In flange joint sealing, when carbon steel flanges or stainless steel flanges are equipped with 304 material 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.
Table.1 carbon content,% room temperature yield strength, MPa, recommended maximum service 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 device is heated up, 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 the high-temperature equipment flange and pipe flange joint, especially the thermal expansion coefficients of the flange and bolt materials are different, the thermal expansion system of the two materials shall be improved as much as possible The number is similar.
It can be seen that 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 materials In this way, it is necessary to improve the strength grade of bolt materials and use high-strength or medium strength alloy steel bolt materials. It is obvious that except cast iron, non-metallic flange or rubber gasket, for semi-metallic and metal gaskets with high pressure grade flange or gasket stress, bolts with low strength materials such as 304 and 316 cannot 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 subject to carbide solution treatment, while CL.2 is subject to strain strengthening treatment in addition to solution treatment. Although there is no fundamental difference between b8cl.2 and b8cl.1 in terms of chemical corrosion resistance, the force relative to b8cl.1 and b8cl.2 For example, the yield strength of b8cl.2 bolt material with a 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. 06cr19ni10 (304) and 06cr17ni12mo2 (316) in domestic bolt material standards are equivalent to b8cl.1 and b8mcl.1. [Note: GB/T150.3 pressure vessel part III design] The bolt material s30408 in is equivalent to b8cl.2; s31608 is equivalent to b8mcl.1.
In view of the above reasons, GB/T150.3 and GB/T38343 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 yield strength of the material and even fracture. Naturally, if there is leakage during the pressure test or start of operation, the bolt force cannot go up and prevent the leakage even if the bolts are tightened continuously. In addition, these bolts are disassembled It can not be reused after installation, because the bolt has been permanently deformed and the cross-section size of the bolt becomes smaller, it is easy to break when it is installed again.
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