From the point of view of hydrogen cracking, it is the supercritical, fully transformed heat
affected zone next to the fusion boundary that has the highest hardness and highest tendency
to form hydrogen cracks. Whether hydrogen cracking would indeed occur or not in a given steel
depends partly on the exact microstructure which, in turn, depends on the steel composition and the
local cooling rate. If these two parameters are accurately known, it becomes possible to design a
welding procedure that will prevent hydrogen cracking. Since one generally knows the composition (or
at least the type) of steel being welded, at this stage it is important to understand what factors
determine the weld zone cooling rate.
The cooling rate indicates how fast the weld zone cools. Therefore, it is measured as the average
decrease in temperature of the weld zone (weld metal or the heat affected zone next to the fusion
boundary) in one second. A cooling rate of 70°C/s is a much higher (or faster) cooling rate than
10°C/s. Conversely, if one looks at the time to cool from 800°C to 500°C, then smaller cooling times
imply high cooling rate and larger cooling times imply slow cooling.