HAZ maximum hardness is estimated by equations described in the following paper:

N. Yurioka et al., :"Prediction of HAZ hardness of steels", Metal Construction, vol19, (1987), p217R

where,
t8/5: welding cooling time between 800 and 500°C (s)

f(B): an increse in HAZ hardenability due to boron, (C <= 0.8%, N <= 0.01%)