The application of hardness testing facilitates the evaluation a material's properties such as strength, ductility and resistance to wear, and helps determine whether a material or material treatment is suitable for its intended in-service application.
Compliance with a relevant specification provides confidence that products are suitable to be entered into service and that the risk of material or component failure is greatly reduced or eliminated. In-service product failure can result in costly consequences which can go far beyond simply inconvenience or financial considerations.
Whichever product we consider, it must be compliant and suitable for use in its intended environment. Hardness verification plays a vital role in that process.
Virtually every manufactured engineering component requires a hardness test of some type, as well as  being necessary for each stage of the manufacturing process. From the receipt of raw material from suppliers, following treatments which form part of the engineering process, to finished product testing, hardness evaluation has an important role to play.
If materials or components undergo any form of heat treatment as part of the manufacturing process, hardness testing can be employed to verify compliance with relevant specifications. Following any surface treatment or coating processes, hardness testing is an ideal way to check conformance.
During the product manufacturing process, if any procedure modifies the properties of the material or surface, it is important to verify that the correct treatment has been implemented and hardness testing can play an important role throughout the production process.
Understanding the characteristics of a material in terms of its strength, wear resistance or toughness, can be achieved by employing a range of hardness testing principles. Quite frequently, hardness testing is combined with a tensile test because these test methods complement each other and help understand the mechanical properties of a material.
Hardness testing is a procedure which determines material hardness, more often than not in a non-destructive way. Hardness testing provides data which illustrates how a material resists indentation by using a series of tests employing a continuous load being applied via a pointed or rounded indenter.
Various options are available for hardness testing procedures but the main principles are; Rockwell, Vickers, Brinell and Knoop.
How to select the optimum hardness testing procedure?
This is based on a number of considerations such as; material type and composition, the geometry and size of the material to be tested, the need to test in accordance with an internal or internationally recognised standard, and the desired accuracy of the test results.
One of the most common and frequently used procedures is the Rockwell hardness test which is used on metals and alloys to Rockwell scales HRB and HRC. The Rockwell test provides a convenient and simple method of determining the hardness of materials.
The Rockwell hardness test measures the indentation and depth of penetration and is widely used on a variety of metals, including aluminium, brass, copper alloys, soft steels and hard carbon steels. When testing softer metals, it is appropriate to use the HRB scale and for harder metals, the HRC scale.
This test method is ideal for high throughput sample testing as it is a cost effective, simple procedure which can be performed by anyone following a short period of training. Additionally, it is non-destructive test which only results in a minor indentation in the sample, meaning that generally the tested product can be utilised and not discarded.
A feature of the Rockwell test is that it provides a direct readout from the Rockwell hardness scale and differs from other hardness tests which require measurements and calculations in order to determine material hardness.
The Rockwell hardness test consists of indenting the test material with a diamond cone or hardened steel ball indenter. Each time a test is performed two loads are applied to the sample being tested. First, the indenter is forced into the test material under a preliminary minor load and this depth is recorded.
With the minor load still applied an additional load is introduced, known as the major load, which increases the depth of penetration on the sample. The major load is then removed, and the force on the sample is returned to the minor load. The increase in the depth of penetration that results from applying and removing the major load is used to calculate the Rockwell hardness value.
Rockwell hardness scales vary according to the type of material being tested and the key differences are the loads and indenters being employed. The result is that both hard and soft materials can be tested on the same machine.
The HRB scale is used on materials such as aluminium, brass and soft steels, using 10 kgf minor loads and 100 kgf major loads. The HRC scale  is used on hard steels, hard cast irons, deep case hardened steel, with 10 kgf minor loads and 150 kgf major loads.
The Rockwell hardness test is generally considered easier to perform compared with other methods such as Vickers or Brinell, and is a commonly used method for quality control purposes in a wide range of industrial processes.
