Vickers hardness from diagonal measurement

The Vickers indenter is a square-based diamond pyramid with an included angle of 136° between opposite faces. Hardness is the test force divided by the sloping surface area of the impression. For that geometry the surface area is d²/(2·sin(136°/2)), and 2·sin(68°) = 1.8544, which is where the constant in HV = 1.8544·F/d² comes from — with F in kgf and the mean diagonal d in millimetres. Because the pyramid is geometrically similar at every depth, HV is very nearly independent of test force above about 1 kgf, which is why a single scale spans annealed aluminium to nitrided steel.

In weld inspection Vickers is the working scale for hardness traverses across the weld metal, HAZ and parent material. The indentations are small enough to sit inside a HAZ only a millimetre or two wide, which no Brinell or Rockwell C indentation can do. Hardness surveys to ISO 15614-1 and hardness limits for sour service to NACE MR0175 / ISO 15156 are both written in HV10, so the load matters as much as the number.

Two diagonals are measured perpendicular to one another and averaged. If they differ by more than 5% on a flat specimen the indentation is distorted — the surface is not perpendicular to the indenter axis, the specimen has moved, or the material is markedly anisotropic — and the reading should be discarded rather than averaged. Surface preparation matters: the measurement is of a sub-100 µm feature, so the specimen must be ground and polished, and any cold-worked or decarburised layer from preparation must be removed or it will be measured instead of the parent metal.

Below about 1 kgf (HV1, HV0.5, HV0.3) the test becomes microhardness. The indentation edges become hard to resolve optically, the indentation size effect makes HV drift upward at low load, and conversions to other scales lose validity. Always report the load with the number: 240 HV10 and 240 HV0.3 are not interchangeable results.

d = (d₁ + d₂) / 2
HV = 1.8544 · F / d²    F in kgf, d in mm
1.8544 = 2·sin(136°/2), the 136° pyramid constant
Diagonal difference = |d₁ − d₂| / d × 100 %   (≤ 5 % on a flat specimen)