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 joints (including flanges, sealing elements and fasteners).
304,304L,316, and 316L are stainless steel grades certified by the American National Standards Institute (ANSI) or American Society for Testing and Materials (ASTM), classified under the 300 series of austenitic stainless steels. Their Chinese counterparts (GB/T) include 06Cr19Ni10 (304),022Cr19Ni10 (304L), 06Cr17Ni12Mo2 (316), and 022Cr17Ni12Mo2 (316L). These grades are collectively referred to as 18-8 stainless steel in industry practice.
As shown in Table 1, the physical-chemical and mechanical properties of stainless steels 304,304L,316, and 316L vary depending on their alloying elements and addition amounts. Compared to conventional stainless steels, they exhibit superior corrosion resistance, heat resistance, and machinability. While 304L shares similar corrosion resistance with 304, its lower carbon content gives it stronger resistance to intergranular corrosion. The molybdenum-containing 316 and 316L grades demonstrate even better corrosion and heat resistance due to added molybdenum. Similarly, 316L’s lower carbon content enhances its resistance to intergranular corrosion. Among austenitic stainless steels, 304,304L,316, and 316L all have relatively low mechanical strength: 304 has a room-temperature yield strength of 205MPa (170MPa for 304L), while 316 and 316L measure 210MPa (200MPa for 316L) respectively. Consequently, bolts made from these grades fall into the low-strength category.
Table 1 Carbon content,% Room temperature yield strength, 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
Why should flange joints not use bolts of one type of material such as 304 and 316?
As mentioned in the previous lectures, flange joint is separated from the sealing surface of two flanges due to the internal pressure, resulting in a corresponding decrease in the stress of the gasket. On the other hand, the gasket stress decreases due to the creep and relaxation of the gasket or the bolt itself creep and relaxation of the bolt force under high temperature, resulting in leakage failure of the flange joint.
In practical applications, bolt force relaxation is inevitable. The initial tightening torque of bolts inevitably diminishes over time. Particularly for flange joints operating under high-temperature and severe cyclic conditions, after 10,000 hours of service, the bolt load loss typically exceeds 50%, with further deterioration occurring as time progresses and temperatures rise.
When flanges and bolts are made of different materials—particularly when the flange is carbon steel and the bolt is stainless steel—the thermal expansion coefficients (TEC) of the two materials differ significantly. For example, stainless steel has a TEC of 16.51×10^-5/℃ at 50°C compared to carbon steel’s 11.12×10^-5/℃. During equipment heating, if the flange’s expansion rate is lower than the bolt’s, the reduced elongation of the bolt after thermal deformation may cause a loss of bolt force, potentially leading to flange joint leakage. Therefore, when installing flanges and pipe joints in high-temperature environments—especially when the materials have differing TECs—it’s crucial to ensure their TECs are as close as possible.
As shown in (1), austenitic stainless steels like 304 and 316 exhibit relatively low mechanical strength. The room-temperature yield strength of 304 is only 205 MPa, while 316 measures 210 MPa. To enhance bolts ‘anti-relaxation and fatigue resistance, measures such as increasing installation force are required. As discussed in subsequent lectures, when maximum installation force is applied, the bolt’s stress must reach 70% of its yield strength. This necessitates using high-strength or medium-strength alloy steel bolts. It becomes evident that for high-pressure flanges or semi-metal/metal gaskets requiring significant stress—excluding cast iron, non-metallic flanges, or rubber gaskets—low-strength materials like 304 and 316 cannot meet sealing requirements due to insufficient bolt force.
It is particularly noteworthy that the American stainless steel bolt material standards categorize 304 and 316 into two classes: 304’s B8 Cl.1 and B8 Cl.2, and 316’s B8M Cl.1 and B8M Cl.2. Cl.1 undergoes carbide solid solution treatment, while Cl.2 additionally incorporates strain hardening treatment. Although B8 Cl.2 and B8 Cl.1 show no fundamental difference in chemical corrosion resistance, B8 Cl.2 demonstrates significantly enhanced mechanical strength compared to B8 Cl.1. For instance, a 3/4-inch diameter B8 Cl.2 bolt material achieves a yield strength of 550MPa, whereas all diameter B8 Cl.1 bolt materials exhibit only 205MPa—more than double the difference. In China’s bolt material standards, grades 06Cr19Ni10 (304) and 06Cr17Ni12Mo2 (316) are equivalent to B8 Cl.1 and B8M Cl.1 respectively. [Note: GB/T 150.3 “Pressure Vessels Part 3: Design” specifies that S30408 bolt material is equivalent to B8 Cl.2, while S31608 is equivalent to B8M Cl.1.]
For the above reasons, GB/T 150.3 and GB/T38343 “Technical Specifications for Flange Joint Installation” stipulate that it is not recommended to use bolts of common 304 (B8 Cl.1) and 316 (B8M Cl.1) materials for pressure equipment flanges and pipe flange joints, especially in high temperature and severe cycle conditions, and should be replaced with B8 Cl.2 (S30408) and B8M Cl.2 to avoid low installation bolt force.
It’s crucial to note that when using low-strength bolt materials like 304 or 316, the bolts may exceed their yield strength and even fracture during installation due to inadequate torque control. Naturally, if leakage occurs during pressure testing or initial operation, further tightening won’t increase the bolt’s load capacity to prevent leakage. Moreover, these bolts can not be reused after disassembly because permanent deformation causes reduced cross-sectional dimensions, making them prone to breakage during reinstallation.
