A material's hardness is a measure of its ability to resist penetration of its surface. The most common of the various hardness tests is the indentation type. This involves applying a specified load onto an indenter of fixed geometry which then makes an impression in the surface of the metal being tested. The degree of hardness is measured by the size of the indentation. Hardness tests that use an indenter include Brinell, Rockwell, and Vickers.
The Vickers Test is also known as the Diamond Pyramid Hardness test. The indenter is made out of a diamond cut in the form of a square-based pyramid that has an angle of 136( between opposite faces. The loads applied can vary from 1 to 120 kilograms with 1, 5, 10, 30, and 50 kg being the most common.
The specimen to be tested is placed on an anvil and brought to within a millimeter of the indenter. When the loaded indenter is released, it goes down and penetrates the specimen's surface and then returns to its original position by means of a cam and weight arrangement. Both the rate at which the indentation is made and the amount of time that the indenter is on the surface are controlled.
The lengths of both of the diagonals in the resulting square-shaped indentation are measured under a microscope. The Vickers hardness number is equal to the applied load divided by the surface area of the indentation or
HV2 P sin (/2) d2
HB P
(% D/2) [D D2 d2]
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Where,
HV = Vickers Hardness P = applied load
= angle between faces of the indenter (136() d = average length of diagonals
The applied load is typically reported along with the Vickers number, e.g. 250 HV where the “10" subscript refers to a 10 kg applied load.10
The Brinell Test consists of applying a constant load (usually 500 or 3000 kg) onto a hardened steel ball-type indenter, 10mm in diameter. The 3000 kg load is used for testing steel and cast irons while the 500 kg load is used for testing softer materials.
The ball indenter penetrates the surface of the test specimen and is held for a specified period of time before the load is released. This generally runs 10-15 seconds for steels and 30 seconds for softer materials. This time insures that all plastic
deformation taking place in the specimen has stopped.
The average diameter of the indentation is calculated from two measurements taken at right angles to each other. The Brinell hardness number is defined as the applied load divided by the surface area of the indentation, or
Where,
HB = Brinell Hardness P = applied load
D = diameter of ball, in mm
d = diameter of indentation, in mm
The Rockwell Test is different from the other two types of hardness tests in that the hardness is determined from the depth of the indentation. One of the most common type of indenters used in Rockwell testing is a diamond ground to a 120( cone with a spherical apex having a 0.2mm radius. This is known as the Brale indenter.
The specimen to be tested is brought up to the indenter and a minor load is applied. The minor load is 10 kg and is applied in order to eliminate backlash in the load train. It also causes the indenter to break through slight surface roughness or foreign matter. The minor load thus greatly increases the accuracy of the test.
The depth of the indenter after the minor load is applied is used as a reference point on the dial gage of the tester. A major load is then applied consisting of a 60, 100,
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or 150 kg load depending on which scale you're using. This, of course, causes the indenter to penetrate even further into the material. The difference in depth between the major and minor loads is automatically registered on the dial gage in the form of a Rockwell Hardness Number. Each division on the gage represents a difference in indentation depth of 0.002 mm.
Various combinations of indenters and loads can be used depending on the hardness and the thickness of the material being tested. Each standard combination of indenter and load constitutes a Rockwell scale. When testing metals, the "C" and "B" scales are the most frequently used. The "C" scale uses a Brale indenter with a 150 kg major load, and the "B" scale a / " diameter ball indenter with a 100 kg load. The "B"116
scale is used for testing materials too soft to be tested using the "C" scale. The applicable scale must be reported when reporting a Rockwell hardness number. For example, 22 HRC tells us that the metal tested has a Rockwell hardness number of 22 when tested using the "C" scale.
Superficial Rockwell testing is similar to the standard Rockwell test except the minor load is 3 kg and the major load is 15, 30, or 45 kg. Both can use the same
indenters. Superficial Rockwell testing is used to determine the hardness of thin sheets, plating, case hardening, and other surface effects.
The Versitron® hardness tester that we sometimes use in our shops is a type of Rockwell tester. Unlike conventional Rockwell machines that use weights to provide the necessary load on the indentor, the Versitron's® load is provided by a spring. This allows the test head to be moved down to the part thus allowing large parts to be tested that would not have fit into a conventional Rockwell machine.
Which is the preferred method of hardness testing? Depends on what you want tested! The Vickers test can check the greatest range of hardness without changing scales, however, it is essentially a laboratory test. Vicker test specimens have to be flat, have exactly parallel faces, and be carefully ground or polished. It is often used for making hardness traverses across weldment cross sections or for other hardness measurements that must be made in a precise location: it is seldom used for production parts. The Brinell test is often used in our shops. The large indentation gives an
accurate hardness of the bulk of the material because it is less sensitive to minor surface irregularities. This also makes surface preparation less critical. The indentation can be measured at any time thus it becomes a permanent record of hardness. Large parts are easily tested. Finally the Brinell hardness test can measure a wide range of hardness values without changing scales. Its main disadvantages are that some parts cannot tolerate the large indentation because of seal areas, thin wall sections, or surface finish requirements, and the fact that a hardness tester must physically measure this indentation. The Rockwell test can quickly and easily test parts that are small enough to fit into the machine. Its small indentation will not deform or disfigure most parts. The main disadvantages are large parts cannot be tested on most
machines and many different scales may have to be utilized depending on the hardness of the material to be tested.
HK P /A P/CL2
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Microhardness testing is hardness testing performed on a very localized area of a specimen using indentor loads of 1 kg or less. The purpose of performing a
microhardness test may be to:
1. Measure the hardness of parts that are too small, thin, etc., for regular hardness testing.
2. Measure the hardness of individual microconstituents in a metal's structure.
3. Measure the hardness of plating, coatings, etc., on the surface of a metal.
4. Measure the effects of carburizing and nitriding operations.
Most microhardness testers are very similar in principle to the Vickers test. The test specimen is ground flat the and test surface ground parallel to the back side. The surface to be tested is then polished. Small specimens may be mounted in a Bakelite® or plastic mold for convenience. The test piece is placed on the X-Y stage of the tester and the stage raised up to the level of the indentor. Looking through a microscope integral with the hardness testing machine, the hardness tester locates the specific area to be hardness tested. He then shifts the stage with the test piece to a point directly under the indenter and trips a lever which causes the indenter to come down, make an indentation, and then retract (all through a cam and weight system). The stage is then shifted back under the microscope and the indentation measured.
Either a Vickers or a Knoop indenter can be used in microhardness testing. The Vickers indenter is identical to the one used in the standard Vickers test. The Knoop indenter is a diamond ground to a pyramidal form that makes a diamond-shaped indentation having an approximate ratio of seven to one between the long and short diagonals (see Figure 7). The pyramid has an included longitudinal angle of 172( 30' and included transverse angle of 130(. The depth of indentation is about / of its1
30
length. The Knoop indenter, because of its shape, can produce accurate results with very light loads. The Knoop hardness number is the ratio of the load applied to the indenter to the projected area of the indentation or
Where, HK = Knoop hardness P = applied load
A = projected area of indentation C = 0.07028
L = measured length of long diagonal in millimeters
The applied load must always be referenced. Microhardness values may change with the applied load at these low load levels because of the differences in the rate of strain hardening as the indenter penetrates the surface.
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Figure 7:
Knoop Hardness Indentation
We frequently use microhardness testing (done with a Knoop indenter) to perform hardness surveys in order to check for banding. Banding is a type of chemical segregation. A banded material, instead of having uniform chemistry, will have
alternating layers of material having different chemistries. Banding may arise from many processing factors including melting practice, hot work, heat treat, etc. It may be
detrimental because the differences in chemistry between the alternating bands may result in non-uniform mechanical properties and less than optimum corrosion
resistance. By performing a hardness survey by making several closely spaced Knoop hardness indentations in a row, we can check the degree of banding in a material. If the hardness results are fairly uniform, then banding is not a problem. If the hardness values show a wide range, then a banding problem exists.