ISO 9001:2015 Certified · Mumbai, India · Exporting to 96 Countries
Alloy steel flanges are forged piping components manufactured from chromium-molybdenum (Cr-Mo) low-alloy steels per ASTM A182 / ASME SA182. The addition of chromium (1–9.5%) and molybdenum (0.5–1.1%) — and in the case of Grade 91, vanadium and niobium — transforms plain carbon steel into a family of alloys that maintain structural integrity at temperatures from 300°C to 650°C where carbon steel flanges lose strength through creep deformation. Alloy steel flanges are the standard material for high-temperature piping in power generation boilers, refinery heaters, hydroprocessing units, and hydrogen manufacturing plants worldwide.
Tesco Steel & Engineering manufactures ASTM A182 / ASME SA182 alloy steel flanges in all Cr-Mo grades — F5 (5Cr-0.5Mo), F9 (9Cr-1Mo), F11 (1.25Cr-0.5Mo-Si), F21 (3Cr-1Mo), F22 (2.25Cr-1Mo), and F91 (9Cr-1Mo-V, Grade 91) — in NPS ½″ through 60″, Class 150 through 2500, in all facing types (RF, FF, RTJ, BW) and flange types per ASME B16.5, B16.47, EN 1092-1, and DIN standards. All flanges are supplied with full material test reports, PMI, hardness certificates, and PWHT records.
ASTM A182 alloy steel grades are classified by chromium and molybdenum content. Higher Cr+Mo content gives greater high-temperature strength, creep resistance, and oxidation resistance — at the cost of more demanding welding and PWHT requirements.
| Grade | UNS | Common Name | Cr (%) | Mo (%) | V (%) | C (Max) | Si (%) | P-No. |
|---|---|---|---|---|---|---|---|---|
| F11 Cl.1 & 2 | K11597 | 1.25Cr-0.5Mo-Si | 1.00–1.50 | 0.44–0.65 | — | 0.05–0.15 | 0.50–1.00 | P4 |
| F21 | K31200 | 3Cr-1Mo | 2.65–3.35 | 0.80–1.06 | — | 0.05–0.15 | ≤0.50 | P4 |
| F22 Cl.1 & 3 | K21590 | 2.25Cr-1Mo | 1.90–2.60 | 0.87–1.13 | — | ≤0.15 | ≤0.50 | P5A |
| F5 Cl.1 & 3 | K41545 | 5Cr-0.5Mo | 4.00–6.00 | 0.44–0.65 | — | ≤0.15 | ≤0.50 | P5A |
| F9 | S50400 | 9Cr-1Mo | 8.00–10.00 | 0.90–1.10 | — | ≤0.15 | 0.25–1.00 | P5B |
| F91 | K90901 | Grade 91 (9Cr-1Mo-V) | 8.00–9.50 | 0.85–1.05 | 0.18–0.25 | ≤0.12 | 0.20–0.50 | P5B |
F91 also contains: Nb 0.06–0.10%, N 0.03–0.07%, Ni ≤0.40%, Al ≤0.02%. These microalloying additions are critical to the Grade 91 creep mechanism — their absence or excess is a known cause of in-service failures.
| Grade | Class / Condition | Min YS (MPa) | Min UTS (MPa) | Min Elong. (%) | Max Hardness |
|---|---|---|---|---|---|
| F11 | Class 1 (Annealed) | 170 | 415 | 20 | HB 143 |
| F11 | Class 2 (N&T) | 310 | 515 | 20 | HB 187 |
| F22 | Class 1 (Annealed) | 205 | 415 | 20 | HB 170 |
| F22 | Class 3 (N&T) | 310 | 515 | 20 | HB 207 |
| F5 | Class 1 (Annealed) | 205 | 415 | 20 | HB 187 |
| F9 | Normalised & Tempered | 205 | 415 | 20 | HB 207 |
| F91 | Normalised & Tempered | 585 | 760 | 20 | HB 248 |
F91 minimum yield strength (585 MPa) is approximately 2.9× higher than F22 Class 3 (310 MPa) and 3.4× higher than F22 Class 1 (205 MPa). This strength advantage — combined with superior creep resistance at 600–650°C — enables significantly lighter and thinner flange designs in high-pressure steam systems.
Unlike duplex stainless steel (where PWHT must NOT be performed), all ASTM A182 Cr-Mo alloy steel grades require mandatory PWHT after welding. The purpose is to: (1) temper the hard, brittle martensite formed in the heat-affected zone (HAZ), (2) relieve residual welding stresses, and (3) bring hardness within NACE MR0175 limits for sour service.
Omitting or incorrectly performing PWHT on F91 flanges is one of the most documented causes of premature creep cracking failure in power plant steam systems. Always specify and verify PWHT records (time, temperature, cooling rate) as part of material documentation.
| Grade | PWHT Temperature (°C) | Min. Hold Time | Cooling | NACE HRC Limit |
|---|---|---|---|---|
| F11 Cl.1 | 675–760 | 1 hr / 25mm | Air or furnace | HRC 22 (HB 237) |
| F11 Cl.2 | 675–760 | 1 hr / 25mm | Air or furnace | HRC 22 (HB 237) |
| F22 Cl.1 | 675–760 | 1 hr / 25mm | Air or furnace | HRC 22 (HB 237) |
| F22 Cl.3 | 675–760 | 1 hr / 25mm, min 30 min | Air or furnace | HRC 22 (HB 237) |
| F5 | 675–760 | 1 hr / 25mm | Air or furnace | HRC 22 (HB 237) |
| F9 | 730–760 | 1 hr / 25mm, min 2 hr | Furnace cool to 315°C | HRC 22 (HB 237) |
| F91 (Grade 91) | 730–800°C (typically 760°C ±15°C) | Minimum 2 hours | Furnace cool to 315°C, then air | Not recommended for sour |
Creep rupture strength (100,000-hour, MPa) at 600°C — the higher the value, the thinner the pipe wall and lighter the flange permitted for the same pressure class. Source: ASME BPVC allowable stress data.
Approximate values for comparative reference. Use ASME BPVC Section II Part D allowable stress tables for design calculations.
| Parameter | F11 / F22 (P4, P5A) | F9 (P5B) | F91 — Grade 91 (P5B) |
|---|---|---|---|
| GTAW Filler Wire | ER80S-B2 (F11) / ER90S-B3 (F22) | ER90S-B9 | EB9 (strictly B9 only) |
| SMAW Electrode | E8018-B2 (F11) / E9018-B3 (F22) | E9015-B9 | E9015-B9 / E9018-B9 |
| Min. Preheat | 150°C | 200°C | 200°C minimum |
| Max. Interpass Temp. | 300°C | 300°C | 300°C |
| PWHT Temperature | 675–760°C | 730–760°C | 730–800°C (typically 760°C ±15°C) |
| PWHT Min. Hold | 1 hr / 25mm thickness | 2 hours minimum | 2 hours minimum |
| Post-Weld Cooling | Air cool from PWHT temp | Furnace cool to 315°C | Furnace cool to 315°C, then air |
Critical F91 note: Non-B9 consumables (e.g., ER90S-B3 used for F22) must never be used for F91 welds — they produce a weld metal with insufficient creep rupture strength that will fail in service. Document all consumable batch certificates alongside the WPS/PQR.
In refinery and chemical plant hydrogen process streams (hydrocracking, hydrotreating, reforming), the combination of elevated temperature and hydrogen partial pressure causes high-temperature hydrogen attack (HTHA) in plain carbon steel: atomic hydrogen diffuses into the steel, reacts with carbon to form methane gas, creating internal voids and decarburisation that destroys mechanical strength. API RP 941 (formerly API 941 "Nelson curves") defines safe operating envelopes for each steel type.
Cr-Mo alloy steels resist HTHA because chromium and molybdenum form stable carbides (Cr₂₃C₆, Mo₂C) that resist dissolution by diffusing hydrogen. The higher the Cr+Mo content, the higher the Nelson curve limit — meaning F5 (5Cr-0.5Mo) can be used at temperatures and H₂ partial pressures where carbon steel and even F22 would suffer HTHA. For hydrogen partial pressures above approximately 3.5 MPa at 450°C, F5 or F9 is the typical engineer's choice to remain safely above the Nelson curve.
F91 and F9 flanges on main steam, hot reheat, and feedwater heater lines in subcritical, supercritical, and ultra-supercritical (USC) coal and gas power plants. F91 is mandated by ASME B31.1 for steam above 565°C.
F22 flanges on vacuum distillation units, delayed cokers, and FCC units. F5 and F9 for hydrotreating and hydrocracking reactors where high H₂ partial pressure requires superior Nelson curve ratings.
Steam methane reformers (SMR) and pressure swing adsorption (PSA) systems operating at high hydrogen partial pressures and temperatures. F5 / F9 are standard for hydrogen-service piping per API 941 Nelson curve requirements.
Ammonia synthesis loops, urea prilling towers, and high-pressure steam headers. F22 and F11 are commonly used in heat exchangers and steam generation systems in fertiliser plants.
F11 and F22 flanges in secondary steam circuits of PWR and BWR reactors per ASME Section III requirements. Tesco supplies nuclear-grade flanges with full traceability documentation.
High-temperature heat exchangers, reactors, and distillation columns in chemical plants. F11 is widely used in moderate-temperature service (below 540°C) where the lower alloy cost offsets the performance premium of F22.
| Standard | Scope |
|---|---|
| ASTM A182 / ASME SA182 | Material standard for forged alloy steel pipe flanges, fittings, and valves for high-temperature service — primary specification for F5 through F91 |
| ASME B16.5 | Dimensional standard for pipe flanges NPS ½″ to 24″, Class 150–2500 |
| ASME B16.47 | Large diameter flanges NPS 26″ to 60″ (Series A & B) |
| ASME B31.1 | Power Piping Code — design, fabrication, PWHT, and inspection requirements for steam systems including F91 service |
| ASME B31.3 | Process Piping Code — refinery, chemical plant, and general process piping |
| ASME BPVC Sec. IX | Welding and brazing qualifications — P-Number assignments and WPS/PQR requirements |
| API 941 (Nelson Curves) | Steels for hydrogen service — defines safe operating limits for Cr-Mo grades in H₂ environments |
| NACE MR0175 / ISO 15156 | Materials for H₂S sour service — hardness limits and heat treatment requirements |
| EN 1092-1 | European flanges — PN-rated alloy steel flanges for EU projects |
Technical questions about ASTM A182 Cr-Mo alloy steel flanges, Grade 91, PWHT, and Nelson curves — answered for engineers and procurement teams.
A five-step engineering decision process for specifying ASTM A182 Cr-Mo alloy steel flanges.
Identify the maximum continuous service temperature. Below 425°C, carbon steel (ASTM A105) is usually sufficient. Above 425°C, move to Cr-Mo alloy steel. Above 600°C, only F9 or F91 (Grade 91) provide adequate creep rupture strength per ASME B31.1 allowable stress tables.
If the process fluid contains hydrogen at elevated temperature and pressure, consult API RP 941 Nelson curves. Plot your operating temperature vs H₂ partial pressure and confirm the selected grade's curve lies safely above your operating point. Above approximately 3.5 MPa H₂ partial pressure at 450°C, F5 (5Cr-0.5Mo) or higher is required.
If H₂S is present, verify the selected grade achieves hardness ≤ HRC 22 (HB 237) in the required heat-treated condition. F22 Class 1 (annealed, HB 170) is the preferred sour-service choice. F91 is generally avoided for H₂S environments due to hardness limitations and PWHT complexity.
All Cr-Mo alloy steel grades require PWHT. Confirm the ASME P-Number (P4 for F11/F21; P5A for F22/F5; P5B for F9/F91) and ensure a qualified WPS/PQR exists. For F91, verify that B9 consumables are specified, preheat ≥200°C is maintained, and PWHT is 730–800°C for a minimum of 2 hours with furnace cool to 315°C.
Confirm the required pressure class (150, 300, 600, 900, 1500, or 2500) against the ASME B16.5 P-T rating table for the alloy group. Alloy steel flanges carry higher allowable pressures than carbon steel at elevated temperatures — use this advantage to justify the material premium. Supply ASME B16.5 P-T rating confirmation with your inquiry to receive an accurate quotation.
F5 · F9 · F11 · F21 · F22 · F91 (Grade 91) — NPS ½″ to 60″ — Class 150 to 2500 — PWHT supplied — Full MTR, PMI, hardness records
Get Quote WhatsAppEvery alloy steel flange dispatched by Tesco Steel & Engineering undergoes individual protection before packing — bore plugs, face caps, plastic wrapping — then is loaded into wooden export crates or polybag-lined cases. PWHT records, MTRs, and PMI certificates are packed inside each crate. Below is a representative selection of our alloy steel flange export dispatch.
