Microstructural Fraction Prediction Model

The t8/5 time tells you how fast the heat dissipates from the weld. This lets you estimate which microstructure will form: lots of martensite on fast cooling, more ferrite and pearlite on slow. Too much martensite makes the weld hard and crack-prone; too soft a microstructure costs strength. You need to hit the cooling time so you get a microstructure that suits the steel and the load. Measure the time or control it via heat input and preheat, and you'll be on the right track.