BRINELL HARDNESS2011-MME-18Hardness TestingAbstract:For Metallurgists, Hardness Testing is a collection of different methods for measuring a definite characteristic of metallic materials, namely:a) The resistance to penetration of a specific Indenter (defined by fixed form and properties),b) Under the application of a certain static forcec) For a definite time,d) Using precise measuring procedures.The result, usually expressed by a number or by a range of numbers, must be qualified by an accepted convention indicating exactly by which one of the possible methods such result was obtained. Hardness so defined is not an intrinsic property of any material, (like density or melting point), it is rather a characteristic deriving from the composition, the thermal and mechanical history of the material, and essentially from the structure (or more properly the microstructure) of the specimen involved.The variety of methods and conditions developed for hardness testing is a consequence of the fact that no single method can cover all the possible degrees of this characteristic. All the methods employed are empirical, in the sense that they were developed by trial and error to satisfy a need, and that they knew their enormous diffusion due to their intrinsic usefulness.

(Brinell hardness Test)Introduction:Brinell hardness test was invented by J.A. Brinell in 1900 using a steel ball indenter with a 10 mm diameter. The steel ball is pressed on a metal surface to provide an impression as demonstrated in figure 1. This impression should not be distorted and must not be too deep since this might cause too much of plastic deformation, leading to errors of the hardness values. Different levels of material hardness result in impression of various diameters and depths. Therefore different loads are used for hardness testing of different materials as listed in table 1. Hard metals such as steels require a 3,000 kgf load while brass and aluminum involve the loads of 2,000 and 1,000 or 500 kgf respectively. For materials with very high hardness, a tungsten carbide ball is utilized to avoid the distortion of the ball.

In practice, pressing of the steel ball on to the metal surface is carried out for 30 second, followed by measuring two values of impression diameters normal to each other using a low magnification macroscope. An average value is used for the calculation according to equation 1

Note: This BHN values has a unit of kgf.mm-2 (1 kgf.mm-2 = 9.8 MPa) which cannot be compared to the average mean pressure on the impression. Generally, the metal surface should be flat without oxide scales or debris because these will significantly affect the hardness values obtained. A good sampling size due to a large steel ball diameter is advantageous for materials with highly different microstructures or microstructural heterogeneity. Scratches or surface roughness have very small effects on the hardness values measured. However, there are some disadvantages of Brinell hardness test. These are errors arising from the operator themselves (from diameter measurement) and the limitation in measuring of too small samples.

If we considered the plastic zone beneath the Brinell indenter, this plastic region is surrounded by elastic material which obstructs the plastic flow. This condition is said to be plane strain compressive where plastic deformation is limited. If the metal is very rigid, the metal flow upwards surrounding the indenter is possible as illustrated in figure 1 a). However this situation is rarely seen because the metal displaced by the indenter is accounted for by the reduced volume of elastic material.Materials and equipment:1. Test Specimen2. Brinell Hardness Testing Machine3. Steel ball of 10mm for indentation4. Traveling MicroscopeExperimental procedure:1. We polished the sample with the help of sand paper and its surface was made clean, otherwise the results should be altered.2. We put the Sample on the anvil and suitable area was selected to obtain the average results.3. The load of 3000kg was applied on the specimen and hold for 10 seconds.4. Two indents were obtained at different positions.5. The diameter of indenter was measured with the help of Traveling Microscope.Observations & Calculations:(A) (B)Diameter of Steel Ball = 10 mm Diameter of Steel Ball = 10 mm Load = 3000 Kg Load = 3000 KgTime of Load = 10 sec Time of Load = 10 sec Diameter of Indentation = 4.5 mm Diameter of Indentation = 4.45 mm HBRs = 178.6 HBRs = 182.1Result:The Hardness of given material with two different diameters i.e with 4.5 and 4.45mm is 178.6 And 182.1 respectively. Conclusion:With utmost care, could be wrong: If the surface is not flat If the surface is covered with a thick scale If the tested material is too thin (less than 9.6 mm or 3/8 ") so that a mark appears on the opposite side. If the tested material is too hard (more than 450 HBS for steel ball or more than 650 HBW for tungsten carbide ball).Discussion:The difference in the HBRs is due to the fact that may be The material was not homogeneous in composition. The surface was not flat or not cleaned properly.PRECAUTIONS:

Diameter of indentation or impression should be seen in the light of lamp or sun. Use high magnification microscope (up to 0.01mm of accuracy) for accurate reading. For at least three readings of Brinell harness number of unknown sample, the specimens length or diagonal should be more than 12 times of the of the impression i.e. 12y ( where y is the size of the impression). And the maximum impression from 10 mm ball is 6mm so we should take 72mm long specimen at least.References: ASM Handbook , Volume 9 ASTM E-140

5