Advances in t8/5 Control, Microstructure–Property Relations, and Intelligent Prediction for Structural Welding

The t8/5 cooling time, defined as the time required for cooling from 800 to 500°C during the welding thermal cycle, is a key parameter for characterizing cooling conditions and phase transformation behavior in the heat-affected zone (HAZ). It significantly influences microstructural evolution and is closely related to the mechanical properties and cold-cracking susceptibility of welded joints. In the welding of thick, high-strength steel plates and large steel structures, accurately determining and stably controlling t8/5 remains difficult due to measurement uncertainty in the high-temperature region near the weld, strong coupling among multiple influencing factors, and limited model generalizability. This review systematically summarizes measurement methods, dominant control factors, microstructure-property relationships, and prediction strategies related to t8/5. Current studies indicate that reliable determination of t8/5 under complex welding conditions still depends on a combined route involving reference-point measurement, model-based calculation, and simulation-assisted validation. Analysis of welding parameters, structural dimensions, thermophysical properties, and environmental conditions shows heat input, preheating temperature, and plate thickness are generally the most sensitive factors, although their effects depend on steel grade, joint geometry, and welding conditions. By linking HAZ subzone characteristics with continuous cooling transformation behavior, this review further shows that t8/5 governs joint performance by modifying the austenite transformation path, microstructure type, and characteristic scale. A short t8/5 promotes martensite and M-A constituents and increases the risk of cold cracking, whereas a long t8/5 may cause softening and reduce load-bearing capacity. Future work should focus on reliable real-time measurement, interpretable hybrid prediction models, and digital-twin-based closed-loop control.