For the sealing of flange joints, why is it not recommended to use 304 material for the bolts?

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In flange joint sealing, when carbon steel or stainless steel flanges are paired with 304 material bolts, leakage problems often occur during operation. This lecture will provide 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 materials commonly used in flanged joints (including flanges, sealing elements, and fasteners). 304, 304L, 316, and 316L are stainless steel grades designated by the American Material Standards (ANSI or ASTM), and they belong to the 300 series of austenitic stainless steels. The grades corresponding to domestic material standards (GB/T) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), and 022Cr17Ni12Mo2 (316L). These types of stainless steel are generally referred to as 18–8 stainless steel. As shown in Table 1, 304, 304L, 316, and 316L exhibit different physical, chemical, and mechanical properties due to the various alloying elements used and their respective amounts. Compared to ordinary stainless steels, they possess excellent corrosion resistance, heat resistance, and machinability. The corrosion resistance of 304L is similar to that of 304, but due to its lower carbon content compared to 304, it has greater 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 superior to those of 304 and 304L. Similarly, since 316L has a lower carbon content than 316, it possesses better resistance to intergranular corrosion. Austenitic stainless steels such as 304, 304L, 316, and 316L have low mechanical strength; the room-temperature yield strength of 304 is 205 MPa, while that of 304L is 170 MPa ； The room-temperature yield strength of 316 is 210 MPa, while that of 316L is 200 MPa. Therefore, the bolts made from them are of low-strength grade. Table 1: Carbon content, %; Yield strength at room temperature, MPa; Recommended maximum operating temperature, °C
304: ≤0.08, 205, 816
304L: ≤0.03, 170, 538
316: ≤0.08, 210, 816
316L: ≤0.03, 200, 538

(2) Why should bolts made of materials such as 304 and 316 not be used in flange joints? As explained in previous sections, flange joints experience separation between their sealing surfaces due to internal pressure, which leads to a reduction in the stress on the gaskets. Additionally, at high temperatures, the gaskets may creep and relax, or the bolts themselves may creep, resulting in a decrease in bolt tension and thus a reduction in gasket stress, ultimately causing leakage and failure of the flange joint. In actual operation, bolt force relaxation is inevitable; the initial tightening force of the bolts always decreases over time. Especially in flange joints subjected to high temperatures and severe cyclic conditions, after 10,000 hours of operation, the bolt load loss often exceeds 50%, and this loss continues to increase over time as well as with rising temperatures. When the flange and bolts are made of different materials, especially when the flange is made of carbon steel and the bolts are made of stainless steel, the difference in their thermal expansion coefficients leads to problems. For example, at 50°C, the thermal expansion coefficient of stainless steel (16.51×10-5/°C) is higher than that of carbon steel (11.12×10-5/°C). When the temperature of the device rises, if the flange expands less than the bolts, then after deformation adjustment, the decrease in the elongation of the bolts results in a reduction in the bolt tension, which can lead to leakage at the flange joint. Therefore, when joining high-temperature equipment flanges and pipe flanges, especially when the coefficients of thermal expansion of the flange and bolt materials differ, it is necessary to make the coefficients of thermal expansion of these two materials as similar as possible. As can be seen from (1), austenitic stainless steels such as 304 and 316 have low mechanical strength; the room-temperature yield strength of 304 is only 205 MPa, and that of 316 is also only 210 MPa. Therefore, in order to improve the resistance of bolts to relaxation and fatigue, measures are taken to increase the tightening force of the bolts. As will be discussed in subsequent lectures, when using the maximum allowable tightening force, it is required that the stress on the bolts reach 70% of the yield strength of the bolt material; hence, it is necessary to raise the strength grade of the bolt material by using high-strength or medium-strength alloy steel bolts. It is evident that, except for cast iron, non-metallic flanges, or rubber gaskets, for flanges with higher pressure ratings or semi-metallic and metallic gaskets that are subject to high stresses, bolts made of low-strength materials such as 304 and 316 fail to meet the sealing requirements due to insufficient bolt strength. It is important to note here that in the U.S. standards for stainless steel bolt materials, 304 and 316 each have two categories: 304 has B8 Cl.1 and B8 Cl.2, while 316 has B8M Cl.1 and B8M Cl.2. Cl.1 has undergone carbide solid solution treatment, while Cl.2, in addition to solid solution treatment, has also been subjected to strain strengthening 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 significantly higher compared to B8 Cl.1; for example, the yield strength of B8 Cl.2 bolts with a diameter of 3/4” is 550 MPa, whereas the yield strength of B8 Cl.1 bolts of all diameters is only 205 MPa, representing a difference of more than twice. The bolt material standards in China, 06Cr19Ni10 (304) and 06Cr17Ni12Mo2 (316), are equivalent to B8 Cl.1 and B8M Cl.1. For the above reasons, GB/T 150.3 and GB/T 38343 \"Technical Specifications for the Installation of Flange Joints\" stipulate that for flanges of pressure equipment and pipe flange joints, it is not recommended to use bolts made of the common materials 304 (B8 Cl.1) and 316 (B8M Cl.1); especially under high-temperature and highly cyclic operating conditions, these should be replaced with B8 Cl.2 (S30408) and B8M Cl.2 in order to avoid low bolt tightening forces. It is worth noting that when low-strength bolt materials such as 304 and 316 are used, even during the installation phase, the bolts may exceed the yield strength of the material due to a lack of torque control, and may even break. Naturally, if a leak occurs during the pressure test or at the time of startup, even if the bolts are tightened further, the bolt tension will not increase enough to stop the leak. Furthermore, these bolts cannot be reused after removal, as they have suffered permanent deformation and their cross-sectional dimensions have decreased, making them prone to breaking when reinstalled.