Three Grades, One Alloy Family

Incoloy 800, 800H, and 800HT are the same base composition — 20% Cr, 30% Ni, 0.4% Ti, 0.4% Al, balance Fe — with one critical difference: carbon content and the resulting solution-anneal temperature. The three grades were developed sequentially by Inco (now Special Metals) starting in the 1950s for the ethylbenzene / styrene, ethylene cracking, and ammonia reformer markets.

What Actually Differs: Chemistry & Heat Treatment

Three subtle but critical differences:

1. Carbon is controlled to 0.05–0.10% in H and HT (vs ≤ 0.10% in 800) — high enough to form a stable M₂₃C₆ carbide network that pins grain boundaries and resists creep deformation.
2. Solution anneal temperature is much higher in H/HT (1,143–1,177°C) to fully dissolve the carbides and titanium carbonitrides, then re-precipitate them as a fine, uniform dispersion during service. This is what gives H and HT their creep strength.
3. Al + Ti is restricted to ≤ 0.70% in HT (and the Al:Ti ratio is specified ≥ 1) to avoid η-phase (Ni₃Ti) formation, which would deplete the matrix of strengthening elements.

ASME Section I & VIII Design Stresses

The ASME Code has long recognized the difference. The maximum allowable stress values (SMTS, “Section II Part D”) for the three grades diverge sharply above ~540°C:

* ASME Code does not list allowable stress for N08800 above 593°C — i.e. it is not Code-accepted for sustained service above ~595°C. Use N08810 or N08811.

At 800°C, the HT grade is rated 23% higher than H — a real engineering difference that allows thinner walls and longer tube runs in the same furnace.

Creep & Rupture Strength at 600–900°C

Creep-rupture data (1% strain in 100,000 h, indicative values) is the design basis for any high-temperature pressure-retaining component:

For comparison: Incoloy 800HT at 800°C has roughly the same 100,000 h rupture strength as 304H stainless, and at 700°C it is within 10% of Inconel 600 — a remarkable level of performance for an iron-based alloy at less than 1/3 the cost of an equivalent nickel-base alloy.

Grain Size: Why It Matters

The grain-size requirement is the single most important fabrication specification for 800H and 800HT. The ASTM grain size number must be 5 or coarser (i.e. lower number, larger grains). Fine grain (8 or higher) is disqualifying for high-temperature ASME service — it can reduce creep life by 50% or more.

To achieve a coarse grain, the mill must:

1. Solution anneal at 1,143°C minimum (1,143–1,177°C for HT).
2. Hold at temperature for 30 minutes per inch of section thickness (or per mill’s qualified procedure).
3. Water-quench rapidly to freeze the coarse grain structure.
4. Verify ASTM grain size on a polished and etched metallographic sample from each heat.

Applications by Temperature

Welding & Fabrication Notes

• Filler metal: ERNiCr-3 (Inconel 82) or ERNiCrCoMo-1 (Inconel 617) for matching high-temperature strength. ER308L is acceptable for low-temperature service but does not deliver equivalent creep strength in the weld zone.
• No PWHT for 800 base grade; PWHT is generally avoided for H/HT because it would refine the grain and destroy creep properties.
• Hot forming at 870–1,150°C, followed by re-anneal at the proper temperature for the grade.
• Cold forming reduces the wall-thickness section — and in some cases can raise hardness above the ASME limit for cold-formed pressure parts (Section VIII requires solution anneal after severe cold work).

Selection Matrix

Stress-Rupture Properties: The Data That Drives the 800/800H/800HT Decision

The single most important distinction between the three Incoloy 800 grades is their long-term creep and stress-rupture behavior at elevated temperature. The ASME Boiler and Pressure Vessel Code Section II Part D publishes the maximum allowable stress values (S) that designers use, but procurement teams need to understand the metallurgical basis:

Values are maximum allowable stress (S) per ASME Section II Part D, 2019 Edition, for temperatures up to the time-independent to time-dependent transition. Above 925°C, all grades enter the creep-limited regime and the differences narrow significantly.

The transition from 800 to 800H/HT typically occurs at about 600°C in pressure-vessel and piping design. Below this, plain 800 is perfectly adequate and more economical (fewer special mill requirements). Above 600°C, the grain-size and chemistry controls of 800H/HT pay for themselves in reduced wall thickness and longer service life.

Case Study: Incoloy 800H in Ethylbenzene Dehydrogenation — 30-Year Operating History

The ethylbenzene-to-styrene process is the largest single market for Incoloy 800H/HT tube, and its operating conditions exemplify why this alloy family dominates the 600–700°C range:

• Temperature: Tube metal temperature 620–650°C at the hot end, with catalyst-bed temperature cycling ±30°C during regeneration.
• Environment: Superheated steam (steam-to-ethylbenzene molar ratio ~8:1) + ethylbenzene vapor + trace styrene monomer + hydrogen (from dehydrogenation). This is a carburizing-oxidizing mixed atmosphere — one of the most aggressive forms of high-temperature corrosion.
• Stress: Tube internal pressure ~1–2 bar(g) at the inlet, declining toward vacuum at the outlet. The primary stress comes from weight (vertical tubes up to 15 meters long) and from thermal expansion restraint at the tube-to-tubesheet joint.

Over a 30-year operating history at a Gulf Coast styrene plant, the original 800H tubes showed:

• General metal loss: ~0.05 mm/yr — essentially negligible. Design margin was 3 mm corrosion allowance; after 30 years the remaining wall was still > 90% of original.
• Carburization depth: ~0.2–0.5 mm from the ID surface — carbon pickup from the cracked hydrocarbon. Acceptable as long as the carburized layer does not exceed 25% of the wall thickness (brittle fracture risk).
• Creep damage: Class A (isolated cavities) to Class B (oriented cavities) at the hottest 2 meters of tube — detected by replication metallography during each turnaround. Tubes retired when Class C (microcracks) appeared, typically at 15–20 years depending on position.
• Sigma phase: Minor amounts at the hot end after 30 years — not a concern for Incoloy 800 chemistry (which is austenitic with ~30% Ni, suppressing sigma formation compared to stainless).

Incoloy 800H in Ethylene Cracking Furnaces — The Most Demanding Application

Ethylene cracking furnace tubes operate at the extreme upper end of the Incoloy 800 family’s capability — metal temperatures of 1,000–1,100°C (well above the ASME design stress tables) with 0.5–2 bar internal pressure and a highly carburizing atmosphere. In this service, 800H/HT is used for:

• Convection-section tubes: Where the temperature is “only” 500–700°C — well within 800H capability. These rarely fail and may last 30+ years.
• Radiant-section (firebox) tubes: Where the metal skin temperature can reach 1,030–1,100°C. At these temperatures, 800H is marginal — most modern crackers specify HP-modified (35Cr-45Ni) centrifugally cast tubes for the firebox and restrict 800H to the convection section and transfer lines.
• Transfer-line exchangers (TLE): The cracked gas exits the furnace at ~850°C and is quenched to ~350°C in 0.02 seconds across Incoloy 800H tube sheets and ferrule plates. This is the most severe thermal-shock service in the entire petrochemical industry — and 800H’s low coefficient of thermal expansion (15.8 × 10⁻⁶/°C) makes it uniquely suited.

If your application requires material for an ethylene plant, contact us for a specific application review — the distinction between “800 for convection” and “HP-modified for radiant” is critical and non-negotiable.

Supply Chain Notes: Availability and Lead Times

The key procurement takeaway: if you need 800HT, plan for 3–5 months lead time — it is not a stock grade at most distributors. 800 and 800H are broadly stocked in tube and plate forms at all major nickel-alloy distributors worldwide. For critical furnace retube projects, we recommend ordering 800HT 6 months ahead of the planned turnaround date.

Sigma Phase in Incoloy 800: Myth vs Reality

A persistent myth in the engineering community holds that Incoloy 800 is “immune to sigma phase embrittlement” because of its high nickel content. This is partially true — but dangerously oversimplified. Understanding the sigma risk is critical for any component expected to operate above 600°C for more than 10,000 hours:

• Sigma phase (FeCr, tetragonal) forms in the 600–900°C range in iron-chromium-nickel alloys with Cr content above ~18%. 800 has ~20% Cr, so sigma is thermodynamically possible.
• The kinetics are SLOW because of the high Ni content (~30%). At 650°C, sigma requires approximately 20,000–50,000 hours to appear in significant quantities — far longer than typical 300-series stainless (which can sigma-embrittle in 500–2,000 hours).
• Practical consequence: For most process heater tubes (design life 100,000 hours), sigma embrittlement IS a concern at the hot end of Incoloy 800H/HT tubes. Tube retirement is often driven by the accumulation of sigma-related creep cavities, not by general wall thinning.
• Detection: Sigma can be detected by metallographic replication (in-situ, during turnaround) or by a simple room-temperature Charpy test on a boat sample. A drop in impact energy from > 100 J (new tube) to < 30 J (aged tube) is the classic sigma signature.

Dissimilar Welding: Incoloy 800 to Carbon Steel and Stainless Steel

In almost every fired heater and reformer, Incoloy 800H/HT tubes are welded to carbon steel or stainless steel components — the tube-to-return-bend joint, the tube-to-manifold connection, or the pigtail-to-header transition. These dissimilar-metal welds (DMWs) require specific filler metals and procedures:

Why ERNiCr-3 (Inconel 82) for 800-to-800 welds? It seems counterintuitive — why not use a matching Incoloy 800 filler? The reason is creep ductility: ERNiCr-3 has 67% Ni and ~20% Cr, giving the weld metal a fully austenitic, high-Ni matrix that is more creep-ductile than a matching 800 composition. In a fired-heater tube subjected to thermal cycling, the higher ductility of the 82 weld absorbs strain that would otherwise concentrate at the HAZ and cause premature cracking. This practice has been standard in the ethylene and styrene industries for 40+ years and is codified in many operator specifications.

Incoloy 800 vs 800H vs 800HT: Procurement Cheat Sheet

For the purchasing manager writing a PO for heater tubes, reformer pigtails, or nuclear steam-generator tubing, here is the decision matrix in one table:

Why Choose Huaxiao Alloy for Your Incoloy 800 Series Procurement

In-Service Inspection of Incoloy 800H/HT Tubes — Turnaround Best Practices

For fired-heater tubes operating at high temperature for years between turnarounds, the inspection plan must be comprehensive:

• Visual inspection: Look for bowing (creep sag), bulging (local overheating), and external oxidation (thick, dark scale indicates the tube ran hotter than design). Any tube with > 2% diameter increase (bulging) should be removed.
• Ultrasonic thickness (UT): Grid measurement every 90° around the circumference at 500 mm intervals along the tube length. Track the minimum wall thickness vs the retirement thickness (typically 50% of original for fired heaters).
• Replication metallography: The gold standard for creep-damage assessment. A small area (~25 mm diameter) is polished and etched in-situ, then a replica is taken with acetate film. The replica is examined under a microscope for creep cavities — Class A (isolated) = OK; Class B (oriented) = plan replacement; Class C (microcracks) = replace immediately.
• Tube-to-tubesheet weld inspection: Dye penetrant (PT) on every tube-to-tubesheet weld. The thermal cycling at the hot tubesheet creates the highest stress in the entire heater — cracks here are the most common cause of unscheduled shutdowns.
• External UT of return bends: Return bends are cast, and cast 800H has a slightly different microstructure (coarser grain, possible microporosity). Inspect every return bend by UT — a single failed return bend can shut down the entire heater pass.

Ordering Incoloy 800H/HT: What to Put on Your PO

For fired-heater tube procurement, go beyond “ASTM B163 N08810” and add these critical supplementary requirements:

1. Grain size: ASTM 5 or coarser (for 800H) or ASTM 2.5–5 (for 800HT) — verified by ASTM E112 on each tube.
2. Annealing temperature: Report the actual furnace temperature and soak time on the MTC, not just “annealed.”
3. Al + Ti content: For 800HT, 0.85–1.20% with actual values reported — not just “within range.”
4. Eddy current test: 100% ET per ASTM E426 with a maximum indication level of 1.5% of the calibration standard — this catches laminations and stringers that ultrasonic might miss.
5. Hydrostatic test: Per ASTM B163 at 1.5× design pressure, held for minimum 10 seconds per tube.
6. End preparation: Specify bevel angle and root face for butt-welding, or plain-end for rolled joints.

Extended FAQ — Incoloy 800 Series Selection

Common Failure Modes of Incoloy 800H/HT Tubes in Fired Heaters

After decades of post-mortem analysis on retired heater tubes, the failure modes are well understood. Understanding them helps procurement and maintenance teams prevent them:

Need Incoloy 800H/HT Tubes for Your Next Turnaround?

We stock Incoloy 800 and 800H seamless tubes in diameters from 1/2″ to 4″ OD, with full ASTM B163 certification and grain-size verification. For 800HT, we arrange mill-direct delivery with 14–20 week lead times. All material includes the original mill MTC with full traceability.

Frequently Asked Questions