The following points highlight the three main tests conducted for determining the hardness of metal. The tests are: 1. Brinell Hardness Test 2. Rebound Test (Shore Scleroscope) 3. Scratch Test.
1. Brinell Hardness Test:
In this test, a standard hardened steel ball is pressed into the surface of the specimen by a gradually applied load which is maintained on the specimen for definite time.
The impression (Fig. 2.50) so obtained is measured by a microscope and the Brinell Hardness Number (B.H.N.) is found out by the following equation:
where, P = Load (kg),
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D = Diameter of ball (mm), and
d = Diameter of indentation (mm).
The test is carried with a hardened steel or carbide ball of 10 mm diameter. A 3000 kg load is used for testing ferrous alloys and alloys of similar hardness. When brass and soft alloys are tested, a 500 kg load is used. The time of loading is specified between 10 and 30 seconds, depending upon the alloy being examined.
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(i) Brinell Hardness Tester:
The hardness test is carried out as follows:
The test piece is placed on the top of the elevating screw and the screw is raised. As the screw moves up, electric connections take place and the reflector throws the light on the surface of the test piece. A sharp view of the steel, showing the surface qualities then appears on the ground-glass screen.
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Now with dash pot set at the correct rate of loading the load lever raised nearly 30°, from which position it automatically moves further till it stops. During this period indenter moves to the position of the test piece and makes indentation/impression.
The lever is pulled back to its original position after about 15 seconds. Simultaneously the microscope objective comes to its normal position over the specimen and thus, the indentation duly magnified 70 times is taken on the screen. The left hand corner of the impression taken on the screen is coincided to convenient scale graduation by knurled thumb screw and right hand corner by the micrometer screw and the reading taken off.
Lastly by applying the equation:
The hardness number is calculated.
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The Brinell hardness number is to some extent an indication of the tensile strength of the metal, which may be ascertained by multiplying the Brinell number by a constant which depends on the character of the metal or alloy. These constants are also listed in the tables.
(i) The Brinell test should be performed on smooth, flat specimens from which dirt and scale have been cleaned.
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(ii) Successive impressions made too close to one another tend to produce high readings (unless there is overlapping) because of work hardening.
(iii) The test should not be made on the specimens so thin that the impression shows through the metal nor should impressions be made too close to the edge of a specimen.
(i) The Brinell test, though more complex than the rebound test, is still simple.
(ii) It does not require the care in surface preparation that rebound (or Shore) and Rockwell test do.
(iii) Its results correlate well with tensile properties.
(iv) Although the effect of modulus of elasticity enters into the hardness figure, it is a less important factor than in the rebound test, where it causes some difficulties.
(v) The impression made by the Brinell machine is large enough to give a fairly representative hardness, not often affected by small soft spots and small hard spots.
(i) A limitation of the Brinell test is the size of the indentation made. The standard Brinell test produces so large an impression that it is considered to be a destructive test under some circumstances.
(ii) Most of the test equipment is heavy, although portable instruments are available.
(iii) The size of the piece which can be tested is limited to the opening between the anvil and penetrator.
(iv) The test is fairly slow, because of the time required to measure the diameter and to determine the hardness.
(v) The Brinell test is not entirely reliable, the hardness indicated often being less than that of the material under test when working with hard materials. Very hard materials may deform the ball and readings become unreliable at over 500 brinell. As a result, hardness testers which employ a conical or pyramid-shaped diamond are often used in place of the Brinell tester.
(ii) Vicker’s Hardness Test:
Here a polished square based pyramid diamond tool with an angle of 136° between the faces is used. This tool under gradual load makes an impression on the specimen. The load when divided by the area of indentation in mm2 gives what is known as pyramid hardness number (D.P.N.).
This is given by the expression-
Where, P = Load in kg,
θ = Angle between the opposite faces, and
d = Mean length of the two diagonals in mm.
The loading on the plunger carrying the diamond is applied by a cam, which is rotated by a drum on which a wire carrying a weight is wound. The load and time during which it is applied are thus mechanically controlled.
1. The advantage of Vicker’s over Brinell is chiefly in greater precision in measurement of the diagonal of the square as compared to the diameter of the circular Brinell impression.
2. The Vicker’s tester can also be used for testing harder materials because it uses a diamond.
1. Vicker’s hardness testers are much more complicated and expensive than either Rockwell or Brinell machines.
2. They may be considered as semi-micro hardness testers because of the small size of the impression.
Note:
The hardness testers in which a diamond shaped impression is produced are even more sensitive than the Vicker’s because of greater precision with which the long diagonal of the diamond can be measured. These are true micro hardness testers whose principle use is in measuring the hardness of constituent particles alloys, thin surface hardened layers, foils, etc.
The Bergsman hardness tester which produces a vicker type indentation is somewhat less expensive than many of the other makes of equipment because it is adaptable to standard metallographs and does not require its own special microscope.
The Rockwell hardness test is probably the most widely used method of hardness testing. Rockwell testers use much smaller penetrators and loads than does the Brinell tester. Four sizes of hard ball from 1/16 in. to 1/2 in. in diameter are available as well as a cone shape diamond.
For testing metallic materials the 1/16 in. ball and the diamond penetrator are most commonly used. The penetrator chuck is mechanically connected to a dial indicator which responds to vertical motion of the penetrator. Since the penetrators are small the specimen should be ground smooth and clean.
The specimen is placed on the anvil of the machine and the penetrator seated by means of 10 kg minor load. The dial indicator is zeroed and then a major load of 60, 100 or 150 kg is applied, forcing the penetrator into the specimen. Upon removal of major load, the indented specimen recovers slightly, and the final depth of penetration is registered directly on the dial indicator as a hardness number.
Various combinations of penetrator and major load are used and designed by a series of letters. The two commonest scales are the HRB scales and the HRC scales, respectively standing for the 1/16” ball with 100 kg load, and the diamond penetrator with the 150 kg major load. In general very hard materials are tested with the diamond penetrator.
Mild steel might have a HRB reading of 90; hardened alloy steel might have a HRC of 55. These are stated as HRB 90 and HRC 55.
Precautions:
(i) Successive impressions should not be superimposed on one another nor be made too close together when making hardness determinations.
(ii) Nor should a measurement be made too close to the edge, or on a specimen so thin that the impression comes through the other side.
(iii) The care required in preparing the surface is greater for Rockwell than for the Brinell test because of smaller Rockwell impression.
(iv) The surface of the specimen should be flat and free from spring action.
(v) Since impression is small it is desirable to take several readings in order to get a representative value for hardness.
(i) It is more flexible than the Brinell; a large number of combinations of indenters and loads make it more useful to test a wider range of materials.
(ii) Rockwell testers are also fitted with a number of fixtures for testing different sizes and shapes of metal parts.
(iii) Rockwell hardness measurements can be made almost as quickly as with the Shore scleroscope because they are read directly from the instrument scale.
(iv) Because of the small size of the Rockwell impression, the test is considered to be non-destructive for most applications.
The Rockwell test is limited by greater care required in preparation of samples but this is offset by its greater sensitivity as compared to Brinell tester.
2. Rebound Test (Shore Scleroscope):
This test was carried out by A.F. shore in 1910. In this test a steel cylinder hammer is dropped from a height of 25 cm. through a glass tube on the surface to be tested. The height of the rebound is used as a measure of hardness of surface. The surface to be tested should be smooth, free from oil and tube should be truly vertical. Shore’s hardness number for steel is 1/6 to 1/7 of Brinell Hardness Number (B.H.N.).
(i) The instrument must be positioned so that the hammer falls and rebounds vertically.
(ii) Measurement should not be made on materials which are too thin.
(iii) Small pieces should be firmly secured to the anvil to avoid absorption of energy by movement of the specimen.
(i) Small size of the indentation (barely visible), speed and portability.
(ii) The instrument can be easily carried to work.
(iii) Also, since it does not require that specimens be subjected to pressure between the penetrator and anvil, it can be used on pieces of unlimited size.
(i) This test does not necessarily give higher readings for harder materials.
(ii) The shore readings do not correlate as well with tensile strength as do Brinell, Rockwell, and Vicker’s hardness from readings.
3. Scratch Test:
A Martens in 1890 defined scratch hardness or the load in grams under which 90° diamond will produce a scratch 0.01 mm wide. According to Mohr’s test, iron and steel range in hardness from 4 to 9 depending upon the heat treatment and the alloy content.