Heat-treat fundamentals for forge dies — austenitize, quench, temper, secondary hardening

The reference path per NADCA #207 and AMS 2759/3 runs as follows.

Preheat at 850°C, with a second preheat at 1010°C for thick sections, held long enough for the part to equalize through-section.

Austenitize at 1010-1030°C for standard H13. Premium grades extend the window. DIEVAR can run up to 1050°C to drive more vanadium into solution. Soak time at austenitize is 30 to 60 minutes once the part is at temperature, scaled to section thickness. Under-soak leaves undissolved carbides at grain boundaries and gives low hardness on quench. Over-soak grows the prior austenite grain, which the rest of the cycle cannot fix.

Quench in vacuum with high-pressure nitrogen at 5 to 10 bar for typical forge die sections. Thick sections require 10 to 20 bar. The critical requirement is cooling through 1010 to 540°C in under 25 to 30 minutes to clear the carbide-precipitation nose on the time-temperature-transformation curve. Slower cooling lets alloy carbides nucleate at prior austenite grain boundaries and produces bainite or pearlite in the section interior.

First temper at 540-595°C, hold two hours minimum or one hour per inch of section. This is the secondary hardening temper.

Second temper at the same temperature or 10 to 20°C higher, two hours minimum. This temper is mandatory, not optional.

Third temper at the same or slightly higher temperature is specified for premium grades and critical applications where retained austenite must sit below 3 percent per NADCA #207 Grade A or AMS 2759/3 acceptance.

Target hardness for hot forge service is 46 to 50 HRC. Hammer dies and impact-dominated work run 44 to 46 HRC. Trim dies and wear-dominated work run 50 to 52 HRC. Temper temperature sets hardness on the secondary hardening curve to within 1 to 2 HRC of target when the cycle is in control.