Incoloy 800 vs 800H vs 800HT Pipe: Which High-Temp Grade Do You Need?

The procurement document lists "Incoloy 800 series" - but when the team requests quotes, three different UNS designations come back: N08800, N08810, and N08811. The composition sheets look nearly identical. The price difference between 800 and 800HT can exceed 18%. And choosing the wrong grade can mean the difference between 100,000 hours of trouble-free service and a premature tube failure that shuts down the furnace.

Q1: What separates Incoloy 800 from 800H and 800HT at the chemistry level?

All three grades share the same base: roughly 30-35% nickel, 19-23% chromium, balance iron. They belong to the same Fe-Ni-Cr family. The differences are in three controlled elements - carbon, aluminum, and titanium - and in grain size requirements.

Here is the full chemistry comparison per ASTM B407:

Three things to notice:

First, 800 allows carbon up to 0.10% with no floor. A heat at 0.03% carbon meets N08800 - but that low carbon actually reduces creep strength at temperatures above 600°C. So "within spec" does not mean "suitable for your application."

Second, 800H tightens carbon to a 0.05–0.10% range. This eliminates the low-carbon heats and ensures enough carbon for carbide precipitation strengthening at grain boundaries - the mechanism that gives 800H its superior creep resistance.

Third, 800HT adds a combined Al+Ti requirement of 0.85–1.20%. This tighter control on gamma-prime precipitation strengthening gives 800HT a further edge in long-term creep rupture life compared to 800H, particularly above 760°C.

Q2: I'm running a furnace at 950°C - which grade survives?

Short answer: 800HT, if your design life exceeds 100,000 hours. 800H will work but with a thinner safety margin. 800 should not be used at this temperature for pressure-retaining components.

Here is why. At 950°C, the dominant failure mode is creep - the slow, progressive deformation of the metal under sustained stress. The 100,000-hour creep rupture strength tells you the stress the material can carry for that duration before fracturing. The three grades diverge sharply at elevated temperature:

Values are approximate from published literature for comparison. Exact values depend on product form, heat treatment, and grain size. Verify against current ASME BPVC and supplier mill certs for design purposes.

At 950°C, 800HT gives you roughly 7 MPa of creep rupture strength at 100,000 hours - more than double what 800 delivers. In a furnace tube designed for, say, 5 MPa hoop stress, that difference is the margin between "designed for the service life" and "running on borrowed time."

For context, a Versalis steam cracker in Priolo, Sicily, running radiant coils at 920–980°C, specified 800HT for its 12-year design life cycle. The alternative - using 800H - would have required thicker walls to compensate for lower creep strength, and the resulting thermal stress from the thicker section could actually reduce fatigue life during thermal cycling.

Q3: My supplier says "800HT covers 800H" - is that true?

This is one of the most common sources of confusion. The short version:

Forward substitution (800H → 800HT): Yes, generally acceptable. If your spec calls for 800H and the supplier offers 800HT, the 800HT meets all chemistry requirements of 800H and then some. The tighter Al+Ti range and carbon floor of 800HT are a subset of the 800H specification. You get better creep performance.

Backward substitution (800HT → 800H): Not automatically. If your spec calls for 800HT, an 800H heat that happens to meet the Al+Ti combined range of 0.85–1.20% and the carbon range of 0.06–0.10% can be dual-certified. But a random 800H heat with Al+Ti of 0.40% does not meet 800HT requirements.

Using 800 where 800H is specified: Not recommended for high-temperature service. A low-carbon 800 heat (C = 0.03%) may technically meet N08800 but will have inferior creep strength. For ASME-coded pressure components, 800 has lower allowable stress values than 800H/800HT at elevated temperatures - your code calculations would not be valid.

The practical takeaway: if you need 800H and your supplier has 800HT in stock at a reasonable premium, take it. If you need 800HT, do not accept 800H without verifying the mill cert meets the Al+Ti and carbon requirements for N08811.

Q4: How does grain size factor into my decision?

Grain size is the second lever - after carbon control - that determines creep performance. ASTM B407 requires 800H to have a grain size of ASTM 5 or coarser (meaning the average grain diameter is ≥ ~64 μm).

Why does grain size matter for your tube selection?

At elevated temperatures (above roughly 0.5 times the melting point in Kelvin, which for these alloys is around 600°C), creep deformation is dominated by grain boundary sliding. Individual grains slide past each other along their boundaries. The more grain boundary area per unit volume, the more sliding occurs.

Coarser grains mean:

Less grain boundary area per unit volume → less sliding → higher creep resistance

Lower fatigue strength at lower temperatures (where crack initiation at boundaries dominates)

Lower impact toughness at room temperature

This is why 800H specifies coarse grain - it is optimized for high-temperature creep, not for ambient mechanical performance. If your application involves thermal cycling with significant low-temperature excursions, the fatigue tradeoff from coarse grains should be evaluated.

For reference:

If your mill cert shows grain size ASTM 7 or finer for material sold as 800H, it does not meet the specification. This is a check you can make directly on the MTR.

Q5: What filler metal do I use for welding these three grades?

The good news: the welding procedure is essentially the same for all three grades. You do not need different filler metals for 800 versus 800H versus 800HT. The base metal chemistry differences are in carbon and gamma-prime formers (Al, Ti), which are controlled in the base metal, not deposited from the filler.

Here is the welding consumable reference for Incoloy 800 series pipe:

Key welding considerations:

Preheat: Not typically required, but ensure moisture-free surfaces.

Interpass temperature: Keep below 150°C to minimize sensitization in the HAZ.

Post-weld heat treatment: Solution anneal at 1150–1205°C (2100–2200°F) followed by rapid cooling if the code or service environment requires it. For furnace tube service, a full solution anneal after welding restores creep properties in the weld zone.

Avoid filler metals with high Fe content - they can crack in the HAZ due to thermal expansion mismatch.

Q6: What temperature can each grade handle before I am in trouble?

For pressure-retaining service, the ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 1 publishes maximum allowable stress values. These are the legal stress limits for code-stamped vessels. Here is a representative comparison:

Representative values for comparison. Exact ASME stress values change with code editions and product form - always verify against the current ASME BPVC edition applicable to your project.

The key observations:

Below 600°C: All three grades perform similarly. 800 is perfectly adequate and typically 10–15% cheaper.

600–760°C: 800H and 800HT pull ahead. 800's allowable stress drops off, meaning you need thicker walls - which costs more material and adds weight.

Above 760°C: 800 is not listed in the ASME code stress table at these temperatures for Section VIII applications. You must use 800H or 800HT.

Above 900°C: 800HT has a measurable but modest advantage over 800H. For design lives exceeding 100,000 hours at 900°C+, 800HT is the preferred choice.

Grade Selection Decision Tree

Here is a practical selection flow that condenses the above into a decision sequence:

Step 1 - What is your sustained design temperature?

≤ 600°C → Go to Step 2A

600–760°C → Go to Step 2B

760°C → Go to Step 2C

Step 2A - Is this a code-stamped pressure component?

Yes, cost-sensitive → Incoloy 800 (cheapest, adequate stress at this temperature)

Yes, long design life (>100,000 hr) → Incoloy 800H (margin for creep)

No (structural/non-pressure) → Incoloy 800

Step 2B - What is your design life requirement?

≤ 100,000 hours → Incoloy 800H (good balance of cost and creep)

100,000 hours → Consider Incoloy 800HT for additional margin

Cost is primary driver, shorter life acceptable → Incoloy 800H

Step 2C - Is the service cyclic (frequent thermal transients)?

Yes, heavy cycling → Incoloy 800H (slightly better fatigue from marginally finer grain)

No, steady-state high temp → Incoloy 800HT (best creep rupture)

Long-term continuous operation (>100,000 hr at >900°C) → Incoloy 800HT(required for margin)

Step 3 - Verify availability and cost:

FAQ

Can I use Incoloy 800 instead of 800H if my temperature is only slightly above 600°C?

Marginal. At 620°C sustained, 800 technically meets ASME stress limits, but the margin is thin. If the furnace has temperature excursions (upset conditions, startup peaks), 800H gives you a safety buffer that 800 does not. The cost premium for 800H is typically 8–12%. For a critical pressure component, that is cheap insurance.

Does 800HT require different welding procedures than 800H?

No. The same filler metals (ERNiCr-3 for GTAW, ENiCrFe-2 for SMAW) and the same procedures work for all three grades. The chemistry differences are in the base metal carbon and Al+Ti content, which do not transfer through the weld pool.

My project requires ASME stamping - which grade is code-compliant?

All three are listed in ASME BPVC Section VIII Division 1, Table 1A. However, 800 has lower allowable stress values at temperatures above 600°C. If your design temperature exceeds 760°C, 800 may not appear in the stress table at all. 800H and 800HT are the standard choices for ASME-coded high-temperature pressure components.

Is 800HT harder to source than 800H?

Yes, typically. 800H is more commonly produced and stocked. 800HT's tighter chemistry window (Al+Ti 0.85–1.20%) means fewer heats qualify. Some mills produce dual-certified 800H/800HT material - if the heat meets 800HT requirements, it can be certified as both. For urgent projects, 800H is the practical choice.

Can a single heat be dual-certified as 800, 800H, and 800HT?

No. 800 allows carbon as low as 0.03% with no floor, while 800H requires 0.05% minimum. A heat at C = 0.03% meets 800 but not 800H. However, a heat at C = 0.08% with Al+Ti = 1.0% can be certified as 800, 800H, and 800HT simultaneously. This is uncommon but possible.

Takeaway for Your Purchase Decision

Need Incoloy 800H or 800HT pipe for your high-temperature project?

HUITONG supplies ASTM B407 seamless pipe in N08810 and N08811, with full ASME SB407 compliance, EN 10204 3.1 MTRs, and sizes from 1/2" to 20". We can also arrange U-bends for heat exchanger applications.

📧 Contact: market@htpipe.com 💬 WhatsApp: +86-19339900201 🌐 Product range: https://www.htsteelpipe.com/incoloy-steel-pipe/

We will tell you honestly if 800 is enough for your temperature - and we will not push 800HT unless your service conditions actually require it.

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