Hardness represents a material's ability to resist the indentation of a hard object on its surface. It is one of the important performance indicators of metallic materials. Generally, the higher the hardness, the better the wear resistance.
The commonly used hardness indicators include Brinell hardness, Rockwell hardness and Vickers hardness.
-Brinell
With a certain load (generally 3000 kg), a hardened steel ball of a certain size (with a diameter generally of 10 mm) is pressed onto the surface of the material. After a period of time, when the load is removed, the ratio of the load to the area of the indentation is the Brinell hardness value (HB), with the unit being kilogram force/mm2 (N/mm2).
-Rockwell
When HB > 450 or the specimen is too small, the Brinell hardness test should not be used but the Rockwell hardness measurement should be adopted instead. It involves using a diamond cone with a 120° apex angle or a steel ball with a diameter of 1.59 or 3.18 mm to press into the surface of the tested material under a certain load. The hardness of the material is determined by the depth of the indentation. Depending on the hardness of the test material, it is represented by three different scales:
HRA: This is the hardness value obtained with a 60kg load and a diamond cone indenter, and is used for materials with extremely high hardness (such as hard alloys, etc.).
HRB: This is the hardness value obtained with a 100kg load and a 1.58mm-diameter hardened steel ball, and is used for materials with lower hardness (such as annealed steel, cast iron, etc.).
HRC: This is the hardness value obtained with a 150kg load and a diamond cone indenter, and is used for materials with very high hardness (such as quenched steel, etc.).
-Vickers
Using a diamond square-ended conical indenter with a tip angle of 136° and a load of 120 kg to press into the material surface, the hardness value (HV) is calculated by dividing the surface area of the material indentation pit by the load value.
As can be seen from the above introduction, Vickers hardness is one of the surface hardness measurement methods. It has conversion relationships with other hardness units.
Hardness refers to the surface hardness. There are no hardness requirements within the material itself. Only mechanical performance requirements (tensile strength, yield strength and impact toughness) .
The surface hardness of bolt should not be higher than the core hardness by 30 Vickers hardness units (approximately 3 HRC). The surface hardness of grade 10.9 should not exceed 390 HV0.3. For those without surface hardness requirements, they belong to low-performance grades such as 3.6, 4.6, 4.8, 5.6, 5.8, and 6.8. Since there is no heat treatment, there are no requirements. Because the surface hardness of these materials is mainly the result of cold heading, baking, turning, etc., and the stress generated therefrom.
When testing the hardness of bolts, for bolts without heat treatment, only the surface hardness needs to be within the qualified range. If the bolt is heat-treated, a large hardness measurement needs to be made on a circular surface at the bottom, with the hardness test conducted at 1/2R of the surface. The hardness should comply with the standard.
It has a surface and a core. The surface refers to the hardness test conducted after removing surface rust and other impurities using Vickers or surface Rockwell hardness testing. The core needs to be tested on the area at the center of the removed part, which is 1/2 of the diameter long, on the surface and the 1/2 part of the core. The difference between the two hardness values should not exceed 30 HV. If the surface hardness is 30 HV higher than the core, it indicates that the surface has undergone carburization, which is not allowed. If the surface hardness is more than 30 HV lower than the core, it indicates that the surface has undergone decarburization, which is also not allowed.
