Why 304 material is not recommended for bolts in flange joint sealing
304, 304L, 316 and 316L are stainless steel codes of American material standard (ANSI or ASTM), belonging to 300 series of austenitic stainless steel. The corresponding grades of GB/T are 06cr19ni10 (304), 022cr19ni10 (304L), 06cr17ni12mo2 (316), 022cr17ni12mo2 (316L). This kind of stainless steel is generally called 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, its intergranular corrosion resistance is stronger. 316L and 316L are molybdenum containing stainless steels. Due to the addition of molybdenum, their corrosion resistance and heat resistance are better than those of 304 and 304L. Similarly, because the carbon content of 316L is lower than that of 316, its resistance to crystal corrosion is better. The mechanical strength of 304, 304L, 316 and 316L austenitic stainless steels is low. The room temperature yield strength of 304 is 205 MPa, and that of 304L is 170 MPa; The room temperature yield strength of 316 is 210 MPa and that of 316L is 200 MPa. Therefore, the bolts made of them belong to the low strength grade.
Table.1 Physicochemical and mechanical properties of 304, 304L, 316 and 316L
|Code of stainless steel||Carbon content,%||Room temperature yield strength, Mpa||Recommended maximum temperature, ℃|
Why should 304 bolts and 316 bolts not be used for flange joints?
As mentioned in the previous sections, firstly, the internal pressure causes the separation of the two flange sealing surfaces of the flange joint, resulting in the corresponding reduction of the gasket stress; secondly, the creep relaxation of the gasket or the bolt itself causes the bolt stress relaxation under high temperature, which also causes the gasket stress to drop, resulting in the leakage failure of the flange joint.
In actual operation, the bolt force relaxation is inevitable, and the initial tightening bolt force will always drop with time. Especially for flange joints under high temperature and severe cycle conditions, the bolt load loss often exceeds 50% after 10000 hours of operation, and decays 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 coefficient of thermal expansion 2 of the bolt and flange materials are different, such as the coefficient of thermal expansion of stainless steel (16.51) at 50 ℃ × The coefficient of thermal expansion of carbon steel is (11.12 10-5/℃) × When the expansion of the flange is less than that of the bolt, the elongation of the bolt decreases after the deformation is coordinated, 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 joint, especially the thermal expansion coefficient of flange and bolt material is different, make the thermal expansion coefficient of both materials close as far as possible.
It can be seen from (1) that the mechanical strength of 304 and 316 austenitic stainless steels is low. The room temperature yield strength of 304 is only 205 Mpa, and that of 316 is only 210 Mpa. Therefore, in order to improve the ability of anti relaxation and anti fatigue of bolts, measures should be taken to improve the installation bolt force. For example, when the maximum installation bolt force is used, the installation bolt stress is required to reach 70% of the yield strength of bolt material. In this way, the strength grade of bolt material must be improved, and high strength or medium strength alloy steel bolt material should be used. It is easy to see that except for cast iron, non-metallic flange or rubber gasket, for semi-metallic and metal gaskets with higher pressure rating flange or gasket stress, 304 and 316 bolts with low strength materials can not meet the sealing requirements due to insufficient bolt force.
In the American Standard for stainless steel bolt materials, 304 and 316 are classified into two categories: B8 CL.1 and B8 CL.2 for 304 and B8 MCL.1 and B8 CL.2 for 316. B8 MCL.1 is treated by solid solution of carbide, while B8 MCL.2 is treated by strain hardening besides solid solution. Although there is no fundamental difference between B8 CL.2 and B8 CL.1 in chemical corrosion resistance, the mechanical strength of B8 CL.2 is quite improved compared with B8 CL.1. For example, the yield strength of B8 CL.2 bolt material with 3/4 “diameter is 550MPa, while that of B8 CL.1 bolt material with all diameters is only 205mpa, which is more than twice the difference. 06cr19ni10 (304) and 06cr17ni12mo2 (316) in domestic bolt material standards are equivalent to B8 CL.1 and B8M CL.1[ Note: the bolt material S30408 in GB/T 150.3 pressure vessels Part 3 design is equivalent to B8 CL.2; S31608 is equivalent to B8M CL.1].
In view of the above reasons, GB/T 150.3 and GB/T 38343 “technical regulations for flange joint installation” stipulate that the bolts made of 304 (B8 CL.1) and 316 (B8M CL.1) materials are not recommended for pressure equipment flange and pipe flange joint, especially in high temperature and severe cycle conditions, B8 CL.2 (s30408) and B8M CL.2 should be replaced to avoid low installation bolt force.
It is worth noting that when 304 and 316 low strength bolt materials are used, even in the installation stage, due to the lack of torque control, the bolt may have exceeded the material yield strength, or even fracture. Naturally, if leakage occurs during pressure test or at the beginning of operation, even if the bolt continues to be tightened, the bolt force will not go up and the leakage cannot be prevented. In addition, these bolts can not be reused after disassembly, because the bolts have produced permanent deformation, and the cross-section size of the bolts becomes smaller, and it is easy to break after re installation.
Source: China High Strength Bolts Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.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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