API 5CT N80 is the most widely specified intermediate-strength casing grade in global oil well construction. With a minimum yield strength of 552 MPa (80,000 psi), it sits above the commodity J55/K55 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 supplies API 5CT N80 casing in both N80-1 and N80Q sub-grades across all standard sizes from 4½ inch through 20 inch, with STC, LTC, BTC, and premium connections. We supply OCTG to oil and gas operators and drilling contractors across Africa, South America and Southeast Asia. This guide covers everything needed to specify, procure, and select N80 correctly.
What Is API 5CT N80?
API 5CT is the American Petroleum Institute specification governing oil country tubular goods for 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 "80" refers to the minimum yield in thousands of psi. N80 is produced in two heat treatment sub-grades: N80-1 and N80Q.
N80 is 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 strength of P110.
N80-1 vs N80Q — The Critical Distinction
The most important specification decision when ordering N80 is the sub-grade. Both N80-1 and N80Q meet identical yield strength requirements but are produced by fundamentally different heat treatments with real consequences for mechanical behaviour in service.
| Property | N80-1 | N80Q |
|---|---|---|
| Heat treatment | Normalised or N+T | Quenched and tempered |
| Microstructure | Ferritic-pearlitic | Tempered martensitic |
| Min yield strength | 552 MPa (80,000 psi) | 552 MPa (80,000 psi) |
| Max yield strength | 758 MPa (110,000 psi) | 758 MPa (110,000 psi) |
| Min tensile strength | 689 MPa (100,000 psi) | 689 MPa (100,000 psi) |
| Hardness limit | None in API 5CT | None in API 5CT |
| CVN toughness | Not required | Not required |
| Cost | Lower | Higher — Q+T premium |
| Best for | Shallow to moderate sweet wells | Deeper wells, higher loads |
When N80Q is worth the premium: Specify N80Q over N80-1 in three scenarios — deeper wells where collapse and burst loads are higher and consistent mechanical properties through the pipe wall matter more; colder environments where low-temperature toughness is a concern; and when a project specification explicitly requires Q+T heat treatment, which some IOC company specifications mandate for all intermediate casing regardless of grade.
When N80-1 is the right choice: For straightforward onshore sweet wells under 2,500 metres with standard load cases, N80-1 delivers the required mechanical performance at lower cost. Most wells in West Africa and South America drilling to moderate depth on sweet reservoirs use N80-1 for production and intermediate strings.
Important for MTC verification: N80-1 and N80Q can have identical chemical composition — the sub-grade is determined entirely by heat treatment, not chemistry. When checking MTC documents, always verify the heat treatment column explicitly. A certificate listing N80 chemistry without specifying Q+T heat treatment is N80-1 by default, regardless of what the purchase order stated.
Chemical Composition
API 5CT sets maximum element limits for N80. Mills formulate their own chemistries within these limits to achieve the required mechanical properties through the specified heat treatment.
| Element | Max % | Note |
|---|---|---|
| Carbon (C) | 0.43 | Higher carbon promotes hardenability |
| Manganese (Mn) | 1.90 | Improves strength and hardenability |
| Molybdenum (Mo) | 0.25 | Improves deep hardenability in heavy wall |
| Chromium (Cr) | 0.50 | Improves hardenability |
| Nickel (Ni) | 0.25 | Improves low-temperature toughness |
| Copper (Cu) | 0.35 | Residual element — limited to control |
| Phosphorus (P) | 0.030 | Embrittlement risk — kept low |
| Sulphur (S) | 0.030 | Lower is better for toughness |
Standard Dimensions and Weights
API 5CT N80 casing is available in all standard casing sizes. The table below shows the most commonly ordered sizes with typical weight ranges.
| OD (inches) | Common Weights (lb/ft) | Wall Range (mm) | Drift Dia. Range (in) | Typical Use |
|---|---|---|---|---|
| 4½ | 9.5–15.1 | 5.21–8.56 | 3.965–3.701 | Production casing, liners |
| 5 | 11.5–18.0 | 5.59–9.19 | 4.435–4.151 | Production casing |
| 5½ | 14.0–23.0 | 5.69–10.54 | 4.887–4.545 | Production casing |
| 7 | 17.0–35.0 | 6.43–13.06 | 6.413–5.879 | Intermediate and production |
| 7⅝ | 24.0–39.0 | 7.62–13.84 | 6.900–6.500 | Intermediate casing |
| 9⅝ | 32.3–53.5 | 8.94–16.13 | 8.845–8.379 | Intermediate casing |
| 10¾ | 32.75–55.5 | 7.09–13.72 | 10.036–9.604 | Surface and intermediate |
| 13⅜ | 48.0–72.0 | 8.38–13.56 | 12.559–12.191 | Surface casing |
| 16 | 65.0–84.0 | 9.53–16.13 | 15.062–14.822 | Surface casing, conductor |
| 20 | 94.0–133.0 | 11.13–16.13 | 18.936–18.542 | Conductor casing |
For full dimension tables including ID, drift diameter, and coupling OD for every weight within each size, see the complete API 5CT casing dimension guide.
Connection Types
N80 casing is compatible with all API 5CT standard connections and premium connection designs.
| Connection | Abbrev. | Seal Type | Joint Efficiency | Typical Use |
|---|---|---|---|---|
| Short Thread Coupling | STC | Compound-dependent | ~60% | Surface casing, low-pressure |
| Long Thread Coupling | LTC | Compound-dependent | ~70% | Intermediate, moderate pressure |
| Buttress Thread Coupling | BTC | Compound-dependent | ~95% | Higher collapse/burst, deep wells |
| Premium Connection | Various | Metal-to-metal | 100%+ | Gas wells, HPHT, critical service |
For 9⅝ inch and 13⅜ inch N80 intermediate casing in moderate-depth onshore wells, BTC is the standard connection — it provides near-pipe-body tension efficiency and better collapse resistance than LTC at a modest cost premium. Never use STC for intermediate or production casing where any meaningful burst or collapse loading exists.
ZC Steel Pipe holds independent patents in premium connection designs qualified to API 5C5 CAL IV. Premium-connected N80 casing is available across all standard sizes.
N80 vs Other API 5CT Grades
| Property | J55/K55 | 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 | No | No | No | Yes — NACE | No |
| Typical depth | Under 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 |
N80 vs L80 — same strength, different application: N80 and L80 share identical minimum yield strength but differ critically in hardness control and sour service qualification. L80 has a mandatory maximum hardness of 23 HRC and is always quenched and tempered — making it NACE MR0175 compliant for H2S wells. N80 has no hardness ceiling and is not sour-service-qualified. For sweet wells, N80-1 is the more economical choice. For any well with confirmed H2S, L80 is mandatory — do not substitute N80.
Sour Service Limitations
N80 is not a sour-service-qualified grade under NACE MR0175 / ISO 15156. API 5CT imposes no maximum hardness limit on N80, and NACE MR0175 limits carbon steel tubulars in H2S service to 22 HRC maximum. Some N80 heats — particularly N80-1 with higher carbon content — can produce areas exceeding this limit, creating susceptibility to sulphide stress cracking (SSC).
Do not substitute N80 for L80 in sour service wells on the basis of equivalent yield strength. L80 is specifically designed for sour service — its 23 HRC hardness cap, mandatory Q+T heat treatment, and NACE certification trail make it the correct specification. Even N80Q, despite its Q+T heat treatment, lacks the hardness ceiling and NACE documentation required for sour service qualification. If H2S partial pressure exceeds 0.0003 MPa — the NACE threshold — specify L80, not N80.
Inspection and Testing Requirements
| Test | 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 | Optional | Per pipe if specified |
| Drift test | Required | Required | Per pipe |
| CVN impact test | Not required | Not required | — |
| MTC (EN 10204 3.1) | Standard | Standard | Per heat |
API 5CT makes NDE optional for N80 unless specified on the purchase order. For production casing in wells above 3,000 metres, or any well with elevated collapse or burst requirements, specify 100% UT of the pipe body as a supplemental requirement. Third-party inspection at the mill — SGS, Bureau Veritas, or TÜV — is standard practice for OCTG orders sourced from China and strongly recommended for EPC projects in Africa and South America with strict documentation requirements.
How to Specify N80 Casing on a Purchase Order
A complete N80 purchase order must include these fields to avoid ambiguity and incorrect deliveries:
- Standard — API 5CT (or ISO 11960)
- Grade — N80-1 or N80Q (not just "N80")
- OD and nominal weight — e.g. 9⅝ inch × 47.00 lb/ft
- Thread type — STC, LTC, BTC, or premium connection designation
- Range — R1, R2, or R3 (most strings are R3)
- PSL level — PSL-1 or PSL-2
- Quantity — in joints or metric tonnes
- Delivery port — for freight planning and lead time
- Inspection scope — third-party witness, supplemental NDE
- MTC level — EN 10204 3.1 or 3.2
Always specify N80-1 or N80Q explicitly. Mixing sub-types in a string is technically permissible under API 5CT but creates traceability problems on projects with strict MTC documentation requirements.