Selection of stainless steel bolt materials

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Today, chemical plants are paying increasing attention to environmental requirements; for many heat exchangers whose components are exposed outside, it is required that the flange bolts be made of stainless steel. Designers who are used to working with 35CrMo steel need to be cautious, as stainless steel bolts may cause leakage issues at the flanges. First, the common causes of leakage in flange joints are: 1. The internal pressure causes the sealing surfaces of the two flanges to separate, resulting in a corresponding reduction in the stress on the gasket. 2. High temperatures cause creep relaxation of the gasket or creep of the bolts themselves, leading to a reduction in bolt tension; this in turn reduces the stress on the gasket, resulting in leakage and failure of the flange joint. In actual operation, bolt force relaxation is inevitable; the initial tightening force of the bolts always decreases over time. Especially in flange joints subjected to high temperatures and severe cyclic conditions, after 10,000 hours of operation, the bolt load loss often exceeds 50%, and this loss continues to increase over time as well as with rising temperatures. When the flange and bolts are made of different materials, especially when the flange is made of carbon steel and the bolts are made of stainless steel, the difference in thermal expansion coefficients between these materials leads to problems. For example, at 50°C, the thermal expansion coefficient of stainless steel is (16.51×10-5/°C), which is higher than that of carbon steel (11.12×10-5/°C). When the temperature of the device increases, if the flange expands less than the bolts, then after deformation adjustment, the elongation of the bolts decreases, resulting in a reduction in bolt tension; this can lead to leakage at the flange joint. Therefore, when joining high-temperature equipment flanges and pipe flanges, especially when the coefficients of thermal expansion of the flange and bolt materials differ, it is necessary to make the coefficients of thermal expansion of these two materials as similar as possible. Austenitic stainless steels have low mechanical strength; the room-temperature yield strength of 304 is only 205 MPa, and that of 316 is also only 210 MPa. Therefore, in order to improve the resistance of bolts to relaxation and fatigue, measures are taken to increase the tightening force of the bolts. When the maximum tightening force is applied, it is required that the stress in the bolts reach 70% of the yield strength of the bolt material; this necessitates an increase in the strength grade of the bolt material, with high-strength or medium-strength alloy steel being used. It is worth noting here that in the U.S. standards for stainless steel bolt materials, 304 and 316 each have two categories: 304 has B8 Cl.1 and B8 Cl.2, while 316 has B8M Cl.1 and B8M Cl.2. Cl.1 has undergone carbide solubilization treatment, while Cl.2, in addition to solubilization treatment, has also undergone strain strengthening treatment. 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 significantly higher compared to B8 Cl.1; for example, the yield strength of B8 Cl.2 bolts with a diameter of 3/4” is 550 MPa, whereas the yield strength of B8 Cl.1 bolts of all diameters is only 205 MPa, representing a difference of more than twice. [Note: The bolt material S30408 specified 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 above reasons, GB/T 150.3 and GB/T 38343 \"Technical Specifications for the Installation of Flange Joints\" stipulate that for flanges of pressure equipment and pipe flange joints, it is not recommended to use bolts made of the common materials 304 (B8 Cl.1) and 316 (B8M Cl.1); especially under high-temperature and highly cyclic operating conditions, these should be replaced with B8 Cl.2 (S30408) and B8M Cl.2 in order to avoid low bolt tightening forces. It is worth noting that when low-strength bolt materials such as 304 and 316 are used, even during the installation phase, the bolts may exceed the yield strength of the material due to a lack of torque control, and may even break. Naturally, if leakage occurs during the pressure test or at the start of operation, even if the bolts are tightened further, the bolt tension will not increase enough to stop the leakage. Furthermore, these bolts cannot be reused after being removed, as they have suffered permanent deformation and their cross-sectional dimensions have decreased, making them prone to breaking when reinstalled. It should be emphasized here that, based on the engineering experience of various companies in this industry, when the operating pressure of the main studs in heat exchanger equipment is above 2 Mpa or when the temperature is high, stainless steel studs need to be equipped with disc spring washers. Since stainless steel bolts are strain-strengthened, external loads can easily cause stress relaxation in the bolts. Think about why GB/T 150 specifies that S30408 studs that have been strain-strengthened after solution treatment can only be used at temperatures up to 50°C