​‌​​‌​

Incoloy 800H / (1.4876) / UNS N08810 / AMS 5766 / AMS 5871

​‌​​‌​

Incoloy 800H, generally known as Alloy 800, is a nickel alloy chosen where high temperatures are involved and mechanical strength is required. Incoloy 800 exists in three different versions that differ in carbon, titanium and aluminium content. The most widely used of these versions is grade 1.4876.

The first of the three Incoloy 800 materials is Incoloy 800 (Alloy 800), also designated 1.4876. When the price of nickel rose in the 1950s, heat resistant materials with a high nickel content became very expensive. Metallurgists therefore began to research and develop a material that contained less nickel while withstanding the same temperatures. Standing up as well as many materials with a higher nickel content, Incoloy 800 creates a price and cost advantage through its lower nickel content.​‌​​‌​

Another widely used version is Incoloy 800H, also known as Alloy 800H. Alloy 800H is the version of Alloy 800 with a slightly higher carbon content, and that small amount of extra carbon gives the material somewhat more strength. Apart from the carbon content there is little difference between Incoloy 800H and Incoloy 800. Alloy 800H is also designated UNS N08810, and in the DIN system it is designated 1.4958.

The third and most distinct version of Incoloy 800 is Incoloy 800HT. The carbon content of this material is similar to that of Alloy 800H, but in addition its aluminium and titanium contents are higher than in the other two versions. This makes Incoloy 800HT somewhat more durable in more demanding and severe environments. Designated UNS N08811, it is also known as 1.4959. Both Incoloy 800H and Incoloy 800HT are designated X10NiCrAlTi32-20 under the EN standards.​‌​​‌​

The essential differences between these three Alloy 800 materials are their temperature capability, grain size and mechanical strength. Incoloy 800 is a nickel alloy recommended for use up to 816 °C; at higher temperatures the 800H and 800HT alloys should be used. The grain size of Alloy 800H and 800HT is specified and is required to exceed defined values, whereas Alloy 800 has no specified grain size. Finally, in terms of temperature capability and mechanical strength, the comparison from strongest to weakest is: Incoloy 800HT > Incoloy 800H > Incoloy 800.

800, 800H and 800HT — the Centrepiece of This Page​‌​​‌​

The three grades have identical nickel, chromium and iron. Everything that separates them is carbon, aluminium plus titanium, grain size and annealing temperature — which is precisely why buyers get it wrong: the top three lines of the mill certificate look the same for all three.

The service-temperature dividing line — and a real divergence. ASTM’s own scope text (which appears verbatim in B407, B408, B409, B514 and B564) says: “Alloy UNS N08800 is normally employed in service temperatures up to and including 1100 °F (593 °C)”, while the other alloys are for “service temperatures above 1100 °F where resistance to creep and rupture is required, and they are annealed to develop controlled grain size for optimum properties in this temperature range”. Against that, the originator’s own bulletin says the H grades are used “especially at temperatures above 1500 °F (816 °C)“. Both figures are real and must not be averaged. The safe procurement rule is ASTM’s: above 1100 °F / 593 °C, order 800H or 800HT, not 800.

What Actually Fails with “800/800H Dual-Certified” Material

800H/800HT dual certification is legitimate and is the normal commercial product. Every 800HT limit sits inside 800H (carbon 0.06–0.10 within 0.05–0.10; Al+Ti 0.85–1.20 within 0.30–1.20), so one heat genuinely satisfies both — the trade calls it “800H/AT”. Accept it.​‌​​‌​

The dual certification that deserves scrutiny is 800/800H. Chemically it can be honest: carbon of 0.05–0.10 % also satisfies 800’s “0.10 % max”. But alloy 800 imposes no grain-size requirement and may be annealed as low as 980 °C. The “800” line on the certificate provides cover for material that was processed as N08800 — Grade 1 anneal, fine grain, no grain-size report. That material is a perfectly valid N08800 and a non-conforming N08810.

The failure is invisible at goods-in. Room-temperature tensile passes — the originator notes that alloy 800 actually has higher room-temperature and short-time elevated-temperature properties than 800H. Chemistry passes. The failure appears in service above 1100 °F, thousands of hours in, as bulging, cavitation and early rupture of a component designed to 800H allowable stresses — because those allowables were derived from coarse-grained material.​‌​​‌​

Practical acceptance rule: a certificate is only 800H if it shows (1) C ≥ 0.05 %, (2) a reported ASTM grain size of 5 or coarser on the certificate, and (3) the actual annealing temperature, ≥1150 °C. If the grain-size line is blank, it is not 800H, whatever the heading says. For 800HT additionally require Al+Ti 0.85–1.20 % with Al and Ti each ≥0.25 %. One more warning: alloy 800 itself has a Grade 2 anneal at about 1150 °C, so “annealed at 2100 °F” alone proves nothing.

Standards by Product Form​‌​​‌​

Three mis-citations, with the correction. (1) ASTM A240 is cited for 800/800H plate; A240’s scope is “chromium, chromium-nickel, and chromium-manganese-nickel stainless steel plate, sheet and strip”, and N08800/N08810/N08811’s presence in its grade table could not be confirmed — do not put A240 on a purchase order for 800H plate; cite B409. (2) ASTM B516 is not an 800H specification: its scope covers N06600, N06601, N06603, N06025, N06045, N06690, N06693 and N06699 — no 800-family alloy at all. The correct welded-tube specification is B515. (3) B514 does not cover N08811: an order saying “800HT welded pipe per B514” is unsatisfiable.

Mechanical Properties, Physical Properties and Welding​‌​​‌​

​‌​​‌​

Specification minimums are not printed on this page. None of the sources reviewed gives the ASTM/ASME minimum tensile, yield and elongation for N08810 or N08811; those live in B409/B407/B408 Table 3. One distributor’s “Yield 200 MPa minimum / Tensile 531 MPa minimum / Elongation 52 % minimum” is not a set of minimums: another distributor publishes the identical three numbers as typical room-temperature values, and a 52 % elongation minimum is not credible for any wrought alloy. Note too that the minimums differ between N08800 and N08810 — the originator states alloy 800 has higher room-temperature properties — so a single shared minimums table for all three grades would be wrong.

Physical properties: density 7.94 g/cm³ · melting range 1357–1385 °C · specific heat (0–100 °C) 460 J/kg·°C · modulus of elasticity 196 GPa at 70 °F. Magnetic behaviour: austenitic and essentially non-magnetic at and above room temperature (µ = 1.014 at 200 Oe); because the Curie temperature is −115 °C, it becomes ferromagnetic only at cryogenic temperatures. A buyer testing with a magnet at ambient will correctly find it non-magnetic; a strongly magnetic response is evidence of the wrong material or of heavy cold work.

Welding​‌​​‌​

Processes: SMAW, GTAW and GMAW. ASME Section IX P-Number: P45. Consumable selection changes with service temperature, and the crossover is the key fact: up to 788 °C (1450 °F) use INCO-WELD A for SMAW and INCONEL Filler Metal 82 (AWS ERNiCr-3) for GTAW/GMAW; above 788 °C use INCONEL Welding Electrode 117 for SMAW and INCONEL Filler Metal 617 (AWS ERNiCrCoMo-1) for GTAW/GMAW. RA330-04 (UNS N08334) is an alternative. Matching-composition filler is not used — both mills point to nickel-base fillers (82 / 617), not an 800H-composition wire, and this is worth saying explicitly because buyers ask for “800H filler”. The AWS classification of INCO-WELD A (commonly quoted as ENiCrFe-2) could not be verified; do not print an AWS class for the covered electrode until it is confirmed.

Post-weld heat treatment carries a genuine, safety-relevant disagreement. One distributor says “no post-weld heat treatment is required”. Another requires, for N08811 entering service above 538 °C, 899 °C for approximately 1 hour per inch of thickness (30 minutes minimum), then air cool, in order to “avoid possible stress relaxation grain boundary cracking“. These directly contradict. Stress-relaxation (reheat) cracking is a well-known problem in coarse-grained creep-resistant austenitics; for high-temperature service follow the specific, mechanistically-argued position.​‌​​‌​

No numeric preheat, interpass temperature or heat-input limits are printed on this page — none could be verified. The welding pitfalls the page should list: (1) stress-relaxation cracking in the coarse-grained HAZ of heavy sections; (2) welding or any hot work in the 1200–1600 °F (650–870 °C) range is prohibited; (3) HAZ sensitisation in the 540–760 °C range; (4) choosing Filler 82 / ERNiCr-3 for service above 788 °C — it is the wrong filler above that line.

Heat Treatment, Fabrication and Service Behaviour​‌​​‌​

Solution anneal at 1150–1200 °C, with time at temperature adjusted to achieve an ASTM grain size of 5 or coarser; 800HT specifically requires a minimum 1149 °C treatment. Stress relief begins at about 540 °C and is virtually complete after 870 °C; the guideline is 1 hour per inch (25 mm) of thickness, or 1½ hours at 870 °C, whichever is greater. Not hardenable by heat treatment: it is strengthened by solid solution, by carbon and carbides and by cold work, and at temperature by controlled coarse grain. Sensitisation window 540–760 °C (the underlying chromium-carbide precipitation runs 540–1095 °C); exposure there opens the alloy to intergranular attack in aggressive media. No claim is made about sigma phase — none of the sources reviewed discusses sigma for 800H, so neither freedom from it nor susceptibility to it should be asserted.​‌​​‌​

Machining: with coated carbide, 33.5–57.9 m/min (110–190 sfpm) at 0.20–0.89 mm/rev with good tool life; with high-speed steel, 10.7–29.0 m/min. “Readily machined by standard methods.” Hot forming 870–1200 °C; heavy forging down to 1010 °C, light working down to 870 °C. “No working should be done between 1200 and 1600 °F (650–870 °C).” That is a hard prohibition and the single most-ignored fabrication rule for this alloy. Cold forming: very ductile, and because the tensile/yield ratio exceeds 2, large amounts of cold work are possible before an intermediate anneal is needed. The corollary: cold-worked material re-annealed below 1150 °C will not meet the 800H grain-size requirement.

Frequently Asked Questions

800, 800H or 800HT — which do I order, and what fails if I get the wrong one?​‌​​‌​

All three share identical nickel (30–35 %), chromium (19–23 %) and iron (39.5 % min), so mill certificates look nearly the same. The differences are four lines: carbon, aluminium-plus-titanium, grain size and annealing temperature. Alloy 800 permits 0.10 % carbon maximum with no minimum, sets no grain-size requirement, and may be annealed as low as about 980 °C. Alloy 800H requires carbon 0.05–0.10 %, Al+Ti 0.30–1.20 %, an anneal at 1150–1200 °C and a grain size of ASTM 5 or coarser. Alloy 800HT tightens carbon to 0.06–0.10 % and Al+Ti to 0.85–1.20 %, with aluminium and titanium each at least 0.25 %, and mandates a minimum 1149 °C heat treatment. The rule ASTM prints in B407, B408, B409, B514 and B564 is unambiguous: N08800 is for service up to and including 1100 °F (593 °C); above that you need N08810 or N08811, because creep and stress-rupture resistance becomes the design basis and those grades are annealed to develop controlled coarse grain. The originator quotes a higher crossover, 1500 °F; take the conservative ASTM figure. What fails if you get plain 800 in an 800H application is not detectable at goods-in. Chemistry passes, room-temperature tensile passes — alloy 800 is actually stronger at room temperature. The failure is creep: fine-grained, low-carbon material has far lower rupture strength, and a component designed to 800H allowable stresses will bulge and rupture thousands of hours early. Always require the reported ASTM grain size on the certificate.

800H versus 330, 601 and 625 for high-temperature service​‌​​‌​

Alloy 800H (N08810) is the pressure-boundary workhorse. It has ASME Boiler and Pressure Vessel Code approval for Section VIII Division 1 construction — the originator cites allowable stresses to 982 °C via Code Case 1983 while a major distributor quotes 899 °C, so treat 899 °C as the conservative design ceiling — with useful oxidation resistance through 982 °C. Choose it whenever the part is a code-stamped pressure component. Alloy 330 (N08330, 34–37 % Ni, 17–20 % Cr, with a deliberate 0.75–1.50 % silicon addition) resists scaling to roughly 1095 °C and oxidation to about 1150 °C. The silicon gives it genuinely better carburisation resistance than 800H. It is the choice for furnace internals, retorts and radiant tubes — but it is not an 800H substitute for coded pressure parts. Alloy 601 (N06601, 58–63 % Ni, 21–25 % Cr and, critically, 1.0–1.7 % aluminium) is the oxidation champion, resisting oxidation to about 1200 °C with a tightly adherent scale that survives thermal cycling. Its ASME allowables are restrictive, though: Section I to only 482 °C and Section VIII Division 1 to 899 °C. Alloy 625 is a strength-and-aqueous-corrosion alloy serving from cryogenic to 982 °C, above which scaling becomes restrictive. Watch the grade: the originator gives Section VIII Division 1 approval as Grade 1 to just 649 °C and Grade 2 to 871 °C. Short version: 800H for coded pressure parts, 330 for carburising furnace hardware, 601 for extreme cyclic oxidation, 625 for strength plus wet corrosion.

What does “800H/800HT dual certified” actually mean, and when should I refuse it?​‌​​‌​

Dual certification to 800H and 800HT together is normal and entirely legitimate. Every 800HT limit sits inside the 800H limits — carbon 0.06–0.10 within 0.05–0.10, Al+Ti 0.85–1.20 within 0.30–1.20 — so a single heat can genuinely satisfy N08810 and N08811 at once, provided the grain size is ASTM 5 or coarser and it was annealed at 1150 °C or above. Major distributors stock exactly this material as one product, “800H/AT”. Accept it. The dual certification that deserves scrutiny is 800/800H. Chemically it can be honest: carbon of 0.05–0.10 % also satisfies alloy 800’s “0.10 % maximum”. But alloy 800 imposes no grain-size requirement and may be annealed as low as 980 °C, so the plain-800 line on the certificate provides cover for material that was processed as N08800 and never developed the coarse grain that 800H’s creep strength and ASME allowable stresses depend on. That material is a perfectly valid N08800 and a non-conforming N08810. Refuse it whenever the certificate does not state, explicitly, a reported ASTM grain size of 5 or coarser and an actual annealing temperature of 1150 °C or higher. A blank grain-size field is disqualifying, regardless of the heading on the document. Two related traps: alloy 800 itself has a second annealed grade at about 1150 °C, so anneal temperature alone proves nothing; and there is no ASTM welded-pipe specification covering 800HT at all — B514 lists only N08120, N08800 and N08810.

​‌​​‌​

​‌​​‌​

​‌​​‌​