When carbon steel or stainless steel flanges are used with 304 material bolts in flange joint sealing, leakage problems often occur during operation. This article will provide a qualitative analysis of this issue.
(1) What are the basic differences between 304, 304L, 316 and 316L materials?
304, 304L, 316 and 316L are the commonly selected stainless steel grades for flange joints (including flanges, sealing elements and fasteners).
304, 304L, 316 and 316L are the stainless steel grades designated by the American material standards (ANSI or ASTM), belonging to the 300 series of austenitic stainless steel.
The corresponding grades in the domestic material standards (GB/T) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), and 022Cr17Ni12Mo2 (316L). Generally, this type of stainless steel is collectively referred to as 18-8 stainless steel.
Due to the differences in the types and amounts of alloying elements added, the physical, chemical and mechanical properties of 304, 304L, 316 and 316L are also different. Compared with ordinary stainless steel, they have excellent 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.
Similarly, because the carbon content of 316L is lower than that of 316, its resistance to intergranular corrosion is even better. The mechanical strength of austenitic stainless steels such as 304, 304L, 316 and 316L is relatively low.
The room temperature yield strength of 304 is 205 MPa, and that of 304L is 170 MPa; the room temperature yield strength of 316 is 210 MPa, and that of 316L is 200 MPa. Therefore, the bolts made of them belong to low-strength bolts.
(2) Why should bolts of type 304 and 316 materials not be used for flange joints?
As mentioned in the previous lectures, the flange joint may fail due to leakage for two reasons: first, the internal pressure causes the two flange sealing surfaces to separate, resulting in a corresponding reduction in gasket stress; second, the gasket creep relaxation at high temperatures or the creep of the bolts themselves leads to bolt force relaxation, which also reduces the gasket stress, causing the flange joint to fail.
In actual operation, the relaxation of bolt force is inevitable. The initial bolt force will always drop over time. Especially for flange joints under high-temperature and severe cyclic conditions, after 10,000 hours of operation, the bolt load loss often exceeds 50%, and it will continue to decline with the extension of time and the increase of temperature.
When the flange and the bolt are made of different materials, especially when the flange is carbon steel and the bolt is stainless steel, due to the different thermal expansion coefficients of the bolt and the flange material, for instance, 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 equipment is heated up, if the expansion of the flange is less than that of the bolt, the bolt will elongate less after deformation coordination, causing the bolt force to relax. This may lead to leakage at the flange joint.
Therefore, when high-temperature equipment flanges and pipe flange joints are used, especially when the thermal expansion coefficients of the flange and the bolt materials are different, it is advisable to make the thermal expansion coefficients of the two materials as close as possible.
From (1), it can be seen that the mechanical strength of austenitic stainless steels like 304 and 316 is relatively low. The room-temperature yield strength of 304 is only 205 MPa, and that of 316 is only 210 MPa. Therefore, to enhance the anti-loosening and anti-fatigue capabilities of bolts, measures such as increasing the installation bolt force are adopted.
As will be discussed in the subsequent lecture, when the maximum installation bolt force is used, the installation bolt stress is required to reach 70% of the bolt material’s yield strength. This necessitates an increase in the strength grade of the bolt material, and the use of high-strength or medium-strength alloy steel bolt materials.
It is obvious that, except for cast iron, non-metallic flanges or rubber gaskets, for flanges or semi-metallic and metallic gaskets with higher pressure ratings or larger gasket stresses, bolts made of low-strength materials like 304 and 316 cannot meet the sealing requirements due to insufficient bolt force.
It is particularly important to note that in the American stainless steel bolt material standards, both 304 and 316 have two categories, namely B8 Cl.1 and B8 Cl.2 for 304, and B8M Cl.1 and B8M Cl.2 for 316. Cl.1 has undergone carbide solution treatment, while Cl.2, in addition to solution treatment, has also undergone strain hardening treatment.
Although B8 Cl.2 and B8 Cl.1 have no fundamental difference in chemical corrosion resistance, compared with B8 Cl.1, the mechanical strength of B8 Cl.2 has been significantly enhanced. For instance, the yield strength of B8 Cl.2 bolt material with a diameter of 3/4″ is 550MPa, while that of all diameters of B8 Cl.1 bolt material is only 205MPa, with a difference of more than twice.
The domestic bolt material standards 06Cr19Ni10 (304) and 06Cr17Ni12Mo2 (316) are equivalent to B8 Cl.1 and B8M Cl.1 respectively. [Note: The bolt material S30408 in GB/T 150.3 “Pressure Vessels – Part 3: Design” is equivalent to B8 Cl.2; S31608 is equivalent to B8M Cl.1.]
For the reasons mentioned above, GB/T 150.3 and GB/T 38343 “Technical Specifications for Installation of Flange Joints” stipulate that bolts made of the commonly used 304 (B8 Cl.1) and 316 (B8M Cl.1) materials are not recommended for flange joints of pressure equipment and pipe flange joints, especially under high-temperature and severe cycling conditions. Instead, B8 Cl.2 (S30408) and B8M Cl.2 should be used 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, if the torque is not controlled, the bolts may have exceeded the material’s yield strength and even broken.
Naturally, if leakage occurs during the pressure test or at the start of operation, even if the bolts are tightened further, the bolt force cannot be increased and the leakage cannot be stopped. In addition, these bolts cannot be reused after disassembly because they have undergone permanent deformation, with the cross-sectional size of the bolts reduced, making them prone to breaking during reinstallation.
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