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The Hardness of the Bolt

Hardness indicates the ability of a material to resist the pressing of hard objects into its surface. It is one of the important performance indexes of metal materials. Generally, the higher the hardness, the better the wear resistance.
 
Commonly used hardness indexes are Brinell hardness, Rockwell hardness and Vickers hardness.
 
1. Brinell hardness (HB)
 
Press a certain size (generally 10mm) hardened steel ball 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 the indentation area is the Brinell hardness value ( HB), the unit is 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 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 be pressed into the surface of the material to be measured 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: It 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: It is a hardened steel ball with a load of 100kg and a diameter of 1.58mm. The hardness obtained is used for materials with lower hardness (such as annealed steel, cast iron, etc.).
HRC: Hardness obtained with 150kg load and diamond cone indenter, used for materials with high hardness (such as hardened steel, etc.).
 
3 Vickers hardness (HV)
 
Press a load within 120kg and a diamond square cone indenter with an apex angle of 136° into the material surface, and divide the surface area of ​​the material indentation pit by the load value, which is the Vickers hardness value (HV).
 
It can be seen from the introduction above that Vickers hardness is one of surface hardness. It has a conversion relationship with other hardness units.
 
Hardness refers to the surface hardness, there is no hardness requirement inside the material. Only mechanical properties are required (tensile strength, yield strength and impact toughness).
 
Why is there no surface hardness in GB/T 3098.1-2000? Doesn't 5.6 indicate surface hardness?
 
The surface hardness should not be higher than the core hardness by 30 Vickers hardness (about 3 HRC). The surface hardness of the 10.9 grade should not be greater than 390HV0.3. As for the surface hardness requirements, it is a low performance grade, such as 3.6 4.6 4.8 5.6 5.8 6.8, because there is no heat treatment, so there is no requirement, because the hardness of his surface is mainly the hardness produced by the stress after cold heading or bakingturning.
 
When testing the hardness of the bolts, the bolts without heat treatment only need to have the surface hardness within the qualified range. If it is a heat-treated bolt, it is necessary to cut a diameter on the surface with a large hardness, and the hardness test is 1/2R on the surface At this point, the hardness meets the standard.
 
It has a surface and a core. The surface refers to the use of Vickers or surface Rockwell to remove the hardness after removing the surface rust. The core should be removed from the surface and the core 1/1/2 The hardness of the two places should be less than 30 HV. If the surface is higher than 30HV, it means that the surface is carburized, which is not allowed. If the surface is lower than 30HV, it means that the surface is decarburized, which is also not allowed.
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