1. Q: What are the primary differences between Incoloy 800, 800H, and 800HT in pipe form?
A: The key differences lie in chemical composition control, heat treatment, and resulting creep-rupture strength at high temperatures.
Incoloy 800 (UNS N08800) is the standard grade with a coarse grain size (ASTM grain size No. 5 or coarser). It offers good oxidation and carburization resistance but has lower creep strength at elevated temperatures. It is typically used in applications below 1100°F (593°C).
Incoloy 800H (UNS N08810) is a controlled version with a fine grain size (ASTM No. 5 or finer) and higher carbon content (0.05–0.10%). It undergoes a solution annealing treatment at 2100°F (1150°C) minimum, which enhances its creep-rupture properties, making it suitable for service above 1100°F, especially in petrochemical and power generation.
Incoloy 800HT (UNS N08811) is a further modification with even higher creep strength. It has a similar carbon range to 800H but includes controlled additions of aluminum (0.15–0.60%) and titanium (0.15–0.60%) to form precipitates. Its solution annealing temperature is higher (min. 2150°F / 1175°C), producing a coarser grain structure that maximizes creep resistance for extreme service conditions, such as superheater tubes and ethylene cracking furnaces.
2. Q: What industrial codes and specifications govern Incoloy 800/800H/800HT pipes?
A: These pipes are covered primarily by ASTM and ASME specifications. The most relevant are:
ASTM B407 / ASME SB407 – Standard specification for seamless nickel-iron-chromium alloy pipe (covers all three grades: N08800, N08810, N08811).
ASTM B408 / ASME SB408 – For bars and shapes, not directly for pipe, but relevant for fittings.
ASTM B163 / ASME SB163 – For seamless condenser and heat exchanger tubes, often applied to Incoloy 800 series for shell-and-tube heat exchangers.
ASTM B514 – For welded pipe (though seamless is more common for high-temperature service).
For pressure applications, ASME Boiler and Pressure Vessel Code Section II, Part D provides allowable stress values. Note that 800H and 800HT have higher allowable stresses at elevated temperatures due to their superior creep strength. When ordering, buyers must specify the grade (e.g., UNS N08810) and required heat treatment conditions.
3. Q: Why is Incoloy 800HT pipe preferred for high-temperature petrochemical furnace applications?
A: Incoloy 800HT pipe is preferred in high-temperature petrochemical furnaces, such as ethylene cracking and steam methane reforming (SMR) units, due to three key reasons:
Superior Creep Rupture Strength – The controlled addition of Al and Ti, combined with high-temperature solution annealing, promotes precipitation of Ni₃(Al,Ti) phases during service, significantly enhancing long-term creep resistance at 1600–1800°F (870–980°C). This prevents tube bulging and failure under sustained high stress.
Excellent Oxidation and Carburization Resistance – The high chromium (19–23%) and nickel (30–35%) content forms a stable, protective Cr₂O₃ scale. This resists carburization – a common failure mode in hydrocarbon cracking where carbon diffuses into the tube wall, leading to embrittlement and metal dusting.
Structural Stability – Unlike austenitic stainless steels (e.g., 310H), Incoloy 800HT maintains phase stability without forming harmful sigma phase even after long-term aging at high temperatures. This ensures toughness and weldability after years of service.
4. Q: What are the welding requirements and common challenges when joining Incoloy 800/800H/800HT pipes?
A: Welding Incoloy 800 series pipes requires careful control to avoid hot cracking and maintain creep strength. Key requirements include:
Filler Metal Selection: Use matching filler metals like ERNiCr-3 (AWS A5.14) or ERNiFeCr-1. In some cases, Inconel 82 or 617 is used for improved high-temperature performance.
Low Heat Input: Maintain interpass temperatures below 200°F (93°C) and use the stringer bead technique to avoid overheating. Excessive heat can cause grain coarsening, reducing the material's creep resistance – especially critical for 800H and 800HT.
Post-Weld Heat Treatment (PWHT): Not always required, but for thick sections or service above 1100°F, a solution anneal (e.g., 2100°F for 800H) may be performed. However, PWHT is often impractical for field welds; instead, controlling heat input and using proper filler metals is emphasized.
Common challenges:
Hot cracking: Due to high Ni content, avoid sulfur contamination and use low heat input.
Sigma phase formation: Not typical in 800 series but can occur if welded with certain stainless fillers – always use nickel-based fillers.
Loss of creep properties: If the base metal's fine grain structure (in 800H) is overwelded, grain coarsening reduces strength. Solution annealing after welding can restore properties but is rarely done in practice.
5. Q: In what specific corrosion environments do Incoloy 800 series pipes outperform 316L stainless steel?
A: The Incoloy 800 series significantly outperforms 316L stainless steel in several high-temperature and aggressive corrosion environments:
High-Temperature Oxidation (above 1500°F / 816°C) – 316L suffers from rapid scaling and spalling due to inadequate chromium diffusion, while Incoloy 800's higher Ni and Cr (32% Ni, 20% Cr vs. 12% Ni, 17% Cr in 316L) forms a more adherent and regenerative oxide layer. Incoloy 800 can withstand intermittent service up to 1800°F (982°C) without significant metal loss.
Carburizing and Nitriding Atmospheres – In petrochemical furnaces or ammonia plants, carbon or nitrogen penetrates 316L, forming chromium carbides or nitrides and causing embrittlement. The higher nickel in Incoloy 800 (30–35%) lowers carbon solubility and diffusion rates, providing far superior carburization resistance.
Chloride Stress Corrosion Cracking (SCC) – While 316L is susceptible to SCC in hot chloride solutions (e.g., 150°C, 100 ppm Cl⁻), Incoloy 800's high nickel content (≥30%) makes it highly resistant to chloride SCC, even in concentrated brines or steam environments. This makes it ideal for heat exchangers in marine or chemical processing where chloride carryover occurs.
Sulfidation – In reducing-sulfur atmospheres at high temperature, 316L rapidly forms iron sulfides. Incoloy 800, though not immune, performs better due to its chromium-rich scale and higher nickel content, which reduces sulfide penetration.
That said, 316L remains more cost-effective and perfectly adequate for lower temperatures (<800°F) and non-carburizing, non-chloride environments. The choice of Incoloy 800 series is justified when long-term reliability is needed under extreme heat or corrosive attack.
