Valve bolt performance grade

Thread Content

The performance grades of bolts used for connecting valve steel structures include over 10 grades such as 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 made of low-carbon alloy steel or medium-carbon steel and have undergone heat treatment (quenching and tempering); they are commonly referred to as high-strength bolts, while the rest are known as ordinary bolts. The performance grade designation of bolts consists of two numbers, which indicate respectively the nominal tensile strength of the bolt material and its yield strength ratio. For example, a bolt with a performance grade of 4.6 means the following: 1) the nominal tensile strength of the bolt material is 400 MPa; 2) the yield strength ratio of the bolt material is 0.6; 3) the nominal yield strength is 400 × 0.6 = 240 MPa. High-strength bolts with a performance grade of 10.9, after heat treatment, can achieve: 1) a nominal tensile strength of 1000 MPa; 2) a yield strength ratio of 0.9; 3) a nominal yield strength of 1000 × 0.9 = 900 MPa. The concept of bolt performance grades is a universally accepted standard – bolts with the same performance grade have identical properties, regardless of their material or place of origin. In design, it is sufficient to specify only the performance grade. The strength grades of 8.8 grade and 10.9 grade refer to the shear stress resistance levels of bolts, which are 8.8 GPa and 10.9 GPa respectively. For a grade of 8.8, the nominal tensile strength is 800 N/mm2 and the nominal yield strength is 640 N/mm2. Generally, bolts have their strength indicated as “X.Y”; where X*100 represents the tensile strength of the bolt, and X*100*(Y/10) represents its yield strength (as per the standard specification, yield strength/tensile strength = Y/10). For example, for a grade of 4.8, the tensile strength of such a bolt is 400 MPa, while the yield strength is 400*8/10 = 320 MPa. Additionally: Stainless steel bolts are usually designated as A4-70, A2-70, etc.; the meaning of these designations is explained separately. Units of measurement: There are mainly two systems for measuring length in the world today. One is the metric system, whose units include meters (m), centimeters (cm), millimeters (mm), etc.; this system is widely used in Europe, China, Japan, and other Southeast Asian countries. The other is the imperial system, whose main unit is the inch, which is equivalent to the old Chinese unit of chi; this system is more commonly used in the United States, the United Kingdom, and other European countries. 1. Metric measurement: (decimal system) 1 m = 100 cm = 1000 mm. 2. Imperial measurement: (octal system) 1 inch = 8 inches; 1 inch = 25.4 mm. 3/8″ × 25.4 = 9.52. 3. For products with a diameter of 1/4″ or less, their diameter is indicated by numbers such as 4#, 5#, 6#, 7#, 8#, 10#, 12#. Threads – A thread is a pattern of uniform spiral protrusions on the external or internal surface of a solid. Based on their structural characteristics and applications, they can be divided into three main categories: (1) Ordinary threads: They have a triangular thread form and are used for connecting or fastening parts. Plain threads are classified into coarse-thread and fine-thread types based on pitch, with fine-thread threads having a higher connection strength. (2) Transmission threads: The tooth profiles include trapezoidal, rectangular, serrated, and triangular shapes, among others. (3) Sealing threads: Used for sealing connections, mainly pipe threads, tapered threads, and tapered pipe threads. II. Thread fit grade: Thread fit refers to the degree of looseness or tightness between mating threads, and the fit grade is a specified combination of tolerances and deviations applied to the internal and external threads. (1) For unified British thread standards, there are three thread grades for external threads: 1A, 2A, and 3A grades, and three grades for internal threads: 1B, 2B, and 3B grades; all of them are clearance fits. The higher the grade number, the tighter the fit. In British standard threads, tolerances are specified only for grades 1A and 2A; the tolerance for grade 3A is zero. Moreover, the tolerance values for grades 1A and 2A are equal. The larger the number of grades, the smaller the tolerance. 1. Classes 1A and 1B are very loose tolerance classes; they are suitable for the tolerance fits of internal and external threads. Classes 2, 2A, and 2B are the most commonly specified thread tolerance grades in the British series of mechanical fasteners. Levels 3, 3A, and 3B: they achieve the tightest fit when screwed together, and are suitable for fasteners with strict tolerances, used in critical designs where safety is important. 4. For external threads, there is a fit tolerance for Classes 1A and 2A; there is no such tolerance for Class 3A. The tolerance for grade 1A is 50% greater than that of grade 2A, and 75% greater than that of grade 3A. For internal threads, the tolerance for grade 2B is 30% greater than that of grade 2A. Grade 1B is 50% larger than Grade 2B, and 75% larger than Grade 3B. (2) For metric threads, there are three thread grades for external threads: 4h, 6h, and 6g. There are three thread grades for internal threads: 5H, 6H, and 7H. (The accuracy classes of JIS threads are divided into three levels: I, II, and III; under normal circumstances, it is Class II.) In metric threads, the fundamental deviations for H and h are zero. The basic deviation of G is positive, while the basic deviations of e, f, and g are negative. 1. H is the commonly used tolerance zone for internal threads; it is generally not used for surface plating, or only a very thin phosphating layer is applied. The basic deviation for position G is used in special cases, such as thicker coatings, and is generally not used very often. 2. g is commonly used to apply thin coatings of 6–9 um thickness; for example, if the product specifications require bolts with a coating thickness of 6 h, the thread tolerance range before coating is set at 6g. 3. The best thread fits are H/g, H/h, or G/h. For the threads of precision fasteners such as bolts and nuts, the standard recommendation is to use a 6H/6g fit. (3) Thread designation. III. Main geometric parameters of self-tapping and self-drilling threads: (1) Major diameter/outer diameter of the thread tooth (d1), which is the diameter of the imaginary cylinder on which the thread teeth coincide. The major diameter of the thread essentially represents the nominal diameter of the thread size. (2) Minor diameter/diameter at the root of the thread (d2): The diameter of the imaginary cylinder along which the roots of the thread threads coincide. (3) Tooth pitch (p): It is the axial distance between the corresponding two points of adjacent teeth on the mid-diameter line. In the imperial system, the thread pitch is indicated by the number of threads per inch (25.4 mm). The table below lists the common specifications for pitch (metric) and number of teeth (imperial). 1. Metric self-tapping screws: Specifications: ST 1.5, 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: 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. Imperial self-tapping screws: Specifications: 4#, 5#, 6#, 7#, 8#, 10#, 12#, 14#. Number of teeth: AB type – 24, 20, 20, 19, 18, 16, 14, 14; A type – 24, 20, 18, 16, 15, 12, 11, 10