SA-672 Grade B70 Carbon Steel Pipe for High-Pressure Service
SA-672 Grade B70 is a high-strength carbon-manganese steel pipe designed for high-pressure, high-temperature applications, particularly in power generation, petrochemical, and heavy industrial sectors where superior strength is required.
Key Characteristics
Material Type: Electric-fusion-welded carbon steel pipe
Primary Standard: ASTM A672 / ASME SA-672 - Electric-Fusion-Welded Steel Pipe for High-Pressure Service at Moderate Temperatures
Grade Designation: B = 240 MPa (35 ksi) minimum yield strength, 70 = 485 MPa (70 ksi) minimum tensile strength
Manufacturing: Electric fusion welding with longitudinal seam + full heat treatment
Primary Use: High-pressure piping in power plants, refineries, and process industries
Chemical Composition (SA-672 Grade B70)
| Element | Content (%) | Notes |
|---|---|---|
| C (Carbon) | ≤ 0.24 | Slightly higher than B65 for increased strength |
| Mn (Manganese) | ≤ 1.35 | Increased from B65 (≤1.20%) for enhanced strength |
| P (Phosphorus) | ≤ 0.035 | Maximum limit |
| S (Sulfur) | ≤ 0.035 | Maximum limit |
| Si (Silicon) | ≤ 0.35 | Deoxidizer, strength enhancement |
| Cu (Copper) | ≤ 0.35 | Optional, for corrosion improvement |
| Ni (Nickel) | ≤ 0.25 | Optional alloying element |
| Cr (Chromium) | ≤ 0.25 | Optional alloying element |
| Mo (Molybdenum) | ≤ 0.08 | Optional alloying element |
| V (Vanadium) | ≤ 0.03 | Optional alloying element |
Note: The sum of Cu, Ni, Cr, and Mo shall not exceed 0.75% unless otherwise agreed upon.
Mechanical Properties
| Property | Requirement | Imperial Equivalent |
|---|---|---|
| Tensile Strength (min) | 485 MPa | 70,000 psi |
| Yield Strength (min) | 240 MPa | 35,000 psi |
| Elongation in 2" (min) | Based on wall thickness: • t ≤ 0.787 in. (20 mm): 28% • t > 0.787 in. (20 mm): 26% |
Heat Treatment Classes
SA-672 Grade B70 is available in different heat treatment conditions:
| Class | Heat Treatment | Typical Applications |
|---|---|---|
| Class 12 | Normalized | Most common for power plant piping |
| Class 13 | Normalized and Tempered | Enhanced toughness applications |
| Class 22 | Stress Relieved Only | Limited to specific applications |
| Class 32 | As-Welded | Rarely used for Grade B70 |
Most Common: Class 12 (Normalized) - provides optimal combination of strength and toughness
Manufacturing Process
Plate Formation: Carbon steel plates formed into cylindrical shape
Welding: Longitudinal seam welded via electric fusion welding
Heat Treatment: Full normalization (or normalization + tempering)
Testing & Inspection: Comprehensive NDE and mechanical testing
Finishing: Ends prepared for welding (typically beveled)
Comparison with Other A672 Grades
| Grade | Min Yield (MPa/ksi) | Min Tensile (MPa/ksi) | Carbon Max (%) | Manganese Max (%) | Relative Strength |
|---|---|---|---|---|---|
| B60 | 240/35 | 415/60 | 0.21 | 1.20 | Standard |
| B65 | 240/35 | 450/65 | 0.22 | 1.20 | Medium |
| B70 | 240/35 | 485/70 | 0.24 | 1.35 | High |
| B75 | 240/35 | 515/75 | 0.27 | 1.35 | Very High |
| C65 | 275/40 | 450/65 | 0.22 | 1.20 | Higher Yield |
Typical Applications
Power Generation:
Main steam lines (moderate temperature sections)
Boiler feedwater lines
High-pressure turbine bypass systems
Header piping for boilers and heat exchangers
Attemperator spray lines
Petrochemical & Refining:
High-pressure process piping
Hydrocracker and reformer piping
Utility steam distribution
Compressor discharge lines
Industrial Applications:
High-pressure hydraulic systems
Compressed air/gas transmission
Process industry high-pressure lines
Size Range and Availability
| Parameter | Typical Range | Notes |
|---|---|---|
| Outside Diameter | 18" to 120" (457 to 3048 mm) | Larger than seamless alternatives |
| Wall Thickness | 0.25" to 4.0" (6.4 to 102 mm) | Thicker walls available |
| Lengths | 20 to 40 ft (6 to 12 m) | Custom lengths possible |
Key Advantage: Economical production of large diameter, heavy wall pipe
Welding and Fabrication
Welding Considerations:
| Aspect | Requirements for B70 |
|---|---|
| Preheating | Required: 200-400°F (95-205°C) for thickness > ¾" (19 mm) |
| Interpass Temp | Max 600°F (315°C) |
| PWHT | Required after fabrication (normalize or stress relieve) |
| Filler Metals | AWS E8018, E9018, or matching strength electrodes |
| Welding Processes | SMAW, GTAW, GMAW, SAW all suitable |
Fabrication Notes:
Cold bending permitted with limitations
Hot bending requires post-bend heat treatment
Cutting: Plasma, oxy-fuel, or machining acceptable
Fit-up: Careful alignment to minimize welding stresses
Quality Control & Testing
Mandatory Tests per ASTM A672:
Hydrostatic Test: Each pipe tested to specified pressure
Tensile Test: From pipe or test plates
Flattening Test: For welded seam evaluation
Radiographic Examination: 100% of weld length
Visual Inspection: All surfaces
Dimensional Checks: OD, wall thickness, length, straightness
Supplemental Tests (Optional):
S1: Additional tension tests
S2: Transverse tension tests
S3: Charpy V-notch impact tests
S4: Ultrasonic examination
S5: Electromagnetic examination
Allowable Stress Values (ASME B31.1)
| Temperature | Allowable Stress (Approximate) | ||
|---|---|---|---|
| °F | °C | ksi | MPa |
| 100 | 38 | 20.0 | 138 |
| 400 | 204 | 20.0 | 138 |
| 600 | 316 | 17.9 | 123 |
| 750 | 399 | 14.7 | 101 |
| 800 | 427 | 13.1 | 90 |
Note: Exact values from ASME Section II, Part D, Table 1A.
Equivalent and Similar Materials
| Standard | Equivalent/Similar |
|---|---|
| ASME | SA-672 B70 (identical) |
| API | No direct equivalent |
| DIN | Similar to DIN 17175 17Mn4 |
| JIS | Similar to JIS G3456 STPT 480 |
| GB | Similar to GB 5310 20MnG |
Advantages of SA-672 B70
High Strength: Superior tensile strength for demanding applications
Cost Effective: More economical than seamless pipe in large diameters
Large Size Availability: Can be produced in sizes difficult to make seamless
Quality Assurance: Rigorous NDE provides high confidence
Weldability: Good with proper procedures despite higher strength
Limitations and Considerations
Temperature Limit: Typically ≤ 800°F (427°C) for continuous service
Cyclic Service: Requires careful fatigue analysis
Corrosion: Not suitable for severe corrosive environments without protection
HAZ Effects: Welded construction creates heat-affected zones
Code Restrictions: Some applications may require additional qualifications
Comparison with Seamless Alternatives
| Parameter | SA-672 B70 (Welded) | A106 B (Seamless) |
|---|---|---|
| Max OD | Up to 120" | Typically ≤ 24" |
| Cost (Large Dia) | Lower | Significantly higher |
| Delivery Time | Longer for custom sizes | Shorter for standard sizes |
| Anisotropy | Directional properties | More isotropic |
| Weld Inspection | Extensive NDE required | Less critical |
Ordering Specification Example
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ASME SA-672 Grade B70 Class 12 Size: 36" OD x 1.5" WT x 40' Long Heat Treatment: Normalized (Class 12) Ends: Beveled 37.5° with 1/16" land Supplemental Requirements: S1, S3, S4 Test Reports: Certified Mill Test Report per ASME Section II
Boiler Code Compliance
ASME Code Acceptance:
Section I: Permitted for boiler external piping with restrictions
B31.1 Power Piping: Fully acceptable
Section VIII Div. 1: Acceptable for pressure vessels
B31.3 Process Piping: Acceptable
Important Notes for Boiler Use:
Requires proper heat treatment documentation
Welding procedures must be qualified
NDE must meet code requirements
Limited to specific temperature-pressure regimes
Summary
SA-672 Grade B70 is a high-strength electric-fusion-welded carbon steel pipe offering excellent mechanical properties for high-pressure applications. With a minimum tensile strength of 485 MPa (70 ksi), it provides superior strength compared to standard carbon steel pipes while maintaining good weldability and toughness.
Its welded construction allows economical production of large diameter, heavy wall pipes needed for main steam lines, feedwater systems, and other critical high-pressure applications in power plants and process industries. While not a "furnace tube" material, it serves essential roles in boiler external piping systems where high strength and reliability are paramount.
The mandatory normalization heat treatment ensures uniform properties and relieves welding stresses, making SA-672 B70 a reliable choice for demanding industrial applications when properly specified, fabricated, and inspected according to applicable codes and standards.
