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Method for expressing hardness of metals
Hardness of metal refers to the resistance of metal surface to plastic deformation caused by extrusion in local volume. The higher the hardness, the stronger the ability of metal to resist plastic deformation, and the more difficult it is for metal to produce plastic deformation. The hardness test method is simple and easy, and does not damage the parts. There are three hardness test methods commonly used in practice: Brinell hardness, Rockwell hardness and Vickers hardness. There is a general conversion relationship between the three hardness test values, and there is a conversion relationship table on the manual of machinery, mechanical technology or metal materials. [Brinell Hardness HB] Brinell hardness is to press the direct D steel ball into the metal surface with the force of load P, and keep it for a certain time. Measure the indentation diameter D on the metal surface, and calculate the indentation area AB based on this, and calculate the force per unit area, which is used as the hardness value of the metal, called Brinell hardness, and recorded as HB. HB=P/AB=P/(π Dh) = 2P/(π D (D-SQD (D2-d2))) Unit: P-kgf, D, h-mm For steel, the steel ball D is 10mm, the load P is 3000kgf, and the pressing time is 10 seconds. The diameter d obtained from the test should be in the range of 0.25 D-0.6 D. The upper limit of Brinell hardness is HB450, which is suitable for measuring the hardness of annealed, normalized, quenched and tempered steel, cast iron and non-ferrous metals. [Rockwell Hardness HR] Rockwell hardness is the most commonly used hardness measurement method in industrial production. Because of its simple and rapid operation, it can directly read out the hardness value without damaging the workpiece surface, and the measurable hardness range is wide. However, Rockwell hardness also has some disadvantages, such as small indentation, segregation of materials and uneven microstructure, which leads to large separation and poor reproducibility of measurement results. Rockwell hardness (HR) is also tested by indentation. The hardness value is expressed by measuring the depression depth. The indenter used in Rockwell hardness test is divided into hard and soft. Hard indenter with apex angle of 120 & ordm; Diamond cone, which is used in hard materials such as hardened steel. HRA with 60kgf load test, the effective range of hardness is> 70, suitable for cemented carbide, surface quenching layer and carburizing layer; HRC is tested with 150kgf load, and the effective hardness range is 20-67 (equivalent to HB230-700), which is suitable for quenched steel and quenched and tempered steel. The soft indenter is made of steel balls with a diameter of 1.588 mm (1/16 "), used in annealed steel, non-ferrous metals, etc., expressed in HRB, and the effective hardness range is 25-100 (equivalent to HB60-230). These three Rockwell hardness are specified in the color of the scale on the dial, with HRA and HRC as black scales and HRB as red scales. [Vickers Hardness HV] Vickers hardness is also measured by using unit stress on indentation area as hardness value. The indenter used for Vickers hardness is that the included angle of cone surface is 136 & ordm; Diamond tetragonal cone. During the test, under the action of load P, a square indentation is pressed on the test surface of the sample. Measure the average length d of two diagonal lines of indentation, so as to calculate the indentation area AV, express the hardness of the sample in P/AV, and express it in HV. HV=P/AV=1. 8544P/d2 The load P can be selected according to different samples, generally 5-100kgf. Advantages and disadvantages of Vickers hardness: Vickers hardness has a continuous and consistent scale; The test load can be chosen arbitrarily, and the obtained hardness values are the same. During the test, the loading pressure is small, the indentation depth is shallow, and the damage to the workpiece is small. It is especially suitable for measuring the hardness of hardened layer and surface chemical treatment of parts, and its accuracy is more accurate than Brinell and Rockwell hardness. But the Vickers hardness test operation is more troublesome, generally rarely used in production, mostly used in laboratory and scientific research.

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