Flange joint sealing – why 304 material is not recommended for bolts?
In flange joint sealing, when carbon steel flanges or stainless steel flanges are equipped with 304 bolts, leakage problems often occur in operation. This lecture will make a qualitative analysis of this.
What are the basic differences between 304, 304L, 316 and 316L materials?
304, 304L, 316 and 316L are commonly used stainless steel materials in flange joints (including flanges, sealing elements and fasteners).
304, 304L, 316 and 316L are stainless steel type codes of American material standard (ANSI or ASTM), belonging to 300 series class I steel of austenitic stainless steel.The brands corresponding to the domestic material standard (GB/T) are 06cr19ni10 (304), 022cr19ni10 (304L), 06cr17ni12mo2 (316), 022cr17ni12mo2 (316L). This kind of stainless steel is generally referred to as 18-8 stainless steel.
Refer to table 1304, 304L, 316 and 316L. Due to different alloying elements and amounts, their physical, chemical and mechanical properties are also different. Compared with ordinary stainless steel, they have good corrosion resistance, heat resistance and processability. The corrosion resistance of 304L is similar to that of 304, but because the carbon content of 304L is lower than that of 304, it has stronger resistance to intergranular corrosion. 316 and 316L are molybdenum containing stainless steels. Due to the addition of molybdenum, their corrosion resistance and heat resistance are better than 304 and 304L.Similarly, because the carbon content of 316L is lower than that of 316, its crystal corrosion resistance is better. 304, 304L, 316 and 316L austenitic stainless steels have low mechanical strength. The room temperature yield strength of 304 is 205MPa and that of 304L is 170Mpa; The room temperature yield strength of 316 is 210MPa and 316L is 200MPa. Therefore, the bolts made of them belong to low strength bolts.
|Material||Carbon content,%||Room temperature yield strength, MPa||Recommended maximum operating temperature, ℃|
Why should bolts of materials such as 304 and 316 not be used for flange joints?
As mentioned in the previous talks, the flange joint is separated from the sealing surfaces of the two flanges due to the internal pressure, resulting in the corresponding reduction of gasket stress. The other is the relaxation of bolt stress due to the creep relaxation of gasket or the creep of bolt itself at high temperature, which also reduces the gasket stress and leads to leakage failure of flange joint.
In actual operation, bolt force relaxation is inevitable, and the bolt force of initial tightening will always drop with time. Especially for flange joints under high temperature and severe cycle conditions, after 10000 hours of operation, the bolt load loss often exceeds 50%, and attenuates with the extension of time and the increase of temperature.
When the flange and bolt are made of different materials, especially when the flange is made of carbon steel and the bolt is made of stainless steel, the thermal expansion coefficient 2 of the bolt and flange materials is different, such as the thermal expansion coefficient of stainless steel at 50 ℃ (16.51 × 10-5 / ℃ specific carbon steel thermal expansion coefficient (11.12) × 10-5 / ℃ is large. After the temperature rise of the device, when the expansion of the flange is less than that of the bolt, after the deformation is coordinated, the bolt elongation decreases, resulting in the relaxation of the bolt force, which may lead to the leakage of the flange joint. Therefore, when high-temperature equipment flange and pipe flange joints, especially the thermal expansion coefficients of flange and bolt materials are different, make the thermal expansion coefficients of the two materials as close as possible.
The mechanical strength of austenitic stainless steels such as 304 and 316 is low, and the room temperature yield strength of 304 is only 205MPa and 316 is only 210MPa. Therefore, in order to improve the anti relaxation and anti fatigue ability of bolts, measures to improve the installation bolt force are taken. For example, when the maximum installation bolt force is used, it will be mentioned in the subsequent forum that the installation bolt stress is required to reach 70% of the yield strength of bolt materiaL.In this way, it is necessary to improve the strength grade of bolt material and adopt high-strength or medium strength alloy steel bolt materiaL. Obviously, except for cast iron, non-metallic flange or rubber gasket, for semi-metallic and metal gaskets with high pressure grade flange or gasket stress, bolts of low-strength materials such as 304 and 316 can not meet the sealing requirements due to insufficient bolt force.
What needs special attention here is that in the American stainless steel bolt material standard, 304 and 316 have two categories respectively, namely B8CL.1 and B8CL. 2 of 304 and B8MCL.1 and B8MCL. 2 of 316. Cl.1 is treated by carbide solution, while CL.2 is treated by strain strengthening in addition to solution treatment. Although there is no fundamental difference in chemical corrosion resistance between B8CL.2 and B8CL.1, the mechanical strength of B8CL.2 is quite improved compared with B8CL.1. For example, the yield strength of B8CL.2 bolt material with diameter of 3/4″ is 550MPa, while the yield strength of B8CL.1 bolt material with all diameters is only 205MPa, which is more than twice the difference between the two. 06cr19ni10 (304) and 06cr17ni12mo2 (316) in domestic bolt material standards are equivalent to B8CL.1 and B8MCL.1[ Note: the bolt material s30408 in GB/T 150.3 pressure vessel part III design is equivalent to B8CL.2; S31608 is equivalent to B8MCL.1.
In view of the above reasons, GB/T 150.3 and GB/T 38343 technical regulations for installation of flange joints stipulate that the usual 304 (B8CL.1) and 316 (B8MCL.1) bolts are not recommended for pressure equipment flanges and pipe flange joints. Especially under high temperature and severe cycle conditions, they should be replaced with B8CL.2 (s30408) and B8MCL.2 to avoid low installation bolt force.
It should be noted that when low-strength bolt materials such as 304 and 316 are used, even in the installation stage, because the torque is not controlled, the bolt may have exceeded the material yield strength or even fracture. Naturally, if there is leakage during the pressure test or operation, even if the bolts are tightened continuously, the bolt force cannot go up and the leakage cannot be stopped. In addition, these bolts cannot be reused after disassembly, because the bolts have been permanently deformed and the cross-section size of the bolts has become smaller, which is easy to be broken when they are installed again.
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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