N80 Casing: API 5CT Specifications, Grades & Sizing Guide
What Is N80 Casing?
N80 is a normalized or normalized-and-tempered grade defined under API 5CT, with a controlled yield strength of 80,000–95,000 psi and a minimum tensile strength of 100,000 psi. N80-1, the base sub-grade, delivers reliable strength and toughness for standard well conditions, making N80 casing pipe one of the most widely used OCTG grades worldwide. API N80 casing conforms strictly to API 5CT chemical, mechanical, and dimensional requirements, ensuring consistent downhole performance. N80 casing & tubing covers sizes from 2⅜″ to 20″, serving production and intermediate string applications across a wide range of well depths. As a versatile N80 steel pipe, it offers excellent weldability and cost-effectiveness for conventional oil and gas wells where extreme corrosion resistance is not required.
- Material Grade: N80-1, N80-Q
- Applicable Product: Casing & Tubing 2⅜″–20″.
- Thread Types: BC, STC / LTC, Premium Connections.
- Available Lengths: R1 / R2 / R3.
| Grade | C (max%) | Mn (max%) | P (max%) | S (max%) | Ni (max%) | Cu (max%) | Heat Treatment |
| N80-1 | 0.34 | 1.90 | 0.020 | 0.010 | — | — | Normalized / N&T |
| N80Q | 0.34 | 1.90 | 0.020 | 0.010 | — | — | Quench & Tempered |
Key Difference: N80-1 and N80Q share identical chemical composition and strength ranges. The critical distinction is that N80Q undergoes quench-and-tempering, which requires Charpy impact testing and a maximum hardness limit — delivering superior toughness for deeper, more demanding wells.
| Grade | Yield Min (psi / MPa) | Yield Max (psi / MPa) | Tensile Min (psi / MPa) | Hardness Max | Impact Test (0°C) |
| N80-1 | 80,000 / 552 | 110,000 / 758 | 100,000 / 689 | — | Not Required |
| N80Q | 80,000 / 552 | 110,000 / 758 | 100,000 / 689 | 22 HRC | Required ≥20J |
API 5CT Specifications
Common N80 Casing Sizes
Standard outside diameters range from 4-1/2″ to 20″, covering every well construction role from production liner to conductor pipe. Below are the most commonly stocked sizes with weight ranges and typical applications.
| OD (in.) | OD (mm) | Wall Thickness Range (mm) | Weight Range (lb/ft) | Typical Application | Common Connections |
|---|---|---|---|---|---|
| 4-1/2″ | 114.30 | 5.21 – 7.37 | 9.50 – 13.50 | Production Liner | STC, LTC, Premium |
| 5″ | 127.00 | 5.59 – 9.19 | 11.50 – 18.00 | Production Liner | STC, LTC, Premium |
| 5-1/2″ | 139.70 | 6.20 – 10.54 | 14.00 – 23.00 | Production Intermediate | STC, LTC, BTC, Premium |
| 6-5/8″ | 168.28 | 7.32 – 10.59 | 20.00 – 28.00 | Intermediate Liner | STC, LTC, BTC |
| 7″ | 177.80 | 6.91 – 12.65 | 17.00 – 38.00 | Production Intermediate | STC, LTC, BTC, Premium |
| 7-5/8″ | 193.68 | 7.62 – 13.72 | 24.00 – 39.00 | Intermediate | STC, LTC, BTC |
| 8-5/8″ | 219.08 | 7.72 – 12.70 | 24.00 – 40.00 | Intermediate Surface | STC, LTC, BTC |
| 9-5/8″ | 244.48 | 7.92 – 13.84 | 32.30 – 53.50 | Intermediate Surface | STC, LTC, BTC, Premium |
| 10-3/4″ | 273.05 | 7.09 – 13.84 | 32.75 – 60.70 | Surface Intermediate | STC, LTC, BTC |
| 11-3/4″ | 298.45 | 8.46 – 14.78 | 42.00 – 65.00 | Surface | STC, LTC, BTC |
| 13-3/8″ | 339.72 | 8.38 – 13.06 | 48.00 – 72.00 | Surface Intermediate | STC, LTC, BTC |
| 16″ | 406.40 | 9.53 – 12.70 | 65.00 – 84.00 | Surface Conductor | STC, BTC |
| 18-5/8″ | 473.08 | 9.53 – 14.27 | 78.00 – 112.00 | Conductor | STC, BTC |
| 20″ | 508.00 | 10.16 – 15.88 | 94.00 – 133.00 | Conductor Surface | STC, BTC |
Thread Connections
API & Premium Connection Types
N80 casing is supplied with API 5CT standard threaded connections or proprietary premium connections, selected based on well pressure, temperature, and structural loads.
| Type | Full Name | Characteristics |
|---|---|---|
| STC | Short Round Thread | Economical, low–medium pressure wells |
| LTC | Long Round Thread | Improved tensile strength over STC |
| BTC | Buttress Thread | High collapse & tensile, deep wells |
| Premium | Proprietary (VAM, TenarisHydril, etc.) | Metal-to-metal seal, gas-tight, high-torque |
All sizes and weights are per API Specification 5CT (10th Edition). Actual availability varies by mill and lead time. Contact BSCO for stock lists, custom wall thicknesses, or premium connection options.
Test Requirements of N80 Casing
| Steel Grade | Mandatory Tests | Supplementary Tests | NDT | Corrosion Test | Key Control Requirements |
| N80-1 | Chemical analysis, Tensile, Hardness, Hydrostatic pressure | Impact | UT | None | Deep well performance |
| N80-Q | Quenched & tempered microstructure |
API 5CT · OCTG Manufacturing
Hot Rolled Process of Seamless N80 Casing
From solid steel billet to finished casing joint — the complete manufacturing flow for API 5CT N80 seamless OCTG products.
Production Flowchart
Seamless Pipe Manufacturing Process
Each stage is performed to API 5CT specification, ensuring dimensional accuracy, metallurgical integrity, and downhole reliability.
1
Round Steel Billet
2
Examination
3
Cut Off
4
Heating Furnace
5
Piercing
6
Elongating
7
Stretch Reducing
8
Straightening
9
Crop End Cutting
10
Heat Treatment
11
Visual Inspection
12
Non-destructive Testing
13
Hydrostatic Test
14
Drift Testing
15
Threading & Coupling
Finished N80 Casing — Ready for Shipment
Seamless & ERW
Pipe Forming
Seamless N80 casing starts from a solid round billet that is furnace-heated and pierced on a Mannesmann mill to create a hollow shell, followed by elongation and stretch-reducing to final dimensions. ERW casing is formed from flat-rolled coil, progressively shaped into a tube and seam-welded by high-frequency resistance welding — no filler metal is added.
N80-1 vs N80Q
Heat Treatment
N80 Type 1 is normalized — heated above its critical temperature and air-cooled — producing uniform grain refinement and the required 80 ksi minimum yield. N80Q is quenched and tempered: water-quenched to form martensite, then tempered at 540–650 °C for superior impact toughness, through-wall uniformity, and collapse resistance in deeper wells.
QC & Finishing
Testing & Threading
Every joint undergoes hydrostatic testing at API 5CT–specified pressure, drift testing with a standardized mandrel to verify bore clearance, and non-destructive examination (UT/EMI). Pipe ends are then machined with API threads — STC, LTC, or BTC — and fitted with couplings and thread protectors before final marking and shipment.
Grade Selection Guide
N80-1 vs N80Q: Choosing the Right Sub-Grade
Same 80 ksi minimum yield — different heat treatment, testing requirements, and performance characteristics.
N80-1
Normalized
Heat Treatment
Normalizing — austenitize + air cool
Mandatory Testing
No Charpy impact, no full-body NDT required
Typical Depth
Sweet-service onshore wells ≤ 2,500 m
Best For
Moderate loads, conventional temperature, cost-sensitive projects
N80Q
Quench & Temper
Heat Treatment
Austenitize → water quench → temper (540–650 °C)
Mandatory Testing
Charpy CVN impact + full-length UT / EMI required
Typical Depth
Deeper wells > 2,500 m, cold regions, high-collapse loads
Best For
Critical strings, Q+T mandated specs (ISO 11960 Annex H, NOC requirements)
Grade Comparison
N80 vs L80 Casing: Key Differences
N80 and L80 share the same yield strength window, but L80 adds a hardness cap and sour-service qualification that make it a fundamentally different product.
N80 (Type 1 / Q)
Sweet-Service Grade
Hardness Limit
No maximum HRC specified by API 5CT
H₂S / Sour Service
Not qualified per NACE MR0175 / ISO 15156 Not Rated
Heat Treatment
N80-1: Normalized / N80Q: Quench & Temper
Best For
CO₂-only or sweet wells with no H₂S exposure
Cost
Lower — standard mill production, wider availability
L80 (Type 1)
Sour-Service Grade
Hardness Limit
Capped at ≤ 23 HRC (max 241 HBW) — SSC prevention
H₂S / Sour Service
Qualified per NACE MR0175 / ISO 15156 Sour Rated
Heat Treatment
Quench & Temper only — mandatory for hardness control
Best For
Wells with H₂S present — SSC-resistant design required
Cost
Higher — stricter process control, hardness testing every joint
Selection rule of thumb: If H₂S partial pressure at any point in the well exceeds the NACE MR0175 threshold (typically ≥ 0.05 psi in the gas phase), L80 — not N80 — must be specified to prevent sulfide stress cracking (SSC). When in doubt, default to L80; the hardness cap provides a built-in safety margin even if the well turns mildly sour over its service life.
“N” = grade family identifier; “80” = minimum yield strength in ksi (80,000 psi / 552 MPa).
N80-1 = normalized, no mandatory impact/NDT; N80Q = quenched + tempered, mandatory Charpy & NDT. Neither is universally better — depends on well depth, environment, and project spec.
N80 yield = 80 ksi vs 55 ksi for J55/K55. Higher burst, collapse, and tensile capacity. Downgrading to J55/K55 unsafe in deeper wells; upgrading is safe but wasteful.
Same yield strength, but L80 has 23 HRC hardness cap + NACE MR0175 sour-service qualification. Use N80 for sweet wells, L80 for H₂S wells.
STC, LTC, BTC (standard API). BTC most common for intermediate/production casing. Premium connections available for gas-tight / HPHT service.
Medium-depth sweet-service wells (~2,000–4,000 m). Onshore and moderate offshore. Not for sour service (use L80) or ultra-deep (use P110).
