The grade you choose for an API 5CT string is a corrosion decision before it is a strength decision. N80 fails in sour service not because it is weak, but because it has no hardness restriction; L80, C90 and T95 exist precisely to stay under the 22 HRC ceiling of NACE MR0175 / ISO 15156.
This article walks through the grade families from K55 to Q125, the three design loads (burst, collapse, tensile) and why the connection — not the pipe body — limits deep strings, plus the PSL levels and paperwork that separate certified pipe from claims.
The string went in clean. Eighteen months later it came back in pieces.
No manufacturing defect. The mill certificate was perfect, the pipe was exactly what the purchase order said. The problem was the purchase order itself: N80 casing, run into a well with 3% H₂S. Nothing about that pipe was wrong — it was simply never designed to survive the environment it was asked to live in.
This is the most expensive mistake in OCTG procurement, and it never shows up at the loading port. It shows up two years later, in a workover report.
At Shanghai Maxmetal, we supply seamless casing and tubing to operators across the Middle East, Central Asia and North Africa. The conversations that save our customers the most money happen before the specification is frozen — not at the quotation stage.
1. Metallurgy — which grade survives the chemistry and temperature of the well?
2. Mechanics — will the pipe body and connection carry the loads you’ll apply?
Get the grade wrong and the pipe corrodes. Get the connection wrong and the string parts at 3,000 metres. Both cost more than the steel.
API 5CT is a family, not a single product. Each grade family exists for a specific combination of depth, pressure and well chemistry.
55,000 psi minimum yield, carbon steel, as-rolled. Cheap, plentiful, and right for shallow wells with benign chemistry — surface casing, intermediate strings, water injection. What neither grade does is tolerate H₂S.
80,000 psi minimum yield. The default for moderately deep, sweet-service wells. It is also the grade most often misapplied — because N80 has no hardness restriction, and hardness is what kills you in sour service.
Same 80 ksi strength as N80, but with a restricted yield window (80–95 ksi) and controlled hardness. That restriction is the entire point: it keeps the steel below the 22 HRC ceiling of NACE MR0175 / ISO 15156, so it resists sulphide stress cracking instead of shattering under it. If your well has H₂S, start here — not with N80.
For sour wells that need more collapse resistance than L80. Quenched and tempered, hardness-controlled. Longer lead times, real price step — which is why they get specified less often than the wells deserve.
110,000 psi minimum yield. The grade for deep, high-pressure, sweet wells where collapse and burst drive the design. We’ve seen tenders specify P110 for wells with measurable H₂S because “it’s the strongest option.” Strength is not corrosion resistance.
Q125 pushes to 125,000 psi for extreme depth — and demands premium connections. Martensitic 13Cr grades handle CO₂ and mild chlorides, but they are not an H₂S solution either.
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If the well contains H₂S, the grade decision is a corrosion decision — not a strength decision. |
Sulphide stress cracking initiates where local hardness exceeds the NACE limit and propagates at stresses far below yield. Pay for the metallurgy, or pay for the workover.
l Burst — internal pressure. Wall thickness and yield strength.
l Collapse — external pressure. Driven by the D/t ratio, and far more sensitive to ovality than most buyers expect.
l Tensile — the string’s own weight. This is where the connection, not the pipe, becomes the limiting factor.
A seamless pipe body is 100% efficient. API round-thread connections are not — STC and LTC joints develop well under pipe-body strength. On deep strings, the connection decides the design. If your string is connection-critical, premium connections with metal-to-metal seals aren’t an upgrade — they’re the only thing that lets the grade you paid for actually be used.
|
Level |
What it adds |
When it’s worth it |
|
PSL1 |
Standard chemistry and mechanical requirements |
Benign service, shallow wells |
|
PSL2 |
Mandatory NDT, tighter chemistry, impact testing |
Default for anything critical |
|
PSL3 |
Enhanced verification for critical service |
Wells where failure cost dwarfs the pipe cost |
Supplementary Requirements (SRs) then stack on top — SR2 through-wall inspection, SR16 hardness testing and others. Each closes a specific failure mode. Specify the SRs your well justifies, then verify them on the certificate, not in the brochure.
l Mill Test Certificate to EN 10204 3.1 — heat-number traceability, actual chemistry, tensile, yield, impact, hardness. The real one, not a summary sheet.
l API monogram verification — the monogram can only be applied under a valid licence. Ask for the number and check it.
l NDT reports — ultrasonic and eddy current, with the standard cited.
l Hardness survey for any sour service order — the single most useful number in the file.
l Pre-loading inspection report with photographs of the heat numbers being shipped.
Documentation is where counterfeit and downgraded material gets caught — and where a supplier’s real capability shows.
Thread damage happens in transit, not downhole. We ship with thread protectors on every joint, couplings torqued to spec, stencilling that survives a sea voyage, and supervised, photographed container stuffing. A pipe that arrives with a dented thread is a pipe you pay to re-thread — or reject.
We are not the cheapest OCTG source, and we don’t try to be:
l ✅ Grade selection support — our engineers review well chemistry and tell you when the tender spec is wrong. Including when it costs us the order.
l ✅ K55 to Q125 — seamless casing and tubing, API round thread, buttress and premium connections.
l ✅ Sour service discipline — hardness verification and certificates that hold up to a third-party audit.
l ✅ Integrated processing — cutting, threading, coating and custom lengths before shipment.
l ✅ Proven logistics — recent shipments include 218 tons of K55 casing held for immediate dispatch, and 76.262 tons of R95 casing moved to Turkmenistan by land bridge (see Internal links above).
Send us the well data — depth, pressure, temperature, H₂S and CO₂, chlorides — and your string design. We’ll come back with a grade recommendation, the connection that makes it work, and a quote that reflects what you actually need.
Because the cheapest casing is the one you never have to pull.
Shanghai Maxmetal Co., Ltd. — seamless pipe, specified correctly.
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