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Valve Fastener Performance Grade
Created on 06.30
Bolt Performance Grades for Steel Structure Connections in Valve Industry
Bolts used in steel structure connections for the valve industry are classified into more than ten performance grades, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. Bolts of Grade 8.8 and above are manufactured from low-carbon alloy steel or medium-carbon steel and are subjected to heat treatment (quenching and tempering). These are generally referred to as high-strength bolts, while all others are commonly termed ordinary bolts.
Interpretation of Bolt Performance Grade Designations
The performance grade designation of a bolt consists of two numbers, which respectively indicate the nominal tensile strength and the yield-to-tensile ratio (yield ratio) of the bolt material.
For example, for a bolt of performance grade 4.6:
- The nominal tensile strength of the bolt material is 400 MPa;
- The yield ratio of the bolt material is 0.6;
- The nominal yield strength of the bolt material is 400 × 0.6 = 240 MPa.
For a Grade 10.9 high-strength bolt, after heat treatment, the material achieves:
- A nominal tensile strength of 1000 MPa;
- A yield ratio of 0.9;
- A nominal yield strength of 1000 × 0.9 = 900 MPa.
The meaning of bolt performance grades is an internationally recognized standard. Bolts of the same performance grade, regardless of differences in material or place of manufacture, possess identical mechanical properties. Therefore, in design, it is sufficient to specify the required performance grade.
Strength Grade Overview
The strength grades 8.8 and 10.9 refer to the shear stress grades of the bolts, specifically 8.8 GPa and 10.9 GPa.
- Grade 8.8: Nominal tensile strength = 800 N/mm²; Nominal yield strength = 640 N/mm².
- In general, bolt strength is denoted by "X.Y", where:
- X × 100 = Tensile strength of the bolt (MPa);
- X × 100 × (Y/10) = Yield strength of the bolt (MPa), as per the designation rule: Yield Strength / Tensile Strength = Y/10.
Example: For a Grade 4.8 bolt:
- Tensile strength = 400 MPa;
- Yield strength = 400 × 8/10 = 320 MPa.
Additional Note on Stainless Steel Bolts:
Stainless steel bolts are typically designated as A4-70 or A2-70, which follow a different interpretation of mechanical properties.
Measurement Systems
Globally, there are two primary systems of length measurement:
- Metric System
- 1 m = 100 cm = 1000 mm
- Imperial System
- 1 inch = 8 fractions (1/8 inch)
- 1 inch = 25.4 mm
- Example: 3/8 inch = 3/8 × 25.4 = 9.52 mm
For sizes smaller than 1/4 inch, products are designated by callout numbers to indicate nominal diameter, such as: 4#, 5#, 6#, 7#, 8#, 10#, and 12#.
Threads – General Definition and Classification
A thread is a continuous helical ridge of uniform cross-section on the external or internal surface of a cylindrical workpiece.
Based on structural characteristics and applications, threads are classified into three main categories:
- General-Purpose Threads (Fastening Threads):
- Transmission Threads:
- Sealing Threads:
Thread Fit Classes
Thread fit refers to the degree of looseness or tightness between mating internal and external threads. The class of fit is a specified combination of allowances and tolerances applied to the internal and external threads.
(1) Unified Inch Threads:
- External threads have three classes: 1A, 2A, and 3A.
- Internal threads have three classes: 1B, 2B, and 3B.
- All are clearance fits; the higher the class number, the tighter the fit.
- For inch threads, deviations are specified only for Classes 1A and 2A; Class 3A has zero deviation, and the deviations for 1A and 2A are equal. The higher the class number, the smaller the tolerance.
- 1A and 1B:
- 2A and 2B:
- 3A and 3B:
- For external threads, Classes 1A and 2A have a fit tolerance, while Class 3A does not. The tolerance of Class 1A is 50% larger than Class 2A and 75% larger than Class 3A. For internal threads, the tolerance of Class 2B is 30% larger than Class 2A; Class 1B is 50% larger than Class 2B and 75% larger than Class 3B.
(2) Metric Threads:
- External threads have three tolerance classes: 4h, 6h, and 6g.
- Internal threads have three tolerance classes: 5H, 6H, and 7H.
- (JIS standard specifies three accuracy grades: I, II, and III; Grade II is generally used.)
For metric threads, the basic deviations for H and h are zero. G has a positive basic deviation, while e, f, and g have negative basic deviations.
- H:
Commonly used tolerance position for internal threads; generally not used with surface coatings, except for very thin phosphating layers.
- G:
Used for special cases requiring thicker coatings, such as heavy plating; rarely used in practice.
- g:
Often used for thin coatings of 6–9 µm. For example, if a product drawing specifies a 6h bolt, the thread before plating is typically machined to the 6g tolerance band.
- Recommended thread fit combinations:
H/g, H/h, or G/h. For precision fasteners such as bolts and nuts, the standard recommends a 6H/6g fit.
(3) Thread Marking: (Reference to standard designation practices.)
Key Geometric Parameters for Self-Tapping and Self-Drilling Threads
- Major Diameter (External Diameter, d₁):
- Minor Diameter (Root Diameter, d₂):
- Pitch (p):
Common Pitch/Thread Count Reference Table
(1) Metric Self-Tapping Threads:
Specification | ST1.5 | ST1.9 | ST2.2 | ST2.6 | ST2.9 | ST3.3 | ST3.5 | ST3.9 | ST4.2 | ST4.8 | ST5.5 | ST6.3 | ST8.0 | ST9.5 |
Pitch (mm) | 0.5 | 0.6 | 0.8 | 0.9 | 1.1 | 1.3 | 1.3 | 1.3 | 1.4 | 1.6 | 1.8 | 1.8 | 2.1 | 2.1 |
(2) Inch Self-Tapping Threads:
Specification | 4# | 5# | 6# | 7# | 8# | 10# | 12# | 14# |
Threads per Inch (AB) | 24 | 20 | 20 | 19 | 18 | 16 | 14 | 14 |
Threads per Inch (A) | 24 | 20 | 18 | 16 | 15 | 12 | 11 | 10 |
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