Effective Case Depth
How is effective case depth measured?
ECD is measured by preparing a metallographic cross-section of the part or witness coupon and running a microhardness traverse at controlled depth intervals. The measured hardness values are plotted against depth, and the depth at which hardness falls to the specified threshold, for example 50 HRC, is interpolated from the curve.
Why does effective case depth matter more than surface hardness alone?
Surface hardness can be high while the hardened layer is too thin. ECD confirms the hard layer is thick enough to resist subsurface shear and bending stresses, which governs many fatigue failures such as spalling and contact fatigue. Without adequate ECD, the case collapses under load even though the surface reads hard.
What process variables affect effective case depth?
In induction hardening, frequency, power input, scan speed, and part geometry control case depth. In carburizing, ECD depends on material hardenability, carbon potential, time at temperature, and quench severity. These variables must be controlled and verified against the specified threshold on the drawing.
Effective case depth (ECD) is the distance from a hardened surface to the depth where hardness drops to a specified threshold, most commonly 50 HRC for carbon and low-alloy steels. It defines the functional load-bearing hardened layer that resists wear and fatigue, and is verified metallographically using a microhardness traverse rather than by surface hardness alone.
In a typical CNC cell, effective case depth determines whether a case-hardened component survives service. Most machining happens before heat treatment, when stock is removed to net shape. After carburizing or induction hardening, final grinding, honing, or hard turning may be needed to hold tolerances. That post-heat-treat stock removal consumes the hardened case. If too much material is removed, remaining ECD at the working surface drops below the drawing requirement, and the part can fail through spalling, flank wear, or contact fatigue. A witness coupon is often run alongside the batch, sectioned, and microhardness-traversed to confirm depth before production continues. CNC programmers and machinists must coordinate with heat treat and QC: leave adequate finish stock, hold consistent pre-hardened geometry, and respect the specified test method. ECD is not just a lab value; it defines the actual load-bearing hardened material at the surface.
Over-machining after heat treat: Finish grinding or hard turning that removes too much stock drops the remaining hardened layer below spec, causing premature flank wear, spalling, or contact fatigue on load-bearing surfaces.
Wrong threshold or wrong specimen: Using a generic 50 HRC cutoff when another value is specified produces false results. A non-representative coupon misleads because hardening response depends on section size, chemistry, and process uniformity.
Confusing effective case depth with total case depth: Effective case depth uses a specified hardness threshold; total case depth ends at core hardness. Confusing them can pass a visible case yet fail because the load-bearing hardened zone is too shallow.