Characteristic Cooling Rate

t8/5 is the time it takes for the weld to cool from 800 to 500 degrees. In that window, it's decided what microstructure you get. If cooling is too fast, hard, crack-prone martensite forms. If it's too slow, the grain grows and toughness suffers. You control this via preheat, heat input, and interpass temperature. Thick material draws heat away faster, so preheating helps. The limits you must adhere to are given by the material manufacturer. Measure the temperature directly at the weld with a contact thermometer, and never cool with compressed air or water.