throughhardening

Throughhardening, also written as through-hardening, is a heat treating process aimed at producing a uniform hardness from surface to core in a steel part. It contrasts with surface-hardening methods such as carburizing, nitriding, or induction hardening, which create a hard outer layer while leaving a softer interior.

The process typically involves heating the steel into the austenitic range and then quenching the entire part in a medium chosen for its ability to transform the entire cross-section to martensite or other hard microstructures. The goal is to achieve similar hardness and strength throughout the thickness, while minimizing distortion and cracking. The quench medium, alloy composition, and section size all influence the final properties.

Hardenability, the steel’s capacity to form martensite throughout thickness, governs through-hardening effectiveness. Alloys with higher hardenability—such as chromium, molybdenum, vanadium, nickel, and silicon, along with adequate carbon content—improve the likelihood of uniform hardness in thicker sections. The Jominy end-quench test is commonly used to assess hardenability for design and processing decisions.

Applications of through-hardened steels include components that require high wear resistance and strength across their entire cross-section, such as gears, shafts, crankpins, and certain tool bodies. Common choices are through-hardenable carbon and alloy steels selected for their balance of hardenability and toughness after tempering. Final tempering is often performed to reduce brittleness and tailor toughness to service conditions.

Limitations include the risk of excessive brittleness if hardness is too high, distortion from quenching, and the need for suitable alloying; in some cases, case hardening remains preferred when a very hard surface with a tougher core is desired.