Heat Input Troubleshooting

Heat Input Problems Quick Answer

Use this when paperwork HI matches the WPS but metallurgy, distortion, or inspector questions say otherwise — or when the number you typed never existed on the plate.

1. Paper Heat Input Is In Range — Travel Was Never Real

What it looks like: WPS log shows 18–22 kJ/mm. Bead appearance, HAZ width, or cracking says the plate saw something else. Operator “estimated” IPM from muscle memory.

Likely causes: Stopwatch includes grind pauses, tack restarts, and torch reposition. Vertical/overhead timed with flat numbers. Weave oscillation ignored — travel along the joint axis is slower than hand speed suggests. Synergic programs change WFS mid-seam without a new travel note.

Fixes: Mark 6–12 inches (or 150–300 mm), arc on, time only the continuous segment, compute IPM or mm/min, then re-run the Heat Input Calculator. For weaves, follow WPS language: most codes want travel along the joint, not torch tip path length. Recalculate whenever position or weave width changes.

2. Dial Amps Look Right — Clamp Meter Disagrees

What it looks like: Machine display or knob says 180 A. Clamp around the electrode lead reads 155 A (or 210 A). HI paperwork used the dial.

Likely causes: Voltage drop in long leads so the inverter works harder or under-delivers. Synergic trim changed actual current. Stick machines with dig/arc-force that pull more than the set amp under load. Reading open-circuit or idle values instead of welding current.

Fixes: Clamp DC current on the electrode cable while welding a scrap coupon at production stickout. Use that amp (and a true volt reading if available) in the calculator. Fix undersized leads with the Welding Cable Size Calculator when sag is the root cause — not by faking HI.

3. ASME Number Accepted on an ISO Procedure (or the Reverse)

What it looks like: Shop runs the ASME path (no efficiency factor, IPM). Customer WPS is ISO-style with η and mm/min. Numbers “look close” until audit.

Likely causes: Mixing IPM into the ISO denominator (off by ~25.4× if you also botch units). Applying TIG η ≈ 0.65 to an ASME line that never used η. Comparing bands across conventions as if they were identical.

Fixes: Open the calculator in the mode printed on the WPS. Convert travel once, deliberately. Never paste an ASME kJ/mm onto an ISO essential-variable table without documenting the conversion path. Details live in the Heat Input Guide — this page only catches the field mix-up.

4. Low Heat Input on Hardable Steel → Cold Cracking

What it looks like: Thin beads, fast travel, “low distortion” pride — then delayed toe cracks or underbead cracks on restrained carbon/alloy steel. Calculator correctly shows <15–20 kJ/mm.

Likely causes: Fast cooling + hydrogen + restraint. Low HI is not automatically “safer.” Missing or inadequate preheat. Wet low-hydrogen rods or contaminated solid wire. High carbon equivalent with no thermal compensation.

Fixes: Raise true heat input (slower travel or more energy) only inside the WPS window — or keep low HI but restore preheat and low-hydrogen practice. Verify preheat with the Preheat Temperature Calculator and guide. Hold interpass so you do not swing from ice-cold to overheated between passes.

5. High Heat Input + Hot Interpass → Toughness / Grain Growth Failures

What it looks like: Production loves wide weaves and slow travel. HI sits 45–70+ kJ/mm. Charpy or CTOD fails; stainless shows heavy HAZ sensitization; aluminum heat-treatable alloys lose strength.

Likely causes: Peak temperature dwell + slow cool coarsens HAZ grains. Max interpass ignored while HI stays high — thermal stack. Weave energy counted wrong so true HI is even higher than the log.

Fixes: Drop HI with faster travel or stringers (see decision below). Enforce interpass maximum with the Interpass Temperature Calculator. If angular distortion appears with the high-HI habit, use Weld Distortion Troubleshooting for sequencing — do not only chase amps.