Stainless steel fabrication for high temperature steam applications

[tamal1475]

Usually for corrosion applications we mandate precautions to be taken against iron contamination during austenitic stainless steel fabrication. Should it apply for high-temperature applications also, as for steam boilers? Normally local iron contamination on an austenitic stainless steel component, in an aqueous corrosive atmosphere, will result in local corrosion leading to failure. Such risks do not exist in high-temperature application where heat-resistant stainless steels are used for their high-temperature properties. I seek comments from learned members and request them to share their experience.

 

[moltenmetal]

One consideration is whether or not the high temperature piping spends any time below 100 C and exposed to water, during shutdown conditions or the like, or due to the ingress of water through insulation cladding. Depending on the service temperature, following exposure the entire material may be sensitized to aqueous corrosion, and bad surface condition won't make that situation any better. Absent these conditions, I haven't seen evidence that surface iron contamination affects the adherence or protectiveness of scales on the surface of stainless steels in high temperature applications to the point that it is a factor in premature failures. Others here may have such evidence.

 

[moltenmetal]

...however...

Once you're into very high temperature services, liquid metal embrittlement can become a concern. Incidental contact with low melting metals such as zinc during fabrication or erection may become a concern once you're above about 1000 F operating temperature.

 

[metengr]

Typically, for boiler construction reheater and superheater tube assemblies containing 3XX series material, iron contamination is not as important or critical especially on the OD surface because of service conditions you mentioned.

If iron contamination is going to happen it would be on the OD surface and not on the ID surface of boiler tubes. With that said, most reputable tube shops know how to handle austenitic boiler tube materials. I would not be concerned about iron contamination on the OD surface especially when I have seen hardties and tie plates attached to the OD surface of austenitic stainless steel using only Grade 22 plate material.