Thermal cycling and heat checking: where the cracks come from and how to slow them

Knowledge check

A sister-die pair runs the same closed-die job on the same press with the same crew. Die A pulls at 80K hits with a fine uniform crazing network on the impression face and no single crack standing out. Die B pulls at 25K hits with a sparser pattern but two cracks 3 mm long running off the parting line. The crew wants to put both back into service after a polish because the dies still hit print. Which die is safe to return and which is not, and what does the difference tell you about the process?