What kind of bolts should be used for stainless steel flanges?
Thread Content
Regarding the issue of bolt materials for stainless steel flanges, many people claim that in order to prevent corrosion, stainless steel bolts must be used for such flanges; however, this is a misconception. When selecting flange bolts, the first requirement is to meet the strength condition; if the strength requirement is not met, the flange will fail, and there’s no point in considering whether corrosion will occur or not. Article 5.8.7 of GB50316-2008 \"Code for Design of Industrial Metal Piping\": Under conditions of severe cycling, the bolts or studs used for flange connections shall be made of alloy steel. Under severe cyclic conditions (where the stress levels inside the pipes change frequently, such as in reciprocating compressor pipelines or PSA separation processes), 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. 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 joints with a nominal pressure of ≤ CL300 that employ non-metallic flat gaskets, and they should not be used in conditions involving severe cyclic loading. Regarding the corrosion issue of alloy steel and stainless steel flanges, alloy steel bolts are coated with rust preventive 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 critical, such as with steel pipes and pipe fittings, that we need to strictly avoid direct contact between stainless steel and other types of steel.
A lot of people have mentioned this issue recently. Let me share my approach: under normal circumstances, I choose high-alloy steel bolts. For certain special applications where acidic gases are released, stainless steel bolts are used; generally, A193B7/A194 2H grades are chosen rather than 304/304. I’ve encountered situations where the bolts made of stainless steel on-site were fine, while those made of alloy steel suffered from corrosion. There are also high-temperature operating conditions, which go beyond the range of use for alloy steel
Does high strength refer to grade 8.8 or above?
Although strength must be considered first, site conditions and the properties of the medium also need to be taken into account. Even if the strength is sufficient, severe corrosion can still pose a safety risk.
Could someone explain stress corrosion? What impact can stress corrosion have when alloy steel bolts come into contact with stainless steel flanges? Under what conditions can this stress corrosion be ignored?
I really haven’t thought about these issues; we just followed the methods that the company has used in the past. I really should consider them more carefully
If stainless steel bolts must be used, strain-hardened stainless steel can be employed; that’s no problem.
Indeed, at the engineering company where I used to work, the requirements for liquid ammonia pipeline systems specified the use of stainless steel; therefore, stainless steel bolts were chosen. Later, it was discovered that the strength of these bolts did not meet the regulatory requirements, so the issue was reported to the company, and all the bolts were replaced with alloy steel ones. The big boss issued a special bonus.
I can treat ordinary high-strength bolts using a new process, allowing them to withstand a neutral salt spray test for over 200 hours