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Hardness is one of the most commonly used mechanical properties of metal materials. Hardness testing is the most rapid and economical test method. But for the hardness of metallic materials, there is not a uniform and clear definition including all test methods. In general, the hardness of a metal is often considered to be the resistance of the material to plastic deformation, scratches, wear or cutting.

For the indentation method, it is also considered to be: the material under certain conditions to resist another body without residual deformation. Both statements exist because hardness is not a simple physical constant. It is not only determined by the macroscopic and microscopic conditions of the material studied (such as the degree of macroscopic deformation, Hot and cold working conditions, microscopic lattice types of metal crystals, lattice constants and binding forces between atoms, etc.) but also depends on the characteristics and conditions of the test. So to speak, for the material to be tested, Hardness is a comprehensive performance index of a series of different physical quantities, such as elasticity, plasticity, plastic deformation strengthening rate, strength, toughness and anti-friction, which are reacted under the action of certain indenter and force. For example, an indenter of the same size and material is pressed on the surfaces of iron and copper with the same amount of test force, After removing the test force, we can see that the indentation depth of iron is shallow, and copper is deep, this surface iron deformation resistance than copper deformation resistance, that is, iron is harder than copper. In fact, this comparison also includes the different elastic, plastic, plastic deformation capacity and deformation strengthening rate of the two materials.

In addition, the meaning of hardness value is different with different test methods. For example, the Brinell hardness test is to compare the resistance of different materials per unit area, and the Rockwell hardness, just in the use of the same ruler conditions, to the value of the size to compare hardness values.

Therefore, more accurate definition to reflect the objective essence of hardness more scientifically, still need to be obtained from experiments and in-depth study of metal macro and micro structure. Nevertheless, on the basis of different test methods and correct application of test principles and conditions, the results obtained are very useful for the correct use of metal materials in all walks of life.

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