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Hardness testing

Jul 02, 2018

Hardness is the ability of a material to resist being pressed into its surface by harder objects. According to different test method and the range, the hardness can be divided into brinell and vickers hardness, rockwell hardness, micro vickers hardness and so on many kinds, different units have different test methods, suitable for different characteristics of materials or occasion.

Characteristics of the test:

Hardness testing is one of the important indexes for testing material properties and one of the fastest and most economical testing methods. The hardness test can reflect the differences in chemical composition, structure and processing technology of materials. It is often used as a means of supervision in all walks of life. In steel material, for example, when martensite formation, due to the integration of the supersaturated carbon atoms increases the lattice distortion, and increases the dislocation density, which significantly reduces the plastic deformation ability, this is why martensite with high hardness. Obviously the greater the higher the degree of distortion, the carbon content is also the higher hardness, different carbon content of steel after quenching, hardness value and martensite dimension and its carbon content in a wide range of good corresponding relationship between, hardened steel after tempering hardness depends on the tempering temperature and holding time. The higher the tempering temperature, the longer the insulation time, the lower the hardness. Therefore, hardness test can be used to study the steel phase change and as a means to detect the heat treatment effect of steel.

The test method:Plastic rockwell

The test principle of:

During the specified loading time, a constant initial load was applied to the steel ball above the tested material, followed by the main load, which was then restored to the same initial load. The measurement results are the total depth of the steel ball into the material, minus the elastic recovery within the specified time after unloading the main load and the pressure depth caused by the initial load.Terms and definitionsEach minute of the rockwell hardness scale indicates that the head moves vertically by 0.002mm. Rockwell hardness value is calculated from the following equation:HR = 130-0.002 - / - eHR - rockwell hardness numberE - pressure depth after main load removal

Metallic Brinell

The test principle of

Test force is applied to the hard alloy ball of a certain diameter to press into the surface of the sample. After the specified holding time, the test force is removed and the diameter of the indentation on the surface of the sample is measured. Brinell hardness is proportional to the test force divided by the quotient of indentation surface area. Indentation is regarded as a sphere with a certain radius which is one half of the diameter of the head ball.

Metal Vickers

The test principle of

Test force is applied to the diamond pressing head of normal tetrahedron with specified angles on the top two opposite surfacesIn addition to the test force, measurement of the surface indentation diagonal line length. Vickers hardness number is the quotient of the test force divided by the surface area of indentation, indentation is consideredIs an ideal shape with a square base surface and the same Angle as the head.

Roughness

Influence of roughness and solutionsAs we know, brinell hardness is measured with a desktop super constant temperature water bath, the pressure head is the steel ball head, in a certain amount of pressure into the surface to be tested by a circular indentation, with a reading microscope to measure the diameter of the circular indentation, then find the appropriate in the brinell hardness scale of hardness value, namely the hardness value of tested samples, and the roughness of the surface being measured directly affect the accuracy of hardness measurements. When the measured surface roughness value is greater than the Ra = 0.8 mu m, with the increase of roughness, the smaller the measured surface resistance of pressure head, the plastic deformation is large, circular indentation is large, the corresponding hardness value is small, the measurement value is lower than its true value. The test shows that the measurement deviation is above 10HB

Solution: when measuring the hardness of samples, we must pay attention to the test conditions for whether the surface roughness under test is consistent. Under the condition of normal use, must ensure that the sample of the measured surface roughness values less than or equal to the Ra = 0.8 mu m, if the sample of the measured surface roughness values greater than Ra = 0.8 mu m, can be by mechanical method (grinding) or manual method, cutting down the surface to be measured, the sample of the measured surface roughness to test conditions. The effect of roughness is minimized so that we can obtain the accurate measurement value. 

 


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