304- The not so good material for exhaust. It is extremely hard and all bends try to return to their original shape. Very hard, and difficult to bend on a mandrel bender without high pressures. A crush type exhaust shop bender will NOT bend this material. It will destroy the material. A JD will not work as the pressures needed are extreme. You can bend very small tube (1" or less) with a .120 wall with a JD and a huge radius, but don't expect good quality. When using this material, it is really hard to cut, manipulate and keep from cracking over time. As an ASE Master Technician, I have repaired many Ferrari's, Maserati's and high dollar car exhaust systems because they use 304 stainless. Since the material is very hard and tries to unbend itself, headers have no place to go when trying to unbend and eventually crack at all the weld seams. Even the systems bought overseas for Subaru's, Honda's and such actually stretch over 1"-3" because the material is trying to straighten out the bends when heated! You can MIG weld, but you need a tri-blend mix with helium and an alloy wire. It works, but doesn't look like show quality. I would recommend TIG welding ONLY for this material. Highly polish-able, but that's the down fall of this material. People buy this material because it looks "pretty". But in reality, it is not the best for exhaust. Sure, a 304 stainless set of headers look cool, but given time, they will crack and your back to square one. If you make an exhaust system with this, make sure you have a lot of room away from any body parts or frame/cross-members. As the material heats up, you will lose room because of the expansion of the material. Life expectancy is 20+, but you will be re-welding cracks over time, especially if you "pie cut" the exhaust parts.
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