Metal Hardness and Testing Methods

Hardness is a key mechanical property of metals, indicating their resistance to deformation, scratching, and wear. In property tables for metals, hardness values are often listed, typically expressed using scales such as Brinell hardness (HB) or Vickers hardness (HV).

These values are determined through indentation tests, in which a fixed load is applied to a metal surface and hardness is evaluated based on the depth or area of the indentation. In simple terms, hardness represents a material’s resistance to deformation.

This article explains the four main metal hardness testing methods—Brinell, Vickers, Knoop, and Rockwell—along with their notations. At the end, you’ll also find a hardness conversion table to compare values across methods.

Types of Metal Hardness Tests

The earliest indentation test was introduced around 1859, using a 3.5 mm impression under load. In 1900, J.A. Brinell standardized what became the Brinell hardness test. Later, the Vickers test (1924) was developed for greater accuracy, followed by the Knoop test (1939) for measuring hardness of thin or brittle materials.

Unlike these methods, which measure indentation size, the Rockwell test (developed between 1908–1914) determines hardness by measuring indentation depth.

Brinell Hardness Test (HB, HBW)

A carbide ball (indenter) of φ1 to φ10 is pressed into the metal surface under a specified load. The indentation diameter is measured with a microscope, and hardness is calculated using formulas or conversion tables.

Because the indentations are large, the Brinell method is best suited for rough or uneven materials like castings. Because the indentation diameter is large, this test method is used for rough-surfaced materials such as castings.

Vickers Hardness Test (HV)

In this method, a diamond pyramid-shaped indenter is pressed into the material under load. The indentation is square-shaped, and its diagonal length is measured with a microscope to calculate hardness.

Since the indentation is very small (around 0.5 mm), the material surface must be polished beforehand for accuracy.

Knoop Hardness Test (HK)

The Knoop test is similar to Vickers but uses an elongated diamond-shaped indenter. Its long diagonal is about three times that of the Vickers indentation, while penetration depth is about half.

This makes the Knoop method suitable for measuring the hardness of thin layers and brittle materials.

Rockwell Hardness Test (HRA, HRC)

Here, a diamond cone or carbide ball indenter is pressed into the surface, and hardness is determined by the depth of indentation. Because the indentation is small (less than 1 mm), the Rockwell test is widely used for quick, on-site evaluations, such as in heat-treatment plants.

The HRC notation is commonly used for quenched steels.

Hardness Conversion Table

Materials with high hardness are resistant to wear but tend to have low toughness and are weaker against impact, so it is important to note that hardness alone does not fully represent the strength of a metal.

Additionally, even metals with low hardness in their raw state can be hardened through heat treatment. When selecting materials, hardness should be considered alongside toughness, grade, and processing method.

The following conversion table provides approximate equivalencies between different hardness scales for reference.

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