Case-hardened parts such as carburized and nitrided components require case depth measurement to verify the process. The common methods fall into two categories: the metallographic method, based on structural change, and the hardness method, based on the hardness gradient. They differ in scope and acceptance criteria and shall be selected according to the product standard.
The Hardness Method
The hardness method relies on the hardness gradient measured on a cross-section normal to the surface. For carburized and hardened parts, GB/T 9450 defines the case hardening depth (CHD) as the perpendicular distance from the surface to the point where the hardness reaches 550HV (or an agreed limit value). Microhardness indents are placed point by point on the polished cross-section, with the load chosen according to case depth and structure (commonly HV1); the case depth is then interpolated from the hardness-versus-depth curve. The method is objective and repeatable, and serves as the reference method for arbitration and routine inspection.
The Metallographic Method
The metallographic method measures case depth from the structural change on an etched cross-section. For slowly cooled carburized parts, the case depth is taken as the hypereutectoid zone plus the eutectoid zone plus one half of the transition zone. It is fast and well suited to quick shop-floor assessment, but it is affected by cooling conditions and interpreter experience, giving lower accuracy than the hardness method. When a quenched structure is examined directly, the boundary is often taken where the martensitic structure changes markedly, and the result is for reference only.
Nitrided Case Measurement
For nitrided parts, the nitrided case depth is determined per GB/T 11354: the metallographic method measures from the structural boundary between the white layer and the diffusion zone after etching, while the hardness method takes the point 50HV above the core hardness as the limit (the agreed value may be adjusted). The nitrided case is thin with a steep hardness gradient, so a smaller load shall be used and the cross-section must be perpendicular to the surface—an inclined section falsely increases the reading. Edge protection during preparation is necessary to avoid bevels that compromise near-surface indentations.
Practical Tips
- For carburized and hardened case depth, the hardness method with a 550HV limit per GB/T 9450 prevails.
- For nitrided cases, follow GB/T 11354; the hardness method uses core hardness plus 50HV as the limit.
- The metallographic method suits quick assessment; disputes are arbitrated by the hardness method.
- The cross-section must be perpendicular to the surface and well polished; specific acceptance follows the product standard and the operating procedure.
MICROCRE learning notes · confirm the method for the specimen.