On a single-pass weld you rarely think about how hot the metal gets between beads, because there is only one bead. The moment a joint needs multiple passes, a new variable appears: how hot the metal is when you start each new pass. That is interpass temperature, and on important work it is controlled just as carefully as preheat. Ignore it and you can quietly weaken a weld that looks perfect on the surface.

What Interpass Temperature Is

When you weld a thick joint, you fill it with several passes stacked on top of each other. Each pass dumps heat into the metal, and the joint stays hot between passes. Interpass temperature is simply the temperature of the metal in the weld zone right before you lay down the next pass.

It is closely tied to preheat, covered in our preheat temperature guide. Preheat is the temperature you bring the metal to before the first pass. Interpass temperature is the running check on metal temperature for every pass after that. In fact, welding procedures usually set the minimum interpass temperature equal to the preheat temperature, so the joint never drops below preheat partway through the weld, and they add a maximum interpass temperature as a ceiling.

Why Both Too Cold and Too Hot Are Problems

Interpass temperature has a window, with a problem at each end.

If the metal gets too cold between passes, you have the same issues preheat is meant to prevent. The weld cools too fast, which on hardenable steels can lead to a brittle structure and hydrogen cracking. This is why the minimum interpass temperature exists, to keep the joint from dropping below the safe preheat level as you work.

If the metal gets too hot, you have the opposite problem. Each pass adds heat, and on a fast-moving welder the joint can climb hotter and hotter as the passes stack up. Too much accumulated heat slows the cooling rate excessively, which can reduce the strength and especially the toughness of the weld metal and the heat-affected zone. High-strength and low-alloy steels are the most sensitive, because their properties depend on cooling within a certain range. This is the same heat-input concern that limits weave bead width on those steels. The maximum interpass temperature is the ceiling that prevents this.

How to Measure It

You cannot judge interpass temperature by eye, so you measure it. A few common tools do the job. Temperature-indicating crayons, often called temp sticks, are marked for a specific temperature and melt when the metal reaches it, giving a simple pass-fail check. A contact thermometer or an infrared thermometer reads an actual number. Temperature-indicating tapes change color at set temperatures.

Where you measure matters. Check the base metal near the weld, a short distance from the joint as the procedure specifies, rather than the glowing weld bead itself, which is far hotter than the surrounding metal you are trying to control. Take the reading before starting each new pass. If the joint is above the maximum, wait and let it cool. If it has dropped below the minimum, it needs reheating before you continue.

Managing It on the Job

Controlling interpass temperature is mostly about pacing and measurement. On a joint that is heating up too fast, the answer is to slow down and let it cool between passes, or to spread your work across the piece so any one area gets a chance to cool. On a joint that is cooling too fast in cold conditions or on heavy sections, you maintain heat with preheat and work efficiently so it does not drop below minimum.

For everyday mild steel hobby work, interpass temperature is rarely critical, and you can weld your passes without much fuss. The moment you move to high-strength steels, code work, or critical structural and pressure joints, it becomes a controlled parameter you measure and record. Knowing what it is and why it matters keeps you from unknowingly cooking the toughness out of an important weld, and it is a routine part of welding to any serious procedure alongside preheat and post-weld heat treatment.