A complete technical comparison of three critical chromium-molybdenum alloy steel forging grades — covering chemical composition, mechanical properties, temperature limits, NACE MR0175 sour service compliance, API 941 Nelson Curve, weldability, PWHT, industry applications, and a structured selection guide for engineers.
When specifying a chromium-molybdenum alloy steel forging for high-temperature, high-pressure service, three grades dominate engineering decisions worldwide: ASTM A182 F11 (1.25Cr-0.5Mo), ASTM A182 F22 (2.25Cr-1Mo), and ASTM A182 F91 (9Cr-1Mo-V). Each occupies a distinct engineering niche defined by operating temperature, pressure rating, weldability, sour-service requirements, and lifecycle cost. This guide delivers an authoritative, side-by-side technical comparison to help engineers, metallurgists, and procurement specialists make the right material selection the first time.
All three grades fall under ASTM A182 / ASME SA182 — the Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, Valves, and Parts for High-Temperature Service. Despite sharing the same standard family, their performance envelopes are dramatically different. The Cr-Mo grading system encodes a fundamental engineering tradeoff: each increment in chromium and molybdenum content delivers superior high-temperature strength and oxidation resistance, at the cost of greater fabrication complexity and material cost.
The table below presents complete ASTM A182 chemical composition requirements for F11, F22, and F91. The defining difference — F91's vanadium (V), niobium (Nb), and nitrogen (N) additions — creates the MX-type carbonitride precipitation hardening mechanism that enables F91's superior creep rupture strength above 600°C.
| Element | F11 · 1.25Cr-0.5Mo | F22 · 2.25Cr-1Mo | F91 · 9Cr-1Mo-V |
|---|---|---|---|
| Carbon (C) | 0.05–0.15% | 0.05–0.15% | 0.08–0.12% |
| Manganese (Mn) | 0.30–0.80% | 0.30–0.60% | 0.30–0.60% |
| Silicon (Si) | 0.50–1.00% | ≤ 0.50% | 0.20–0.50% |
| Chromium (Cr) ★ | 1.00–1.50% | 2.00–2.50% | 8.00–9.50% |
| Molybdenum (Mo) ★ | 0.44–0.65% | 0.87–1.13% | 0.85–1.05% |
| Vanadium (V) | — | — | 0.18–0.25% |
| Niobium / Columbium (Nb) | — | — | 0.06–0.10% |
| Nitrogen (N) | — | — | 0.03–0.07% |
| Aluminium (Al) max | — | — | 0.04% |
| Phosphorus (P) max | 0.025% | 0.025% | 0.020% |
| Sulfur (S) max | 0.025% | 0.025% | 0.010% |
★ Chromium and molybdenum are the primary alloying elements governing high-temperature strength and oxidation resistance. The F91 additions of vanadium, niobium, and nitrogen deliver creep rupture strength approximately 3× greater than F22 at temperatures above 600°C by forming fine MX-type carbonitride precipitates (principally VN and NbC) that pin dislocation networks within the tempered martensite matrix. F11 and F22 rely solely on solid-solution strengthening from Cr and Mo — which is why their effective temperature ceilings are significantly lower.
| Mechanical Property | F11 · Class 2 | F22 · Class 3 | F91 |
|---|---|---|---|
| Min Tensile Strength | 485 MPa (70 ksi) | 655 MPa (95 ksi) | 725 MPa (105 ksi) |
| Min Yield Strength (0.2% offset) | 275 MPa (40 ksi) | 517 MPa (75 ksi) | 585 MPa (85 ksi) |
| Min Elongation (2 in/50 mm) | 20% | 20% | 20% |
| Min Reduction of Area | 45% | 45% | 45% |
| Max Hardness (NACE MR0175) | 22 HRC / 237 HBW | 22 HRC / 237 HBW | Not applicable |
| Typical Hardness Range | 143–187 HBW | 170–255 HBW | 197–269 HBW |
| Standard Heat Treatment | Normalize + Temper | Normalize + Temper | Normalize + Temper |
| Dual Certification | A739 Grade B11 | A739 Grade B22 | — |
ASTM A182 F22 has three classes: Class 1 (annealed, 207 MPa min yield — rarely specified), Class 3 (normalized and tempered, 517 MPa min yield — the dominant industrial specification), and an unlisted Class with higher strength for special applications. Always confirm Class 3 with your forging supplier. Receiving Class 1 material where Class 3 was intended is a serious specification error. For a full list of available shapes, dimensions, and delivery standards, see F22 forging shapes and specifications from Jiangsu Liangyi.
Continuous service temperature capability is the most critical differentiator between these three grades. The chart below visualises the usable temperature range for sustained high-pressure service in the creep-limited regime, normalised to F91's 650°C ceiling. Above these limits, the steel's creep rupture strength falls to levels that would require unacceptable wall thickness increases or component redesign.
Above their respective temperature limits, two failure mechanisms dominate. Creep is time-dependent plastic deformation under sustained stress — components deform and ultimately rupture at stresses well below room-temperature yield strength. The high chromium content of F91 (9% Cr) is the primary oxidation inhibitor, explaining why F91 is used in steam environments exceeding 600°C where F22 (2.25% Cr) would experience unacceptable surface scaling. For ultra-supercritical power plants targeting steam temperatures above 600°C, F91 is the only viable ASTM A182 Cr-Mo grade.
| Welding Parameter | F11 | F22 | F91 |
|---|---|---|---|
| Minimum Preheat | 150–200°C (300–390°F) | 200–250°C (390–480°F) | 200–260°C (390–500°F) |
| Max Interpass Temperature | 315°C (600°F) | 315°C (600°F) | 300°C (570°F) |
| PWHT Temperature Range | 675–760°C | 675–760°C | 730–800°C |
| PWHT Mandatory? | Strongly recommended | Yes — mandatory | Yes — mandatory & critical |
| Filler Metal (SMAW) | E8018-B2 | E9018-B3 | E9015-B9 |
| Filler Metal (GTAW / TIG) | ER80S-B2 | ER90S-B3 | ER90S-B9 |
| Overall Weldability Rating | Good — routine | Moderate — standard PWHT | Complex — specialist required |
| H₂ Bake-Out (thick sections) | Not mandatory | Recommended >50mm | Required |
F91 welding is a specialised discipline that has caused numerous documented in-service failures when performed without proper procedure qualification. The martensitic transformation occurs in a narrow temperature window. Failure to maintain preheat throughout the weld cycle — or cooling too rapidly before PWHT — creates untempered martensite zones with hardness values that can exceed 400 HBW and near-zero toughness. EPRI guidance consistently notes that F91 weld failures are almost always procedural, not material defects.
When welding F22 forgings, never allow the assembly to cool below preheat temperature before PWHT. For sections above 50 mm thickness, a post-weld hydrogen bake-out at 300–350°C for 2–4 hours is recommended per AWS D10.8 before initiating PWHT at 675–760°C. Failure to bake out hydrogen is the most common cause of hydrogen-induced delayed cracking in thick-section F22 weldments.
For oil and gas applications involving hydrogen sulfide (H₂S) — classified as sour service — material selection must comply with NACE MR0175 / ISO 15156. This standard limits the maximum hardness of Cr-Mo low alloy steels to 22 HRC (237 HBW) to prevent sulfide stress cracking (SSC). SSC is a form of hydrogen embrittlement that can cause catastrophic brittle fracture at stresses below the nominal yield strength.
F91 is not listed in NACE MR0175 for sour service because its 9% chromium martensitic microstructure, even after correct PWHT, typically has hardness levels exceeding the 22 HRC NACE limit when heat-treated to develop full high-temperature mechanical properties. For sour service in oil and gas, F22 is the highest-performing Cr-Mo grade qualified under NACE MR0175, making it the definitive choice for wellhead equipment, BOP bodies, and high-pressure valve components in H₂S-containing environments. Jiangsu Liangyi supplies NACE MR0175-compliant 2.25Cr-1Mo open die forgings and seamless rolled rings for these critical applications.
In hydroprocessing and refinery service, all three grades are assessed against the API 941 Nelson Curve for resistance to High-Temperature Hydrogen Attack (HTHA). HTHA occurs when atomic hydrogen diffuses into the steel at elevated temperature and partial pressure, reacting with carbides to form methane that creates internal fissures and leads to brittle failure. The fundamental principle is: higher chromium content = larger safe operating envelope on the Nelson Curve.
1.25Cr-0.5Mo Nelson Curve Line: Provides a moderate safety margin. Suitable for catalytic reformer piping, atmospheric distillation headers, and refinery duties where hydrogen partial pressures are modest. Do not use in high-pressure hydroprocessing reactors where conditions approach or exceed the F11 Nelson Curve boundary.
2.25Cr-1Mo Nelson Curve Line: The industry standard for high-pressure hydroprocessing reactors, hydrocracking units (HCU), and hydrotreating reactors. Most critical refinery reactor nozzles, flanges, and tube sheets at hydrogen partial pressures above the F11 safe zone are specified as F22. Widely used in hydrogen service at temperatures up to 550°C.
9Cr-1Mo Nelson Curve Line: Provides the largest Nelson Curve safety margin of the three grades. Less commonly used in refinery environments than F22 due to complex weldability, but appropriate for the most aggressive HTHA-risk service conditions where maximum hydrogen resistance combined with high-temperature capability are simultaneously required.
Following the 2010 Tesoro Anacortes refinery explosion — attributed to HTHA failure in carbon steel heat exchanger tubing operating beyond its Nelson Curve limit — API 941 was substantially revised with more conservative boundaries. Engineers specifying forgings for hydrogen service above 260°C (500°F) must consult the most current edition of API 941. Consider upgrading from F11 to F22 for any equipment operating close to the F11 Nelson Curve boundary — the incremental material cost is negligible versus the consequences of an HTHA failure.
Use this 6-step structured decision framework to identify the correct ASTM A182 Cr-Mo grade. Work through the questions in order — the first criterion that eliminates a grade is almost always the most critical engineering constraint.
| Cost Factor | F11 | F22 | F91 |
|---|---|---|---|
| Raw material (index) | 1.0× baseline | 1.4–1.7× | 2.5–3.5× |
| Forging process premium | Low | Low–Medium | High (tight window) |
| Heat treatment complexity | Standard | Standard | Elevated (precise cycle) |
| Welding fabrication premium | Baseline | +15–25% | +60–100% |
| NDT and inspection | Standard | Standard | Enhanced (critical) |
| Standard lead time (open die) | 15–25 working days | 20–35 working days | 25–45 working days |
| F22 value vs F11/F91 | — | Best overall value | — |
F22 consistently delivers the best total cost of ownership across the widest range of industrial applications. Its alloy cost premium over F11 is modest, its weldability is manageable with standard qualified procedures, and its temperature and pressure capability covers the majority of oil & gas, nuclear, and conventional power generation requirements. Specifying F91 where F22 is thermally adequate is a common engineering overspecification error that adds significant cost — in both material and fabrication — without engineering benefit.
The selection between ASTM A182 F11, F22, and F91 is a structured engineering decision, not a preference. The summary below provides definitive final guidance for the most common application scenarios.
1.25Cr-0.5Mo · Max 450°C · Cost-optimised service
Operating temperatures are below 450°C, hydrogen partial pressure is moderate (within the F11 Nelson Curve), sour service hardness limits are achievable with standard heat treatment, and cost minimisation is a primary constraint. Ideal for subcritical steam piping, moderate-duty flanges, and refinery sections without severe hydroprocessing exposure. If conditions approach the F11 Nelson Curve boundary at any point in the operating range, upgrade to F22.
2.25Cr-1Mo · Up to 550°C · Industry standard for critical service
You need the best combination of high-temperature strength, sour service qualification, API 6A compliance, and manageable weldability. F22 is the correct choice for oil & gas wellhead equipment, BOP components, hydroprocessing reactors, nuclear forgings, and power generation components up to 550°C. It is the most versatile and widely-used Cr-Mo forging grade globally, and should be the first choice for most critical industrial applications where temperatures and conditions fall within its performance envelope. Jiangsu Liangyi manufactures F22 forgings from 30 kg to 30,000 kg with lead times of 20–35 days — view capacity, lead times, and get a quote.
9Cr-1Mo-V · Up to 650°C · Maximum creep performance
Service temperatures exceed 550°C — as in supercritical and ultra-supercritical steam power plants — or when maximum creep rupture strength is required for long-term elevated-temperature service. Accept the higher material cost and weld complexity as the engineering price of the performance gain. Do not use F91 in sour (H₂S) service environments under any circumstances — it is not listed in NACE MR0175 and is not compliant regardless of heat treatment condition. When specifying F91, ensure fabricators hold current F91-specific Welding Procedure Specifications (WPS) and have recent production qualification records (PQR).
Established in 1997 in Jiangyin, Jiangsu, China. ISO 9001:2015 certified manufacturer of ASTM A182 F11, F22, and F91 open die forgings and seamless rolled rings. All parts manufactured in accordance with applicable international standards. Annual capacity 120,000 tons · Max single piece 30,000 kg · Ring OD up to 6,000 mm · 50+ countries served.