In a series of Hot Seat articles, we are going to discuss the various methods of mechanical property testing. We will be covering hardness testing, tensile testing, and fracture toughness testing. In this article and next month’s article, we will discuss hardness testing.
Hardness testing is probably the most common type of mechanical test performed in the United States — perhaps the most common worldwide. The term hardness is poorly defined and is relative to the measuring device.
There are three basic types of hardness tests: the scratch test, the indenter, and the dynamic rebound test. The scratch test is familiar to mineralogists, who use the Mohs scale. The Mohs scale is the relative hardness of 10 minerals arranged in order. Talc is the softest (Mohs 1), and diamond is the hardest (Mohs 10). Most hard metals fall in the range of Mohs 4-8. There is inadequate differentiation along the scale to be of much use to a metallurgist. In dynamic tests, an indenter is dropped on the material, and hardness is defined as the energy of impact. The Mohs scale is commonly used for rubbers and polymers. One exception is the Shore sceleroscope, which is used for metals. The indenter type of hardness test is the most widely accepted for metals.
In the indenter type of hardness test, an indenter is pressed into the material and released, and either the diameter or the depth of the impression is measured. The load and the impression measurement determine the hardness. As a hardness impression is made, there is a plastic zone around the hardness indentation that is surrounded by undisturbed elastic material. This elastic zone hinders plastic flow. As the plastic region is constrained by the elastic region, the compressive strength of the material in the area of the hardness impression is higher than the value of simple compression. This is a classic problem in plasticity and should be able to be explained by slip line theory [1]. However, in some types of hardness testing, the impression is asymmetric, so that slip line theory is not applicable [2]. This is really an elastic–plastic boundary problem, best explained from the Hertz theory of contact stresses [3]. This model accounts for the material displaced by the indenter by the decrease (by compression) in volume of the elastic underlying material. No upward flow around the indenter is predicted, which agrees with observation. This explanation is the basis for all indention hardness tests used for metallic materials.
Brinnell Test
The Brinnell test [4] was first proposed in 1900 by Johan August Brinell, and has since become widely accepted throughout the world. It is primarily used on unmachined surfaces of casting and forging, as well as other types of materials. It is accomplished by indenting the surface with a 10-mm steel ball with a 3,000-kg load. For soft metals, a 500-kg load is usually used because otherwise the impression is too deep. For hard metals, a tungsten-carbide ball is used to prevent distortion of the indenter. The diameter of the round indentation is measured with a low-power microscope after loading. At least two measurements of the diameter are made, and the results are averaged. To ensure that accurate measurements are made, the surface must be free of dirt and scale. The hardness expressed as a Brinnell hardness number (BHN) is determined from the equation:
where F is the applied load (kg), D is the diameter of the indenter (10mm), and d is the measured diameter of the impression (Figure 1).
The Brinnell hardness could also be calculated by measuring the depth of the impression, t:
Because of the large size of the impression made in the Brinnell test, it averages out any local inhomogeneities. It also precludes the testing of small objects or objects in which the Brinnell impression can be a site of crack initiation. Therefore, it is commonly used for castings, forgings, or raw stock. The size of the piece tested for Brinnell hardness should be at least 10 times the depth of the impression. Because of the plastic zone surrounding the impression and the elastic constraint, additional Brinnell hardness should not be measured any closer to the impression than four times the impression diameter. The distance from an edge when taking a hardness reading should also be at least four times the impression diameter.
The biggest source of error in Brinnell testing, is determining the diameter of the impression. Light sanding of the surface to remove scale or other debris is often done to improve repeatability.
Rockwell Hardness Test
The Rockwell hardness test [5] is very widely used in the United States because of its speed and its freedom from errors by operating technicians. The impression is small, making it possible to test a wide variety of parts.
In this test, a minor load of 10kg is applied to the part to seat the indenter and part; then the major load is applied. After 30 seconds, the depth of the impression is measured and exhibited on a rotary dial or, in newer machines, on a digital readout. Some machines are equipped with printer ports or USB interfaces for communicating with a computer. The scale used in Rockwell testers is based on 100 divisions, with each division equal to a depth of 0.00008 in. The scale is reversed, so that a small impression results in a high hardness number. The number from the Rockwell test is an arbitrary number that is only consistent within the same scale. This is unlike the Brinnell and Vickers hardness tests, which provide numbers that are based on mass per unit area (kg/mm2).
More than one indenter can be used with the Rockwell test. The most common is the Brale indenter, which is a 120° diamond cone and is used for the Rockwell C test. Other indenters used are 1/16” and 1/8” diameter steel balls. Major loads can be varied from 60 to 150kg. If a carburized surface is measured, the hardness reading may be lower than expected because of a greater depth of impression.
The material supporting the test piece may also influence the hardness reading. As a rule of thumb, it is wise to test only test pieces that are at least 10 times as thick as the depth of the impression [6]. If the impression of the hardness indenter shows on the other side of the tested piece, the test is obviously invalid. Because the scale on a Rockwell hardness machine is arbitrary, it is necessary to ensure that one machine records the same hardness as another machine. This is accomplished using standardized test blocks calibrated by the manufacturer. Generally, three test blocks of values throughout the range are adequate to maintain the hardness machine in calibration.
The superficial tester operates in an identical fashion to the Rockwell hardness tester. In fact, they look very similar. Two types of indenters are used: a 1/16” diameter steel ball (used for surface hardnesses of brasses, aluminum sheet, etc.) and the N-Brale penetrator. This Brale penetrator is like the one used in the standard Rockwell hardness tests except the spherical end is shaped to higher tolerances. The minor load used to seat the indenter and specimen is 3 kg. The major loads used are 15, 30, and 45 kg. The major load is applied for 30 seconds, and the depth of the penetration is measured.
The scale of the superficial Rockwell test is arbitrary like that of the Rockwell test, except that each division represents 0.001 in. in depth. The scale is reversed, so deeper impressions mean lower hardness numbers. As the loads used on the superficial Rockwell tester are different from those used for the Rockwell tests, different scales were established defined by the load and indenter as shown in Table 1.
Conclusions
In this article, we have discussed some of the various hardness testing methods. Each type of hardness test is really geared toward a specific purpose. The Rockwell testers are suitable for general purpose testing of parts and sheet, while the Brinell tester is suitable for larger components, or raw materials, where the large indentation will not affect function.
Next month’s installment will discuss the two remaining common hardness tests. Vickers hardness, widely used throughout the world, is used for both large-component testing as well as finer measurements of microstructure at very small loads. Finally, the Knoop hardness test is used for very fine hardness measurements of case depth or microstructural phase fields.
Should you have any questions or comments regarding this or have suggestions for further articles, please contact the editor or myself.
References
- R. Hill, The Mathematical Theory of Plasticity, Oxford, UK: Clarendon Press, 1950.
- M. C. Shaw and G. J. DeSalvo, “A new approach to plasticity and its application to blunt two dimensional indenters,” J. Eng. Ind., vol. 92, p. 469, 1970.
- S. Timoshenko and J. N. Goodier, Theory of Elasticity, New York: McGraw-Hill, 1972.
- ASTM International, “ASTM E10-18 Standard Test Method for Brinell Hardness of Metallic Materials,” ASTM International, West Conshohocken, PA, 2018.
- ASTM International, “E18-19 Standard Test Methods for Rockwell Hardness of Metallic Materials,” ASTM International, West Conshohocken, PA, 2019.
- G. L. Kehl, The Principles of Metallographic Laboratory Practice, New York: McGraw-Hill, 1949.
