The selection of flange connection bolts and nuts should take into account factors such as pipeline operating pressure, operating temperature, medium type, and gasket type. For example, in the flange connection of low-pressure highly toxic medium pipelines sealed with spiral wound gaskets, although the operating pressure and temperature of the pipeline are not high, the high specific pressure required to form the initial seal of the spiral wound gasket requires the fasteners to bear a large load. Therefore, in this case, high-strength alloy steel materials are required for the fasteners.
Classifications of bolts and nuts
Bolts and nuts are divided into two categories: commercial grade and specialized grade, depending on their application scopes.
Bolt materials to standardize their mechanical properties are not used for commercial grade bolts and nuts, but rather the mechanical properties of the finished bolt are used to mark its performance level. For bolts and nuts with the same performance level, regardless of the difference in material and origin, only the performance level should be selected in the design.
Special grade bolts and nuts refer to bolts and nuts specifically designed for certain industries, such as the chemical, petrochemical, and nuclear power industries. Special grade bolts and nuts require the material and mechanical properties to comply with specific standards, and their identification is usually indicated by the material grade. The performance grade is equivalent to the mechanical properties of the material. Full threaded bolts: 35CrMo, 0Cr18Ni9, A193, B8 CL. Alloy steel bolts should be made of high-quality steel, with the letter An added after the material grade, such as 35CrMoA and 25CrMoVA.
According to the different structural types, bolts and nuts can be divided into two categories: hexagonal bolts and nuts and double headed bolts and nuts (also known as bolts and nuts), and double headed bolts and nuts are further divided into threaded and non-threaded types.
Hexagonal head bolts and nuts: it is also known as single-sided bolts and nuts. When the tension is high, it is easy to break at the connection between the screw and the screw head, and cannot be used on medium and high pressure flanges. It is often used in conjunction with flat welding flanges and non-metallic gaskets for milder operating conditions. The commonly used materials for hexagonal bolts and nuts are BL3 or Q235B.
Double headed bolts and nuts: it's also known as double-sided bolts and nuts. It can be used for medium and high pressure flanges, making it easy to tighten from both sides. It is often used in conjunction with butt welded flanges under harsh operating conditions. Among them, because there is no change in cross-sectional shape on threaded double headed bolts, their load-bearing capacity is strong. Non-threaded double headed bolts and nuts have relatively weaker load-bearing capacity.
The nuts used for flange bolts are generally hexagonal, divided into two types: A and B. The surface of the A-shaped nut in contact with the connected part is flat, and only the hexagon on the other side is rounded, while both sides of the B-shaped nut are rounded. The material of the nut is often determined based on the bolt material it matches, and these combinations are specified in general standards. In general, the nut material should be slightly lower than the bolt material and ensure that the nut hardness is about HB30 lower than the bolt hardness.
According to the manufacturing method, bolts and nuts can be divided into coarse bolts and nuts, semi-refined bolts and nuts, and refined bolts and nuts. Except for the threaded part, the rest of the coarse bolts and nuts are not machined and can withstand low tightening force. The size specifications of the bolts are expressed as "bolt diameter x bolt length".
The strength grade and material strength grade of bolts and nuts mainly refer to commercial grade carbon steel fasteners, while commercial grade stainless steel bolts are distinguished by performance grade and material group.
The common strength grades for carbon steel bolts and nuts are 4.8, 5.8, 6.8, 8.8, 10.9, and 12.9. The corresponding bolts and nuts are: Grade 4, Grade 6, Grade 8, Grade 10, Grade 12.
Generally, bolts and nuts below grade 8.8 are called ordinary bolts and nuts, while bolts and nuts above grade 8.8 (including grade 8.8) are considered high-strength bolts. The difference is that high-strength bolts and nuts require quenching and tempering heat treatment. At present, there are three main materials for standard parts on the market: carbon steel, stainless steel, and copper.
Carbon steel
We distinguish low carbon steel, medium carbon steel, high carbon steel, and alloy steel based on the carbon content in the carbon steel material.
Low carbon steel with a C% ≤ 0.25% is commonly referred to as A3 steel in China. Abroad, it is commonly referred to as 1008101510181022, etc. It's mainly used for products without hardness requirements such as grade 4.8 bolts, grade 4 nuts, and small flange bolts and nuts. Generally, 1008 materials are used for ordinary grade products, such as 4.8 grade flange bolts and nuts and ordinary grade nuts. 1015 is usually used for hanging ring flange bolts and nuts. 1018 is generally used for making machine flange bolts and nuts, but there are also those used for making self-tapping flange bolts and nuts; (Note: Self tapping flange bolts and nuts are mainly made of 1022 material.)
Medium carbon steel with a content of 0.25%< c% ≤ 0.45% is commonly referred to as 35 # and 45 # steel domestically, while it is commonly referred to as 1035, ch38f, 1039, 40acr, etc. in foreign countries. It's mainly used for grade 8 nuts, grade 8.8 bolts, and grade 8.8 hexagon products. High carbon steel C%>0.45%. Currently, it is not widely used in the market.
Alloy steel
Adding alloy elements to plain carbon steel to enhance some special properties of the steel, such as 35, 40 chromium molybdenum, SCM435 (Japanese steel grade, corresponding to domestic 35CrMo), 10B38. 1045/10B21/40Cr make 10.9 grade flange bolts and nuts. Use 40Cr/SCM435 grade 12.9 flange bolts and nuts.
Stainless steel
Performance levels are divided into 45, 50, 60, 70, 80: groups are represented by one letter for stainless steel.
A represents austenite (18% Cr, 8% Ni) with good heat resistance, corrosion resistance, and weldability. A1，A2，A4.
C represents martensite, and 13% Cr has poor corrosion resistance, high strength, and good wear resistance. C1，C2，C4.
F represents ferrite, and 18% Cr has better forging properties and stronger corrosion resistance than martensite;
D represents duplex (austenite ferrite).
At present, the imported materials in the market are mainly Japanese products. Mainly divided into SUS302, SUS304, and SUS316 by level.
Example: A2-70 represents A2 group steel, and 70 represents a product strength grade of 70. The tensile strength of bolts, flange bolts and nuts, and studs reaches 700MPa or above, and the guaranteed load of nuts meets the requirements of grade 70. After loading the corresponding load force and maintaining it for 15 seconds, the nuts should not trip or break. The naming rules for bolts, flange bolts and nuts, and studs: For low-carbon austenitic stainless steel with a carbon content of less than 0.03%, the letter "L" can be added for the naming rules of nuts. For example: A4L-80.
Copper
The commonly used materials are brass and zinc copper alloy. H62, H65, and H68 copper are mainly used as standard components in the market.
Key points for selecting flange bolts: When selecting bolts for flange connections, the type, size, number, and material grade of the bolts should be determined.
The type, material, quantity, and size of bolts used for flanges mainly depend on the nominal diameter, nominal pressure, and working temperature of the flange, and can be selected according to the corresponding flange technical standards.
The "Code for Construction of Industrial Metal Pipeline Engineering" GB50235-2019 standard stipulates that bolt connections should be installed in the same direction, all nuts should be screwed into the bolts, and the tightened bolts should be flush with the nuts.
The hardness of the nut should not exceed that of the bolt.
In general, no washer is provided. When the thread length on the screw is too short to tighten, a flat washer can be added, and the washers should not be stacked together.
Selection of flange connection bolts for pipelines
Although the chemical industry, petrochemical industry, nuclear power industry and other industries have specialized bolts and nuts, commercial grade bolts are also used under certain conditions.
Fastener materials for steel pipe flanges are also divided into high-strength, medium strength, and low strength materials. High strength bolts include 8.8, 35CrMo, 25Cr2MoV, etc. Medium strength bolts include A2-70, A4-70, B8-2, B8M-2, etc. Low strength bolts include 0Cr17Ni12Mo2 (316), 0Cr18Ni9 (304), 5.6, A4-50, A2-50, etc.
The biggest feature of commercial grade fasteners is that they maintain mechanical properties but not chemical composition, and their chemical composition is relatively free.
The original wording of GB/T 3098.6 is: Unless otherwise agreed upon by the purchaser and the manufacturer in advance, the chemical composition within the specified range of steel groups shall be ultimately chosen by the manufacturer. And, in addition to specifying the content range, the chemical composition mentioned in the standard refers to the maximum value.
People usually consider "A2 group" as 304 class materials and "A4 group" as 316 class materials. This can be said verbally, but if it falls on paper, such as design documents and purchase documents, it cannot be written casually and must be strictly marked according to the GB/T3098.6 regulations.
Moreover, in fact, there are differences between the special grade fasteners 304 and 316 for austenitic stainless steel in standards such as A2, A4, and HG/T20634 (material standard GB/T1220). Firstly, in terms of chemical composition, there are differences between A2 and 304, A4 and 316, especially in the Ni content of A2, and both A2 and A4 contain Cu. Austenitic stainless steel is not easy to cut due to its high ductility and work hardening properties. And an appropriate amount of Cu can reduce its cold work hardening tendency and improve its cold work formability. Secondly, the special grade fasteners 304 and 316 in HG/T20634 are considered as low strength fasteners.
Moreover, for pipe flanges, according to HG/T20635, commercial grade fasteners can only be used in non-toxic, non-flammable, and non-severe circulation situations. The chemical composition not only affects the mechanical and processing properties of materials, but also affects their corrosion resistance. Although A2 and A4 have similar chemical compositions to 304 and 316, there are still differences, mainly due to anti-corrosion requirements.
