Introduction to bolts:
The flanges are drilled to receive bolts which, if of the proper material, will develop the full flange strength and result in a satisfactory joint at the service intended.
The term ‘bolting material’ covers bolts, studs and stud bolts.
As per ASME B16.5, one can use either square head machine bolts or studs for making a joint. Bolts and studs are available in either fully threaded or partially threaded condition. The fully threaded structure ensures an equal distribution of elongation when the bolts are tightened and avoid concentration of stresses that exist in partially threaded stock due to varying cross-sections.
Also, studs are preferred over bolts due to ease of insertion during assembly. At places, like bolting of two valves together, there may not be sufficient clearance available for inserting bolts due to valve body contour and use of studs is the only possible solution
Classification of bolting material:
Since bolting material does not come in contact with fluid, its material compatibility with fluid is not important. The selection of bolt material is determined based on service conditions and it is wasteful to specify expensive alloys when carbon steel material is entirely suitable. It is important for bolting material to have good tensile stress.
As per ASME B16.5, bolting material has been divided into three categories as follows:
i. High strength bolting:
Bolting materials having allowable stresses not less than those for ASTM A193 grade B7 are listed as high strength bolts. ASME B16.5 Table 1B is enclosed for reference.
These and other materials of comparable strength may be used in any flanged joint.
ii. Intermediate strength bolting:
Bolting materials listed as intermediate strength in Table 1B, and other bolting of comparable strength, may be used in any flanged joint, provided the user verifies their ability to seat the selected gasket and maintain a sealed joint under expected operating conditions.
iii. Low strength bolting:
Bolting materials having not more than 30 ksi specified minimum yield strength are listed as low strength in Table 1B. These materials and others of comparable strength shall be used only in Classes 150 and 300 joints, and only with gaskets of group no. 1a. Flanged joints using low strength carbon steel bolts shall not be used above 400°F or below -20°F.
Applications of bolts:
i. For flanged joints in piping system where one or both flanges are of cast iron, bolts / studs conforming to ASTM A307 Gr. B and nuts conforming to A194 Gr. 2 are usually used.
Equivalent IS standards for such fasteners is IS 1367 CI. 6.6 / IS 1367 CI. 6.
ii. For carbon steel flanges having ASME 150 rating on non-critical services, bolts / studs conforming to ASTM A307 and nuts conforming to ASTM A194 Gr. 2 or fasteners conforming to IS 1367 CI. 6.6 / IS 1367 CI. 6 are usually used.
iii. For carbon steel flanges on process and critical services involving medium and moderately high pressures and temperatures, bolts / studs conforming to ASTM A193 Gr. B7 along with heavy hexagonal head nuts conforming to ASTM A194 Gr. 2H are usually used.
iv. For very severe service conditions involving high temperatures upto 1100°F bolts /studs conforming to ASTM A193 Gr. B7 along with nuts conforming to ASTM A194 are usually used.
v. For low temperature carbon steel flanges, studs conforming to ASTM A320 Gr. L7 along with nuts conforming to ASTM A194 Gr. 4 are usually used.
vii. Sometimes in plastic piping, handling corrosive fluids, it is recommended that the fasteners are supplied with cadmium plating or galvanized coating. This is in order to have corrosion resistance against spillages, etc.
Dimensional standards for bolts:
Dimensional standards usually adopted for fasteners are as follows:
Studs bolts: ASME B 18.2.1
Studs: DIN 976 Type B / DIN 934
Nuts: ASME B 18.2.2
Male Threads: ASME B 1.1 C1. 2A
Coarse series (UNC)
Female Threads: ASME B 1.1 C1. 2A
Bolting material shall not be used beyond temperature limits specified in the governing code.
- Repair welding of bolting material is prohibited.
- These bolting materials may be used with all listed materials and gaskets.
- These bolting materials may be used with all listed materials and gaskets, provided it has been verified that a sealed joint can be maintained under rated working pressure and temperature.
- These bolting materials may be used with all listed materials that are limited to Classes 150 and 300 joints. See para. 5.4.1 for required gasket practices.
- These materials may be used as bolting with comparable nickel and special alloy parts.
- This austenitic stainless steel material has been carbide solution treated but not strain hardened. Use A194
- Nuts may be machined from the same material or may be of a compatible grade of ASTM A194.
- Maximum operating temperature is arbitrarily set at 500°F, unless material has been annealed, solution annealed or hot finished, because hard temper adversely affects design stress in the creep rupture stage.
- Forging quality not permitted, unless the producer last testing or working these parts, tests them as required for other permitted conditions in the same specification and certifies their tensile, yield and elongation properties to equal or exceed the requirements for one of the other permitted conditions.
10. This ferritic material is intended for low temperature service. Use A194 Grade 4 or Grade 7 nuts.
11. This austenitic stainless steel material has been carbide solution treated and stain hardened. Use A194 nuts of corresponding material.
12. This carbon steel fastener shall not be used above 400°F or below -20°F. See also note (4). Bolts with drilled or undersized hands shall not be used.
13. Acceptable nuts for use with quenched and tempered bolts are A194 Grades 2 and 2H. Mechanical property requirements for studs shall be the same as those for bolts.
14. This special Alloy is intended for high temperature service with austenitic stainless steel.