Hardness conversion (HRC, HB, HV, HRB)

How it works

There is no exact formula between hardness scales: each test presses a different indenter into the metal with a different load. The figures come from the conversion tables in ASTM E140 (Tables 1 and 2, non-austenitic steels), checked 25/10/2026, with straight-line interpolation between rows. Readings between whole numbers are rounded as E140 recommends.

Tensile strength is estimated as about 3.45 × HB in MPa. That is a widely used rule of thumb for carbon and low-alloy steels, not part of E140, so treat it as a rough guide EST.

Don’t use these tables for austenitic stainless steel, cast iron, non-ferrous metals or case-hardened parts with a thin skin: each has its own relationship. If a drawing specifies a scale, test on that scale.

Quick guide

• About 15–20 HRC: annealed tool steel, easy with HSS.
• 28–32 HRC: pre-hardened bar such as 4140 / EN24T. HSS works at low speed; carbide is easier.
• 45–55 HRC: needs carbide or CBN, light cuts and a rigid set-up.
• 58–64 HRC: hardened tool steel. Grind, or hard-turn with CBN.

Source: ASTM E140, Standard Hardness Conversion Tables for Metals (Tables 1 and 2). Values are for reference only.