N80 and L80 sit at the same rung of the API 5CT grade ladder — identical minimum yield strength of 552 MPa (80,000 psi), both widely used for intermediate and production casing — and are regularly treated as interchangeable by procurement teams pricing a new well. They are not interchangeable. The four differences that separate them — yield ceiling, tensile minimum, hardness control, and sour service qualification — are the differences that determine whether a casing string survives H₂S exposure or fails by sulfide stress cracking (SSC) within months of completion.
ZC Steel Pipe supplies both N80 and L80 casing across a full OD and weight range, with BTC and premium connections available for both grades, to operators across West Africa, the Middle East, and Southeast Asia. This guide covers where the two grades agree, where they diverge, and the specific conditions that make one the correct choice and the other a compliance or integrity risk.
What we see on batch quotes: The N80 vs L80 question comes up in almost every batch quote for a sweet service intermediate casing string. The assumption we hear most often is that L80 is the "more specified" grade and therefore somehow stronger. The tensile data is consistently a surprise: N80 has a higher minimum tensile strength — 689 MPa (100 ksi) — than L80, which is only 655 MPa (95 ksi). We show buyers the API 5CT table. The follow-up question is always the same: then why does L80 cost more? Because the value in L80 is the 23 HRC hardness ceiling and the sour service qualification, not the structural load ratings.
Side-by-Side Comparison
The table below covers all four grade variants — N80-1, N80Q, and L80-1 — against the properties that matter most for selection decisions. Read the tensile and hardness columns carefully: the tensile numbers are counterintuitive, and the hardness column explains the entire sour service story.
| Property | N80-1 | N80Q | L80-1 |
|---|---|---|---|
| Min yield strength | 552 MPa (80 ksi) | 552 MPa (80 ksi) | 552 MPa (80 ksi) |
| Max yield strength | Not specified | Not specified | 655 MPa (95 ksi) |
| Min tensile strength | 689 MPa (100 ksi) | 689 MPa (100 ksi) | 655 MPa (95 ksi) |
| Heat treatment | Normalize or N+T | Quench and temper | Quench and temper |
| Max hardness | No limit | No limit | 23 HRC (241 HBW) |
| Hardness testing | Not required | Not required | 100% per joint |
| Sour service (NACE MR0175) | No | No | Yes |
| Chemistry — C max | Not restricted | Not restricted | 0.43% |
| Chemistry — Mn max | Not restricted | Not restricted | 1.90% |
| Chemistry — Si max | Not restricted | Not restricted | 0.45% |
| Relative cost | Baseline | +5–10% | +10–20% |
N80-1 is the most common and lowest-cost variant. N80Q adds quench-and-temper processing for tighter mechanical property control but shares N80-1's absence of any hardness requirement. L80-1 tightens yield through a maximum ceiling, accepts a lower tensile floor in exchange, and adds the hardness cap that makes sour service qualification possible.
For the full mechanical properties and chemistry table across all API 5CT grades, see the API 5CT specification tables →
Heat Treatment and Microstructure
N80-1 can be delivered normalized, normalized-and-tempered, or quench-and-tempered — the mill chooses. Normalizing heats the pipe above the upper critical temperature and air-cools it, which refines the grain structure but leaves hardness variable. Depending on carbon content and cooling rate, N80-1 produced this way can land anywhere between 18 and 28+ HRC. There is no API 5CT requirement to measure or control it.
N80Q is always quench-and-tempered. Water or oil quenching from austenitizing temperature creates a martensitic microstructure; tempering then reduces hardness and restores toughness. The result is more consistent mechanical properties than N80-1. But — critically — API 5CT sets no hardness ceiling for N80Q either. Better consistency is not the same as controlled compliance.
L80-1 uses the same Q&T cycle as N80Q, but adds a 23 HRC maximum with mandatory 100% per-joint testing. The chemistry restrictions (C max 0.43%, Mn max 1.90%, Si max 0.45%) are there to ensure the steel can reach the yield target with Q&T while staying under the hardness ceiling. The whole package — controlled chemistry, defined heat treatment, verified hardness — is what earns L80 its place in ISO 15156-2 as a sour service material.
The Tensile Strength Difference
N80 carries a higher minimum tensile than L80: 689 MPa (100 ksi) against L80's 655 MPa (95 ksi). This is not an error. It follows directly from L80's yield ceiling.
L80 must stay below 655 MPa (95 ksi) on the maximum yield. To keep all production within a narrow yield band, the mill uses lower-carbon heats and controlled cooling — which reduce the potential tensile ceiling. The API 5CT tensile minimum for L80 reflects this: 655 MPa (95 ksi), set below N80's 689 MPa. N80, with no maximum yield specified, can be made from higher-carbon heats that achieve higher tensile naturally.
The structural implication for casing design: both grades use the same 80 ksi minimum yield in burst, collapse, and tensile load calculations. N80's higher tensile floor does not translate to a higher design load for the string — the minimum governs. What N80's higher tensile does indicate is that N80-1 pipe can, and regularly does, sit at 95–110 ksi actual yield in the field.
The hidden design risk in N80-1: A casing string designed to 80 ksi minimum could contain N80-1 pipe actually sitting at 95–110 ksi yield yield. This doesn't affect burst or collapse design (which use minimum), but it means the actual hardness of delivered pipe can be anywhere from 20 to 28+ HRC — no specification requires the mill to measure it. In a sweet well, that variation is irrelevant. The moment H₂S appears — even trace H₂S from an unexpected zone — uncontrolled hardness above 22 HRC creates the conditions for SSC. The grade that passed API 5CT receiving inspection with no anomalies becomes a well integrity problem.
Sour Service — The Critical Divide
API 5CT Specification, 11th Edition designates L80 Type 1 as a sour service grade and lists it in ISO 15156-2 (NACE MR0175) as acceptable for H₂S service when the 23 HRC maximum hardness is documented. N80 — neither N80-1 nor N80Q — appears in that list. The reason is structural: without a maximum hardness requirement and without 100% joint testing, there is no mechanism to ensure compliance with the SSC-resistance threshold that NACE MR0175 / ISO 15156-2 requires.
NACE MR0175 / ISO 15156-2 sets the sour service threshold for H₂S partial pressure at 0.0003 MPa (0.05 psia). That is a very low number — reachable by trace H₂S in a formation gas phase, not just high-H₂S reservoirs. Wells that show gas during drilling with any H₂S indication are already past that threshold.
One detail the API 5CT table does not make obvious: L80's API maximum hardness is 23 HRC, but NACE MR0175 carbon steel limit is 22 HRC. A pipe at 23 HRC is API-compliant but not NACE-compliant. When ordering L80 for sour service, specify a 22 HRC maximum on the purchase order — not the API default. We flag this on every L80 sour service order we process, because not every supplier or buyer distinguishes between the two limits.
For a structured decision framework across all sour service grades — L80, C90, T95, C110 — see OCTG sour service grade selection →
Burst Pressure Worked Example
To make the structural equivalence concrete: consider 9-5/8" 47 lb/ft casing, OD = 9.625 inches, wall thickness = 0.472 inches. The Barlow internal yield pressure formula per API 5CT is:
P = 0.875 × (2 × Y_min × t / OD)
where 0.875 is the API wall tolerance factor and Y_min is the minimum yield strength in psi.
For both N80 and L80, Y_min = 80,000 psi:
P = 0.875 × (2 × 80,000 × 0.472 / 9.625) P = 0.875 × (75,520 / 9.625) P = 0.875 × 7,847 P = 6,870 psi
The burst rating is identical — 6,870 psi — for N80 and L80 in this size and weight. This is not a coincidence; it is the definition of what these grades share. Both are 80 ksi minimum yield grades. For a purely sweet well, specifying L80 instead of N80 adds 10–20% to material cost and delivers no additional structural load capacity. The structural equivalence in sweet service is real, and it matters for cost-driven procurement.
When NOT to Use N80
Any H₂S present in the well. The NACE MR0175 / ISO 15156-2 threshold is 0.0003 MPa (0.05 psia) — far below what most engineers consider a "sour" well. Any confirmed H₂S in the gas phase exceeds this. N80 without hardness control cannot be documented as compliant, and SSC failure in N80 under H₂S exposure is a known and documented failure mode.
Uncertain reservoir data. If geological data cannot definitively rule out H₂S — exploration wells, appraisal wells, or wells in formations with historically variable H₂S presence — the cost of specifying N80 is not justified. The 10–20% material premium for L80 is a fraction of one day's rig cost.
Wells where the operator's project specification requires NACE MR0175 compliance by default. Some operators, particularly on deepwater and HPHT projects, require sour service material for all production and intermediate casing regardless of measured H₂S. N80 cannot satisfy that requirement.
Any well where a gas show during drilling included H₂S. Once H₂S has been detected in the wellbore at any point, the completion string should be treated as a sour service environment.
When NOT to Use L80
Confirmed sweet service wells with documented H₂S-free reservoir data. When geochemical sampling and well history conclusively show no H₂S, L80's sour service qualification adds cost without adding load capacity or life. N80-1 provides identical burst, collapse, and tensile ratings at lower material cost.
Cost-constrained projects with shallow, simple sweet wells. The structural equivalence is real: for a sweet water injection well or shallow production casing string with no sour service concern, the L80 premium is not warranted by the engineering.
Applications where yield scatter matters more than sour qualification. For some deep HPHT wells, the engineer wants maximum yield predictability — which pushes toward T95 or P110, not L80. L80's ceiling is 655 MPa (95 ksi); wells requiring higher burst and collapse ratings will need a higher-grade string regardless of sour service requirements.
Procurement Trap and Correct PO Language
The most expensive N80 vs L80 mistake we see does not happen in the grade selection meeting — it happens on the purchase order, weeks later, after a project specification has been approved and procurement has moved on.
The scenario: a purchase order reads "N80 casing, 9-5/8" 47 lb/ft, BTC, API 5CT." The well has historical gas shows that include trace H₂S — flagged in the geological report, noted in the HAZOP, but not carried through to the casing material specification. The pipe arrives on site, passes API 5CT receiving inspection without issues (N80 has no hardness requirement, so there is nothing to fail), and is run in hole. Six months later, the company man requests NACE MR0175 sour service certification to satisfy a change in operator well design standards. N80 cannot be retroactively certified for sour service. Replacing production casing after completion — with the rig mobilized, formation perforated, and production equipment installed — is an emergency with day-rate costs that dwarf the original material savings.
The correct PO language for sour service intermediate casing:
Standard: API 5CT L80 Type 1, [OD] × [weight] lb/ft Connection: BTC, Range 2 (or specify premium connection) PSL level: PSL-2 Maximum hardness: 22 HRC (product analysis — NACE MR0175 limit, stricter than API 23 HRC) MTC: EN 10204 3.2, with per-joint hardness records SR15A: SSC test per NACE TM0177 Method A (if required by operator spec)
For confirmed sweet service N80:
Standard: API 5CT N80-1 (or N80Q), [OD] × [weight] lb/ft Connection: BTC, Range 2 PSL level: PSL-1 MTC: EN 10204 3.1
The type designation (N80-1 vs N80Q) matters because the mill's heat treatment choice affects mechanical property consistency. If consistent toughness is needed in cold environments, specify N80Q explicitly.
Cost and Supply
| Grade | Relative cost | Primary cost driver |
|---|---|---|
| N80-1 | Baseline | Simplest heat treatment; no hardness testing |
| N80Q | +5–10% | Mandatory Q&T; better consistency |
| L80-1 | +10–20% | Q&T + 100% per-joint hardness testing + tighter chemistry controls |
The L80 premium is real but modest relative to total well cost. At a typical day rate for offshore drilling, one additional day caused by a material non-conformance costs more than the entire L80 premium on a full casing string. For any well where sour service cannot be ruled out, the selection should be L80 — the engineering and economic case are the same.
Supply availability for both grades is strong across standard API 5CT casing sizes (4-1/2" through 20"). Lead times for N80 and L80 in standard BTC connections are comparable; premium connections on either grade add 4–8 weeks depending on connector manufacturer. ZC Steel Pipe maintains inventory in the most common intermediate casing sizes for both grades. Contact us with OD, weight, grade, PSL level, sour service requirements, and quantity for current availability and lead time.
For structural load data across the full API 5CT grade ladder, use the AI Pipe Grade Selector → or see the API 5CT specification tables →
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Frequently Asked Questions
What is the difference between N80 and L80 casing?
N80 and L80 have the same minimum yield strength of 552 MPa (80,000 psi) but differ in four critical ways: maximum yield ceiling, tensile strength, hardness control, and sour service qualification. N80 has no maximum yield limit (N80-1) and a minimum tensile of 689 MPa (100 ksi). L80 has a maximum yield of 655 MPa (95 ksi) and a minimum tensile of 655 MPa (95 ksi). L80 has a mandatory 23 HRC hardness ceiling and is qualified for sour service per NACE MR0175/ISO 15156-2 — N80 is not.
Can N80 be used in sour service wells?
No — N80 is not acceptable for sour service environments per NACE MR0175/ISO 15156-2. N80-1 normalized pipe has no hardness control and typically exceeds the 22 HRC NACE limit. Even N80Q is not specifically qualified for H₂S service. When H₂S is present at any significant partial pressure, L80, T95, or C110 must be specified instead.
Does N80 have higher tensile strength than L80?
Yes — N80 has a higher minimum tensile strength than L80. API 5CT requires a minimum tensile of 689 MPa (100 ksi) for N80, compared to 655 MPa (95 ksi) for L80. The difference arises from L80's tighter yield ceiling (max 655 MPa / 95 ksi): constraining the maximum yield forces a lower tensile floor. N80's uncapped yield allows higher-hardness heats that contribute to higher tensile. Despite the higher tensile, N80 is not 'stronger' in the design sense — both grades design to the same 80 ksi minimum yield for burst and collapse.
Why is L80 more expensive than N80?
L80 typically costs 10-20% more than N80 due to mandatory quench and temper heat treatment, 100% joint hardness testing, and tighter chemistry controls for sour service qualification. For sweet service wells where sour qualification is not required, N80 provides identical structural performance at lower cost.
What is N80Q and how does it differ from N80-1?
N80-1 is the normalized or normalized-and-tempered variant of N80 — the most common and least expensive. N80Q is quench and tempered, giving tighter mechanical property control and lower hardness than N80-1. N80Q is sometimes specified where better toughness is needed but sour service qualification is not required. Neither N80-1 nor N80Q qualifies for H₂S sour service — only L80 and higher grades do.
Can L80 and N80 be mixed in the same casing string?
Technically yes since they have the same minimum yield strength. However mixing is generally avoided because it complicates material traceability. More critically, if H₂S is encountered, any N80 in the string creates an SSC risk. Best practice is to use a single grade throughout each string to simplify design, procurement, and inspection records.
What connection types are standard for N80 and L80?
Both N80 and L80 use STC, LTC, and BTC API connections, with BTC being standard for most production and intermediate casing. Premium connections are available for both grades and required for gas wells or HPHT applications. For sour service L80 strings, the connection thread compound must also be H₂S-compatible.
When should I upgrade from N80 to L80?
Upgrade to L80 any time H₂S is present in the well, including trace amounts. The NACE MR0175/ISO 15156-2 sour service threshold is very low — H₂S partial pressure above 0.0003 MPa (0.05 psia) requires sour service material qualification. L80 should also be considered for wells where reservoir conditions are uncertain and H₂S cannot be ruled out. For sweet service wells with confirmed H₂S-free reservoir data, N80 provides equivalent structural performance at lower material cost.