Hardness Testing

The hardness of the metal is often considered as a resistance to permanent indentation. Although hardness testing of materials, especially metals, does not directly indicate performance properties, it is widely used for material evaluation due to its simplicity relative to direct measurement.

The most popular method of measuring hardness is by pressing a very hard pointed tool (an indenter) into the test surface using a specific force (Load) for a definite time interval (dwell). The hardness number is then calculated from the area of the resulting indent (Vickers, Brinell and Knoop) or from the depth of indentation (Rockwell and Rockwell Superficial)

Hardness Indented block

What are the different types of hardness tests?

Many methods of applying the load and quantifying the resistance to deformation have been suggested and used in industries during the past century. The hardness scales most commonly used today are:

v Brinell Hardness (J. Brinell, 1900)

v Vickers Hardness (R. Smith / G. Sandland, 1925)

v Rockwell Hardness (S. Rockwell, 1919)

v Rockwell Superficial Hardness (S. Rockwell, 1921)

v Shore hardness (A. Shore, 1907)

v Knoop Hardness (F. Knoop, 1939)

Which hardness Tester is widely used?

The internationally standardized (ISO) hardness Scales are Brinell, Vickers, Rockwell and Rockwell Superficial. Hardness is relative and thus has no fundamental absolute standard, which is completely different from other physical quantities such as length.

In practice, hardness is often used as a substitute measure to estimate other physical characteristics. Of all the hardness-testing methods in use today, the Rockwell family is by far the most popular, with Rockwell ‘C’ (officially HRC for Hardness Rockwell C) being the favourite scale in use.

Why is hardness testing universally used?

The answer lies in two aspects of hardness testing

1.   Such tests are easy to perform and relatively non-destructive
2.   Hardness Testing correlates well with other physical properties of materials.

 Hardness Testing

Thus the hardness test is a simple and non-destructive method of determining the suitability of a workpiece for its intended use.

Characteristics of metals that relate to hardness

Ø Ability to resist indentation

The ability of components such as bearing races and escapment wheels are subject to indentation forces that can be correlated directly with indentation-type hardness testing methods. The most common application is predicting the ability of ball-bearing races to resist so called Brinelling, which is indentation from balls under impact and high static load conditions.

Ø Abrasion resistance

The Mohs hardness number scale is based on the hardness of materials (from talc to diamond) and corresponds roughly to logarithmic values of indentation hardness. The ability of metals to resist abrasive wear shows a similar non-linear relationship, in which a large increase in indentation hardness corresponds to a lesser increase in scratch and abrasion resistance.

Ø Yield Strength

The correlation between hardness and yield strength is positive and significant but departs sharply from a straight-line relationship for metals that have been treated to increase hardness.

Ø Impact Strength

Within a specific alloy or pure metal type, impact strengths are inversely related to hardness (i.e., harder metals are more brittle). There is no direct correspondence between impact strength and indentation hardness, which is therefore only useful in this regard when comparing the same alloy or metal.

Ø Malleability and ductility

These properties are generally inversely proportional to hardness. As with impact strength, comparisons are only useful between samples of the same alloy or metal.

Indentation hardness testing methods are useful when a test specimen is too large or too heavy to move. In 1900, Brenell’s idea of using a ɸ10mm steel ball to indent a specimen under a static load as high as 3000 kgf was the beginning of modern hardness testing methods. This is called the indentation method and Rockwell, Vickers and Knoop all operates on the same principle.