MY3200 Hardness Measurement


The most frequently used methods of measuring hardness involve indenting the surface of a material with an indentor of a suitable shape and material and measuring the size of the impression that results. The indentors used are specially hardened steel balls (Brinell and some Rockwell scales), pyramidal diamonds (Vickers and Knoop) or conical diamonds (some Rockwell scales). In this experiment, the characteristics of the Vickers hardness test will be determined, the results compared with the Rockwell test and the Vickers microhardness test utilised to obtain the hardness of individual microconstituents.

Brinell Hardness Number (HB)

The Brinell hardness number is given by the ratio of the load to the curved indentation area



where d is the diameter of the indentation, D the diameter of the indentor and W, the applied load. The diameter of the ball is usually 10mm (3/8”) but can be smaller. The load used depends on the material but is usually 3000 kg for steel, 1000 kg for copper aloys and 500 kg for Aluminium alloys. When balls smaller than 10mm are used, th eload is reduced such that P/D2 is a constant.The time of loading is 15s. The approximate tensile strength of the material is 3.5x the Brinell number. The brinell hardness test does not work well for hard materials because of flattening of teh steel ball.

Vickers Hardness (HV)

The Vickers hardness uses a square pyramidal diamond identor with an angle of 136° between opposite faces. The hardness is simply the ratio of the load to the surface area of the indentation so


where d is the length of the diagonal of the indentation. Loads used range from 5 to 120 kg and the indentation is measured using a low power optical microscope.



Rockwell Hardness

Popular in the US and Europe as a production/inspection machine, the Rockwell test uses either a conical diamond indentor or a hardened steel ball indentor with a 1.58mm (1/16th”) diameter. The indentor is pressed into the surface of the specimen by a preload of 10 kg and the depth measuring device set to zero. The test load is then applied. The depth to which the indentor penetrates under the test load is directly measured on a dial gauge. The Rockwell hardness number is meaningless unless the scale is also listed. e.g.. RC=70 means a diamond indentor and 150kg load.

Experimental

Using the steel sample that you prepared, measure the size of the Vickers indentation hardness using loads of 5, 10 ,20 and 50 kg. Use 3 indentations per load and measure both diagonals of the indentation. Plot the mean hardness vs. load using a spreadsheet. Choose a scale appropriate to the material tested and measure the Rockwell hardness at least 3 times.

The Vickers microhardness tester uses the same geometry and loading as the Vickers test but much smaller loads allowing the indentations to be placed within individual phases. Using a steel sample containing ferrite and pearlite, measure the hardness of each of the two microconstituents. Combine the results obtained from each of the plain carbon, annealed steels.

Determine the relationship between the hardness of the steel and the individual hardnesses of the ferrite and pearlite and the volume fraction of each phase. What type of rule of mixtures best describes the results.

There are two possible mixture rules, one where the individual components act in series ie.



and one where the components act in parallel ie.



This requires that you know the macroscopic Vickers hardness of each of the steels examined in the first laboratory, the microhardness of the ferrite and pearlite in a steel that has approximatey 50 vol% of ferrite/pearlite and the volume fraction of pearlite in each steel.