Austenitizing and quenching a medium- or high-carbon steel so that all or nearly all of the cross section transforms to martensite. It needs enough hardenability for the section size, or the result is a hard skin over a softer core. Typical through-hardened parts are 4140 and 4340 shafts, 52100 bearing races and tool steel dies, always followed by tempering to the hardness on the print.
These are through-hardened, 40 to 44 HRC all the way in, so don't send them to carburize.
Also heard as
- through-hardening
- thru-hardening
- through hardened
Do not mix it up with
- Case: A through-hardened part is hard across its section, while a case-hardened part has a hard surface layer over a softer core.
Related terms
Hardenability
A steel's ability to form martensite to a given depth, set by its chemistry and grain size, not its hardness.
Q&T
Quench and temper: hardening steel by quenching, then tempering it back to a set strength and toughness.
Case
The hardened outer layer of a case-hardened part, as opposed to its softer, tougher core.
Quench
To cool hot metal rapidly in a liquid or gas so that it hardens, or the act of doing so.