API 5CT N80 Casing Pipe — N80-1 vs N80Q Grades & Specifications

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 occupies the gap between the commodity J55/K55 grades and the sour-service-qualified L80 — covering the broad middle of onshore and shallow offshore programmes where sweet service conditions apply and cost efficiency matters. Two sub-grades exist within the N80 designation: N80-1 and N80Q. The distinction between them is not academic — it determines what heat treatment the mill applies, and in deeper wells, it determines the through-wall toughness profile of the string.

The most frequent order error we see on N80 purchase orders is the buyer writing "N80" without specifying the sub-grade. Under API 5CT, N80-1 is the default — but N80-1 may be supplied normalised, normalised-and-tempered, or quench-and-tempered. A buyer who needed Q+T properties for better through-wall toughness in a deeper well, but did not write "N80Q" on the PO, may receive N80-1 produced by normalising. The MTC will be correct; the microstructure will not be what was intended. The other pattern we see frequently is an engineer switching from N80 to L80 under the assumption that L80 provides better pressure containment. It does not — both grades have 80 ksi minimum yield. L80 costs more and provides something different: hardness control and NACE documentation. If H₂S is not in the picture, N80-1 is the right choice and the cost difference is unrecovered.

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, to oil and gas operators and drilling contractors across Africa, South America, and Southeast Asia.

What Is API 5CT N80?

API Specification 5CT, 11th Edition, is the American Petroleum Institute specification governing oil country tubular goods for well construction. N80 is a Group 1 casing grade within that standard — "Group 1" meaning it covers both the N80-1 and N80Q sub-grades under a shared yield strength envelope. The "80" in the designation refers to the minimum yield in thousands of psi: 80,000 psi, or 552 MPa.

N80 sits above J55 and K55 (55,000 psi minimum yield) and shares the same 80 ksi minimum yield as L80, but without the hardness ceiling that L80 carries. Above N80, the next rung in the sweet-service grade ladder is P110, which starts at 110,000 psi minimum yield and is designed for deeper or higher-load well programmes. N80 is the default specification for intermediate and production casing in sweet 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.

The two sub-grades exist because the API committee recognized that normalising and quench-and-tempering produce genuinely different microstructures — and that buyers needed a way to specify which one they required without creating an entirely separate grade. N80-1 leaves the heat treatment choice to the mill. N80Q removes that choice and mandates Q+T.

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 they are produced by fundamentally different heat treatment routes with real consequences for mechanical behaviour under load.

A point that causes confusion: N80-1 is not restricted to normalising. API 5CT permits N80-1 to be produced by normalising, normalised-and-tempering, or quench-and-tempering. N80Q, by contrast, permits only quench-and-tempering. The defining characteristic of N80Q is not that Q+T is permitted — it is that Q+T is the only permitted route.

The practical consequence of the three permitted routes for N80-1 is that two different N80-1 heats from two different mills can have completely different microstructures while both passing the API 5CT yield test. A normalised N80-1 and a Q+T N80-1 will both show yield at or above 552 MPa — but their fracture toughness, particularly through the wall thickness, can differ substantially. For shallow wells with modest burst and collapse loading, this difference may be immaterial. For deeper wells where the casing is under sustained axial load, or in environments with cyclic thermal loading, the Q+T microstructure provides meaningfully better toughness.

When to specify N80Q: Use N80Q when the casing design requires consistent Q+T properties — for deeper wells where through-wall toughness matters, in colder environments where low-temperature ductility is a concern, or when an IOC company specification mandates Q+T heat treatment for all intermediate casing regardless of grade. Specify it on the PO: "API 5CT N80Q."

When N80-1 is correct: 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 African onshore programmes and South American land drilling at moderate depth use N80-1 for production and intermediate strings without issues.

Chemical Composition

API 5CT sets maximum element limits for N80. The limits are narrow: only phosphorus and sulphur carry explicit maxima. All other elements — carbon, manganese, silicon, chromium, nickel, molybdenum, and others — are unrestricted by the standard.

The consequence of this open chemistry is significant for procurement: N80 mechanical properties vary more across different mill heats than for grades where chemistry is tightly controlled. Grades like L80 and T95 carry explicit carbon equivalent limits and tighter alloy restrictions precisely because their sour service or HPHT applications require predictable microstructure. N80 does not have those constraints — which is why mills have latitude to formulate very different chemistries and still produce pipe that passes the API 5CT yield and tensile tests. When consistent mechanical behaviour across multiple heats matters — for a multi-string programme or a project with tight collapse design margins — requesting yield histograms from the mill before placing the order is worth the time.

For the complete grade ladder with tensile, hardness, and chemistry limits, see the API 5CT specification tables →

To match a grade to your well conditions, use the AI Pipe Grade Selector →

Standard Dimensions and Weights

API 5CT N80 casing is available in all standard casing sizes from 4½ inch through 20 inch OD. The table below shows the most commonly ordered sizes with typical weight ranges and drift diameter ranges.

The two most common sizes in intermediate casing programmes are 9⅝ inch (47 lb/ft being the most frequently ordered weight) and 13⅜ inch. Production strings in land wells typically use 5½ inch or 7 inch. For full dimension tables including ID, drift diameter, and coupling OD for every weight within each size, see the API 5CT specification tables → or use the Barlow pressure calculator → to check burst ratings at your chosen weight.

Worked Burst Pressure Calculation

To understand what N80's yield strength buys in terms of pressure containment — and to illustrate exactly where N80 versus L80 is not a relevant distinction — consider a standard 9⅝ inch × 47 lb/ft intermediate casing string.

For this pipe, the OD is 9.625 inches and the nominal wall thickness is 0.472 inches. Using the API 5C3 burst pressure formula:

P = 0.875 × (2 × Yp × t / D)

Where Yp is minimum yield strength in psi, t is wall thickness in inches, D is OD in inches, and 0.875 is the API 5C3 mill tolerance factor.

N80 at 9⅝" × 47 lb/ft: P = 0.875 × (2 × 80,000 × 0.472 / 9.625) P = 0.875 × 7,841 P = 6,860 psi (68.6 MPa)

For context, the same geometry in J55 (55,000 psi minimum yield): P = 0.875 × (2 × 55,000 × 0.472 / 9.625) P = 0.875 × 5,391 P = 4,720 psi (32.6 MPa)

N80 provides 45% more burst resistance than J55 at identical geometry. That gap is meaningful for intermediate strings where reservoir pressure has not depleted and kick scenarios must be designed for.

Now the critical comparison: L80, also at 80,000 psi minimum yield, produces the same result — 6,860 psi for this pipe. The burst rating is identical. The difference between N80 and L80 is not pressure containment. It is hardness control and sour service qualification. Engineers who specify L80 in sweet-service wells on the assumption that it provides better burst performance are paying a cost premium for a benefit they are not receiving. N80-1 is the correct sweet-service choice at this yield level; L80 is the correct choice when H₂S is present.

Connection Types

N80 casing is compatible with all API 5CT standard connections and premium connection designs.

BTC is the industry default for 9⅝ inch and 13⅜ inch N80 intermediate casing in onshore wells. It provides near pipe-body tensile efficiency and better collapse resistance than LTC at a modest cost premium. STC should not be used for intermediate or production casing where burst or collapse loading is meaningful — its thread engagement is insufficient for those load cases. Premium connections are used when the connection must be gas-tight independent of thread compound, in gas wells where the compound can be displaced by produced gas over time, or in deviated wells where connection integrity under combined axial and bending load is critical.

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

The table makes clear that N80 and L80 are the same grade for burst and collapse calculations — both use 80,000 psi minimum yield. L80's narrower maximum yield ceiling (655 MPa versus N80's 758 MPa) means L80 is in practice a tighter yield band, which aids string design predictability — but the minimum burst and collapse ratings for the same geometry are identical. P110 is a fundamentally different well condition: at 110 ksi minimum yield, it is designed for deep, high-load applications where N80 is structurally insufficient.

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 — either sub-grade — and NACE MR0175 restricts carbon steel tubulars in H₂S service to 22 HRC maximum. Some N80 heats, particularly those produced by normalising with higher carbon content, can develop areas exceeding this limit, creating susceptibility to sulphide stress cracking (SSC).

A point that surprises some engineers: N80Q's Q+T heat treatment does not qualify it for sour service. The distinction between N80Q and L80 is not the heat treatment — both are Q+T. The distinction is the hardness ceiling and the NACE documentation trail. L80 carries a mandatory 23 HRC maximum hardness and the testing records to prove it; N80Q does not. NACE MR0175 requires both the hardness limit and the documented verification. N80Q provides neither. If H₂S partial pressure exceeds 0.0003 MPa (0.05 psi) — the NACE MR0175 threshold — specify L80. Substituting N80Q on cost grounds in a sour well is non-compliant regardless of what the Q+T microstructure looks like on a given heat.

When Not to Use N80

Six conditions where N80 is the wrong specification:

• Any well with confirmed H₂S partial pressure above 0.0003 MPa (0.05 psi). Specify L80. N80 — neither sub-grade — qualifies under NACE MR0175 / ISO 15156. This is a compliance issue, not a preference.

• When the project specification requires a documented maximum hardness. N80 has no hardness limit and no hardness testing requirement under API 5CT. If a company specification, project QAQC plan, or regulatory requirement mandates a maximum HRC value on the MTC, N80 cannot satisfy it. Specify L80.

• Deep wells where a casing design program requires Q+T heat treatment confirmation on the MTC. N80-1 cannot guarantee Q+T — the mill may supply normalised pipe and be fully API-compliant. Specify N80Q (if sour service is not a factor) or L80 (if it is).

• HPHT wells where collapse and burst require 110 ksi yield or above. N80's maximum yield is 758 MPa (110 ksi) — at the ceiling, not the floor. L80 caps at 655 MPa (95 ksi) and P110 starts at 758 MPa (110 ksi). For high-pressure deep wells where the design load requires assured 110 ksi yield, P110 is the correct grade. N80 at 80 ksi minimum is insufficient.

• Gas wells requiring gas-tight connections where the connection premium would offset grade savings. If the connection must be a premium type regardless of grade, the cost argument for N80 over L80 weakens. Evaluate the full string cost including connections before assuming N80-1 is the economical choice.

• Strings where yield consistency across multiple heats matters for string design calculations. Because N80's chemistry is almost entirely unrestricted, yield strength can vary widely across heats from different mills or even different production runs at the same mill. If the casing design is sensitive to yield variation — tight collapse design factors, high-deviation wells with combined load — consider L80 or specify a supplemental yield histogram requirement.

Inspection and Testing Requirements

The absence of mandatory hardness testing is the most important line in this table. API 5CT does not require hardness testing for N80, and the MTC will reflect this — the hardness column will be blank or marked N/A. This is compliant. It is also the reason N80 cannot satisfy sour service requirements and cannot be used where a project specification mandates hardness documentation. NDE — ultrasonic testing of the pipe body and ends — is optional under API 5CT for N80 unless specified on the purchase order. For production casing in wells above 3,000 metres, or any well with elevated burst or collapse requirements, add 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 recommended for EPC projects in Africa and South America where project documentation requirements are strict.

Procurement Trap — The Sub-Grade Omission

This is the single most consistent specification error on N80 orders, and it costs engineers and procurement teams real money when it surfaces during MTC review.

Wrong PO language: "API 5CT N80, 9⅝" × 47 lb/ft, BTC"

What the mill ships: N80-1 produced by normalising — the cheapest permitted route under API 5CT. The MTC will correctly list "N80 Type 1" and show tensile test results meeting the 552 MPa minimum. Everything on the MTC is accurate and compliant. The microstructure is ferrite-pearlite, not tempered martensite.

What to write instead (if Q+T is required): "API 5CT N80Q, 9⅝" × 47 lb/ft, BTC, Q+T heat treatment required, EN 10204 3.1 MTC"

If sour service is ever possible in the well's life: Switch to L80. Do not use N80Q for wells where H₂S may be encountered — even occasionally or unexpectedly. The MTC trail required by NACE MR0175 does not exist for N80Q, and retroactively qualifying a string that has been installed is not an option.

The sub-grade omission is especially common on repeat orders where the first order was written correctly by the drilling engineer but subsequent reorders are placed by procurement from a simplified spreadsheet. Building the sub-grade into the item description in the procurement system — not just a free-text PO note — is the fix.

How to Specify N80 Casing on a Purchase Order

A complete N80 purchase order must include these fields to avoid ambiguity and incorrect deliveries:

1. Standard — API 5CT (or ISO 11960), 11th Edition
2. Grade and sub-grade — N80-1 or N80Q explicitly (never "N80" alone)
3. Heat treatment — State "Q+T required" when ordering N80Q; state "N or NT acceptable" when N80-1 is specified and normalising is acceptable
4. OD and nominal weight — e.g. 9⅝ inch × 47.00 lb/ft
5. Thread type — STC, LTC, BTC, or premium connection designation with full qualification reference
6. Range — R1, R2, or R3 (most casing strings are R3)
7. PSL level — PSL-1 or PSL-2
8. Quantity — in joints or metric tonnes
9. Inspection scope — third-party witness (SGS / BV / TÜV), supplemental NDE requirements
10. MTC level — EN 10204 3.1 (standard) or 3.2 (third-party witnessed)

Always specify N80-1 or N80Q explicitly. A purchase order that carries only "N80" is legally ambiguous and will default to N80-1 at the mill. Mixing sub-grades in a string is technically permissible under API 5CT but creates traceability complications on projects with strict MTC documentation requirements — avoid it unless the design explicitly allows it.