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Hardness Tester Basics for Machinists

Guide · Updated October 27, 2026

Machinists usually check hardness to confirm heat treatment or material before cutting. Rockwell testing is the shop standard. It measures how deep an indenter goes under a set load.

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Step by step

Pick the scale: Use HRC (diamond cone, 150 kgf) for hardened steel and HRB (1/16 in ball, 100 kgf) for softer steel and brass.
Prep the sample: Grind or file off scale and decarburized skin, and make sure the faces are flat and parallel.
Check with a test block: Test a certified hardness block first.
Test: Space indents several indent diameters from each other and from edges. Take several readings.
Record: Record the average and the scale.

Common errors

Thin parts: the anvil effect raises readings.
Curved surfaces need a correction.
Testing over scale or decarb gives low readings.

Worked example: choosing a scale and checking the tester

For hardened tool steel, use Rockwell C with a diamond indenter, 10 kgf minor and 150 kgf major load. Before testing parts, test a certified test block, for example one marked 62.5 ± 1.0 HRC. Take three readings; if they average 62.3, the tester is within tolerance. For soft steel, brass and aluminum below about 20 HRC, switch to Rockwell B with a 1/16 in ball and 100 kgf major load.

ASTM E18 calls for the test piece to be thick enough that no mark shows on the back, about 10 times the indent depth, and for indents to be at least 3 diameters apart center to center and at least 2.5 diameters from any edge. On round parts, a curvature correction is added: small diameters read low. A 0.25 in diameter shaft tested on the C scale needs a correction of roughly 1 to 3 HRC depending on hardness, taken from the table in the standard.

Clean scale and decarburized skin off the test spot with a file or fine grit first; decarb on heat-treated steel can read several points low, which makes a good part look soft.

Quick shop check

Rockwell C uses a 150 kgf total load with a diamond cone; Rockwell B uses 100 kgf with a 1/16 in ball. Verify the tester on a certified test block before use: if a 62 HRC block reads 61.2, the tester is out 0.8 points, which is outside many tolerances of about ±1. Take at least three readings and space them at least about 3 indent diameters apart.

FAQ

What is HRC?

Rockwell C: a diamond cone at 150 kgf, used for hardened steel.

HRC vs HRB?

HRB uses a ball and a lower load for softer metals. HRC is for hard steels.

How do I check a hardness tester?

Use certified test blocks on the scale you're testing.

Can a file test replace a tester?

Only roughly. A file that skates suggests very hard steel, but it gives no number.

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General shop information only — follow your gage manufacturer's instructions and your quality system.