Weld Cooling Time and the Thickness Transition

Which is where austenite transforms and where a weld's final microstructure -- its hardness, its toughness, and its susceptibility to hydrogen cracking -- is decided. Cool too fast and the result is untempered martensite; cool too slowly and grain growth costs toughness. Every lever a welding procedure pulls -- heat input, preheat, interpass temperature -- is pulled to land this number in the right window. The two regimes are the part that is easy to get wrong. In a thick plate the heat escapes into three dimensions and the thickness stops mattering, so the cooling time is independent of it. In a thin plate the heat has nowhere to go through the thickness, the flow is two-dimensional, cooling is SLOWER, and it depends on one over the thickness squared, which is sharp. The two expressions cross at a transition thickness and each governs on its own side, which is the same as saying the larger of the two is the real one. At 1.0 kJ/mm with no preheat the thick-plate figure is 5.29 s, and a 10 mm plate is below the 14.7 mm transition so two-dimensional flow governs at 11.4 s -- more than twice as long. A 20 mm plate is above the transition and the 3D figure of 5.29 s governs instead. Getting the regime backward on that example is a factor of two in either direction. Preheat is the other lever: raising it from 20 to 150 C takes the thick-plate time from 5.29 to 7.85 s, a 48% slower cool from preheat alone, which is exactly what preheat is for. An engineering estimate; the welding procedure specification, the carbon-equivalent screen, and a qualified welding engineer govern.

3D: t8/5 = (6700 - 5 T0) x Q x [1/(500 - T0) - 1/(800 - T0)] x F3. 2D: t8/5 = (4300 - 4.3 T0) x 1e5 x Q^2 / d^2 x [1/(500 - T0)^2 - 1/(800 - T0)^2] x F2. The transition thickness is where the two are equal; the larger value governs.

The t8/5 weld cooling-time model in its two- and three-dimensional heat-flow forms, with the transition thickness where they cross, by name -- the formulation published in EN 1011-2 and the standard welding-metallurgy references, cited and not reproduced. Heat input in kJ/mm, temperatures in degrees Celsius, and thickness in millimetres, which is the notation this model is universally written in. The welding procedure specification, the carbon-equivalent screen, and a qualified welding engineer govern.

The heat-flow model is public welding metallurgy, cited to its published formulation; the heat input, preheat, thickness, and joint shape factors are the procedure's own values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: heat_input_kj_mm, preheat_c, thickness_mm, f2, f3, t85_3d_s, t85_2d_s, transition_mm, governing_s

• Two regimes 3D above the transition thickness (independent of d), 2D below it (goes as 1/d^2 and cools SLOWER); the larger value governsEN 1011-2 formulation
• Metric notation kJ/mm, degrees Celsius, and millimetres -- the units this model is universally written in, with no customary formwelding metallurgy literature
• Shape factors F2 and F3 account for the joint geometry and are entered from the procedureEN 1011-2 formulation