Every tube with a weld and a formed end has two possible sequences. Form first, then weld, or weld first, then form. Drawings rarely say which, because the drawing is a description of a finished part and sequence is a process decision. The choice still has to be made, and it changes the part.
Most of the time forming first is correct, because the weld is then made on material that is already in its final shape and nothing disturbs it afterwards. The exceptions are real, though, and they are worth knowing before a drawing is released.
Forming after welding pulls the joint through a shape change. The weld and its heat affected zone are the least ductile material in the assembly, so they resist that change and the strain goes instead to the boundary between weld and parent metal, which is where a crack will start.
The bead itself is also a geometric feature. A proud bead inside a tube becomes a fold when the end is swaged, and a fold inside an exhaust is both a restriction and a trap for condensation. A bead on the outside becomes a mark that the die presses into the surface.
Then there is thickness. A weld is usually slightly thicker than the parent metal and often slightly harder. Expanding across it thins the parent on either side while the weld stays put, which produces a step in the wall exactly where load is highest.
Welding distorts a tube; forming distorts it as well. Done in the wrong order the two distortions add, and the part comes out with an oval end and a bowed axis. Done in the right order the forming can actually correct a small amount of weld distortion, because the die imposes roundness on the end.
That is the strongest argument for forming after welding on a part that must be round at the joint. The die does not care what the tube did during welding; it closes on the outside and produces a circle. The correction is limited to the formed length, but the formed length is where the joint lives.
Some parts cannot be formed after welding because there is nothing to hold. A tube welded into an assembly with a flange, a bracket or a second branch cannot be pushed into a die without loading the whole fabrication, and the fixture needed to support it costs more than the part.
Others cannot be formed afterwards because the weld is the datum. If the joint position is set by a welded flange, forming the end afterwards moves material relative to that datum, and the part no longer fits the jig it was built in.
Forming before welding keeps the weld zone out of the die entirely. The end is made in clean, unhardened material, the transition is formed in one continuous wall, and the weld is placed afterwards in a position chosen for access and inspection rather than for clearance.
It also lets the formed end be trimmed square before anything is welded to it, which matters because a formed end is rarely square off the tool. Trimming after welding means machining a joint that has already been made, with the heat affected zone moving as you cut.
For most of the exhaust work we build, this is the sequence: form, trim, fit, weld. It is not a rule about craft, it is a rule about which operation is easier to correct. Forming errors are correctable before welding and expensive after.
If a part must be formed after welding, the weld should be dressed before forming rather than after. A dressed weld presents a smooth surface to the die, which reduces the chance of a fold or a gouge, and it removes the proud bead that would otherwise be pressed into the wall.
Inspection then has to happen twice. Once before forming, to confirm the weld is sound, and once after, to confirm the forming did not damage it. A dye penetrant check on the transition after forming is the usual way to catch a crack that opened during the operation rather than in service.
The heat affected zone is not a line, it is a band, and its width depends on the weld and how it was made. Within that band the material is softer or harder than the parent depending on the alloy and the cooling, and either way it is different, which is what forming dislikes.
This is why a formed end placed a short distance from a weld behaves better than one placed across it, even when both are technically possible. Distance lets the strain distribute before it reaches the band, and it keeps the die away from the one region that cannot be annealed back to matching condition without redoing the weld.
Sequence is the cheapest thing to agree and the most expensive thing to discover. The five points below settle it, and each of them can be answered before a tube is cut.
The welds that meet a formed end are covered under TIG stainless steel welding, the tube itself is listed in the exhaust pipe range, our second pipe listing and the flared pipe work, and the systems that combine both are under stainless steel exhaust systems and titanium exhaust systems.
