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how much do you know about bolt strength grades?

1.What is the strength of a bolt?

In material mechanics, strength refers to the ability of a material to resist damage (excessive elastic deformation, plastic deformation, or fracture) when subjected to external forces. It is an important indicator in the mechanical properties of materials, usually used in engineering design to ensure the safety and reliability of structures. The material strength can be divided into tensile strength, yield strength, compressive strength, shear strength, fatigue strength, etc. according to different load conditions and failure modes.

For bolted connections, they mainly bear axial working loads in most working scenarios. Therefore, the bolt strength we generally refer to mainly refers to the tensile strength and yield strength of the bolt. The tensile strength of a bolt refers to its maximum ability to resist fracture under tensile load, and is a key indicator to ensure the reliability of bolt connections. If the tensile force on the bolt exceeds its tensile strength, the bolt will fracture, leading to connection failure. The yield strength of a bolt refers to the stress value at which the bolt begins to undergo significant plastic deformation during the tensile process.

In practical use, it is generally required that bolts do not undergo significant plastic deformation under working loads to ensure the tightness and stability of the connection. In some complex alternating stress scenarios, in addition to considering the tensile strength and yield strength of bolts, it is also important to focus on the fatigue strength of bolts. The fatigue strength of a bolt refers to the maximum stress at which it can withstand countless cycles of alternating loads without experiencing fatigue failure. Fatigue strength reflects the reliability of a bolt under long-term repeated stress and is an important indicator for preventing fatigue fracture.

2.Why is it necessary to classify the strength of bolts?

Can bolts used in household appliances be used in engineering structures? Obviously, the strength of bolts used in household appliances is much lower than the engineering standards. Therefore, the strength of bolts should be classified to suit different scenarios of bolt connections. In addition to the above reasons, there are many other reasons. Screw Lord has compiled the following reasons for the classification of bolt strength.

2.1 Satisfying diverse application requirements 

By dividing strength, bolts can ensure corresponding mechanical properties in different application scenarios, thereby meeting diverse usage needs. For example, in high-strength applications such as heavy machinery and bridge structures, high-strength bolts are required to ensure the stability and safety of the connection; In general applications or non critical connections, bolts with slightly lower strength can be chosen to reduce costs.

2.2 Unified production and use standards

Dividing the strength of bolts can unify production and use standards, facilitating bolt procurement, use, and management worldwide. The International organization for Standardization (ISO) and relevant organizations in various countries have gradually developed standards for bolt strength grades, which ensure that bolts of the same performance grade, regardless of their material and origin, have the same mechanical properties.

2.3 Improve product quality and safety

Dividing the strength of bolts can ensure that they meet specific mechanical performance requirements during the design and manufacturing process, thereby improving product quality and safety. High strength bolts undergo special heat treatment processes, resulting in higher tensile strength and yield strength, which can maintain the stability and reliability of the connection when subjected to large loads. This helps to reduce safety accidents and property damage caused by bolt failure.

2.4 Optimize material utilization and cost control

By dividing the strength of bolts, suitable bolt grades can be selected based on actual application needs, thereby optimizing material utilization and cost control. Although high-strength bolts are relatively expensive, their use in applications that require heavy loads can reduce the number and size of bolts, and lower the weight and cost of the overall connection structure. In general applications, using bolts with slightly lower strength can further reduce costs.

3. What are the strength grades of bolts based on?

The strength grade classification of bolts is based on national or international standards, and commonly used standards mainly include GB/T 3098.1-2010 "Mechanical Properties of Fasteners - Bolts, Screws, and Studs" ISO 898-1: 2013 "Mechanical Properties Part 1: Bolts, Screws, and Studs" and ASTM A307 "Standard Specification for Carbon Steel Bolts and Studs".

3.1 GB /T 3098.1-2010 "Fasteners Mechanical properties Bolts, screws and studs"

This is a Chinese national standard that specifies the mechanical performance requirements and performance levels of bolts, including tensile strength, yield strength, hardness, and other indicators. This standard applies to bolts, screws, and studs with nominal diameters ranging from M1.6 to M39. For example, a bolt with a performance level of 4.8 has a nominal tensile strength of 400MPa, a yield ratio of 0.8, and a nominal yield strength of 400 × 0.8=320MPa.

3.2 ISO 898-1:2013 Mechanical properties - Part 1: Bolts, screws and studs

This is a standard developed by the International organization for Standardization (ISO), which is similar to GB/T 3098.1 and is widely adopted internationally. By specifying the nominal tensile strength value of the bolt material and the flexion ratio to divide the strength grade, such as 8.8 indicates that the nominal tensile strength of the bolt material reaches 800MPa level, and the flexion ratio is 0.8.

3.3 ASTM A307 Standard Specification for Carbon Steel Bolts and studs

This is a standard developed by the American Society for Testing and Materials (ASTM) for carbon steel bolts and bolts for general use. The bolts are divided into two grades, A and B. Grade A is suitable for bolts with a diameter not exceeding 1 inch, while Grade B is suitable for bolts with a diameter exceeding 1 inch. The tensile strength and yield strength requirements for A-grade bolts are relatively low, while the requirements for B-grade bolts are relatively high.

There may be some differences between different standards, but overall, the strength grades are classified based on performance indicators such as tensile strength, yield strength, and hardness of bolts.

4. Bolt grade code how to interpret?

According to the Chinese national standard GB/T 3098.1-2010, the strength grades of bolts are mainly divided into ten levels: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9. Among them, bolts of grade 8.8 and above are usually made of low-carbon alloy steel or medium carbon steel, and undergo heat treatment (quenching+tempering). These bolts are called high-strength bolts; Bolts below grade 8.8 are often made of low-carbon steel material and generally do not require special heat treatment. They have higher strength than high-strength bolts and are classified as ordinary bolts.

We can see that the bolt grade code is generally represented in the form of "X.Y", where X represents 1/100 of the nominal tensile strength value of the bolt material, that is, X multiplied by 100 is the tensile strength of the bolt, in MPa (megapascals); Y represents the yield strength ratio of bolt material, which is the ratio of yield strength to tensile strength. It is usually expressed in decimal form, but is directly expressed as an integer or approximate integer in the bolt grade code.

Taking common 4.6 and 8.8 grade bolts as examples:

 4.6 grade bolts: 

nominal tensile strength: 400MPa (4 × 100); 

Bending strength ratio: 0.6; 

Nominal yield strength: 400MPa × 0.6=240MPa; 

Grade 8.8 bolt: 

nominal tensile strength: 800MPa (8 × 100); 

Bending strength ratio: 0.8; 

Nominal yield strength: 800MPa × 0.8=640MPa.

5. What are the main influencing factors of bolt strength grade?

The strength grade of bolts is mainly influenced by material selection, manufacturing process, and heat treatment process. The selection of materials determines the basic mechanical properties, among which carbon content and alloying elements such as manganese (Mn), chromium (Cr), molybdenum (Mo), etc. affect hardness, strength, and toughness. In terms of manufacturing processes, forging, cold heading and rolling, as well as machining accuracy, are crucial for refining metal grains, reducing defects, and ensuring the expected preload force during assembly. Heat treatment, especially quenching and tempering, is a key step in improving bolt strength by adjusting hardness and increasing toughness to prevent brittle fracture; Normalization and annealing are used to improve the internal structure and eliminate internal stress. The correct selection of materials and appropriate processing and heat treatment methods are the foundation for producing high-quality and high-strength bolts.

6. How to choose bolt strength grade?

6.1 Determine the strength level according to the load

The selection of bolt strength grade should be based on the load situation. If subjected to static load, the required fastening force for the connection can be selected. For small loads, lower strength bolts such as 4.8 grade can be selected, while for large loads, bolts of 8.8 grade or above can be selected. If subjected to dynamic loads such as vibration, impact, etc., high-strength bolts such as grade 10.9 or 12.9 should be selected to prevent bolt loosening or fatigue fracture, and anti loosening measures should be taken.

6.2 Select the connection component based on its features

The choice of bolt strength class should be based on the characteristics of the connecting parts. The hardness of the connected parts is high, in order to prevent the deformation and damage of the bolts when tightening, it is necessary to select high-strength bolts; The connected parts are soft and can be selected with relatively low strength grades. In addition, for important connections that are not allowed to loosen or fail, such as Bridges, pressure vessels, etc., bolts with high strength and reliable quality should be selected.

6.3 Select based on working environment conditions

The working environment conditions affect the selection of bolt strength grade. In high temperature environment, it is necessary to consider the high temperature strength and creep performance of the material, choose superalloy bolts or high-strength bolts with special heat treatment; Low temperature environment to prevent bolt brittle break, select low temperature toughness and strength grade. In corrosive environment, in addition to taking anti-corrosion measures such as galvanized and nickel plating, corrosion resistant material bolts, such as stainless steel bolts, should be selected according to the properties and concentration of corrosive media; For occasions with high strength requirements, high-strength stainless steel bolts or high-strength carbon steel bolts with anti-corrosion treatment can be selected.

6.4 Select this option based on installation and removal requirements

The choice of bolt strength level is related to installation and disassembly requirements. The connection should be disassembled frequently. In order to avoid damage or thread slip during disassembly, bolts with moderate strength and good toughness should be selected to facilitate multiple use; For one-time installation and requiring large fastening force, higher strength class bolts can be selected, but pay attention to controlling the tightening torque to prevent the bolt from breaking over load.

6.5 Consideration of cost and economy

Cost and economy should be considered when selecting bolt strength grade. Under the premise of meeting the requirements of use, it is necessary to consider the cost comprehensively, and the price of high-strength grade bolts is usually higher, so it is necessary to weigh the relationship between performance and cost to avoid excessive pursuit of high strength and waste costs. For general connections with low strength requirements, lower strength class bolts can be used to reduce costs.