Alloy Steel Pipe Grades: Chrome-Moly OCTG and Boiler Tube Guide

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:

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)

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).

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

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.

The T-prefix versus P-prefix distinction is not cosmetic. T91 (ASTM A213, tube) and P91 (ASTM A335, pipe) share identical UNS K91560 chemistry — 9Cr-1Mo-V-Nb — but are governed by different standards with different product form requirements and different code acceptance criteria.

Wall Thickness Design: T91 vs P22 at Supercritical Steam Conditions

The allowable stress advantage of T91 over P22 at elevated temperature directly determines wall thickness — and therefore pipe weight and material cost. The following calculation uses ASME Section I allowable stresses.

Service conditions: 20-inch (508 mm) OD steam header, 24 MPa operating pressure, 593°C (1100°F) operating temperature.

Allowable stresses from ASME Section II Part D:

• T91 allowable stress S = 103 MPa at 593°C
• P22 allowable stress S = 62 MPa at 593°C

Wall thickness formula (thin-wall approximation, per ASME): t = P × D / (2 × S × E), where E = 1.0 for seamless pipe.

T91 requires 59.2 mm wall versus P22's 98.4 mm — a 40% reduction in wall thickness and approximately 40% less weight per metre. For a 1-km supercritical steam header, the material saving alone justifies the T91 premium at current plate and tube pricing. This is why supercritical and ultra-supercritical power plants standardised on T91 from the 1990s onward: the engineering economics are decisive, not just the temperature capability.

OCTG Alloy vs Boiler Tube Alloy — Key Differences

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?

For the full CO2 and H2S CRA selection framework, see CRA Grade Selection Guide →

Power Generation — Which Boiler Tube Grade?

When NOT to Use Chrome-Moly Alloy Grades

Grade selection errors with chrome-moly alloys tend to follow predictable patterns. The table below covers the most common conditions where a chrome-moly grade is either the wrong choice or requires an explicit upgrade decision before specifying.

The most expensive mistake in this table is the first row. L80-13Cr is a CO2 corrosion-resistant grade, and procurement teams that select it for wells with concurrent H2S often do so because the 13Cr label implies general corrosion resistance. It does not — the passive Cr2O3 film that resists CO2 attack is attacked by H2S above NACE MR0175/ISO 15156 threshold partial pressures. When both CO2 and H2S are present above those thresholds, the correct approach is Super 13Cr (lower C, higher Mo, added Ni) or duplex stainless, not standard L80-13Cr.

Alloy Steel Pipe Failure Modes to Specify Against

The three failure modes below are the most frequent sources of in-service failures with chrome-moly alloy grades in both oilfield and power plant service. Each has a specific procurement fix that costs nothing to implement.

Failure Mode 1 — Under-tempered T91 Brittle Fracture

Mechanism: T91 tube normalised at correct temperature but tempered at 700°C (below the 730°C minimum per ASTM A213). Martensite is incompletely converted to tempered martensite; the tube retains elevated residual stress and reduced impact toughness. Under the thermal cycling of boiler startup/shutdown, the under-tempered zone initiates a brittle fracture at a notch (weld toe, mechanical damage mark, or corrosion pit).

Diagnostic: Hardness is within the 250 HB limit but at the upper range (230–250 HB). Impact energy from Supplementary Requirement S5 testing is below the minimum for the specified test temperature. PMI confirms 9Cr-1Mo-V-Nb chemistry (correct), but MTC shows no tempering temperature entry or a value below 730°C.

Fix: Specify ASTM A213 T91 with documented tempering temperature ≥ 730°C on MTC as a mandatory hold point before goods acceptance. Reject any MTC where the tempering temperature field is blank, "per standard," or below 730°C. Cost: zero — it is a paperwork check.

Failure Mode 2 — T95 Hardness Exceedance at Sour Service Well

Mechanism: T95 casing supplied with HRC 27 at one cross-section test location (API 5CT requires a minimum of one hardness test per heat at 2 locations). The excess hardness was within a few Rockwell points of the NACE limit (HRC 25.4) and within the normal scatter of a compliant heat. In the well, H2S at 0.005 MPa partial pressure diffuses into the HAZ of connections during well loading. SSC initiates at the hardest zone within 3–6 months.

Diagnostic: Connection failure under static well load with no overpressure event. Metallographic cross-section shows transgranular cracking with no deformation (brittle SSC signature). Hardness mapping of the failed connection zone identifies the hard spot.

Fix: Specify hardness testing at all API 5CT Section 10 required locations (minimum 2 per heat, 4 per heat for supplementary testing); add "HRC ≤ 25.4 at each test location, not average" to the PO. For high-H2S partial pressure (> 0.01 MPa), require supplementary SSC testing per NACE TM0177 Method A.

Failure Mode 3 — L80-13Cr Used in H2S-Present Well

Mechanism: L80-13Cr tubing run in a gas condensate well where H2S is present at 0.002 MPa partial pressure. The grade was selected for CO2 corrosion resistance (which it provides). The passive Cr2O3 film that resists CO2 does not resist H2S; at H2S partial pressures above the NACE MR0175/ISO 15156 threshold (typically 0.0003 MPa for 13Cr), SSC initiates at inclusions, welds, and connection surfaces.

Diagnostic: Failure within the first 12 months of production, concentrated at connections and machined surfaces (not in the pipe body). Fracture surface shows intergranular cracking without corrosion pitting on the pipe body — distinguishable from CO2 pitting (which occurs on the pipe body, not at connections).

Fix: Confirm H2S partial pressure against the NACE MR0175/ISO 15156 limits for 13Cr before grade selection. When H2S is above the threshold, upgrade to Super 13Cr (lower C, higher Mo, added Ni) which has a higher H2S resistance envelope, or to duplex stainless steel for severe combined CO2/H2S environments.

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.

Wrong PO: "ASTM A213 T91 boiler tube, OD 50.8mm × 6.3mm wall, seamless."

What ships: The mill applies normalise and temper heat treatment but does not document the tempering temperature on the MTC. Tempering at 700°C instead of the 730°C minimum per ASTM A213 produces a tube with correct chemistry, dimensions, and tensile properties but with under-converted martensite — brittle at thermal cycling. The hardness may appear within the 250 HB maximum limit because tempering at 700°C versus 730°C produces only approximately 10 HB difference.

Correct PO additions: "ASTM A213 T91, normalised and tempered, tempering temperature minimum 730°C documented on MTC; Al ≤ 0.02% (excess Al destroys creep resistance — verify separately from tensile and hardness); Charpy impact test per ASTM A213 Supplementary Requirement S5 if required by project specification."

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 →