1. Limit hardness and interpolation
The definition of CHD has not changed: the perpendicular distance from the surface to the limit hardness (550 HV by default). What changed is that evaluation is now specified as normative linear interpolation — valid points must exist on both sides of the limit, and the crossing is interpolated between them.
In practice this makes “crossing 550 HV” a hard requirement of the layout. Stopping at 520 HV, or having a single point near 560 HV, does not permit a compliant calculation.
2. Spacing constraints
The new standard is explicit about indentation placement:
- the first indentation at least 2.5 diagonal lengths from the surface;
- adjacent indents at least 3 diagonals apart, to avoid mutual strain-hardening effects;
- the ±0.15 mm band around 550 HV refined to 0.05 mm.
The last point is where most problems occur: running a uniform 0.1 mm pitch to save time often straddles the limit band, degrading interpolation accuracy and repeatability.
3. More than 10 % between lines? Check preparation first
When two measurement lines differ by more than 10 %, the standard requires re-preparation and re-testing. The first instinct is to blame the material; far more often the cause is preparation:
- edge rounding or collapse displacing the first indentation;
- residual polishing deformation raising near-surface hardness;
- a line not perpendicular to the surface normal, stretching apparent depth.
Rule out preparation before discussing process.
4. What the report must state
A report acceptable to customers and auditors needs at least: standard number and edition, test force, specimen condition (tempered), sampling position and method, the two points used for interpolation, the CHD result and the acceptance basis.
One sentence is often omitted but matters: hardness-method CHD is not metallographic case depth, and the two are not interchangeable.
Our approach
Luore Technology is building a hardened-depth testing system around GB/T 9450-2025 and GB/T 5617-2025, linking sampling, preparation, gradient acquisition, interpolation and non-conformance review into one traceable flow, with work instructions, report templates, raw records and gradient sheets.
The system is currently in internal operation and refinement; laboratory accreditation has not been applied for. We use it as an internal quality-control tool and are glad to open the method and records to customers who wish to audit them.