Intro
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Heat treating tool steel
Austenitize, quench, temper. What each step does and why skipping the second temper cracks parts in service.
When your heat treater calls and asks "what hardness?", what number do you give them and why?
Heat treatment is the base process for hardening tool steel. Three stages, each doing a specific job. Get any of them wrong and the tool fails before it sees production.
Austenitize. Heat the steel above its transformation temperature (typically 1500-1900°F depending on the grade) and hold it there long enough for the carbon in the steel to dissolve uniformly into the iron crystal structure. The grade matters: D2 austenitizes around 1825-1875°F, H13 around 1850-1900°F, A2 around 1750-1800°F. Different grades, different temperatures. You don't pick the number — the grade does.
Quench. Cool the part rapidly to lock the dissolved carbon in place. Done right, the steel transforms into a structure called martensite — very hard, but brittle. The quench rate depends on the grade: D2 and A2 are "air-hardening" (cool in still or moving air), H13 likes oil or polymer, O-series grades need oil. Quench too slow and you get pearlite (soft). Quench too fast on the wrong grade and you crack the part.
Temper. Reheat the as-quenched part below the transformation temperature (usually 300-1000°F) and hold it. This relieves the brittleness while keeping most of the hardness. It also transforms any "retained austenite" — leftover unhardened material — that would otherwise transform later in service and crack the part on its own schedule.
Here's the part most people new to the trade get wrong: the highly-alloyed grades (D2, H13, A2, S7, and the high-speed steels) require multiple tempering cycles — typically two, sometimes three. Per Aobo Steel and Gateway Metals technical guides, this is because the first temper transforms most of the retained austenite into fresh martensite, which then needs its own temper to relieve. Skip the second cycle and you've got a ticking time bomb.