Steel selection by application — hot-work, warm-work, and the special cases

Producer datasheets share a common structure and a common set of marketing tells. Read four things, in order.

Read the temper resistance curve first. A useful hot-work steel for forge service holds hardness within 2-3 HRC of as-tempered after 100 hours at 600°C. A vendor curve that stops at 500°C or shows only "hardness versus tempering temperature" without a hold-time axis is hiding the relevant data. Compare across grades at a fixed exposure (often 600°C, 100 hours) and a fixed starting hardness.

Read the hot hardness curve second. This is hardness measured at temperature, not after cooling. A grade that holds 40 HRC at 600°C is a different animal from a grade that drops to 30 HRC at 600°C, even if both temper-curve to 48 HRC at room temperature. Vendor datasheets sometimes blur the two. Confirm the units and the test method.

Read fracture toughness third. K_IC values in the 30-50 MPa√m range are typical for hot-work tool steel at forging hardness, and the spread between standard and premium can be 30-50%. Premium grades like DIEVAR push K_IC toward the high end at the same hardness, which is exactly the property that resists gross cracking and heat-check initiation. Vendors often publish Charpy impact data instead of K_IC. The Charpy number is useful relative comparison within one vendor's lineup but not directly comparable across vendors with different sample geometry.

Read machinability rating last. Vendor machinability ratings are relative numbers indexed against the vendor's own standard grade, not against a universal benchmark. A "70% machinability versus standard H13" claim from one supplier and a "75% machinability versus our reference" claim from another are not the same number. On a complex cavity, machinability differences of 20-30% drive real shop hours.

Two outputs that almost never appear on a producer datasheet but matter for the decision: the as-delivered carbide segregation rating (NADCA #207 grade A-G, with A being the best) and the microcleanliness rating per ASTM E45 or DIN 50602. Both should be on the certificate of the steel you actually receive, even if they are not on the marketing page. AISI grade specifications are not standalone documents in 2026. The H-series grades (H11, H13, H21, etc.) are defined inside ASTM A681, and that is the citable reference. NADCA #207 (current edition 2018) is the de facto acceptance standard the forge industry borrows from die casting.