API 5CT: Grades, Connections and What a Shop Must Check

API 5CT is the American Petroleum Institute specification for casing and tubing. It sets the steel grades, dimensions, tolerances, testing and marking a joint must meet before it goes downhole. It also defines the API connections — round thread and buttress for casing, non-upset and external-upset for tubing — and the product specification levels that…

API 5CT is the American Petroleum Institute specification for casing and tubing. It sets the steel grades, dimensions, tolerances, testing and marking a joint must meet before it goes downhole. It also defines the API connections — round thread and buttress for casing, non-upset and external-upset for tubing — and the product specification levels that decide how much testing each joint receives.

At a glance

Question	Short answer
What does it cover?	Casing, tubing, pup joints and couplings for wells
What does it not cover?	Line pipe. That is API 5L, a different product for a different job
Grades	H40 through Q125, arranged in four groups by strength and service
Connections	Round and buttress for casing, NU and EU for tubing. Thread form and gauging sit in API 5B
Testing levels	PSL-1, PSL-2 and PSL-3, in rising order of required testing
Where shops meet it	Receiving inspection, re-threading, and every make-up record

What API 5CT actually governs

The specification is written around one question: will this joint survive the well it is going into?

It fixes the chemistry and mechanical properties of the steel, the outside diameter and wall thickness with their tolerances, the heat treatment condition, the non-destructive examination required, the hydrostatic test, and the marking that lets a joint be traced back to the heat it was rolled from. A joint that meets all of it can carry the API monogram. A joint that meets most of it cannot.

For a workshop the practical consequence is narrower. You will rarely audit a mill. You will constantly handle the paperwork the mill produced, and you will be the last person to touch the connection before it is run. Reading how casing, tubing and drill pipe differ is the starting point; API 5CT is what makes two of those three a controlled product.

The grades, and what the letters mean

A grade is a letter and a number. The number is the specified minimum yield strength in thousands of psi.

The letter distinguishes grades that share a yield number. Sometimes that is the service condition and heat treatment — L80 against N80, both 80 ksi minimum yield. Sometimes it is only tensile strength: J55 and K55 have the same minimum yield, but K55 requires 95 ksi minimum tensile against J55’s 75 ksi. The number alone never identifies the grade.

Group	Grades	What the group is for
Group 1	H40, J55, K55, N80, R95	General service. The bulk of ordinary casing and tubing
Group 2	M65, L80, C90, T95, C110	Restricted yield and sour service. Hardness is controlled, not just strength
Group 3	P110	High strength, deeper and higher pressure wells
Group 4	Q125	Highest strength in the specification

Two details in that table cause most of the confusion in a threading shop.

The first is that Group 2 is not simply “stronger than Group 1”. It is controlled. Every Group 2 grade carries a maximum hardness limit as well as a minimum yield, and the standard controls hardness variation through the wall as well as the peak value, because hardness is what drives sulfide stress cracking in sour service.

The second is that L80 is not one material. L80 Type 1 is a carbon steel; L80 9Cr and L80 13Cr are chromium alloys with very different corrosion behaviour, different cost, and different handling requirements on the bench. “L80” on a tally is not a complete description.

How the API 5CT size tables are structured

Most of the day-to-day use of the specification is reading its dimensional tables, and they trip people up because a single outside diameter is not one product.

Column	What it is	Why it matters on an order
Label 1	The size designation — effectively the outside diameter	The number everyone quotes, and the least complete on its own
Label 2	The weight designation, nominal weight per foot	Separates the wall thicknesses available at that OD
Wall thickness	The actual wall for that weight designation	Drives pressure rating and what the jaws are clamping
Grade	H40 to Q125	Sets strength, hardness control and handling
Connection	The end finish available for that size and grade	Not every connection exists at every size and grade
Drift diameter	The guaranteed minimum bore, checked with a drift mandrel	Whether the tools the well needs will pass through

The practical rule that follows is simple: one OD carries several weight, wall, grade and connection combinations, so an order naming only the size is not an order. Name all four, and state the drift requirement if the well has one. The current tables come from API directly rather than from a reseller’s summary.

Connections: what 5CT specifies and what it does not

API 5CT covers connections for both products. For casing: short round thread (STC), long round thread (LTC), buttress (BTC), and extreme-line (XC), which is integral rather than coupled. For tubing: non-upset (NU) and external-upset (EU), plus integral-joint tubing in the small sizes. It references API 5B for the thread form, dimensions and gauging practice.

Factor	Effect to consider	What the team should use for a decision
Thread form	Round thread seals differently from buttress; buttress carries more tension	The well design, not shop preference. Confirm against the running programme
Gauging	Thread inspection is a separate discipline governed by API 5B	Whether your shop holds calibrated gauges and someone trained to read them
Premium connections	The connection is outside 5CT; the pipe body usually is not. Grade, chemistry and body dimensions still follow 5CT, with a licensed thread cut on it	The licensor’s own procedure for the thread. Never assume an API rule applies to it
Make-up values	API connections have published torque guidance; premium connections do not follow it	The published figure for that exact connection, recorded per joint

A premium connection is a licensed product with its own make-up procedure, its own torque-turn signature and its own acceptance criteria. Applying an API assumption to it is how connections get galled. The difference between buttress and the round threads is the right place to start if the connection family on the tally is unfamiliar.

PSL levels, and why they appear on the order

Product specification levels set how much testing a joint receives. PSL-1 is the base requirement. PSL-2 and PSL-3 add progressively more non-destructive examination, more traceability and tighter documentation.

The level is normally a purchasing decision made long before the pipe reaches a workshop, and mills often stencil PSL 2 or PSL 3 in the body marking — but the stencil is not the record. The certificate is, and that is what to check against the order. If a customer specifies PSL-2 and the certificates say PSL-1, that is a supply problem, not an inspection finding, and it needs raising before anything is threaded or made up.

When API 5CT is the right specification

• The pipe is going into a well as casing or tubing.
• The connection is an API form and the well design does not demand a gas-tight premium seal.
• The service is not severely sour, or the grade selected already carries the restricted-yield control.
• The buyer wants a monogrammed, traceable product with a recognised certificate.

When another specification may be the better fit

Situation	Why 5CT may not be enough	Where to look instead
Pipeline or flowline	5CT is for downhole product	API 5L, the line pipe specification
Gas-tight sealing required	API connections are not designed as gas-tight seals	A licensed premium connection and its own procedure
Severe sour service	Grade alone does not cover materials qualification	NACE MR0175 / ISO 15156 alongside the grade
Drill string	Drill pipe is a different product with different rules	API 5DP, and the inspection class system
Corrosion-resistant alloys	CRA handling and jaw contact needs specific control	The material supplier’s handling requirement

Where the specification meets the bench

Most of API 5CT is a mill’s problem. Three parts of it become a workshop’s problem the moment the pipe arrives.

Factor	Effect to consider	What the team should use for a decision
Grade hardness limits	Restricted-yield grades tolerate less surface disturbance than plain carbon grades	The grade on the certificate, before selecting jaw or die contact
Marking and traceability	Stencil and die-stamp carry the heat number. Lose it and the joint loses its identity	Whether the heat number was transcribed into the job record at receiving, before slings, jaws or thread protectors touch that end
Dimensional tolerance	OD and ovality are within tolerance at the mill, not necessarily after transport	A measured check at receiving, not the certificate alone

A joint can leave the mill fully compliant and arrive out of round after a long sea freight and a rough offload. Measuring an out-of-round end is a receiving task, and it is cheaper than discovering it during make-up.

Make-up, and the part the standard leaves to you

API 5CT tells you what the joint is. It does not tell you that your make-up was good.

A connection can be made up to a correct final torque and still be wrong, because a final number says nothing about how the torque was reached. A connection that climbed early because the threads were picking up and one that rose cleanly to its shoulder can stop at the same figure. What separates them is the shape of torque against turns, which is why the values belong in a documented make-up torque specification and the result belongs in a record per joint.

For a shop threading or making up API product at volume, that record is what a customer audits. Torque-turn recording, a curve the operator can see while it happens, and an exportable report against the joint identity are what turn a claim into evidence. That capability is what a workshop torque bench with recorded make-up exists to provide.

What to send when you ask for a quotation

If you are specifying equipment to thread or make up this product, these are the fields that decide the answer:

• OD range and wall thickness across everything on the rack
• Grades handled, with the L80 type stated explicitly if L80 is on the list
• Connection family — API form, or the premium licensor by name
• Make-up and break-out torque required at the top of that range
• Surface and jaw type, and any CRA or coated product in the mix
• Daily workload — joints per shift, not joints per year
• Report format your customer accepts
• Workshop layout and the power available at the bench

Send those eight fields to our engineering team and you will get a specification back rather than a price list.

Frequently asked questions about API 5CT

What is API 5CT?

The API specification for casing and tubing. Pipe made under it can carry the API monogram, which is a licence held by the manufacturer and issued by API — so a buyer confirms compliance from the certificate and the licence, not from the pipe body alone.

What is the difference between API 5CT and API 5L?

5CT is downhole product — casing and tubing that must survive well pressure, tension and the environment inside a well. 5L is line pipe for transporting fluids at surface. Different duty, different grades, different tests. They are not interchangeable.

What grades does API 5CT cover?

H40 through Q125, in four groups. Group 1 is general service, Group 2 carries restricted yield and hardness control for sour conditions, Group 3 is P110 and Group 4 is Q125. The number is minimum yield in thousands of psi.

What connections does API 5CT cover?

For casing, short round thread, long round thread, buttress, and extreme-line, which is integral rather than coupled. For tubing, non-upset and external-upset, plus integral-joint in the small sizes. Thread form, dimensions and the gauging practice in API 5B sit in that specification.

What do PSL-1, PSL-2 and PSL-3 mean?

Product specification levels — how much testing and documentation a joint receives, rising from PSL-1 to PSL-3. The level is set at purchase and confirmed in the certificates, whatever the body stencil says.

Are premium connections covered by API 5CT?

The connection is not. The pipe body normally still is — 5CT grade and 5CT body requirements, with a licensed thread cut on it. Premium connections are proprietary designs with their own make-up procedures and acceptance criteria, and API torque guidance does not apply to them.

Is L80 a single material?

No. The type is stated on the mill certificate and the purchase order, not on the pipe stencil, so an order or tally naming only “L80” needs the type confirmed before anyone treats the material as plain carbon steel and picks jaw contact to match.

Which edition of API 5CT applies?

Editions are revised periodically, so the applicable one is whichever the purchase order names. Confirm the edition against the order and the certificate rather than assuming the newest, and obtain the current text from API directly.

Closing checklist

• Name size, weight, wall, grade and connection on the order — the OD alone is not an order
• State the drift requirement if the well has one
• Confirm the grade and the type — “L80” alone does not identify the material
• Check the PSL level on the certificate against what the order specified, not the body stencil
• Treat every Group 2 grade as hardness-controlled, not simply stronger
• Record the heat number at receiving, before handling can damage the marking
• Measure OD and ovality at receiving; the certificate describes the joint as it left the mill
• Confirm whether the connection is an API form or a licensed premium design before choosing any make-up value
• Keep a per-joint make-up record, because the final torque figure alone does not show how it got there