API 5CT K55 Casing Pipes

Product Introduction

API 5CT K55 is a medium-low carbon steel grade oil and gas casing/tubing specified by the American Petroleum Institute (API) 5CT standard. It has a minimum yield strength of 379 MPa (55 ksi) and a minimum tensile strength of 655 MPa. It undergoes normalizing or normalizing and tempering treatment, with stricter impurity control than J55, resulting in superior toughness and stability. Primarily used in shallow to medium-depth well surface and technical casing, it supports the wellbore, isolates pressure zones, and prevents collapse. Suitable for non-acidic, medium internal pressure, and external extrusion conditions, it commonly comes in diameters of 4½"–20". Short round, long round, or trapezoidal threads are available, offering high cost-effectiveness and wide applicability.

API 5CT K55 casing pipe Specifications

Key Mechanical Properties of API 5CT K55 Casing Pipes

K55 is a medium-strength carbon steel grade under API 5CT with a yield strength range of 379–552 MPa and a minimum tensile strength of ≥655 MPa - notably higher than J55 (≥517 MPa), making K55 a direct upgrade. Both grades share the same chemistry (P, S ≤0.030%), but K55 delivers significantly better tensile performance, allowing it to replace J55 (not vice versa). Elongation and impact toughness requirements are comparable to J55, though K55's elongation is slightly lower. Hardness is controlled per API 5CT limits. This balanced combination of strength, toughness, and cost makes K55 one of the most widely used casing grades globally for shallow to medium-depth oil and gas wells

API 5CT K55 OCTG casing Required Tests

All K55 casing must pass hydrostatic test, flattening test, tensile test, hardness test (ASTM E18/E10), and SSC test as per API 5CT. For PSL-1, these are the mandatory requirements with no impact or NDT obligations. For PSL-2, additional tests are required: Charpy impact test at 21°C (70°F) with minimum 20J (27J for full-size specimen), NDT inspection to Level L4 (via UT, MFL, ECT, or MPI) for longitudinal imperfections on both inner and outer surfaces, full-length wall thickness measurement (25% coverage minimum), and grain size determination (ASTM E112). K55 belongs to Group 1, so impact test temperature is 21°C (not 0°C like other groups). Chemical analysis (P, S ≤0.030%) is also mandatory for all PSL levels.

API 5CT K55 oil well Casing steel pipe Application

API 5CT K55 OCTG Casing Applications:

K55 casing is primarily used in shallow to medium-depth wells with moderate mechanical demands, including conventional oil and gas wells, coalbed methane wells, geothermal wells, and water wells. It is the go-to grade for non-sour (non-H₂S) environments where cost-effectiveness is a priority, offering higher tensile strength (≥655 MPa) than J55, making it suitable for wells requiring greater structural stability and collapse resistance. K55 is widely applied in onshore drilling operations (which account for 80% of global OCTG demand) and is also used in steam injection and water injection wells. However, it is not recommended for sour service or H₂S-containing environments - for those, N80Q or higher grades should be used instead.

API 5CT K55 casing pipe certifications

API 5CT K55 OCTG casing process line

FAQ

1.What is the difference between K55 and J55?
Same chemistry and yield strength, but K55 has higher tensile strength (≥655 vs ≥552 MPa) and requires mandatory heat treatment (normalizing), giving more consistent properties. K55 can replace J55, but not vice versa.

2.What is the packaging requirement?

Steel strapping + thread protectors on every connection. Threads must be smooth, free of burrs, cracks, or damage. Surface defects (folds, seams, cracks) must be removed to a depth not exceeding 12.5% of nominal wall thickness.

3.Why is K55 becoming less popular compared to L80/N80Q?

No sour service capability - as more wells encounter H₂S, K55 is being phased out in favor of N80Q/L80.

Lower yield strength (max 552 MPa) vs L80 (min 552 MPa) - L80 gives the same minimum yield but with much higher tensile and better toughness.

Cost gap is shrinking - the price difference between K55 and L80 has narrowed to ~$10–20/ton, making the upgrade economically viable.

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