Free tool · no sign-up
t8/5 cooling time calculator
Enter heat input, working temperature, plate thickness and the two joint shape factors, and read the cooling time from 800 °C to 500 °C for both heat-flow regimes at once — with the transition thickness that decides which of them describes your joint.
No acceptance band is built in: the t8/5 window belongs to the steel supplier and the WPS. Type it in and the tool reports your margin at both ends.
Two regimes and the thickness that separates them
Three-dimensional t8/5 = (6700 − 5·T₀) · Q · [1/(500 − T₀) − 1/(800 − T₀)] · F3
Two-dimensional t8/5 = (4300 − 4.3·T₀) · 10⁵ · (Q²/d²) · [1/(500 − T₀)² − 1/(800 − T₀)²] · F2
T₀ = preheat or interpass temperature in °C · Q = heat input in kJ/mm · d = plate thickness in mm · F2, F3 = joint shape factors
The transition thickness follows by setting the two expressions equal and solving for d. Below it the two-dimensional result is the larger of the two and describes the joint; above it the three-dimensional result takes over and thickness stops mattering. That is why the governing value is always the longer time, and why a calculator that reports only one regime can be wrong by a factor of two without ever looking wrong.
How to calculate t8/5 cooling time
Four values off the WPS and the drawing, then one decision about heat flow.
- Enter the heat input the steel receives. In kJ/mm, after the process thermal efficiency factor. Entering arc energy instead overstates the cooling time by the size of that factor, which is up to 40 % for TIG.
- Enter the working temperature. The preheat or interpass temperature the joint is actually at when the pass is laid, not the ambient shop temperature. It appears in every term of both expressions.
- Enter the plate thickness. The thickness of the member the heat is flowing into. This is the input that decides which regime governs.
- Set the joint shape factors. F2 and F3 from Annex D of the standard for the joint type. Leaving them at 1.0 calculates the reference case and understates the cooling time of a joint surrounded by more metal.
- Read the governing value against your window. Type the minimum and maximum t8/5 the steel supplier gave you. The tool reports which regime governs, the transition thickness, and how much margin you have at each end.
- Heat input has to be heat input. Both expressions take the energy the steel receives, so arc energy needs the process thermal efficiency factor applied first. The heat input calculator does exactly that, and the heat input guide explains which quantity each standard calls by which name.
- T₀ is the temperature at the moment of the pass. Not the preheat that was reached an hour ago. On a multi-run weld the interpass temperature drifts down through a break, and every term of both expressions moves with it.
- The shape factors are not decoration. A fillet in a T-joint drains heat into three members instead of two, and leaving F2 and F3 at 1.0 quietly assumes a run-on plate. Take the values from Annex D for the joint you are actually welding.
- The window has two ends. Too fast hardens the heat-affected zone; too slow coarsens the grain and costs low-temperature toughness — and on quenched and tempered steel it softens the zone the steelmaker engineered. The t8/5 and microstructure article works through both failure modes.
- Predicted is not measured. These relationships assume a clean geometry and a steady arc. HeatCam IR-C records the actual cooling curve of the joint, which is the only version an investigation can use.
For the metallurgy behind the window — what the cooling rate does to the microstructure — read heat input, cooling rate and t8/5.
Questions engineers ask about this calculation
What does t8/5 actually measure?
The time the weld metal and the heat-affected zone spend cooling from 800 °C to 500 °C. That interval is chosen because it is where austenite decomposes in structural steels, so it decides whether the result is a tough ferritic structure or hard martensite. Everything else on a WPS that touches cooling — preheat, interpass temperature, heat input, plate thickness, joint shape — acts on the joint only through this one number.
Why does the same weld give two different cooling times?
Because heat leaves the joint in a different number of directions depending on how much metal there is. In a thin plate the heat spreads sideways only, which is two-dimensional flow and gives a time proportional to Q²/d². In a heavy section it also flows downwards, which is three-dimensional and gives a time proportional to Q with no thickness term at all. Both expressions are always computable; only one describes the joint in front of you.
How do I know whether two-dimensional or three-dimensional heat flow applies?
By the transition thickness, the thickness at which both expressions return the same time. Below it the joint is thermally thin and the two-dimensional value applies; above it the three-dimensional value does. Because the two-dimensional time falls as 1/d² while the three-dimensional time does not depend on thickness at all, this is the same as taking the longer of the two — which is what this calculator reports as the governing value, along with the transition thickness for your conditions.
Is a longer cooling time always better for weld quality?
No, it is a window with a floor and a ceiling. Cool too fast and the heat-affected zone hardens and becomes crack-sensitive. Cool too slowly and the grains coarsen, which costs toughness at low temperature, and on quenched and tempered or thermomechanically rolled steels it also softens the very zone the steelmaker engineered. That is why a steel supplier issues a t8/5 range rather than a minimum, and why raising heat input to escape a preheat requirement can fail the joint from the other side.
A calculated cooling time is a prediction. A thermal camera gives you the real one.
HeatCore AI watches the joint through the cooling curve and scores every pass against the thermal window the procedure requires, so a t8/5 that drifted because the interpass temperature dropped is caught on the pass it happened, not in a macro-section weeks later.
