Sustained exposure to oxidising or carburising atmospheres above 600 °C, where grain boundary attack and scale adhesion govern service life.
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.
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%).
Inconel 601 has good resistance to heat and corrosion, especially outstanding to oxidation at elevated temperatures up to 1200℃. It also has high strength, good fabricability and good resistance to aqueous corrosion.
Inconel 600 has good resistance to corrosion and high temperature, and has high strength and good fabricability. It resists chloride-ion-incurred stress corrosion cracking, sulfur compounds and oxidizing conditions at high temperatures.
Inconel X750 is an age-hardenable nickel-chromium alloy. It has good corrosion and oxidation resistance and high strength at temperatures up to 700℃. It also has good properties down to cryogenic temperatures. Its corrosion resistance performance is similar to Inconel 600.
Hastelloy X is a nickel-chromium-iron-molybdenum alloy with an exceptional combination of high temperature strength, oxidation resistance and fabricability. It shows good ductility after prolonged exposure at temperatures up to 870℃ for 16000 hours.
Haynes 230 combines excellent high temperature strength, outstanding resistance to oxidizing environments up to 1149℃ for prolonged exposure, premier resistance to nitriding environments, and excellent long term thermal stability.
Hastelloy S has excellent thermal stability, low thermal expansion and excellent oxidation resistance at 1093℃. It has good high temperature and thermal fatigue strength. It retains its strength and ductility after aging at temperatures of 427 to 871℃.
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.
Grade 800 has a relatively high carbon content and a fine grain, which makes it the weaker choice for creep rupture above about 760 °C. 800H and 800HT were developed specifically to fix that.
Chromium alone cannot build a protective scale at these temperatures. Nickel is what carries the resistance, and above roughly 1,000 °C you need an alloy with an aluminium or chromium addition as well.
