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Lesson 05·Heat Treatment

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

What actually happens in the cycle that takes an H13 block from 22 HRC anneal to 48 HRC working hardness, why two tempers are mandatory, and how to read a heat-treat chart and catch a missed secondary-hardening peak.

9 min readLesson 5 of 13

Tying it together

Reading a heat-treat chart

A heat-treat chart for H13 plots tempering temperature on the x-axis and hardness on the y-axis, typically as a family of curves at different austenitize temperatures or different temper times. Three features matter.

The secondary hardening peak is the local maximum near 510-540°C where hardness exceeds the as-quenched baseline by 1 to 2 HRC. Tempering below 480°C misses the peak and leaves the alloy carbides un-precipitated.

The temper resistance shoulder is the plateau between 540 and 595°C where hardness drops slowly. This is the operating range for forge die tempering. A setpoint on this shoulder lets the heat treater hit hardness within 1 HRC and gives the substrate the secondary hardening structure it needs for service.

The drop-off above 595°C is where carbide coarsening accelerates and hardness crashes by 4 to 6 HRC over a 50°C interval. A vendor who tempers at 620°C to land 44 HRC has not put the part on the secondary hardening shoulder. They have over-tempered it past the shoulder, and the structure carries coarse carbides instead of the fine coherent precipitates the steel needs.

NADCA #207 acceptance is hardness range with tolerance, microstructure showing tempered martensite with fine alloy carbides and no bainite or pearlite, retained austenite below 3 percent for premium grades, and prior austenite grain size at ASTM 6 or finer. AMS 2759/3 covers the same ground for aerospace hot-work steels with tighter documentation requirements. Both call out witness coupons in every load.

What this means on the shop floor

For a press die at 200 mm section, the standard call is vacuum austenitize at 1020°C with a 30 minute soak, 6 bar nitrogen quench, double temper at 555°C to 48 HRC. For a hammer die at 400 mm section, the call moves to 10 bar quench, 45 to 60 minute austenitize soak, double temper at 580°C to 44 HRC, with a third temper to drive retained austenite below 3 percent. For an aluminum forge die where the die surface stays below 500°C, the cycle is the standard H13 path tempered to 50 HRC for higher wear resistance, since secondary softening at die-face temperature is not the dominant loss mechanism. For an Inconel hot-forge die at extreme service temperature, the substrate is usually not H13 and the cycle is whatever the alloy supplier specifies, but the principle of controlled quench, secondary hardening temper, and multiple tempers still applies.

The heat treater's cycle trace is the only document that proves the cycle actually ran as specified. Require it on every PO.

Pushback questions for the vendor

  1. What is your austenitize temperature, soak time, and the calibration record for the thermocouple placed on the part rather than the furnace?
  2. What gas quench pressure are you running for my section, and can you show the GQH curve that says it is sufficient?
  3. How many tempers, at what temperatures, with what hold time per temper, and what is the documented retained austenite limit on the witness coupon?
  4. Can you provide the full cycle trace, the witness coupon micrograph showing prior austenite grain size and microstructure, and a microhardness reading at the core on heavy sections?

Common confusions

A high surface hardness reading on the certificate does not prove the core is in spec. On any section above 100 mm, require a core hardness reading from a saw cut on a witness coupon processed in the same load.

A single temper at the right temperature is not equivalent to two tempers at a lower temperature. The two tempers do different metallurgical jobs, and skipping either leaves a structure that fails in service.

Tempering above 595°C to land hardness in spec is over-tempering, not soft-tempering. The hardness number is correct but the structure is wrong. Insist on a temper setpoint on the secondary hardening shoulder.

Stress relief at 650-700°C is not part of the heat treat. It is a separate operation that runs after rough machining and before austenitize, and it should appear as its own line on the print. Skipping it produces unpredictable distortion on the final quench.

Up next: distortion budget.

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