Properties of MetalsDOE-HDBK-1017/1-93PHYSICAL PROPERTIESHardnessis the property of a material that enables it to resist plastic deformation, penetration,indentation, and scratching. Therefore, hardness is important from an engineering standpointbecause resistance to wear by either friction or erosion by steam, oil, and water generallyincreases with hardness.Hardness tests serve an important need in industry even though they do not measure a uniquequality that can be termed hardness. The tests are empirical, based on experiments andobservation, rather than fundamental theory. Its chief value is as an inspection device, able todetect certain differences in material when they arise even though these differences may beundefinable. For example, two lots of material that have the same hardness may or may not bealike, but if their hardness is different, the materials certainly are not alike.Several methods have been developed for hardness testing. Those most often used are Brinell,Rockwell, Vickers, Tukon, Sclerscope, and the files test. The first four are based on indentationtests and the fifth on the rebound height of a diamond-tipped metallic hammer. The file testestablishes the characteristics of how well a file takes a bite on the material.As a result of many tests, comparisons have been prepared using formulas, tables, and graphsthat show the relationships between the results of various hardness tests of specific alloys. Thereis, however, no exact mathematical relation between any two of the methods. For this reason,the result of one type of hardness test converted to readings of another type should carry thenotation " converted from" (for example "352 Brinell converted from RockwellC-38").Another convenient conversion is that of Brinell hardness to ultimate tensile strength. Forquenched and tempered steel, the tensile strength (psi) is about 500 times the Brinell hardnessnumber (provided the strength is not over 200,000 psi).Nickelis an important alloying element. In concentrations of less than 5%, nickel will raise thetoughness and ductility of steel without raising the hardness. It will not raise the hardness whenadded in these small quantities because it does not form carbides, solid compounds with carbon.Chromiumin steel forms a carbide that hardens the metal. The chromium atoms may alsooccupy locations in the crystal lattice, which will have the effect of increasing hardness withoutaffecting ductility. The addition of nickel intensifies the effects of chromium, producing a steelwith increased hardness and ductility.Copperis quite similar to nickel in its effects on steel. Copper does not form a carbide, butincreases hardness by retarding dislocation movement.Rev. 0Page 27MS-02
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