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What do 4 and 8 on the bolt stand for?

There are more than 10 grades of bolts for steel structure connection, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. The bolts of grade 8.8 and above are made of low carbon alloy steel or medium carbon steel and heat treated (quenched and tempered), which are generally called high strength bolts, and the others are called ordinary bolts.
There are two parts in the performance grade of bolt, which are the nominal tensile strength value and yield strength ratio of bolt material. 48 is 4.8.
For example, the bolt with performance grade 4.8 means:

  • 1. The nominal tensile strength of bolt material is 400MPa;
  • 2. The yield ratio of bolt material is 0.8;
  • 3. The nominal yield strength of bolt material is 400 × 0.8=320MPa.

After heat treatment, the material of high-strength bolt with performance grade of 10.9 can reach:

  • 1. The nominal tensile strength of bolt material reaches 1000MPa;
  • 2. The yield ratio of bolt material is 0.9;
  • 3. The nominal yield strength of bolt material is 1000 × 0.9 = 900MPa.

The meaning of bolt performance grade is an international standard. Bolts with the same performance grade have the same performance regardless of their material and origin. Only performance grade can be selected in design.
The so-called strength grades of 8.8 and 10.9 refer to the shear stress grades of 8.8gpa and 10.9gpa8.8. The nominal tensile strength is 800N / mm2, and the nominal yield strength is 640n / mm2. Generally, the strength of bolts is expressed by “x.y”, X * 100 = the tensile strength of the bolt, X * 100 * (Y / 10) = the yield strength of the bolt (because according to the marking regulations: yield strength / tensile strength = Y / 10, The tensile strength of the bolt is 400MPa if it is grade 4.8; The yield strength is 400 * 8 / 10 = 320MPa.
Another: stainless steel bolts are usually marked as a4-70, A2-70, which has another meaning. Measurement: there are two main measurement units of length in the world, one is metric system, and the measurement units are meter (m), centimeter (CM), millimeter (mm), etc., which are widely used in Southeast Asia such as Europe, China and Japan, and the other is British system, and the main measurement unit is inch It is widely used in the United Kingdom and other European and American countries.

  • 1. Metric measurement: (10 system) 1m = 100cm = 1000mm
  • 2. English Measurement: (octal) 1 inch = 8 inch,1 inch = 25.4mm,3/8 ¢¢× 25.4=9.523

The following products are named by serial numbers:
For example: 4, 5, 6, 7, 8, 10, 12.

Source: China Bolts Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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