Long design life, elevated temperature, and a documentation regime that is stricter than the material itself.
Almost every alloy decision comes back to these. Get them right and the grade usually picks itself.
General corrosion, pitting, crevice, stress-corrosion cracking, oxidation, creep. Each one excludes different alloys. Deciding which mechanism governs removes most of the shortlist immediately.
Nominal conditions are not design conditions. Peak temperature, peak concentration, upset conditions and shutdown states are what actually consume the material.
The same grade carries different ASTM specifications for bar, plate, pipe and forgings. Name the form with the grade, or accept whatever the mill has in stock.
The grades that suit this service, with their UNS numbers and the specifications they are supplied to.
Inconel 625 has excellent strength at temperatures up to 816℃. At higher temperatures, its strength is generally lower than that of other solid solution strengthened alloys.
Inconel 718 is an age-hardenable nickel-chromium alloy that combines corrosion resistance with high strength and good fabricability. It has high creep-rupture strength at temperatures up to 700℃. Its excellent relaxation resistance contributes to its application in springs.
Incoloy 800H has a restricted carbon content range of 0.05 to 0.10%, which is in the upper portion of that of Incoloy 800, and is annealed at 1149 to 1177℃ (Incoloy 800 is annealed at 983 to 1038℃). These differences impart it to have higher stress rupture and creep properties than Incoloy 800.
Incoloy 800HT ensures higher stress rupture and creep properties compared to Incoloy 800H as a result of restricting the carbon content further in the range of 0.06 to 0.10% (Incoloy 800H is 0.05 to 0.10%), and the aluminum plus titanium content at 0.85 to 1.20% (Incoloy 800H is 0.30 to 1.20%).
Hastelloy C-276 has excellent resistance to localized corrosion, stress corrosion cracking, and to both oxidizing and reducing media, thus making it suitable for a wide variety of chemical process environments, including ferric and cupric chlorides, hot contaminated media (organic and inorganic),…
Invar 36 is a 36% nickel-iron alloy notable for its uniquely low coefficient of thermal expansion which is roughly one-tenth that of carbon steel in the range from room temperature to around 200℃. It also retains good strength and toughness at cryogenic temperatures.
The wrong grade in the right service fails quietly for a long time and then all at once. These are the pairings that go wrong most often.
The low expansion behaviour of Invar depends on its magnetic transformation near 279 °C. Above that the expansion coefficient rises sharply and the whole point of the alloy is lost.
In this sector the certification is the product. A correct alloy in an incomplete document package is not deliverable.
