Why Is It Not Recommended to Use 304 Material for Bolts and Nuts?
(1)304, 304L
What is the basic difference between 316 and 316L materials?
304, 304L, 316 and 316L are commonly used stainless steel materials in flange joints, including flanges, sealing components, and fasteners.
304, 304L, 316 and 316L are stainless steel grade codes in the American Standards for Materials (ANSI or ASTM), belonging to the 300 series of Class I steels of austenitic stainless steel. The grade corresponding to the domestic material standard (GB/T) is 06 Cr19Ni10(304), 022Cr19Ni10(304L), 06Cr17Ni12Mo2(316), 022Cr17Ni12Mo2(316L). This type of stainless steel is commonly referred to as 18-8 stainless steel.
304, 304L, 316, and 316L, their physical, chemical, and mechanical properties are also different due to the addition of alloying elements and their 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 due to its lower carbon content, it has stronger resistance to inter-granular corrosion. 316 and 316L are molybdenum containing stainless steels, which have better corrosion resistance and heat resistance than 304 and 304L due to the addition of molybdenum element.
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 316 and 316L is low, with a room temperature yield strength of 205 MPa for 304 and 170 MPa for 304L. The room temperature yield strength of 316 is 210MPa, and 316L is 200MPa. Therefore, the bolts and nuts made with them belong to the low strength class of bolts and nuts.
(2) Why should flange joints not use bolts and nuts 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. The second reason is that the gasket creep relaxation or bolt creep under high temperature causes bolt force relaxation, which also reduces gasket stress and leads to leakage 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 cycling conditions, after 10,000 hours of operation, the bolt load loss often exceeds 50% and decays with the continuation of time and 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 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, 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, which may lead to leakage of the flange joint.
Therefore, when connecting high-temperature equipment flanges and pipe flanges, especially when the thermal expansion coefficients of the flange and bolt materials are different, it is advisable to make the thermal expansion coefficients of the two materials as close as possible.
The mechanical strength of austenitic stainless steels such as 304 and 316 is low. The room temperature yield strength of 304 is only 205 MPa, and 316 is also only 210 MPa. Therefore, in order to improve the ability of bolts and nuts 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, it is required that the installation bolt stress reach 70% of the bolt material yield strength. Therefore, it is necessary to increase 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, for semi metallic and metallic gaskets with high pressure ratings and high stress, low-strength material bolts and nuts such as 304 and 316 cannot meet sealing requirements due to insufficient bolt force.
It should be noted that in the US stainless steel bolt material standard, there are two categories for 304 and 316, 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 B8 Cl.1 bolt material with all diameters is only 205MPa, which is more than twice the difference between the two. The 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 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 Installation of Flange Joints" stipulate that it is not recommended to use bolts and nuts made of the usual 304 (B8 Cl.1) and 316 (B8M Cl.1) materials for flange and pipe flange joints of pressure equipment. Especially in high temperature and severe cycling 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 low-strength bolt materials such as 304 and 316, even during the installation phase, due to the lack of torque control, the bolt may have exceeded the yield strength of the material or even fractured. Naturally, if there is a leak during the pressure test or during operation, even if the bolt is tightened further, the bolt force cannot increase and the leak cannot be prevented. In addition, these bolts and nuts cannot be reused after disassembly, as they have already undergone permanent deformation and the cross-sectional size of the bolts and nuts has decreased, making them prone to breakage when reinstalled.
