9Cr-1Mo | UNS K90941 | DIN 1.7386 | ASME SA182 | Max 649°C | Extreme Sulfidation Resistance | McConomy Relative Rate 0.01–0.03 | Class 150–2500
ASTM A182 F9 is a 9Cr-1Mo ferritic/martensitic alloy steel covered under ASME/ASTM A182 — the standard specification for forged or rolled alloy-steel pipe flanges, fittings, and valves for high-temperature service. Designated UNS K90941 and equivalent to DIN 1.7386 (X12CrMo9-1), F9 contains 8.00–10.00% chromium and 0.90–1.10% molybdenum, placing it at the top of the standard Cr-Mo steel family.
The 9% chromium content gives F9 outstanding sulfidation resistance — the highest of any standard Cr-Mo grade below the modified P91 steel — and excellent oxidation resistance up to and beyond 650°C. This makes F9 the material of choice in refinery circuits where F5 (5Cr-0.5Mo) corrosion allowances are exceeded by severe sulfur-laden streams at elevated temperatures.
F9 flanges are supplied in the normalized and tempered (N&T) condition per ASTM A182, delivering minimum UTS 585 MPa and YS 380 MPa — substantially higher than F5 (415 MPa / 205 MPa) while maintaining the full service temperature capability of 649°C. They are manufactured and supplied by Tesco Steel & Engineering across ASME B16.5 (NPS ½–24, Class 150–2500) and ASME B16.47 (NPS 26–60) in all standard face types and schedules.
Understanding where F9 sits in the Cr-Mo family is essential for correct material selection. The family spans from ASTM A105 carbon steel (no alloying) through to the advanced F91 (modified 9Cr-1Mo-V), with F9 occupying the highest-chromium standard grade:
The following composition limits apply per ASTM A182 / ASME SA182 for Grade F9:
| Element | Min % | Max % | Significance |
|---|---|---|---|
| Carbon (C) | — | 0.15 | Kept low to avoid carbide embrittlement; governs weldability |
| Manganese (Mn) | 0.30 | 0.60 | Deoxidiser; lower than F11 to control hardenability |
| Silicon (Si) | 0.50 | 1.00 | Deoxidation; contributes to oxidation resistance |
| Phosphorus (P) | — | 0.030 | Controlled for temper embrittlement resistance |
| Sulfur (S) | — | 0.030 | Low sulfur for weld toughness and cleanliness |
| Chromium (Cr) | 8.00 | 10.00 | Primary sulfidation and oxidation resistance element |
| Molybdenum (Mo) | 0.90 | 1.10 | Elevated Mo (vs F5's 0.44–0.65%) for high-temp strength & corrosion resistance |
Note: P + Sn + Sb + As (temper embrittlement elements) should be specified ≤ 0.020% aggregate for critical service flanges to resist step-cool embrittlement during slow cooling through 375–575°C.
ASTM A182 F9 is supplied normalized and tempered (N&T) only. The higher Cr and Mo content compared to F5 translates to improved strength at both ambient and elevated temperatures:
| Property | F9 (9Cr-1Mo) | F5 (5Cr-0.5Mo) | F22 (2.25Cr-1Mo) | F11 Cl.2 (1.25Cr-0.5Mo) | A105 (C Steel) |
|---|---|---|---|---|---|
| UTS (min) | 585 MPa | 415 MPa | 415 MPa | 485 MPa | 485 MPa |
| YS (min) | 380 MPa | 205 MPa | 205 MPa | 275 MPa | 250 MPa |
| Elongation (min) | 20% | 20% | 20% | 20% | 22% |
| Hardness (max) | 241 HBW | 241 HBW | 241 HBW | 207 HBW | 187 HBW |
| Heat Treatment | N&T | N&T | N&T | N&T or A | N or N&T |
| Max Service Temp | 649°C | 649°C | 649°C | 593°C | 538°C |
| ASME B16.5 Group | 5.2 | 5.1 | 3.3 | 3.1 | 1.1 |
F9's UTS 585 MPa significantly exceeds F5's 415 MPa at ambient. Combined with its superior high-temperature strength retention, F9 delivers higher allowable stresses across the full temperature range up to 649°C.
Sulfidation — the reaction of steel with sulfur compounds (primarily H₂S) at elevated temperatures — is the primary corrosion mechanism in refinery processing units handling crude oil and its derivatives. API 939-C (Modified McConomy curves) is the industry reference for predicting sulfidation corrosion rates as a function of chromium content and temperature.
F9's 9% chromium content places it in the highest-resistance tier among standard Cr-Mo carbon steels. The following table shows relative sulfidation rates at a representative refinery operating temperature:
| Material | Cr Content | Relative Sulfidation Rate (API 939-C; carbon steel = 1.00) |
Typical Application |
|---|---|---|---|
| Carbon Steel (A105) | None | 1.00 (baseline) | Low-temp, low-sulfur only |
| A182 F11 (1.25%Cr) | 1.25% | ~0.30 | Mild sulfur <260°C |
| A182 F22 (2.25%Cr) | 2.25% | ~0.15 | Moderate sulfur; hydrocracker duty |
| A182 F5 (5%Cr) | 5% | ~0.05–0.10 | Heavy sulfur; vacuum columns, cokers |
| A182 F9 (9%Cr) ★ | 9% | ~0.01–0.03 | Extreme sulfur; highest Cr standard Cr-Mo grade |
| 316L Stainless Steel | 16–18% | Minimal | Aqueous / mild high-temp sulfur |
At 370°C in a stream with moderate H₂S partial pressure, F9 corrodes at approximately 3–5 times slower than F5 and 10–30 times slower than carbon steel. This dramatic improvement — driven purely by the additional 4% chromium over F5 — is what justifies the use of F9 when corrosion rate calculations based on F5 exceed the design corrosion allowance for the projected equipment life.
F9 flanges are manufactured in all ASME B16.5-recognised face types. In high-temperature refinery service, the face type selection is driven by the gasket type and the operating conditions:
| Face Type | Code | Gasket | Typical F9 Application |
|---|---|---|---|
| Raised Face | RF | Spiral wound (SS + graphite filler) | Standard refinery piping; most common for F9 |
| Ring Type Joint | RTJ | Oval / octagonal ring (same material or softer) | High-pressure Class 600–2500; FCC, hydrocracker |
| Flat Face | FF | Full-face gasket | Non-metallic lined equipment; low-pressure utility |
| Large Male / Female | LM/F | Flat ring / spiral wound | Vessel nozzle connections requiring alignment |
| Large Tongue / Groove | LT/G | Flat ring enclosed | Heat exchangers; containment-critical joints |
| Small Tongue / Groove | ST/G | Flat ring enclosed | High-pressure compact piping systems |
| Nubbin | — | Soft gasket | Special applications per purchaser specification |
RTJ flanges are standard for Class 900 and above in critical F9 service. The ring groove finish for F9 flanges should be 1.6 μm Ra (63 μin) or smoother per ASME B16.5. Ring material should be AISI 316 stainless steel or Alloy 625 for hydrogen-bearing streams.
| Standard | Designation | Notes |
|---|---|---|
| ASTM / ASME (Flanges) | A182 / SA182 Grade F9 | Forged flanges, fittings, valves |
| UNS | K90941 | Unified Numbering System |
| ASTM (Pipe) | A335 Grade P9 | Seamless ferritic alloy steel pipe |
| ASTM (Fittings) | A234 Grade WP9 | Wrought alloy steel fittings |
| ASTM (Castings) | A217 Grade C12 | Alloy steel castings for pressure service |
| DIN / EN | 1.7386 / X12CrMo9-1 | German/European standard |
| ASME B16.5 Group | Group 5.2 | Pressure-temperature ratings table |
| Weld Filler — GTAW | ER80S-B8 | AWS A5.28 — 9Cr-1Mo matching wire |
| Weld Filler — SMAW | E8018-B8 | AWS A5.5 — 9Cr-1Mo low-hydrogen electrode |
| Weld Filler — FCAW | E91T1-B8 | AWS A5.29 — for positional welding |
Flange dimensions are set by ASME B16.5 regardless of material. The table below gives Class 150 Weld Neck Raised Face (WNRF) dimensions for standard NPS sizes. Full dimensional tables for all classes (150–2500) and face types are available on our Flange Dimensions page.
| NPS | OD (mm) | BC (mm) | Bolts (no.) | Bolt ⌀ (mm) | Flange Thick. (mm) | Approx. Wt. (kg) |
|---|---|---|---|---|---|---|
| ½" | 88.9 | 60.3 | 4 | 15.7 | 9.7 | 0.4 |
| ¾" | 98.4 | 69.8 | 4 | 15.7 | 11.2 | 0.6 |
| 1" | 107.9 | 79.4 | 4 | 15.7 | 12.7 | 0.8 |
| 1½" | 127.0 | 98.4 | 4 | 15.7 | 14.3 | 1.3 |
| 2" | 152.4 | 120.6 | 4 | 19.0 | 15.9 | 2.2 |
| 3" | 190.5 | 152.4 | 4 | 19.0 | 19.0 | 4.0 |
| 4" | 228.6 | 190.5 | 8 | 19.0 | 22.4 | 7.0 |
| 6" | 279.4 | 241.3 | 8 | 22.2 | 25.4 | 13.0 |
| 8" | 342.9 | 298.4 | 8 | 22.2 | 28.6 | 21.0 |
| 10" | 406.4 | 362.0 | 12 | 25.4 | 31.8 | 36.0 |
| 12" | 482.6 | 431.8 | 12 | 25.4 | 35.0 | 54.0 |
| 14" | 533.4 | 476.2 | 12 | 28.6 | 38.1 | 75.0 |
| 16" | 596.9 | 539.7 | 16 | 28.6 | 41.4 | 105.0 |
| 18" | 635.0 | 577.8 | 16 | 31.7 | 44.4 | 135.0 |
| 20" | 698.5 | 635.0 | 20 | 31.7 | 47.6 | 165.0 |
| 24" | 812.8 | 749.3 | 20 | 35.0 | 50.8 | 270.0 |
Larger sizes NPS 26–60 available per ASME B16.47 Series A & B. Custom sizes, special bores and non-standard schedules on request. Request dimensional drawings.
| Parameter | Requirement / Value |
|---|---|
| Filler — GTAW (TIG) | ER80S-B8 (AWS A5.28) — 9Cr-1Mo matching |
| Filler — SMAW (Stick) | E8018-B8 (AWS A5.5) — low-hydrogen, dry before use |
| Filler — FCAW | E91T1-B8 (AWS A5.29) — for flat/horizontal |
| Minimum Preheat | 175°C (350°F); 200°C (390°F) for t > 25 mm or high restraint |
| Interpass Temperature | 175–320°C — maintain throughout; do not allow to cool |
| PWHT Temperature | 730–790°C (1350–1455°F) |
| PWHT Hold Time | Minimum 1 hour per 25 mm wall thickness, minimum 1 hour total |
| PWHT Heating / Cooling Rate | ≤ 150°C/hour above 427°C; slow cooling through 575–375°C critical for temper embrittlement prevention |
| Pre-PWHT cool-down | Allow weld to cool to 80–100°C before starting PWHT (martensite transformation must complete) |
| Hardness after PWHT | ≤ 22 HRC (≤ 250 HBW) per NACE MR0175 for sour (H₂S) service |
| Temper Embrittlement Risk | Control P + Sn + Sb + As; avoid slow cooling through 375–575°C in service |
| Dissimilar-metal weld to A105 or A335 P22 | Use Alloy 82/182 buttering or ER90S-B3 transition; seek metallurgical engineering review |
ASTM A182 F9 belongs to ASME B16.5 Material Group 5.2. Its pressure-temperature ratings are higher than both Group 5.1 (F5) and Group 3.1 (F11) at temperatures above 450°C, reflecting the superior high-temperature strength of the 9Cr-1Mo composition. Representative Class 150 ratings are shown below; full tables for all pressure classes are available from ASME B16.5:
| Temperature | Class 150 (psi) | Class 300 (psi) | Class 600 (psi) | Class 900 (psi) | Class 1500 (psi) | Class 2500 (psi) |
|---|---|---|---|---|---|---|
| –20 to 50°C | 275 | 720 | 1440 | 2160 | 3600 | 6000 |
| 100°C | 250 | 655 | 1310 | 1965 | 3275 | 5455 |
| 200°C | 230 | 600 | 1200 | 1800 | 3000 | 5000 |
| 300°C | 215 | 560 | 1120 | 1680 | 2800 | 4665 |
| 400°C | 200 | 520 | 1040 | 1560 | 2600 | 4335 |
| 500°C | 175 | 455 | 910 | 1365 | 2275 | 3790 |
| 593°C | 145 | 380 | 760 | 1140 | 1900 | 3165 |
| 649°C | 115 | 300 | 600 | 900 | 1500 | 2500 |
Values are indicative. Always verify against ASME B16.5 Table 2-1.1 Group 5.2 for your specific design code edition. Group 5.2 ratings at 649°C are substantially higher than Group 1.1 (A105 carbon steel), which is derated to near-zero above 538°C.
F9 flanges are specified wherever extreme sulfur content, very high temperatures, or both simultaneously exceed the capability of lower-chromium Cr-Mo grades. Key applications include:
| Standard | Scope |
|---|---|
| ASTM A182 / ASME SA182 | Specification for forged alloy steel flanges — material standard for F9 |
| ASME B16.5 | Pipe flanges and flanged fittings, NPS ½–24, Classes 150–2500 |
| ASME B16.47 | Large diameter steel flanges, NPS 26–60 |
| ASME B16.20 | Metallic gaskets including ring joint gaskets for F9 RTJ flanges |
| ASME B16.25 | Butt-welding ends geometry for F9 weld neck and similar flanges |
| ASME Section IX | Welding qualification — WPS/PQR requirements for F9 welds |
| ASME B31.3 | Process piping design code referencing F9 P-T ratings |
| API 939-C | Avoiding environmental cracking; Modified McConomy curves for sulfidation |
| NACE MR0175 / ISO 15156 | Hardness limits (≤ 22 HRC) for sour H₂S service after PWHT |
| ASTM A335 | Seamless ferritic alloy steel pipe — P9 (companion grade to F9 flange) |
| ASTM A234 | Wrought alloy steel fittings — WP9 (companion grade) |
| ASTM A217 | Alloy steel castings — C12 (companion casting grade) |
| EN 10204 / 3.1 / 3.2 | Mill test report requirements — 3.1 standard, 3.2 for critical service |
| MSS SP-44 | Steel pipeline flanges — alternative standard for large-bore F9 flanges |
| Parameter | Range / Options |
|---|---|
| Size | NPS ½" to NPS 60" (½–24 per B16.5; 26–60 per B16.47) |
| Pressure Class | 150, 300, 600, 900, 1500, 2500 (ASME B16.5); PN 6–PN 400 (EN) |
| Flange Types | Weld Neck (WN), Slip-On (SO), Blind (BL), Socket Weld (SW), Threaded (TR), Lap Joint (LJ), Long Weld Neck (LWN), Reducing, Spectacle Blind, Paddle Blind |
| Face Types | Raised Face (RF), Ring Type Joint (RTJ), Flat Face (FF), Large/Small Tongue & Groove, Large/Small Male & Female |
| Schedule / Wall | Sch 40, 80, 120, 160, XS, XXS; custom bore to order |
| Heat Treatment | Normalized and Tempered (N&T) — mandatory for F9 |
| Testing | Hydrostatic test, PMI, hardness survey (post-PWHT), UT, RT, MPT, Charpy impact on request |
| Documentation | EN 10204 3.1 MTR standard; 3.2 available; NACE / PED / ATEX certificates on request |
| Surface Finish | Raised face: 3.2–6.3 μm Ra (125–250 μin) ASME B16.5 serrated; RTJ groove: ≤ 1.6 μm Ra |
For a custom quote or to request our F9 stock list, please submit an inquiry or contact us via WhatsApp at +91-9223366922.
Questions sourced from AI search platforms, engineering procurement queries, and refinery materials-engineering practice.
ASTM A182 F9 is a 9Cr-1Mo alloy steel (UNS K90941) while F5 is 5Cr-0.5Mo (UNS K41545). The additional ~4% chromium in F9 roughly doubles its oxidation resistance and reduces sulfidation corrosion by a further 3–5 times over F5 per API 939-C Modified McConomy curves. F9 also has significantly higher ambient-temperature strength (UTS 585 MPa vs 415 MPa for F5), higher molybdenum (1.0% vs 0.5%), and correspondingly better high-temperature creep resistance. The trade-off is higher preheat requirements and more stringent PWHT control.
Yes — PWHT at 730–790°C is mandatory for ALL pressure-containing welds on F9 with no thickness exemption. The high hardenability of the 9Cr-1Mo composition produces a nearly fully martensitic HAZ upon air cooling, regardless of weld size. The as-welded HAZ hardness commonly exceeds 350–400 HV, far above the 250 HBW NACE limit and highly susceptible to hydrogen-induced cracking. Pre-weld cooling to 80–100°C before commencing PWHT is also required to ensure full martensite transformation.
ASME B16.5 Group 5.2 rates ASTM A182 F9 to a maximum temperature of 649°C (1200°F). At 649°C, Class 150 F9 flanges are rated at approximately 115 psi; Class 600 at approximately 460 psi. Above 649°C, F9 enters the range where oxidation and creep rates accelerate significantly and the specification no longer provides ratings.
F9 is the "classic" 9Cr-1Mo (UNS K90941). F91 is the "modified" 9Cr-1Mo-V-Nb-N (UNS K91560), developed in the 1970s to achieve dramatically higher creep strength through vanadium carbide and MX-type precipitate strengthening. Both grades have similar ambient UTS (~585 MPa) and similar sulfidation/oxidation resistance (same ~9% Cr). F91's advantage emerges above 500°C in creep-limited designs — power plant steam headers, hydrocracker reactors — where its allowable stress is 2–3× higher than F9. F9 remains preferred for sulfidation-dominated applications where creep is not the critical factor, and it is simpler and less costly to weld, inspect and PWHT correctly.
The matching filler metals are ER80S-B8 (GTAW/GMAW, AWS A5.28) and E8018-B8 (SMAW, AWS A5.5) — both contain nominally 9Cr-1Mo chemistry matching the base metal. FCAW equivalent is E91T1-B8. Do NOT use B6 (F5 matching) or B9 (F91 matching) consumables for F9 welds without metallurgical engineering review. Hydrogen-controlled (low-hydrogen) electrodes must be dried at 300–350°C for a minimum 1 hour before use.
ASTM A182 F9 is classified under ASME B16.5 Material Group 5.2. This group provides pressure-temperature ratings that are higher than Group 5.1 (F5) at temperatures above approximately 450°C, reflecting F9's superior elevated-temperature strength. At ambient temperature, Group 5.2 Class 150 rates at 275 psi — the same as most other Cr-Mo groups since ambient strength governs the bolt/flange combination at low temperature.
F9 requires a minimum preheat of 175°C (350°F), with 200°C (390°F) for sections over 25 mm or highly restrained joints. This is the same minimum as F5 but in practice F9 often demands the higher 200°C value more frequently due to its greater hardenability and martensite start temperature. The high Cr + Mo content of F9 produces a hard, brittle martensitic HAZ unless the cooling rate is slowed by adequate preheat and the weld is immediately PWHT'd before ambient hydrogen diffusion can trigger HAZ cracking.
Yes, F9 is permitted for sour service under NACE MR0175 / ISO 15156 provided the finished hardness (base metal + HAZ + weld) does not exceed 22 HRC (250 HBW). This hardness requirement is only reliably met after correct PWHT at 730–790°C. The relatively high as-tempered hardness of 9Cr-1Mo means that inadequate PWHT frequently produces hardness above 250 HBW in the HAZ, causing non-conformance. Full hardness survey across weld + HAZ + base metal is mandatory for sour service F9 welds.
The principal equivalents are: UNS K90941 (Unified Numbering System); DIN 1.7386 / X12CrMo9-1 (German/European); ASTM A335 P9 (companion pipe); ASTM A234 WP9 (companion fittings); ASTM A217 C12 (castings). Japanese JIS equivalent is SFVA F9 under JIS B 2220. Always verify chemical composition limits independently as equivalents are not always identical in minor element ranges.
Like all ferritic Cr-Mo steels, F9 is susceptible to temper embrittlement — a loss of toughness resulting from segregation of trace impurities (P, Sn, Sb, As) to grain boundaries during slow cooling through 375–575°C. In service, this appears as a shift of the ductile-to-brittle transition temperature (DBTT) to higher values, increasing the risk of brittle fracture during cold startups. Mitigation: specify low-impurity heats (P + Sn + Sb + As ≤ 0.020%), avoid planned slow cooling through the critical range, and conduct a step-cool embrittlement susceptibility assessment (Bruscato X factor or J factor calculation) for critical service. The step-cool test per API 934-A provides direct verification.
| Grade | A182 F9 |
| UNS | K90941 |
| DIN | 1.7386 |
| Alloy | 9Cr-1Mo |
| UTS (min) | 585 MPa |
| YS (min) | 380 MPa |
| Max Temp | 649°C |
| Hardness (max) | 241 HBW |
| Heat Treatment | N&T |
| ASME B16.5 Grp | 5.2 |
| Preheat (min) | 175°C |
| PWHT Range | 730–790°C |
| Filler (GTAW) | ER80S-B8 |
| Filler (SMAW) | E8018-B8 |
| Cr Content | 8.00–10.00% |
| Mo Content | 0.90–1.10% |
McConomy Relative Rate
Carbon Steel: 1.00 (baseline)
F11 (1.25Cr): ~0.30
F5 (5Cr): ~0.05–0.10
F9 (9Cr): ~0.01–0.03 ★
Per API 939-C (Modified McConomy)
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