A hot pass is the second weld bead laid down in a multi-pass welding joint, deposited directly over the root pass. It’s a standard step in pipe welding and other heavy joint work, and it serves several purposes at once: burning out imperfections from the root bead, adding critical thickness to prevent cracking, and improving the metallurgical quality of the joint. The name comes from the fact that it’s typically run while the root pass is still warm, or “hot.”
Where the Hot Pass Fits in the Welding Sequence
Multi-pass welds follow a specific order. The root pass goes in first, fusing the two pieces of metal at the very bottom of the joint. The hot pass comes next, covering the root. After that come one or more fill passes to build up the joint, and finally the cap pass on top to finish it off.
The hot pass is especially important in pipeline welding, where joints are made on pipe in fixed or rotating positions and the root pass alone isn’t thick enough to hold up as the weld cools. Without the added thickness from the hot pass, the thin root bead is vulnerable to cracking during cooldown. On many job sites, welding procedures require the hot pass to be completed before a welder is allowed to stop at the end of a shift for exactly this reason.
What the Hot Pass Actually Does
The hot pass pulls double duty as both a structural addition and a cleanup step. Its main functions break down like this:
- Burns out imperfections. The root pass, especially when run with a cellulosic rod like E6010, often leaves behind trapped slag lines called “wagon tracks” along the edges of the bead. The hot pass melts through these, cleaning them out so they don’t become defects buried inside the finished joint. Experienced welders describe being able to see leftover slag from the root pass burning out as they lay the hot pass.
- Adds thickness. A root pass by itself is thin and fragile. The hot pass beefs it up, giving the joint enough cross-section to resist cracking as the metal contracts during cooling.
- Improves the root bead profile. The heat from the hot pass can slightly push the root pass material through to the inside of the pipe, giving a smoother, more consistent contour on the interior surface.
- Tempers the heat-affected zone. The area around any weld bead undergoes grain coarsening from the heat, which makes it more brittle. When the hot pass reheats this zone, it refines the grain structure. Research in multi-pass welding has shown that this reheating effect can cut the average grain size roughly in half, bringing it back close to the original base metal. Since coarse grains in the heat-affected zone are the primary cause of reduced fracture toughness in welded joints, this refinement meaningfully strengthens the weld.
Protecting a Thin Root Pass
One scenario where the hot pass is especially critical: when the root pass is welded with one process and the fill passes use another. A common example is laying a root pass with TIG (GTAW) for precision, then switching to stick welding (SMAW) for the fill and cap because it’s faster. The problem is that a stick electrode dumps significantly more heat into the joint than TIG. Running a stick fill pass directly on a delicate TIG root bead can blow right through it. The solution is to first run a TIG hot pass over the root, thickening it enough to survive the transition to the higher-heat process.
Rods and Techniques
In pipeline work using stick welding, the root pass is commonly run with E6010 electrodes, which dig deep and penetrate well. The hot pass can also be run with E6010, taking advantage of its aggressive arc to burn out slag and wagon tracks. Alternatively, many welders switch to E7018 for the hot pass, using a weave technique with a 3/32-inch electrode. The choice depends on the welding procedure specification for the job.
Travel speed on a pipeline hot pass typically falls between 12 and 24 inches per minute. Going too slow dumps excessive heat into the thin root bead and risks burn-through. Going too fast can cause undercut along the edges or leave a bead that sits on top of the joint without fusing properly into it. If the puddle is running too hot, welders often switch to a drag angle and push in moderately, forcing the puddle to chase the arc rather than pooling up and melting through.
Preparation matters too. Before running the hot pass, many welders grind the root bead lightly to ensure no slag remains on the surface. This is especially important when switching from E6010 root to E7018 hot pass, since E7018 doesn’t have the same aggressive slag-burning action.
Temperature Control Between Passes
The temperature of the metal when you start the hot pass matters. Welding codes specify both minimum preheat temperatures and maximum interpass temperatures. For mild steel, the general guideline is to keep the interpass temperature within about 150°F of the preheat temperature. Many procedures cap the maximum interpass temperature at 500°F, though the absolute upper limit for most mild steels before you risk damaging the metal’s properties is around 1,200°F.
If welding gets interrupted and the joint cools below the minimum interpass temperature, the welder has to reheat it with a torch before continuing. This is why the hot pass is often run promptly after the root, while the joint still holds enough heat to stay within the required range.
Common Hot Pass Problems
The biggest risk during the hot pass is burn-through, where too much heat melts completely through the root bead and leaves a hole. The root cause is always excessive heat input, but that can come from several directions: amperage set too high, travel speed too slow, poor weaving technique, or an improper joint fit-up that left too wide a gap in the root.
To avoid burn-through, reduce amperage if the puddle looks too fluid, increase travel speed slightly, and avoid unnecessary weaving. Proper joint preparation before the root pass pays dividends here too. A root gap that’s too wide or walls that have been ground too thin leave less material for the hot pass to work with. Some welders use a heat sink or backing material on the inside of the joint when burn-through is a persistent concern.
Internal undercut is the other defect to watch for. This happens when the arc erodes the sidewalls of the joint without filling them in, leaving a groove that becomes a stress point in the finished weld. It’s typically caused by traveling too fast or holding the arc too far to one side during the pass.