Hardness Testing : Defining Hardness Easily

Used to characterize materials such as verifying the heat treatment of part, hardness is material’s resistance to the permanent indentation.

By Augustine Quek

Hardness is an empirical test, not a material property, as several different hardness value, given the same material. Generally, three types of hardness measurements have been defined: scratch hardness, indentation hardness and rebound or dynamic hardness.

Scratch Hardness
The ability of various minerals and other materials to scratch one another determines a metal’s rating, measured according to the Moh’s scale. This consists of 10 standard minerals arranged according to their ability to be scratched. The softest mineral in this scale is talc (scratch hardness 1), while diamond has a hardness of 10. The Mohs’ scale is not well suited for metal since the intervals are not widely spaced in the high hardness range. Most hard metals fall in the Mohs’ hardness range of 4 to 8.

In dynamic hardness measurements, the indenter impacts the metals surface and the hardness is expressed as the energy of impact. The Shore scleroscope is a dynamic-hardness tester, which consist of dropping a diamond tipped hammer inside a glass tube under the force of its own weight. This is done from a fixed height and onto the test specimen. The height of the hammer’s rebound travel is measured on a scale of arbitrary chosen Shore scleroscope does not mark the test material.

Many other hardness tests use a specifically shaped indenter, which is harder than the test sample that is pressed into the sample’s surface. Either the depth or size of the indent determines a hardness value.

Brinell Hardness
In 1990, J A Brinell proposed the Brinell Hardness test, which consists of indenting the metal surface with a 10-mm-diameter steel ball at a load of 3000 kg mass. For soft metals, load are reduces to 500 kg to avoid deep impressions. On tests of very hard metals, a tungsten carbide ball is used to minimize the indenter’s distortion.

The full load is normally applied for 10 to 15 seconds on iron and steel and for at least 30 seconds on other metals. The diameter of the indentation left in the test material is measured with a low powered microscope. The Brinell Hardness number is calculated by dividing the load applied by the surface area of the indentation. The formula shown below gives the Brinell hardness number (BHN), in unit of Mpa.

The impression’s diameter is the average of two readings at right angels and the use of a Brinell hardness number table can simplify the determination of the Brinell hardness. A well-structured Brinell hardness number reveals the test conditions. For example, “75 HB 10/500/30” means the a Brinell Hardness of 75 was obtained using a 10mm diameter hardened steel with a 500 kilogram load applied for 30 seconds.

Compared to other hardness test methods, the Brinell ball makes the deepest and widest indentation, so the test averages the hardness over a wider amount of material, which will more accurately account for multiple grain structure and any irregularities in the materials.

This method is the best for achieving the bulk or micro-hardness of a material, particularly those materials with heterogeneous structures. Surface scratches and roughness do not affect the Brinell test significantly. Typical values for BHN are 35 Mpa for aluminum, 120 Mpa for mild steel, and 1250 Mpa for stainless steel> however, small objects cannot be properly tested due to the large size of the Brinell impression.

Meyer Hardness Test
Another hardness test, proposed by Meyer in 1908, used the definition of hardness based on the projected area of the impression rather than the surface area. The mean pressure between the surface of the indenter and the indentation is equal to the load divided by the projected area of indentation. Like Brinell’s definition, Meyer Hardness has units of Mpa. It is less sensitive to the applied load than Brinell hardness. For example, Meyer hardness does not change with load for a cold-worked material, while Brinell hardness decrease as the load increases.

The empirical relation between the load and the size of the indentation is called Meyer’s law, and is given by P = kdn’, where n’ s the slope of the straight line obtained when log P is plotted against log d, and k is the value of P at d = 1.

Fully annealed metals have a value of about 2.5 for n’, while n’ is about two for fully strain-hardened metals. This parameter is roughly related to the strain-hardening coefficient in the true-stress-true-strain curve. The exponent in Meyer’s law is approximately equal to the strain-hardening coefficient plus 2.

Vickers Hardness
Developed in the 1920s, the Vickers Hardness test is another method for measuring hardness of materials. Calculations for the Vickers test are independent of the size of the indenter, and the indenter can be used for all materials. The Vickers Hardness test uses a square-base diamond pyramid as the indenter, with the angle between opposite faces of the pyramid at 136°. This angle approximates the ideas ratio of indentation diameter to ball diameter in the Brinell hardness test. Its hardness number (usually abbreviated as DPH, VHN or VPH) is defined as the load divided by the surface area of the indentation.

In testing, this area is obtained from measuring the lengths of the impression’s diagonals. The DPH may be determined from the following equation:

This test can be used for all metals. Given a load, it provides a continuous scale of hardness, from soft metals with a DPH of 5 to hard materials with a DPH of 1.500.

Rockwell Hardness Test
The Rockwell Hardness Test consists of indenting the test material with a diamond cone or hardened steel ball indenter. The indenter is forced into the test material under a preliminary minor load. When equilibrium has been reached an indicating device, which follows the movements of the indenter and responds to changes in depth of penetration of the indenter, is set to a datum position.

While the preliminary minor load still applied, an additional major load is added, resulting in an increase in penetration. When equilibrium has again been reached, the additional major load is removed but the preliminary minor load is maintenaned. Removal of the additional major load allows a partial recovery, so reducing the depth of penetration. The permanent increase in depth of pentration, resulting from the application and removal of the additional major load is used to calculate the Rockwell Hardness number.

This agrees with other hardness number. But unlike the Brinell and Vickers hardness designations, which have unit of Mpa, the Rockwell Hardness number are arbitrary.

Major loads of 60, 100 and 150 kg are used. Since the Rockwell hardness depends on the load and indenter, specifying the combination is necessary. Rockwell hardness number is prefixed with a letter (A, B or C) to indicate the combination of load and indenter for the hardness scale employed.

For example, hardened steel is tested on the C scale with the diamond indenter and a 150-kg major load. This scale ranges from about RD 20 to RC 70. Softer materials are tested on the B scale with a 1/16-in-diameter steel ball and a 100-kg major load. This scale ranges from RB 0 to RB 100. The A scale (diamond penetrator, 60-kg major load) is for materials ranging from annealed brass to cemented carbides.

The Rockwell Hardness test offers speed and accuracy. It distinguishes small hardness differences in hardened steel. The indentation the result from this test a small, so testing finished heat-treated parts leaves no damage.

A Rockwell Hardness Tester has the capacity to apply a load of up to 103 Newtons (or 100 kg of force), putting a dent in most metals on one point. Most modern testers also utilize the robotics and electronics technology to its full advantage.

Microhardness Tests
Microhardness testing machines can perform operations such as measuring of hardness gradient at a carburised surface, determining the hardness of a microstructure, or checking the hardness of a delicate watch gear.

The Knoop’s Hardness method, developed by the National Bureau as Standards, is a popular hardness test. The Knoop indenter is a diamond ground to a pyramidal form. It produces a diamond-shaped indentation with the long and short diagonals in the approximate ratio of 7:1. Its hardness number (KHN) is the applied load divided by the unrecovered projected area of indentation.

The shape of the Knoop indenter allows placing indentations closer together, compared to using a square Vickers indentation. This is useful when measuring the hardness of thin layers or testing brittle materials where the fracture is proportional to the volume of stressed material.

In the 1980s, more sensitive hardness testing methods were needed, as the advent of technologies enabled hardening of surfaces using ion implanted region could be only 100-200 nm deep. The UMIS 2000 operates in the milli-Newton range (10-3 Newtons). Applications include the assessment of high tech surface coatings on cutting tools, dies, bearings, integrated circuits and artificial human joints.

Hardness At High Temperatures
Development of alloys with high-temperature strength has fuelled the interest in measuring hardness of metals at elevated temperatures. Hot hardness indicates these alloys’ potential usefulness.

The temperature dependence of hardness could be expressed by

Two straight lines with different gradients appear after plotting log H versus temperature for pure metals. The change in the gradient occurs at a temperature that is about one-half the melting point of the metal being tested.

Similar behaviors is found in plots of the logarithm of the tensile strength against temperature. The constant A derived from the low-temperature branch of the curve can be equivalent to the intrinsic hardness of the metal. Values of A for different metals are correlated with the heat content of their liquid forms at the melting point. The correlation is sensitive to the metal’s crystal structure.

The Constant B, derived from the slope of the curve, is the temperature coefficient of hardness. B is related in a rather complex way to the rate of change of heat content with increasing temperature. With these correlations, it is possible to calculate fairly well the hardness of a pure metal as a function of temperature up to about one-half its melting point.

The Future Of Hardness Testing
With advanced nanotechnology, nanoindenters have been developed. These can deliver loads in micro-Newtons (10-6 Newtons). Recent events have also warranted the need for primary hardness test machines to provide standards from which other machines and test blocks can drive their hardness reading.

Many countries have started establishing traceable hardness standards and accreditation procedures. In future, all hardness measurements could be coordinated through national agencies and accredited bodies.

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