Seamless vs. ERW Piping 4

[gthomas]

I have questions about ERW vs. seamless SST piping. We currently use seamless piping (4" Sch. 40 304SST)for duct burners and would like to switch to ERW due to cost savings. This 'piping' can span from 20' to 40' w/ supports and is basically a gas manifold. There has been concerns raised that ERW piping might have problems due to the heat from the welding reducing the ability of the chrome in the steel to resist going into the carbide phase when heated. Basically they are saying that welded pipe does not perform as well as seamless at very high temperatures. I can't seem to get a definate temp. limit or what the "performance"/longevity would be.. Any help is appreciated. No codes are applied to this product.

 

[kstaylor]

I believe what you are describing is a phenomena known as sensitization. This occurs in a material (304SS in this case) where the carbon in the material matrix reacts with the chromium to form chrome carbides, thus depleting the amount of chrome available for corrosion resistance. The temperature range where sensitization occurs is roughly 900-1400°F. If your material is exposed to a temperature in this range for a length of time it will become sensitized. The heat of welding will result is some amount of sensitization around the welds but the parent material away from weld joints can become sensitized as well given the right temperature exposure.

Sensitization can be dealt with in several ways. First, a chemically stabilized grade of SS can be used, like 347SS or 321SS, although both have other issues to be concerned about as well. Second, a low carbon grade of material can be used like 304LSS or 316LSS. The amount of carbon in the material is reduced so the onset of sensitization is delayed. Given enough time at temperature however, L grade materials will eventually sensitize. The third method is to put the material through a solution anneal heat treat process after welding. This will put the carbides back into solution, essentially un-sensitizing the material. However, exposure to temperature in the sensitization range will cause sensitization again. Solution anneal works best when combined with use of the stabilized grades of stainless steel.

Whether a sensitized material will corrode depends on the service environment that it is exposed to. In the refining industry polythionic stress corrosion cracking of sensitized SS is a major concern. The sensitized material corrodes along the chrome-depleted grain boundaries resulting in intergranular stress corrosion cracking.

You need to determine if your service will result in sensitization of the material you want to use and then determine of sensitization will be detrimental. For your duct burner application you might find that your seamless material already is sensitized. The use of liquid penetrant examination in the field will show whether the material is cracked or not (may need longer dwell time for tight cracks). Field metallography will allow you to determine if you have a sensitized material without resorting to cutting out samples for lab analysis.