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

H13 holds its hardness as long as the surface stays under the last tempering temperature. The standard build sequence for a forge die tempers H13 to 540-595°C depending on the target hardness. Once the die is in service, the working ceiling is the last tempering temperature minus a margin, typically 30°C. A die tempered at 540°C should not see surface temperatures above 510°C in production. A die tempered at 595°C should stay under 565°C.

The reason is over-tempering. Steel that sits above its last tempering temperature continues to temper. The reaction is time-and-temperature driven, the same way the original temper was. Run a die 50°C above its last temper for a full shift and you have effectively re-tempered the surface at the higher temperature, with a corresponding drop in surface hardness. Run it 100°C above and the drop is faster and larger. The case does not come back. Recovery requires pulling the die, re-heat-treating the substrate, and re-finishing.

The lower end of the window matters too, but for a different reason. A cold die hitting a hot billet is the heat-checking failure mode from Lesson 2. The operational range for an H13 forge die in production is roughly 200°C on the low end (sustained bulk after preheat and cooling) to 450°C on the high end of the surface equilibrium between hits, with brief contact spikes that can reach 500-600°C and fall back within seconds. The ceiling on the brief spike is still the temper-minus-30°C rule. Spike duration matters, but the surface metallurgy does not care whether the over-temperature lasted five seconds or five minutes if it happens every cycle for eight hours.