API 5CT C110 Casing Pipe — Specifications, Sour Service & Supplier Guide

C110 occupies a precise engineering slot that no other standard API casing grade fills: the highest yield strength available in a grade explicitly listed as sour-service-qualified under NACE MR0175 / ISO 15156-2. T95 tops out at 95 ksi minimum yield. P110 reaches 110 ksi minimum but is disqualified from any H2S environment. C110 delivers 110 ksi minimum with sour service qualification intact — a combination that exists specifically because there are wells deep enough and pressurised enough that T95 cannot contain the wellbore, yet H2S partial pressure is present at levels that eliminate P110 from the design envelope.

The practical consequence of that narrow positioning is that C110 is not a commodity grade. Mills that produce it routinely are far fewer than those producing T95 or P110. Lead times are longer, documentation requirements are more demanding, and buyers who do not specify it correctly create problems that take considerably longer to resolve than a misspecified T95 order. When we receive a C110 inquiry, the first thing we establish is whether the buyer genuinely needs C110 or whether the well conditions can be met with T95 at a heavier wall.

ZC Steel Pipe supplies API 5CT C110 casing to PSL-2 in seamless form, primarily for operators and EPC contractors working in deep sour wells across West Africa, the Middle East, and South America. Connections range from BTC to premium metal-to-metal seal depending on the service environment and wellbore geometry.

What Is API 5CT C110?

C110 is defined in API Specification 5CT, 11th Edition as a casing grade in Group 3 — the sour service group that also contains L80 and T95. The designation C110 reflects the controlled yield range: minimum 758 MPa (110 ksi), maximum 828 MPa (120 ksi). Quench and temper heat treatment is mandatory. No other heat treatment route is permitted under API 5CT.

The grade's engineering identity is defined by three characteristics that distinguish it from every adjacent grade in the API ladder.

First, it is the highest-yield sour-service-rated casing grade in API 5CT. L80 offers 552 MPa (80 ksi) minimum. T95 offers 655 MPa (95 ksi) minimum. C110 offers 758 MPa (110 ksi) minimum — matching P110 on the minimum yield floor — while retaining NACE qualification. That alignment with P110's yield floor is not coincidental: C110 was designed to offer a sour-service-rated alternative at a yield level that previously required accepting P110's disqualification from H2S service.

Second, C110 has a controlled maximum yield. The 828 MPa (120 ksi) ceiling is not a guideline — it is a specification requirement. A heat that produces above 828 MPa is non-conforming for C110. This ceiling matters for string design: unlike P110, where over-strength is an acceptable outcome, C110 string calculations must account for both ends of the yield range.

Third, C110's chemistry is explicitly controlled by API 5CT for both Chromium and Molybdenum — mandatory ranges, not just maxima. The S_max of 0.005% is the tightest sulphur limit in the entire API 5CT grade list, tighter even than T95's 0.010%. This is not incidental; it reflects that C110 is designed for more severe H2S environments where sulphide inclusion control is correspondingly more critical.

Mechanical Properties

Read the hardness comparison carefully. Unlike T95, where the API limit (25.4 HRC for Type 1) and the NACE limit (22 HRC) create a 3.4 HRC gap that is the source of the most common sour service specification error in OCTG procurement, C110's API limit and NACE limit are the same value: 29.0 HRC. A C110 heat that meets API 5CT on hardness meets the NACE MR0175 hardness requirement. There is no separate hardness ceiling to overlay on top of API conformance — the API limit is the NACE limit.

The minimum tensile strength of 793 MPa (115 ksi) provides a meaningful margin above the minimum yield of 758 MPa, confirming that the Q+T process has produced adequate work hardening and toughness in the microstructure. Verify this on every MTC: a tensile result close to or below the yield result would indicate an anomalous heat that warrants rejection.

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 imposes mandatory chemistry limits for C110. Unlike P110 — which API 5CT does not restrict for Carbon or Manganese — C110 carries explicit alloy controls designed to deliver the hardenability required for Q+T heat treatment without exceeding the hardness ceiling.

The Mo and Cr entries deserve particular attention. Both have mandatory minimums and maximums — not just maxima. A heat that produces Molybdenum below 0.25% is non-conforming for C110, even if it meets every other chemical requirement. The same applies to Chromium below 0.40%. Verify that the MTC shows both elements within their specified ranges, not merely at or below an upper limit.

The S_max of 0.005% demands specific attention. This is the tightest sulphur limit in the entire API 5CT specification — lower than T95's 0.010%, and six times lower than P110's 0.030%. Achieving 0.005% S consistently in production requires dedicated ladle metallurgy: vacuum degassing and calcium treatment are standard methods for desulphurisation to this level. Calcium treatment converts elongated manganese sulphide inclusions into globular calcium aluminosulphide inclusions that are significantly less active as hydrogen-induced cracking initiation sites. The practical supply consequence is that not every mill producing sour service OCTG has the ladle metallurgy equipment to hit 0.005% S reliably across a full heat. Confirm the mill's demonstrated sulphur capability — not just the specification commitment — before placing the order.

Burst Pressure Example

The API 5C3 internal yield pressure formula (Barlow approximation with the 0.875 correction factor) is:

P_burst = 0.875 × (2 × SMYS × t / D)

Where SMYS is the specified minimum yield strength in psi, t is the nominal wall thickness in inches, and D is the OD in inches.

For a 7" 23 lb/ft C110 string — a common intermediate casing weight — the wall thickness is 0.317 inches and D is 7.000 inches:

P_burst = 0.875 × (2 × 110,000 × 0.317 / 7.000) = 0.875 × (69,740 / 7.000) = 0.875 × 9,963 = 8,720 psi

The same size and weight in T95 (SMYS = 95,000 psi) produces:

P_burst = 0.875 × (2 × 95,000 × 0.317 / 7.000) = 0.875 × (60,230 / 7.000) = 0.875 × 8,604 = 7,530 psi

The 1,190 psi additional burst capacity from C110 versus T95 at 7" 23 lb/ft represents a 15.8% improvement. For a sour gas well where T95 leaves an insufficient design factor against shut-in wellhead pressure, C110 restores the margin without requiring a move to heavier wall or a larger casing programme. The same 15.8% advantage carries through all sizes at constant D/t ratio. Use the Barlow pressure calculator → to work through your full size and weight range.

Sour Service Qualification Under NACE MR0175 / ISO 15156-2

NACE MR0175 / ISO 15156-2 governs the use of carbon and low-alloy steels in oil and gas production equipment exposed to H2S. C110 is one of the few high-strength grades explicitly listed in that standard as acceptable for sour service, subject to its 29.0 HRC maximum hardness requirement.

The significant qualification in the standard is that C110's acceptability is conditional: the hardness limit of 29.0 HRC must be confirmed on the production heat, not assumed from the grade designation alone. A C110 pipe that has not been hardness-tested to confirm it falls at or below 29.0 HRC has not been NACE-qualified — it has only been API-compliant. The MTC must show actual hardness survey results, not just a statement that the grade meets API 5CT.

The comparison between C110 and the other sour-service grades in the API ladder is instructive for understanding where the NACE hardness trap does and does not apply:

L80 and C110 both have API limits aligned with the NACE limit — no gap exists for either grade. T95 Type 1 has the documented trap: the API limit permits heats that NACE would reject. For C110, the trap is not in the hardness value; it is in whether the hardness survey documentation appears on the MTC in a format the receiving inspector can verify per heat.

One additional point: NACE MR0175 / ISO 15156-2 acceptance is also contingent on the heat treatment being as-manufactured Q+T. Post-weld heat treatment zones, field repairs, or connection threading operations that alter the local hardness above 29.0 HRC void the NACE qualification in that region. This is particularly relevant for premium connection threading — confirm with the connection supplier that their threading process has been qualified to maintain base material hardness within the NACE limit.

When NOT to Use C110

C110 has a well-defined service envelope. Outside that envelope it is either insufficient for the conditions or unnecessarily expensive and hard to source.

When the well is sweet (no H2S) — In a confirmed sweet well with no H2S at concentrations triggering NACE MR0175, C110's tight sulphur limit, controlled Cr-Mo chemistry, and sour service documentation requirements add cost and lead time without delivering any engineering benefit. P110 at the same minimum yield costs less, is more widely available, has shorter lead times, and requires simpler documentation. Q125 is available if higher yield is needed. There is no justification for specifying C110 in a confirmed sweet service environment.

When T95 yield strength is adequate for the casing design — C110 should not be specified unless the casing design actually requires 110 ksi minimum yield in a sour environment. If the burst and collapse calculations close with T95 at adequate design factors, specifying C110 is over-engineering: higher cost, fewer mills, longer lead times, more demanding documentation, and a smaller pool of replacement pipe in the market if a joint needs to be swapped during running. Do the calculation first, then select the grade.

When the 120 ksi maximum yield ceiling is restrictive for the design — C110's 828 MPa (120 ksi) maximum yield is a hard ceiling that some designs cannot accommodate. If a string design must not exceed a certain yield for dimensional stability reasons but the designer is working close to or at 120 ksi, the controlled maximum becomes a risk. Over-yield heats above 828 MPa must be rejected as non-conforming. If the design can tolerate uncapped yield — P110's profile — and the well is sweet, P110 is the simpler choice.

When severe sour service conditions exceed the carbon steel limits in NACE MR0175 / ISO 15156-2 — NACE MR0175 / ISO 15156-2 defines environmental limits within which carbon and low-alloy steel tubulars are acceptable. Wells with very high H2S partial pressures combined with low pH and elevated chloride concentrations may fall outside the envelope where carbon steel grades — including C110 — are qualified. For those conditions, 13Cr or CRA alloy tubulars are the engineering answer, not a higher-strength carbon steel.

C110 vs T95 vs Q125 — Grade Selection

The selection decision maps to three distinct conditions.

Use T95 when H2S is present at levels requiring NACE MR0175 qualification, and burst and collapse calculations close with 95 ksi minimum yield at adequate design factors. T95 Type 2 is the clean specification for sour service — more widely available and better-stocked at most mills than C110.

Use C110 when H2S is confirmed in the reservoir fluid, NACE MR0175 compliance is mandatory, and T95's 95 ksi minimum yield is genuinely insufficient for the wellbore pressure profile. This is not a common situation; most deep sour wells can be designed around T95 with appropriate wall thickness selection. When it does apply, C110 is the correct choice — there is no alternative within the API 5CT carbon steel grade list that provides 110 ksi with sour service qualification.

Use Q125 when the well is confirmed sweet and maximum yield strength is required for collapse or burst resistance. Q125's uncapped maximum yield and lack of hardness restriction make it unsuitable for any H2S environment where the NACE threshold is exceeded.

Standard Sizes

C110 is produced in casing sizes from 4½" through 20" OD in API 5CT — the full range covering production to surface casing. In practice the grade is almost exclusively used in production and intermediate casing applications where yield requirement is highest.

Availability decreases significantly at larger OD sizes. Mills that run C110 routinely — meaning as a scheduled heat, not as a special order requiring a dedicated heat number — typically do so in the 4½" to 9⅝" range. Above 9⅝", expect to negotiate a minimum order quantity with the mill, and plan for lead times that reflect the time required to schedule and execute a dedicated Q+T heat cycle for a non-standard production size.

The 5½" and 7" sizes in standard production casing weights represent the practical centre of C110 demand. If you are sourcing C110 in these sizes with standard connections, availability from established mills is reasonable with adequate lead time. If you need C110 in sizes outside this range — particularly in non-standard weights — establish availability before the casing programme is locked.

Purchase Order Guidance

What to Specify on the PO

A complete C110 sour service purchase order must include:

1. Standard — API Specification 5CT, 11th Edition (or ISO 11960)
2. Grade — C110 (no type designation is used for C110; there is only one variant unlike T95)
3. OD and nominal weight — e.g. 7 inch × 23.00 lb/ft
4. Connection type — BTC or premium connection designation; if premium, name the specific connection form
5. Range — R1, R2, or R3 (production casing is typically R3)
6. PSL level — PSL-2 mandatory for all sour service C110
7. Supplementary requirements — SR2 (Charpy V-notch at project temperature), SR15 (NACE hardness qualification with individual survey results per heat)
8. MTC level — EN 10204 3.1 minimum; EN 10204 3.2 (third-party witnessed) required by most IOC project specifications
9. Third-party inspection scope — hardness survey witness, NDE verification, dimensional check at the mill before shipment
10. Heat treatment record — Q+T records as a separate MTC line item, not just a grade designation statement

The Procurement Trap

Wrong PO language: "API 5CT C110, PSL-2, 7" × 23 lb/ft, BTC, EN 10204 3.1"

What the mill ships: C110 produced to API 5CT, with hardness at or below 29.0 HRC, Sulphur at or below 0.005%, Cr-Mo chemistry within limits. The MTC will state "Conforms to API 5CT C110." This is fully API-compliant.

What is missing: The MTC has no individual hardness survey records — only a grade conformance statement. The operating company's receiving inspector requires per-heat hardness values to verify NACE MR0175 compliance. Without those values on the MTC, verification is impossible and the pipe goes on quality hold at the yard.

What to write instead: "API 5CT C110, PSL-2, 7" × 23 lb/ft, BTC, EN 10204 3.2, third-party witness hardness survey per NACE MR0175 / ISO 15156-2, individual hardness values per heat required on MTC, Charpy V-notch testing at −10°C per SR2, heat treatment records as separate MTC line item"

The difference is the explicit requirement for individual hardness survey records. A mill that is genuinely set up for C110 sour service production will generate this documentation routinely. A mill that adds C110 to its grade portfolio without the corresponding hardness documentation process will deliver a paper gap that holds your shipment at the receiving yard.

Named Failure Modes

Sulphide stress cracking at threaded connections — C110 at 110 ksi yield in a high-H2S environment is susceptible to SSC at stress concentration points, and the highest local stress in a casing string is at the thread root of a connection. SSC at connections typically manifests as a through-crack from the last engaged thread into the pipe wall, driven by the combination of residual threading stress, makeup torque stress, and atomic hydrogen absorbed from the H2S environment. The risk is elevated when the connection threading has work-hardened the surface locally above 29.0 HRC, when the thread compound creates a local electrochemical environment that accelerates hydrogen uptake, or when makeup torque was applied in cold conditions where subsequent thermal cycling imparts additional thread stress. Mitigation requires confirming that the connection threading process has been qualified to maintain surface hardness within the NACE limit, and specifying a NACE-qualified thread compound explicitly on the purchase order.

Over-yield rejection during receiving inspection — Because C110 has a maximum yield of 828 MPa (120 ksi), heats that produce over-yield pipe are non-conforming and must be rejected. The characteristic failure mode in procurement terms is that the MTC shows a yield of 840 MPa on a heat that otherwise meets all dimensional, chemistry, and hardness requirements. This heat cannot be accepted as C110. It cannot be re-labelled as P110 for the sour service application without a full NACE MR0175 review — P110 is not NACE-qualified. Over-yield pipe requires return to the mill or reallocation to a non-sour application if the receiving project permits it. Verify the maximum yield on every C110 heat before accepting any shipment.

What to Verify on the C110 MTC

References

• API Specification 5CT, 11th Edition — Specification for Casing and Tubing (American Petroleum Institute)
• ISO 11960 — Petroleum and Natural Gas Industries: Steel Pipes for Use as Casing or Tubing
• NACE MR0175 / ISO 15156 — Materials for Use in H2S-Containing Environments in Oil and Gas Production
• API Technical Report 5C3 — Equations and Calculations for Casing, Tubing, and Line Pipe Used as Casing or Tubing