Die-temperature management during the run: thermal profiling in plain English

A real die-surface trace over one forging cycle has four phases. Contact with the billet drives a fast spike, often 200-400°C above the between-hit temperature, lasting tenths of a second. The billet releases and the surface starts to cool radiatively. The lubricant spray hits and drops the surface another 50-150°C in a fraction of a second through evaporative cooling. The die equilibrates between hits, which on a typical hammer or press cycle is 5-30 seconds.

Across a shift, the equilibrium temperature drifts. The first part of the shift, the die is climbing from its preheat soak toward its working equilibrium. By an hour in, the equilibrium is roughly stable. By mid-shift, if the cooling spray and lubricant are holding, the equilibrium is steady. If the spray nozzle drifts or partially clogs, the equilibrium creeps up over the shift, and the contact spikes climb with it. The first hour of a shift is rarely where the temperature problem shows up. Hour four or five, after the operator stopped looking, is where it shows up.