Interpass temperature is the temperature of the joint at the moment the next pass starts. Not the temperature the arc reaches, not the temperature the part will settle at, but the condition the metal is in when the welder lays the following bead on top of the previous one.
It matters because every pass is welded onto a body that has already been changed by the pass before it. On a heavy section a single pass barely shifts the bulk temperature, so the second pass starts almost cold. On thin wall the first pass can leave the surrounding metal hot enough that the second pass, run at identical settings, delivers a completely different thermal cycle.
That is the whole argument in one sentence: the same procedure applied to a hot part is not the same procedure. Interpass control exists to make sure the welder knows which of the two situations he is in.
Steel and titanium both change structure as they heat and cool, and the structure that results depends on how fast the change happened and how long the material sat in the sensitive range. Welding a second pass onto hot metal slows the cooling of the first, which gives the first pass more time in that range than it would have had on its own.
The effect compounds. Each successive pass adds heat to a body that is already warm, so later passes cool more slowly than earlier ones, and the section ends up with a gradient of structures rather than a uniform one. On a multi-pass joint that difference can be read in a hardness traverse.
There is a second effect that has nothing to do with metallurgy. Hot metal is soft metal. A part welded while still hot is more willing to move under its own contraction, so restraint that would have held a cold joint will let a hot one creep.
Titanium reacts with the atmosphere at temperatures well below its melting point, which is why every titanium weld is a gas problem before it is a heat problem. Interpass temperature matters here for a different reason than it does in steel: a hot part that is exposed between passes keeps absorbing whatever the shielding is failing to exclude.
In practice that means a titanium joint should be cool enough to handle comfortably before the next pass begins, and the shielding should stay on it while it cools. Pulling the trailing shield away the moment the arc stops is the most common way a good root becomes a grey one.
Our TIG titanium welding parts work treats the cooling period as part of the weld rather than as downtime, because on titanium the weld is not finished when the arc goes out.
Stainless is more forgiving than titanium and less forgiving than mild steel. The concern is the region just outside the fusion line, where the steel has been hot enough to change but not hot enough to melt, and where a slow cooling cycle can leave it in a condition that is less corrosion resistant than the parent metal.
That is the argument for keeping interpass temperature down on stainless: not because the weld will crack, but because the material beside the weld can lose some of the property the buyer specified. It is a slow, invisible effect, and it shows up as discolouration, tea staining or pitting months later rather than as a failed test.
Counter-intuitively, extremely fast cooling is not a safe default either. It produces hard, crack-sensitive structures. The target is a controlled middle, which is exactly what an interpass limit is protecting.
Most small shops have no contact thermometer at the bench, and the honest answer is that they do not need one for routine work. A gloved hand held close to the joint, or a brief touch on a cooler part of the assembly, gives a usable yes or no. If the part is uncomfortable to hold, it is too hot to weld onto.
Temperature-indicating crayons and surface probes are the next step up, and they are worth having on titanium and on any job where the customer asks for records. They turn a judgement into a recorded fact, which matters when a part comes back and nobody can remember how it was run.
The standard advice is to let the part cool in still air. That is usually right, and it is not always practical. A production line with a fixture tied up in one part will be tempted to cool faster, and the methods available range from a fan to a damp rag to a water dip.
Water and wet rags are the risky ones. Quenching a hot stainless joint can leave it in a condition that is harder and more crack-sensitive than intended, and on titanium the thermal shock is worse. Compressed air is gentler than water and still faster than still air, but it also blows the shielding away, which is a serious problem if the part is still hot enough to react.
A fan moving ambient air across the part is the compromise most shops settle on. It shortens the wait without changing the cooling mechanism, and it keeps the shielding envelope intact if it is directed at the back of the part rather than the weld.
Interpass limits get ignored in three predictable situations. The first is repair work, where a weld is ground out and re-run on a part that is still warm from service or from the original weld. The second is a long joint welded in one continuous run, where there is no defined pass boundary so nobody thinks about the temperature between them.
The third is the small part with several fittings close together. Each fitting is a separate joint and each is welded while the previous one is still hot, so the last fitting on the part sees a completely different condition from the first. Nothing in the drawing tells the welder to wait, which is why the sequence has to come from the shop.
The fix is procedural and cheap. Group the joints, define which ones must cool, and write the wait into the routing so it is visible as a step rather than as a welder's private decision. You can see how such assemblies end up on our titanium fab components and exhaust pipe pages.
Interpass control is invisible in a finished part, so the questions have to go to the process rather than to the product. These are the ones that separate a shop that thinks about it from one that has read about it.
Tigindustry has welded titanium and stainless assemblies in Shanghai since 2009, with more than 50 employees across four factories, and the interpass habit is one of the reasons repeated parts stay predictable. You can see the range on our titanium exhaust system and TIG stainless steel welding pages, or browse by alloy under shop by material.
