Steel and preheat
Cooling time t8/5 from 800 °C to 500 °C (BS EN 1011-2 Annex D)
- where
- cooling time with three-dimensional heat flow (s)
- preheat or interpass temperature (°C)
- heat input (kJ/mm)
- shape factor for 3D heat flow
- cooling time with two-dimensional heat flow (s)
- plate thickness (mm)
- shape factor for 2D heat flow
- governing cooling time from 800 to 500 °C (s)
- transition thickness (mm)
Explain this equation
Estimates how long a weld takes to cool from 800 °C to 500 °C, the range where the steel's final structure and hardness are set. Use it when a procedure or steel supplier gives a target t8/5 window.
In thick plate heat flows away in all three directions, so cooling time grows with heat input Q but not with thickness. In thin plate heat can only spread sideways, so cooling is slower and depends on (Q/d)². The two temperature brackets come from integrating the cooling between 800 and 500 °C; a higher preheat T₀ makes both brackets larger, so cooling is slower. F₂ and F₃ adjust for joint shape. The longer of the two times governs, and d_t is the thickness where they are equal.
Both equations come from the classic moving point source (thick plate) and line source (thin plate) heat flow models, with constants for steel.
Sense check: 1.5 kJ/mm on 10 mm plate at 20 °C gives about 8 s for thick-plate flow and about 26 s for thin-plate flow; the transition thickness is about 18 mm, so the plate behaves as thin and 26 s governs.
Watch out: use the shape factors for the actual joint type (fillet welds cool faster than bead-on-plate). The results are estimates for unalloyed and low-alloy steels.
Estimates how long the weld and heat-affected zone take to cool from 800 °C to 500 °C. Steel suppliers give t8/5 windows for high-strength and quenched-and-tempered steels.
Enter your values to see the result.
Good to know
Thin plate cools slowly by two-dimensional heat flow; thick plate cools faster by three-dimensional flow. The standard says to use the larger of the two results, which is the same as choosing by the transition thickness dt. Shape factors from BS EN 1011-2: bead on plate 1.0/1.0; fill runs of butt welds 0.9/0.9; single-run fillet in a corner joint F2 0.9–0.67, F3 0.67; single-run fillet in a T-joint F2 0.45–0.67, F3 0.67. Longer t8/5 means a softer HAZ but lower toughness and strength in high-strength steels.
Common mistake
Using arc energy instead of heat input Q, or room temperature when the work is preheated. T0 is the actual plate temperature when the run is laid.
Limits
Simplified analytical model for ferritic steels. Accuracy is roughly ±20 %; follow the steel maker’s t8/5 limits and the qualified WPS.
Sources
- BS EN 1011-2:2001 Welding – Recommendations for welding of metallic materials – Part 2: Arc welding of ferritic steels (Annex C: CE and CET preheat methods; Annex D: cooling time t8/5)
- Dillinger welding calculator help: CET, EN 1011-2 preheat formula and validity range (CET 0.2–0.5 %, d 10–90 mm, HD 1–20 ml/100 g, Q 0.5–4.0 kJ/mm), t8/5 2D/3D equations, transition thickness and shape factors F2/F3
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the welding procedure specification (WPS), BS EN ISO 15614/9606, BS EN 1090, BS EN 1993 and manufacturers’ data. Structural welds need qualified welders and engineered designs.