T95 vs C90 vs R95 Casing: Sour Service Grade Comparison

T95, C90, and R95 are three API 5CT grades that a procurement engineer will encounter together on sour-well casing programmes — but only two of them belong in a sour string. T95 and C90 are controlled-hardness sour service grades qualified under NACE MR0175 / ISO 15156. R95 is a general-service grade that happens to share T95's exact strength window, which is precisely why it is the one most often specified by mistake. The grades line up neatly on a yield chart and diverge completely on the property that matters in H₂S.

ZC Steel Pipe exports T95 and C90 sour service casing with PSL-2 documentation to gas wells across North Africa, the Middle East, and South America, and supplies R95 into the non-sour medium-depth wells where its strength-to-cost ratio is genuinely attractive. The confusion between these three grades shows up on real purchase orders often enough to justify laying the distinction out in full.

Where These Three Grades Sit in API 5CT

All three are defined in API Specification 5CT, 11th Edition. Two of them — C90 and T95 — are Group 2 grades, the API classification reserved for restricted-yield, controlled-hardness sour service steels produced exclusively by quench and temper. R95 is a Group 1 grade, the general-service category that also contains H40, J55, K55, N80, and L80-1.

That group assignment is the first signal. Group 2 exists because sour service requires a tight yield window and a hardness ceiling that Group 1 grades are not obligated to meet. C90 and T95 carry those constraints. R95 carries the strength but none of the sour-service metallurgy, and its service_classification in the API grade data is simply "general."

The practical consequence: C90 and T95 are selected by wells; R95 is selected by loads. If a well is sour, the grade choice is between C90 and T95. If a well is not sour, R95 enters as a lower-cost way to reach 95 ksi yield.

Mechanical Properties — Side by Side

Source: API Specification 5CT, 11th Edition (December 2023)

The table makes the trap visible. R95 and T95 are identical on every strength row — same minimum yield, same maximum yield, same minimum tensile. Read only those three lines and the grades are interchangeable. The difference lives in the last three rows: R95 has no hardness limit and no NACE qualification, while both sour grades are held to 25.4 HRC.

C90 differs from T95 in a subtler way. Its yield window sits one 5-ksi step lower (90–105 ksi vs 95–110 ksi) and its minimum tensile is 5 ksi lower, but the hardness ceiling is the same 25.4 HRC. C90 is not a "softer" grade than T95 in the sour-resistance sense — it is a lower-strength grade with the same hardness discipline.

Chemical Composition — Where the Sour Qualification Comes From

Source: API Specification 5CT, 11th Edition. "Not restricted" means API 5CT does not set a limit on that element for that grade.

The chemistry explains why R95 cannot be a sour grade regardless of its strength. Its sulphur limit is 0.030% — three times the 0.010% ceiling applied to C90 and T95. Sulphur controls the sulphide inclusion content of the steel, and those inclusions are the initiation sites for hydrogen-induced cracking and sulphide stress cracking. A grade that permits 0.030% sulphur is not built to resist H₂S, and no heat treatment recovers that. R95 also allows carbon to 0.45% (0.55% when oil-quenched) and manganese to 1.9%, giving it cheap hardenability — exactly the lean, hard-transforming chemistry that sour service is designed to avoid.

T95's mandated minimums are the other half of the story. Where C90 only permits chromium and molybdenum, T95 requires at least 0.4% Cr and 0.25% Mo. Those additions raise temper resistance so the mill can develop 95 ksi minimum yield through quench and temper while holding hardness under the ceiling. C90's lower 90 ksi target can be reached without mandating the alloy, which is part of why C90 is the more economical of the two sour grades where its strength is enough.

For the full grade ladder with tensile, hardness, and chemistry limits across all API 5CT grades, see the API 5CT specification tables →. To match a grade to your well conditions, use the Pipe Grade Selector →.

The Hardness Trap — API 5CT vs NACE

C90 and T95 both carry an API 5CT maximum hardness of 25.4 HRC. That number is not the sour service limit. NACE MR0175 / ISO 15156-2 restricts carbon and low-alloy steels in sour service to 22 HRC maximum. The two figures do not agree, and the gap between them is where sour wells get the wrong pipe.

A T95 or C90 joint produced at 24 HRC is fully within the API grade limit. It passes mill inspection, the MTC shows a compliant hardness, and the pipe ships. But 24 HRC is above the NACE ceiling, and in an H₂S environment that joint is susceptible to sulphide stress cracking. The grade designation did its job — the pipe is genuine T95 — and the well still gets steel that will crack, because the API hardness limit and the NACE hardness limit are different requirements.

R95 sidesteps this discussion by having no API hardness limit at all — which is another way of saying it was never meant for the conversation. A grade with no hardness ceiling cannot be brought into NACE compliance by adding a PO clause, because the underlying chemistry does not support it. The hardness trap applies to C90 and T95, the two grades that are close enough to compliance that the gap is easy to miss. For the full treatment of this failure mode, see the API 5CT and NACE MR0175 hardness trap guide →.

Worked Burst Comparison — Why R95 Reads Like T95

Because R95 and T95 share the same 655 MPa (95 ksi) minimum yield, they produce the same pressure ratings at identical geometry. The API 5C3 burst relationship (Barlow approximation with the 0.875 wall-tolerance factor) is:

P = 0.875 × (2 × Yp × t / D)

For a 7″ 26 lb/ft casing — OD 7.000 in, wall 0.362 in — the three grades at their minimum yield strengths compute as:

T95 (Yp = 95,000 psi): P = 0.875 × (2 × 95,000 × 0.362 / 7.000) = 0.875 × 9,826 = 8,600 psi

R95 (Yp = 95,000 psi): P = 0.875 × (2 × 95,000 × 0.362 / 7.000) = 0.875 × 9,826 = 8,600 psi

C90 (Yp = 90,000 psi): P = 0.875 × (2 × 90,000 × 0.362 / 7.000) = 0.875 × 9,309 = 8,150 psi

T95 and R95 land on exactly the same 8,600 psi rating — the calculation cannot tell them apart, because minimum yield is the only grade input and it is identical. C90 comes in about 5% lower at 8,150 psi, reflecting its 90 ksi minimum yield. This is the arithmetic behind the substitution error: a purchasing engineer running a burst check will see R95 satisfy the same design pressure as T95 and conclude the grades are equivalent. The calculation is correct and the conclusion is wrong, because the burst formula contains no term for sour service. Run the same check across your full size range with the Barlow pressure calculator →.

When NOT to Use Each Grade

Do not use R95 in any sour well. Its 0.030% sulphur limit and absence of hardness control make it unqualified for H₂S service under NACE MR0175 / ISO 15156-2. Matching strength does not make it a sour grade.

Do not use C90 where the design requires 95 ksi minimum yield. C90 tops out at a 90 ksi minimum; if the burst or collapse analysis needs 95 ksi, C90 is under-strength and T95 is the correct sour grade.

Do not use T95 where C90's strength is sufficient and cost matters. T95's mandated Cr–Mo chemistry makes it the more expensive sour grade. On a sour well whose loads are within C90's envelope, specifying T95 pays an alloy premium the design does not need.

Do not use C90 or T95 above 22 HRC in sour service, regardless of the 25.4 HRC API allowance. The NACE limit governs, and pipe produced between 22 and 25.4 HRC is API-compliant but not sour-compliant.

Do not use any of the three for severe sour service without checking the temperature and H₂S partial-pressure envelope. C90 and T95 are qualified for defined sour conditions, not unlimited ones; wells beyond their envelope move to grades such as C110 or to CRA material. Screen the conditions with the OCTG sour service grade selection guide →.

Purchase Order Guidance

The recurring error on these three grades is ordering by strength when the well is sour. A PO that reads "API 5CT 95 ksi casing" or "R95 or T95" leaves the grade open, and a mill quoting competitively will supply R95 — the cheaper 95 ksi grade — and be fully compliant with the order as written. In a sour well, that pipe is unqualified for the service it was bought for, and nothing on the MTC will flag the problem because R95 is not required to report a hardness result at all.

The correct PO for a sour application names the sour grade and the NACE requirement together:

• Grade and standard: "API 5CT T95" (or C90), 11th Edition — not "95 ksi" and not "R95 or equivalent."
• Sour service clause: "22 HRC maximum, pipe body and coupling, per NACE MR0175 / ISO 15156-2" as a separate line item.
• Product specification level: PSL-2, to invoke Charpy V-notch impact testing, full-length UT, and drift testing — most sour project specifications require it.
• Certification: EN 10204 3.1 MTC as a minimum; 3.2 (third-party witnessed) where the project specification or market practice requires it.

On acceptance, verify three things on the MTC before releasing the consignment: the grade line matches the PO grade exactly; the reported hardness is at or below 22 HRC, not merely below the 25.4 HRC API ceiling; and the heat treatment is recorded as quench and temper. A T95 or C90 heat reported at 24 HRC should be held and referred to the project metallurgist — it is API-compliant and sour-non-compliant, and accepting it puts unqualified pipe in an H₂S string. For a deeper look at the T95-versus-lower-grade decision, see the L80 vs T95 sour gas well selection guide →.