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Why is 304 material not recommended for bolts?
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(1) What are the basic differences between 304, 304L, 316 and 316L materials?
304, 304L, 316 and 316L are commonly used stainless steel materials for flange joints (including flanges, sealing elements and fasteners).
304, 304L, 316 and 316L are stainless steel grade codes in the American material standard (ANSI or ASTM), belonging to the 300 series of austenitic stainless steel. The corresponding grades in the domestic material standard (GB/T) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), 022Cr17Ni12Mo2 (316L). This type of stainless steel is usually referred to as 18-8 stainless steel.
As shown in Table 1, 304, 304L, 316, and 316L have different physical, chemical, and mechanical properties due to the addition of alloying elements and the amounts added. Compared to ordinary stainless steels, they offer excellent corrosion resistance, heat resistance, and processability. 304L's corrosion resistance is similar to that of 304, but because 304L has a lower carbon content than 304, it has greater resistance to intergranular corrosion. 316 and 316L are molybdenum-containing stainless steels. Due to the addition of molybdenum, their corrosion and heat resistance are superior to those of 304 and 304L. Similarly, because 316L has a lower carbon content than 316, it has 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, bolts made from these materials are classified as low-strength bolts.
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 shouldn't flange joints use bolts made of materials such as 304 and 316?
As mentioned in the previous lectures, flange joints are caused by the separation of the two flange sealing surfaces due to internal pressure, which causes the gasket stress to decrease accordingly. Secondly, the bolt force relaxes due to creep relaxation of the gasket or the creep of the bolt itself at high temperature, which also causes the gasket stress to decrease, 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 exposed to high temperatures and intense cycling conditions, bolt load loss often exceeds 50% after 10,000 hours of operation, and this loss decreases with time and increasing temperature.
When flanges and bolts are made of different materials, especially when the flange is carbon steel and the bolts are stainless steel, the thermal expansion coefficients of the bolts and flange materials differ. For example, at 50°C, the thermal expansion coefficient of stainless steel (16.51×10⁻⁵/°C) is greater than that of carbon steel (11.12×10⁻⁵/°C). When the equipment heats up, if the flange expands less than the bolt, the deformation will be balanced, and the bolt elongation will decrease, causing the bolt force to relax, potentially leading to leakage in the flange joint. Therefore, for high-temperature equipment flanges and pipe flange joints, especially when the thermal expansion coefficients of the flange and bolt materials are different, it is best to keep the thermal expansion coefficients of the two materials as close as possible.
As can be seen from (1), the mechanical strength of austenitic stainless steels such as 304 and 316 is low. 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 capabilities of bolts, measures are taken to increase the installation bolt force. For example, in subsequent lectures, it will be mentioned that when the maximum installation bolt force is used, the installation bolt stress is required to reach 70% of the bolt material yield strength. In this way, the strength grade of the bolt material must be increased, and high-strength or medium-strength alloy steel bolt materials must be used. Obviously, except for cast iron, non-metallic flanges or rubber gaskets, for flanges with higher pressure ratings or semi-metallic and metallic gaskets with greater gasket stress, low-strength material bolts such as 304 and 316 cannot meet the sealing requirements due to insufficient bolt force. It is important to note that in the US stainless steel bolt material standard, 304 and 316 have two categories: 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 both solution treatment and strain hardening. Although there is no fundamental difference between B8 Cl.2 and B8 Cl.1 in terms of chemical corrosion resistance, the mechanical strength of B8 Cl.2 is 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 B8 Cl.1 bolt material of all diameters is only 205MPa, which is more than twice the difference. 06Cr19Ni10(304) and 06Cr17Ni12Mo2(316) in the 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 3" is equivalent to B8 Cl.2; S31608 is equivalent to B8M Cl.1.
For the above reasons, GB/T 150.3 and GB/T38343, "Technical Regulations for Flange Joint Installation," stipulates that the use of standard 304 (B8 Class 1) and 316 (B8M Class 1) bolts for pressure equipment flanges and pipe flange joints is not recommended. Especially in high-temperature and severe cycling conditions, B8 Class 2 (S30408) and B8M Class 2 should be used instead to avoid low installation bolt force.
It is worth noting that when using low-strength bolt materials such as 304 and 316, even during installation, due to lack of torque control, the bolts may exceed their yield strength and even break. Naturally, if leakage occurs during pressure testing or initial operation, further tightening will not increase the bolt force and prevent leakage. Furthermore, these bolts cannot be reused after removal due to permanent deformation and reduced cross-sectional dimensions, making them more susceptible to breakage during reinstallation.