Explanation of terms of metal materials (V): Hardness

Hardness indicates the ability of a material to resist the pressure of a hard object into its surface. It is one of the important performance indexes of metal materials. Generally, the higher the hardness, the better the wear resistance. Common hardness indicators include Brinell hardness, Rockwell hardness and Vickers hardness:

1. Brinell hardness (HB): press a hardened steel ball of a certain size (generally 10mm in diameter) into the material surface with a certain load (generally 3000kg) and keep it for a period of time. After unloading, the ratio between the load and its indentation area is the Brinell hardness value (HB), in kilogram force / mm2 (n / mm2)

2. Rockwell hardness (HR): when HB > 450 or the sample is too small, the Brinell hardness test cannot be used and Rockwell hardness measurement is used instead. It uses a diamond cone with a top 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 calculated from the depth of the indentation. According to the hardness of test materials, different indenters and total test pressure can be used to form several different Rockwell hardness scales. Each scale is indicated after the Rockwell hardness symbol HR with a letter. The commonly used Rockwell hardness scales are a, B and C (HRA, HRB and HRC). Among them, C scale is the most widely used.

HRA: it is the hardness obtained by using a 60kg load diamond cone press. It is used for materials with extremely high hardness (such as cemented carbide, etc.).

HRB: the hardness obtained by using 100kg load and 1.58mm diameter hardened steel ball, which is used for materials with low hardness (such as annealed steel, cast iron, etc.).

HRC: it is the hardness obtained by using 150kg load and diamond cone press. It is used for materials with high hardness (such as quenched steel, etc.).

3. Vickers hardness (HV): press the material surface with a load within 120kg and a diamond square cone press with a top angle of 136 °, and divide the surface product of the material indentation pit by the load value, which is the Vickers hardness value (HV). Hardness test is one of the simplest mechanical performance test methods. In order to replace some mechanical property tests with hardness test, a more accurate conversion relationship between hardness and strength is needed in production. Practice has proved that there is an approximate corresponding relationship between various hardness values of metal materials and between hardness value and strength value. Because the hardness value is determined by the initial plastic deformation resistance and continuous plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value.