API 5CT N80 - Specification for Casing and Tubing (N80 Grade)
API Specification 5CT N80 is a high-strength quenched and tempered casing/tubing grade widely used for intermediate and production strings in oil and gas wells where higher mechanical properties are required than J55/K55 can provide.
Key Characteristics
| Feature | Description |
|---|---|
| Material | Carbon-Manganese Steel (Quenched & Tempered) |
| Manufacturing | Seamless (SMLS) or Electric Resistance Welded (EW) |
| Heat Treatment | Quenched and Tempered (Mandatory) |
| Primary Use | Intermediate & production casing, tubing |
Grade Designation Meaning
"N" = Normalized or Quenched & Tempered category
"80" = Minimum yield strength in kpsi (80,000 psi = 552 MPa)
Note: There are TWO TYPES of N80:
N80-1 (Type 1): Standard grade
N80 Q (Type Q): Quenched and tempered only
Mechanical Requirements
Tensile Properties:
| Property | Requirement |
|---|---|
| Yield Strength | 552-758 MPa (80-110 ksi) |
| Tensile Strength, min | 689 MPa (100 ksi) |
| Yield/Tensile Ratio | ≤ 0.85 |
| Elongation, min | Based on formula in spec |
Hardness Limitations:
Maximum Hardness: 263 HBW or 27.5 HRC
Typical range: 220-260 HBW
NOT suitable for sour service without special processing
Chemical Requirements
API 5CT does not specify chemistry for N80, but manufacturers control composition to achieve mechanical properties. Typical ranges:
| Element | Typical Range (%) | Purpose |
|---|---|---|
| Carbon (C) | 0.20 - 0.40 | Base strength |
| Manganese (Mn) | 1.00 - 1.60 | Hardenability, strength |
| Phosphorus (P), max | 0.025 | Toughness |
| Sulfur (S), max | 0.025 | Weldability |
| Silicon (Si) | 0.15 - 0.35 | Deoxidizer |
| Key Point: | Chemistry optimized for quench & temper response |
Manufacturing & Processing
Production Sequence:
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Steel Making → Pipe Forming (SMLS/EW) → Quenching (water/oil) → Tempering (600-700°C) → Sizing/Straightening → Testing
Heat Treatment Details:
Quenching: Rapid cooling from austenitizing temperature
Tempering: Reheating to 600-700°C to achieve ductility
Microstructure: Tempered martensite or bainite
Result: High strength with reasonable toughness
N80 Types:
| Type | Heat Treatment | Characteristics |
|---|---|---|
| N80-1 | Normalized or Q&T | General purpose |
| N80 Q | Quenched & Tempered only | Better properties |
| Difference: | N80 Q has tighter yield range (80-95 ksi) |
Testing Requirements
| Test | Requirement | Frequency |
|---|---|---|
| Tensile Test | Mandatory | 2 per heat treat lot |
| Hardness Test | Optional | Per lot |
| Hydrostatic Test | Every joint | 100% |
| Impact Test | Optional | For low temperature |
| Flattening Test | Welded only | Per lot |
| SSC Test | If specified | Special order |
Hydrostatic Test Pressure:
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P = (2 × 0.80 × Ymin × t) / D Where Ymin = 80,000 psi for N80 Minimum duration: 5 seconds
Primary Applications
1. Oil & Gas Wells:
Production Casing: Most common application
Intermediate Casing: Through high-pressure zones
Tubing: For high-rate producers
Liner Pipe: Both drilling and production liners
2. Typical Well Configurations:
| Casing String | Typical N80 Application |
|---|---|
| Surface | Rare (overkill, expensive) |
| Intermediate | Through pressure transitions |
| Production | Primary application |
| Liner | Both drilling and production |
3. Depth & Pressure Capability:
| Parameter | Capability |
|---|---|
| Depth | Up to 15,000 ft (4,500 m) |
| Internal Pressure | Higher than J55/K55 |
| Collapse Resistance | Better due to higher strength |
| Tension Capacity | Supports longer/heavier strings |
Comparison with Other Grades
Strength Comparison:
| Grade | Yield Strength (ksi) | Strength vs N80 |
|---|---|---|
| J55 | 55-80 | 31% weaker |
| K55 | 55-80 | 31% weaker |
| N80 | 80-110 | Baseline |
| L80 | 80-95 | Similar but sour service |
| P110 | 110-140 | 38% stronger |
| Q125 | 125-150 | 56% stronger |
When to Select N80:
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Required Yield > 80 ksi? → Yes → Consider N80 Sour Service? → Yes → Use L80, not N80 Cost Sensitive? → Yes → Consider if J55/K55 sufficient Deep Well? → Yes → N80 good for intermediate depths
Design Considerations
1. Connection Selection:
| Connection Type | N80 Application |
|---|---|
| API Round Thread | Moderate depth/simpler wells |
| API Buttress | Better for compression/tension |
| Premium Connections | Recommended for critical wells |
| Integral Connections | No coupling, better clearance |
2. Pressure Ratings (Example: 5½" 17 lb/ft):
| Rating | Value | Advantage vs J55 |
|---|---|---|
| Internal Yield | 7,600 psi | ~60% higher |
| Collapse Pressure | 8,500 psi | ~70% higher |
| Joint Strength | 464,000 lb | ~50% higher |
| Biaxial Performance | Better | More design flexibility |
3. Triaxial Design Factors:
Higher allowable stresses than J55/K55
Better collapse under tension (biaxial effects)
More design margin for unexpected loads
Sour Service Limitations
⚠️ Critical Warning:
Standard N80 is NOT SUITABLE for sour (H₂S) service!
Reasons:
Hardness typically > 22 HRC (exceeds NACE limits)
No chemistry control for SSC resistance
Quenched & tempered microstructure susceptible to SSC
Alternatives for Sour Service:
L80: Same strength range, hardness controlled
C90: Higher strength with SSC resistance
C95/T95: Enhanced SSC resistance
If N80 Must Be Used in Sour Service:
Special heat treatment to achieve ≤ 22 HRC
Chemistry control (low sulfur, calcium treatment)
Extensive SSC testing qualification
Considered "special N80" not standard
Failure Modes & Prevention
1. Sulfide Stress Cracking:
Cause: H₂S + hardness > 22 HRC
Prevention: Avoid in sour service, use L80 instead
Monitoring: Regular inspection in borderline cases
2. Stress Corrosion Cracking:
Cause: Chlorides + stress + temperature
Prevention: Control environment, coatings
Critical in: High-temperature wells
3. Connection Failures:
Cause: Over-torque, improper make-up
Prevention: Torque monitoring, thread inspection
For N80: Higher strength allows higher make-up torque
4. Corrosion Fatigue:
Cause: Cyclic loading in corrosive environment
Prevention: Corrosion inhibition, load management
N80 advantage: Higher fatigue strength than lower grades
Economic Considerations
Cost Factors:
| Factor | Impact |
|---|---|
| Material Cost vs J55 | 50-70% higher |
| Heat Treatment Cost | Additional 15-20% |
| Connection Cost | Similar to other grades |
| Transportation | Same as other casing |
| Running Cost | Similar to other grades |
Cost-Benefit Analysis:
text
Cost Premium of N80 vs J55: ~60% Strength Increase: ~45% Result: Better strength-to-cost ratio than premium grades Typical Payback: Reduced wall thickness, longer service life
Life Cycle Costs:
Higher initial investment
Lower replacement frequency
Better performance in demanding wells
Reduced workover costs
International Equivalents
| Region | Equivalent Standard | Notes |
|---|---|---|
| ISO | ISO 11960 N80 | Identical |
| Russia | GOST 632 N80 | Similar |
| China | SY/T 6194 N80 | API adoption |
| Europe | EN 10208-2 L360NB | Similar strength |
Ordering Information
Essential Specifications:
Specification: API 5CT
Grade: N80 or N80-1 or N80 Q
Type: Casing or Tubing
Size: OD and Weight (e.g., 7" 26 lb/ft)
Length: Range 1, 2, or 3
Thread: Specify API or premium
End Finish: Plain, thread protectors
Supplementary: Special testing if required
Sample Purchase Order:
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API 5CT N80 Casing, 5½" OD, 17 lb/ft, Range 3, Buttress Thread Connection (BTC), Quenched & Tempered (N80 Q type), Full traceability MTC with heat numbers, For sweet service only (non-sour).
Quality Assurance
Required Documentation:
Mill Test Certificate with mechanical properties
Heat treatment records (time, temperature, cooling rate)
Hydrostatic test records
Hardness test reports (if specified)
Traceability to heat and heat treat lot
Inspection Requirements:
Visual inspection: 100%
Dimensional checks: Critical dimensions
Thread inspection: API gauges
NDE: Optional but recommended for critical wells
Marking Requirements:
Each joint marked with:
API 5CT N80
Heat number
Heat treat lot number
Weight designation
Manufacturer ID
Country of origin
Industry Best Practices
For Optimal Performance:
Specify N80 Q for more consistent properties
Use premium connections for critical wells
Consider internal coatings for corrosion protection
Verify hardness if near sour service conditions
Review manufacturer's quality system
Design Recommendations:
Utilize higher strength to reduce wall thickness
Consider collapse under tension in design
Account for temperature effects on strength
Design for combined loads (triaxial analysis)
Common Specifications
Popular Sizes & Weights:
| OD (inch) | Common Weights (lb/ft) | Typical Application |
|---|---|---|
| 4½" | 11.6, 12.6, 13.5 | Tubing, liner |
| 5½" | 15.5, 17.0, 20.0 | Production casing |
| 7" | 23.0, 26.0, 29.0 | Production casing |
| 9⅝" | 40.0, 43.5, 47.0 | Intermediate casing |
| 13⅜" | 54.5, 61.0, 68.0 | Intermediate casing |
Bottom Line: API 5CT N80 is the workhorse high-strength casing/tubing grade for demanding oil and gas wells, providing 45-70% higher strength than J55/K55 at a reasonable cost premium. Its quenched and tempered microstructure delivers excellent mechanical properties for intermediate and production strings in sweet (non-sour) service, making it one of the most widely used grades worldwide for medium to deep wells.
