1. Introduction
A217 cast steels WC6 and WC9 (industry shorthand for the 1¼Cr–½Mo and 2¼Cr–1Mo cast grades, respectively) are purpose-designed low-alloy Cr–Mo steels for pressure-retaining components in elevated-temperature service.
WC6 is typically specified where good toughness and moderate creep strength are required up to roughly ~520–540 °C;
WC9 provides higher long-term strength and oxidation resistance and is used where service temperatures and creep demand approach ~550–580 °C.
Successful use of these materials depends as much on foundry practice, heat treatment and welding discipline as on the nominal chemistry—poor processing is the root cause of most field failures.
This review compares WC6 vs WC9 from metallurgy and properties through fabrication, service use, competing alternatives, and practical procurement guidance.
2. What are A217 Alloy Cast Steels WC6 and WC9?
ASTM A217 Standard Context
ASTM A217 / ASME SA217 is the globally recognized specification governing martensitic and austenitic alloy cast steels
used in pressure-retaining components—valves, flanges, fittings, headers, and reactors—exposed to high-temperature service (≥343 °C / 650 °F).
- Historical Note: First issued in 1937, the standard has undergone continuous refinement, with the 2024 revision updating composition tolerances, heat treatment requirements,
and mechanical property ranges to align with modern energy infrastructure, including ultra-supercritical power generation and advanced petrochemical reactors. - Within the standard, WC6 and WC9 fall under the martensitic Cr–Mo alloy family.
Unlike austenitic grades (e.g., C12, CN7M) that rely on high nickel (>9 wt%) for corrosion resistance,
martensitic alloys contain low Ni (<0.5 wt%) and derive their performance primarily from chromium (Cr) and molybdenum (Mo) additions.
This fundamental distinction makes WC6/WC9 more suitable for high-load, creep-limited environments, where austenitics—though more corrosion resistant—would soften or lose strength.
3. Chemical Composition of A217 WC6 vs WC9
The performance distinction between WC6 and WC9 alloys lies primarily in their chemical composition, which governs microstructure evolution, creep resistance, oxidation behavior, and weldability.
Nominal Composition Ranges (ASTM A217)
| Element | WC6 (1.25Cr–0.5Mo) (wt%) | WC9 (2.25Cr–1Mo) (wt%) | Function in Alloy |
| Carbon (C) | 0.15 – 0.30 | 0.15 – 0.30 | Provides martensitic hardenability and forms carbides for strength; excessive carbon risks brittleness. |
| Manganese (Mn) | 0.50 – 1.00 | 0.50 – 1.00 | Improves hardenability and acts as a deoxidizer; too much reduces creep strength. |
| Silicon (Si) | 0.50 – 1.00 | 0.50 – 1.00 | Enhances oxidation resistance (SiO₂ film) and strengthens ferrite matrix. |
| Chromium (Cr) | 1.00 – 1.50 | 2.00 – 2.50 | Improves oxidation and corrosion resistance; stabilizes carbides (M₇C₃, M₂₃C₆). |
| Molybdenum (Mo) | 0.44 – 0.65 | 0.90 – 1.20 | Provides creep resistance; forms Mo₂C carbides to resist grain boundary sliding. |
| Nickel (Ni) | ≤ 0.50 | ≤ 0.50 | Residual element; improves toughness but limited to prevent retained austenite. |
| Sulfur (S) | ≤ 0.030 | ≤ 0.030 | Controlled impurity; excess causes hot cracking during casting/welding. |
| Phosphorus (P) | ≤ 0.030 | ≤ 0.030 | Controlled impurity; excess leads to temper embrittlement in service. |
| Iron (Fe) | Balance | Balance | Forms the ferritic/martensitic matrix. |
4. Mechanical Properties & Elevated-Temperature Behavior of A217 WC6 vs WC9
Room-Temperature Mechanical Properties
Both WC6 and WC9 alloys are designed to provide high strength and toughness at ambient and moderate service conditions.
The values below are from ASTM A217 requirements and industrial practice after standard heat treatment.
| Property | WC6 (1.25Cr–0.5Mo) | WC9 (2.25Cr–1Mo) | Remarks |
| Tensile Strength (MPa) | 485 – 655 | 585 – 760 | WC9 has higher Cr & Mo → stronger carbide strengthening. |
| Yield Strength (0.2% offset, MPa) | ≥ 275 | ≥ 380 | Higher Cr/Mo in WC9 increases yield resistance. |
| Elongation (%) | 18 – 22 | 17 – 20 | WC6 slightly more ductile; WC9 slightly stronger but less ductile. |
| Hardness (HB) | 150 – 190 | 170 – 220 | WC9 tends to be harder, reflecting higher carbide density. |
| Charpy V-Notch Impact Energy (J, RT) | 40 – 60 | 35 – 50 | WC6 retains slightly better toughness at room temperature. |
Elevated-Temperature Strength & Creep Resistance
In high-temperature service, creep rupture properties are the critical design parameter for pressure-retaining components such as valves, headers, and piping.
| Property | WC6 (1.25Cr–0.5Mo) | WC9 (2.25Cr–1Mo) | Remarks |
| Max Continuous Service Temp (°C) | ~538 °C (1,000 °F) | ~595 °C (1,100 °F) | WC9 tolerates higher temperatures due to 2.25% Cr + 1% Mo. |
| 100,000 h Creep Rupture Strength @ 538 °C | ~85 MPa | ~120 MPa | WC9 exhibits ~40% higher creep rupture resistance. |
| 100,000 h Creep Rupture Strength @ 595 °C | Not recommended (rupture <50 MPa) | ~75 MPa | WC9 is suitable up to 595 °C; WC6 loses strength. |
| Oxidation Resistance | Moderate | High | Cr content (2.25% in WC9) forms more protective Cr₂O₃ film. |
5. Processing Technology of A217 WC6 vs WC9
The successful manufacture and deployment of ASTM A217 Grade WC6 and WC9 alloy cast steels depends on precisely controlled processing technology.
Because these alloys are used in critical, high-temperature, pressure-retaining components such as valves, headers, turbine casings, and reactor housings, even small deviations in processing can lead to premature failure.
Welding: Preventing brittle martensite and cracking
- Preheat: Thick sections require preheat (commonly 180–250 °C) to slow cooling and reduce hydrogen-induced and martensite formation.
Exact preheat depends on thickness, section restraint, and welding procedure qualification. - Consumables: Use low-hydrogen electrodes / filler metals specifically qualified for Cr–Mo service and creep applications.
Select fillers compatible with base metal chemistry and required post-weld properties. - Interpass temperature control: Maintain within qualified limits to avoid local hardening.
- PWHT (Post Weld Heat Treatment): Mandatory in most high-temperature service cases.
PWHT restores temper to HAZ and reduces residual stress—common practice is tempering/soaking in the 600–700 °C range (procedure must be qualified;
time at temperature depends on section thickness). Field PWHT must be executed per a qualified WPS/PQR. - Avoiding brittle martensite: Rapid cooling can form untempered martensite in HAZ—hence preheat and PWHT are indispensable.
Machining: Overcoming hardness and workability
- Structure after HT: Tempered martensite/bainite has relatively high strength; use appropriate carbide tooling, low cutting speeds and flood coolant.
- Distortion control: Machining should account for possible distortion when removing restraint—stress-relief heat treatment sequencing and finishing passes minimize warpage.
- Surface integrity: Avoid surface grinding temperatures that can re-harden surfaces.
Casting Considerations
WC6 and WC9 are often manufactured as large sand-cast components (valves, steam chests, turbine casings up to 10 tons).
Casting requires meticulous process control to avoid metallurgical defects.
- Melting practice: For critical castings, use VIM/VAR or argon-shielded melting to control impurities and inclusion content. Clean melts reduce fatigue and creep initiation sites.
- Gating and risering: Design for directional solidification, adequate feeding and chills to eliminate shrinkage porosity.
Castings for pressure service often require radiographic acceptance levels. - Heat treatment after casting: Normalize/anneal cycles relieve stresses and refine microstructure prior to tempering.
Final tempering produces the desired balance of strength/toughness. - NDT: Radiography, ultrasonic testing and acceptance criteria per code required for pressure components.
6. Heat Treatment & Surface Treatment of A217 WC6 vs WC9
Heat Treatment
The performance of ASTM A217 WC6 (1.25Cr–0.5Mo) and WC9 (2.25Cr–1Mo) alloys is critically dependent on heat treatment, which governs their microstructure, mechanical properties, and high-temperature service life.
| Step | WC6 (1.25Cr–0.5Mo) | WC9 (2.25Cr–1Mo) | Purpose |
| Austenitizing | 900–955 °C (1,650–1,750 °F), hold 2–4 h | 930–980 °C (1,710–1,800 °F), hold 2–4 h | Dissolve carbides, homogenize chemistry, refine grains |
| Quenching | Air cool or oil spray for thick sections | Air cool (smaller castings), oil/polymer for heavy sections | Avoid retained austenite, minimize cracking |
| Tempering | 660–705 °C (1,220–1,300 °F), 2 cycles | 675–740 °C (1,245–1,360 °F), 2 cycles | Precipitate secondary carbides, improve creep resistance, reduce brittleness |
| PWHT (welding) | 621–677 °C (1,150–1,250 °F) | 650–705 °C (1,200–1,300 °F) | Relieve stresses, temper HAZ martensite |
Surface Treatment
Although WC6 and WC9 provide inherent oxidation and creep resistance, surface engineering can extend component life in corrosive or erosive environments.
| Treatment | Method | Benefit | Typical Application |
| Shot Blasting / Grit Blasting | High-velocity abrasive particles | Removes oxide scale, improves surface cleanliness, enhances fatigue life | Post-heat treatment cleaning |
| Nitriding | Gas or plasma nitriding (500–550 °C) | Improves surface hardness (up to 900 HV), wear resistance | Valve seats, moving parts in turbines |
| Aluminizing | Pack cementation or vapor deposition | Forms protective Al₂O₃ layer, increases oxidation resistance >600 °C | Power plant superheaters, petrochemical reactors |
| Chromium-Rich Overlay Welding | Hardfacing with high-Cr electrodes or strip cladding | Enhances hot corrosion and erosion resistance | Boiler valves, refinery equipment |
| Diffusion Coatings (Al, Si, Cr) | High-temperature diffusion process | Improves hot corrosion and carburization resistance | Furnace components |
| Thermal Spray Coatings (HVOF, Plasma) | WC-Co, Cr₃C₂-NiCr cermet coatings | Resists erosive slurry and steam impingement | Pump impellers, slurry valves |
7. Typical Applications of A217 WC6 vs WC9
A217 WC6 and WC9 alloys are martensitic Cr-Mo low-alloy steels engineered for high-temperature, high-pressure service.
Their combination of tempered martensite microstructure, creep strength, and thermal stability makes them indispensable in power generation, petrochemical, and process industries.
Power Generation Industry
WC6 (1.25Cr–0.5Mo):
- Subcritical steam service (≤538 °C)
- Components:
-
- Boiler headers and elbows
- Superheater and reheater elements
- Turbine casing sections for intermediate pressures
WC9 (2.25Cr–1Mo):
- Supercritical and ultra-supercritical steam (538–595 °C)
- Components:
-
- High-pressure superheater and reheater headers
- Steam chest valves
- Turbine inlet casings
Petrochemical and Refinery Equipment
- WC6:
-
- Furnace components (tube sheets, combustion chambers)
- Intermediate-temperature heaters (≤538 °C)
- WC9:
-
- Reactor and heater tubes operating up to 595 °C
- Catalyst bed support structures
- High-pressure petrochemical valves
Steam and Heat Transfer Equipment
- Headers and Manifolds: Both WC6 and WC9 are widely used in steam headers where temperature and pressure fluctuate cyclically.
- Heat Exchanger Components: Tube sheets, baffles, and end plates require creep resistance and thermal fatigue tolerance, making these alloys ideal.
- Boiler Valves and Fittings: Swing, gate, globe, and check valves use WC6 or WC9 depending on operating temperature.
Other Industrial Applications
- Pressure Vessels: Small to medium vessels for subcritical/critical steam in industrial power generation.
- Pump Casings and Turbine Components: High-pressure pumps in petrochemical and nuclear applications.
- Furnace and Kiln Components: Supports and internal structures exposed to elevated temperatures for extended durations.
Comparative Service Envelope
| Alloy | Max Continuous Service Temp | Typical Pressure | Typical Components | Recommended Surface Treatment |
| WC6 | 538 °C (1,000 °F) | 30 MPa (4,350 psi) | Subcritical boiler headers, valves, turbine casing sections | Nitriding, aluminizing, shot blasting |
| WC9 | 595 °C (1,100 °F) | 30 MPa (4,350 psi) | Supercritical boiler/reheater headers, valves, high-pressure turbines | Overlay welding, aluminizing, shot blasting |
8. Advantages and Limitations of A217 WC6 vs WC9
Understanding the advantages and limitations of WC6 and WC9 is critical for engineers and designers selecting materials for high-temperature, high-pressure industrial components.
Advantages
| Feature | WC6 (1.25Cr–0.5Mo) | WC9 (2.25Cr–1Mo) | Notes |
| High-temperature strength | Excellent up to 538 °C | Superior up to 595 °C | WC9 is preferred for supercritical steam |
| Tempered martensite microstructure | Good toughness, ductility | Slightly higher strength, slightly lower ductility than WC6 | Ensures reliability under pressure and thermal cycling |
| Creep resistance | Suitable for subcritical service | Optimized for long-term supercritical applications | WC9 exhibits 10–15% higher creep rupture life at elevated temperatures |
| Cost-effectiveness | Lower alloy content → reduced cost | Higher alloy content → increased material cost | Budget-sensitive applications may favor WC6 |
| Fabrication flexibility | Easier welding and machining due to lower Cr/Mo | Higher hardness and Cr content → requires more careful welding and machining | Preheat and PWHT required for both, but WC9 is more demanding |
| Corrosion/oxidation resistance | Adequate for moderate steam and chemical environments | Improved due to higher Cr content | Surface treatments further enhance performance |
Limitations
| Limitation | WC6 | WC9 | Mitigation / Notes |
| Maximum service temperature | Limited to 538 °C | 595 °C max | Exceeding limits accelerates creep and may lead to deformation |
| Weldability | Moderate; preheat and PWHT required | More sensitive; higher hardness and Cr require stricter welding control | Use low-hydrogen consumables, maintain interpass temperature |
| Machinability | Good for heat-treated condition | Slightly lower due to higher hardness | Use carbide/CBN tooling and optimized cutting parameters |
| Stress corrosion cracking (SCC) | Susceptible in H₂S or chloride-rich environments | Similar susceptibility, slightly higher Cr offers marginal improvement | Avoid service with H₂S >50 ppm or Cl⁻ >100 ppm |
| Cost | Economical | More expensive due to higher alloy content | Use WC6 when high-temperature creep is not critical |
9. Comparison with Competing Materials
When selecting high-temperature, pressure-retaining materials, engineers often evaluate WC6 and WC9 against alternative alloy steels and stainless steels.
Key Competing Materials
- Carbon Steel (CS): Low-alloy, economical, suitable for low-to-moderate temperatures (<400 °C), but poor creep and corrosion resistance.
- Chromium-Molybdenum Steel Plates (e.g., ASTM A335 P11/P22): Forged or welded pressure pipe material, higher creep resistance than CS, less expensive than WC9 castings.
- Austenitic Stainless Steels (304, 316, 321, 347): Excellent corrosion resistance, suitable for moderate temperatures (≤650 °C), lower strength and creep resistance compared with WC9.
- Nickel Alloys (Inconel 600/625, Hastelloy): Outstanding corrosion and high-temperature strength (up to 700–1,000 °C), but very expensive and difficult to fabricate.
- Other Low-Alloy Cast Steels (e.g., ASTM A217 Grade C12, CN7M): Austenitic cast steels, good corrosion resistance but lower strength for high-pressure service.
Comparative Performance Table
| Property / Feature | WC6 (1.25Cr–0.5Mo) | WC9 (2.25Cr–1Mo) | Carbon Steel | Cr-Mo Steel (P22) | Austenitic Stainless (316/321) | Nickel Alloys (Inconel 625) |
| Max Service Temp (°C) | 538 | 595 | 400 | 565 | 600 | 980 |
| Creep Strength | Moderate | High | Low | Moderate | Low | Very High |
| Tensile Strength (MPa) | 500–600 | 550–650 | 400–500 | 500–600 | 500–600 | 700–900 |
| Charpy Impact @ 20°C (J) | >40 | >40 | 30–50 | 40–50 | 40–80 | 50–100 |
| Oxidation Resistance | Moderate | Good | Poor | Moderate | Good | Excellent |
| Corrosion Resistance | Moderate | Good | Poor | Moderate | Excellent | Excellent |
| Weldability | Moderate | Moderate (requires strict preheat/PWHT) | Excellent | Good | Excellent | Difficult |
| Cost | Medium | High | Low | Medium | High | Very High |
| Fabrication Complexity | Moderate | High | Low | Medium | Medium | Very High |
| Typical Applications | Boilers, valves, subcritical/supercritical headers | Supercritical/reheater headers, turbine casings | Low-pressure vessels, piping | Pressure piping, moderate temp headers | Corrosive service, moderate temp | Extreme high-temp reactors, chemical processing |
10. Conclusion
A217 WC6 vs WC9 are the workhorses of mid-high-temperature pressure systems, enabling the safe, efficient operation of power plants, refineries, and petrochemical facilities worldwide.
Their success stems from:
- Targeted Alloying: Cr and Mo deliver oxidation and creep resistance tailored to 400–595°C service, the most common range for industrial high-temperature pressure applications.
- Proven Heat Treatment: Tempered martensite microstructure balances strength, toughness, and stability—validated by decades of ASTM/ASME testing and field service.
- Cost-Effectiveness: A middle ground between low-performance carbon steels and high-cost advanced alloys, minimizing LCC while meeting safety standards.
While advanced alloys (e.g., P91, nickel-based superalloys) are displacing WC6/WC9 in ultra-high-temperature (>600°C) applications, WC6/WC9 remain irreplaceable for 400–595°C service—where their performance, fabricability, and cost align with industrial needs.
For engineers and procurement teams, success with WC6/WC9 hinges on strict adherence to ASTM/ASME standards for composition, heat treatment, and fabrication—ensuring these alloys deliver their full 15–25 year service life.
FAQs
Can WC6 and WC9 be welded together or to carbon steel?
Yes, but joints must be engineered: use compatible filler metals, preheat, interpass controls and PWHT.
Dissimilar metal joints require attention to matching thermal expansion, galvanic issues and HAZ metallurgy. Follow qualified WPS/PQR and code requirements.
What PWHT is typical after welding?
Field practice commonly uses tempering PWHT in the 600–700 °C range.
Exact soak temperature/time depends on thickness and must follow qualified procedure; always consult supplier/code.
How long will a WC9 valve body last at 550 °C?
Service life depends on stress, cycle, environment and casting quality.
WC9 is designed for longer creep life than WC6 at elevated temperatures, but predicting life requires creep-rupture data and design stress; perform fitness-for-service analyses for critical components.
Are WC6/WC9 suitable for corrosive chloride-rich environments?
They are not the best choice for severe chloride corrosion (pitting/SSC). Duplex stainless steels or nickel alloys are preferable where chloride stress corrosion is a concern.
What inspections are essential on delivery?
Require chemical analysis (MTC), tensile and hardness (as specified), radiography/UT for pressure castings, dimensional checks and heat-treatment records. Where applicable, impact testing and PMI are prudent.
