What's inside
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Interpass temperature is the temperature of the weld and nearby base metal immediately before you deposit the next pass. Controlling it keeps the weld within the procedure’s qualified limits. Too much heat can reduce strength, increase distortion, enlarge the heat-affected zone, and encourage hydrogen cracking in susceptible steels. Too little heat can cause lack of fusion, poor tie-in, and trapped slag.
The correct target is not the same for every job. The welding procedure specification (WPS), filler-metal classification, base-metal grade, thickness, preheat requirement, and service conditions all matter. Treat the WPS as the controlling document; general temperature ranges are useful only when no more specific instruction is available.
What Interpass Temperature Means
Measure interpass temperature after cleaning the previous pass and before starting the next one. Measure near the weld, usually on the parent metal beside the joint or in the weld area specified by the WPS. Do not measure the cooling arc or a spot several inches away from the joint.
A procedure may specify a maximum interpass temperature such as 300°F (150°C), or a range such as 150–250°F (65–120°C). A minimum interpass temperature may also be required. If the joint is below the minimum, preheat it before welding. If it is above the maximum, stop and allow it to cool.
Interpass temperature is different from preheat. Preheat is established before welding begins and is maintained through the work. Interpass temperature is checked between passes. On a thick joint, the joint can still be warm from preheating while the weld zone becomes considerably hotter after several passes.
Why Temperature Control Matters
Excessive interpass heat reduces the cooling rate. In some steels and heat-treated alloys, that can produce an undesirable microstructure or lower mechanical properties. In stainless steel, prolonged high heat can increase distortion, discoloration, and the risk of sensitization in grades that are vulnerable to carbide precipitation. Aluminum loses heat rapidly but can suddenly become difficult to control as the joint approaches its melting range.
Low interpass temperature has different problems. A cold joint may pull heat from the arc so quickly that the edges do not fuse. Stick electrodes can leave slag trapped between passes, while MIG welds can show cold lap or poor sidewall fusion. On low-alloy and high-strength steels, insufficient preheat or uncontrolled cooling can contribute to hydrogen cracking.
Heat input and interpass temperature are related but not interchangeable. A short, hot weld can have a high instantaneous heat input while the surrounding joint remains relatively cool. Conversely, repeated passes over a large joint can raise the overall temperature even when each individual pass is small.
How to Measure It
For occasional repair work, temperature-indicating crayons are inexpensive and practical. Use a crayon rated for the required temperature and mark the base metal beside the weld. The mark melts or changes appearance when the metal reaches its rated temperature. They are less precise than electronic instruments, and residue can contaminate a clean stainless or aluminum surface.
Contact thermocouples and handheld infrared thermometers provide faster readings for production work. A thermocouple is generally more dependable on shiny metal. Infrared devices can be useful, but polished stainless steel and aluminum reflect surrounding heat, producing misleading readings unless emissivity is set correctly. Measuring a dull, oxidized spot or applying high-temperature tape or paint intended for infrared measurement improves consistency.
Keep the measuring point consistent. If the procedure specifies a distance from the joint, use that distance every time. Check several locations on a large fabrication because one end may be below the minimum while the center is already above the maximum. A temperature gun should not replace a written measurement routine.
| Method | Best use | Strengths | Limitations |
|---|---|---|---|
| Temperature crayon | Shop repairs and occasional fabrication | Low cost, simple, no batteries | Approximate; leaves marking material |
| Contact thermocouple | WPS-controlled fabrication | Repeatable and accurate at the contact point | More expensive; requires proper placement |
| Infrared thermometer | Fast checks over large joints | Noncontact and quick | Emissivity and reflection can cause errors |
| Visual estimation | Almost never | Free | Unreliable and unsafe for qualified work |
Ways to Control Heat
Start with the correct preheat method. Use an electric resistance heater, induction heater, or gas torch where permitted by the procedure. Heat broadly and evenly rather than creating a small hot spot directly beside the weld. Check both sides of accessible joints. A temperature blanket or insulation can slow heat loss in cold or windy conditions, but it cannot substitute for measuring the metal.
Control the welding process as well. Use the amperage, voltage, travel speed, electrode diameter, and bead size stated in the WPS. A wider weave usually puts more heat into the joint than a narrow stringer bead. On many structural and pressure-work procedures, stringer beads are preferred because they make heat input easier to control.
Plan the pass sequence before striking an arc. Skip welding, backstepping, and alternating sides can spread heat and reduce distortion. Do not immediately fill a long joint from one end to the other if the first passes are already close to the maximum temperature. Break the work into sections, allowing completed areas to cool while another section is welded.
Forced cooling is a procedure decision, not an automatic shortcut. Air movement may be acceptable in some shop work, but water, wet rags, or compressed air can cool a weld too quickly and may promote cracking in hardenable steels. Never quench a weld unless the qualified procedure specifically allows it.
Equipment and Buying Advice
If you weld mild steel occasionally, a temperature crayon set and a basic contact thermometer are usually enough. Browse welding temperature-indicating crayons for a low-cost starting point. Choose ratings that cover both your minimum preheat and maximum interpass requirement.
For repeated fabrication, a contact thermocouple thermometer is a better purchase than relying on an inexpensive infrared gun. If you do buy an infrared unit, choose one with adjustable emissivity and use it consistently on the same surface finish. Contact thermocouple thermometers cost more, but the added repeatability matters when records or inspections are required.
Heat blankets and electric preheaters are worthwhile for thick plate, outdoor work, and low-alloy steel. For small mild-steel brackets, they are often unnecessary expense. Your welding heat blanket should be rated for the temperature involved and kept clear of spatter and flammable materials.
A Practical Workflow
Before welding, read the WPS and record the required preheat and maximum interpass temperature. Clean the joint, establish even preheat, and verify it at multiple points. After every pass, remove slag and spatter, then measure the joint before restarting. If it exceeds the limit, stop welding and let it cool naturally while protecting it from drafts when required.
Record actual temperatures, pass sequence, and any unusual delays on critical work. If the temperature cannot be maintained, change the sequence, reduce bead size, lower heat input within the qualified range, or use approved heating equipment. Do not guess based on the color of the metal—bright red heat is already far beyond the interpass limits used for most structural welding.