Total Case Depth

Total case depth (TCD) is the perpendicular distance from a hardened surface to the point where hardness, microstructure, or chemistry no longer differs from the core. In practice, it is where the hardness gradient levels off and further traverse readings match core hardness scatter. TCD is a metallurgical inspection metric used after carburizing, nitriding, induction hardening, or carbonitriding to verify surface-layer extent.

On the shop floor, total case depth becomes critical after a part leaves the CNC machine and returns from heat treatment. For gears, shafts, bushings, cams, or wear surfaces, TCD tells heat-treat and QA teams whether the hardened layer extends deep enough to carry contact loads without the part transitioning too early into a soft core. It is measured on a sectioned, polished, and etched coupon or cross-section, often with a microhardness traverse when visual contrast is insufficient. This is not a machining parameter: CNC depth of cut is material removed per pass, while TCD belongs to metallurgical inspection. A frustrating scenario is a batch of hardened gears showing shallow contact fatigue; a traverse reveals the case stops too early in the root even though the flank looks fine. Drawings must specify TCD, cutoff criterion, standard, and location, because TCD always exceeds effective case depth.