Describes high-carbon steel straight out of the quench, before tempering: maximum hardness, often 64 to 67 HRC, with high internal stress and almost no toughness. Glass-hard parts are handled gently and tempered fast, because they can crack just sitting on the bench. Toolmakers and knifemakers use it as the starting point before drawing back to working hardness.
Those punches are glass hard right now, don't drop them, get them in the temper before lunch.
Also heard as
- glass-hard
- as-quenched
- full hard
Related terms
Martensite
The hard, needle-like phase that forms when austenite is quenched fast enough to trap its carbon.
File hard
Hard enough that a sharp file skates across the surface without biting, roughly 58 HRC and up.
Snap temper
A quick, low-temperature temper given right after quenching to keep crack-prone parts from splitting.
Tempering
Reheating hardened steel below its lower critical temperature to trade some hardness for toughness.