Weld Cooling Time t8/5
Computes how long the heat affected zone takes to cool from 800 °C to 500 °C, the t8/5 parameter of EN 1011-2, from the heat input, the preheat temperature and the joint shape factor. The formula multiplies the term (6700 − 5 × preheat temperature) by the heat input, by the difference between the reciprocals of (500 − T₀) and (800 − T₀), and by the shape factor tabulated in the standard, which is 1.0 for a bead deposited on a plate and drops to about 0.9 for a butt weld and 0.67 for a fillet weld on a T-joint. Austenite decomposes in that range, so t8/5 decides the microstructure of the joint: cooling too fast forms martensite and opens the door to cold cracking, cooling too slowly coarsens the grain and destroys impact toughness, and most structural steels call for something between 5 and 25 seconds. The three-dimensional heat flow equation was adopted, valid when the plate is thick relative to the weld bead; in thin plate the flow is two-dimensional and t8/5 grows with the square of the heat input rather than in proportion to it. Enter the heat input, the preheat temperature and the joint shape factor.
Result
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t8/5 Cooling Time: The Number That Rules the HAZ
Cold cracking in the heat affected zone rarely shows up while the arc is burning. It shows up hours later, with the part already in the yard, sometimes only at the next day inspection. At the other end of the scale, a joint that cools too slowly reaches the Charpy test below the specified energy and fails the whole qualification. Between those extremes lies a window measured in seconds: how long the metal beside the bead takes to fall from 800 °C to 500 °C. The steel maker publishes the range; the welder controls heat input, preheat and geometry.
For three-dimensional heat flow, EN 1011-2 gives t8/5 = (6700 − 5·T0) · Q · [1/(500 − T0) − 1/(800 − T0)] · F3. Q is the heat input in kJ/mm, already multiplied by the thermal efficiency of the process: 0.8 for stick and MIG/MAG, 1.0 for submerged arc, 0.6 for TIG. T0 is the preheat or interpass temperature, whichever is higher when the arc strikes. F3 is the tabulated shape factor: 1.0 for a bead on plate, 0.9 for a butt weld and 0.67 for a fillet on a T-joint, where the third plate opens one more path for heat to drain. At 1.5 kJ/mm, 150 °C and F3 of 1.0, t8/5 comes out at 11.8 s.
The three-dimensional equation assumes a plate that is thick relative to the bead. Transition thickness for the default values lands near 20 mm; under that, flow turns two-dimensional, t8/5 starts growing with the square of the heat input and the joint cools more slowly than this formula suggests. The calculator never asks for thickness, so checking that limit falls to you. The classic unit slip is entering heat input in J/mm: 1500 in the field trips the warning, since the check rejects anything above 20 kJ/mm. Subtler is mixing up arc energy with heat input — dropping the 0.8 efficiency turns 1.2 kJ/mm into 1.5 and 9.4 s into 11.8 s.
Frequently asked questions
At 1.5 kJ/mm, 150 °C and F3 of 1, the result is 11.8 s. Is that acceptable?
Is heat input the number shown on the machine display?
Why does a fillet on a T-joint cool faster than a butt weld?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.