(1) What are the basic differences between 304, 304L, 316 and 316L materials?
304, 304L, 316 and 316L are stainless steel materials commonly used in flange joints (including flanges, sealing elements and fasteners).
304, 304L, 316 and 316L are stainless steel grade codes of American material standards (ANSI or ASTM), which belong to the 300 series of austenitic stainless steel. The grades corresponding to the domestic material standards (GB/T) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), 022Cr17Ni12Mo2 (316L). This type of stainless steel is usually collectively referred to as 18-8 stainless steel.
| Galvanized steel |
304, 304L, 316 and 316L have different physical, chemical and mechanical properties due to different alloying elements and amounts. 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 because the carbon content of 304L is lower than that of 304, its ability to resist 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. In the same way, because the carbon content of 316L is lower than that of 316, its resistance to crystal corrosion is better. 304, 304L, 316 and 316L austenitic stainless steels have low mechanical strength. The room temperature yield strength of 304 is 205MPa and 304L is 170MPa; the room temperature yield strength of 316 is 210MPa and 316L is 200MPa. Therefore, bolts made from them belong to the low-strength class of bolts.
Table 1 Carbon content, % Yield strength at room temperature, 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 in the previous lectures, the first reason for flange joints is that the sealing surfaces of the two flanges are separated due to the internal pressure, causing a corresponding reduction in gasket stress; the second reason is that the bolt force is relaxed due to the creep relaxation of the gasket or the creep of the bolt itself at high temperatures. , which also reduces the stress of the gasket, causing the flange joint to leak and fail.
In actual operation, bolt force relaxation is inevitable, and the initial tightened bolt force will always decrease over time. Especially for flange joints under high temperature and severe cycle conditions, after 10,000 hours of operation, the bolt load loss will often exceed 50%, and will decay as time continues and the temperature increases.
When the flange and bolts are made of different materials, especially when the flange is made of carbon steel and the bolts are stainless steel, the thermal expansion coefficients of the bolt and flange materials are different. For example, the thermal expansion coefficient of stainless steel at 50°C (16.51×10-5/ ℃) is larger than the thermal expansion coefficient of carbon steel (11.12×10-5/℃). After the device heats up, when the expansion of the flange is less than the expansion of the bolt, after the deformation is coordinated, the bolt elongation decreases, causing the bolt force to decrease. Looseness may cause the flange joint to leak. Therefore, when connecting high-temperature equipment flanges and pipe flanges, especially when the thermal expansion coefficients of flange and bolt materials are different, the thermal expansion coefficients of the two materials should be as close as possible.
It can be seen from (1) that 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 ability of bolts to resist relaxation and fatigue, measures are taken to increase the installation bolt force. For example, in subsequent forums, 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. , so it is necessary to improve the strength level of bolt materials and use high-strength or medium-strength alloy steel bolt materials. It is obvious that, in addition to cast iron, non-metallic flanges or rubber gaskets, for flanges with higher pressure levels or semi-metallic and metal gaskets with large gasket stress, low-strength material bolts such as 304 and 316, due to the bolt force Not enough to meet sealing requirements.
What needs special attention here is that in the American stainless steel bolt material standards, 304 and 316 have two categories respectively, namely B8 Cl.1 and B8 Cl.2 of 304 and B8M Cl.1 and B8M Cl.2 of 316. Cl.1 has undergone carbide solid solution treatment, while Cl.2 has undergone strain strengthening treatment in addition to solid 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 is greatly improved compared to B8 Cl.1, such as B8 Cl.2 with a diameter of 3/4". The yield strength of bolt material is 550MPa, while the yield strength of B8 Cl.1 bolt materials of all diameters is only 205MPa, which is more than twice the difference. 06Cr19Ni10 (304) and 06Cr17Ni12Mo2 (316) in domestic bolt material standards are different from B8 Cl.1 is equivalent to B8M Cl.1. [Note: The bolt material S30408 in GB/T 150.3 "Pressure Vessel Part 3 Design" 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/T 38343 "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) for pressure equipment flanges and pipe flange joints. .1) Bolt materials, especially under high temperature and severe cycle conditions, 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 low-strength bolt materials such as 304 and 316, even during the installation stage, the bolt may exceed the material yield strength or even break due to the lack of torque control. Naturally, if leakage occurs during the pressure test or at the start of operation, even if the bolts are continued to be tightened, the bolt force will not increase and the leakage will not be prevented. In addition, these bolts cannot be reused after being disassembled, because the bolts have been permanently deformed and the cross-sectional size of the bolts has become smaller, and they are prone to twisting and breakage when reinstalled.
