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Weld cooling time t8/5 and preheat

Thin plate cools slower than thick plate, not faster — and the thickness at which that flips is where most t8/5 calculations go wrong. This one computes both heat-flow models and tells you which applies.

Welding parameters

Joint

For the preheat calculation

CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40 — not the same as CE(IIW). Carbon equivalent calculator →

Cooling time t8/5

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Arc energy / heat input Q—
Transition thickness—
2D (thin) / 3D (thick)—
Preheat, CET method—

Where this one comes from. Unlike the other calculators here, the arithmetic on this page is not taken from the METALLAI engine — the platform screens weldability by carbon equivalent and hydrogen-cracking risk, and does not model weld thermal cycles. These are the published EN 1011-2 expressions, implemented directly. Treat the output as a planning estimate, not a WPS.

The two heat-flow models

Heat leaving a weld pool has two ways to go: sideways through the plate, or downward through its thickness. Which dominates decides the whole calculation.

Q = k · U · I / (v · 1000) kJ/mm 3D thick plate, heat escapes downward too: t8/5 = (6700 − 5·T₀) · Q · [1/(500−T₀) − 1/(800−T₀)] · F3 — independent of thickness 2D thin plate, heat trapped in plane: t8/5 = (4300 − 4.3·T₀) · 105 · (Q²/d²) · [1/(500−T₀)² − 1/(800−T₀)²] · F2 — rises sharply as the plate gets THINNER

Because the 3D result does not depend on thickness and the 2D result falls as 1/d², the two curves cross exactly once. That crossing is the transition thickness, and the correct answer is whichever model gives the longer time — below the transition that is 2D, above it that is 3D.

The counter-intuitive part. Halving plate thickness in the thin regime roughly quadruples t8/5. Welders who assume thin sheet chills quickly and needs more preheat have it backwards: in the 2D regime the risk is a slow cooling rate and coarse-grained, low-toughness heat-affected zone.

Joint shape factors, EN 1011-2

JointF2 (2D)F3 (3D)
Bead on plate / run-on plate1.01.0
Butt weld between plates0.90.9
Fillet weld, T-joint0.450.67
Fillet weld, corner0.90.67

Preheat by the CET method

Tp = 697·CET + 160·tanh(d/35) + 62·HD0.35 + (53·CET − 32)·Q − 328 Valid for CET 0.20–0.50 % d 10–90 mm HD 1–20 ml/100 g Q 0.5–4.0 kJ/mm

Note the sign on the last term: at low CET, more heat input reduces the required preheat, because a slower cooling rate lets hydrogen diffuse out and gives a softer heat-affected zone. The calculator flags any input outside the envelope rather than extrapolating.

Questions

What is t8/5 and why 800 to 500 °C?
Because that window covers most of the austenite decomposition in structural steels. It is the single number that best predicts the resulting heat-affected-zone microstructure and hardness. Too short and you get martensite plus hydrogen cracking risk; too long and grain coarsening costs you toughness.
What t8/5 should I be aiming for?
Whatever the steel and consumable data sheets specify for that combination — commonly a window somewhere in the region of 5–25 s for ordinary structural grades, but that is a rule of thumb, not a specification. Thermomechanically rolled and quenched-and-tempered steels have much tighter windows than a plain S235.
Is arc energy the same as heat input?
Not quite. Arc energy is U·I/v; heat input multiplies it by the thermal efficiency k of the process. Reporting arc energy as heat input overstates the energy reaching the plate by 20–40 % for most arc processes. This calculator applies k and shows the result it used.
Does this replace a WPS or a procedure qualification?
No. It is a planning estimate from published formulae. Qualification is a test, and the preheat you actually apply should come from your welding procedure, your steel supplier's recommendation and the relevant code — not from a web page.

Screen the parent metal first

METALLAI computes CE(IIW) and Pcm, flags hydrogen-cracking risk, and predicts the parent metal's properties from composition and route — the inputs this page assumes you already have.

Open the app — free Carbon equivalent calculator