Hardness Testing Guide

HRC. HV.
HB. Shore.
When each applies.

Q: Rubber and plastic hardness?

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.

Q: Does higher hardness mean better?

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.

Q: Traceability and certification?

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.

Hardness is the most widely specified mechanical property — but also the most misunderstood. Different scales for different material ranges, different tests for different material types. Here's the honest reference.

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01 · Scales overview

Which scale when?

Each hardness test has a specific range where it works well. Using the wrong scale gives unreliable results.

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

02 · HRC details

Rockwell C (HRC).

The most common hardness specification for steels. Used extensively in machining, heat treatment, tooling.

20-30

Soft steel

Mild steel, annealed tool steel. Easily machined. Not wear resistant.

30-40

Tempered structural

Quenched and tempered steels for structural use. Good fatigue life. Still machinable with effort.

40-50

General hardened

Typical hardened steel parts — machinable by grinding, difficult to mill. Good wear resistance.

50-60

Hardened tool steel

Fully hardened tool steels. Requires grinding or EDM for machining. Excellent wear resistance.

60-65

Hard cutting tools

HSS cutting tools, hardened bearings. Extremely wear-resistant. Brittle at upper end.

65-70

Extreme

Carbide tools, extreme wear applications. Very brittle — limited to specific applications.

03 · Conversions

Approximate equivalents.

Not exact — hardness conversions are approximate because different tests measure different material behavior. Use as rough guide, not precise equivalent.

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)

04 · When hardness matters

Applications by hardness.

Why specify hardness

• Wear resistance — harder materials last longer against abrasion
• Deformation resistance — prevents indentation, surface flow
• Fatigue life — surface hardness affects crack initiation
• Machinability — harder = more difficult to machine
• NACE compliance — oil and gas limits hardness for sour service
• Quality verification — confirms heat treatment occurred properly

Typical specs

• Gears: 58-62 HRC on tooth surfaces (carburized)
• Bearings: 58-62 HRC (52100 ball bearing steel)
• Tool steels: 58-65 HRC depending on application
• Shafts: 28-32 HRC for general, 55+ HRC at bearing journals
• NACE sour service: 22 HRC maximum on wetted surfaces
• Aerospace fasteners: 180-200 HV typical

FAQ

Why are hardness conversions inexact?

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 vs bulk hardness?

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 testing?

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.

Rubber and plastic hardness?

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.

Does higher hardness mean better?

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.

Traceability and certification?

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.