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

The earlier lessons in this course and Course #2 each act on one or more of the three contributors. Putting them in order of leverage on the shop floor:

Preheat verification (Lesson 2). Closes the gradient on the early cycles. Acts on ΔT per cycle. A die preheated to 300°C and verified by surface-to-core delta before the first hit takes its early thermal cycles at half the ΔT of a die loaded cold. The early cycles are the most damaging cycles the die will ever see. Preheat is the cheapest, fastest lever available on every job.

Cooling-spray and lube management (Lesson 3 and Lessons 5-6). Holds the die-bulk temperature steady across the shift. Acts on ΔT per cycle. A spray nozzle that drifts or partially clogs across a shift drives bulk temperature up, which raises the contact spike, which raises ΔT. A lube film that fails locally lets metal-to-metal contact dump heat into the die, which has the same effect. Both effects are localized, which is why heat-check networks are denser in some regions of the die face than in others.

Surface treatment (Course #2). Nitriding extends the time before crazing initiates by raising the surface hardness and the cumulative-damage threshold. The Behrens group at Hannover (Materials, 2022) reported an 80 percent durability uplift from nitriding versus untreated H13. A compound layer that is too thick is its own problem under thermal cycling, because the brittle ε-rich layer crazes more aggressively than the diffusion zone underneath. The Course #2 distinction between a thin γ'-dominant compound layer for impact-loaded dies and a thicker ε-rich layer for adhesive-wear-dominated dies applies directly here. Surface treatment acts on the initiation threshold, not on the cycle ΔT.

Substrate selection. A finer-carbide H13 grade (the B-grade in the Calvo-García work) doubles the corrosion-fatigue life of the A-grade. This is a die-build decision, not a shop-floor decision, and Course #3 covered the substrate selection logic. The shop floor cannot change substrate after the die is in service. It can demand the substrate grade and the heat-treat cert on the next die build.

What this stacks to: most of the leverage the shop floor has on heat-check rate is on ΔT per cycle, through preheat and through thermal management across the shift. Peak temperature is mostly fixed. Cycle frequency is mostly fixed. The substrate and the surface treatment were locked in before the die hit the press. ΔT is where the work is.