SA691 Grade 2 1/4CR Chrome-Molybdenum Alloy Steel Boiler Pipe
Product Overview
SA691 Grade 2 1/4CR (commonly written as 2.25CR) is an intermediate chromium-molybdenum alloy steel pipe manufactured to the ASTM/ASME SA691 specification for electric-fusion-welded (EFW) high-temperature and high-pressure piping. This grade, with its nominal composition of 2.25% chromium and 1% molybdenum, is one of the most widely used and versatile creep-strength-enhanced ferritic steels in the power and process industries. It offers an optimal balance of high-temperature strength, weldability, and resistance to oxidation and hydrogen attack.
Primary Applications
High-Pressure Steam Piping: Main steam lines, hot reheat lines, and superheater outlets in supercritical and subcritical power plants.
Petrochemical & Refining: High-temperature reactor transfer lines, heater tubes, and piping in hydroprocessing units (excellent resistance to hydrogen attack).
Heat Recovery Steam Generators (HRSG): High-pressure evaporator and superheater sections.
High-Temperature Heat Exchangers: Tubes and shells in severe service.
Industrial Boilers: Critical high-pressure headers and interconnecting piping.
SA691 2.25CR Chemical Composition & Mechanical Properties Table
| Category | Property / Element | Specification / Value |
|---|---|---|
| Standard & Grade | Standard Designation | ASTM/ASME SA691 |
| Grade | 2 1/4CR (Compositionally equivalent to ASTM A335 P22 / P22 Type 2) | |
| Chemical Composition | Carbon (C) | 0.05 - 0.15% |
| Manganese (Mn) | 0.30 - 0.60% | |
| Phosphorus (P) | 0.025% max | |
| Sulfur (S) | 0.025% max | |
| Silicon (Si) | 0.50% max | |
| Chromium (Cr) | 2.00 - 2.50% | |
| Molybdenum (Mo) | 0.87 - 1.13% | |
| Mechanical Properties | Tensile Strength, min | 415 MPa (60,000 psi) |
| Yield Strength (0.2% Offset), min | 205 MPa (30,000 psi) | |
| Elongation, min | ≥20% (in 2 inches / 50mm) for standard wall | |
| Heat Treatment | Condition | Normalizing and Tempering is standard. Normalizing: 900-1000°C (1650-1830°F); Tempering: ≥675°C (≥1250°F). |
| Manufacturing Process | Type | Electric Fusion Welded (EFW) followed by full heat treatment. |
Elevated Temperature Performance Data
| Property | Value / Range | Remarks / Source (Typical) |
|---|---|---|
| Maximum Recommended Continuous Metal Temp | 593°C (1100°F) | Standard limit for long-term service; ASME B31.1 |
| Allowable Stress (at 538°C / 1000°F) | ~103 MPa (~15,000 psi) | ASME Section II, Part D reference value |
| Allowable Stress (at 593°C / 1100°F) | ~55 MPa (~8,000 psi) | Demonstrates strength retention |
| Oxidation Resistance Limit | Excellent up to 600°C (1110°F) | Forms stable Cr-rich oxide layer |
| Creep Rupture Strength (100,000 hrs) | ~100 MPa at 538°C | Key reason for widespread use |
| Excellent Resistance To | Hydrogen Attack (HTHA), Sulfidation, Thermal Fatigue |
Key Technical Characteristics & Considerations
Superior High-Temperature Strength: The 2.25Cr-1Mo composition provides exceptional creep and rupture strength in the critical 450-600°C (840-1110°F) range, making it the material of choice for main steam lines in traditional power plants.
Resistance to Hydrogen Attack (HTHA): This grade is highly resistant to high-temperature hydrogen attack at partial pressures and temperatures defined by the Nelson Curves (API RP 941). This property is paramount for its use in refinery hydroprocessing units.
Weldability and Fabrication:
Preheat: Mandatory, typically 175-250°C (350-480°F).
Post-Weld Heat Treatment (PWHT): Essential. Performed at 690-750°C (1275-1380°F) for a minimum of 1 hour per inch of thickness to restore toughness and relieve stresses.
Filler Metals: E9018-B3 (SMAW), ER90S-B3 (GTAW/GMAW) for matching composition.
Requires careful control of interpass temperature and heat input.
Microstructural Stability: Supplied in the normalized and tempered condition, resulting in a microstructure of tempered bainite, which ensures long-term stability and creep resistance. Prolonged exposure in the 375-575°C (700-1070°F) range can lead to temper embrittlement; chemistry control (limiting Si, Mn, P, Sn) is often specified to mitigate this (e.g., "P22 Type 2" requirements).
Quality and Inspection Requirements:
Mandatory Testing: Hydrostatic/NDE, chemical analysis, tension tests.
Common Supplementary Tests: Ultrasonic Examination (SA791 S6), Radiographic Examination (S5), Hardness Traverses across welds.
Advanced Requirements: For critical applications, Charpy V-Notch impact testing and creep testing may be specified.
Economic and Design Advantage: As an EFW pipe, SA691 2.25CR offers significant cost savings over seamless A335 P22 for large diameters and heavy walls, while meeting the same performance criteria for most applications.
Design, Specification, and Procurement Notes:
Clear Specification: Use "SA691 Grade 2 1/4CR" or "SA691 Gr. 2.25CR". For enhanced resistance to temper embrittlement, specify "Grade 2 1/4CR with P22 Type 2 chemistry limits" (lower Si, P).
Code Compliance: The material is accepted under ASME B31.1 (Power Piping) and B31.3 (Process Piping). Always verify the latest allowable stress values from ASME Section II, Part D.
Life Assessment: For existing plants, remaining life assessment often focuses on creep damage accumulation and oxidation wastage.
Documentation: Ensure the Certified Mill Test Report (CMTR) includes full heat treatment records, actual chemical analysis (especially for tramp elements), and results of all specified tests.
Disclaimer: This summary provides general engineering data. For all design, fabrication, and procurement activities, it is imperative to consult the latest official ASTM/ASME SA691 specification, the relevant ASME Boiler and Pressure Vessel Code Sections, applicable piping codes (B31.1/B31.3), and project-specific technical requirements. Material properties can vary between manufacturers and heats.
