The hardness of the bolt

Hardness refers to the ability of a material to resist the pressing of hard objects 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.

1. Brinell hardness (HB)

Press a hardened steel ball of a certain size (usually 10mm in diameter) into the surface of the material with a certain load (usually 3000kg) and keep it for a period of time. After the load is removed, the ratio of the load to the indentation area is 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, Brinell hardness test cannot be used and Rockwell hardness measurement can be 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, which is pressed into the surface of the material to be tested under a certain load, and the hardness of the material is obtained from the depth of the indentation. According to the hardness of the test material, it can be expressed in three different scales:

HRA: It is the hardness obtained with a 60kg load and a diamond cone indenter, used for extremely hard materials (such as cemented carbide, etc.).

HRB: It is the hardness obtained by using a 100kg load and a hardened steel ball with a diameter of 1.58mm. It is used for materials with lower hardness (such as annealed steel, cast iron, etc.). (Guide: Stainless steel numbering principle)

HRC: It is the hardness obtained with a 150kg load and a diamond cone indenter. It is used for materials with high hardness (such as hardened steel, etc.).

3 Vickers hardness (HV)

With a load of less than 120kg and a diamond square cone indenter with an apex angle of 136°, it is pressed into the surface of the material, and the surface area of u200bu200bthe material indentation pit is divided by the load value to obtain the Vickers hardness value (HV).

From the above introduction, we can see that Vickers hardness is one of the surface hardness. There is a conversion relationship between it and other hardness units.

Hardness refers to the surface hardness, and 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? Does 5.6 not indicate the surface hardness-the surface hardness should not be 30 Vickers hardness (about 3 HRC) higher than the core hardness. 10.9 The surface hardness of grade should not be greater than 390HV0.3. As for those without 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 cold forging or The hardness produced by the stress after baking\turning.

When testing the hardness of bolts, the bolts without heat treatment only need to make the surface hardness within the qualified range. If the bolts are heat-treated, the hardness should be large on the surface without a diameter. The hardness test is on the surface At 1/2R, the hardness meets the standard.

It has a surface and a core. The surface refers to the hardness of Vickers or Rockwell surface after removing the surface rust, etc. The core should be on the surface and the core after removing the 1/2 diameter of the long part. Hardness at the half of the part. The difference between the two hardnesses cannot exceed 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. of. Look at this 3098 carefully.

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