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

The quench from austenitize temperature does not produce 100 percent martensite. A fraction of the austenite remains untransformed at room temperature because the martensite finish temperature for H13 sits below ambient. Typical retained austenite after quench is 10 to 20 percent by volume.

During the first temper the alloy carbides precipitate out of the martensite, but the retained austenite is also conditioned. As carbides nucleate inside the retained austenite islands, the local matrix is depleted of carbon and alloying, which raises the martensite start temperature of those islands above room temperature. On cooling from the first temper, that conditioned austenite transforms to fresh untempered martensite. The structure leaving the first temper now contains hard brittle untempered martensite that was not present at the start.

The second temper does two jobs at once. It transforms the conditioned retained austenite that did not convert on first cooling, and it tempers the fresh martensite that formed from the conditioned austenite during the first temper's cool-down. A single temper leaves either untransformed retained austenite or untempered fresh martensite in the part. The retained austenite transforms slowly under thermal cycling at die-face temperatures, the die grows dimensionally by 0.05 to 0.15 percent, and the new martensite cracks under impact load before it ever sees the temper that should have stabilized it. A third temper catches the small fraction of austenite that survived the second cycle on heavy or asymmetric sections.