Through Hardening
How is through hardening different from case hardening?
Through hardening makes the whole part hard, while case hardening creates a hard outer layer with a softer core. Through hardening is preferred for uniform strength, while case hardening is chosen when a tough core is needed.
Why temper after through hardening?
Quenching raises hardness but also raises brittleness. Tempering reduces brittleness and adjusts the hardness to a usable level for service.
What machining strategy is common for through-hardened parts?
Rough machine in the annealed or prehard condition, heat treat, then finish only critical features. Leave approximately 0.25–0.40 mm (0.010"–0.015") stock for the post-heat-treat pass, depending on part geometry and process.
Through hardening is a heat-treatment process that hardens the entire cross-section of a metal part by heating above critical temperature, quenching, and tempering. It creates uniform hardness from surface to core, common in steels like 4140, 4340, A2, D2, and O1, and is used when parts require consistent strength throughout, such as tooling and structural components.
Through hardening is selected when a part’s load path runs through the full section, making a soft core a failure mode. Typical parts include shafts, cutters, punches, wear blocks, and heavily loaded fixtures. On the shop floor, the workflow begins with rough CNC machining in the annealed or prehard condition, leaving approximately 0.010" to 0.015" stock on critical areas. After heat treatment, the part is inspected for distortion from quenching. Final machining uses rigid fixturing, sharp carbide or coated tools, and controlled feeds and speeds to hold tolerances only on datum faces, bores, and wear surfaces. This approach minimizes tool wear and manages quench distortion, ensuring the uniform hardness delivers consistent mechanical response under impact, bending, or multi-directional wear.
Quench distortion on thin or asymmetrical sections: Thin or asymmetrical sections can warp during quenching, causing critical features to drift out of tolerance. This is why such features are left oversize for finishing after hardening.
Cracking or brittleness from improper tempering: If a part is quenched but not tempered correctly, it becomes too brittle for service. Tempering after quench is essential to achieve usable toughness and hardness balance.
Trying to finish-machine fully hardened stock with general-purpose tooling: After hardening, cutting forces rise and tool life plummets. Shops must use carbide tooling and limit post-hardening work to finishing passes to hold tolerances.