Which scale when?
| Scale | Range | Best for | Indenter |
|---|---|---|---|
| Rockwell C (HRC) | 20-70 HRC | Hardened steels, tool steels | Diamond cone 120° |
| Rockwell B (HRB) | 20-100 HRB | Soft steels, copper, brass, aluminum | 1/16" steel ball |
| Rockwell A (HRA) | 20-88 HRA | Tungsten carbide, thin hard parts | Diamond cone |
| Vickers (HV) | 5-3000 HV | Universal range, thin parts | Diamond pyramid 136° |
| Brinell (HB) | 50-650 HB | Large parts, castings, forgings | 10mm steel or carbide ball |
| Shore A | 0-100 Shore A | Soft rubber, elastomers | Blunt pin, spring loaded |
| Shore D | 0-100 Shore D | Hard rubber, soft plastics | Sharp pin, spring loaded |
| Mohs | 1-10 Mohs | Minerals (scratch test) | Comparative scratching |
| Knoop (HK) | Same as HV | Microhardness, thin layers | Rhombic pyramid |
Each hardness test has a specific range where it works well. Using the wrong scale gives unreliable results.
Rockwell C (HRC).
Soft steel
Mild steel, annealed tool steel. Easily machined. Not wear resistant.
Tempered structural
Quenched and tempered steels for structural use. Good fatigue life. Still machinable with effort.
General hardened
Typical hardened steel parts — machinable by grinding, difficult to mill. Good wear resistance.
Hardened tool steel
Fully hardened tool steels. Requires grinding or EDM for machining. Excellent wear resistance.
Hard cutting tools
HSS cutting tools, hardened bearings. Extremely wear-resistant. Brittle at upper end.
Extreme
Carbide tools, extreme wear applications. Very brittle — limited to specific applications.
The most common hardness specification for steels. Used extensively in machining, heat treatment, tooling.
Approximate equivalents.
| HRC | HRB | HV | HB | Approximate UTS (MPa) |
|---|---|---|---|---|
| — | 60 HRB | 108 | 105 | 365 |
| — | 80 HRB | 150 | 150 | 515 |
| — | 100 HRB | 240 | 240 | 800 |
| 22 HRC | — | 250 | 240 | 820 |
| 30 HRC | — | 300 | 285 | 980 |
| 40 HRC | — | 390 | 375 | 1290 |
| 50 HRC | — | 515 | N/A (beyond HB range) | 1750 |
| 60 HRC | — | 720 | N/A | 2400+ |
| 65 HRC | — | 830 | N/A | (brittle) |
Not exact — hardness conversions are approximate because different tests measure different material behavior. Use as rough guide, not precise equivalent.
Applications by hardness.
Why specify hardness
Typical specs
FAQ
Different tests measure different material responses. Rockwell measures depth of indent under load. Vickers measures surface area of diamond-shape indent. Brinell measures diameter of ball indent. Each responds differently to material elasticity, work hardening, and heterogeneity. For heat-treated carbon steels, conversions are reasonable within 10%. For other materials (copper, aluminum, brass, hardened tool steels), conversions may differ 20%+. Use direct test for the material when precision matters.
Surface hardness can differ significantly from bulk (core) hardness. Case-hardened parts: surface 60+ HRC, core 30 HRC. Nitrided parts: surface 1000+ HV, core 300 HV. Induction-hardened shafts: hardened zone 50+ HRC, unhardened 25 HRC. When specifying hardness, clarify whether surface or bulk is required. Standard Rockwell testing measures surface hardness — for bulk, cross-section and test.
Microhardness (HV or HK) uses very small indenter load (< 1000g) — measures hardness of specific features: case-hardened layers (depth profile), individual phases in microstructure, thin coatings, heat-affected zones. Required for: carburization case depth verification, coating hardness specification, quality verification of specialty heat treatments. Standard macro-hardness tests can't distinguish these features.
Shore hardness for rubbers and soft plastics. Shore A for rubbers (gaskets, tires, seals): 20A very soft (rubber bands), 50A medium (soft O-rings), 80A hard (tire tread). Shore D for hard rubber and soft plastic: 60D (Delrin), 80D (hard plastics). Specific durometer affects sealing, friction, impact properties. Specify by application requirements.
No. Higher hardness = better wear resistance but increased brittleness, harder machining, reduced fatigue life. A 65 HRC part may wear well but fracture under impact; a 45 HRC part wears faster but tolerates impact. Specify hardness to match application: wear service → higher HRC, impact service → moderate HRC with ductility, general structural → lower HRC. Over-specifying hardness is a common mistake.
For quality-critical parts, hardness testing documentation: calibrated testers traceable to national standards (NIST), calibration records, test location specified on drawing, test force and anvil specified, multiple locations on each part. For aerospace and medical work, witnessed testing available. Certificates per EN 10204 3.1 include hardness verification data.
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