Cooling time t8/5 calculator
How long the joint takes to fall from 800 to 500 °C decides what structure forms in the heat-affected zone. It is the parameter a welding procedure hides behind its heat input range and preheat temperature.
The formulas in EN 1011-2
The standard gives two, because heat leaves in two different ways.
Three-dimensional — thick material, heat escapes in every
direction:
t8/5 = (6700 − 5·T₀) · Q · (1/(500−T₀) − 1/(800−T₀)) · F3
Two-dimensional — thin plate, heat spreads only in the
plane:
t8/5 = (4300 − 4.3·T₀) · 10⁵ · Q²/d² · (1/(500−T₀)² − 1/(800−T₀)²) · F2
Which one applies is settled by the transition thickness — the thickness at which both give the same answer. The calculator works it out and shows it beside the result: below it the 2D formula rules, above it the 3D one. Under three-dimensional flow the thickness drops out of the result altogether — adding more material changes nothing.
The shape factor
A joint sheds heat the worse, the more walls meet at one place. The standard captures that with the coefficients of Table D.1.
| Joint type | F2 | F3 |
|---|---|---|
| Run on plate | 1.0 | 1.0 |
| Butt weld | 0.9 | 0.9 |
| Fillet weld, corner joint | 0.9 | 0.67 |
| Fillet weld, T-joint | 0.45–0.67 | 0.67 |
For the T-joint the standard gives F2 as a range. The calculator takes the upper value: it yields a longer cooling time, that is, the less cautious estimate — and that is the one worth checking with a measurement, not the safe one.
What counts as a good time
| t8/5 | What it means |
|---|---|
| under 5 s | A quenched structure in the HAZ, hardness above 380 HV, cold cracking risk |
| 5–25 s | The working range for unalloyed and low-alloy structural steels |
| over 25 s | Coarse grain, falling toughness, and on heavy sections distortion as well |
For quenched and tempered high-strength steels — S690, S890 — the maker states its own range, typically around 6–15 s, and it is easier to fall outside it than on S355. There the material data sheet governs, not a general table. The same goes for stainless steels: what matters is not hardening but the time spent in the carbide precipitation range.
Where it earns its keep
- Choosing the preheat. If the preheat temperature comes out high and there is nothing to heat with, t8/5 shows how much has to be made up with heat input.
- Qualifying a procedure. The WPQR records the actual heat input; converting it to t8/5 tells you whether the toughness and hardness stand a chance.
- Investigating cracks. A cold crack in the HAZ is almost always too short a cooling time together with hydrogen. Check both, not just the electrode brand.
Frequently asked questions
Why 800 to 500 °C in particular?
In that band the austenite turns into the structure the joint will live with. Fast, and you get martensite: hard and prone to cold cracking. Slow, and the grain coarsens and toughness drops. Below 500 °C the transformation is over, above 800 °C it has not started, so that is the stretch worth measuring.
Where does the heat input for the formula come from?
From the heat input calculator: current, voltage and travel speed give Q = k · U · I · 60 / (v · 1000). Enter the value that already includes the thermal efficiency, not the arc energy.
What if the time comes out too short?
Two levers: preheat and heat input. Preheat is the stronger one, because it enters the formula twice — as the factor in front and inside the temperature differences themselves. Raising the current also works, but there you soon hit the upper limit in the procedure.
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