Measure of how deep a steel will harden under a given quench, as opposed to how hard it can get. Maximum hardness comes from carbon; hardenability comes mainly from manganese, chromium, molybdenum, nickel and boron, plus coarser austenite grain. It is measured with the Jominy end-quench test or expressed as an ideal critical diameter. Low hardenability means a soft core or a harsher quench, with more distortion and cracking risk.
Purchasing swapped 4140 for 1045 to save a buck and now they're asking why the cores are soft, that's hardenability.
A 1045 and a 4340 reach similar surface hardness, but only the 4340 hardens deep into a 3-inch bar quenched in oil.
Do not mix it up with
- Rockwell hardness: Hardness is how hard a given spot reads; hardenability is how deep a steel can be hardened for a given quench.
Related terms
Jominy test
End-quench test that measures a steel's hardenability by spraying water on one end of a hot bar.
DI
Ideal critical diameter: the largest bar that would reach 50 percent martensite at its center in a perfect quench.
Through hardening
Hardening a steel part so martensite forms across the whole section, not just at the surface.
Quench severity
How fast a quenchant pulls heat out of steel, usually rated by the Grossmann H value.
CCT diagram
Continuous-cooling-transformation chart predicting which structures form as steel cools at various steady rates.