Why is 304 material not recommended for bolts?
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(1) What are the basic differences between 304, 304L, 316, and 316L materials? 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 stainless steel grades designated by the American Material Standards (ANSI or ASTM), and they belong to the 300 series of austenitic stainless steels. The grades corresponding to domestic material standards (GB/T) are 06Cr19Ni10 (304), 022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), and 022Cr17Ni12Mo2 (316L). This type of stainless steel is generally referred to as 18–8 stainless steel. As shown in Table 1, 304, 304L, 316, and 316L exhibit different physical, chemical, and mechanical properties due to the various alloying elements used and their respective amounts. Compared to ordinary stainless steels, they possess excellent corrosion resistance, heat resistance, and machinability. The corrosion resistance of 304L is similar to that of 304, but due to its lower carbon content compared to 304, it has greater resistance to intergranular corrosion. 316 and 316L are molybdenum-containing stainless steels; due to the addition of molybdenum, their corrosion resistance and heat resistance are superior to those of 304 and 304L. Similarly, since 316L has a lower carbon content than 316, it possesses better resistance to intergranular corrosion. Austenitic stainless steels such as 304, 304L, 316, and 316L have low mechanical strength; the room-temperature yield strength of 304 is 205 MPa, while that of 304L is 170 MPa ； The room-temperature yield strength of 316 is 210 MPa, while that of 316L is 200 MPa. Therefore, the bolts made from them are of low-strength grade. Table 1: Carbon content, %; Yield strength at room temperature, MPa; Recommended maximum operating temperature, °C304: ≤0.08, 205, 816
304L: ≤0.03, 170, 538
316: ≤0.08, 210, 816
316L: ≤0.03, 200, 538
(2) Why should bolts made of materials such as 304 and 316 not be used in flange connections? As explained in previous sections, in flange connections, the internal pressure causes the sealing surfaces of the two flanges to separate, resulting in a corresponding reduction in the stress on the gaskets. Additionally, at high temperatures, the gaskets may creep and relax, or the bolts themselves may creep, leading to a decrease in the bolt tension and thus a reduction in the stress on the gaskets, which can result in leakage and failure of the flange connection. 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 their thermal expansion coefficients leads to problems. For example, at 50°C, the thermal expansion coefficient of stainless steel (16.51×10-5/°C) is higher than that of carbon steel (11.12×10-5/°C). When the temperature of the device rises, if the flange expands less than the bolts, then after deformation adjustment, the reduction in the elongation of the bolts results in a decrease in the bolt tension, which 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. As can be seen from (1), austenitic stainless steels such as 304 and 316 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. As will be discussed in subsequent lectures, when using the maximum allowable tightening force, it is required that the stress on the bolts reach 70% of the yield strength of the bolt material; hence, it is necessary to raise the strength grade of the bolt material by using high-strength or medium-strength alloy steel bolts. It is evident that, aside from cast iron, non-metallic flanges, or rubber gaskets, for flanges with higher pressure ratings or semi-metallic and metallic gaskets that are subject to high stress, bolts made of low-strength materials such as 304 and 316 cannot meet the sealing requirements due to insufficient bolt strength. It is important to note 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. The 06Cr19Ni10 (304) and 06Cr17Ni12Mo2 (316) specified in China’s bolt material standards are equivalent to B8 Cl.1 and B8M Cl.1. [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 bolts made of the common materials 304 (B8 Cl.1) and 316 (B8M Cl.1) are not recommended for use in flanges of pressure equipment and pipe flange joints; especially under high-temperature and highly cyclic operating conditions, these should be replaced with B8 Cl.2 (S30408) and B8M Cl.2 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 a leak occurs during the pressure test or at the time of startup, even if the bolts are tightened further, the bolt tension will not increase enough to stop the leak. 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.
The main reasons why 304 material is not recommended for bolts are as follows: 1. Insufficient mechanical properties: Both 304 and 316 materials have low yield strengths, which results in bolts made from these materials being of low strength. To prevent bolts from loosening or suffering fatigue fractures under high pressure and high temperature conditions, it is often necessary to use bolt materials with higher strength. 2. Effect of thermal expansion coefficient: Due to the different thermal expansion coefficients of various materials, when the coefficients of the flange and bolts differ, temperature changes cause the bolts to extend or contract at different rates, leading to variations in bolt tension and potentially resulting in leaks at the flange joint. 3. Corrosion resistance: Although 304 and 316 possess good corrosion resistance, their resistance may decrease in certain high-temperature and high-pressure environments due to changes in temperature and operating conditions. The main differences between 304, 304L, 316, and 316L materials lie in their chemical composition and mechanical properties: The primary difference between 304 and 304L is the carbon content; 304L has a lower carbon content than 304, which gives it better resistance to intergranular corrosion compared to 304. - Molybdenum is added to 316 and 316L, giving them better corrosion resistance and heat resistance compared to 304 and 304L. - Similarly, 316L has a lower carbon content than 316, so its resistance to intergranular corrosion is better than that of 316. - According to the data in Table 1, 304 and 316 have low yield strengths and are not suitable as bolt materials for applications with high requirements. Therefore, to ensure the reliability of flange joints, especially under high-temperature or severe cyclic conditions, it is necessary to avoid using 304 (B8 Cl.1) and 316 (B8M Cl.1) as bolt materials. Higher-strength bolt materials can be chosen, such as B8 Cl.2 (S30408) and B8M Cl.2; these materials, after appropriate heat treatment and strain hardening, possess higher yield strengths and are better able to resist relaxation and stress corrosion cracking. .
The main reason for trying to avoid using austenitic stainless steel bolts is as follows: 1, they are expensive; 2, their strength is relatively low; 3, many manufacturers cut corners by not performing surface treatment or heat treatment. It’s okay for one bite, but the second bite kills it. It’s difficult to cut through the bolt.
Stainless steel threads tend to get stuck, preventing them from being moved up or down; the only solution is to cut them off, which is a real nightmare for operators.
What does Article 3 mean? What does it mean to bite to death? Could you please explain it? Thank you
Due to their similar and low strength, the bolt thread and the nut thread end up pressing against each other. The threaded fastener is damaged and cannot be tightened or removed properly.