Do stainless steel bolts must be used for stainless steel flanges?
There are often many owners who force the contractor to rectify on the ground that stainless steel flange using carbon steel or alloy steel bolts will cause corrosion, but it is difficult to explain the numerous grievances of the contractor. But is it really hard to explain?
Actually, it’s easy to explain.
- 1. Article 5.8.7 of GB50316-2000(2008) code for design of industrial metal pipelines: under the condition of severe cycle, bolts or double-headed studs for flange connection shall be made of alloy steel.
- Note: Under the condition of severe cycle (refers to the occasion where the internal stress of pipeline changes greatly, such as reciprocating compressor pipeline and PSA separation, etc.), if high-strength alloy steel bolts are not used, but stainless steel and other low-strength bolts, according to the fatigue theory, due to the low strength of stainless steel bolts, the corresponding allowable cycle times are smaller when the amplitude of alternating stress is larger, long-term use will lead to sudden fatigue fracture of bolts, causing significant danger.
- 2. Article 4.0.1 and 4.0.2 of HG/T20634-2009 “fasteners for steel pipe flange” clearly indicate that the common 304, 316 stainless steel bolts are low strength bolts and can only be used when flange supporting non-metallic flat gaskets for CL150/cl300.
- The reason is: the pre-tightening force that the low-strength bolt can bear is limited, and the flange with higher pressure grade and the semi-metal and metal gasket with greater rigidity are used with stainless steel bolts, the sealing effect may not be achieved because of insufficient pre-tightening force.
- 3. HG/T20635-2009 provisions for selection of steel pipe flanges, gaskets and fasteners 3.3.7: low-strength fasteners are only used for nominal pressure≤cl300, and are flange joints of non-metallic flat gaskets and should not be used under severe cycle conditions.
- 4. Alloy steel bolts have been coated with anti-rust oil at the time of delivery, which can isolate direct contact with stainless steel flange. For the flange, what we need is its strength and rigidity (that is, the thickness of the flange plate and the thickness of the neck of the flange), weak corrosion caused by the contact between alloy steel bolts and stainless steel flanges will not have any impact on its performance, only for steel pipes and pipe fittings and other occasions, we must strictly avoid direct contact between stainless steel and other steel grades.
In view of the above four points, most design institutes in China have completely abandoned the adoption of stainless steel bolts in their common material design regulations and material grade lists. Therefore, stainless steel flange must use stainless steel bolts, which is a big mistake.
Source: China Stainless Steel Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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