7 most common hardness tests: |
It isn’t hard to test a material’s hardness …once you settle on a definition of hardness.
As a property of every solid material, hardness is much desired by application requirement but very elusive in terms of a single standard or description. In most instances, it is defined as the property that enables a material to resist plastic deformation, typically via penetration by another object. However, it can also mean stiffness, temper or resistance to bending, scratching, abrasion or cutting. Hardness is confused as stiffness (e.g., the element Osmium is stiffer than diamond, but not as hard as diamond).
Thus, in materials science, there has been a consensus that there are three principal operational definitions of hardness:
• Scratch hardness – Resistance to fracture or plastic (permanent) deformation due to friction from a sharp object
• Indentation hardness – Resistance to plastic (permanent) deformation due to a constant load from a sharp object
• Rebound hardness – Height or speed of the bounce of an object dropped on the material (related to elasticity)
Measuring Hardness
Hardness is not an intrinsic material property because there are no precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure. Hardness has long been determined according to resistance by the material to scratching or cutting. An example would be material B scratches material C but not material A. Alternatively, material A scratches material B slightly and scratches material C heavily.<
Usually, to obtain a hardness value is to measure the depth or area of an indentation left by an indentor of a specific shape with a specific force applied for a specific time (Fig. 1). There are several principal standard test methods to express the relationship between hardness and the size of the impression or the rebound velocity on specific materials. Vickers, Rockwell, Brinell and Leeb are the most common scales. For practical and calibration reasons, each of these methods is divided into a range of scales defined by a combination of applied load and indentor geometry or, for the rebound method, by the weight of the impact body.
Most Common Hardness Tests
Here are some of the most widely employed hardness tests.
The primary Rockwell test standard involves the indenting of the test material with a diamond cone (HRC) or hardened (tungsten) steel ball indentor (HRB, etc.) applying a preload of 10 kgf first, followed by a main test force of 60, 100 or 150 kgf (Fig. 2).
Rockwell Superficial (HR scales)
A superficial form of Rockwell testing involves indenting the test material with a diamond cone or hardened (tungsten) steel ball indentor, depending on the scale chosen. The superficial Rockwell scales use lower force and shallower impressions on brittle and very thin materials, applying a preload of 3 kgf, first followed by a main test force of 15, 30 or 45 kgf.
The Vickers test involves indenting the test material with a diamond indentor in the form of an upside-down perfect pyramid with a square base and an angle of 136 degrees between opposite faces (Fig. 3). The material is subjected to test forces from 1 to 120 kgf. A microscope or USB camera is used to visualize and measure the indentation.
Micro-Vickers is a variation of Vickers testing that calls for the indenting of the test material with a diamond indentor in the form of an upside-down perfect pyramid with a square base and an angle of 136 degrees between opposite faces. The material is subjected to test forces usually not exceeding 1 kgf. A precision microscope or high-resolution USB camera is used to visualize and measure the indentations. Magnifications up to 600x are most common. However, magnifications up to 1,000x are becoming popular.
The Knoop method of hardness testing also involves indenting the test material with an “elongated” diamond pyramid subjected to test forces usually not exceeding 1 kgf. Like the micro-Vickers methods, a precision microscope or high-resolution USB camera is used to visualize and measure the indentations. Magnifications up to 600x are the most common.
A 1-, 2.5-, 5- or 10-mm-diameter hardened-steel or carbide ball subjected to a load/force ranging from 1 to 3,000 kg is used to indent the material in the Brinell method. A microscope or USB camera is used to visualize and measure the rather large indentations (Fig. 4).
The Leeb (HL) method is employed primarily for portable hardness testing. An impact body that has a spherical tungsten-carbide tip is impelled onto the test surface by spring force. The impact creates a plastic deformation of the surface – an indentation – due to which the impact body loses part of its original speed (or energy). Consequently, the softer the material is, the more speed will be lost at rebound of the impact body. Leeb is applicable for a wide variety of components. Minimum test requirements should be followed.
Rockwell Hardness Test Further Explained
Metallurgist Stanley P. Rockwell devised the Rockwell hardness test in Syracuse, N.Y., circa 1919, in order to quickly determine the effects of heat treatment on steel bearing races. The Brinell hardness test, invented in 1900 in Sweden, was slow, not useful on fully hardened steel and left impressions that were too large to be considered nondestructive. Rockwell collaborated with an instrument manufacturer to commercialize his invention and develop standardized testing machines.
The Rockwell Hardness test is a hardness measurement based on the net increase in depth of impression when a load is applied. Hardness values are commonly given in the A, B, C, R, L, M, E and K scales. The higher the value in each of the scales means the harder the material.
Hardness has been variously defined as resistance to local penetration, scratching, machining, wear or abrasion. In the Rockwell method of hardness testing, the depth of penetration of an indentor under certain arbitrary test conditions is determined. The indentor may either be a steel (carbide) ball of some specified diameter or a spherical diamond-tipped cone of 118-degree angle and 0.2-mm tip radius, also called an indentor. The type of indentor and the test load determine the hardness scale (A, B, C, etc.).
A minor load of 3 kg or 10 kg is first applied, causing an initial penetration and holding the indentor in place. The dial is then set to zero, and the major load is applied. Upon removal of the major load, the depth reading is taken while the minor load is still on. The hardness number may then be read directly from the scale. The Rockwell scale characterizes the indentation hardness of materials through the depth of penetration of an indentor, loaded on a material sample and compared to the penetration in some reference material. It is one of several definitions of hardness in materials science. Its hardness values are noted by HRX with “X” being the letter for the scale used. Hardness is related to strength because both are measures of the pressure it takes to get plastic deformation to occur in materials.
Vickers Hardness Test Further Explained
Smith and Sandland developed the Vickers hardness test in 1924 at Vickers Ltd. as an alternative to the Brinell method to measure the hardness of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indentor, and the indentor can be used for all materials irrespective of hardness.
The basic principle, as with all common measures of hardness, is to observe the material’s ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH).
The hardness number can be converted into units of Pascals, but it should not be confused with a pressure, which also has units of Pascals. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force. Therefore, it is not a pressure. The hardness number is not really a true property of the material and is an empirical value that should be seen in conjunction with the experimental methods and hardness scale used. When doing the hardness tests, the distance between indentations must be more than 2.5 indentation diameters apart to avoid interaction between the work-hardened regions.
Brinell Hardness Test Further Explained
The Brinell scale characterizes the indentation hardness of materials through the scale of penetration of an indentor loaded on a material test-piece. Proposed by Swedish engineer Johan August Brinell in 1900, it was the first widely used and standardized hardness test in engineering and metallurgy.
Typical tests use a 10-, 5-, 2.5- or 1-mm-diameter steel ball as an indentor with a test force starting at 1 kgf up to 3,000 kgf (29 kN). For softer materials, a lower force is used; for harder materials, a tungsten-carbide ball is substituted for the steel ball. After the impression is made, a measurement of the diameter of the resulting round impression (d) is taken (Fig. 4). It is measured to +/-0.05 mm using a low-magnification microscope. The hardness is calculated by dividing the load by the area of the curved surface of the indention. The area of a hemispherical surface is arrived at by multiplying the square of the diameter by 3.14159 and then dividing by 2. IH
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