Why is it not recommended to use 304 material for bolts when sealing flange joints?

When carbon steel or stainless steel flanges are equipped with 304 material bolts in flange joint sealing, leakage problems often occur during operation. This article 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 commonly used stainless steel materials in flange joints (including flanges, sealing elements, and fasteners).
304, 304L, 316, and 316L are stainless steel grade codes in material standards (ANSI or ASTM), belonging to the 300 series of austenitic stainless steel in Class I. The grades corresponding to domestic 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 18-8 stainless steel.
Refer to Tables 1304, 304L, 316, and 316L. Due to the different addition of alloying elements and amounts, their physical, chemical, and mechanical properties also differ. 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 due to its lower carbon content, it has stronger resistance to intergranular corrosion. 316 and 316L are molybdenum containing stainless steels, and due to the addition of molybdenum, their corrosion and heat resistance are superior to 304 and 304L. Similarly, due to the lower carbon content of 316L compared to 316, its resistance to crystal corrosion is superior. The mechanical strength of austenitic stainless steels such as 304, 304L, 316, and 316L is low. 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 210MPa, while that of 316L is 200MPa. Therefore, the bolts made with them belong to the low strength level of bolts.
Table 1 Carbon Content,% Room Temperature Yield Strength, MPa Recommended Maximum Operating 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 made of materials such as 304 and 316? 
As mentioned earlier, the flange joint is caused by the separation of the sealing surfaces of the two flanges due to internal pressure, resulting in a corresponding reduction in gasket stress. Secondly, it is caused by the relaxation of gasket creep or bolt creep under high temperature, which also leads to a decrease in gasket stress, resulting in leakage and failure of the flange joint.
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 cyclic conditions, after 10000 hours of operation, the bolt load loss often exceeds 50% and decays over time and with 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. For example, at 50 ℃, the thermal expansion coefficient of stainless steel (16.51 × 10-5/℃) is larger than that of carbon steel (11.12 × 10-5/℃). After the device is heated up, when the expansion of the flange is smaller than that of the bolt, after deformation coordination, the elongation of the bolt decreases, causing relaxation of the bolt force, It may cause leakage at the flange joint. Therefore, when connecting high-temperature equipment flanges and pipe flanges, especially when the thermal expansion coefficients of flange and bolt materials are different, try to make the thermal expansion coefficients of the two materials as close as possible.
As shown in (1), the mechanical strength of austenitic stainless steels such as 304 and 316 is low, with a room temperature yield strength of only 205 MPa for 304 and 210 MPa for 316. Therefore, in order to improve the ability of bolts to resist relaxation and fatigue, measures should be taken to increase the installation bolt force. For example, in the subsequent lecture, it will be mentioned that when using the maximum installation bolt force, the installation bolt stress is required to reach 70% of the yield strength of the bolt material. Therefore, it is necessary to improve the strength grade of the bolt material and use high-strength or medium strength alloy steel bolt materials. It is evident that, except for cast iron, non-metallic flanges, or rubber gaskets, low strength materials such as 304 and 316 bolts cannot meet the sealing requirements due to insufficient bolt force for semi metal and metal gaskets with high pressure rating flanges or gaskets with high stress.
Special attention should be paid here that there are two categories for 304 and 316 in the stainless steel bolt material standard, namely B8 Cl.1 and B8 Cl.2 for 304 and B8M Cl.1 and B8M Cl.2 for 316. Cl.1 undergoes carbide solution treatment, while Cl.2 undergoes strain strengthening treatment in addition to solution treatment. Although there is no fundamental difference in chemical corrosion resistance between B8 Cl.2 and B8 Cl.1, the mechanical strength of B8 Cl.2 has been significantly improved compared to B8 Cl.1. For example, the yield strength of B8 Cl.2 bolt material with a diameter of 3/4 "is 550MPa, while the yield strength of all diameter B8 Cl.1 bolt materials is only 205MPa, which is more than twice the difference between the two. The 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 "Design of Pressure Vessels Part III" 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 the Installation of Flange Joints" stipulate that it is not recommended to use the usual 304 (B8 Cl.1) and 316 (B8M Cl.1) bolts for pressure equipment flanges and pipe flange joints. Especially in high temperature and severe cyclic conditions, they should be replaced with B8 Cl.2 (S30408) and B8M Cl.2 to avoid low installation bolt force.
It is worth noting that when using high-strength bolt materials such as 304 and 316, and even during the installation phase, due to the lack of torque control, the bolt may have exceeded the material's yield strength and even fracture. Naturally, if leakage occurs during the pressure test or during operation, even if the bolts are tightened further, the bolt force cannot increase and cannot prevent leakage. In addition, these bolts cannot be reused after disassembly, as they have already undergone permanent deformation and the cross-sectional size of the bolts has decreased, making it easy to break them when reinstalled.