Vicker hardness test

MSC STUDENTS

1. VICKERS HARDNESS TEST By Dr. K. Senthilarasan, Assistant professor , Department of Physics E.G.S Pillay Arts & Science College, Nagapattinam.

2. VICKERS HARDNESS TEST The test makes the use of a diamond square based pyramid indenter with 136  angles between the opposite faces. The load range is variable from 5 to 120 kg in steps of 5 kg. It provides a continuous scale of hardness and so it is widely accepted for research works. Vickers hardness testing machine is somewhat similar to the Brinell hardness testing machine.

3. A piston and dashpot of oil is used for controlling the rate and duration of the loading. The test is performed by placing the specimen on the anvil and is raised till it close to the indenter point. The load is then gradually applied to the indenter and then removed. The loading time is about 20 seconds. After the anvil is lowered a microscope is swung over the specimen. The diagonal of the square indentation is measured to an accuracy of 0.001 mm.

4. Vickers hardness number P= applied load in kg A= surface area of indentation in sq.mm D= length of diagonal in mm  = angle between opposite faces of diamond pyramid (136  )  

5. First figure b (i) shows the perfect indentation which is in the form of a square by a perfect diamond indenter.

6. The pincushion indentation is observed with annealed metals and result in an overestimates of the diagonal length. This is due to sinking in of the metal around the flat faces of the pyramid. The barrel shaped indentation is found in cold worked metals . It results from ridging or piling up of the metal around the faces of the indenter. The diagonal measurement in this case produces a low value of the contact area so that hardness numbers are higher than we expected. But empirical correction for these effects is available.

7. Advantages: The indentations always have the same shape whatever be the load. This makes the measurement independent of load The hardness of hardened steel or soft materials can be determined using large or small load respectively. This test is very useful for testing hardness of polished or nitrides surface due to small impression made on the test. This test is accurate and is suitable for metals as thin as 0.15mm. Hardness at elevated temperatures can be determined using Vickers Hot hardness tester in which the indenter is made of sapphire. Hardness is decreased exponentially with absolute temperature of the material H= Ae - BT where, A, B are constants.

8. Disadvantage: It takes more time for each measurement and so it is not suitable for industries. When irregularities are present then the measurement taken by Vickers shows more deviation than those of Brinell test.

9. Thank you