Hardness is related to residual stress, not to hydrogen cracking. When the usual welding standards (AWS, ASME, API) were first written, there was no way to measure the residual stress. Hardness was then used to make a comparison, especially after stress relieving (PWHT): if the hardness was less than a certain value that experience showed was acceptable, PWHT was OK. If the hardness was higher, the PWHT was to be repeated.
Now it's possible to measure the residual stress in a weld by X ray difraction, and there are portable instruments that allow you to do that on the welded piece at site, i.e., you don't need to get the welded piece to the laboratory.
However, as far as I know, the standards havn't still taken that advance into consideration.
Giovanni S. Crisi
Sao Paulo - Brazil