The term "alloy steel pipe" covers two distinct product families that rarely overlap: chrome-moly grades used in oil and gas OCTG to resist CO2 corrosion or control sour-service hardness, and chrome-moly grades used in power generation boilers to maintain strength at extreme steam temperatures. Understanding which family applies to your application — and which standard governs the specification — is the first decision in alloy pipe procurement.
ZC Steel Pipe manufactures alloy steel pipe and tube across both families: API 5CT OCTG grades including L80-13Cr, T95, C90, and C110 for sour-service and CO2-corrosion well completions, and ASTM A213 boiler tube grades T11, T22, T91, and T92 for power generation and industrial boiler applications. Supply markets include the Middle East, Africa, South America, and Southeast Asia, with EN 10204 3.1 MTCs and third-party inspection available on all orders.
What Makes Steel Pipe "Alloy Steel"?
ASTM A941 defines alloy steel as steel containing specified quantities of alloying elements — typically manganese greater than 1.65%, silicon greater than 0.60%, or deliberate additions of chromium, molybdenum, nickel, vanadium, or other elements. In practice, "alloy steel pipe" in the oil and gas and power industries refers specifically to grades with intentional chromium and/or molybdenum additions that extend performance beyond what carbon steel can achieve.
The engineering purpose of the alloying:
| Element | Effect in OCTG | Effect in Boiler Tube |
|---|---|---|
| Chromium (Cr) | Passive oxide film — CO2 and H2S corrosion resistance | Oxidation resistance, creep resistance at high temperature |
| Molybdenum (Mo) | Hardenability — enables Q+T to high strength while staying under NACE HRC limits | Solid-solution strengthening — raises creep resistance above 500°C |
| Vanadium (V) | Grain refinement, secondary hardening in Q+T heat treatment | Carbide precipitation strengthening in T91/T92 |
| Nickel (Ni) | Toughness in 13Cr and higher CRA grades | Low-temperature toughness in select grades |
Chrome-Moly OCTG Grades (API 5CT, 11th Edition)
API Specification 5CT, 11th Edition (December 2023) defines the Cr-Mo-containing OCTG grades in two distinct categories: CRA grades (corrosion-resistant alloys, where chromium forms a passive film) and sour-service alloy grades (low Cr-Mo content, controlled hardness for H2S service).
All mechanical values from API 5CT, 11th Edition.
CRA Grades — L80 Family (13Cr, 9Cr, 3Cr)
| Grade | Cr Content | Min Yield (MPa / ksi) | Max Yield (MPa / ksi) | Min Tensile (MPa / ksi) | HRC Max |
|---|---|---|---|---|---|
| L80-3Cr | 3% Cr nominal | 552 / 80 | 655 / 95 | 655 / 95 | 23.0 |
| L80-9Cr | 9% Cr nominal | 552 / 80 | 655 / 95 | 655 / 95 | 23.0 |
| L80-13Cr | 12–14% Cr | 552 / 80 | 655 / 95 | 655 / 95 | 23.0 |
All three grades are quenched and tempered (Q+T) only. The 13Cr grade has a detailed chemistry requirement: C 0.15–0.22%, Mn 0.25–1.00%, Cr 12.0–14.0%, Ni max 0.50%, P max 0.020%, S max 0.010%. The passive chromium-oxide film that provides corrosion resistance begins to form reliably at approximately 12% Cr, which is why L80-13Cr is the standard CRA entry grade — L80-9Cr is a compromise grade for mild CO2 environments, and L80-3Cr for very mild CO2 with essentially no H2S.
For detailed L80-13Cr specifications and CO2 corrosion performance, see L80-13Cr Casing and Tubing Specifications →
For the complete API 5CT grade ladder with all chemistry and hardness limits, see the API 5CT specification tables →
Sour-Service Low-Alloy Grades — T95, C90, C110
These grades do not form a passive film. Their Cr and Mo additions serve a different purpose: enabling quench-and-temper heat treatment to high yield strength while controlling the resulting hardness within NACE MR0175 limits. Without Cr-Mo additions, achieving 95–120 ksi yield via Q+T would produce hardness above NACE limits, risking sulphide stress cracking (SSC).
| Grade | Min Yield (MPa / ksi) | Max Yield (MPa / ksi) | Min Tensile (MPa / ksi) | HRC Max | Cr Range | Mo Range |
|---|---|---|---|---|---|---|
| C90 | 621 / 90 | 724 / 105 | 689 / 100 | 25.4 | 0.4–1.5% | 0.25–0.85% |
| T95 | 655 / 95 | 758 / 110 | 724 / 105 | 25.4 | 0.4–1.5% | 0.25–0.85% |
| C110 | 758 / 110 | 827 / 120 | 793 / 115 | 29.0 | 0.4–1.5% | 0.25–1.00% |
C110 has the tightest S limit in the group (S ≤ 0.005%) — tighter than T95 (S ≤ 0.010%) — reflecting its application in deep, high-H2S partial pressure environments where sulphide inclusion morphology is critical to SSC resistance.
For T95 sour service specifications and NACE hardness guidance, see API 5CT T95 Casing Pipe Specs →
To match OCTG alloy grade to your well conditions, use the AI Pipe Grade Selector →
Chrome-Moly Boiler Tube Grades (ASTM A213)
ASTM A213 covers seamless ferritic and austenitic alloy steel boiler and heat exchanger tubes. The chrome-moly ferritic grades are designated with a T-prefix (tube form). The equivalent pipe grades under ASTM A335 carry a P-prefix (P11, P22, P91). Chemistry is identical between corresponding T and P grades — the standard and product form differ.
All mechanical values from the ASTM A213 article ASTM A213 T11, T22, and T91 → and verified against the standard.
ASTM A213 Chrome-Moly Grade Summary
| Grade | Alloy | Min Tensile (MPa) | Min Yield (MPa) | HB Max | Max Service Temp. (°C) |
|---|---|---|---|---|---|
| T11 | 1.25Cr-0.5Mo-Si | 415 | 205 | 163 | 565 |
| T22 | 2.25Cr-1Mo | 415 | 205 | 163 | 593 |
| T91 | 9Cr-1Mo-V-Nb | 585 | 415 | 250 | 650 |
| T92 | 9Cr-0.5Mo-1.8W-V-Nb | 620 | 440 | 250 | 675 |
T91 and T92 have a martensitic microstructure (higher hardness limit of 250 HB) compared to the ferritic/bainitic T11 and T22 (163 HB max). T91 must be normalised and tempered per strict ASTM A213 requirements — incorrect heat treatment is the leading cause of in-service T91 failures.
OCTG Alloy vs Boiler Tube Alloy — Key Differences
| Criterion | API 5CT OCTG Alloy (L80-13Cr, T95, C110) | ASTM A213 Boiler Tube (T11, T22, T91) |
|---|---|---|
| Governing standard | API Specification 5CT, 11th Edition | ASTM A213 / ASME SA-213 |
| Product form | Casing and tubing pipe | Heat exchanger and boiler tubes |
| Primary loading | Collapse, burst, tension | Internal steam pressure; thermal cycling |
| Corrosion mechanism | CO2, H2S, chloride attack | Oxidation, steam-side corrosion, fireside ash corrosion |
| Hardness limit | HRC 23–29 (NACE MR0175) | HB 163–250 (ASTM A213) |
| Connection threading | API BTC, LTC, premium connections | Not threaded — expanded, rolled, or welded into tube sheets |
| Interchangeable? | No | No |
These two families use related alloy chemistry but are never interchangeable. A T91 boiler tube cannot be threaded and run in a well string — it is not designed for the combined loads, connection integrity, or H2S resistance required by API 5CT. An L80-13Cr casing pipe cannot replace a T91 boiler tube — it does not meet the high-temperature creep requirements of ASME Section I.
Grade Selection Guide
Oilfield OCTG — Which Alloy Grade?
| Well Condition | Recommended Alloy Grade | Reason |
|---|---|---|
| CO2 corrosion, low/no H2S | L80-13Cr | Passive Cr film resists CO2; NACE-compliant HRC 23 limit |
| Mild CO2, no H2S | L80-3Cr or L80-9Cr | Lower cost than 13Cr where full CRA not required |
| H2S, low CO2, medium strength | C90 or T95 | Controlled hardness within NACE limits at 90–110 ksi |
| HPHT, high H2S partial pressure | C110 | Highest API 5CT alloy grade; tight S and hardness control |
| High CO2 and H2S combined | Super 13Cr or duplex CRA | Standard L80-13Cr limits in high H2S; upgrade required |
For the full CO2 and H2S CRA selection framework, see CRA Grade Selection Guide →
Power Generation — Which Boiler Tube Grade?
| Steam Temperature | Recommended Grade | Reason |
|---|---|---|
| Up to 500°C | Carbon steel (SA-192, SA-210) | Adequate creep strength; lower cost |
| 500–565°C | T11 (1.25Cr-0.5Mo) | Adequate at moderate superheat; widely used for economisers |
| 565–600°C | T22 (2.25Cr-1Mo) | Higher creep resistance; standard for legacy sub-critical boilers |
| 600–650°C | T91 (9Cr-1Mo-V) | Required for supercritical boilers; 60–70% more allowable stress than T22 at 600°C |
| Above 650°C | T92 (9Cr-0.5Mo-1.8W-V-Nb) | Ultra-supercritical applications where T91 is marginal |
Purchase Order Guidance
Specifying Alloy Steel Correctly
The single largest source of procurement errors with alloy steel pipe is specifying the chemistry without the standard and product form. "9Cr-1Mo pipe" is ambiguous — it could refer to T9 boiler tube (ASTM A213), P9 pipe (ASTM A335), or L80-9Cr OCTG (API 5CT). Each is manufactured to different dimensions, different mechanical requirements, and different inspection protocols. A PO must specify:
- Applicable standard (API 5CT, ASTM A213, ASTM A335)
- Grade designation as defined in that standard (e.g., T95, not "95 ksi chrome-moly")
- Product form (casing, tubing, boiler tube, pipe)
- Heat treatment condition (Q+T for OCTG; normalised and tempered for T91)
- Required documentation: EN 10204 3.1 MTC, chemical analysis, hardness test results
The Heat Treatment Trap
Alloy steel pipe that has not received correct heat treatment is one of the most dangerous materials in the supply chain — it passes visual inspection and dimensional check, and its chemistry is correct, but its mechanical properties and microstructure are wrong. For T95 and C110 OCTG, Q+T is mandatory; any pipe delivered without documented heat treatment records should be rejected regardless of chemical analysis. For T91 boiler tube, normalising temperature, tempering temperature, and hold time must all be on the MTC — under-tempered T91 is brittle and will fail under thermal cycling.
What to Verify on the MTC
- Heat treatment type, temperature, and soak time documented
- Hardness test result (HRC for OCTG grades, HB for boiler tube grades) — within standard limits
- Chemical analysis — confirm Cr and Mo content within the grade specification
- Tensile test: yield, tensile, and elongation at room temperature
- For T91/T92: aluminium content (Al ≤ 0.02% is critical — excess Al destroys creep resistance)
- For sour-service OCTG (T95, C90, C110): sulphur content within specification; SSC testing if required by project
- Pipe dimension certification: OD, wall thickness, length, weight
For the full MTC review procedure and what each field means, see Pipe Mill Test Certificate Guide →