Bolted Flange

Classification Of Bolted Flange

According to the scope of application, bolts are divided into two categories: commercial grade and particular grade.

Commercial-grade bolts do not use bolt materials to regulate their mechanical properties but use the mechanical properties of the finished bolts to mark their performance grades. Regardless of the difference in their materials and origins, bolts of the same performance grade only use the performance grade in design.

Particular grade bolts are dedicated to certain industries, such as bolts and nuts devoted to chemical, petrochemical, nuclear power, and other industries.

Special-grade bolts require the material to be marked, and the mechanical properties must meet the requirements of specific standards. The material grade usually indicates its identification, and the performance grade is equivalent to the mechanical properties of its material, such as full thread studs: 35CrMo, 0Cr18Ni9, A193, B8 CL.2.

Alloy steel studs should be made of high-quality steel; that is, the letter A should be added after the material grade, such as 35CrMoA or 25CrMoVA.

According to the different structural types, bolts can be divided into two categories: hexagonal head bolts and stud bolts (also known as studs), and stud bolts are divided into through-thread and non-through-thread.

Hexagonal head bolts: also called single-sided bolts, when the tension is large, 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 with flat welding flanges and non-metallic gaskets for milder operating conditions. The commonly used materials for hexagonal head bolts are BL3 or Q235B;

Stud bolts: also called double-sided bolts. It can be used for medium- and high-pressure flanges, and it is convenient for tightening from both sides. It is often used with butt welding flanges under harsh operating conditions. Because there is no change in the cross-sectional shape of the through-thread stud bolts, their bearing capacity is strong. The non-through-thread stud bolts have relatively weak bearing capacity.

The nuts used for bolted flange are generally hexagonal and are divided into two types: AB. 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. Both sides of the B-shaped nut are rounded.

The nut material is often determined according to the bolt material it matches, and these combinations are specified in general standards. Generally, the nut material should be slightly lower than the bolt material, and the nut hardness should be about HB30 lower than the bolt hardness.

Bolts can be divided into coarse bolts, semi-finished bolts, and fine bolts according to the manufacturing method. Except for the threaded part, the rest of the coarse bolts are not processed, and the tension that can be borne is not high. The size specification of the bolt is expressed as “bolt diameter x bolt length”.

Bolted Flange Grade And Material

Strength grade mainly refers to commercial-grade carbon steel fasteners, and commercial-grade stainless steel bolts are distinguished by performance grade and material group.

The common strength grades of carbon steel bolts are 4.8, 5.8, 6.8, 8.8, 10.9, and 12.9. The nuts are 4, 6, 8, 10, and 12.

Bolts below grade 8.8 are generally called ordinary bolts, while those above grade 8.8 (including grade 8.8) are high-strength bolts. The difference is that high-strength bolts need to be tempered and heat-treated.

At present, standard parts on the market mainly include three materials: carbon steel, stainless steel, and copper.

When it comes to carbon steel, there are various types distinguished by their carbon content. These include low-carbon, medium-carbon, high-carbon, and alloy steel. 1. Low carbon steel, with a C%≤0.25% carbon content, is commonly known as A3 steel in China and internationally recognized as 1008, 1015, 1018, 1022, and similar classifications.

Mainly used for products without hardness requirements such as 4.8-grade bolts, 4-grade nuts, and small screws. Generally, 1008 material is used to make ordinary-grade products, such as 4.8-grade screws and ordinary-grade nuts; 1015 is generally used for eyebolts; 1018 is generally used for machine screws, and of course, it is also used for self-tapping screws; (Note: Self-tapping screws are mainly made of 1022 material.)

Medium carbon steel with a carbon content between 0.25% and 0.45% is referred to as 35 and 45 steel in China and known as 1035, ch38f, 1039, 40acr, etc. internationally. This type of steel is primarily used for producing grade 8 nuts, grade 8.8 bolts, and grade 8.8 hexagonal products.

Alloy steel: Alloy elements are added to ordinary carbon steel to increase some special properties of steel: such as 35, 40 chromium-molybdenum, SCM435 (Japanese steel grade, corresponding to domestic 35CrMo), 10B38. 1045/10B21/40Cr is used to make 10.9 grade screws; 40Cr/SCM435 is used to make 12.9 grade screws.

Fangsheng Screws (a trademark brand in Taiwan) use SCM435 chromium-molybdenum alloy steel, the main components of which are C, Si, Mn, P, S, Cr, and Mo.

Currently, the imported materials on the market are mainly Japanese products. According to the grade, they are mainly divided into SUS302, SUS304, and SUS316.

Example: A2-70 indicates A2 group steel, 70 represents the product strength grade of 70, the tensile strength of bolts, screws, and studs reaches more than 700MPa, the guaranteed load of nuts meets the requirements of 70 grade, and the nuts should not be disengaged or broken when the corresponding load force is loaded and maintained for 15s.

For low-carbon austenitic stainless steel with a carbon content of less than 0.03%, the letter “L” can be added. For example: A4L-80.

Critical Points For Selecting

Bolted Flange

When selecting bolts for flange connection, the type, size, number, and material grade of the bolts should be determined.

The type, material, quantity, and size of the bolts used for the flange mainly depend on the flange’s nominal diameter, nominal pressure, and operating temperature, and they can be selected according to the corresponding flange technical standards.

The number of bolts is usually a multiple of 4, which is convenient for tightening with the cross method.

The hardness of the nut should not exceed the hardness of the bolt.

Generally, washers are not provided. When the thread length on the screw is too short to be tightened, a flat washer can be added, and washers must not be stacked.