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

The numeric targets are the part most shops shortcut. For H13 hot-work forge dies running carbon and low-alloy steel billets, the preheat target is 200-300°C surface temperature before the first hit. For aluminum forging, where die-surface temperatures during the cycle are lower but the lubricant flash-off behavior matters, the typical preheat target is 300-400°C. Both ranges sit well below the substrate's last tempering temperature (commonly 540-595°C for H13 forge dies). Preheat is not heat treatment. It is bringing the die into the operating window the process was tuned for.

Soak time is the underspecified variable. A rule of thumb used in the FIA-archived preheat literature: soak time after the surface reaches target equals roughly the section thickness in inches multiplied by 30-60 minutes per inch, with the longer end of the range for dense geometries and the shorter end for thinner sections. A 6-inch section reaches target on the surface long before the core does. A 30-minute soak after the surface meter shows 300°C is the floor for that section. 60 minutes is safer.

The verification is not the surface temperature on its own. The verification is the surface-to-core delta, which should be below 50°C before the first hit. That measurement requires a contact thermocouple in a tapped well in the die back-face read against the surface reading, or an equivalent pair of points the operator can return to shift after shift. AMS 2750G (current edition, superseding 2750E) is the SAE aerospace pyrometry standard, cited inside CQI-9 for heat-treat audits. The same instrument expectations apply to in-house preheat verification. A contact thermocouple or IR gun that has not been calibrated against a known reference inside the last twelve months is not a measurement instrument. It is a guess.