Introduction to Hardness: Hardness refers to the ability of a material to resist the pressing of a hard object into its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.
- Brinell hardness (HB) Press a hardened steel ball of a certain size (generally 10mm in diameter) into the surface of the material with a certain load (generally 3000kg), and keep it for a period of time. After the load is removed, the ratio of the load to its indentation area , which is the Brinell hardness value (HB) in kilogram force/mm2 (N/mm2).
- Rockwell hardness (HR) When HB>450 or the sample is too small, the Brinell hardness test cannot be used and the Rockwell hardness measurement is used instead. It uses a diamond cone with an apex angle of 120° or a steel ball with a diameter of 1.59 and 3.18mm to press into the surface of the tested material under a certain load, and the hardness of the material is obtained from the depth of the indentation. According to the different hardness of the test material, it is expressed in three different scales:
- HRA: is the hardness obtained by using a 60kg load and a diamond cone indenter. It is used for materials with extremely high hardness (such as cemented carbide, etc.).
- HRB: Hardened steel balls with a load of 100kg and a diameter of 1.58mm are used to obtain the hardness, and are used for materials with lower hardness (such as annealed steel, cast iron, etc.).
- HRC: It is the hardness obtained by using a load of 150kg and a diamond cone indenter. It is used for materials with high hardness (such as hardened steel, etc.).
- Vickers hardness (HV) is pressed into the surface of the material with a load within 120kg and a diamond square cone indenter with an apex angle of 136°, and the surface area of the material indentation pit is divided by the load value, which is the Vickers hardness HV value (kgf/mm2).
Note: A, B, C in HRA, HRB, HRC, etc. in Rockwell hardness are three different standards, called scale A, scale B, scale C. Rockwell hardness test is one of several common indentation hardness tests used today. The initial pressure of the three scales is 98.07N (10kgf), and finally the hardness value is calculated according to the indentation depth. Scale A uses a ball-cone diamond indenter, then pressurized to 588.4N (60kgf); Scale B uses a steel ball with a diameter of 1.588mm (1/16 inch) as the indenter, and then pressurized to 980.7N (total 100kgf); while the scale C uses the same spherical cone rhombus as the scale A as the indenter, but the force after pressing is 1471N (total 150kgf). Therefore, scale B is suitable for relatively soft materials, while scale C is suitable for harder materials. Practice has proved that among various hardness values of metal materials, there is an approximate corresponding relationship between the hardness value and the strength value. Because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value. However, the conversion relationship of various materials is not consistent.
Hardness test is the most simple and easy test method in mechanical property test. In order to replace some mechanical performance tests with hardness tests, a more accurate conversion relationship between hardness and strength is required in production. Practice has proved that among various hardness values of metal materials, there is an approximate corresponding relationship between the hardness value and the strength value. Because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value.
Using the Brinell hardness indentation diameter to directly convert the Rockwell hardness of the workpiece In the production site, due to the limitation of the testing instrument, the Brinell hardness tester is often used to measure the hardness of large quenched parts. If you want to know the Rockwell hardness value of the workpiece, the usual method is to measure the Brinell hardness value first, and then find out the corresponding Rockwell hardness value according to the conversion table, which is obviously a bit cumbersome. Then, can the Rockwell hardness value of the workpiece be directly calculated according to the indentation diameter of the Brinell hardness tester? The answer is of course yes. According to the Brinell hardness and Rockwell hardness conversion table, an empirical formula that is simple to calculate and easy to remember can be summarized: HRC = (479-100D)/4, where D is the Φ10mm steel ball indenter pressed on the workpiece under a pressure of 30KN Indentation diameter measurements on . The error between the value calculated by this formula and the converted value is in the range of 0.5 to -1. This formula is very convenient to use in the field. You may wish to try it.
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