API 5CT C90 Casing Pipe — Sour Service Specifications & Selection

C90 purchase orders come to us infrequently compared to L80 and T95 — and for good reason. The grade occupies a narrow yield window (90–105 ksi) that most well designs either don't require or over-specify past. When a customer does order C90, it is almost always because a drilling engineer's string design calculation landed squarely in that 90–105 ksi zone, and T95 was rejected on cost or because the string design tools showed the 110 ksi maximum of T95 wasn't needed. That precision of specification is actually a supply risk: C90 is not stock at most mills. The customers who order it tend to know exactly what they need, but the procurement timeline catches them by surprise.

ZC Steel Pipe supplies API 5CT C90 casing to PSL-2 with full sour service documentation — EN 10204 3.2 MTC, third-party inspection, hardness survey per heat. We supply to operators and EPC contractors running sour-service wells in West Africa, East Africa, and the Middle East. C90 goes out as a project-order item with adequate lead time built into the schedule; we do not advertise it as stocked grade.

What Is API 5CT C90?

C90 is defined in API Specification 5CT, 11th Edition / ISO 11960 as a Group 2, restricted-yield casing and tubing grade. Three properties define its position in the sour service grade sequence.

Restricted yield range with a controlled ceiling. The yield window of 621–724 MPa (90–105 ksi) is bounded at both ends. The 621 MPa minimum is the design floor — below this, the grade fails to meet yield requirements. The 724 MPa maximum is the ceiling that enables NACE MR0175 qualification. High-strength steel above certain yield thresholds becomes progressively susceptible to sulphide stress cracking; restricting the maximum yield limits that risk while preserving the mechanical performance needed for the well design. This is why "restricted-yield" is not a generic label — it describes the actual qualification mechanism.

Maximum hardness of 25.4 HRC. API Specification 5CT mandates 25.4 HRC maximum (255 HBW) for C90. Hardness is the proxy for microstructural susceptibility to hydrogen embrittlement: above 25.4 HRC, Q+T carbon-alloy steel becomes increasingly vulnerable to sulphide stress cracking in H2S environments. For C90, the API hardness limit and the applicable NACE MR0175 / ISO 15156-2 limit align — there is no gap between API compliance and NACE qualification, which is one of C90's operational advantages over T95 Type 1.

Mandatory quench and temper heat treatment. API 5CT prohibits any other heat treatment for C90. Normalised, normalised-and-tempered, or as-rolled pipe cannot be designated C90. The Q+T microstructure is what achieves the combination of controlled yield ceiling and low hardness simultaneously.

Group 2 classification. C90 requires more stringent qualification testing than Group 1 grades — additional Charpy V-notch impact testing, hardness surveys across the pipe wall, and tighter chemistry controls. This is not a bureaucratic distinction: Group 2 testing reflects the safety-critical consequences of a sour service failure and the tighter process control required to achieve consistent restricted-yield performance.

Mechanical Properties

All values from API 5CT 11th Edition.

The maximum yield of 724 MPa (105 ksi) is as operationally important as the minimum. A C90 heat that tests above 724 MPa on the tensile report is non-conforming and must be rejected — not upgraded to T95. The restricted-yield ceiling is a qualification condition, not an advisory. Verify both the minimum and maximum yield on every C90 MTC before acceptance.

For the complete grade ladder with tensile, hardness, and chemistry limits, see the API 5CT specification tables →

To match a grade to your well conditions, use the AI Pipe Grade Selector →

Chemical Composition

API 5CT 11th Edition chemistry limits for C90:

The sulphur limit of 0.010% maximum is the most consequential chemistry restriction for sour service. Non-metallic sulphide inclusions — elongated MnS stringers produced during hot rolling — act as crack initiation sites for both sulphide stress cracking and hydrogen-induced cracking. The 0.010% limit reduces the total sulphide inclusion inventory. Mills producing C90 for sour service complement this with calcium treatment, which modifies the morphology of residual sulphide inclusions from elongated stringers to spherical particles that are significantly less effective as crack initiators. Calcium treatment records should be requested on the MTC; this practice is standard for qualifying mills but not universally documented without a specific PO requirement.

The phosphorus maximum of 0.020% is similarly tighter than Group 1 grade limits (where P max is 0.030%). Phosphorus segregates to grain boundaries during solidification and, at concentrations above 0.020%, degrades the fracture toughness of the tempered martensitic microstructure — the same microstructure that the Q+T heat treatment is designed to produce. Tight phosphorus control is part of achieving consistent Charpy impact values across the pipe body.

Compare C90's chemistry constraints to L80 Type 1: L80 carries C max 0.43%, Mn max 1.90%, no Mo or Cr restrictions, P max 0.030%, and S max 0.030%. These are broad, service-indifferent limits. C90's chemistry is purpose-built for sour service from the specification level up.

The molybdenum footnote in API 5CT is worth noting: Mo has no minimum requirement for C90 when wall thickness is less than 17.78 mm. For heavier wall C90 (t ≥ 17.78 mm), Mo contributes to hardenability at depth in the cross-section, helping the Q+T heat treatment achieve through-wall hardness uniformity. Thin-wall pipe achieves through-wall hardness more readily from Cr alone; the Mo minimum becomes relevant as wall thickness increases.

Burst Pressure Example

The Barlow formula per API Bulletin 5C3 is the standard method for estimating internal yield pressure for comparison purposes:

P_burst = 0.875 × (2 × SMYS × t / D)

The factor 0.875 is a 12.5% reduction for wall thickness manufacturing tolerance per API 5CT. SMYS is the specified minimum yield strength.

Example: 5-1/2", 17 lb/ft C90 casing

From API 5CT:

• OD (D) = 5.500 inches
• Nominal wall thickness (t) = 0.304 inches
• SMYS = 90,000 psi (621 MPa)

Calculation:

P_burst = 0.875 × (2 × 90,000 × 0.304 / 5.500)

= 0.875 × (54,720 / 5.500)

= 0.875 × 9,949

= 8,710 psi (rounded to nearest 10 psi)

For comparison, L80 in the same 5-1/2" 17 lb/ft size produces:

P_burst = 0.875 × (2 × 80,000 × 0.304 / 5.500) = 7,742 psi

The 968 psi difference — roughly 12.5% more burst resistance from C90 — reflects the 10 ksi SMYS difference between the two grades at the same geometry. In a well design where the maximum anticipated surface pressure at shut-in exceeds L80's burst rating at the selected wall thickness, C90 provides the next increment of capacity within the sour service grade sequence. To use the Barlow calculator for your specific OD, weight, and SMYS combination, see the Barlow Pressure Calculator →

Sour Service Qualification — NACE MR0175 / ISO 15156

NACE MR0175 / ISO 15156-2 governs the use of carbon and low-alloy steels in H2S-containing environments. C90 is specifically identified as a restricted-yield grade in the standard. For NACE qualification, C90 casing must:

• Be supplied to PSL-2 with full chemistry certification against API 5CT Group 2 limits
• Carry a hardness survey on the MTC confirming ≤ 25.4 HRC across the pipe body and at the pipe ends
• Be accompanied by an EN 10204 3.2 inspection certificate from a named independent inspection body
• Have sulphur ≤ 0.010% confirmed on the heat chemistry report
• Carry calcium treatment records confirming sulphide shape control

The confirmation process on the MTC matters. A hardness survey that lists a single hardness value for the lot is insufficient. The MTC should carry hardness results from multiple test locations per pipe — typically body mid-length and both ends — with all results confirmed below 25.4 HRC. When a third-party inspector witnesses this test at the mill, the 3.2 certificate documents the witness scope, not just the results.

For wells with elevated H2S partial pressure or temperatures above 65°C, supplementary HIC testing per NACE TM0284 may be specified. This is not a standard API 5CT requirement for C90 — it is an operator-driven supplementary requirement that must appear explicitly on the purchase order. The applicable conditions are defined in ISO 15156-2 and depend on the H2S partial pressure, temperature, and in-situ pH at the point of service.

One clarification that comes up frequently: C90 does not have the T95 Type 1 / Type 2 distinction. There is one C90 grade, and its API hardness limit of 25.4 HRC does not create a gap with the NACE qualification limit in the way that T95 Type 1 does. A C90 heat that passes the API hardness test is NACE-qualified for hardness. This is not an ambiguity to manage — it is part of why C90 was designed as a distinct grade from T95.

When NOT to Use C90

When the well is sweet. C90's restricted chemistry and Group 2 testing requirements come with a price and supply premium over N80Q or P110. For sweet wells — no H2S, no NACE MR0175 requirement — N80Q or P110 achieves better pressure performance at better availability and cost. C90's chemistry controls buy sour service capability; there is no engineering benefit to those controls in a sweet environment.

When L80 is sufficient. If the string design calculation shows the critical section needs less than 90 ksi minimum yield, L80 covers the requirement with better mill availability, shorter lead times, and no Group 2 testing overhead. Use C90 only when L80's 95 ksi yield ceiling is demonstrably insufficient for the design loads.

When the project schedule cannot accommodate 14–20 week lead times. C90 is a specialty grade. Most mills do not carry it in stock at all sizes. If the project schedule requires tubular delivery in less than 10–12 weeks, C90 may not be achievable from a qualified mill. Specifying C90 under schedule pressure sometimes forces buyers to accept unverified material from distributors without mill-direct MTC traceability — which is a sour service risk, not a solution.

When T95 mill availability is confirmed and the string design tolerates the higher maximum yield. T95's maximum yield of 758 MPa (110 ksi) is higher than C90's 724 MPa (105 ksi). For most well designs, this difference is not an engineering problem — the string design tools show the higher maximum is acceptable. If T95 Type 2 is available on a shorter lead time or at better price, and the design envelope can accommodate the T95 yield range, T95 is operationally preferable for most projects despite the nominally higher yield ceiling.

C90 vs L80 vs T95 — Grade Selection

Reading the comparison table

L80's 23.0 HRC API hardness limit is actually lower than C90's 25.4 HRC — L80 is a more conservative restriction, which reflects L80's position as a mild sour service grade for lower H2S partial pressures. L80's S max of 0.030% is three times C90's 0.010% limit, which means L80 can carry more sulphide inclusions — appropriate for mild service, but insufficient for the more aggressive conditions where C90 is deployed.

T95 Type 2's 22 HRC limit is stricter than C90's 25.4 HRC, which is counterintuitive given T95's higher yield ceiling. The NACE qualification path for T95 is harder to achieve than for C90 because producing a 95–110 ksi grade while keeping hardness below 22 HRC requires tighter Q+T process control. That is why T95 Type 2 carries a Mo minimum of 0.25% — the Cr-Mo alloying is specifically designed to achieve hardenability without requiring the hardness. C90, with its lower yield ceiling, achieves NACE-compatible hardness at the 25.4 HRC limit without needing the Mo minimum.

The narrow window where C90 is distinctly correct

C90 is specifically correct when all three conditions hold simultaneously: H2S is present at a level requiring NACE MR0175 compliance; the string design minimum yield requirement exceeds L80's 95 ksi maximum; and the design envelope's maximum yield tolerance is at or below 105 ksi. Engineers who routinely specify T95 as the default for all above-L80 sour service work are not wrong to do so — T95 covers C90's territory while providing additional margin — but they accept a higher yield maximum than the design may require, which can complicate wellbore integrity models that assign stress loads based on minimum yield. When the model is sensitive to the maximum yield of the casing string, C90's tighter ceiling (105 ksi vs 110 ksi) may be the specification reason.

Standard Sizes

C90 is available in API 5CT standard casing OD ranges. The grade is most commonly ordered in intermediate sizes where 90–105 ksi yield provides a meaningful step above L80 in deep string design:

Confirm size, weight, and connection availability with the mill before committing C90 to a project schedule. Not all sizes are produced at all qualified mills, and lead times for non-stocked items run 12–20 weeks from order placement. The 5-1/2" and 7" sizes have the broadest C90 availability; 4-1/2" and 9-5/8" are more restricted.

Purchase Order Guidance

The procurement trap — "C90 from stock"

The most common C90 procurement failure is accepting stock pipe from a distributor without verifying that the MTC traces to a qualified mill production heat. The sequence goes like this: the project is under schedule pressure, a distributor offers C90 from their yard, the buyer accepts because the offer is faster than a mill order, and the MTC that arrives is a reissued document that does not carry a mill heat number, heat treatment record, or hardness survey traceable to the pipe in the shipment.

Sour service OCTG without verifiable heat-level documentation — including a confirmed hardness survey per heat — is not qualified for H2S service, regardless of what the paperwork says. The risk is sulphide stress cracking in a well where the operating company believed the pipe was NACE-qualified. Reject any C90 for sour service that cannot produce an original mill MTC with hardness survey values traceable to the production heat.

Named failure mode: hardness scatter above 25.4 HRC

API 5CT permits hardness to be measured at specific locations on a sample from each heat. A heat can pass the API hardness test — with all measured values below 25.4 HRC — and still contain regions of higher hardness in the pipe body that were not sampled. This is not a fraud scenario; it is a consequence of hardness variation across a large production heat. The mitigation is a more intensive hardness survey covering multiple locations per pipe (not per heat), which is achievable through supplementary requirement SR15 or equivalent mill instructions on the purchase order. For C90 sour service supply, the hardness survey scope should cover body mid-length and both ends for each pipe, not a sample-based per-heat measurement. Confirm the survey scope in the inspection plan before production begins.

Minimum sour service C90 purchase order line items

• Grade: API 5CT C90 PSL-2
• OD and nominal weight (confirm availability before placing order)
• Connection: BTC minimum; premium connection for high-combined-load strings
• Range: R2 or R3 per project requirement
• Documentation: EN 10204 3.2 inspection certificate with named independent inspection body
• Hardness survey: 25.4 HRC maximum, measured at body and ends of each pipe (not per-heat)
• Chemistry: Full heat chemistry per API 5CT 11th Edition Group 2 limits, including P ≤ 0.020% and S ≤ 0.010%
• Calcium treatment records confirming sulphide shape control
• Charpy V-notch impact testing (SR2) at temperature specified by project
• NDE: SR13 (seamless verification) if seamless supply is required
• Hardness test method: Rockwell C scale, reported to 0.1 HRC resolution

Do not accept a 3.1 MTC (mill self-certified) for sour service C90. The industry standard for H2S service tubulars is EN 10204 3.2 with an independent inspection body — named on the certificate — who witnessed the hardness and Charpy testing at the mill. The certificate number and inspector name should be on the MTC. If the MTC carries a 3.1 designation without a third-party witness signature, return it and request re-inspection under 3.2 scope before accepting the consignment.