Step 3 of 4
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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.
Step 3 of 4Healthy crazing versus runaway network
The gold-set distinction between normal and early-warning patterns is the visual call the inspector has to make every time a die comes off the press. The peer-reviewed literature and the H. James Henning trade-press writing on the same topic converge on the same visual rules.
A healthy crazing pattern at 40K hits looks like cracked mud or alligator skin. The cracks are shallow, typically below 0.1 mm deep, with spacing typically below 0.5 mm. The network is dense and uniform across the highest-temperature regions of the die face. No single crack stands out from the network in length or depth. The pattern lives in the case layer and does not propagate into the diffusion zone or the substrate. A die in this state has surface fatigue accommodating the thermal cycle. It is doing what it is supposed to do.
A runaway network at 25K hits has a different signature. The pattern is sparser overall, but one or more cracks have propagated out of the network. A single crack 1-3 mm long, deeper than the surrounding pattern, often running perpendicular to local material flow or aligned with a stress concentrator like a sharp corner or a parting-line edge, is no longer surface fatigue. It is structural crack growth into the diffusion zone or the substrate, driven by some combination of mechanical fatigue from the forging load and continued thermal cycling. A directional pattern of parallel lines, rather than a random network, is the other early-warning shape. Parallel-line dominance means mechanical fatigue is now the driver, not thermal cycling alone.
The Walters and Van Tyne piece in the FIA library (Forge, 2008) and the Henning Forging Magazine columns make the same call from the practical side. A network is the expected end state. A network with one or more cracks leaving it is the call to dye-pen the area, photograph for trend tracking, and decide whether to pull. The shop habit of "we always see some cracks at this hit count" is fine when the cracks are the network. It is dangerous when one of the cracks is not.
Quick check
An inspector finds a 2 mm crack on a die at 60K hits that was not present at the 55K inspection. The crack is in a low-stress region of the impression and the rest of the surface shows the expected dense crazing. Is this an early-warning crack?