The Different Scales of Hardness Testing

Hardness testing systems provide a quick and economical method of testing mechanical properties; all materials can be tested for hardness provided the correct method of test is adopted. Hardness testing offers valuable information for research and production application where material properties are an important consideration.

The hardness test measures a materials resistance to indentation and is a good reference for properties such as resistance to abrasion. Typical applications are for automotive heat treatment studies, protective and hard coating applications in aerospace and weld inspection in advanced fabrications. In this article we intend to look at the five main types of hardness testing, their history and a brief description of their industrial applications.

Brinell

The Brinell hardness testing method is the first method invented for standardising hardness, from which other hardness measurement methods have since been derived. Originally invented by Dr. J. A. Brinell in Sweden in 1900, a steel or carbide ball is used of diameter 1mm to 10mm depending on the requirements for hardness test. The ball is loaded with upto 3000Kg to create an impression or indentation in the surface, the size of the indent is measured and converted to a hardness value.

Brinell testing is valuable in foundry applications where the testing of inhomogeneous material is required such as large cast iron castings. The advantage of the large ball is that the components hardness is measured across a range of microstructural features and this enables consistent hardness values to be achieved. While the Brinell test is considered to be less versatile than that of the Vickers or Rockwell, it is more effective at testing materials such as forgings and castings.

Vickers

The Vickers hardness test was originally developed in 1924 by Smith and Sandland at Vickers Ltd, as an alternative to the Brinnell test, which was considered to be rather inaccurate and limited in application. The resulting Vickers test on the other hand, retained the same process as the Brinell but by using a pyramid shaped diamond rather than a steel ball indenter, they were able to deliver a more consistent and versatile hardness test.

Rockwell

The Rockwell test was thought up by a Viennese professor Paul Ludwik in his 1908 publication Die Kegelprobe, which directly translates as ‘The Cone Trial’. However, it wasn’t until 1914 that the commercial potential of the test was realised by brothers Stanley and Hugh Rockwell when they applied for a patent. By the 1920’s the commercial production of Rockwell testers had started, and the test is still commonly used today. One of the main strengths of the Rockwell is the small area of indentation needed, unlike the Brinell test. It is also much easier to use as readings can be given directly, without the need for complex calculations and equations.

Microhardness and Knoop

In some cases where only a small material sample is available or needed for testing, then a micro hardness may be used in place of a macro test. Due to the small sample size, the test requires the use of a microscope to measure the indent size following the test. The principle of the micro hardness test is the same as used for the Vickers with loads below 1 Kg and a facility to measure indents at magnifications up to 1000x can be required.

The ability to use such a small sample is both a strength and weakness of the test. On the one hand, only a small sample is needed, however, due to the need to measure the indent optically the material needs to be metallographically prepared prior to testing. The test is also slower than the Vickers test, taking roughly the same time as a Brinell test. rockwell hardness tester for sale