Intro
14%
Thermal cycling and heat checking: where the cracks come from and how to slow them
Why every forge die heat-checks, which of the three contributors a shop can actually move, and how to tell a healthy crazing pattern at 40K hits from a runaway network at 25K.
A shop runs two H13 closed-die inserts in rotation on the same connecting-rod job. The dies were built to the same print, nitrided in the same load, and started service the same week. Die A reaches 80K hits with a fine dense crazing network across the impression face, the kind of pattern that looks like cracked mud under a magnifier, with no individual crack standing out. Die B is pulled at 25K hits because two cracks 3 mm long are running off the parting line into the impression, well outside the rest of the surface pattern. The crew shrugs and says heat checking happens. They are not wrong. They are not finished, either. The dies were not running the same process, even though the job was the same.
Heat checking on a hot forge die is inevitable. The rate at which it accelerates from healthy crazing to a runaway network is not. This lesson is about the difference.