Hardened depth,
judged by the standard

Why build a testing system at all

A salt bath is a process variable; hardened depth is the outcome you actually sell. Without a repeatable way to measure the outcome, every improvement to the medium is an opinion.

We wrote the testing system for two reasons: to close our own service loop — filtration and concentration control should be judged by hardness results, not by the colour of a beaker — and to give customers documents they can audit.

Two standards, two work instructions

• GB/T 9450-2025 — carburised and carbonitrided case depth (CHD), evaluated from the surface to the 550 HV limit by linear interpolation.
• GB/T 5617-2025 — surface hardened depth (SHD), with the limit hardness derived from the drawing minimum (HL = A × HV, A = 0.8 by default).

Each has its own work instruction, report template, raw record, gradient record sheet and a simplified 8D non-conformance review.

Where the errors actually come from

In practice, most CHD disputes are not caused by the furnace. They come from preparation and layout: edge rounding that displaces the first indent, residual polishing deformation that inflates near-surface hardness, a measurement line that is not perpendicular to the surface, or an indentation pitch too coarse to straddle the limit hardness.

The system therefore fixes three things in writing: minimum distance from the surface (2.5 diagonals), minimum spacing (3 diagonals), and mandatory refinement to 0.05 mm within ±0.15 mm of the limit.