Hardness Conversion Chart
HRC · HB · HV · HK · Tensile

How hardness conversion works

Every hardness test pushes a different shaped indenter into the material under a different load and measures the result a different way. That is why you cannot just plug one number into a formula and get another. The conversions are empirical, built from thousands of comparison tests on real steel. ASTM E140 is the standard everybody uses.

Rockwell C (HRC)

Diamond cone, 150 kgf. This is the scale you see on almost every heat-treat drawing. Valid from HRC 20 up to 68. Fast to test, portable testers exist, and you do not need a polished surface. If the print says “HRC 28-32,” the heat treater checks with a Rockwell C tester and that is the end of it.

Rockwell B (HRB)

1/16″ steel ball, 100 kgf. For softer material that would bury the diamond cone: annealed steel, brass, copper, softer aluminum alloys. Useful from HRB 0 to 100. Past 100 the ball deforms and the number is junk. If you are testing something that hard, switch to HRC.

Brinell (HB)

10mm carbide ball, 3000 kgf. Makes a big impression, 3 to 6mm across, so you are averaging roughness and grain variation over a larger area. Mill certs almost always report Brinell because it is more forgiving of surface condition and less sensitive to local variations in castings or forgings.

Vickers (HV)

Diamond pyramid, measured under a microscope. Works from dead soft to tool-steel hard without changing the indenter, HV 100 up to 2000+. You see it in labs for micro-hardness work: coating thickness checks, case depth profiles, weld HAZ surveys. Less common on the shop floor.

Knoop (HK)

Elongated diamond pyramid that makes a long, shallow mark. Used for micro-hardness on thin coatings, ceramics, and glass where a deeper indentation would crack the specimen. Mostly a lab tool. If your drawing calls out Knoop, you are probably working on something with a PVD or CVD coating.

The tensile strength rule of thumb

For carbon and alloy steels: tensile strength in psi is roughly 500 times the Brinell number. So HB 300 is about 150 ksi. Machinery’s Handbook publishes this relationship and it holds up well from HRC 20 to 55. Above 55 the steel is too brittle for a meaningful tensile test, so the number stops being useful. Do not apply this to stainless, aluminum, or anything cold-worked.

When conversion tables go wrong

ASTM E140 says right up front that these numbers are approximate. They assume standard quench-and-temper heat treatment on carbon or alloy steel. Three things will throw them off:

Cold work. If the part was cold-drawn or cold-rolled instead of heat-treated, the hardness-to-tensile relationship shifts. The tables were not built from that data.

Case hardening. A carburized or nitrided surface has a hardness gradient. The Rockwell test reads one depth, the Brinell test reads another. The conversion assumes uniform hardness through the test zone.

Wrong material. The tables are for steel. Stainless, nickel alloys, tool steels with heavy vanadium or tungsten, and non-ferrous metals all indent differently. A Brinell-to-HRC conversion on 304 stainless will give you a number, but it might not mean what you think it means.