Why Your PWHT Pads Keep Creating Hot Spots

Why Your PWHT Pads Keep Creating Hot Spots Even When the Controller Says 620°C

I’ve spent twelve years on fabrication yards and plant turnarounds around Shanghai—mostly pipe shops in Baoshan, Songjiang, and the outer chemical parks—watching post-weld heat treatment on carbon and chrome-moly welds. The complaint that shows up most is not that the ceramic pad won’t heat. It’s that the hardness readings or the temperature chart look uneven, one side of the weld is soft, and the crew blames the controller or the thermocouple. Nine times out of ten the flexible ceramic heating pad has crushed or broken alumina beads, the wire is exposed in spots, and the heat is no longer uniform.

PWHT ceramic beads are sintered high alumina beads, usually around 95% Al₂O₃, threaded with multi-strand 80/20 nickel-chromium wire to make a flexible mat. A standard 60 V pad runs about 2.7 kW at 45 A and is rated for element temperatures up to roughly 1050°C. For carbon-steel stress relief we normally soak at 600–650°C. alumina beads do three jobs: keep the live wire insulated from the pipe, transfer heat into the steel, and let the pad bend around a weld. When ceramic beads crack or get crushed under tight banding, the wire sits closer to the surface in those spots, local temperature climbs, and other areas lag. You can have the control thermocouple reading a perfect 620°C while 100 mm away the weld is 40–50°C hotter or cooler.

Last winter I was on a thick-wall carbon-steel pipe job in a Baoshan fabrication yard. They were doing PWHT on 20-inch, 25 mm wall butt welds. The ceramic pad heaters were standard 60 V units, wrapped with ceramic fiber blanket and banded tight. After the first cycle the hardness traverse showed soft zones on one side of two welds. We pulled the pads. Several rows of beads were cracked or missing, the NiCr wire was visible and slightly oxidized in those spots, and the blanket had been compressed hard against the damaged area. We ran a quick check with extra thermocouples: the control point held 615–625°C, but 80 mm toward the damaged beads the surface hit 670°C while the opposite side stayed around 580°C. Lab-style resistance checks on the damaged pad showed higher local current density where beads were gone. The intact spare pads from the same batch measured even across the mat.

We replaced the bad pads with ones that had been stored flat and banded with wider, softer straps plus an extra layer of blanket only where needed for insulation, not for crushing. On the next welds the extra thermocouples stayed within 15°C of the set point through the soak. Hardness came out uniform. The yard kept the tighter inspection on bead condition after that. The original pads had probably been stepped on or over-banded on earlier jobs; once a few beads break, the wire heats harder in that spot and the problem gets worse each cycle.

The ceramic itself is tough when it is intact. Sintered alumina ceramic beads handle the temperature and transfer heat well. The failures I see are mechanical—crushed beads from foot traffic, tight steel banding that point-loads the mat, or pads rolled and stored so the beads crack before they ever see a weld. Moisture can add another variable if the pads sit in the open, but the hot-spot pattern is usually broken beads and exposed wire. A ceramic pad heater for post weld heat treatment only works when the heating element is still evenly supported.

A few things I check on every job now. Look at the pwht beads before you wrap the weld—if more than a couple are cracked or missing in one area, set that pad aside. Use wide banding or soft straps and don’t crank them until the blanket is flattened to nothing. Place the control thermocouple on sound ceramic, not over a repaired spot, and add a second couple on thick-wall or critical welds until you trust the setup. Keep pads off the ground and out of forklift paths. And if your alumina beads PWHT heater is giving uneven hardness or the chart looks good but the weld doesn’t, pull the pad and count the broken beads before you recalibrate the controller. On the Baoshan pipe the simple pad swap and lighter banding turned inconsistent hardness into uniform results. Most yards I visit get the same improvement once they stop treating the flexible ceramic heating pad as something that can be abused and still deliver even heat.