SA691 Grade 1 1/4CR Chrome-Molybdenum Alloy Steel Boiler Pipe
Product Overview
SA691 Grade 1 1/4CR (commonly written as 1.25CR) is a low chromium-molybdenum alloy steel pipe manufactured according to the ASTM/ASME SA691 specification for electric-fusion-welded (EFW) piping in high-temperature service. This grade contains approximately 1.25% chromium and 0.5% molybdenum, offering improved high-temperature strength and oxidation resistance compared to carbon steels (like SA106 Grade C) while remaining more economical than higher chromium grades like 5CR and 9CR. It serves as a transitional material between carbon steel and medium alloy steels.
Primary Applications
Power Generation: Boiler tubes, steam lines, and auxiliary piping in subcritical power plants (up to ~540°C / 1000°F)
Petrochemical: Process piping, heater tubes, and heat exchanger shells in moderate temperature services
Refinery: Transfer lines, catalytic cracker piping
Industrial Boilers: High-pressure steam headers and distribution lines
SA691 1.25CR Chemical Composition & Mechanical Properties Table
| Category | Property / Element | Specification / Value |
|---|---|---|
| Standard & Grade | Standard Designation | ASTM/ASME SA691 |
| Grade | 1 1/4CR (Compositionally similar to ASTM A335 P11) | |
| 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 - 1.00% | |
| Chromium (Cr) | 1.00 - 1.50% | |
| Molybdenum (Mo) | 0.44 - 0.65% | |
| Mechanical Properties | Tensile Strength, min | 415 MPa (60,000 psi) |
| Yield Strength, min | 205 MPa (30,000 psi) | |
| Elongation, min | ≥30% (in 2 inches / 50mm) for standard wall thickness | |
| Hardness, max | 163 HBW (typical) | |
| Heat Treatment | Condition | Full Annealing or Normalizing and Tempering |
| Manufacturing Process | Type | Electric Fusion Welded (EFW) |
Temperature & Performance Characteristics
| Property | Value / Range | Remarks |
|---|---|---|
| Recommended Service Temperature | Up to 580°C (1075°F) | For long-term service; short-term peaks higher |
| Maximum Metal Temperature | ~595°C (~1100°F) | For intermittent service |
| Oxidation Resistance Limit | Good up to 565°C (1050°F) | Superior to carbon steel, inferior to 5CR+ grades |
| Minimum Design Strength (at 540°C) | ~115 MPa (~16,700 psi) | ASME B31.1 allowable stress value |
| Key Advantage | Cost-effective upgrade from carbon steel for elevated temperature service |
Key Technical Features & Considerations
Balanced Performance: Provides a cost-effective performance boost over plain carbon steel (SA106) for services between 455-565°C (850-1050°F), where carbon steel's strength declines significantly.
Weldability & Fabrication:
Preheat Required: Typically 150-200°C (300-400°F) minimum
Post-Weld Heat Treatment (PWHT): Mandatory at 595-720°C (1100-1330°F) to relieve welding stresses and temper the heat-affected zone (HAZ)
Filler Metal: E8018-B2 (SMAW) or equivalent for matching composition
Good cold forming properties when properly heat-treated
Microstructure: Typically supplied in a normalized and tempered condition, providing a fine-grained microstructure of tempered bainite or ferrite with carbides, ensuring good toughness and creep strength.
Corrosion & Oxidation:
The 1.25% Cr content forms a more stable and protective oxide layer (Cr₂O₃) than plain carbon steel, offering better resistance to steam oxidation and some reducing atmospheres.
Not suitable for wet H₂S service or strong acidic environments without proper evaluation.
Quality & Testing Requirements:
Standard Tests: Hydrostatic or non-destructive electric test, tension tests, chemical analysis
Optional/Specified Tests: Ultrasonic examination (UT), radiographic examination (RT), hardness surveys
Heat Treatment Verification: Mill test reports must certify the heat treatment cycle
Comparisons & Selection Guidance:
vs. SA106 Gr.C: Higher strength at temperature, better oxidation resistance. Used when metal temperatures exceed ~425°C (800°F).
vs. SA691 5CR: Lower cost, adequate for many subcritical applications. Selected when temperatures don't justify 5CR's superior oxidation resistance (>595°C).
Often used for headers, manifolds, and large-diameter piping where welded construction is more economical than seamless.
Design & Procurement Notes:
When specifying, clearly denote the grade as "SA691 Grade 1 1/4CR" or "SA691 Grade 1.25CR".
Review project-specific code cases and limitations (e.g., ASME Boiler and Pressure Vessel Code, Section I).
For critical service, consider invoking supplementary requirements of SA691, such as S5 (radiographic examination) or S6 (ultrasonic examination).
Ensure the Material Test Report (MTR) includes the actual heat treatment parameters (temperature and time).
Disclaimer: This information is for general reference. For engineering design and procurement, always consult the latest edition of the ASTM/ASME SA691 specification, applicable ASME B31.1 Power Piping or ASME B31.3 Process Piping codes, and project-specific technical requirements. The properties and limits can vary based on the specific heat of steel and manufacturing process.
