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Interpass temperature is the temperature of the weld and nearby base metal before you deposit the next pass. Controlling it matters most on stainless steel, nickel alloys, aluminum, and thick joints where repeated TIG passes can steadily heat the work. If the joint gets too hot, the weld may lose corrosion resistance, distort, crack, or become difficult to control.
The target is not simply “as cool as possible.” You need enough heat for proper fusion, but not so much accumulated heat that the joint exceeds the procedure limit. A good TIG setup therefore includes a way to measure temperature, a defined maximum interpass temperature, and a cooling plan.
What Interpass Temperature Means
Measure interpass temperature after one weld pass is complete and before the next pass starts. The measurement should be taken on the weld area or adjacent base metal, normally close to the weld but not directly on a glowing or freshly molten surface.
Do not confuse interpass temperature with preheat. Preheat is the minimum temperature maintained before welding. Interpass temperature is usually a maximum limit. A stainless steel procedure might specify a minimum preheat of 50°F and a maximum interpass temperature of 300°F, for example. The actual values depend on the alloy, thickness, joint design, filler metal, and applicable welding code.
When no formal procedure exists, use the material manufacturer’s guidance rather than guessing. Stainless steel often benefits from a relatively low interpass limit, while carbon steel may tolerate higher temperatures. Aluminum can require careful control because it conducts heat rapidly and hides its temperature until the surface suddenly becomes too hot to handle.
Why Temperature Control Matters
Excessive heat input can produce a wide, shallow bead, undercut, distortion, and burn-through. On stainless steel, high heat and slow cooling can encourage distortion, heavy heat tint, and reduced corrosion resistance near the weld. On aluminum, the oxide layer and high thermal conductivity make it easy to increase amperage or slow travel speed until the part becomes unstable.
Repeated passes also change the joint as you work. The puddle may become more fluid, the arc may wander, and the same amperage may produce a larger bead than it did on the first pass. This is why a sound root pass does not guarantee that later passes will behave the same way.
Too little temperature can cause its own problems. A cold joint may resist fusion, especially at the toes of a fillet weld or at the sidewalls of a groove. The solution is controlled heat, not automatically maximum amperage. Keep the torch angle, arc length, travel speed, and filler addition consistent while watching the temperature between passes.
How to Measure Interpass Temperature
A contact temperature crayon is inexpensive and practical for many steel jobs. Choose a crayon rated near your required limit and mark the base metal beside the joint. When the mark melts, the metal has reached that temperature or higher. Crayons are less useful when you need several readings or when the surface is difficult to access.
An infrared thermometer is faster and reusable, but shiny stainless steel and aluminum can give inaccurate readings because they reflect infrared radiation. Improve consistency by measuring a dull, clean area or applying a small patch of flat black high-temperature paint away from the weld. Keep the thermometer at the same distance and angle each time. A high-end infrared unit does not automatically solve emissivity errors.
A contact thermocouple or surface probe is the better choice when a welding procedure requires documented readings. It costs more and needs good contact with the work, but it is generally more repeatable on reflective metals. Allow enough time after the pass for the surface reading to stabilize, while following the procedure’s stated measurement location.
| Measuring method | Best use | Main limitation |
|---|---|---|
| Temperature crayon | Simple steel fabrication and occasional checks | One approximate threshold; marks can be hard to read |
| Infrared thermometer | Fast, repeated shop measurements | Reflective metal can produce false readings |
| Contact probe or thermocouple | Procedure qualification and recorded work | Higher cost and slower handling |
Ways to Control Heat Between Passes
Start by limiting heat input during each pass. Use the lowest amperage that gives reliable fusion, keep the arc short, and avoid lingering at the toes. A steady travel speed is more useful than moving quickly and stopping repeatedly. For long welds, a skip or back-step sequence can spread heat, but only if the joint and distortion requirements allow it.
Clean the interpass area thoroughly. Remove oxide, smut, and surface contamination with a suitable stainless brush or abrasive reserved for that alloy. Contamination can make you add unnecessary heat while trying to correct an erratic puddle.
When the temperature is too high, stop welding. Let the joint cool naturally, or use controlled cooling approved for the material and procedure. A fan can help in some general fabrication work, but forced air can create uneven cooling and may be prohibited on critical welds. Do not quench a hot weld with water unless the welding procedure specifically permits it; rapid cooling can increase cracking or distortion risks.
Copper chill bars and backing bars can pull heat from the joint and support the puddle. Copper is especially useful for thin aluminum or sheet work, but it must fit tightly and be positioned so it does not contaminate the weld. A heavy fixture can also act as a heat sink. This is often a better investment than simply buying a higher-amperage welder.
Equipment Worth Buying
For occasional repair work, a basic welding temperature crayon is usually enough. It is cheap, does not need batteries, and works well when your maximum temperature is a single clear limit.
If you weld stainless, aluminum, or multi-pass joints regularly, an infrared temperature thermometer saves time. Choose a unit with adjustable emissivity if possible, but still verify readings on reflective metal. For code work or production records, spend more on a surface thermocouple temperature probe rather than relying on an inexpensive infrared meter.
Keep a written temperature limit at the welding station. Record the alloy, thickness, filler, amperage range, minimum preheat, maximum interpass temperature, and the location where readings are taken. That prevents a common failure: one welder measuring the thick fixture while another measures the hot weld zone.
A Practical TIG Routine
Before striking the arc, confirm the material and the required temperature range. Tack the joint, then measure the starting temperature if preheat is specified. Deposit a controlled pass, clean it, and inspect the bead before measuring near the weld.
If the reading is below the maximum, continue with the next pass while the joint is stable. If it is above the limit, wait and recheck at the same measurement location. Adjust travel speed, amperage, pass size, or weld sequence if the joint repeatedly overheats. Smaller stringer beads usually provide better heat control than wide weaving.
Finally, watch the weld rather than the thermometer alone. Excessive heat often shows up as a collapsing puddle, excessive discoloration, distortion, or a bead that spreads farther than expected. Temperature measurement confirms the condition; consistent TIG technique is what keeps the heat where it belongs.