N80 in the API 5CT Casing Family
N80 is a medium-strength casing and tubing grade defined in API 5CT with a specified minimum yield strength of 80,000 psi (552 MPa). It occupies the middle of the grade ladder, between the soft J55/K55 grades and the high-strength P110, and offers the best strength-to-cost balance for medium-depth wells where collapse and burst loads are moderate. It is produced seamless in most oil country applications and is available in two types: N80-1 and N80-Q.
N80-1 versus N80-Q
The two types share the same strength range but differ in how they are manufactured and heat treated:
N80-1:
may be produced by hot rolling, normalizing, or normalizing and tempering at the manufacturer's discretion, provided all mechanical requirements are still met.
N80-Q:
must be quenched and tempered (Q&T), which gives a more uniform microstructure, better toughness and tighter mechanical consistency through the wall.
Recommendation:
for higher loads, sour-influenced environments or critical wells, N80-Q is preferred for its controlled, repeatable properties.
Mechanical Requirements (API 5CT)
| Property | N80-1 / N80-Q |
|---|---|
| Yield strength | 552-758 MPa (80-110 ksi) |
| Tensile strength (min) | 689 MPa (100 ksi) |
| Hardness (max) | ~23 HRC |
| Elongation | Per API 5CT formula |
| Manufacturing process | Seamless (welded production permitted only within defined limits) |
Typical Sizes and End Finishes
Outside diameter:
4.5" to 13.375" (114.3-339.7 mm) in common casing sizes, with larger diameters available by mill.
Wall thickness:
selected per design collapse and burst calculations for the specific well.
Connections:
STC, LTC or BTC threaded-and-coupled, or premium gas-tight connections for critical wells.
Length range:
R1 (6.10-7.01 m), R2 (8.53-9.75 m) or R3 (11.28-12.19 m).
Well Service Considerations
N80 casing is the standard choice for intermediate and production casing in medium-depth wells with moderate pressures and temperatures. For hydrogen sulfide service, N80 is generally not permitted unless the H2S partial pressure is very low and hardness is verified; where sour conditions are expected, L80 with its restricted hardness range is the safer selection. N80-Q is also widely used as tubing in pumping and gas-lift wells where fatigue resistance matters, and it responds well to drift and pressure testing during completion.
Quality Assurance
Each heat of N80 casing is verified with chemical analysis, tensile tests, hardness surveys and flattening tests. Every pipe is hydrostatically tested, and drift testing confirms that the internal diameter is clear. Stencilled marking includes the API monogram, grade, type, size, weight and heat number, and the Mill Test Certificate provides full traceability from steelmaking to the finished joint.
Frequently Asked Questions
What is the difference between N80-1 and N80-Q?
N80-Q must be quenched and tempered, giving more uniform microstructure and toughness; N80-1 can be supplied in other heat-treated or hot-rolled conditions that still meet the mechanical limits.
What strength does N80 casing provide?
It has a yield strength range of 552-758 MPa (80-110 ksi), placing it between J55/K55 and P110 in the API 5CT grade ladder.
Can N80 be used in sour service?
N80 is not the preferred choice for H2S service; API 5CT recommends sour-service grades such as L80 with controlled hardness and special testing.
Is N80 casing seamless?
It is normally produced seamless for maximum integrity; API 5CT permits welded production in defined cases, but seamless dominates oil country applications.
What connections are available for N80 casing?
Standard threaded-and-coupled connections (STC, LTC, BTC) plus premium gas-tight connections for high-pressure and offshore wells.
How is N80 casing inspected before delivery?
Through chemical and mechanical testing per heat, hydrostatic testing of every joint, drift testing, dimensional checks and visual surface inspection, all documented on the mill certificate.
