Brief introduction of Incoloy alloy

Incoloy is a trademark, and the alloy it is most often used for is one chemistry that has carried four names. A short introduction that lists three of those names as three kinds of alloy has described a filing system and called it a material family.

The distinction has a practical edge. The 800 branch of the trademark is bought against three UNS numbers, and the numbers differ in carbon, in aluminium plus titanium, in grain size, and in the temperature at which the material is annealed. Two of those differences are invisible in a chemical analysis report, and one of them appears in no certificate at all unless the purchase order asks for it.

There is also a number that circulates widely and deserves an explanation. Alloy 800 is often described as serviceable to about 593 °C. That figure is real, and it belongs to a condition that stopped being sold under that name in the 1960s. Recovering it is the quickest way to understand why the 800 family looks more complicated than it is.

The publisher of the trademark maintains a technical bulletin index for the family, and that index lists sixteen documents, running from the 800 series through corrosion grades, a low-expansion series, and a mechanically alloyed grade. A page that divides the family into three kinds has described one branch of it.

Reading that table as a menu is the wrong move. Each row answers a different failure mode, and the failure mode decides the branch before the grade is chosen. Oxidation above 900 °C points at the 800 line or at 803. Sulphuric acid points at 825 or 020. Chloride pitting at ambient temperature points at 27-7MO or 945X. A clearance-controlled assembly at cryogenic temperature points at 903, 907, or 909. A single enquiry that asks for Incoloy without naming the failure has asked for a brand.

Alloy 800 entered the ASME Boiler and Pressure Vessel Code in 1963, under Code Case 1325. The case did not list one material. It listed two grades, distinguished by the temperature at which the material was annealed, and it published a separate set of design stresses for each.

Grade 1 was annealed at approximately 1,800 °F (980 °C). Grade 2 was annealed at approximately 2,100 °F (1,150 °C). The case listed design stresses for Grade 1 to 1,100 °F (593 °C) and for Grade 2 to 1,500 °F (816 °C). Those two figures are the origin of almost every temperature claim about the 800 family, including the ones that are now attached to the wrong name.

The renaming is documented in the alloy bulletin, and it happened in one step. When Code Case 1325-7 was issued for the higher-carbon material, the term 800H came into common use, Grade 2 no longer needed to be mentioned because 800H had replaced it, and the material that had been called Grade 1 became, simply, 800.

So the 593 °C figure did not survive as a property of the family. It survived as the ceiling of the lower-temperature anneal, and that anneal is now the defining treatment of plain 800. A datasheet that applies 593 °C to 800H and to 800HT has taken the older of the two code ceilings and placed it on the two grades that exist precisely to exceed it. The internal contradiction gives it away: the same sentence says the higher grades are chosen where more creep resistance is required, and gives them the same temperature.

The names in circulation are not synonyms, because two of them are trademarks and one of them is a producer designation. That is the reason a single alloy appears under four labels, and it is the reason a purchase order that copies a catalogue heading can fail at the certificate stage.

The commercial consequence is simple. A mill outside the trademark group can make the same chemistry and cannot call it Incoloy or 800HT. It sells the same material under the UNS number, under a national designation, or under a house name such as 800AT. All of those routes reach the same composition, and none of them carries the trademark owner’s data package with it.

For a buyer, the practical rule is that the UNS number is the only label that survives a change of supplier. The trade name travels with a licence. The national designation travels with a standards edition. When a quotation uses a house name, the question to ask is which UNS number the certificate will state, and the answer belongs on the order before the heat is melted.

The differences are small in chemical terms and large in mechanical terms, and the reason is that two of them act on the same mechanism. Carbon raises the creep and rupture strength of this alloy, and the alloy bulletin records that Special Metals had been melting to a carbon range of 0.05 to 0.10 % for years before the code recognised the practice, because that range sits in the upper part of the 800 specification.

Grain size does the same work by a different route. A coarse grain reduces the amount of grain boundary available for sliding at temperature, which is why 800H is defined by a carbon range and by an average grain size of ASTM 5 or coarser. That is the requirement most likely to be missing from a supplier’s paperwork, because grain size is a metallographic result and it does not appear in a chemical analysis.

Aluminium and titanium act through a third route. They form a fine dispersion that raises creep resistance, and the 800HT grade holds both elements in a narrower and higher band than 800H: 0.25 to 0.60 % of each, with the sum held between 0.85 and 1.20 %, against a sum of 0.30 to 1.20 % for 800H. A grade that meets the sum but distributes it unevenly is not the same material, which is why the individual minimums are specified as well.

The heat treatment completes the set. 800HT requires an anneal at 1,149 °C (2,100 °F) minimum, and that figure is not a processing preference. It is the treatment that puts the carbon and the grain structure where the allowable stresses assume they are.

The relationship between the top two grades is asymmetric, and the bulletin states it without qualification. Alloy 800HT meets all the requirements for UNS N08811 and for UNS N08810, and can be certified to either or to both. Material produced to UNS N08810 cannot be certified as UNS N08811 unless it meets the additional requirements for that designation.

That asymmetry is worth reading twice, because it creates two failure modes in a purchase file. The first is a buyer who needs N08811 and accepts a dual-certified N08810 certificate without checking which number the mechanical results were generated against. The second is a buyer who needs N08810 and pays for N08811 capacity without needing the higher allowable stresses.

The reason 800HT can carry the higher stresses is a matter of record and not a matter of marketing. The bulletin explains that the grade was introduced because Special Metals held aluminium and titanium in the upper part of the range and obtained higher creep and rupture properties than competing 800H, and that retaining those properties required a restricted chemistry inside the 800H limits together with the higher annealing temperature. The maximum allowable stresses for the grade are published in ASME Code Case 1987.

The nickel figure and the precipitation statement both need a narrower form. Neither is invented, and both are loose enough to mislead a specification.

The nickel content of the 800 branch is a band of 30.0 to 35.0 %, and the resistance to chloride stress corrosion cracking that the bulletin attributes to the alloy rests on that nickel level. A quotation that states 32 % has quoted a point inside the band and cannot be inspected against a limit. The band belongs in the specification, and the point belongs in a marketing sentence.

The precipitation statement needs replacing with the two windows the bulletin actually documents. Chromium carbides precipitate in the alloy between 540 and 1,095 °C (1,000 and 2,000 °F). Exposure between 540 and 760 °C (1,000 and 1,400 °F) can render the alloy susceptible to intergranular corrosion in certain aggressive environments, which is the sensitisation that the same bulletin compares to other austenitic alloys.

Those two windows matter for the applications the alloy is bought for. A heat-treating fixture, a furnace muffle, or a weld heat-affected zone can all sit inside 540 to 760 °C long enough for the boundary chemistry to change. The consequence is not a loss of strength at temperature. It is a loss of corrosion resistance in the affected band, which appears later and elsewhere.

Five items turn a brand name into a material, and the 800 family shows why each one is necessary.

The first is the UNS number. It is the only identifier that survives a change of supplier, and it is the identifier the certificate will carry.

The second is the code case, because the design stresses come from one. A vessel designed to the allowable stresses of Code Case 1987 needs material certified to N08811, and material certified to N08810 will not support the calculation.

The third is the grain size, for 800H and for 800HT. It is a metallographic result, it does not appear in a chemical analysis, and the grade definition depends on it. A purchase order that omits it can receive fine-grained material that satisfies every element on the chemistry report.

The fourth is the heat treatment, stated as a temperature with a minimum. For 800HT that figure is 1,149 °C, and a certificate that reports an anneal below it has reported a nonconformance in the same line that reports compliance.

The fifth is the certificate type. An EN 10204 3.1 certificate reports results from the actual heat, and the grade requirements that are measured and not analysed have to be listed on it, which is why the grain size and the annealing temperature belong in the inspection plan and not in a covering email.

DLX supplies the 800 branch in bar, wire, strip, and tube from Changzhou, Jiangsu, produced against the UNS number named on the order, with the grain size and the annealing temperature reported where the grade depends on them. An EN 10204 3.1 certificate is available with the heat number traceable to the mechanical and metallographic results.

The same works cover the corrosion branch of the family, including 825 and 020, where the molybdenum and copper additions answer a different question and the temperature ceiling is set by phase stability and not by creep.

No. It is a trademark registered by Inco in 1952 and now held by Special Metals, applied to a family that the publisher documents across sixteen technical bulletins. The grade is the number: N08800, N08810, N08811, N08825, and so on. An enquiry that names only the trademark has named a brand.

Carbon, grain size, and the aluminium plus titanium band, together with the annealing temperature. 800 allows carbon to 0.10 % and does not specify a grain size. 800H holds carbon at 0.05 to 0.10 % and requires a grain size of ASTM 5 or coarser. 800HT keeps the grain requirement, narrows aluminium and titanium to 0.25 to 0.60 % each with a sum of 0.85 to 1.20 %, and requires an anneal at 1,149 °C minimum.

From Code Case 1325, issued in 1963, which listed design stresses for Grade 1 to 1,100 °F (593 °C) and for Grade 2 to 1,500 °F (816 °C). Grade 1 was the material annealed at about 980 °C, and it became plain 800 when Grade 2 became 800H. The figure is the ceiling of the lower-temperature anneal, not a property of the family.

800HT meets the requirements of both UNS N08811 and UNS N08810 and can be certified to either or to both. Material produced to N08810 cannot be certified as N08811 unless it meets the additional requirements of that designation. The certificate direction is one way, and the allowable stresses are higher on the N08811 side.

It is a producer designation used in catalogues for the aluminium and titanium restricted grade, and the composition behind it is N08811. The name exists partly because 800HT is a trademark of the alloy’s originator, so a mill outside that group needs another label for the same chemistry. Ask which UNS number the certificate will state.

825 or 020, both of which carry copper and molybdenum. The 800 branch is bought for oxidation and carburisation resistance at temperature, and its corrosion resistance is general and not sulphuric-acid specific. The temperature ceiling on the corrosion grades is set by phase stability and not by creep, and it is far lower than the creep ceiling of the 800 branch.

The 800 family is one chemistry carrying four names, and the names record a history and not four materials. Grade 1 and Grade 2 were conditions in a 1963 code case, with ceilings of 593 °C and 816 °C. The higher-carbon condition became 800H. The lower-temperature anneal became plain 800. The restricted-chemistry variant became 800HT, which is a trademark, which is why a fourth name exists for the same composition outside the trademark group.

What separates the three UNS numbers is small in chemistry and decisive in service: carbon, grain size, the aluminium plus titanium band, and the annealing temperature. Two of those four are absent from a chemical analysis report, and one of them is absent from every certificate unless the order asks for it. A purchase order that names the brand, the UNS number, the code case, the grain size, and the annealing temperature has bought a material. One that names a trade name has bought a conversation.

Special Metals, INCOLOY alloy 800 technical bulletin, publication SMC-04600, 2004, for Code Case 1325, the Grade 1 and Grade 2 annealing temperatures and their design stress ceilings of 593 °C and 816 °C, the 816 °C service statement, the chemistry of N08800, and the chromium carbide and sensitisation windows.

Special Metals, INCOLOY alloys 800H and 800HT technical bulletin, for the carbon range of 0.05 to 0.10 %, the aluminium plus titanium band of 0.85 to 1.20 % for N08811, the 1,149 °C minimum heat treatment, the grain size requirement of ASTM 5 or coarser, Code Case 1987, and the one-way certification between N08811 and N08810.

Special Metals, INCOLOY technical bulletin index, for the breadth of the trademark family and the UNS numbers listed against each alloy.

ASME Boiler and Pressure Vessel Code, Code Case 1325 and its revision 1325-7, for the original design stresses and the higher stresses granted to the higher-carbon material.

ASME Boiler and Pressure Vessel Code, Code Case 1987, latest revision, for the maximum allowable stresses of UNS N08811.

ASTM B409, standard specification for nickel-iron-chromium alloy plate, sheet, and strip, for the 800 family in flat product form.

ASTM B408, standard specification for nickel-iron-chromium alloy rod and bar, for the same family in long product form.

SAE and ASTM unified numbering system, for the assignment of N08800, N08810, and N08811, and the German designation 1.4876 recorded in the alloy bulletin.

The figures quoted here are the published values of the documents named, and they are not a certificate for a delivered lot. Code cases are revised, and the revision named in a design calculation governs the material that calculation accepts. Grain size and annealing temperature are reported only when the purchase order requires them, and their absence from a certificate is not evidence that they were met. Where a name is a trademark, the right to use it rests with the holder and its licensees, and a UNS number is the identifier that remains valid across suppliers. Product specifications are revised, and the edition named on the order governs. Nothing here is a substitute for an engineering release or for the qualification testing agreed between buyer and supplier.