Pre-heat strategy: getting the die to operating temperature without killing it

Heat checking is driven by thermal gradient and the rate at which that gradient cycles. The peer-reviewed review literature on H13 thermal fatigue is consistent on this point. The surface compressive stress on impact and the surface tensile stress on release are functions of the local temperature swing, and the early cycles on a cold die are the most damaging cycles the die will ever see. A 1100°C billet contacting a 25°C die surface drives a surface spike of several hundred degrees Celsius in milliseconds, against a bulk that is still cold. The substrate underneath the surface has not moved. That is the worst-case thermal-shock condition, and it seeds the heat-check pattern the die will carry for the rest of its service.

Preheating closes the gradient. With the die at 300°C, the surface temperature swing per hit is smaller, the gradient between surface and bulk is smaller, and the die spends its early hits in a regime the H13 was heat-treated to survive. The "scrap parts" at the front of the run are scrap because the die was the wrong temperature for the process. They also represent a payment to the die in cumulative thermal-fatigue damage. Both losses are real, and both are avoidable.