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Hardness, hardness units

   Hardness - is a measure of the performance of soft and hard materials. It can be understood as the ability of a material to resist elastic deformation, plastic deformation or damage, and the ability of the material to resist residual deformation and reverse damage. Hardness is not a simple physical concept, but a comprehensive mechanical performance indicator such as material elasticity, plasticity, strength and toughness. There are many methods for hardness testing, the principle is different, and the measured hardness values ​​and meanings are not exactly the same. The most commonly used is the static load pressing hardness test, namely Roche hardness (hra | hrb | HRC), Brinell hardness (HB), Vickers hardness (HV), the value of which indicates the ability of the material surface to resist hard object pressure. The Leeb hardness (HL) and the Shore hardness (HS) are hardness tests of the rebound method, and their values ​​represent the magnitude of the metal elastic deformation work. Therefore, hardness is not a simple physical quantity, but a comprehensive performance index reflecting the elasticity, plasticity, strength and toughness of the material.
 
   Cloth hardness | HB: The hardness of the cloth (HB) is a test load of a certain size. A hardened steel ball or carbide ball of a certain diameter is pressed into the surface of the test metal for a specified time, then unloaded, and the diameter of the surface indentation measured is measured. . The cloth hardness value is a quotient obtained by dividing the load by the spherical area of ​​the indentation. Generally: a certain load (usually 3000kg) to a certain size (typically 10mm in diameter) of hardened steel balls into the surface of the material, for a period of time, after the load, the load ratio is the area of ​​the indentation, that is, the Brinell hardness value (HB) The unit is kg force / mm 2 (n / mm 2).

Hardness, hardness units

Vickers Hardness | HV : Vickers Hardness (HV) is pressed into the surface of the material with a load of less than 120kg and a diamond square cone press with a top angle of 136°, divided by the surface area of the material indentation pits by the load value, that is, the Vickers hardness value (HV). It is suitable for the determination of hardness of larger workpieces and darker surface layers. Vickers Hardness has a small load of Vickers hardness, test load 1.961-49.03n, it is suitable for thinner workpiece, tool surface or coating hardness determination, microscopic hardness, test load less than 1.961N, suitable for metal foil, extremely thin surface layer hardness determination.

Shaw Hardness | HA/HD : a steel pressure needle with a certain shape, which is pressed vertically into the surface of the specimen under the action of the Test force, and when the surface of the pressing foot is fully fitted with the surface of the specimen, the tip of the pressure needle has a certain outstretched length l to the foot plane, The size of the L value is used to characterize the hardness of Shaw, and the larger the L value, the lower the hardness of the shore, the higher the reverse. 

SHOW HARDNESS | HS: SHOW HARDNESS TEST is a dynamic load test method. Its principle is to drop a weight with a certain quality of diamond or alloy steel ball from a certain height to the surface of the sample, and to measure the hardness value according to the height of the rebound of the weight hammer. The symbol is HS. The higher the weight bounces back, the harder the surface measurement is. The hardness of A90 diamond drill and D45 quenched steel.

Rockwell hardness | HR: Rockwell hardness is determined by the depth of indentation plastic deformation. Take 0.002mm as a unit of hardness. When HB > 450 or sample is too small, Rockwell hardness measurement can not be used instead of Brinell hardness test. It is a diamond cone with a top angle of 120 degrees or a steel ball with a diameter of 1.59 and 3.18 mm. It is pressed into the surface of the material under certain load, and the hardness of the material is calculated from the depth of the indentation. According to the different hardness of the test material, it can be expressed by three different scales:

HRA: The hardness obtained by using 60 kg load and diamond cone indenter is used for materials with very high hardness, such as cemented carbide.

HRB: Hardness obtained by hardening a steel ball with a diameter of 1.58 mm and a load of 100 kg. It is used for materials with lower hardness, such as cast iron.

HRC: Hardness obtained by using 150 kg load and diamond cone indenter for materials with high hardness, such as quenched steel, etc.

Hardness testing is the simplest and easiest test method in mechanical performance testing. In order to replace some mechanical performance tests with hardness tests, a more precise conversion relationship between hardness and strength is required in production. Practice has shown that there is an approximate relationship between the hardness and strength of metal materials. Since the hardness value is determined by the initial plastic deformation resistance and the sustained plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance and the higher the hardness value.


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