C110 occupies a specific and important position in the API 5CT grade ladder: it is the highest-strength casing grade with explicit sour service qualification under NACE MR0175/ISO 15156-2. Where T95 provides insufficient yield strength for the casing design and P110 or Q125 cannot be used due to H2S presence, C110 is the correct engineering solution. It is a grade that requires careful specification — controlled chemistry, mandatory heat treatment, and hardness verification are not optional on C110 orders.
ZC Steel Pipe manufactures C110 casing in seamless form across the standard API size range. This guide covers the full API 5CT specification for C110, its sour service qualification under NACE MR0175, the critical comparison with adjacent grades T95 and Q125, and the procurement requirements that distinguish a compliant C110 order from a non-compliant one.
1. API 5CT C110 Mechanical Properties
C110 is defined in API 5CT as a single-type grade — unlike T95 (Type 1 and Type 2) or L80 (L80-1, L80-9Cr, L80-13Cr), there is no subtype designation. The mechanical requirements are:
| Property | C110 Requirement |
|---|---|
| Minimum yield strength | 110,000 psi (758 MPa) |
| Maximum yield strength | 140,000 psi (965 MPa) |
| Minimum tensile strength | 125,000 psi (862 MPa) |
| Maximum hardness | 30 HRC / 286 HBW |
| Elongation (min) | 13% |
| Heat treatment | Quench and temper (Q&T) mandatory |
The yield strength window of 110–140 ksi is relatively wide, which gives manufacturers flexibility in heat treatment. However, the hardness ceiling of 30 HRC is a hard limit — any C110 pipe exceeding 30 HRC is non-conforming regardless of yield strength.
Charpy Impact Testing
API 5CT requires Charpy V-notch impact testing for C110. Minimum absorbed energy requirements apply at 21°C (70°F) per API 5CT Table C.68/Table E.68. For sour service applications, some project specifications impose additional low-temperature Charpy requirements — confirm with your project specification whether ambient temperature testing is sufficient or whether low-temperature testing is required.
2. Chemical Composition Requirements
C110 has controlled chemistry requirements that distinguish it from non-sour grades:
| Element | Maximum (% by weight) |
|---|---|
| Carbon (C) | 0.35 |
| Manganese (Mn) | 1.20 |
| Molybdenum (Mo) | 1.00 |
| Chromium (Cr) | 1.50 |
| Phosphorus (P) | 0.020 |
| Sulfur (S) | 0.010 |
| Silicon (Si) | 0.45 |
| Nickel (Ni) | 0.99 |
| Copper (Cu) | 0.99 |
| Vanadium (V) | 0.12 |
| Niobium (Nb) | 0.10 |
| Titanium (Ti) | 0.06 |
The low sulfur maximum (0.010%) and low phosphorus maximum (0.020%) are critical for sour service — both elements promote hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) when present above these levels. Verify mill test reports against these limits on every C110 order.
3. Sour Service Qualification — NACE MR0175/ISO 15156-2
C110 is one of the few casing grades above 95 ksi yield strength that is explicitly listed in NACE MR0175/ISO 15156-2 (Table B.1) for use in H2S-containing oilfield environments. The qualification conditions are:
- Maximum hardness: 30 HRC (296 HBW) — same as the API 5CT limit
- Heat treatment: quench and temper mandatory
- Chemistry: must comply with API 5CT C110 composition limits
- SSC Region: C110 is qualified for use in NACE MR0175 Region 3 (higher H2S partial pressure) subject to hardness compliance
This is the critical distinction between C110 and P110: P110 is not listed in NACE MR0175 and cannot be used in H2S-containing environments. C110 at 110 ksi minimum yield is the highest-strength grade with standard NACE MR0175 qualification.
Hardness Trap Comparison
A common procurement error is assuming all API 5CT sour service grades have the same hardness threshold under NACE MR0175. They do not:
| Grade | API 5CT Max Hardness | NACE MR0175 Limit | Gap |
|---|---|---|---|
| L80 | 23 HRC | 23 HRC | None |
| T95 | 25.4 HRC | 22 HRC | 3.4 HRC — significant |
| C110 | 30 HRC | 30 HRC | None |
| P110 | No limit | Not qualified | N/A |
C110 is unusual in that its NACE MR0175 hardness limit matches the API 5CT limit exactly — there is no gap between the two standards as there is with T95. This makes C110 compliance verification more straightforward than T95, where pipes passing API 5CT hardness inspection may still fail NACE MR0175 qualification.
4. C110 vs T95 vs Q125 — Grade Selection
| Property | T95 | C110 | Q125 |
|---|---|---|---|
| Min yield strength | 95 ksi (655 MPa) | 110 ksi (758 MPa) | 125 ksi (862 MPa) |
| Max yield strength | 110 ksi (758 MPa) | 140 ksi (965 MPa) | 150 ksi (1034 MPa) |
| Min tensile strength | 105 ksi (724 MPa) | 125 ksi (862 MPa) | 135 ksi (931 MPa) |
| Max hardness (API) | 25.4 HRC | 30 HRC | No limit |
| NACE MR0175 qualified | Yes (22 HRC limit) | Yes (30 HRC limit) | No |
| Heat treatment | Q&T mandatory | Q&T mandatory | Q&T mandatory |
| Sour service use | Yes | Yes | No |
| Typical application | Moderate H2S, standard pressure | High pressure, H2S present | High pressure, sweet wells |
When to specify C110 over T95
Specify C110 when:
- Casing design requires yield strength above 95 ksi
- H2S is present at partial pressures requiring NACE MR0175 compliance
- T95 wall thickness required to achieve pressure rating is impractical or uneconomical
- High-pressure deep wells with sour gas zones
When to specify T95 over C110
Specify T95 when:
- 95 ksi yield strength is sufficient for the casing design
- H2S partial pressure is moderate — T95 with 22 HRC NACE limit provides an additional hardness safety margin
- Project specification mandates T95 explicitly
When to specify Q125 over C110
Specify Q125 when:
- The well is confirmed sweet (no H2S at NACE MR0175 threshold)
- Maximum yield strength is required for collapse resistance in deep, high-pressure wells
- Ultra-deep wells where the strength-to-weight ratio of C110 is insufficient
5. Standard Sizes and Weights
C110 is available in the full range of API casing sizes. Common production and intermediate casing dimensions:
| OD (inches) | Weight (lb/ft) | Wall Thickness (inches) | ID (inches) |
|---|---|---|---|
| 4-1/2 | 9.50 | 0.205 | 4.090 |
| 4-1/2 | 11.60 | 0.250 | 4.000 |
| 5 | 11.50 | 0.220 | 4.560 |
| 5 | 15.00 | 0.296 | 4.408 |
| 5-1/2 | 14.00 | 0.244 | 5.012 |
| 5-1/2 | 20.00 | 0.361 | 4.778 |
| 7 | 17.00 | 0.231 | 6.538 |
| 7 | 23.00 | 0.317 | 6.366 |
| 7 | 26.00 | 0.362 | 6.276 |
| 7-5/8 | 24.00 | 0.300 | 7.025 |
| 7-5/8 | 33.70 | 0.430 | 6.765 |
| 9-5/8 | 36.00 | 0.352 | 8.921 |
| 9-5/8 | 47.00 | 0.472 | 8.681 |
| 13-3/8 | 54.50 | 0.380 | 12.615 |
| 13-3/8 | 68.00 | 0.480 | 12.415 |
Contact ZC for sizes and weights not listed above. C110 is produced to order — lead times and minimum order quantities apply.
6. Connection Options for C110
C110 is available with the following connection types:
Standard API connections:
- BTC (Buttress Thread Casing) — standard choice for most C110 applications
- LTC (Long Thread Coupling) — where higher tensile efficiency is required
- STC (Short Thread Coupling) — limited use with C110 due to lower performance
Premium connections: Premium connections are strongly recommended for C110 in sour service applications where:
- High H2S partial pressure requires gas-tight seal integrity
- Deviated or horizontal wellbores impose bending loads on connections
- High axial loads from deep setting depths require high tensile efficiency
Premium connections for C110 must be independently qualified for sour service. Confirm connection body and coupling material, heat treatment, and hardness compliance with your connection manufacturer. ZC Steel Pipe holds independent patents in premium connections — contact us for sour-service-qualified premium connection options for C110.
7. Inspection and Testing Requirements
Every C110 order should specify the following inspection and testing requirements:
Mandatory per API 5CT:
- Full-length electromagnetic inspection (EMI) or ultrasonic testing (UT) for body defects
- Drift test — mandatory for all casing
- Hardness testing — mandatory, reported on MTR, maximum 30 HRC
- Charpy V-notch impact testing — mandatory per API 5CT
- Hydrostatic test — mandatory
- Visual and dimensional inspection
Additional for sour service:
- Hardness mapping — consider specifying hardness testing at multiple locations per pipe (body, upset ends) to confirm uniformity
- Sulfide stress cracking (SSC) testing per NACE TM0177 — required for some project specifications
- Hydrogen-induced cracking (HIC) testing per NACE TM0284 — required where sour water contact is anticipated
- Full material traceability — heat number, pipe number, MTR correlation
Specify third-party inspection (TPI) witness points for hardness testing and Charpy testing on all C110 orders for sour service wells. These are the two properties most likely to be disputed in the event of a field failure.
8. Procurement Checklist
When placing a C110 purchase order, include the following as a minimum:
- Grade: API 5CT C110
- OD and weight per foot (or wall thickness)
- Length range: R1 (16–25 ft), R2 (25–34 ft), or R3 (34–48 ft)
- Connection type: BTC / LTC / premium (specify thread form)
- End finish: plain end, threaded and coupled (T&C), or threaded one end (TOE)
- Applicable standard: API 5CT, latest edition
- PSL level: PSL-1 or PSL-2 (PSL-2 adds additional NDE and dimensional requirements)
- Sour service statement: "C110 shall comply with NACE MR0175/ISO 15156-2 requirements including maximum hardness 30 HRC"
- Inspection level: API 5CT inspection level 1, 2, or 3
- Third-party inspection: TPI witness requirements for hardness, Charpy, and hydrostatic test
- Mill test report (MTR): full chemical and mechanical properties, heat number, hardness results
- Coating/thread compound: confirm dope type compatible with sour service connection
9. Frequently Asked Questions
What is API 5CT C110 casing pipe?
C110 is a high-strength, sour-service-rated casing grade defined in API 5CT. It has a minimum yield strength of 110,000 psi (758 MPa) and a maximum yield strength of 140,000 psi (965 MPa), with a hardness limit of 30 HRC maximum. C110 is designed for high-pressure wells containing H2S where T95 strength is insufficient but the environment precludes use of P110 or Q125.
What is the difference between C110 and T95 casing?
C110 offers higher yield strength (110 ksi min vs 95 ksi min for T95) while maintaining sour service compliance under NACE MR0175. T95 has a stricter hardness limit (25.4 HRC API / 22 HRC NACE) versus C110's 30 HRC maximum. C110 is the correct choice when T95 yield strength is insufficient for the casing design but the well contains H2S that rules out P110.
Is C110 approved for sour service under NACE MR0175?
Yes. C110 is explicitly listed in NACE MR0175/ISO 15156-2 for use in H2S-containing environments, subject to its 30 HRC hardness limit and mandatory quench-and-temper heat treatment. Unlike P110, which is not sour-service rated, C110 is manufactured with controlled chemistry specifically to meet NACE MR0175 requirements.
What is the hardness limit for C110 under NACE MR0175?
API 5CT specifies a maximum hardness of 30 HRC for C110. NACE MR0175/ISO 15156-2 accepts C110 at this same hardness level — unlike T95, there is no gap between the API and NACE hardness limits for C110. Always confirm the applicable NACE MR0175 edition and any project-specific hardness requirements with your metallurgist before finalising grade selection.
What sizes is C110 casing available in?
C110 casing is available in standard API casing sizes from 4-1/2 inch to 20 inch OD. ZC Steel Pipe supplies C110 in seamless form across the standard size range — contact us with your OD, weight per foot, and connection requirements for availability confirmation.
Can C110 be used with premium connections?
Yes. C110 is commonly specified with premium connections in sour service wells where high H2S partial pressure, high pressure, and deviated wellbore geometry require both sour-service-rated material and a high-performance connection. Premium connections for C110 must themselves be qualified for sour service.
What is the difference between C110 and Q125 casing?
Q125 has a higher minimum yield strength (125 ksi vs 110 ksi for C110) but is not rated for sour service under NACE MR0175. C110 is the sour-service-rated alternative for high-strength applications where H2S is present. If the well is sweet, Q125 provides higher strength. If H2S is present at NACE MR0175 threshold levels, C110 is the correct grade.