Effective Case Depth: 550 HV, ISO 2639 and ISO 18203:2026 Explained

Case depth is how deep below the surface the steel stays above a hardness threshold before dropping to core hardness.

Too shallow and the skin wears through to soft metal fast; too deep and it can get brittle or waste time/cost.

Effective case depth (CHD) uses a hardness threshold: the distance from the surface to where hardness falls to the specified value. Total case depth uses the structural/compositional transition toward the core, whose boundary may be less sharp.

A bare “0.5 mm” is therefore incomplete. State effective or total depth, units, test location and hardness criterion together.

ISO 2639:2002 defined depth for carburized/carbonitrided cases, and many legacy drawings still use a 550 HV callout. ISO now lists that edition as withdrawn.

ISO 18203:2026, published in January 2026, covers case hardening depth, surface hardening depth, nitriding hardness depth and total thickness for a broader range of thermal and thermochemical processes.

Do not silently change a customer drawing that still cites ISO 2639. Obtain a technical clarification on whether to retain its 550 HV criterion or update the governing requirement.

Section the specified location, mount and polish it, then make microhardness indents from near the surface toward the core with controlled spacing and test force.

Plot hardness against distance. The intersection with the agreed threshold—550 HV in many legacy carburized-case callouts—is the effective depth. Microstructure helps explain the layer but does not replace the hardness method when the callout requires it.

Use more than one traverse at critical areas because corners, thread roots and different sections can develop different depths.

Carburizing is controlled by furnace temperature, time and carbon potential.

Induction is controlled by frequency, power and time.

A traceable report should show lot, section location, test force, indent distances, hardness profile, threshold and each traverse—not only one final number.

Legacy-style example: Effective case depth 0.30–0.50 mm at 550 HV, section A on drawing, two traverses per part, three parts per lot, with surface/core hardness and an indent-location image.

Induction-hardened and nitrided layers need process-appropriate names and criteria; do not automatically copy the 550 HV carburizing criterion. Follow the customer drawing and governing standard.