API 5CT L80 is a very common grade of steel casing and tubing used in the oil and gas industry. It is defined by the American Petroleum Institute (API) Specification 5CT.
The key characteristic of L80 is its balanced combination of strength and resistance to Sulfide Stress Cracking (SSC), making it suitable for moderately sour environments (environments containing Hydrogen Sulfide, H₂S).
Key Features of L80:
Minimum Yield Strength: 80,000 psi (80 ksi) - The "80" in the name.
SSC Resistance: It belongs to API 5CT's Group 2, which comprises grades with mandatory hardness limits and SSC resistance requirements. The maximum allowable hardness is 23 HRC.
Types: L80 is available in different metallurgical types to suit specific corrosive environments:
L80 Type 1: Standard chromium-molybdenum steel.
L80 9Cr: Contains about 9% Chromium for improved corrosion resistance.
L80 13Cr: A martensitic stainless steel offering excellent resistance to CO₂ corrosion.
Product Parameters & Technical Data Tables
The following tables summarize the key mechanical and chemical properties as per API 5CT.
Table 1: Mechanical Properties of API 5CT L80
| Property | Value | Notes | |
|---|---|---|---|
| Grade | L80 | - | |
| Specification | API 5CT | - | |
| Yield Strength (min.) | 80,000 psi | 552 MPa | |
| Yield Strength (max.) | 95,000 psi | 655 MPa | |
| Tensile Strength (min.) | 95,000 psi | 655 MPa | The minimum tensile strength requirement is specific to certain wall thicknesses. |
| Hardness (max.) | 23 HRC | Critical for SSC resistance. Rockwell C Scale. | |
| Elongation | Varies with wall thickness | As per API 5CT formula. |
Table 2: Chemical Composition Requirements (Weight % Maximum, unless range is indicated)
This table shows the base requirements. Note that 9Cr and 13Cr have significantly different chemistries.
| Element | L80 Type 1 | L80 9Cr | L80 13Cr | Notes |
|---|---|---|---|---|
| Carbon (C) | 0.40% max | 0.15% max | 0.15% max | Lower carbon improves weldability and toughness. |
| Manganese (Mn) | 1.90% max | 0.40-1.00% | 0.25-1.00% | - |
| Phosphorus (P) | 0.020% max | 0.020% max | 0.020% max | Impurity, kept low. |
| Sulfur (S) | 0.010% max | 0.010% max | 0.010% max | Impurity, kept very low for SSC resistance. |
| Silicon (Si) | 0.45% max | 0.50% max | 0.50% max | - |
| Chromium (Cr) | - | 8.00-10.00% | 12.00-14.00% | Key element for corrosion resistance. |
| Molybdenum (Mo) | - | 0.90-1.10% | - | Enhances strength and corrosion resistance. |
| Nickel (Ni) | 0.25% max* | 0.50% max | 0.50% max | *Max is 0.25% for SSC resistance. |
Table 3: Typical Dimensional Data (Casing)
API 5CT L80 is manufactured in a wide range of sizes. The table below shows common examples.
| Nominal Size (O.D.) | Weight (lb/ft) | Wall Thickness (in) | Drift Diameter (in) | Internal Yield Pressure (psi) *Approx. |
|---|---|---|---|---|
| 4-1/2" | 11.60 | 0.271 | 3.965 | 7,260 |
| 5-1/2" | 17.00 | 0.304 | 4.857 | 6,340 |
| 7" | 23.00 | 0.317 | 6.366 | 5,480 |
| 9-5/8" | 40.00 | 0.395 | 8.835 | 5,220 |
| 10-3/4" | 40.50 | 0.350 | 9.978 | 4,080 |
Note: Internal Yield Pressure is a calculated value based on minimum yield strength and can vary. Always refer to the manufacturer's spec sheet for precise values.
Common Applications
Production Casing/Tubing: In oil and gas wells with moderate levels of H₂S and/or CO₂.
Intermediate Casing: Across zones that may contain sour gas.
Tubing: As the conduit for production fluids where corrosion resistance is needed beyond what carbon steel (like J55, N80-1) offers.
Important Distinctions
L80 vs. N80: While both have the same minimum yield strength (80 ksi), N80 is a Group 1 grade with no mandatory SSC resistance requirements. It is generally cheaper but should not be used in sour service. L80 is the specified choice for such environments.
L80 13Cr vs. Standard L80: L80 13Cr is essentially a stainless steel and is selected specifically for wells with high partial pressures of CO₂ to combat sweet corrosion.
Conclusion
API 5CT L80 is a versatile, mid-range grade that provides an essential balance for engineers designing wells where corrosion is a concern. Its standardized properties ensure reliability and interchangeability from different manufacturers worldwide. For critical applications, always consult the full API 5CT specification and the manufacturer's test certificates (Mill Test Certificates - MTC).
