Fastener Wikipedia-What is the hardness test of bolts?

 

The hardness testing of bolts is one of important quality control process to ensure the fastener to meet mechanical properties.

 

 1.What means of hardness of bolt

 

Hex bolt hardness measures a bolt's resistance to deformation, scratching, and wear, indicating its material strength and proper heat treatment. It ensures the fastener can handle specific stress loads without stripping.

 

2. Purpose of Hardness Testing

     

• Material Hardness: Verifies the bolt's resistance to deformation,scratching and wear.
• Heat Treatment Validation: Confirms proper quenching, tempering, or case hardening processes.
• To Match with nut ：The core principle of matching bolts and nuts is that "the grade of the nut should be equal to or higher than that of the bolt."  We need to confirm whether the hardness of the bolts falls within grade range in standard, in order to ensure that they can be used together with the correct nuts.

 

3. Hardness Testing Methods/Advantage/Application

 

Usually,we used the hardness test machine.See the below picturer.

 

There are three kinds of hardness Method for fastener.

 

Rockwell Hardness (HRC):

 

• Method:  Uses a 120° diamond cone,the hardness is calculated based on the depth.
• Advantages: direct reading, high speed, suitability for production lines, and minimizes indentation.
• Application: Suitable finished products and high-hardness materials (such as high strength bolt,nut etc)

 

Brinell Hardness (HB):

 

• Method: Uses a tungsten carbide ball. 
• Advantages: Effective for rough or curved surfaces but after test will left larger marks.
• Application: Often used for larger bolts or raw materials.

 

Vickers Hardness (HV):

 

• Method: Uses a 136° diamond square pyramid indenter to measure the diagonal length of the indentation.
• Advantages: The range is extremely wide, from the softest to the hardest materials, including thin sheets and carburized layers.
• Application: Often used for flat sheet, such us washers.
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4. Hardness Test Precautions

 

 

For bolts without heat treatment, it is sufficient to ensure that the surface hardness is within the qualified range.

For bolts with heat treatment, a surface needs to be cut at the end of the diameter for hardness testing. The specific location is at the center 1/2 radius of the surface. Ensure that the hardness meets the corresponding standards.

 

The hardness test of bolts is divided into two parts: surface hardness and core hardness. The surface hardness test is conducted after removing impurities such as rust from the surface.The core hardness test requires removing the surface of the bolt for 1/2 of its diameter, and then conducting the test at the surface and 1/2 radius of the core. The difference in hardness between the two should not exceed 30 HV. If the surface hardness is 30 HV higher, it indicates the presence of surface carburization, which is not allowed. Conversely, if the surface hardness is 30 HV lower, it may be surface decarburization, which is also not allowed.

 

• 5. Relatived Standards

   

      ASTM Standards: ASTM F606 , A Standard Test Method of Bolt (cover the hardness)

      ISO/DIN Standards:ISO898-1,Mechanical properties of bolt made of carbon steel and alloy steel  (cover the hardness)

      SAE Standards: SAE J429,Mechanical and Material Requirements for Steel bolt (cover the hardness)

•  Prevents failures due to brittleness (overhardening) or weakness (underhardening).
•  Ensures consistency in mass production and compliance with international standards.

 

In our factory,ZJB Fastener, we will carried on strict quality control throughout the entire production process. Every batch of products need through professional hardness testing to ensure consistent performance, reliability, and compliance with international standards. We are committed to delivering superior quality you can trust. More information pls check our website and about us .

 

Relatived information

 

What is the hardness test of nuts

 

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