Why Is It Not Recommended to Use SS304 for Sealing Bolts of Flange Connections?

When the carbon steel or stainless steel flange is equipped with SS 304 bolts in the flange's sealing, leakages often occur in the operating state.
(1) What are the differences between 304, 304L, 316 and 316L?
304, 304L, 316 and 316L are commonly used stainless steels in flange connections, including flanges, sealing elements and fasteners. 304, 304L, 316 and 316L are the stainless steel grades of American Standard of Testing Materials (ANSI or ASTM), which belong to the 300 series of austenitic stainless steel. The corresponding Chinese material standards (GB/T) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), and 022Cr17Ni12Mo2 (316L). Usually, this type of stainless steel is collectively referred to as 18 to 8 stainless steel. ‍
 
304, 304L, 316 and 316L have different physical, chemical and mechanical properties due to the different alloying elements and addition amounts (Table 1). Compared with ordinary stainless steel, they have good corrosion resistance, heat resistance and processing properties. The corrosion resistance of 304L is similar to that of 304. However, the carbon content of 304L is lower than that of 304. Its resistance to intergranular corrosion is better. 316 and 316L are stainless steels containing molybdenum. Their corrosion resistance and heat resistance are better than those of 304 and 304L due to the addition of molybdenum. In the same way, because the carbon content of 316L is lower than that of 316, its resistance to crystal corrosion is better. Austenitic stainless steels such as 304, 304L, 316 and 316L have low mechanical strength. The yield strength of 304 at a room temperature is 205MPa and that of 304L is 170MPa. The yield strength of 316 at room temperatures is 210MPa and that of 316L is 200MPa. Therefore, the bolts made from them are bolts with low strength.
 
Table 1 Chemical and mechanical properties of different stainless steels

 
(2) Why aren't bolts used for flange connections made from good materials such as 304 and 316?
As mentioned above, the sealing surfaces of the two flanges are separated due to internal pressure, which causes the gasket's stress to be reduced accordingly. The bolt's force looses due to the creep of the gasket under high temperatures or the creep of the bolt itself, which also reduces the stress of the gasket and even causes the flange connection to leak.
 
In actual operation, the loosening of the bolt force 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's load loss will often exceed 50%, and it will attenuate with the continuation of time and increase of temperatures.
 
For the flange and bolt made from different materials, especially when the flange is made from carbon steel and the bolt stainless steel, the thermal expansion coefficients of the material of the bolt and flange are different. The thermal expansion coefficient of stainless steel at a temperature of 50°C is 16.51×10-5/℃, which is greater than that of carbon steel (11.12×10-5/℃). After the device being heated up, when the expansion of the flange is less than that of the bolt, after the deformation is coordinated, the bolt's elongation decreases and causes the bolt's force to loose. This may cause the flange joint to leak. Therefore, when the high-temperature equipment flange is connected to pipe flanges, the thermal expansion coefficients of materials of flanges and bolts are especially 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. The yield strength of 304 at room temperatures is only 205MPa, and that of 316 is only 210MPa. Therefore, in order to improve the ability of bolts to resist relaxation and fatigue, measures are taken to increase the mounting bolt force. When the maximum mounting bolt force is adopted, it is required that the installation bolt stress reaches 70% of the yield strength of the bolt's material. In this way, the strength of the bolt's material must be improved, and materials of alloy steel bolts with high strength or medium strength must be used. It's noticeable that bolts made from 304 and 316 should be used for flanges with high pressure rating, semi-metal and metal gaskets with great stress.
 
What needs special attention here is that there are two categories of 304 and 316 in the material standard of American stainless steel bolts, namely 304 B8 Cl.1 and B8 Cl.2 and 316 B8M Cl.1 and B8M Cl.2. Cl.1 has undergone carbide solution treatment, while Cl.2 has undergone strain strengthening treatment in addition to the solution treatment. Although there is no fundamental difference between B8 Cl.2 and B8 Cl.1 in chemical resistance, the mechanical strength of B8 Cl.2 is considerably improved compared to B8 Cl.1. For example, the yield strength of the bolt made from B8 Cl.2 of 3/4 inch is 550MPa, and that of the bolt made from B8 Cl.1 is only 205MPa, which is more than double. The domestic bolt materials 06Cr19Ni10(304) and 06Cr17Ni12Mo2(316) are the same as B8 Cl.1 and B8M Cl.1. Please note that the bolt's material which is S30408 is equivalent to B8 Cl.2 and S31608 to B8M Cl.1 in GB/T 150.3 "Design of Pressure Vessel Part III".
 
Given the above reasons, GB/T 150.3 and GB/T38343 Technical Regulations of Installation of Flange Connections stipulate that the bolts made from 304 (B8 Cl.1) and 316 (B8M Cl) are not recommended for pressure-bearing equipment flanges and pipe flange joints, especially in high-temperature and severe cycle conditions, which should be replaced with B8 Cl.2 (S30408) and B8M Cl.2 to avoid low mounting bolt force.
 
It is worth noting that bolts made from materials with low strength such as 304 and 316 may have exceeded the material's yield strength and even break during the installation stage because the torque is not controlled. Of course, when there is a leak in the pressure test or the start of the operation, even if the bolts are tightened continuously, the bolt force will not increase and the leakage cannot be prevented. In addition, these bolts cannot be reused after disassembly. Because the bolts have been permanently deformed and the cross-sectional size of the bolts becomes smaller, the bolts are prone to breakage after installation.