Hardness
Quick Definition
Hardness is a material's resistance to permanent indentation. It is measured by pressing a hard indenter into the surface under a set load, and reported on a scale such as Rockwell (HRC), Brinell (HB) or Vickers (HV). Each scale has its own number, so a hardness value must always name its scale.
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Hardness explained
A hardness test presses a hard tip into a surface and measures the mark. A smaller or shallower mark means a harder material. There is no single hardness unit. A reading of 60 on the Rockwell C scale and 60 on the Brinell scale describe very different materials, so numbers can only be compared through a conversion table.
Synonyms & Related Terms
Formula
HV = 1.8544 × F ÷ d² (F in kgf, d in mm)
HV = 0.1891 × F ÷ d² (F in N, d in mm)
Real-World Example
A gear tooth in case-carburised steel may be specified at 58–62 HRC at the surface with a softer core. A surface reading checks the case only, not the core.
Which test measures what
| Test | Indenter | What is measured | Typical use |
|---|---|---|---|
| Rockwell (HRC, HRB) | Diamond cone or steel ball | Depth under load | Heat-treated steel, production checks |
| Brinell (HB) | 10 mm ball, usually | Width of the mark | Castings, forgings, coarse grains |
| Vickers (HV) | Diamond pyramid | Diagonal of the mark | Thin sections, fine work, any hardness |
| Knoop (HK) | Long diamond | Long diagonal | Coatings, thin layers |
| Shore (A, D) | Spring-loaded pin | Depth | Rubber and plastics |
| Mohs | Scratch against minerals | Which scratches which | Minerals in the field |
Rockwell C is used from about 20 to 70 HRC. Softer steel moves to Rockwell B, because the cone gives poor readings at low hardness.
A Vickers test, step by step
A 30 kgf load leaves a mark with a mean diagonal of 0.50 mm.
HV = 1.8544 × 30 ÷ 0.50² = 222 HV
Using newtons, F = 294.2 N and 0.1891 × 294.2 ÷ 0.25 = 222 HV, the same answer. Put newtons into the kgf formula and you get a result 9.8 times too high. Write the result with its load: 222 HV 30.
A Brinell test
A 10 mm ball at 3,000 kgf leaves a mark 4.0 mm wide.
HB = 2F ÷ (π × D × (D − √(D² − d²))) = 229 HB
Thicker sections suit Brinell because the wide mark averages over the grains. On a thin sheet, the anvil below affects the reading.
From hardness to strength: a rough guide
For carbon and low-alloy steels, tensile strength is often estimated from Brinell hardness:
Tensile strength in MPa ≈ 3.45 × HB (or psi ≈ 500 × HB)
A 200 HB steel comes out near 690 MPa. Expect a scatter of 10% or more. The rule does not apply to cast iron, aluminium or very hard steel. Hardness can estimate strength, but it cannot replace a tensile test where a part's strength matters. The hardness conversion calculator moves between scales, and conversions are approximate too.
Why hardness and toughness pull apart
A hard steel resists wear and indentation, but it can be brittle. Tempering gives up some hardness for toughness. That is why a gear may have a case at 58–62 HRC for wear and a softer core for shock. The heat treatment guide explains how each step changes hardness. A surface reading on a case-hardened part tells you about the case only, not the core.
Why It Matters
Hardness testing is one of the most used quality checks in metalworking, because it is quick and leaves only a small mark. It shows whether heat treatment worked. It does not replace a tensile test where strength matters.
Industry Applications
Sources
- ISO, ISO 6507-1 — Metallic materials, Vickers hardness test, Part 1: Test method.
- ASTM International, ASTM E140 — Standard Hardness Conversion Tables for Metals.
- ISO, ISO 18265 — Metallic materials, conversion of hardness values.
Related Standards
Frequently Asked Questions
What is hardness?
Resistance to permanent indentation, measured on a named scale.
What is the difference between hardness and strength?
Hardness is a surface response. Strength is the load a section carries. They are related in many steels but are not the same.
Which is harder, 60 HRC or 500 HV?
60 HRC. On conversion tables, 500 HV is close to 49 HRC. Compare on the same scale.
What is surface hardness?
The hardness of the outer layer, often raised by carburising or nitriding.
Does a hardness test damage the part?
It leaves a small mark, so test where a mark is acceptable.
Is hardness the same as toughness?
No. A very hard material may crack easily.
See Also
Related Glossary Terms
Tensile Strength
Tensile strength, or ultimate tensile strength (UTS), is the highest stress a material can carry in tension: the maximum force in a tensile test divided by the original cross-section. Ductile metals reach it before they neck and break.
Yield Strength
Yield strength is the stress at which a material stops springing back and starts to deform permanently. For metals without a sharp yield point it is defined as the stress that leaves 0.2% permanent strain — the 0.2% proof stress, R_p0.2.
Brittleness
Brittleness is a material's tendency to fracture with little or no plastic deformation, absorbing little energy before it breaks. It shows up as low elongation and reduction of area in a tensile test and low absorbed energy in a Charpy impact test.
Tempering
Tempering is a heat treatment applied to quenched steel by reheating to a temperature below the critical point and slow cooling, to reduce brittleness and relieve stresses while retaining acceptable hardness.
Quenching
Quenching is the rapid cooling of a metal from a high temperature by immersion in a cooling medium such as water, oil, or air, to lock in a hardened microstructure.
Ductility
Ductility is how far a material can be stretched before it breaks. It is measured in a tensile test, as percent elongation or percent reduction of area. Steel, copper and aluminium are ductile; glass and cast iron are not.