Materials
CORTEN (Most Common)
CORTEN (trademark of US Steel Corporation) is a low-alloy steel that is generally considered a “weathering” steel since it builds up an initial layer of corrosion that inhibits further corrosion of the material. Nearly all of the rectangular metal expansion joints we have supplied have been constructed from CORTEN. The specific CORTEN alloy used for rectangular metal expansion joints is generally classified under the ASTM A 606 designation (thin sheet). While CORTEN provides a longer service life than ordinary carbon steel, it is still susceptible to corrosion where pockets of liquid reside, such as the bottom of the expansion joint. For this reason, we recommend that rectangular metal expansion joints be well insulated on the outside. This reduces the potential for the formation of acidic condensates which could collect in the bottom of the expansion joint.
Stainless Steel & Other Higher Alloys
Rectangular metal expansion joints can be constructed from various grades of stainless steel from Type 304 to Types 316 and 321, as well as “H” alloy versions (such as 304 H). Care should be taken when using stainless steel bellows in conjunction with carbon steel flanges for high-temperature applications because the thermal expansion of the stainless steel bellows is much higher than the carbon steel flanges. This may cause the stainless steel bellows to buckle and fail.
Sometimes, customers try to use stainless steel bellows, thinking that it has an improved service life for acidic corrosion. Type 304 stainless offers only a 10% improvement in service life over CORTEN when acidic corrosion is involved. When the differences in thermal expansion come into play, the service life of a stainless steel rectangular metal expansion joint will probably be less than a similarly designed CORTEN material. There are other stainless steel alloys that offer better resistance to acidic corrosion, like Type 317 but these alloys are very expensive.
We have also constructed rectangular metal expansion joints using INCOLOY 800 H bellows and Type 304 H flanges for special high-temperature applications in a Fluid Bed Combustor.
Common Applications
Coal Mill, Economizer Hopper, Primary Air (PA), Secondary Air (SA), Tempering Air, Windbox Feed.
