Preheat Temperature Guide — CE, Cracking Risk & Verification

Why Preheat Matters — Hydrogen Cracking and Cooling Rate

Cold cracking (hydrogen-assisted cracking) shows up hours or days after the weld looks fine. Three ingredients drive it: diffusible hydrogen (moisture in consumables, humidity, contamination), a hardable microstructure in the HAZ, and restraint / residual stress. Preheat attacks the second by slowing cooling so the HAZ spends less time in the transformation range that produces brittle martensite — and gives hydrogen more time to escape while the metal is still hot.

Thick plate pulls heat away faster than thin sheet. High carbon and alloy content raise hardenability. Highly restrained joints (heavy fixtures, thick members, intersecting welds) raise stress. That is why “A36 under 3/4" in a warm shop” and “1½" quenched-and-tempered in January” are not the same preheat problem — even if both are “carbon steel.”

For how preheat sits next to interpass (minimum vs maximum thermal control), see Preheat vs. Interpass Temperature. This page stays on calculating and verifying minimum preheat.

What the Calculator Computes

IIW carbon equivalent

The Preheat Temperature Calculator uses the International Institute of Welding (IIW) formula:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

Use actual chemistry from the mill test report (MTR) when you have it. Without an MTR, use specification maximums — conservative, but safer than guessing mid-range chemistry. Silicon is collected in the form for reference; the IIW CE path above is what drives the result.

Thickness + CE → minimum preheat (°F)

After CE is known, the calculator looks up a minimum preheat by thickness bands (roughly ≤3/8", ≤3/4", ≤1½", and heavier), with higher CE rows demanding hotter minimums. Very high CE uses a capped escalation rather than pretending a single shop table covers every alloy.

Process and ambient adjustments

• GMAW / GTAW / SAW — calculator subtracts ~25°F from the table value (lower hydrogen / different heat patterns vs stick in many shop assumptions). Still confirm against your WPS.
• Ambient below 32°F — floor of 70°F minimum (AWS D1.1-style cold-weather rule).
• Ambient below 0°F — floor of 150°F.

The same run also suggests a CE-based maximum interpass ballpark. Treat that as a companion hint — lock the real max with the Interpass Temperature Calculator and your WPS.

How to Apply and Verify Preheat on the Job

1. Get chemistry — MTR preferred; else ASTM max chemistry into the calculator.
2. Measure thickness at the joint — use the governing thickness the code/WPS defines (often the thicker member in a T-joint).
3. Heat the zone — torch, induction, or oven per shop practice. Heat a band wide enough that the joint does not quench into cold metal 1" away.
4. Soak — thin plate reaches temperature fast; thick sections need time for heat to soak through the thickness, not just the surface. If the face is hot and the back is cold, you do not have preheat — you have a gradient.
5. Verify temperature before the first arc (see methods below).
6. Maintain — if the joint cools below minimum between fit-up and welding, reheat. Minimum preheat is a floor for the start of welding, not a one-time torch kiss.

Measurement methods

• Tempilstik / temperature crayons — mark 2–3" from the weld centerline on clean metal. Melt = at or above that stick’s rating. Carry the stick that matches your minimum (and often one for max interpass).
• Contact pyrometer / thermocouple — best for WPS verification. Probe on clean base metal near the joint, not in slag or the puddle.
• Infrared gun — fast for surveys; emissivity errors on shiny, mill-scale, or freshly ground surfaces are common. Cross-check IR with a crayon or contact reading when the number matters.

Common Mistakes

• Guessing CE from the grade name alone — “A36” spans chemistry; use the heat’s MTR.
• Surface-hot, core-cold thick plate — no soak time through thickness.
• Trusting IR on shiny stainless or ground steel without a contact/crayon check.
• Skipping preheat because ambient “feels fine” while CE and thickness demand heat.
• Heating only the top of a T-joint — stem and flange both need the thermal floor.
• Using damp low-hydrogen electrodes on a high-CE joint — preheat cannot fully save bad hydrogen control.
• Ignoring cold-weather floors — below 32°F, AWS D1.1-style rules raise the minimum even on “easy” steels.

Frequently Asked Questions

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