What Are The Grades of OCTG Pipe?

OCTG — Oil Country Tubular Goods — covers the casing, tubing, and drill pipe that make up every oil and gas well. Every foot of it is manufactured to a grade defined by API 5CT, the governing specification from the American Petroleum Institute. The grade determines yield strength, heat treatment, hardness limits, and critically, whether the pipe can be used where hydrogen sulphide (H₂S) is present.

ZC Steel Pipe manufactures and exports OCTG pipe across the full range of API 5CT grades — H40 through Q125 — in both seamless and ERW forms for casing and tubing applications. This guide is a working reference for engineers and procurement managers: what each grade delivers, where it is specified, and how to choose between them.

1. API 5CT: The Standard That Governs OCTG Grades

API 5CT organises grades into four product specification levels (PSL1 and PSL2), with PSL2 adding more stringent requirements for chemical composition, mechanical property consistency, and NDE. Most project specifications for critical wells — HPHT, sour service, deepwater — require PSL2. For routine well construction, PSL1 is standard.

Each grade designation encodes the grade's minimum yield strength in thousands of psi. P110 means minimum yield of 110,000 psi (758 MPa); L80 means minimum yield of 80,000 psi (552 MPa). The letter prefix carries additional meaning — restricted yield range, heat treatment type, or corrosion-resistance category — explained in the grade-by-grade section below.

2. Complete Grade Reference Table

The table below covers all standard API 5CT grades. Yield and tensile values are in MPa (psi equivalents follow). Hardness limits apply to the pipe body. Grades marked with ✓ under "Sour" are approved for H₂S service per NACE MR0175/ISO 15156.

S = Seamless; W = Welded (ERW). Yield/tensile minimums per API 5CT latest edition. Hardness limits are per pipe body — coupling hardness limits differ. "No limit" for sour column means the grade is not approved for H₂S environments above 0.05 psia.

3. Grade-by-Grade Breakdown

H40 — Shallow, Low-Pressure Surface Casing

H40 is the lowest-strength API 5CT grade with a minimum yield of just 276 MPa (40,000 psi). No specific heat treatment is required. It is primarily used for surface casing strings in shallow, low-pressure onshore wells — conductor pipe in soft formations, surface casing where regulatory requirements set the depth rather than pressure design. H40 is rarely seen in procurement specifications for anything beyond surface strings; its cost advantage over J55 is marginal in most markets.

J55 & K55 — The Workhorses for Shallow to Medium Wells

J55 and K55 share the same minimum yield strength (379 MPa / 55,000 psi) but differ in minimum tensile strength: J55 requires 517 MPa (75,000 psi) and K55 requires 655 MPa (95,000 psi). Neither requires heat treatment. J55 is the most widely used grade globally for surface and intermediate casing in onshore wells at moderate depth and pressure. K55 is specified for couplings and applications where higher tensile strength is needed without moving to N80. Neither grade is sour-rated.

N80 — High Volume, Medium-Depth Casing

N80 is the most widely produced high-strength API 5CT grade, with a minimum yield of 552 MPa (80,000 psi). It comes in two subtypes — N80 Type 1 (N80-1) and N80Q — which share the same mechanical property requirements but differ in heat treatment:

• N80-1: Normalized or hot-finished. No quench and temper required. More economical, widely available. Used for the majority of N80 casing in routine well designs.

• N80Q: Quench and tempered. More consistent microstructure, better toughness distribution, tighter hardness control. Specified where uniformity matters or where intermediate sour conditions require controlled hardness.

N80 is not sour-rated. Its wide yield range (552–758 MPa / 80,000–110,000 psi) means hardness is not controlled, which makes it susceptible to sulfide stress cracking in H₂S environments. Full N80 specification guide →

L80 — The Sour Service Standard Grade

L80 is the most important grade to understand in the OCTG family. It has the same minimum yield as N80 (552 MPa / 80,000 psi) but with a restricted maximum yield (655 MPa / 95,000 psi) and a mandatory hardness ceiling of 23 HRC (241 HBW). Quench and tempering is mandatory. These controls are what qualify L80 for sour service under NACE MR0175/ISO 15156 — the restricted strength and hardness limits reduce susceptibility to sulfide stress cracking.

L80 comes in three metallurgical variants:

• L80 Type 1: Carbon-manganese steel. The standard sour service casing grade for sweet-to-moderate-H₂S environments. Seamless and welded.

• L80 9Cr: 9% chromium steel. Improved CO₂ corrosion resistance over Type 1. Seamless only. Used in CO₂-rich environments where Type 1 would corrode excessively but full 13Cr is not yet required.

• L80 13Cr: 13% chromium stainless steel. Excellent CO₂ and mild H₂S resistance. Seamless only. The default tubing grade for many CO₂-producing oil wells. More expensive and requires specific weld procedure for connection make-up. L80-13Cr metallurgy guide →

N80 vs L80 — full grade comparison →

C90 — Sour Service Between L80 and T95

C90 fills the gap between L80 and T95 with a yield range of 621–724 MPa (90,000–105,000 psi) and a hardness limit of 25.4 HRC. It is seamless only, Q&T mandatory, and sour-rated. C90 is less commonly stocked than L80 and T95 and tends to be specified for specific well designs where the yield strength of L80 is insufficient but the full strength of T95 is not required.

T95 — Sour Service at High Strength

T95 is the highest-strength sour-rated API 5CT grade (Q125 is not sour-rated), with a yield range of 655–758 MPa (95,000–110,000 psi) and hardness limit of 25.4 HRC. It is seamless only, mandatory Q&T, and the standard upgrade path from L80 for wells where H₂S is present but design pressure exceeds what L80 can support. T95 carries a significant price premium over L80 and requires SSC and HIC testing for sour service qualification. L80 vs N80Q vs T95 sour service decision matrix →

P110 — High Strength for Deep Wells

P110 is the standard high-strength grade for deep, high-pressure wells with minimum yield of 758 MPa (110,000 psi). It is mandatory Q&T and available in seamless and welded form. P110 is not sour-rated — its wide hardness range makes it susceptible to SSC in H₂S environments. It is the grade of choice for intermediate and production casing in deep wells where collapse and burst calculations under N80/L80 cannot meet design factors, provided H₂S partial pressure remains below 0.05 psia.

For a full review of P110 economics and operational constraints: API 5CT P110 Casing Pipe — Specifications & Application Guide →

Q125 — HPHT, Maximum Strength

Q125 is the highest-strength standard API 5CT grade with a minimum yield of 862 MPa (125,000 psi). It is seamless only, mandatory Q&T, and not sour-rated. Q125 is specified almost exclusively for ultra-deep, ultra-high-pressure (HPHT) wells where P110 cannot meet collapse or burst design requirements. It requires tighter chemistry controls than other grades, and welding or heat treatment of Q125 connections requires qualified procedures. Q125 is not a stocked commodity grade — it is produced to order with longer lead times than standard grades.

4. Sour Service Grades & H₂S Limits

5. Grade Selection Decision Guide

Use this decision framework to narrow down the correct API 5CT grade for a given casing or tubing string. The four key inputs are: H₂S partial pressure, well depth/pressure, temperature, and CO₂ content.

For a detailed grade selection workflow with design examples: How to Choose OCTG Material — Complete Grade Selection Guide →

6. Connection Types by Grade

The connection type is part of the OCTG specification and must be matched to the grade and application. API 5CT defines three standard connection types for casing, plus a generic category for premium connections.

Premium connections provide metal-to-metal sealing that API threads cannot achieve, which is critical for gas-tight applications and sour wells where any leak path is unacceptable. ZC Steel Pipe holds independent patents on premium connection designs for OCTG casing and tubing.

For detailed connection guidance: BTC Casing Explained →  |  Premium Connections — Usage Conditions & Advantages →  |  OCTG Connections TCO Guide →

7. Casing & Tubing Length Ranges

API 5CT defines three standard length ranges for casing and tubing. These are range designations for the pipe body — the joint length including the coupling is slightly longer. Always confirm which range your well design calls for before ordering.

R3 is the preferred range for offshore and deep onshore wells because longer joints mean fewer connections to make up, reducing rig time and the number of potential leak points. R2 is most common in onshore applications. API 5CT casing length specifications — full guide →

8. Frequently Asked Questions