What the Term Covers
An Incoloy pipe is a nickel-iron-chromium pipe made from the 800-series family of alloys. The name is not a single grade. For pressure piping and heat-transfer equipment the family splits into three controlled-chemistry variants, and confusing them is the most common specification error in the field: N08800 for general corrosion service, and the two high-carbon variants N08810 and N08811 for elevated-temperature creep service.
All three share the same base chemistry, a face-centred-cubic austenitic structure and roughly 30-35% nickel, which is enough to make them immune to chloride stress corrosion cracking under normal operating conditions and to give them good resistance to polythionic acid attack in refinery service.
Grade Differences That Matter
| Property | N08800 | N08810 | N08811 |
|---|---|---|---|
| Nickel, % | 30.0-35.0 | 30.0-35.0 | 30.0-35.0 |
| Chromium, % | 19.0-23.0 | 19.0-23.0 | 19.0-23.0 |
| Iron, % | 39.5 min | 39.5 min | 39.5 min |
| Carbon, % | 0.10 max | 0.05-0.10 | 0.06-0.10 |
| Aluminium, % | 0.15-0.60 | 0.15-0.60 | 0.15-0.60 |
| Titanium, % | 0.15-0.60 | 0.15-0.60 | 0.15-0.60 |
| Aluminium + Titanium, % | - | - | 0.85-1.20 |
The carbon content is what separates the grades. Higher carbon, combined with a coarser grain size specified as ASTM E112 grain size 5 or coarser, produces the creep and stress-rupture strength needed in furnace tubing, reformer outlet headers and steam superheaters. The aluminium plus titanium addition in the 800HT variant takes the same idea further and stabilises the structure against embrittlement during very long exposure.
Standard Specifications by Product Form
Pipe and tube for the 800 series are ordered against the following ASTM and equivalent ASME specifications:
| Product form | Specification |
|---|---|
| Seamless tube | ASTM B163 (ASME SB163) |
| Seamless pipe and tube | ASTM B407 (ASME SB407) |
| Welded pipe | ASTM B514 (ASME SB514) |
| Welded tube | ASTM B515 (ASME SB515) |
| Rod and bar | ASTM B408 (ASME SB408) |
| Plate, sheet and strip | ASTM B409 (ASME SB409) |
| Pipe fittings | ASTM B366 (ASME SB366) |
The ASME versions carry the same technical requirements as the ASTM originals and are required whenever the component is to be stamped under the ASME Boiler and Pressure Vessel Code. A pipe that is dual certified to B407 and SB407 is therefore no more expensive to produce, only more useful to the fabricator who has to present material certificates to an inspector.
Service Conditions and Corrosion Behaviour
The 800 series is selected for two distinct duties, and the choice of grade depends on which one dominates. In aqueous or wet-process service the controlling properties are resistance to chloride stress corrosion cracking, to caustic and to oxidising-and-reducing acid mixtures; here N08800 is normally sufficient. In dry, high-temperature service the controlling properties are creep and rupture strength, carburisation resistance and structural stability; here N08810 or N08811 is required.
Typical elevated-temperature applications run from about 540 C to 900 C. Above that range the alloy is still used for less demanding duty, but oxidation and grain growth begin to dominate the design. In steam-methane reforming and ethylene pyrolysis furnaces, the tube grade, wall thickness and grain size are usually fixed by the furnace licensor on the basis of rupture data rather than by corrosion allowance.
Ordering and Quality Points to Confirm
Because the three grades are visually identical, purchase documents should state the UNS number, the product specification, the heat-treatment condition and the grain-size requirement. Points that regularly cause rejected deliveries:
- Grade substitution: N08800 supplied against an order for N08810 or N08811. The chemistry report will show the carbon content, so the mismatch is detectable but only if the certificate is checked against the purchase order.
- Missing grain-size statement for the high-carbon grades.
- Solution treatment temperature and cooling rate not recorded, which makes the creep properties of the finished pipe unverifiable.
- Test certificate type. For European projects a certificate to EN 10204 3.1 is normally required; for ASME work, the ASTM or ASME product specification marks must be shown on the certificate together with the heat number.
Where welding is involved, filler metal selection follows the parent grade, and the procedure qualification should reflect the actual grain size because heat input influences the creep ductility of the weld metal and the heat-affected zone.
Frequently Asked Questions
Q: Is Incoloy 800 the same as Inconel 600?
A: No. Inconel 600 is a nickel-chromium-iron alloy with a minimum of 72% nickel, while Incoloy 800 contains only 30-35% nickel with iron as the balance. They look similar on a drawing but their corrosion and high-temperature behaviour differ.
Q: Which grade should be used for a heat exchanger in a caustic environment?
A: The 800 series is a common choice because of its resistance to caustic and to chloride stress corrosion cracking; the exact grade depends on whether the design is governed by corrosion allowance or by creep strength.
Q: Can 800 and 800H pipe be welded together?
A: Yes, and they are frequently joined in furnace and header assemblies. The welding procedure must be qualified for the combination, and testing should reflect the properties of the weaker of the two grades.
Q: What does dual certification mean on a pipe certificate?
A: It means the same heat has been tested and reported against two specifications, for example ASTM B407 and ASME SB407, or against two grades where the chemistries overlap. It does not mean the pipe has better properties.
Q: Does the 800 series suffer from sensitisation at weld heat-affected zones?
A: The high-carbon grades are more susceptible than the low-carbon grade because of the higher carbon level, and intergranular attack can occur in strongly oxidising acid service after welding. Where that risk exists, the low-carbon N08800 variant is preferred.
