API 5CT K55 is a medium-strength carbon steel casing grade with a yield strength range of 379 to 552 MPa (55,000 to 80,000 psi) and a minimum tensile strength of 655 MPa (95,000 psi). The standard fixes only the phosphorus and sulphur limits and leaves the rest of the chemistry to the manufacturer, so K55 is bought on its mechanical properties and its dimensions rather than on a fixed composition.
Download API 5CT K55 Casing data sheet (PDF) One-page summary of the supply range, chemistry, mechanical properties and inspection documents on this page. The standard is the governing document.
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Size range and specification
- Outside diameter 4-1/2 inch – 20 inch (114.3 – 508 mm)
- Most quoted sizes 4-1/2, 5, 5-1/2, 7, 9-5/8, 10-3/4 and 13-3/8 inch
- Wall thickness 5.21 – 22.22 mm, or by weight designation
- Length R1 4.88 – 7.62 m, R2 7.62 – 10.36 m, R3 10.36 – 14.63 m
- Ends STC, LTC, BTC, or premium connection
- Surface treatment Bare, varnish, phosphate, or as specified
- Manufacturing process Seamless, ERW, or welded
- Delivery standard API 5CT / ISO 11960, threads to API 5B
- Delivery condition Not fixed by the grade; the steelmaking route is the manufacturer’s choice provided the mechanical requirements are met
How this grade is supplied
- Inspection Inspected by the producing mill’s own inspection department, which is independent of production, against the delivery standard above. Third-party inspection — SGS, BV or TUV, or an inspector you send yourself — can be arranged at the mill or before shipment; give us the dates and the scope at the order stage.
- Certificate A mill test certificate to EN 10204 3.1 showing the heat number, the chemical composition and the mechanical results is standard on every order. EN 10204 3.2, countersigned by the inspector you nominate, is available on request.
- Documents with the goods The mill test certificate and the packing list travel with the shipment, and the heat number is traceable from cast through to the finished product.
- Packing Sea-worthy export packing: bundled by size with end protection, corrosion protection on request, stencilled with the size, grade and heat number.
- MOQ One bundle or one truckload; mixed sizes within a bundle accepted.
- Lead time Counted from order confirmation, not from the enquiry. Stock sizes 5–10 days; mill run or non-stock 3–5 weeks. The delivery window is fixed on the quotation.
- How we supply Sourced through partner mills; the producing mill is named on the quotation.
API 5CT K55 chemical composition (%)
| Element | Requirement | Element | Requirement |
|---|---|---|---|
| Phosphorus, P | 0.030 max | Carbon, C | not specified |
| Sulphur, S | 0.030 max | Manganese, Mn | not specified |
| Silicon, Si | not specified | Chromium, Cr | not specified |
| Nickel, Ni | not specified | Copper, Cu | not specified |
K55 is one of the API 5CT grades whose chemistry is left open. The standard fixes the two elements that damage toughness and weldability – phosphorus and sulphur, each at 0.030 percent maximum – and leaves carbon, manganese and the alloying elements to the manufacturer, who is free to reach the required yield and tensile strength by whatever steelmaking route is economical. Two consequences follow for a buyer: a mill certificate for K55 will not look the same from one mill to another, and the grade should be specified by its mechanical properties and dimensions rather than by an expected chemistry. Published tables quoting a carbon or manganese range for K55 are describing a particular mill’s practice, not a requirement of the standard.
Source: chemical composition requirements of API 5CT / ISO 11960. The standard is the governing document.
API 5CT K55 mechanical properties
| Grade | Yield strength | Tensile strength | Elongation | Hardness |
|---|---|---|---|---|
| K55 (API 5CT) | 379 – 552 MPa (55,000 – 80,000 psi) | 655 MPa min (95,000 psi) | per the API 5CT formula | not specified at delivery |
K55 has a yield strength range rather than a minimum only, so an over-strength heat is as much a deviation as an under-strength one. Elongation is calculated from the specimen cross-sectional area using the formula in API 5CT rather than being tabulated as a single figure. The standard sets no hardness limit for the grade as delivered; where the casing is to be used in sour service, NACE MR0175 / ISO 15156 limits the material to 22 HRC (250 HV), and that requirement is normally carried on the order.
Source: tensile requirements of API 5CT / ISO 11960. The standard is the governing document.
What Is API 5CT K55 Casing?
API 5CT K55 is a carbon steel casing grade for oil and gas wells, with a yield strength between 379 and 552 MPa and a minimum tensile strength of 655 MPa. It is a medium-strength grade: above the H40 and J55 grades used for the shallowest and lightest strings, and below the N80, L80 and P110 grades that deeper wells require. K55 is the grade that shows what an API 5CT grade name means – the number is the nominal minimum yield strength in thousands of psi, and the letter describes the strength class the grade belongs to.
K55 versus J55
K55 and J55 share the same yield strength range, 379 to 552 MPa, and the same chemistry requirements, which is to say only the phosphorus and sulphur limits. They are told apart by tensile strength: J55 requires a minimum of 517 MPa and K55 requires 655 MPa. The higher tensile requirement gives K55 more margin in tensile loading and in collapse, so it is chosen where the string load or the setting depth is beyond what J55 would carry with an economic wall thickness. Because the two grades overlap in yield, the tensile requirement is the number to check on the certificate.
Where K55 Casing Is Used
K55 is specified for surface and intermediate casing strings in shallow to medium-depth wells, for production casing in low-pressure wells, and for water, observation and geothermal wells where the loads are moderate and the higher-strength grades would be unnecessary cost. It is also used for conductor and structural pipe in some wells. The decision between K55 and a higher grade is made from the collapse, burst and tensile load cases for each string: the design normally takes the lightest and cheapest grade that passes all three.
Connections, Couplings and Accessories
K55 casing is supplied with short round thread (STC), long round thread (LTC) or buttress thread (BTC) couplings as standard, with premium connections available where gas-tightness or a higher torque capacity is needed. The coupling is a separate API 5CT item and must match the grade and the thread form of the pipe, and couplings in a higher grade than the pipe are used where the connection is the limiting item. Accessories ordered with the string – couplings, thread protectors, centralizers, pup joints and crossover subs – are part of the same delivery and should be specified against the same standard.
Ordering and Documentation
Because the grade leaves chemistry open, a K55 order should be written on the dimensions, the weight designation, the thread form, the length range and the mechanical requirements, plus any additional requirement the well needs, such as a hardness limit for sour service or a specific impact test. The mill certificate for K55 should be checked for the yield and tensile results against the range and the minimum, for the phosphorus and sulphur values against 0.030 each, and for the heat number traceability. Where the casing is destined for a sour well, the NACE MR0175 / ISO 15156 hardness limit belongs on the purchase order rather than being assumed.
PSL1 or PSL2 for K55
| Requirement | PSL1 | PSL2 |
|---|---|---|
| Chemical composition | Only P and S at 0.030% max | Only P and S at 0.030% max, with PSL2 traceability and process controls |
| Mechanical properties | Yield 379 – 552 MPa; tensile 655 MPa min | Same mechanical limits |
| Impact test | Not required unless agreed | Not required by the grade itself unless supplementary requirements are invoked |
| NDT and traceability | Basic product traceability | Additional inspection and traceability under PSL2 rules |
Source: API 5CT PSL1 and PSL2 requirements. The standard is the governing document.
Order fields for K55 casing
- Grade and PSL API 5CT K55, PSL1 or PSL2
- Size and weight Outside diameter, wall thickness or weight designation
- Length range R1, R2, R3 or fixed length
- Thread STC, LTC, BTC, or specify coupling and drift
- Tests Impact temperature and energy, hardness, sour-service limits if required
- Documents EN 10204 3.1 or 3.2, plus project inspection plan
Typical applications
- Surface casing strings
- Intermediate casing in shallow to medium-depth wells
- Production casing in low-pressure wells
- Water, observation and geothermal wells
- Conductor and structural casing
Standards covered
Frequently asked questions
What is the difference between J55 and K55 casing?
They have the same yield strength range, 379 to 552 MPa, and the same chemistry limits – only phosphorus and sulphur at 0.030 maximum. The difference is minimum tensile strength: J55 requires 517 MPa and K55 requires 655 MPa. K55 therefore carries more tensile capacity and more collapse resistance for the same wall, and it is chosen where the string load or depth puts the J55 option too close to its limit.
What chemistry does API 5CT require for K55?
Only phosphorus and sulphur, each at 0.030 percent maximum. Carbon, manganese and the alloying elements are not specified for K55, so the manufacturer is free to reach the required yield and tensile strength by any economical route. The practical consequence is that the chemistry on a K55 mill certificate varies from mill to mill, and that the grade is bought on mechanical properties and dimensions rather than on composition.
Can K55 casing be used in sour service?
It can, but only where the delivered material meets the hardness limit that NACE MR0175 / ISO 15156 sets for carbon steels, which is 22 HRC (250 HV). API 5CT does not itself set a hardness limit for K55, so the requirement has to be written into the purchase order and verified on the certificate rather than assumed. Where the well is sour, the hardness limit and the accompanying test results are part of what is being bought.
Where to go next
- OCTG: Casing and Tubing – which string, grade, connection and documents the well needs
- API 5CT Casing Pipe – API 5CT well casing
- API 5CT N80Q Casing – quenched and tempered well casing
- API 5CT Casing Coupling – casing couplings to API 5CT
- Premium Connection Casing and Tubing – gas-tight premium connections
- Oil & Gas – the four products in the oil and gas chain
- Sour Service – what changes when H2S is present
- Steel Pipe Standards & Grades – every grade page in this section
- Get a Quote – send the grade, size, quantity and destination port
- API 5CT Tubing Pipe – API 5CT production tubing
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