Fastener Testing

Why is hardness testing important and why do we do it? Critically, if we did not do it, what would happen? John Cross, general manager at Rotech Laboratories and Phoenix Materials Testing in the UK, takes us through the answers to these questions.

Hardness testing characterises a material’s properties and verifies a product to a particular specification or requirement. A hardness test can give an indication of a material’s strength and wear resistance. Compliance provides confidence that products are suitable to go to market and reduces the risk of failure in service, as well as the legal and commercial consequences that could follow. The seriousness of which, depends on the nature and use of the product.

The products could be anything from tiny fasteners, the smallest of screw components such as those that hold eyeglasses together, to extra-large bolts for use in heavy industry. This includes fasteners used in nuclear settings, to ensure containment of radioactive substances, which are obviously highly safety critical; or bolting used in petrochemical applications ensuring the safe delivery of millions of barrels of oil – failure of which could have severe environmental consequences.

Whichever product we are talking about, it must be compliant and suitable to go to market. Hardness verification plays an important part of that process. Virtually every engineering component that is manufactured requires a hardness test of some form, as well as it being required for each stage of the production process – from the raw material received from suppliers; after industrial treatments as part of the engineering process; as well as at final product testing before shipping to the customer.

If a material or product has undergone any form of heat treatment as part of its processing, then it again needs to be tested and verified. If engineers have carried out any surface treatment or coating processes, such as nitriding – which involves diffusing nitrogen into the surface of a metal to create a case hardened surface to increase its hardness and corrosion resistance – hardness testing is a primary way to assess conformance. If you perform any action that modifies the properties of the material or surface of products, it is imperative to verify that the right treatment has been carried out. Hardness testing plays a crucial role in this process.

What is hardness testing?

It is important to understand the characteristics of a material in terms of its strength, resistance to wear, and toughness, and this can be achieved via various hardness tests. Hardness testing often goes ‘hand in glove’ with tensile testing because it is focussed on understanding the mechanical properties of a material. Hardness testing is a measurement that establishes how hard a material is, in many instances, in a non-destructive way. This method of testing provides data to show how a material resists indentation through a series of tests in controlled conditions with a continuous load being applied by a pointed or rounded indenter.

There are varied options of hardness testing and the key tests conducted by mechanical and metallurgical engineers include:

• Rockwell.
• Brinell.
• Vickers macro and microhardness.
• Knoop microhardness.

 What type of hardness test to choose?

The key decision factors are what is the type of material? What is the material’s composition? What is the size/area of the material to be tested?  What are the required standards to be met?  And how accurate do the results need to be? Rotech’s engineers have a wealth of experience and knowledge to advise on the right type of hardness test needed for each sample.

Brinell hardness testing

For uneven or rough surfaced materials, Rotech’s recommendation would be Brinell hardness testing for bulk hardness. Brinell works incredibly well when testing forgings, castings and larger coarse material samples, as well as bulk metal. Rotech is able to use a greater diameter indenter, which helps to eliminate any variability due to localised, structural differences and surface inconsistencies.

These inconsistencies and roughness would provide varied readings according to where the test is carried out on the surface material. Due to the wider indentation over the surface area, Brinell hardness testing gives a more representative measurement. Non-ferrous metals, pre-heat treated steel, forgings, and castings, may benefit from Brinell hardness tests.

Rotech works with the following Brinell hardness testing standards and specifications:

• 5mm ball diameter and 750kg load.
• 10mm ball diameter and 3,000kg load.
• BS EN ISO 6506-1.
• ASTM E10.

Brinell hardness testing needs less preparation than more focussed test methods. It is a relatively straightforward procedure and is mainly used on rough structured materials, such as forged metals or cast irons.

Rotech uses a 5mm or 10mm round carbide ball, which makes an indentation and has an especially high load (typically 750kg and 3,000kg). We then measure the indentation, which is analysed using a microscope to find an average result via the Brinell hardness number scale.

Vickers macro hardness testing

Vickers hardness testing is used on smaller, thinner layers of metals and materials and involves reasonably light loads. It is a popular test method for all metals as it has the most extensive hardness scale measurement range compared to all other hardness testing procedures. It is incredibly versatile and covers a broad range of materials and different properties.

When conducting a Vickers hardness test, Rotech utilises a pyramid shaped diamond indenter. This is placed under a load and the indentation is measured. Rotech can also employ considerably lighter loads and can extend the use to softer materials. The method used depends on the shape, size and precise required location of the hardness test.

Rotech works with the following Vickers hardness testing standards and specifications:

• BS EN ISO 6507-1 (HV1, HV5, HV10).
• ASTM E92 (HV1, HV5, HV10).
• Low force Vickers hardness tests (HV0.3, 0.5, and 1).

To perform a Vickers hardness test, Rotech uses light loads on thin, small samples that require a polished, smooth surface prior to testing. The pyramid-shaped diamond indenter is placed on a section of the material and a constant load (macro hardness 1kg – 10kg, low force 0.3kg – 1kg) is applied for a precise period of time. This is always conducted in line with international standards. Rotech then measures the diagonal lengths of the indentation using the Vickers hardness scale to understand the hardness of the material.

Rockwell hardness testing

The Rockwell hardness test is used on metals and alloys to Rockwell scales HRB and HRC. This is a frequently used hardness test method, because it is a quick, simple method of determining the hardness of materials. Rockwell hardness testing measures the indentation and depth of penetration and is generally used on a variety of metals, which include aluminium, copper alloys, brass, soft steels and hard carbon steels. For the softer metals, Rotech uses the HRB scale and for the harder metals it uses the HRC scale. This is a good testing option for bulk metal hardness determinations, because of the loads used and it is a cost-effective, simple operation. It is non-destructive test and only leaves a minor indentation in the sample.

Rockwell testing provides a direct readout from the Rockwell hardness scale. This differs to other hardness tests, which require measurements and calculations for verification of the hardness levels.

To carry out a Rockwell hardness test a diamond cone or hard steel ball is used to apply stress to the sample. Initially, a minor load is applied to create an indentation within the material to measure the penetration depth. Secondly, a major load is then applied to the sample and the depth of penetration is measured. Finally, the minor load is applied. The permanent deformation of the sample material is determined by the difference between the first and final measurements. The Rockwell hardness scale is then used to analyse the results.

The Rockwell hardness scales are varied according to the type of material in test. The key differences are the loads and indenters used. This means that harder and softer materials can be tested. The HRB scale is used on materials such as aluminium, brass and soft steels, with 10kgf minor loads and 100kgf major loads applied. HRC is used on hard steels, hard cast irons, deep case hardened steel, with 10kgf minor loads and 150kgf major loads applied.

Microhardness testing (Knoop and Vickers)

This method of hardness determination is used on materials that are very small or thin, require microstructural hardness analysis or examination of surface treatments. 

Microhardness testing is ideal for coating hardness, surface hardness, case depth, hardness of fragile or brittle metals, as well as microstructural hardness variations.

Knoop testing is used for very low loads and perfect if the sample is quite brittle or for smaller, more delicate samples – for example thin metal sheets or thin metal layers. An elongated pyramid diamond is pressed into the surface of the material to create an indentation. This diamond has a long and narrow shape and is perfect for use on delicate samples to create longer, shallower indentations and this provides an accurate microhardness test result.

Knoop hardness testing is carried out to the following standards:

• ASTM E384 micro indentations.
• BS EN ISO 4545-1.

The Vickers micro tests are performed using a constant load with a symmetrical diamond-shaped indenter as opposed to the elongated four-sided pyramid employed in Knoop testing. Vickers microhardness testing is used for samples that require very low load applications.

Vickers microhardness testing is carried out to the following standards:

•  BS EN ISO 6507-1.
•  ASTM E384 Micro indentations.

Although arguably hardness testing is the most widely used form of quality control check, the type of test applied will depend on a number of factors and constraints. Rotech’s team of experienced test engineers can advise on the nature and type of test required to ensure products meet the required standards and are fit for purpose in a global marketplace.