API 5CT Steel Grades of Oil Casing Pup Joints

Oil casing pup joints are indispensable connectors in oil and gas well completion, mainly used for supplementary connections between downhole casing pipe or between casing pipe and tools. Its performance directly affects wellbore integrity and operational safety. According to the API 5CT standard, the commonly used materials for casing pup joints are J55, K55, N80, L80, C90, T95, P110 and other grades of steel.

API 5CT casing pup joint steel grade overview

Note: The above values are based on the typical requirements of API 5CT, and there are slight differences between different manufacturers or standard versions.

Note: Anti-sulfur performance rating is based on NACE TM0177 standard (saturated H₂S solution test).

Detailed characteristics of each material

1. J55, K55 - entry-level carbon steel

Chemical composition: C content is 0.27-0.33%, Mn 0.30-0.60%, P and S are both less than 0.035%

Mechanical properties: Yield strength is 379 MPa, tensile strength is 420-600 MPa, and impact toughness is average.

Features and applications: Low price, easy processing, mainly used for shallow wells or temporary casing pipe; not suitable for high pressure and high corrosion environment. The short-cut casing pipe of this material can meet basic needs, and it can be matched with common low-pressure well seamless pipe and other pipes to achieve partial connection functions of downhole pipes.

2. N80 - intermediate alloy reinforced steel

Chemical composition: C 0.33-0.39%, Mn 0.60-1.20%, trace Cr and Mo enhance strength and toughness.

Mechanical properties: Yield strength is 552 MPa, tensile strength is 620–780 MPa, and hardness is controllable.

Features and applications: Balanced strength and toughness, commonly used in medium-deep well sections; has a certain resistance to CO₂ or H₂S corrosion. In medium-pressure well sections, N80 casing pup joints can form a complete downhole tubing system with ordinary carbon steel pipe.

3. L80 — Corrosion-resistant alloy steel

Chemical composition: Add 1.5–2.5% Cr or add Mo to N80.

Mechanical properties: Similar to N80, but superior in high-temperature corrosion performance.

Features and applications: Specially used in corrosive formations containing H₂S and CO₂; high safety factor for oil fields.

4. C90, T95 — High-strength alloy steel

Chemical composition: C90 contains C~0.40–0.45%, Cr/Mo alloy about 1–2%; T95 fine-tunes heat treatment on this basis.

Mechanical properties: C90 yield strength is 620 MPa, T95 yield strength is 655 MPa; both have good plasticity and toughness.

Features and applications: Suitable for high-temperature and high-pressure wells, good creep resistance; the wall thickness of the pup joint can be made thin to reduce weight. There are similar requirements in precision tube manufacturing.

5. P110 — Ultra-high-strength steel

Chemical composition: C 0.40–0.50%, Mn 0.60–1.20%, Cr, Mo, V micro-alloyed.

Mechanical properties: Yield ~758 MPa, tensile ~830–960 MPa, hardness can reach about HB 330.

Features and applications: Used in ultra-deep wells or extreme high-pressure environments; difficult to process and the highest cost.

Selection recommendations

1. Well depth and pressure

Shallow wells (<3000m): J55/K55

Medium-deep wells (3000~5000m): N80/L80

Ultra-deep wells (>5000m): C90/T95/P110

2. Corrosive environment

H₂S content>50ppm: mandatory use of L80/T95

CO₂ partial pressure>0.2MPa: 13Cr or duplex stainless steel is required

3. Economic efficiency

Conventional development wells: N80 is preferred

High-sulfur gas fields: L80-13Cr doubles the cost, but can reduce the number of well repairs

4. Special working conditions

Salt rock layer creep: select T95 (anti-external extrusion pressure ≥80 MPa)

Shale gas fracturing: P110 with thick wall design (such as TP140V, higher steel grades have been gradually replaced)

Summary

The safety and reliability of oil casing pup joints are inseparable from the precise grasp of material properties. From J55 to P110, the higher the steel grade, the thinner the wall thickness can be, the higher the yield strength and tensile strength, but the cost and processing difficulty also increase simultaneously. In practical applications, flange supplier and valve supplier should also be asked to provide matching connector materials according to system requirements to ensure the safe operation of the entire oil and gas system.

Read more: What Is The Minimum Length of Welded Pipe Pup Joints?