There are three industry acceptable methods to determine case hardening depth.
All normally are done on a test coupon hardened with the deliverable part.
1) Chemical, where the part of concern is hardened and a coupon from the same material is not. Both parts are machined and the chips are chemical composition analyzed to determine depth from carbon content from the differences.
2) Mechanical, where the effective depth can be determined by hardness testing in the 50 HRC range. There should be multiple strikes done along an area and the effective depth calculated (RMS). It is normal to have a range of hardness’s measured – materials are not perfect. Normally, a material coupon is included with the deliverable part where the testing is actually done. There are several configurations for testing possible where the coupon is machined with a taper the strikes done along that taper.
3) Visual method:
a) Prepare the coupon sample by fracturing. Examine at a magnification not to exceed 20 diameters with no further preparation.
b) Fracture and then Etch - Water quench product or samples directly from the carburizing temperature. Fracture and etch in 20% ****** acid in water for a time established to develop maximum contrast. Rinse in water and read while wet.
c) Fracture cut and then rough grind prepare specimen by either fracturing, or cutting and rough grinding. Etch in 10% nital for a period of time established to provide a sharp line of demarcation between case and core. Examine at magnification not to exceed 20 diameters (Brinell glass) and read all the darkened area for approximate total case depth.