Why are bolts not recommended with 304, 304L, 316, and 316L materials? What are the fundamental differences?

304, 304L, 316, and 316L are the stainless steel materials commonly used in flange fittings, including flanges, sealing elements, and #fasteners .

304, 304L, 316, and 316L are the #stainless steel grade codes of the American Standards for Materials (ANSI or ASTM) and belong to the 300 series of steels of austenitic stainless steel. The grades corresponding to the domestic material standard (GBT) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), 022Cr17Ni12Mo2 (316L). Usually, this type of stainless steel is collectively referred to as 18-8 stainless steel. 304 material

(2) Why should flange joints not use #bolts of materials such as 304 and 316?

As mentioned in the previous lectures, the flange joint is separated by the internal pressure, causing the gasket stress to decrease accordingly, and secondly, the bolt force is relaxed due to the creep relaxation of the gasket at high temperature or the creep of the bolt itself, which also reduces the gasket stress, so that the flange joint leakage failure.

In practice, bolt force relaxation is inevitable, and the initial tightened bolt force will always drop over time. Especially for flange joints in high temperature and severe cycling conditions, after 10,000 hours of operation, the bolt load loss will often exceed 50%, and decay with the continuation of time and the increase of temperature.

When the #flange and the bolt are different materials, especially when the flange is carbon steel and the bolt is stainless steel, because the thermal expansion coefficient of the bolt and the flange material is different, such as the thermal expansion coefficient of stainless steel at 50 °C (16.51×10-5/°C) is larger than the thermal expansion coefficient of carbon steel (11.12×10-5/°C), 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 is reduced and the bolt force is relaxed, which may lead to leakage of the flange joint. Therefore, when the high temperature equipment flange and pipe flange joint, especially the thermal expansion coefficient of the flange and bolt materials are different, the thermal expansion coefficient of the two materials is similar as much as possible.

It can be seen from (1) that the mechanical strength of 304 and 316 austenitic stainless steels is low, the room temperature yield strength of 304 is only 205MPa, and 316 is only 210MPa. Therefore, in order to improve the ability of bolts to resist relaxation and fatigue, measures to improve the strength of the installation bolt are taken, such as in the subsequent forum will talk about the use of the maximum installation bolt force, the installation bolt stress is required to reach 70% of the bolt material yield strength, so it is necessary to increase the strength level of the bolt material, using high-strength or medium-strength alloy steel bolt material. It is easy to see that in addition to cast iron, non-metallic flanges or rubber gaskets, for semi-metallic and metallic #gaskets with higher pressure flanges or gasket stresses, 304, 316 and other low-strength material bolts cannot meet the sealing requirements due to insufficient bolt force.

Of particular concern here is that there are two categories of 304 and 316 in the US stainless steel bolt material standard, namely B8 Cl.1 and B8 Cl.2 of 304 and B8M Cl.1 and B8M Cl.2 of 316. Cl.1 is treated with carbide solution, while Cl.2 is treated with strain intensification in addition to the solution. Although there is no fundamental difference in chemical resistance between B8 Cl.2 and B8 Cl.1, the mechanical strength of B8 Cl.2 is quite improved compared to B8 Cl.1, such as the yield strength of B8 Cl.2 bolt material with a diameter of 3/4" is 550MPa. In contrast, all diameter B8 Cl.1 bolt materials yield strength is only 205MPa. The difference between the two is more than twofold. 06Cr19Ni10 (304), 06Cr17Ni12Mo2 (316) in the domestic bolt material standard, which is comparable to B8 Cl.1 and B8M Cl.1. [Note: The bolt material S30408 in GB/T 150.3 "Pressure vessels - Part 3 Design" is equivalent to B8 Cl.2; The S31608 is comparable to the B8M Cl.1.