How a UCI (Ultrasonic) Hardness Tester Works — and Where It Wins

An ultrasonic hardness tester reads a frequency shift, not a bounce, which is why it survives on parts too thin for an impact device.

The UCI principle: a resonating rod with a 136° Vickers diamond

The probe holds a slender rod tipped with a 136° Vickers diamond, driven at its resonance frequency. Press it in and the contact area changes, shifting that frequency: softer material, bigger shift. The mark is small, so an ultrasonic hardness tester is minimally invasive, not non-destructive. The standard calls the result a comparative hardness value and the test superficial: surface layer, not bulk.

Test load and probe choice (HV 1 / HV 2 / HV 5)

The KH271 ultrasonic (UCI) hardness tester measures by the UCI method defined in GB/T 34205-2017: parts from 0.3 kg and 2 mm, multi-point calibration across three user-defined materials, and storage for 600 groups. It is calibrated for steel and cast steel, cast aluminum alloy and pure copper; other families need your own test block, because the three custom materials cover families you have verified, not a license to measure anything. It ships with a 2 kgf (20 N) probe; 1 kgf (10 N) and 5 kgf (50 N) probes are available at Ra < 3.2 µm, < 5 µm and < 10 µm.

Direction: the UCI hardness tester probe must be perpendicular; no correction is applied. Accuracy: 4–8 % relative error and 5–9 % repeatability, on a calibrated test block under reference conditions.

Where UCI is the only practical option

• Thin wall. Tube and sheet under 5 mm: at 2 mm, UCI goes where type D cannot.

• Small parts. Bolts, pins, bearing races: 0.3 kg minimum.

• Weld and HAZ. The case that decides most purchases: a type D impact device leaves a mark about 0.54 mm across at 300 HV, while the UCI hardness tester's Vickers mark maps across the zone.

• Case-hardened layers. Nitrided and carburized cases run tenths of a millimeter deep; an ultrasonic hardness tester reaches them.