What is the performance grade of valve bolts?

The performance grade of bolts for valve steel structure connection can be divided into more than 10 grades, such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9 and 12.9. Among them, grade 8.8 of bolts and above are made of low-carbon alloy steel or medium carbon steel and heat-treated (quenching and tempering), which are generally called high-strength bolts, and the rest are commonly referred to as ordinary bolts.

20201203020824 35286 - What is the performance grade of valve bolts?

The bolt performance grade label consists of two parts, which respectively represent the nominal tensile strength and yield strength ratio of bolt material.
For example:
The meaning of bolts with performance grade 4.6 is as follows:

  • 1. The nominal tensile strength of bolt material is 400MPa;
  • 2. The yield strength ratio of bolt material is 0.6;
  • 3. The nominal yield strength of bolt material is 400 × 0.6 = 240mpa.

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

  • 1. The nominal tensile strength of bolt material reaches 1000MPa;
  • 2. The yield strength 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. The performance of bolts with the same performance level is the same regardless of the difference of material and origin, so only performance grade can be selected in design.
The so-called grade 8.8 and grade 10.9 are the shear stress grades of 8.8GPA and 10.9GPA.

  • 8.8: nominal tensile strength 800n/mm2, nominal yield strength 640n/mm2.

Generally, “x.y” is used to express the strength of a bolt. X * 100 = the tensile strength of the bolt, and X * 100 * (Y / 10) = the yield strength of the bolt (because according to the marking regulations: yield strength / tensile strength = Y / 10).
If grade 4.8, the tensile strength of the bolt is 400MPa; the yield strength is 400 * 8 / 10 = 320MPa.
In addition, stainless steel bolts are usually marked as A4-70 and A2-70.
Measurement: there are two main measurement units of length in the world. One is the metric system, and the measurement units are meter (m), centimeter (CM) and millimeter (mm). They are widely used in Europe, China, Japan and other Southeast Asian regions. The other is the British system, with the main unit of measurement of inch, which is equivalent to the market size of the old system in China, and is more used in the United States, Britain and other European and American countries.

  • 1. Metric measurement: (10 base) 1m = 100 cm = 1000 mm;
  • 2. English Measurement: (8 base) 1 inch = 8 English minutes, 1 inch = 25.4 mm 3 / 8 ¢ x 25.4 = 9.52;
  • 3. For products less than 1 / 4 ¢ or less, they are denoted by serial numbers, such as: 4 ¢, 5 ¢, 6 ¢, 7 ¢, 8 ¢, 10 ¢ and 12 ¢.


Screw thread is a kind of shape with uniform helix bulge on the cross section of the outer or inner surface of a solid. It can be divided into three categories according to its structural characteristics and uses.

  • (1) Common thread: the tooth shape is triangle, which is used to connect or fasten parts. The common thread can be divided into coarse thread and fine thread according to pitch, and the connection strength of fine thread is higher.
  • (2) Transmission thread: trapezoidal, rectangular, saw shaped and triangular.
  • (3) Sealing thread: used for sealing connection, mainly pipe thread, taper thread and taper pipe thread.

Thread fit grade:

Thread fit is the loose or tight size between screw threads, and the fit grade is the specified combination of deviation and tolerance acting on internal and external threads.
(1) For unified inch screw thread, there are three thread grades for external thread: 1a, 2a and 3a, and three grades for internal thread: 1b, 2b and 3b, all of which are clearance fit. The higher the grade number, the tighter the fit. In English screw threads, the deviation is specified only for class 1A and 2A, class 3A is zero, and class 1A and 2A are equal. The larger the number of grades, the smaller the tolerance.

  • 1. Grade 1a and 1b, very loose tolerance grade, suitable for tolerance fit of internal and external threads.
  • 2. Grade 2a and 2b are the most common thread tolerance grades specified for English series mechanical fasteners.
  • 3. Grade 3A and 3b, spin to form the tightest fit, suitable for tight tolerance fasteners, for safety critical design.
  • 4. For external threads, there is a fit tolerance for grades 1a and 2a, but not for class 3a. Class 1A tolerances are 50% greater than class 2A tolerances and 75% larger than class 3A tolerances. For internal threads, class 2B tolerances are 30% greater than 2A tolerances. Grade 1b is 50% larger than grade 2B and 75% higher than grade 3B.

(2) Metric thread, external thread has three thread grades: 4h, 6h and 6G, and internal thread has three thread grades: 5h, 6h and 7h. (the precision grade of Japanese standard thread is divided into three levels: I, II and III, and generally it is grade II). In metric thread, the basic deviation of H and H is zero. The basic deviation of F and G is negative.

  • 1. H is the position of tolerance zone commonly used for internal thread, which is not used as surface coating or extremely thin phosphating layer. The basic deviation of G position is used in special occasions, such as thick coating, which is seldom used.
  • 2. G is often used for plating 6-9um thin coating. For example, if the product drawing requires 6h bolt, the tolerance zone of 6G is adopted for the thread before plating.
  • 3. The best combination of thread matching is H / g, H / h or g / h. for the screw thread of refined fasteners such as bolts and nuts, 6h / 6G matching is recommended in the standard.

(3) Thread marking

The main geometric parameters of self tapping and self drilling thread are as follows:

  • (1) The major diameter / tooth diameter (D1) is the diameter of the imaginary cylinder with the top of the thread overlapping. The major diameter of the thread basically represents the nominal diameter of the thread size.
  • (2) , minor diameter / tooth base diameter (D2): it is the imaginary cylinder diameter with the thread base overlapping.
  • (3) Distance between teeth (P): the axial distance between two points corresponding to the middle meridian line of adjacent teeth. In English, the number of teeth per inch (25.4mm) is used to indicate the pitch.

Source: China Bolts Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading flange 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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