What are the specific applications where Incoloy 800H round seamless tube

1. Q: What are the primary differences in chemical composition, heat treatment, and creep strength between Incoloy 800, 800H, and 800HT round seamless tubes?

A:
All three grades are based on the same nickel-iron-chromium system (Ni 30–35%, Cr 19–23%, Fe balance), but controlled differences in carbon content, grain size, and precipitation-hardening elements create distinct performance levels for high-temperature service.

Incoloy 800 (UNS N08800):

Carbon content: ≤0.10% (no lower limit)

Grain size: no specific requirement (typically fine-grained)

Aluminum + Titanium: 0.15–0.60%

Strengthening mechanism: Solid-solution with limited carbide precipitation

Typical creep strength (100,000 hr rupture at 700°C): ≈ 35 MPa

Maximum service temperature: 600°C (1112°F) for load-bearing applications

Incoloy 800H (UNS N08810):

Carbon content: 0.05–0.10% (strictly controlled)

Grain size: minimum ASTM No. 5 (coarse grain)

Aluminum + Titanium: 0.15–0.60%

Strengthening mechanism: Controlled grain size + uniform M₂₃C₆ carbide precipitation at grain boundaries

Typical creep strength (100,000 hr rupture at 700°C): ≈ 55 MPa

Maximum service temperature: 900°C (1652°F)

Incoloy 800HT (UNS N08811):

Carbon content: 0.06–0.10%

Grain size: minimum ASTM No. 5

Aluminum + Titanium: 0.85–1.20% (significantly higher)

Strengthening mechanism: Coarse grain + M₂₃C₆ carbides + fine Ti(C,N) carbonitrides that resist coarsening

Typical creep strength (100,000 hr rupture at 700°C): ≈ 70 MPa

Maximum service temperature: 980°C (1796°F)

Key manufacturing difference:
800 is typically supplied in the solution-annealed condition (1100–1200°C, rapid cool) with no further heat treatment. 800H and 800HT require a final solution anneal at 1150–1200°C (2100–2190°F) followed by rapid cooling to achieve the specified coarse grain structure. This high-temperature anneal dissolves carbides and allows controlled grain growth, which is essential for creep resistance.

Selection guidance:

Use 800 for service below 600°C where creep is not a concern.

Use 800H for service between 600–900°C under static loads.

Use 800HT for the most demanding high-temperature applications (ethylene cracking, steam methane reforming) or where thermal cycling is severe.

---

2. Q: Why is Incoloy 800H / 800HT round seamless tube the preferred material for steam methane reforming (SMR) furnace outlet pigtails and transfer lines?

A:
Steam methane reforming (SMR) is the primary industrial process for hydrogen production. The outlet pigtails and transfer lines carry reformed gas (H₂, CO, CO₂, H₂O, residual CH₄) from the radiant section at temperatures of 800–950°C (1472–1742°F) and pressures of 15–35 bar. These conditions create a unique combination of creep, thermal fatigue, and corrosion challenges.

Why 800H / 800HT is specified:

1. Creep rupture strength at temperature:
SMR outlet piping experiences constant internal pressure (hoop stress) at temperatures where most alloys rapidly deform. The controlled carbon and coarse grain structure of 800H/800HT provide a 100,000-hour creep rupture strength of approximately 40–50 MPa at 900°C. This allows designers to use reasonable wall thicknesses (typically 4–8 mm for 4–8 inch piping) with safe stress levels.

2. Resistance to thermal fatigue:
SMR furnaces undergo frequent start-ups and shutdowns (sometimes weekly for maintenance). The coarse-grained structure of 800H/800HT provides better thermal fatigue resistance than fine-grained 800. The high nickel content (30–35%) also maintains ductility after long-term aging, preventing brittle fracture during thermal cycles.

3. Carburization resistance:
The reformed gas contains carbon monoxide and methane, which can carburize many alloys, leading to embrittlement and cracking. Incoloy 800H/800HT forms a stable, slow-growing Cr₂O₃ scale that resists carbon ingress. The controlled silicon content (typically 0.3–0.7%) further enhances carburization resistance by forming a sub-scale SiO₂ layer.

4. Oxidation resistance:
The 19–23% chromium content provides excellent resistance to high-temperature oxidation. Even in the presence of steam (which can accelerate oxidation of some alloys), 800H/800HT maintains a protective scale.

5. Fabricability:
SMR pigtails require complex bends and weldments. 800H/800HT tubes can be cold or hot bent, and welded using standard techniques (GTAW with ERNiCr-3 filler). Post-weld heat treatment is not required, simplifying field fabrication.

Failure modes avoided:

800 (fine-grained) would suffer from creep rupture within 2–3 years due to grain boundary sliding.

310 stainless steel would carburize and become brittle within 12–18 months.

Alloy 600 would perform similarly but at significantly higher cost.

Field experience:
Incoloy 800HT seamless tubes are standard for SMR pigtails in hydrogen plants worldwide, with typical service lives of 8–12 years. Replacement is usually due to creep distortion (bulging) or thermal fatigue cracking after 80,000–100,000 hours, rather than catastrophic failure.

---

3. Q: What are the recommended welding practices and filler metals for joining Incoloy 800H / 800HT round seamless tubes, and is post-weld heat treatment required?

A:
Incoloy 800H and 800HT are readily weldable using common arc welding processes, but proper filler metal selection and technique are essential to maintain high-temperature strength.

Welding processes:

GTAW (TIG) – Preferred for thin-wall tube and root passes. Provides best control of heat input and weld pool.

GMAW (MIG) – Suitable for fill and cap passes on thicker walls.

SMAW (stick) – Acceptable for field welding where GTAW equipment is unavailable.

Filler metal recommendations:

For 800H to 800H welding: ERNiCr-3 is recommended. It provides a weld metal with approximately 70–80% nickel, 20% chromium, and 2–3% niobium. The high nickel content maintains ductility, while niobium prevents hot cracking.

For welding 800H to dissimilar metals (e.g., to stainless steel 310 or 347):
Use ERNiCr-3 or ERNiCrFe-6. The high nickel filler accommodates differential thermal expansion between the alloys.

Welding precautions:

No preheating required – Preheating is unnecessary and may promote grain coarsening in the heat-affected zone (HAZ).

Interpass temperature – Maintain below 150°C (300°F). Excessive interpass temperatures can cause sensitization or unwanted carbide precipitation.

Low heat input – Use 0.5–1.5 kJ/mm. Stringer beads (no weaving) and multiple thin passes produce the best microstructure.

Back-purging – For tube welding, back-purge with argon to prevent oxidation of the root pass. Oxidized root beads have reduced creep strength.

Shielding gas – 100% argon for GTAW. For GMAW, use argon-helium mixtures (75% Ar + 25% He) to improve penetration.

Post-weld heat treatment (PWHT):
Generally NOT required for 800H/800HT tubes in high-temperature service. The as-welded structure retains adequate creep strength for most applications.

However, PWHT (solution annealing at 1150–1200°C followed by rapid cooling) may be specified for:

Heavily cold-worked tube that is subsequently welded (restores ductility)

Components that require maximum creep strength in the weld region

Service conditions with severe thermal cycling (the PWHT homogenizes the weld microstructure)

Important note: If PWHT is performed, the entire tube assembly must be heat-treated uniformly. Localized PWHT (e.g., torch heating of a weld) is ineffective and can cause localized grain growth or distortion.

NACE requirement: 800H/800HT are not typically used in sour wet service. For high-temperature hydrogen service (e.g., reformer outlet), no NACE restrictions apply.

---

4. Q: What are the specific applications where Incoloy 800H round seamless tube is mandated over standard 800, and where is 800HT required instead of 800H?

A:
The choice between 800, 800H, and 800HT depends on operating temperature, stress level, and expected service life.

Applications mandating Incoloy 800H over 800:

Applications mandating Incoloy 800HT over 800H:

Comparative service life example (ethylene cracking furnace TLE at 950°C, 5 MPa):

Cost-benefit analysis:
800HT seamless tube typically costs 10–20% more than 800H, but the extended service life (often double) makes it cost-effective for critical, difficult-to-replace components. For easily accessible piping at moderate temperatures (600–750°C), 800H remains the standard choice.

Selection rule of thumb:

T < 600°C, no creep concern → 800

600°C < T < 850°C, continuous service → 800H

T > 850°C, or thermal cycling, or > 5 MPa stress → 800HT

T > 950°C → 800HT is minimum; consider cast alloys or refractory metals for extreme conditions

---

5. Q: What are the critical heat treatment requirements for Incoloy 800H and 800HT round seamless tubes, and how do they affect microstructure and properties?

A:
Unlike many precipitation-hardening alloys, Incoloy 800H and 800HT achieve their creep strength through controlled grain size and carbide distribution, not through aging. However, proper solution annealing is critical.

Solution annealing – the critical heat treatment:

For Incoloy 800H:

Temperature: 1150–1200°C (2100–2190°F)

Time: 15–60 minutes (depending on wall thickness)

Cooling: Rapid (water quench or forced air)

Resulting grain size: Minimum ASTM No. 5 (coarse)

For Incoloy 800HT:

Temperature: 1150–1200°C (2100–2190°F)

Time: 15–60 minutes

Cooling: Rapid (water quench typically required)

Resulting grain size: Minimum ASTM No. 5, with uniform Ti(C,N) carbonitrides

Why this specific heat treatment is essential:

Grain size control – The high-temperature anneal dissolves all carbides and allows grains to grow to the specified coarse size (ASTM No. 5 corresponds to approximately 64–128 µm average diameter). Coarse grains reduce grain boundary area, which minimizes grain boundary sliding - the primary creep mechanism at high temperatures.

Carbide dissolution and reprecipitation – During solution annealing, all M₂₃C₆ carbides dissolve. Upon cooling, fine carbides reprecipitate uniformly along grain boundaries. These carbides pin dislocations and prevent grain boundary movement during service.

Carbonitride formation (800HT only) – The higher titanium and aluminum content in 800HT forms stable Ti(C,N) carbonitrides during cooling. These particles are much more resistant to coarsening than chromium carbides, providing long-term creep strength even after 50,000–100,000 hours of service.

Consequences of improper heat treatment:

Is post-service heat treatment possible?
After long-term service (e.g., 50,000 hours at 850°C), the carbide structure coarsens, and creep strength decreases. It is theoretically possible to restore properties by re-solution annealing, but this is rarely practical for installed tube due to:

Size and geometry constraints (furnace capacity)

Oxidation scale removal requirements

Risk of distortion during reheating

Cost (often exceeds replacement cost)

Practical guidance:

Always purchase 800H/800HT tube from qualified mills that certify grain size and solution annealing parameters.

Do not perform additional heat treatment on finished tubes unless specifically approved by the manufacturer.

If field bending or forming is required, perform the operation in the solution-annealed condition (soft). Cold work followed by stress relief at 900–950°C is not equivalent to full solution annealing and will not restore creep strength.

Inspection verification:
For critical applications (ethylene cracking, SMR), verify the following on the mill test certificate:

Grain size (ASTM No. 5 minimum, measured per ASTM E112)

Carbon content (0.05–0.10% for 800H; 0.06–0.10% for 800HT)

Aluminum + Titanium (0.15–0.60% for 800H; 0.85–1.20% for 800HT)

Mechanical properties at room temperature and elevated temperature (if specified)

Final note: 800H and 800HT are not age-hardenable. Attempting to perform a low-temperature aging treatment (e.g., 600–700°C) will not increase strength and may actually reduce ductility by coarsening carbides prematurely. The only heat treatment that matters is the initial solution anneal.