Cooling time t8/5

The interval from 800 °C to 500 °C is not an arbitrary round number — it brackets the range in which austenite decomposes into ferrite, bainite or martensite in carbon and low-alloy steel. Everything else on a welding procedure specification that touches thermal cycle — preheat temperature, interpass temperature, heat input, plate thickness, joint geometry — acts on the final microstructure only through the cooling time it produces across this one window. Cool too fast through it and the result is hard, crack-susceptible martensite; cool too slowly and grain growth reduces toughness even though hardness looks acceptable.

The same weld can have two different cooling times depending on how heat escapes the joint, and qualification work has to know which one governs. A thin plate loses heat in two directions, spreading through the sheet; a thick plate loses heat in three directions, spreading into the surrounding mass as well. EN 1011-2 Annex D gives separate solutions for each regime and a transition thickness that says which one applies to a given joint — below it, two-dimensional flow dominates; above it, three-dimensional flow takes over and the joint cools more slowly for the same heat input. Reading a single formula off a datasheet without checking which regime the section thickness falls into is a common source of a calculated value that disagrees with what a thermal camera actually measures on the part.

Two production facts follow from this. First, the variables that shorten cooling time — low preheat temperature, low heat input, a thick, three-dimensional section — are exactly the combination that pushes a joint toward hydrogen cracking if the material is hardenable, so t8/5 and hydrogen-cracking risk are read together, not separately. Second, a value calculated from nominal WPS parameters is an estimate of what should happen, while a value measured with a temperature-calibrated thermal camera on the actual joint — tracking a region of interest as it passes through the window — is a measurement of what did happen, and the two are worth comparing on every procedure change rather than assumed to agree.