1. Article 5.8.7 of GB50316-2000 (2008) version of "Industrial Metal Pipe Design Code": Under severe cyclic conditions, the bolts or studs used for Stainless Steel Flange connections should be made of alloy steel.

Note: Under severe cyclic conditions (referring to occasions where the internal stress of the pipeline changes alternately a lot, such as reciprocating compressor pipelines, PSA separation, etc.), if high-strength alloy steel bolts are not used, but low-strength bolts such as stainless steel are used, according to fatigue theory, due to the low strength of stainless steel bolts, the corresponding allowable number of cycles is small when the alternating stress amplitude is large, and long-term use will cause the bolts to suddenly fatigue and break, causing serious danger.

2. Articles 4.0.1 and 4.0.2 of HG/T20634-2009 "Fasteners for Steel Pipe Flanges" clearly state that the common 304 and 316 stainless steel bolts that meet the GB/T1220 standard are low-strength bolts and can only be used when the CL150/CL300 flange is equipped with non-metallic flat gaskets.

The reason is: low-strength bolts can only bear limited preload. When stainless steel bolts are used in conjunction with flanges with higher pressure levels and greater rigidity or semi-metallic and metal gaskets with greater hardness, the sealing effect may not be achieved due to insufficient preload.

3. Article 3.3.7 of HG/T20635-2009 "Regulations on the Selection of Steel Pipe Flanges, Gaskets and Fasteners": Low-strength fasteners are only used for nominal pressures ≤ CL300. Flange joints with non-metallic flat gaskets should not be used in severe cyclic conditions.

4. Alloy steel bolts are already coated with anti-rust oil when delivered, which can isolate direct contact with stainless steel flanges. For flanges, what we need is their strength and rigidity (i.e. the thickness of the flange plate and the thickness of the flange neck). The slight corrosion caused by the contact between alloy steel bolts and stainless steel flanges will not have any effect on their performance. Only for occasions such as steel pipes and pipe fittings that emphasize sealing, we must strictly avoid direct contact between stainless steel and other steel types.