Hardness Testing Our Test Equipment
  • Avery 1412 Rockwell hardness testing machine. Rockwell scales B & C (UKAS Calibrated)
    • Maximum test piece height/diameter: 200mm
    • Throat depth: 150mm
    • Maximum test piece length: Approx 1300mm
    • Maximum test piece weight: Manual handling only
  • Portable (King) Brinell hardness tester. (10/3000 UKAS Calibrated) with chain adaptor for large pieces.
    • Maximum test piece height/diameter: 350mm in stand or apporox 1000mm using chain adaptor
    • Throat depth: 100mm
    • Maximum test piece length: Unlimited
    • Maximum test piece weight: Unlimited
  • Vickers-Armstrong pedestal hardness testing machine.Vickers HV30 & HV 10 (UKAS Calibrated)
    • Maximum test piece height/diameter: 380mm
    • Throat depth: 135mm
    • Maximum test piece length: Approx 1300mm
    • Maximum test piece weight: Manual handling only

Hardness Testing Methods Hardness testing of materials can be summed up as a repeatable method which produces a comparable value of a material's ability to resist plastic deformation.

When a uniform force is applied to two materials, one hard like tool steel and one other soft like aluminium or copper then it’s logical to assume that the softer materials will experience a greater amount of plastic deformation than the harder one. This is the practical basis for almost all hardness testing.

There are several types of hardness testing commonly in use and many of these can operate over several different scales for the different ranges of hardness found in various materials. The basic principle is to apply a known force to a special shaped tool called an indentor which results in a small area of plastic deformation to the test sample, the measurement of either the width or depth of the deformation can then be used to determine the hardness of the material on a given scale.

Another thing to consider when choosing a hardness testing method is the size of deformation created, which is commonly known as a spot. Generally the greater the area of the spot the more accurate your test result will be due to an averaging effect over the area, however the drawback to this is that it could result in unacceptable damage to the part due to a) the large size of the spot and 2) the high forces that need to be used in order to create the large spot. Often, such tests (whilst offering better accuracy) are not used due to unacceptable levels of damage that are caused to the part and the trade off between smaller sized spots and less mechanical damage but somewhat less accuracy and repeatability of results is deemed acceptable.

It has been found that the hardness values of metals correlates to the tensile strength of the sample, and so if the hardness of a metal can be determined it may be possible to make an estimated guess of the materials tensile strength.

The three main types of test that are commonly used for metallic hardness testing are.

  • Rockwell
  • Brinell
  • Vickers
Below is a brief description of each of the methods along with some advantages and disadvantages of the test

Rockwell Hardness Test

The Rockwell hardness testing method was invented in the USA by Hugh Rockwell and Stanley Rockwell around the time of World War One. Hugh and Stanley were not thought to be related and it was purely coincidental that they both shared the same surname. Over the course of the next decade Stanley Rockwell made several improvements to the method and later teamed up with Charles Wilson to commercialise and standardise the Rockwell method.

The Rockwell hardness testing method is unique in that it uses two loads, an initial minor load followed by a major load. The minor load is first applied to the test piece, and the depth display zeroed, then the major load is applied for a short period of time, then removed. After the major load is removed the depth of the indentation will be greater than before, the Rockwell hardness is calculated from the difference between the depth of indentation from before and after the application of the major load.

The indentation is created via a special conical diamond indenter often called a brale or a 1/8” or 1/16” tungsten carbide ball bearing mounted in a housing. The type of indentation used and the values of the major and minor forced give rise to the different Rockwell Hardness scales. For example the commonly used HRC scale uses the brale diamond indentor, a minor load of 10kgf and a major load of 150 kgf.

The advantages of the Rockwell Hardness method are as follows:
  • Unlike other methods the hardness value is easily read from the display on the machine, so operator error is greatly reduced compared to methods which rely on measurement microscopes.
  • Numerous Rockwell scales are available for testing materials of differing hardness, with the most popular for metallic materials being HRB for softer metals like aluminium and soft steels and HRC for harder engineering steel alloys.
  • The indentation left by the Rockwell method is very small and often barely noticeable so is often suitable for use on finished items.
The disadvantages of the Rockwell Hardness method are that:
  • The machine is very sensitive and therefore is not portable.
  • Only small parts which can easily be moved by hand are suitable for testing.
  • The accuracy of the results is highly sensitive to movement of the test piece so parts need to be rested firmly against the lower anvil and evenly balanced. Parts which can rock or slightly move about will nearly always give poor and inconsistent results. Although a wide range of different shaped anvils are available or can be made for specific products.
Brinell Hardness Test

The Brinell hardness test was invented in 1900 by Johan August Brinell and was the first standardised hardness test to be used in the fields of engineering and metallurgy.

The method works by clamping the test piece between an anvil base and a ball bearing of a known diameter (typically 10mm) a known force is then applied (typically equivalent to 3000kg) and the ball bearing is pushed into the test piece. After at least 10 seconds the force is removed and the test piece taken out of the machine.

A special microscope is then used to read the diameter of the resulting indentation. This microscope will typically be able to read indentations up to 6mm with an accuracy of 0.05mm. Once the diameter of the indentation has been measured a simple converstion chart is used to give the correct Brinell Hardness valve for the test piece.

The advantages of the Brinell Hardness method are as follows:
  • Some Brinell testing machines are portable so that accurate hardness measurements can be made with the part in-situ or test equipment can be taken on-site.
  • A wide range of accessories are available (such as a chain adaptor) which allow a wide range of parts to be tested both large and small.
  • Due to the large indentation made by the ball bearing, the brinell test is one of the most accurate tests as an averaging effect takes place over the area of the indentation and is therefore less susceptible to inaccurate results resulting from a small indentation hitting a local hard or soft spots in the material.
  • In the case of any dispute or should anyone wish to read the hardness value again at a later date, the spot can easily be located and re-measured using a brinell microscope to confirm that the hardness value.
The disadvantages of the Brinell Hardness method are as follows:
  • The large size of the indentation (typically around 3-5mm diameter) and the requirement for a flat surface to be ground means that often the method is unsuitable for finished products. As the indentation would affect the usability of the part.
  • As the forces involved are typically quite high the Brinell method is often not suitable for thin materials or small parts.
  • The accuracy of the test results is highly dependent upon the correct use of the equipment, particularly the measuring microscope and the indentation at times may be challenging for an inexperienced technician to read accurately. An appropriately trained and experienced technician is essential to obtain satisfactory results.
Vickers Hardness Test

The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland who both worked for the engineering conglomerate Vickers. The test was designed to be a viable alternative to the Brinell method.

The method works by applying a known force to a pyramid shaped diamond indentor for a period of around 10 to 15 seconds. Once the load has been removed the distance from corner to corner (across corners) of the square shaped indentation is measured using a high magnification microscope to an accuracy of 0.01mm. With a typical indentation having a size of around 0.5mm

The Vickers hardness test is one of the most versatile methods for testing hardness and the same indentor is used regardless of scale. With the applied force being the only variable. Applied forces are usually in the range of 5kg to 100kg, with 10kg and 30kg being most common.

A special application of the Vickers test is that of a micro-Vickers hardness test. Applied forces can range from 10 grams to 1kg and these low force tests are often used on ceramics, foils and other situations where a very small indentation is desirable.

The advantages of the Vickers Hardness method are as follows:
  • The small indentation size and relatively low forces used mean that the Vickers test is ideally suited for small and thin parts or areas where the indentation has to be kept as inconspicuous as practically possible.
  • A single scale suitable for a wide range of materials ranging from very soft to very hard.
  • The use of a fixed mounted microscope means that there is less scope for operator error when compared to a manual portable Brinell microscope.
The disadvantages of the Vickers Hardness method are as follows:
  • Generally Vickers hardness testing machines are not portable.
  • Only small parts which can easily be moved by hand are suitable for testing.
  • In order to read the indentation accurately a very smooth surface finish is required, so some surface dressing or polishing may be required in order to achieve good results.