Selection of wound gaskets for stainless steel flanges
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Regarding stainless steel flanges and the outer rings of wound gaskets, what are the guidelines? Should an outer ring made of carbon steel be used or one made of stainless steel? I’ve seen cases where either type is used, as well as carbon steel rings coated with zinc. If the temperature is not low, could zinc have an adverse effect on stainless steel?
The outer ring does not come into contact with the medium; by default, carbon steel is used unless otherwise specified. Stainless steel is chosen when the temperature of the transported medium is relatively high
1. General standards stipulate that, unless specified otherwise by the user, the outer ring is made of carbon steel, but it must undergo spraying, electroplating, or other surface treatments. Specific standards include HG/T20610, HG/T20631, SH/T3407, etc. 2. The outer ring is used only for centering and generally does not come into direct contact with the internal medium. Therefore, except for conditions such as high and low temperatures (taking heat transfer into account), flange joints buried underground, immersion in corrosive media, and severe atmospheric corrosion, which require consideration of the effects on carbon steel materials, carbon steel can be used for the outer ring. 3. Additionally, since the flange joint is a single unit, the material of the bolts also needs to be considered. As you mentioned, for example, if the bolt is made of stainless steel and the outer ring of the gasket is carbon steel coated with zinc, then when the stainless steel bolt comes into contact with the galvanized outer ring, liquid metal embrittlement (LME) may occur; this factor needs to be taken into account during design.
According to the third point you mentioned, will the galvanized outer ring cause what is known as LME on the stainless steel flange itself?
It can happen as long as there is contact. Zinc has a relatively low melting point; the melting point of pure zinc is around 420°C. At room temperature, zinc is hard and brittle, but it becomes ductile at 100 to 150°C. When the temperature exceeds 210°C, zinc becomes brittle again. You need to consider that in high temperatures or in case of a fire, zinc may soften or melt and drip onto the flange. Therefore, if it is chosen, there are risks involved. Of course, what was mentioned above is just one possibility; when designing, you need to take various possibilities into account and ensure that risks are under control.
In principle, stainless steel should not come into contact with zinc, as this can easily cause corrosion of the stainless steel