Stainless steel flanges must use stainless steel bolts is a big mistake

Many owners often force the contractor to make rectifications on the grounds that the use of carbon steel or alloy steel bolts for stainless steel flanges will cause corrosion. But is it really hard to explain?

Article 5.8.7 of GB50316-2000 (2008) “Code for Design of Industrial Metal Piping”: Under severe cyclic conditions, the bolts or studs used for flange connection should be made of alloy steel.

Editor’s Note: Under severe cyclic conditions (referring to occasions where the internal stress of the pipeline changes frequently, such as reciprocating compressor pipelines, PSA separation, etc.), if high-strength alloy steel bolts are not used, stainless steel and other low-strength bolts are used.

According to the fatigue theory, due to the low strength of stainless steel bolts, the corresponding allowable cycle times are small when the alternating stress amplitude is large. Long-term use will cause the bolts to suddenly fatigue and break and cause serious danger.

Article 4.0.1 and 4.0.2 of HG/T20634-2009 “Fasteners for Steel Pipe Flanges” clearly indicate that the common 304 and 316 stainless steel bolts that meet the GB/T1220 standard are low-strength bolts. When used for CL150/CL300 flanges with non-metallic flat gaskets.

The reason is that the pre-tightening force that low-strength bolts can withstand is limited, and flanges with higher pressure ratings and greater rigidity or semi-metal and metal gaskets with higher hardness may reach the limit due to insufficient pre-tightening force when using stainless steel bolts. No sealing effect.

Article 3.3.7 of HG/T20635-2009 “Steel Pipe Flange, Gasket, Fastener Selection Regulations”: Low-strength fasteners are only used for flanges with nominal pressure ≤ CL300 and non-metallic flat gaskets The joint should not be used in severe cycling conditions.

Alloy steel bolts have been coated with anti-rust oil when they are delivered to isolate direct contact with stainless steel flanges. For flanges, what we need is its strength and rigidity (that is, the thickness of the flange and the thickness of the flange neck). The weak corrosion caused by the contact between alloy steel bolts and stainless steel flanges will not produce any performance Influence, we must strictly avoid direct contact between stainless steel and other steel grades only when sealing is emphasized on steel pipes and pipe fittings.