Flange Joint Sealing - Why is 304 material not recommended for bolts?
When carbon steel or stainless steel flanges are used with 304 material bolts in flange joint sealing, leakage problems often occur during operation. This lecture will make a qualitative analysis of this.
(1) What are the basic differences between 304, 304L, 316 and 316L materials?
304, 304L, 316 and 316L are the stainless steel grades commonly used in flanged joints including flanges, sealing elements and fasteners.
304, 304L, 316 and 316L are the stainless steel grade designations of the American Standard for Materials (ANSI or ASTM), which belong to the 300 series of austenitic stainless steels. The grades corresponding to the domestic material standards (GB/T) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), 022Cr17Ni12Mo2 (316L). This type of stainless steel is usually collectively referred to as 18-8 stainless steel.
See Table 1, 304, 304L, 316 and 316L have different physical, chemical and mechanical properties due to the addition of alloying elements and amounts. Compared with ordinary stainless steel, they have good corrosion resistance, heat resistance and processing performance . 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 resistance to intergranular corrosion is stronger. 316 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. In the same way, because the carbon content of 316L is lower than that of 316, its ability to resist crystal corrosion is better. Austenitic stainless steels such as 304, 304L, 316 and 316L have low mechanical strength. The room temperature yield strength of 304 is 205MPa, 304L is 170MPa; the room temperature yield strength of 316 is 210MPa, and 316L is 200MPa. Therefore, the bolts made of them belong to the low-strength grade bolts.
Table 1 Carbon content, % Room temperature yield strength, MPa Recommended maximum service temperature, ℃
304 ≤0.08 205 816
304L ≤0.03 170 538
316 ≤0.08 210 816
316L ≤0.03 200 538
(2) Why should flange joints not use bolts of materials such as 304 and 316?
As mentioned in the previous lectures, the flange joint firstly separates the sealing surfaces of the two flanges due to the action of internal pressure, resulting in a corresponding decrease in the stress of the gasket, and secondly, the relaxation of the bolt force due to the creep relaxation of the gasket or the creep of the bolt itself at high temperature , also reduces the stress of the gasket, so that the flange joint leaks and fails.
In actual operation, bolt force relaxation is inevitable, and the initial tightening bolt force will always drop over time. Especially for flange joints under high temperature and severe cycle conditions, after 10,000 hours of operation, the bolt load loss will often exceed 50%, and it will attenuate with the continuation of time and the increase of temperature.
When the flange and the 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 material of the bolt and the flange is different, such as the thermal expansion coefficient of stainless steel at 50°C (16.51×10-5/ ℃) is larger than the thermal expansion coefficient of carbon steel (11.12×10-5/℃). After the device is heated up, when the expansion of the flange is smaller than the expansion of the bolt, after the deformation is coordinated, the elongation of the bolt decreases, causing the force of the bolt to decrease. If there is any looseness, it may cause leakage in the flange joint. Therefore, when the high-temperature equipment flange and pipe flange are connected, especially the thermal expansion coefficients of the flange and bolt materials are different, the thermal expansion coefficients of the two materials should be as close as possible.
It can be seen from (1) that the mechanical strength of austenitic stainless steel such as 304 and 316 is low, and the room temperature yield strength of 304 is only 205MPa, and that of 316 is only 210MPa. Therefore, in order to improve the anti-relaxation and anti-fatigue ability of bolts, measures to increase the bolt force of the installation bolts are taken. For example, when the maximum installation bolt force is used in the follow-up forum, it is required that the stress of the installation bolts reaches 70% of the yield strength of the bolt material , so that the strength grade of the bolt material must be improved, and high-strength or medium-strength alloy steel bolt materials are used. Obviously, except for cast iron, non-metallic flanges or rubber gaskets, for semi-metallic and metal gaskets with higher pressure grade flanges or gaskets with greater stress, bolts made of low-strength materials such as 304 and 316, due to bolt force Not enough to meet the sealing requirements.
What needs special attention here is that in the American stainless steel bolt material standard, 304 and 316 have two categories, namely B8 Cl.1 and B8 Cl.2 of 304 and B8M Cl.1 and B8M Cl.2 of 316. Cl.1 is solid solution treated with carbides, while Cl.2 undergoes strain strengthening treatment in addition to solid solution treatment. Although there is no fundamental difference in chemical resistance between B8 Cl.2 and B8 Cl.1, the mechanical strength of B8 Cl.2 is considerably improved relative to B8 Cl.1, such as B8 Cl.2 with a diameter of 3/4" The yield strength of the bolt material is 550MPa, while the yield strength of the B8 Cl.1 bolt material of all diameters is only 205MPa, the difference between the two is more than twice. The domestic bolt material standards 06Cr19Ni10(304), 06Cr17Ni12Mo2(316), and B8 Cl.1 is equivalent to B8M Cl.1. [Note: The bolt material S30408 in GB/T 150.3 "Pressure Vessel Part Three 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/T38343 "Technical Regulations for Flange Joint Installation" stipulate that the flanges of pressure equipment and pipe flange joints are not recommended to use the usual 304 (B8 Cl.1) and 316 (B8M Cl. .1) Bolts of materials, especially in high temperature and severe cycle conditions, should be replaced with B8 Cl.2 (S30408) and B8M Cl.2 to avoid low installation bolt force.
It is worth noting that when low-strength bolt materials such as 304 and 316 are used, even during the installation stage, because the torque is not controlled, the bolt may have exceeded the yield strength of the material, or even fractured. Naturally, if leakage occurs during the pressure test or start of operation, even if the bolts continue to be tightened, the bolt force will not go up and the leakage cannot be stopped. In addition, these bolts cannot be reused after being disassembled, because the bolts have undergone permanent deformation, and the cross-sectional size of the bolts has become smaller, and they are prone to breakage after reinstallation.