Controlling product quality with hardness testing

Many new products entering the market today are based on new materials and processes. These new materials are playing increasingly important roles, supporting innovation and boosting competitiveness in technology driven industries.

The manufacturing processes and quality requirements for products made using high-tech materials call for a comprehensive knowledge of process technology in addition to thorough knowledge of the material’s properties. Among the various mechanical and optical tests used to determine the characteristics of a material and its suitability for a given application, hardness testing is one of the most critical. Hardness is defined as the mean pressure a material will support. Hardness testing is typically undertaken to assess resistance to plastic deformation, a value of tremendous importance to the determination of part quality in a wide range of industries and applications.

Exposure to loads, pressures and extreme temperatures in manufacturing has the potential to affect the performance of parts comprised of metals and metal alloys. Due to complex specimen geometry and linear correlation between hardness and tensile strength in metals, hardness testing is often the best way of establishing that components will survive and perform in their intended applications. As a result, hardness testing has become an essential part of the quality control process.

Accurately determining the hardness of a material for any given application involves several factors including the type of material, specimen geometry, surface conditions, exposure to heat treatment processes, and production requirements. A range of different hardness tests may be applied to determine different hardness values for the same test specimen. Selection of the type of hardness test is often influenced by end-customer requirements, as hardness measurements are commonly reported values on specification sheets accompanying the delivery of manufactured goods.

Among the most common tests used to determine material hardness are the Rockwell hardness test, the Brinell hardness test, and the Vickers hardness test. These three tests measure hardness by determining resistance to the penetration of a non-deformable ball, cone, or four-sided pyramidal indenter. Each test determines the depth to which the indenter will sink into the material, at a defined load, within a specific period of time. load, within a specific period of time. All three test methods are defined in ISO and ASTM standards. For suppliers to the aerospace industry, compliance with the requirements of Nadcap is often also necessary.

The Rockwell hardness test is a measurement based on the net increase in depth of impression as a load is applied. In the Rockwell method of hardness testing, the depth of penetration of an indenter following application of a minor load and a major load is measured. The indenter may either be a tungsten carbide ball of some specified diameter or a spherical diamond-tipped cone of 120° angle and 0.2mm tip radius, called a Brale indenter. The type of indenter and the test load determine the hardness scale which is expressed in letters such as A, B, C, and so forth.

Brinell hardness is determined by applying a tungsten carbide sphere of a specified diameter at a specified load into the surface of a material and measuring the diameter of the residual indentation post-test. The Brinell hardness number, or simply the Brinell number, is obtained by dividing the load used, in kilograms, by the actual surface area of the indentation, in square millimetres.

In the Vickers test the surface is subjected to a pre-defined load exerted by a four-sided pyramidal diamond indenter for a standardised length of time referred to as the dwell time. The diagonal of the resulting indentation is measured under a microscope and the Vickers Hardness value read from a conversion table.

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