Case Depth
How is case depth measured in a machining or heat-treat context?
Case depth is commonly checked by sectioning the part and performing hardness testing across the cross-section, or by metallographic inspection after etching. Hardness is mapped inward from the surface until the drop from the hard case to the softer core establishes the effective depth.
Is case depth the same as depth of cut in CNC machining?
No. Depth of cut is the material removed per pass during machining. Case depth is a metallurgical property that describes the thickness of the hardened layer produced by case hardening after heat treatment.
What materials are commonly case hardened, and when should case depth be specified?
Low-carbon and alloy steels are common because they can develop a hard surface while retaining a tough core. Case depth should be specified whenever surface wear resistance, rolling contact fatigue life, or impact durability depends on a hardened skin over a ductile core.
Case depth is the thickness of the hardened surface layer produced by case hardening, measured from the surface inward to the depth where hardness drops to the specified core or transition hardness. In manufacturing, parts are machined first, then heat treated, and critical surfaces may be finish ground afterward to maintain size.
Case depth governs how much wear-resistant material remains over a tough, ductile core. For CNC-machined gears, shafts, cam profiles, bearings, and drivetrain components, the print specifies a target case depth such as 0.8–1.2 mm plus surface and core hardness. That combination determines whether the part survives cyclic load, sliding wear, and contact stress without brittle failure. Case depth also drives process planning because carburizing, nitriding, or induction hardening can distort dimensions and shift stock, so bores, faces, and datum surfaces often need post-heat-treat grinding. If the case is too shallow, the component wears through the hardened layer early, exposing the soft core and causing scuffing, pitting, or tooth-flank failure. If the case is too deep, cycle time and cost rise, and the part may lose toughness near the core as the hardened zone extends beyond the duty-cycle requirement. Machinists must balance allowances against final case depth to leave enough hardened layer.
Under-case depth after hardening: A gear tooth or shaft journal wears through the hardened layer because the specified case depth was too shallow for the real contact pressure, leading to pitting, adhesive wear, or spalling.
Excessive stock removal after heat treat: Machining cuts away the hardened layer on a critical surface because the stock allowance was not matched to the case depth, so the final part lacks specified wear resistance at bearing or sealing land.
Distortion and measurement error: The hardened layer can be non-uniform around corners, edges, and thin sections, and heat-treat distortion shifts effective case depth relative to finished geometry, causing false confidence if inspection is done on one cross-section only.