Must stainless steel flanges be secured with stainless steel bolts?

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◆Introduction: Often, many property owners insist that contractors make corrections on the grounds that using carbon steel or alloy steel bolts with stainless steel flanges will cause corrosion; the contractors, despite feeling wronged, find it difficult to explain why this is the case. But is it really hard to explain? ◆It’s actually easy to explain. According to Article 5.8.7 of the GB50316-2000 (2008) version of the \"Code for Design of Industrial Metal Piping\", under conditions of severe cycling, the bolts or studs used for flange connections should be made of alloy steel. Editor’s note: Under severe cyclic conditions (where the stress levels inside the pipes change frequently, such as in reciprocating compressor pipelines or PSA separation systems), if low-strength bolts such as stainless steel bolts are used instead of high-strength alloy steel bolts, fatigue theory dictates that due to the lower strength of stainless steel bolts, the allowable number of cycles is reduced when the amplitude of alternating stresses is high. Prolonged use can lead to sudden fatigue fracture of these bolts, posing a serious risk. Articles 4.0.1 and 4.0.2 of HG/T20634-2009 \"Fasteners for Steel Pipe Flanges\" specify that the common 304 and 316 stainless steel bolts that meet the GB/T1220 standard are low-strength bolts, and can only be used with non-metallic flat gaskets in flanges rated at CL150/CL300. The reason is that low-strength bolts can only withstand a limited amount of preload; flanges with high pressure ratings and high stiffness, or semi-metallic and metallic gaskets with high hardness, may fail to achieve a proper sealing effect when used with stainless steel bolts, due to insufficient preload. 3. Article 3.3.7 of HG/T20635-2009 \"Specifications for the Selection of Steel Pipe Flanges, Gaskets, and Fasteners\": Low-strength fasteners are only to be used in flange connections with a nominal pressure of ≤ CL300 that employ non-metallic flat gaskets, and they should not be used in applications subject to severe cyclic loading. 4. The alloy steel bolts are coated with anti-rust oil at the time of delivery, which prevents direct contact with the stainless steel flanges. For flanges, what is important is their strength and stiffness (i.e., the thickness of the flange face and the thickness of the flange neck). The slight corrosion that occurs as a result of contact between alloy steel bolts and stainless steel flanges has no impact on their performance. It is only in applications where sealing is crucial, such as with steel pipes and pipe fittings, that we need to strictly avoid direct contact between stainless steel and other types of steel. 5. In light of the above four points, the vast majority of design institutes in China have completely eliminated the use of stainless steel bolts in their standard material design specifications and material grade tables.