OCTG casing and tubing grades are the shorthand the oil and gas industry uses for pipe strength, and API 5CT is the standard that defines them. The current version is the 11th edition, published in December 2023, with a Monogram effective date of January 1, 2025, and Addendum 1 in May 2025.
Its ISO equivalent, ISO 11960, fixes the chemical composition, heat treatment, mechanical properties, and testing each grade must meet. OCTG stands for Oil Country Tubular Goods: drill pipe, casing, and tubing. OCTG drill pipe turns the drill bit and circulates drilling fluid during the drilling process, then comes out. Casing and tubing stay.
This reference covers every grade in the current edition: what the code means, how the groups differ, where casing pipe and production tubing part ways, and what else travels with the grade on an order for OCTG products. It draws on the experience of Imex Canada, which has supplied OCTG casing and tubing to API 5CT for more than 30 years.
Key Takeaways
- The grade number is the minimum yield strength in ksi, nothing more.
- Grade omits diameter, wall, connection, and pressure ratings.
- Suffixes matter: N80-1 will not substitute for N80-Q.
- Group 2 grades carry the hardness caps sour service requires.
- The 11th edition added L80-3Cr; Addendum 1 added C125.
- Imex Canada supplies casing J55 to Q125 and tubing J55 to P110-HC.
What the Grade Designation Actually Tells You
Take P110. The letter “P” is the API grade letter, an identifier, not a measurement. The number “110” is minimum yield strength in thousands of pounds per square inch, or ksi, so P110 yields no lower than 110 ksi, or 758 MPa.
That number is a floor, not a target, and the yield achieved at the mill can land higher. API 5CT also caps maximum yield on several grades to keep the steel from getting too hard, which is critical in sour wells.
The grade says nothing about outside diameter, wall thickness, weight per foot, connection, collapse rating, burst rating, or corrosion limits. Two joints of L80 can withstand very different internal pressure, since those ratings come from geometry.
Reading the Suffixes
Several grades carry a suffix, and it changes what you get:
- “-1” as in N80-1 or L80-1 means the standard requirement for chemistry and processing.
- “-Q” as in N80-Q means quenched and tempered: the same yield class with tight tolerances on mechanical properties.
- “-3Cr”, “-9Cr”, and “-13Cr” on L80 mean roughly 3, 9, or 13 percent chromium in the alloy, often with molybdenum, for carbon dioxide service. L80-3Cr is new in the 11th edition for mildly corrosive downhole applications.
N80-1 and N80-Q share the same 80 ksi number but are not interchangeable. If the design assumed quenched and tempered pipe and the yard sends N80-1, the strings match on paper and differ downhole.
The API 5CT Grade Groups
API 5CT has long sorted grades into four groups by strength level and service environment, not yield alone. Recent editions reorganized how the groups are presented, but the logic still works for screening.
Group 1 — H40, J55, K55, N80, R95
H40 is the entry-level grade at 40 ksi, about 276 MPa, for the shallowest work: conductor and surface strings that stabilize the top of the borehole.
J55 has a 55 ksi minimum yield, 379 MPa, for shallow oil and gas wells at low pressure. K55 shares that yield floor but carries a higher minimum tensile strength requirement. The difference shows up in burst-governed strings, where the extra tensile margin changes behavior near failure.
N80-1 and N80-Q both sit at 80 ksi, about 552 MPa, the medium-depth workhorse of Group 1. R95 sits above them at 95 ksi, 655 MPa. It is quenched and tempered and gives T95-level strength without the sour-service controls. It answers the case where N80 is short and P110 is more than the well needs.
One grade you may see on old drawings is M65, at 65 ksi. API eliminated it in the 10th edition, so it cannot be supplied to current API 5CT. J55 or N80 covers the intent.
Group 2 — L80, C90, T95, C110
Group 2 is the sour-service family, adding hardness caps and heat-treatment controls on top of the strength requirement. The caps are maximum mean values: 23 HRC for L80, 25.4 HRC for C90 and T95, 29 HRC for C110, and 34 HRC for C125, with retest thresholds slightly above each.
L80-1 delivers 80 ksi, about 552 MPa, with a maximum yield and a hardness limit. What buyers pay for is dependable hardness control, not strength. The chromium versions add excellent resistance to carbon dioxide.
C90 reaches 90 ksi, about 621 MPa, and T95 reaches 95 ksi, 655 MPa, both for higher-strength sour service applications. C110 sits at 110 ksi, 758 MPa, and still needs review against actual partial pressures. For the worst corrosive environments, duplex stainless steel and nickel alloys sit outside API 5CT.
Group 3 — P110
P110 has a 110 ksi minimum yield strength, or 758 MPa. It is the high-strength workhorse for deep, high-pressure wells and is widely stocked as both casing pipe and OCTG tubing.
Standard P110 is not a sour-service grade: with no Group 2 hardness cap, it is ruled out where hydrogen sulfide is present.
Group 4 — Q125, C125
Q125 has a 125 ksi minimum yield, about 862 MPa. It is mainly a casing grade for extreme conditions, where nothing lower satisfies the loads.
Addendum 1 added a second grade at this strength in May 2025: C125. Like Q125, it is a casing grade at 125 ksi, but the C prefix signals controlled hardness and defined sulfide stress cracking tests. It fills the gap above C110 where Q125 cannot serve because of sour conditions.
At this strength level, handling discipline matters more. The steel tolerates notches and hard slip marks poorly, so durability depends on tight tolerances and careful handling.
Connection choice moves to premium connections, since API threads may not carry the full pipe body rating.
Complete OCTG Grade Comparison Table
| Grade | Casing / Tubing | Min. Yield Strength (ksi / MPa) | Typical Application |
| H40 | Both | 40 / 276 | Shallow, low pressure |
| J55 | Both | 55 / 379 | Shallow, low pressure |
| K55 | Both | 55 / 379 | Shallow wells, burst margin |
| N80-1 | Both | 80 / 552 | Medium depth |
| N80-Q | Both | 80 / 552 | Medium depth, quenched |
| R95 | Both | 95 / 655 | Sweet service above N80 |
| L80-1 | Both | 80 / 552 | Sour service, hardness capped |
| L80-3Cr | Both | 80 / 552 | Mild CO₂ service |
| L80-9Cr | Both | 80 / 552 | Moderate CO₂ |
| L80-13Cr | Both | 80 / 552 | CO₂ with higher chlorides |
| C90 | Both | 90 / 621 | Higher-strength sour |
| T95 | Both | 95 / 655 | High-strength sour |
| C110 | Both | 110 / 758 | High-strength sour, capped |
| P110 | Both | 110 / 758 | Deep, high-pressure, non-sour |
| C125 | Casing | 125 / 862 | Very high strength, hardness capped |
| Q125 | Mainly casing | 125 / 862 | Very deep, high load |
OCTG Casing vs. Tubing: Same Grade, Different Job
Casing pipe protects the structural integrity of the borehole and isolates formations. Cemented in place, its loads are external pressure and the dead weight of the string below. Most wells run several: a conductor, a surface casing over shallow water zones, an intermediate string over trouble zones, and the production string.
Tubing runs inside the casing and carries the flow for oil and gas production. Production tubing is retrievable and contacts well fluids for the life of the well, so corrosion resistance and sealing matter more for tubing pipes than for casing.
J55, K55, N80, L80, and P110 come in both forms. What changes: the dimensional and testing requirements, the connections, and the wall and weight ranges. Specifying only “L80” leaves the order incomplete.
Matching OCTG Grades to Well Conditions
Depth and Axial Load
The deeper the string, the more dead weight the top joints carry in axial tension. That pushes a design from 55 ksi grades toward 110 and 125 ksi. Tapered strings are the answer: a higher grade where tension peaks, stepping down deeper in the hole.
Collapse and Burst Requirements
Burst resistance tracks yield strength and wall thickness. Collapse resistance is governed mostly by geometry, especially the diameter-to-wall ratio. So a higher grade does not automatically solve a collapse problem; in the elastic range, wall thickness is the critical variable.
H₂S Exposure and Sour Service
Sour-service grades exist because of sulfide stress cracking. Hydrogen sulfide drives atomic hydrogen into the steel, and hydrogen embrittlement can crack hard steel under tension with little warning.
That is why L80-1, C90, T95, C110, and C125 carry hardness caps, while standard P110 and Q125 do not. NACE MR0175 and ISO 15156 set the limits by partial pressure, temperature, and pH.
CO₂ Corrosion
Carbon dioxide dissolved in water forms carbonic acid and drives localized corrosion, deep pits rather than even wall loss. Chlorides add crevice corrosion under scale and at connection shoulders.
That is where the 3Cr, 9Cr, and 13Cr chemistries earn their premium in wet gas production and in carbon capture injection wells, where free water makes dense CO₂ aggressive.
Temperature
Temperature is one of the environmental factors that can undo a good grade choice. Heat derates yield strength, so a string designed on surface values runs short at depth. Cold cuts toughness.
Beyond the Grade: Specs That Complete the OCTG Order
The grade is one line of a specification. Before the order goes out, confirm every item below:
- Outside diameter and nominal weight per foot
- Wall thickness and the diameter-to-thickness ratio
- Connection type: API threads (BTC, LTC, STC, EUE, NUE) or premium threads for downhole applications
- Collapse and burst ratings against expected well loads
- Temperature profile, plus H₂S and CO₂ partial pressures
- Well pressure, including test and stimulation
- Length range, end finish, and coating requirements
- Certification to API standards, mill test reports, inspection
Common OCTG Grade Selection Mistakes
Most trouble on incoming orders comes from a few assumptions that look fine on paper and cost money downhole.
- Treating J55 and K55 as identical because both show 55 ksi
- Substituting N80-1 for N80-Q on strength parity alone
- Assuming a higher grade always means a safer string
- Reading hardness as a bonus when sour service calls for a cap rather than super hardness
- Ignoring the connection after sizing the pipe body correctly
- Specifying a sour-service grade but skipping the hardness testing behind it
- Carrying a withdrawn grade such as M65 forward from an old template
A Practical Selection Sequence
Working in this order keeps the environment from becoming an afterthought.
- Define well geometry, depth, and expected loads.
- Characterize the fluids: H₂S, CO₂, chlorides, and temperature.
- Screen out every grade the environment disqualifies.
- Size for collapse and burst with a design factor.
- Select the grade that meets the remaining load cases at the lowest total cost.
- Match the connection to the load and sealing requirement.
- Set inspection and certification requirements before releasing the order.
Where to Source the OCTG Grade You Specify
Once grade, size, and connection are settled, the order still has to be filled correctly. Imex Canada supplies OCTG casing and tubing to API 5CT: casing from J55 through Q125 and tubing from J55 through P110-HC, where HC means high collapse, plus mill proprietary grades for sour service and corrosion resistance.
Our team has spent more than 30 years on tubular supply. We can check your grade and connection against the well’s loads and fluids, supply full mill test documentation, and deliver to the site.
Specifying OCTG Grades: The Short Version
An API 5CT grade is where an OCTG pipe specification starts, not where it ends. It tells you how the pipe was manufactured and tested, while geometry, connection, and certification decide whether the string performs. Read the table with your load and fluid data, and the right grade narrows quickly.
If you have any questions about OCTG casing and tubing grades or want a second look at a string design before you release the order, contact Imex Canada. We are happy to help.