Core Hardness
How is core hardness verified on a case-hardened part?
The part is sectioned and polished, then hardness is measured progressively inward from the surface using Vickers or Knoop indentation. This maps the hardness gradient and confirms the value at the interior, which surface readings cannot reveal.
Why do specs often call out both case hardness and core hardness?
Surface hardness alone only indicates wear resistance. Designers specify both because a hard exterior with a controlled interior toughness profile is required to balance wear resistance, load strength, and dimensional stability in components such as gears and shafts.
What happens if the core hardness is too high in a gear blank?
The gear loses toughness and becomes more sensitive to cracking during quenching and in service. Tooth roots and hub transitions act as stress concentrators where bending loads can trigger fracture and premature fatigue failure.
Core hardness is the hardness of the interior of a metal part, measured away from the surface, used to verify that heat-treated components retain a tough, ductile center after case hardening. In manufacturing it contrasts with case hardness: the hardened surface provides wear resistance, while the softer core preserves impact toughness and prevents brittle failure.
On the shop floor, core hardness dictates the sequence. A gear blank or shaft is rough-machined in the soft state, then sent through carburizing, nitriding, induction hardening, or quench-and-temper. That cycle creates a hard surface while leaving the center tough enough to absorb shock loads. After heat treatment, the drawing often demands a verified core hardness value, which means sectioning a sample or using indirect testing because a surface reading cannot prove the interior stayed ductile through the full section. The specified core range, commonly 25–35 HRC for quenched-and-tempered steels, also affects finish machining. Once the core hardens, cutting forces climb and tool wear accelerates, so stock removal is limited to light finishing passes with carbide or coated inserts. A machinist who understands this balance will schedule inspection and finishing operations around the heat-treat step rather than trying to hog material out of a hardened core.
Overhardening the section: If the heat-treat cycle penetrates too deeply, the core becomes hard and brittle. This invites impact cracking and reduced fatigue life, especially at fillets and stress risers.
Measuring only the surface: A part can pass a surface hardness check while the core remains under-hardened or uneven. This leads to false acceptance of gears and shafts that later deform, brinell, or fail in service.
Machining after hardening with the wrong setup: Trying to remove significant stock from a hard core with standard tooling causes chatter, chipping, and rapid wear. The part may also drift dimensionally because finish machining should have occurred before heat treatment.