API 5CT N80 is the world's most widely used intermediate-strength casing grade — the practical backbone of global oil well construction. With a minimum yield strength of 552 MPa (80,000 psi), N80 sits above the commodity K55/J55 grades and below the sour-service-qualified L80 and high-strength P110, covering the broad middle of onshore and shallow offshore well programmes where sweet service conditions apply and cost efficiency matters.
ZC Steel Pipe manufactures API 5CT N80 casing in both N80-1 and N80Q sub-grades, across all standard sizes from 4½" through 20", with API 8-round, BTC, and proprietary premium connections. We supply OCTG to oil and gas operators and drilling contractors across Africa, the Middle East, and South America. This guide covers everything needed to specify, procure, and select N80 correctly.
CONTENTS
What Is API 5CT N80?
N80-1 vs N80Q — The Critical Distinction
Chemical Composition
Mechanical Properties
Standard Dimensions and Weights
Connection Types
N80 vs K55, L80, and P110 — Grade Selection
Sour Service Limitations
Inspection and Testing Requirements
FAQ
1. What Is API 5CT N80?
STANDARD DEFINITION — API 5CT N80API 5CT (Specification for Casing and Tubing) is published by the American Petroleum Institute and governs oil country tubular goods (OCTG) for oil and gas well construction. N80 is a Group 2 casing grade within API 5CT, defined by a minimum yield strength of 552 MPa (80,000 psi) and a maximum yield of 758 MPa (110,000 psi). The "N" designates the grade family and the "80" refers to the minimum yield in thousands of psi. N80 is produced in two heat treatment sub-grades: N80-1 and N80Q.
N80 was historically one of the first medium-strength casing grades to achieve broad global adoption. It remains the default grade for intermediate and production casing in sweet (non-sour) wells at moderate depth — typically 1,500 to 4,000 metres — where the well design does not require the tighter hardness control of L80 or the higher collapse resistance of P110.
N80 is used as both casing (run in the annular space between the drilled hole and the well tubing string) and occasionally as production tubing for larger bore completions. The same grade designation covers both applications under API 5CT.
2. N80-1 vs N80Q — The Critical Distinction
The most important specification decision when ordering N80 is the sub-grade — N80-1 or N80Q. Both meet the same minimum yield requirement but are produced by fundamentally different heat treatments with real consequences for mechanical behaviour in service.
N80-1
Heat treatment: Normalised (N) or Normalised + Tempered (NT)
Microstructure: Ferritic-pearlitic
Min yield: 552 MPa (80,000 psi)
Max yield: 758 MPa (110,000 psi)
Hardness limit: None specified in API 5CT
Toughness: Not specified — no CVN requirement
Cost: Lower — less energy-intensive process
Best for: Shallow-to-moderate sweet wells, cost-sensitive programmes
N80Q
Heat treatment: Quenched and Tempered (Q+T)
Microstructure: Tempered martensitic
Min yield: 552 MPa (80,000 psi)
Max yield: 758 MPa (110,000 psi)
Hardness limit: None specified in API 5CT
Toughness: Better and more consistent through-wall
Cost: Higher — Q+T process premium
Best for: Deeper wells, higher collapse loads, better toughness needed
Engineering Insight — When N80Q Is Worth the PremiumThe practical case for specifying N80Q over N80-1 comes down to three scenarios: (1) deeper wells where collapse and burst loads are higher and consistent mechanical properties through the pipe wall matter more; (2) colder operating environments where low-temperature toughness is a concern — N80Q's tempered martensite is inherently tougher than N80-1's ferrite-pearlite; (3) when the project specification explicitly requires Q+T heat treatment, which some IOC company specifications mandate for all intermediate casing regardless of grade. If your well is straightforward, onshore, sweet, and under 2,500 metres, N80-1 is usually the right economic choice. If any of those conditions change, reconsider.
3. Chemical Composition
API 5CT does not prescribe a fixed chemistry for N80 — instead it sets maximum limits by element. Within those limits, mills formulate their own chemistries to achieve the required mechanical properties through the specified heat treatment. The following are the API 5CT maximum chemistry limits for N80:
| Element | N80-1 Max (%) | N80Q Max (%) | Note |
| Carbon (C) | 0.43 | 0.43 | Higher C promotes hardenability |
| Manganese (Mn) | 1.90 | 1.90 | Improves strength and hardenability |
| Molybdenum (Mo) | 0.25 | 0.25 | Improves deep hardenability for heavy wall |
| Chromium (Cr) | 0.50 | 0.50 | Improves hardenability and corrosion resistance |
| Nickel (Ni) | 0.25 | 0.25 | Improves toughness at low temperature |
| Copper (Cu) | 0.35 | 0.35 | Residual element — limited to control |
| Phosphorus (P) | 0.030 | 0.030 | Embrittlement risk — kept low |
| Sulphur (S) | 0.030 | 0.030 | MnS inclusions — lower is better for toughness |
Procurement Note — Chemistry Is Not Enough to Verify Sub-GradeN80-1 and N80Q can have identical chemical composition — the sub-grade is determined entirely by heat treatment, not chemistry. When verifying MTC documents, always check the heat treatment column explicitly. A certificate that lists N80 chemistry but does not specify Q+T heat treatment is N80-1 by default, regardless of what the order said. This is a common source of supply errors when sourcing N80Q from secondary market or non-mill sources.
4. Mechanical Properties
| Property | N80-1 | N80Q | Unit |
| Minimum yield strength | 552 | 552 | MPa (80,000 psi) |
| Maximum yield strength | 758 | 758 | MPa (110,000 psi) |
| Minimum tensile strength | 689 | 689 | MPa (100,000 psi) |
| Minimum elongation | See API 5CT table | See API 5CT table | % (formula by WT and grade) |
| Hardness — max | Not specified | Not specified | HRC / HBW |
| CVN toughness | Not required | Not required | J |
| Heat treatment | N or NT | Q+T | — |
5. Standard Dimensions and Weights
API 5CT N80 casing is available in all standard casing sizes. The following table shows the most commonly ordered sizes with standard weights. Wall thickness increases with weight per foot for the same OD.
| OD (inches) | Common Weights (lb/ft) | Wall Thickness Range (mm) | Drift Diameter (inches) | Typical Use |
| 4½" | 9.5 – 15.1 | 5.21 – 8.56 | 3.965 – 3.701 | Production casing, liners |
| 5" | 11.5 – 20.8 | 5.59 – 9.19 | 4.408 – 4.044 | Production casing |
| 5½" | 14.0 – 26.8 | 5.69 – 10.54 | 4.892 – 4.276 | Production casing |
| 7" | 17.0 – 38.0 | 6.43 – 13.06 | 6.331 – 5.595 | Intermediate and production casing |
| 7⅝" | 24.0 – 45.3 | 7.62 – 13.84 | 6.875 – 6.184 | Intermediate casing |
| 9⅝" | 32.3 – 61.1 | 8.94 – 16.13 | 8.835 – 8.097 | Intermediate casing |
| 10¾" | 32.75 – 65.7 | 7.09 – 13.72 | 10.192 – 9.456 | Surface and intermediate casing |
| 13⅜" | 48.0 – 85.0 | 8.38 – 13.56 | 12.565 – 12.093 | Surface casing |
| 16" | 65.0 – 109.0 | 9.53 – 16.13 | 15.124 – 14.124 | Surface casing, conductor |
| 20" | 94.0 – 133.0 | 11.13 – 16.13 | 19.124 – 18.124 | Conductor casing |
6. Connection Types
N80 casing is manufactured with the pipe body to API 5CT and threaded with the connection type specified on order. API 5CT defines three standard connection types, and N80 is compatible with all premium connection designs applied by the mill or a licensed threading facility.
| Connection | Abbrev. | Thread Form | Seal Type | Efficiency | Typical Application |
| Short Thread Coupling | STC | API 8-round tapered | Compound-dependent | ~60% joint | Surface casing, low-pressure applications |
| Long Thread Coupling | LTC | API 8-round tapered | Compound-dependent | ~70% joint | Intermediate casing, moderate pressure |
| Buttress Thread Coupling | BTC | Buttress (flat-load, tapered-stab) | Compound-dependent | ~95% pipe body | Higher collapse/burst, deeper wells |
| Premium Connection | Various | Proprietary | Metal-to-metal | 100% pipe body or higher | Gas wells, HPHT, directional, critical service |
Procurement Note — BTC vs LTC for N80 Intermediate CasingFor 9⅝" and 13⅜" N80 intermediate casing in moderate-depth onshore wells, BTC is the standard connection choice — it provides near-pipe-body tension efficiency and better collapse resistance than LTC at a modest cost premium. LTC is occasionally specified for surface casing where loads are low and cost pressure is high. Never use STC for intermediate or production casing where any meaningful burst or collapse loading exists — its compound-dependent seal is inadequate for pressure isolation across cement. This is a basic but commonly ignored rule on cost-driven rig programmes.
7. N80 vs K55, L80, and P110 — Grade Selection
| Property | K55 / J55 | N80-1 | N80Q | L80 | P110 |
| Min yield (MPa) | 379 | 552 | 552 | 552 | 758 |
| Max yield (MPa) | 552 | 758 | 758 | 655 | 965 |
| Heat treatment | None required | N or NT | Q+T | Q+T | Q+T |
| Hardness max | None | None | None | 23 HRC | None |
| Sour service (NACE) | No | No | No | Yes — qualified | No |
| API group | 1 | 2 | 2 | 2 | 3 |
| Typical depth range | <1,500 m | 1,500–3,500 m | 1,500–4,000 m | 1,500–4,000 m | 3,000–6,000+ m |
| Relative cost | Lowest | Low | Moderate | Moderate–High | Highest |
Engineering Insight — N80 vs L80: Same Strength, Very Different ApplicationsN80 and L80 are frequently confused because they share identical yield strength requirements. The practical difference is hardness control and sour service qualification — L80 has a mandatory maximum hardness of 23 HRC in API 5CT, which when combined with NACE MR0175 compliance makes it the correct grade for H₂S-containing wells. N80 has no hardness ceiling, which means some heats can exceed the NACE limit and fail in sour environments. For sweet wells, N80-1 is the more economical choice. For any well with confirmed or possible H₂S, specify L80 — do not attempt to qualify N80 for sour service without independent test data.
Related: API 5CT L80 Casing Pipe — Grades, Specs & Sour Service → | API 5CT P110 Casing Pipe — Specs & Supplier →
8. Sour Service Limitations
N80 is not a sour-service-qualified grade under NACE MR0175 / ISO 15156. The reason is straightforward: API 5CT does not impose a maximum hardness limit on N80, and NACE MR0175 limits carbon and low-alloy steel tubulars in H₂S service to a maximum of 22 HRC. Some N80 heats — particularly N80-1 with higher carbon content and incomplete normalisation — can produce areas exceeding this limit, creating susceptibility to sulphide stress cracking (SSC).
Critical Engineering Point — Do Not Substitute N80 for L80 in H₂S ServiceThe temptation to substitute N80 for L80 in a sour service well on the basis of equivalent yield strength is a well-documented engineering error. L80 is specifically designed for sour service — its 23 HRC hardness cap, mandatory Q+T heat treatment, and the mill's obligation to certify compliance with NACE MR0175 make it the correct specification. N80 has none of these controls. Even N80Q, despite its Q+T heat treatment, lacks the hardness ceiling and NACE documentation trail required for sour service certification. If H₂S partial pressure exceeds 0.0003 MPa (0.05 psi) — the NACE threshold — specify L80, not N80.
9. Inspection and Testing Requirements
| Test / Inspection | N80-1 | N80Q | Frequency |
| Tensile test | Required | Required | Per heat and per lot |
| Hardness test | Not mandatory | Not mandatory | — |
| Flattening test | Required | Required | Per pipe |
| Hydrostatic test | Required | Required | Per pipe |
| NDE (UT/EM) | Optional unless specified | Optional unless specified | Per pipe if required |
| Drift test | Required | Required | Per pipe |
| CVN impact test | Not required | Not required | — |
| MTC (EN 10204 3.1) | Standard | Standard | Per heat |
Procurement Note — When to Specify Additional NDE for N80API 5CT makes NDE optional for N80 unless the purchaser specifies it in the order. For shallow surface casing in low-risk wells, this is acceptable. For production casing in wells above 3,000 metres, or in any well with elevated collapse or burst requirements, specify 100% UT of the pipe body and EM inspection of the weld area (for ERW) as supplemental requirements on the purchase order. The cost of additional NDE on a casing string is negligible compared to the cost of a casing failure at depth. Third-party inspection at the mill (SGS, Bureau Veritas, TÜV) is standard practice for larger OCTG orders sourced from Chinese mills.
10. Frequently Asked Questions
What is API 5CT N80 casing pipe?
API 5CT N80 is a medium-strength oil country tubular goods (OCTG) casing grade with a minimum yield strength of 552 MPa (80,000 psi) and a maximum yield of 758 MPa (110,000 psi). It is produced in two sub-grades — N80-1 (normalised or normalised-and-tempered) and N80Q (quenched and tempered) — and is used for intermediate and production casing in sweet (non-sour) onshore and shallow offshore oil and gas wells at depths typically between 1,500 and 4,000 metres.
What is the difference between N80-1 and N80Q?
N80-1 is heat-treated by normalising or normalising-and-tempering, producing a ferritic-pearlitic microstructure at lower cost. N80Q is quenched and tempered, producing a tempered martensitic microstructure with better and more consistent toughness through the pipe wall. Both sub-grades have identical yield strength requirements. N80Q is preferred for deeper wells, higher mechanical loading, and colder environments; N80-1 is the economical default for standard onshore sweet wells.
Can N80 casing be used in sour service wells?
No — N80 is not suitable for sour service without additional qualification that API 5CT does not require mills to perform. NACE MR0175 limits carbon steel tubulars in H₂S service to 22 HRC maximum hardness. N80 has no hardness ceiling in the standard and some heats can exceed this limit. For wells with any H₂S content above the NACE threshold (0.0003 MPa partial pressure), specify L80 — the sour-service-qualified equivalent at the same yield strength.
What is the difference between N80 and L80 casing?
N80 and L80 share identical minimum yield strength (552 MPa / 80,000 psi) but differ critically in hardness control and sour service qualification. L80 has a maximum hardness of 23 HRC per API 5CT and is always quenched and tempered — making it compliant with NACE MR0175 for sour service when properly certified. N80 has no hardness limit and is not sour-service-qualified. For sweet wells, N80-1 is the more economical choice; for any H₂S-containing well, L80 is mandatory.
What sizes are available in API 5CT N80 casing?
API 5CT N80 casing is available in all standard casing sizes from 4½" through 20" OD. The most commonly ordered sizes are 5½", 7", 9⅝", and 13⅜" for production, intermediate, and surface casing applications. Multiple wall thickness options (expressed as weight per foot) are available for each OD, and N80 can be supplied with STC, LTC, BTC, or premium connections per order.
What connections are available for N80 casing?
N80 casing is available with all API 5CT standard connections — STC (Short Thread Coupling), LTC (Long Thread Coupling), and BTC (Buttress Thread Coupling) — as well as premium connections with metal-to-metal seals for gas wells, directional wells, HPHT applications, and any service where API thread sealing reliability is insufficient. ZC Steel Pipe holds independent patents in premium connection designs and can supply premium-connected N80 casing on order.
Source API 5CT N80 Casing from ZC Steel Pipe
ZC Steel Pipe manufactures API 5CT N80-1 and N80Q casing in all standard sizes from 4½" through 20", with STC, LTC, BTC, and premium connection options. Full MTC documentation, third-party inspection (SGS, Bureau Veritas, TÜV), and supplemental NDE available on request. We hold independent patents in premium connection technology and supply OCTG to oil and gas operators across Africa, the Middle East, and South America.
Custom chemistry controls, additional hardness testing, and project-specific supplemental requirements available. Minimum order quantities and lead times quoted on request.
mandy.w@zcsteelpipe.com
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