1. Q: What are the primary differences between Incoloy 800, 800H, and 800HT in terms of chemical composition and high-temperature strength?
A: The key differences lie in carbon content, grain size, and creep rupture strength.
Incoloy 800 has a carbon content of ≤0.10%, with no strict control over grain size. It is suitable for moderate temperatures up to 600°C (1112°F).
Incoloy 800H features a controlled carbon range of 0.05–0.10% and a minimum average grain size of ASTM No. 5, ensuring better creep resistance at temperatures above 650°C (1202°F).
Incoloy 800HT further optimizes creep properties by adding up to 1.0% aluminum and titanium (combined) while maintaining the same carbon and grain size control as 800H. This makes 800HT the preferred choice for service at 700–900°C (1292–1652°F), such as in petrochemical cracking tubes and superheater supports.
2. Q: Why is Incoloy 800H/800HT widely used in steam methane reforming (SMR) furnaces for hydrogen production?
A: SMR furnaces operate at extreme temperatures (up to 950°C / 1742°F) and pressures, with corrosive atmospheres containing steam, hydrogen, and carbon oxides. Incoloy 800H/800HT offers:
High creep rupture strength under long-term static loads, preventing tube bulging or failure.
Resistance to carburization and metal dusting due to a stable austenitic matrix and protective oxide layer (Cr₂O₃ + SiO₂).
Good thermal fatigue resistance against frequent start-ups and shutdowns.
Industry standards (e.g., NACE TR 8-304) recommend 800HT for outlet pigtails and transfer lines where the combination of thermal stress and corrosive gases is most severe.
3. Q: Can Incoloy 800 series be welded without post-weld heat treatment (PWHT)? What precautions are necessary?
A: Yes, all three grades are weldable using common techniques (GTAW, SMAW, SAW). However, Incoloy 800H and 800HT generally do not require PWHT because of their stable austenitic structure and controlled carbon content, which minimizes the risk of carbide precipitation and intergranular corrosion.
Precautions include:
Using matching filler metals (e.g., ER NiCr-3 or ER NiCrCoMo-1) to maintain creep strength.
Avoiding overheating to prevent excessive grain coarsening.
For highly restrained joints or thick sections, a solution annealing at 1150–1200°C (2102–2192°F) followed by rapid cooling may be applied to restore ductility.
In contrast, the standard Incoloy 800 (non-H) may require PWHT if used in sensitization-prone environments.
4. Q: How does Incoloy 800HT perform in molten salt environments, such as concentrated solar power (CSP) plants?
A: In CSP plants using nitrate or carbonate salt mixtures (e.g., Solar Salt: 60% NaNO₃ + 40% KNO₃) at 500–700°C, Incoloy 800HT exhibits excellent resistance to hot corrosion and chlorides. Its high chromium (20–23%) and nickel (30–35%) content form a stable oxide scale that resists salt-induced attack.
Long-term tests (up to 10,000 hours) show that 800HT maintains tensile strength and microstructural stability, with negligible intergranular attack. Compared to stainless steels (e.g., 347H), 800HT suffers less from stress corrosion cracking (SCC) caused by chloride impurities in molten salts. Consequently, it is often selected for thermal storage tank shells and heat exchanger tubing in CSP systems.
5. Q: What industry standards and certifications apply to Incoloy 800/800H/800HT for pressure vessel and boiler applications?
A: The key standards include:
ASME Boiler and Pressure Vessel Code:
Incoloy 800: ASME SB-163 (tubing), SB-366 (fittings), SB-407 (pipe), with allowable stress values listed in Section II, Part D.
800H and 800HT: Covered under ASME Code Case 2225 (for 800H) and Code Case 2159 (for 800HT), allowing higher design stress at elevated temperatures.
ASTM specifications:
B407 (seamless pipe and tube), B408 (bar/rod), B409 (plate/sheet/strip).
NACE MR0175/ISO 15156: For sour gas environments, both 800H and 800HT are acceptable when properly solution annealed.
EN 10216-5 (European standard for seamless tubes).
For nuclear applications, 800H is also listed in RCC-MRx (French code) for intermediate heat exchangers in very-high-temperature reactors (VHTR). Always consult the latest revision, as allowable stresses may differ between grades due to creep data updates.
