GB/T 1591-2009 Grade A (Q345A) Double-Sided Submerged Arc Welding (DSAW) Steel Pipe
⚠️ Important Clarification: Grade A Status
GB/T 1591-2009 Grade A (Q345A) is a recognized material grade, but it is important to note that Grade A is generally not recommended for critical structural applications . According to Chinese structural steel standards, Q345A does not guarantee impact toughness, making it unsuitable for:
High-rise building structures
Primary load-bearing components in cold climates
Applications requiring guaranteed low-temperature performance
Critical welded structures
For most structural applications, Q345B or higher grades (C, D, E) are preferred due to their guaranteed impact toughness at specified temperatures .
Basic Overview
GB/T 1591-2009 Grade A (Q345A) Double-Sided Submerged Arc Welding (DSAW) steel pipe is a low-alloy high-strength structural steel pipe manufactured to the Chinese national standard. The "Q345A" designation indicates a minimum yield strength of 345 MPa (50,000 psi) for thicknesses ≤ 16mm .
The DSAW (also known as LSAW - Longitudinal Submerged Arc Welding) process is a high-integrity welding method where both inside and outside welds are applied, forming a single high-quality weld nugget. This process is particularly well-suited for large-diameter, thick-wall pipes used in structural, piling, and general engineering applications .
Name Explanation
| Part | Meaning |
|---|---|
| GB/T | Chinese national standard (Guobiao/Tuijian) |
| 1591-2009 | Standard number for Low-Alloy High-Strength Structural Steels (2009 edition; note that GB/T 1591-2018 is the newer version, which replaced Q345 with Q355) |
| Grade A | Lowest quality grade – no guaranteed impact toughness; highest allowable P (0.045%) and S (0.045%) among Q345 grades |
| DSAW | Double-Sided Submerged Arc Welding – both inside and outside welds are applied; the welding arc is submerged in granular flux during welding |
| Also Known As | LSAW (Longitudinal Submerged Arc Welded) – when the weld seam runs longitudinally along the pipe |
Key Specifications
| Attribute | Description |
|---|---|
| Material Standard | GB/T 1591-2008/2009 – Low-Alloy High-Strength Structural Steels (2008 edition; 2009 version is common in industry references) |
| Steel Grade | Q345A – Minimum yield strength 345 MPa (for thickness ≤ 16mm) |
| Quality Level | Grade A – No impact testing required; highest allowable phosphorus and sulfur content among Q345 grades |
| Material Type | Low-alloy high-strength structural steel (HSLA) |
| Manufacturing Process | DSAW (Double-Sided Submerged Arc Welding) / LSAW (Longitudinal Submerged Arc Welding) |
| Forming Methods | JCOE (progressive J-C-O forming) or UOE (U-ing, O-ing, Expanding) |
| Size Range | 219 mm to 1829 mm (8" to 72") OD – typical for DSAW |
| Wall Thickness | 5 mm to 40 mm (up to 50 mm for piling applications) |
| Length | 6 m to 12.3 m standard; up to 18 m available |
Chemical Composition (GB/T 1591 Q345A)
| Element | Maximum % | Notes |
|---|---|---|
| Carbon (C) | 0.20% | Low carbon for good weldability |
| Silicon (Si) | 0.55% | Deoxidizer |
| Manganese (Mn) | 1.00-1.70% | Provides strength; higher than Q235 |
| Phosphorus (P) | 0.045% | Highest among Q345 grades; controlled for weldability |
| Sulfur (S) | 0.045% | Highest among Q345 grades; controlled for toughness |
| Vanadium (V) | 0.02-0.15% | Micro-alloying for grain refinement |
| Niobium (Nb) | 0.015-0.060% | Micro-alloying for strength |
| Titanium (Ti) | 0.02-0.20% | Micro-alloying for grain refinement |
Note: Grade A has the least stringent chemical controls among Q345 grades, with no specified aluminum content requirement and the highest allowable phosphorus and sulfur limits .
Q345 Grade Comparison (Quality Levels)
| Grade | Impact Test Temperature | P max (%) | S max (%) | Al min (%) | Key Feature |
|---|---|---|---|---|---|
| Q345A | Not required | 0.045 | 0.045 | Not specified | Most economical; general structures |
| Q345B | +20°C (27-34J min) | 0.035-0.045 | 0.035-0.045 | Not specified | General construction, mild climates |
| Q345C | 0°C (34J min) | 0.030-0.035 | 0.030-0.035 | ≥0.015 | Colder climates, outdoor structures |
| Q345D | -20°C (34J min) | 0.025-0.030 | 0.025-0.030 | ≥0.015 | Severe cold environments |
| Q345E | -40°C (27J min) | 0.020-0.025 | 0.015-0.020 | ≥0.015 | Extreme cold, critical applications |
*Grade A is the only Q345 grade that does not guarantee impact toughness. It is also being phased out in favor of Q355 series under GB/T 1591-2018 .*
Mechanical Properties (GB/T 1591 Q345A)
| Property | Q345A Requirement | Condition |
|---|---|---|
| Yield Strength (min) | 345 MPa | Thickness ≤ 16mm |
| 325 MPa | 16mm < t ≤ 35mm | |
| 295 MPa | 35mm < t ≤ 50mm | |
| 275 MPa | 50mm < t ≤ 100mm | |
| Tensile Strength | 470-630 MPa | All thicknesses |
| Elongation (min) | 21% | Thickness ≤ 16mm |
| Charpy Impact | Not required | Grade A has no impact testing requirement |
| Hardness | Moderate | Typical for HSLA steel |
Note: As thickness increases, yield strength decreases. For thickness over 100mm, consult manufacturer for specific values.
Dimensional Specifications
| Parameter | Range / Tolerance |
|---|---|
| Outside Diameter | 219 mm to 1829 mm (8" to 72") – typical DSAW range |
| Wall Thickness | 5 mm to 40 mm (up to 50 mm for piling) |
| Diameter Tolerance | ±0.75% of specified OD (typical) |
| Wall Thickness Tolerance | +15% / -12.5% of nominal (typical) |
| Length | 6 m to 12.3 m standard; up to 18 m available |
DSAW/LSAW Manufacturing Process
Double-Sided Submerged Arc Welding (DSAW), also known as Longitudinal Submerged Arc Welding (LSAW), is a high-efficiency process used primarily for large-diameter steel pipes :
Process Description
Welding arc is submerged in granular flux during welding, protecting the weld from atmospheric contamination
Both inside and outside welds are required, usually accomplished in separate processes
The separate welds consume a portion of the other, resulting in a single high-quality weld nugget
Forming Methods
| Method | Description | Typical Diameters |
|---|---|---|
| JCOE | Progressive J-C-O forming steps (J-shape → C-shape → O-shape), expanded after welding | 406-1626 mm (16"-64") |
| UOE | Plate pressed into U-shape, then O-shape, expanded after welding | 508-1118 mm (20"-44") |
Process Steps
Plate Selection – Steel plates meeting GB/T 1591 Q345A requirements
Edge Preparation – Edges are beveled to create a V-shaped groove
Forming – Steel plates are formed into cylindrical shapes using JCOE or UOE presses
Tack Welding – Formed plates are tack-welded to maintain shape
Double Submerged Arc Welding – Pipe seam welded on both interior and exterior surfaces
Cold Expansion – Pipe may be expanded up to 1.5% to achieve final OD dimension
Ultrasonic Inspection – Seam inspected by continuous ultrasonic automatic flaw detection
Hydrostatic Pressure Test – Each pipe undergoes hydrostatic pressure testing
Finishing – End processing, beveling, coating, and marking according to specifications
Testing & Inspection Requirements
| Test Type | Requirement | Notes |
|---|---|---|
| Chemical Analysis | Per heat lot | Verifies composition limits per GB/T 1591 |
| Tensile Test | Per lot | Verifies yield and tensile strength |
| Flattening Test | Required | Checks ductility and weld integrity |
| Bend Test | Required | Verifies weld integrity |
| Charpy Impact Test | Not required for Grade A | Only Grades B, C, D, E require impact testing |
| Hydrostatic Test | Each pipe | Pressure integrity verification |
| Ultrasonic Inspection | 100% of weld seam | Continuous automatic flaw detection |
| X-ray Inspection | For "T" joints | Where strip butt welds intersect spiral seams |
| Dimensional Inspection | 100% | Per standard tolerances |
| Mill Test Certificate | EN 10204 Type 3.1B | Provided with full test results |
Delivery Conditions
Q345A DSAW pipes can be supplied in various delivery conditions :
AR – As Hot Rolled only
TMCP – Thermal Mechanical Control Processing (enhances toughness)
N – Normalized (improves grain structure)
Q+T – Quenched and Tempered (for maximum strength uniformity)
Note: Grade A is typically supplied in as-rolled condition, as impact properties are not required.
Applications
Important Note: Due to its lack of guaranteed impact toughness, Q345A has limitations in structural applications :
| Application | Suitability | Notes |
|---|---|---|
| General Structural Engineering | ✓ Suitable | Building frames, columns, trusses for non-critical applications |
| Low-Pressure Fluid Transmission | ✓ Suitable | Water, gas, and oil pipelines (non-critical) |
| Piling Foundations | ✓ Limited | For light to medium loads in non-critical applications |
| Bridge Construction | ✗ Not Recommended | Requires guaranteed impact toughness (use Grade B/C/D) |
| High-Rise Buildings | ✗ Not Recommended | A级钢不适用于高层钢结构 |
| Pressure Vessels | ✓ Limited | For non-critical, ambient temperature service |
| Offshore Platforms | ✗ Not Recommended | Requires low-temperature toughness (use Grade D/E) |
| Cold Climate Structures | ✗ Not Recommended | No guaranteed low-temperature performance |
Q345A vs. Q345B Comparison
| Feature | Q345A | Q345B | Significance |
|---|---|---|---|
| Impact Testing | Not required | Mandatory (27-34J at +20°C) | Q345B guarantees room temperature toughness |
| Phosphorus (P) max | 0.045% | 0.035-0.045% | Q345B may have tighter controls |
| Sulfur (S) max | 0.045% | 0.035-0.045% | Q345B may have tighter controls |
| Typical Application | Non-critical, indoor structures | General construction, mild climates | Q345B is preferred for most applications |
| Relative Cost | Lowest | Low | Q345A is more economical |
*The key differentiator is impact toughness – Q345A has no guaranteed impact properties, while Q345B guarantees 27-34J at +20°C .*
Q345 vs. Q235 Comparison
| Property | Q345A | Q235B | Significance |
|---|---|---|---|
| Yield Strength (min) | 345 MPa | 235 MPa | Q345 offers 47% higher strength |
| Tensile Strength (min) | 470 MPa | 370 MPa | Q345 has higher tensile strength |
| Material Type | Low-alloy HSLA | Carbon steel | Q345 has better strength-to-weight ratio |
| Typical Application | Higher-load structures | General construction | Q345 for more demanding applications |
Coating Options
Q345A DSAW pipe can be supplied with various coatings for corrosion protection:
| Coating Type | Best For | Key Features |
|---|---|---|
| 3LPE (3-layer polyethylene) | Buried pipelines | Most common; excellent corrosion protection, high impact resistance |
| FBE (Fusion Bonded Epoxy) | Oil & gas pipelines | Strong adhesion, chemical resistance |
| Coal Tar Epoxy | Marine environments | Heavy-duty protection |
| Varnish / Anti-rust oil | Temporary protection | Short-term corrosion protection during transport |
| Bitumen Coating | Buried service | Cost-effective protection |
| Black Oil Coating | Standard mill finish | Temporary protection |
End Finishes
Available end treatments include:
Plain ends (PE) – standard for most applications
Beveled ends (BE) – for field welding (30° bevel angle)
Threaded ends – for mechanical connections
International Equivalents
| Standard | Equivalent Grade | Notes |
|---|---|---|
| ASTM | A572 Grade 50 | Similar yield strength, different impact requirements |
| EN 10025 | S355JR | Similar strength (355 MPa yield) |
| ISO 630 | E355 | International standard equivalent |
| JIS G3106 | SM490 | Japanese standard |
Note: International equivalents may have different impact testing requirements. Direct substitution requires engineering evaluation.
Key Advantages
| Advantage | Description |
|---|---|
| High Strength | 345 MPa yield – 47% higher than Q235, allowing reduced wall thickness or higher load capacity |
| Excellent Weldability | Low carbon content (≤0.20%) and controlled carbon equivalent ensure good weldability |
| Good Formability | Suitable for cold bending, flanging, and shaping without cracking |
| Large Diameter Capability | DSAW produces pipes from 8" to 72" OD – ideal for large-diameter applications |
| High Weld Integrity | Double-sided welding creates a single high-quality weld nugget with 100% UT inspection |
| Most Economical Q345 Grade | Lowest cost among Q345 grades; no impact testing required |
| Micro-alloyed | Contains Nb, V, Ti for grain refinement and improved properties |
| Wide Availability | Commonly stocked by Chinese manufacturers |
Important Considerations
1. Grade A Limitations
Grade A has NO guaranteed impact toughness
Not recommended for: High-rise buildings, bridges, offshore platforms, or any application requiring low-temperature toughness
Not suitable for cold climates or dynamic loading applications
2. Standard Version Note
GB/T 1591-2009 has been superseded by GB/T 1591-2018, which replaced Q345 with Q355 (minimum yield strength adjusted to 355 MPa)
When ordering, specify the required version year to ensure compliance
For new projects, Q355 series is recommended
3. Grade Selection Recommendations
Grade A – Only for non-critical, indoor applications where impact toughness is not required
Grade B – General construction, mild climates (guarantees +20°C impact)
Grade C – Outdoor structures, temperate climates (guarantees 0°C impact)
Grade D – Cold climates, demanding applications (guarantees -20°C impact)
Grade E – Arctic, extreme cold environments (guarantees -40°C impact)
4. Product Standard
While GB/T 1591 defines the material grade, the finished DSAW pipe product is typically manufactured to product standards such as:
GB/T 9711 – Petroleum and natural gas industries line pipe
SY/T 5037 – Spiral submerged arc welded pipe for low-pressure fluid service
API 5L – International line pipe standard (when specified)
5. Pressure Limitations
Q345A is suitable for low to medium pressure applications
For higher pressure requirements, consult design codes for allowable stress values
6. Temperature Limitations
Maximum recommended service temperature: 350°C (662°F)
Short-term exposure up to 450°C possible
No guaranteed low-temperature toughness – not suitable for sub-zero applications without engineering evaluation
7. Delivery Conditions for Higher Grades
For applications requiring enhanced properties, higher grades (B, C, D, E) are available with additional heat treatment options:
Normalizing (N)
Thermo-mechanical control processing (TMCP)
Quenching and tempering (Q+T)
8. Coating for Corrosion Protection
For buried applications, specify 3LPE or FBE coating
For temporary protection, varnish or anti-rust oil is sufficient
9. Third-Party Inspection
Available inspection services include SGS, BV, Lloyds
10. Weldability
Low carbon content (≤0.20%) ensures excellent weldability
Suitable for all standard welding methods (SMAW, GTAW, GMAW, SAW)
No post-weld heat treatment required for most thicknesses
Preheat: 100-150°C recommended for thicker sections
11. Minimum Order Quantity
Typical MOQ: 1 piece
Larger orders available from manufacturers
Summary
GB/T 1591-2009 Grade A (Q345A) Double-Sided Submerged Arc Welded (DSAW) steel pipe is a low-alloy high-strength structural steel pipe with minimum yield strength of 345 MPa (50,000 psi) – approximately 47% higher than Q235. However, Grade A has no guaranteed impact toughness and is generally not recommended for critical structural applications such as high-rise buildings, bridges, or cold climate structures .
Key features:
Material standard: GB/T 1591-2009 Q345A (being replaced by Q355 in GB/T 1591-2018)
Grade A – No impact testing required; highest allowable P (0.045%) and S (0.045%) among Q345 grades
DSAW manufacturing produces pipes from 8" to 72" OD with wall thickness up to 40 mm
Double-sided welding creates a single high-quality weld nugget
JCOE or UOE forming methods ensure precise dimensional accuracy
100% ultrasonic inspection of weld seam ensures quality
Excellent weldability and good formability
Micro-alloyed with Nb, V, Ti for grain refinement
Most economical Q345 grade
Common applications include:
General structural engineering for non-critical applications
Low-pressure fluid transmission (water, gas, oil)
Light to medium-load piling foundations
Industrial equipment and machinery frames
Sheet metal and light structural elements
When ordering, specify: GB/T 1591-2009, Grade Q345A, DSAW (Double-Sided Submerged Arc Welded), Size (OD x WT), Length, End Finish (plain/beveled), Delivery Condition (AR/TMCP/N/Q+T), Coating Requirements, and Mill Test Certificate .
Important Recommendation: For most structural applications requiring guaranteed impact toughness, specify Q345B (+20°C), Q345C (0°C), Q345D (-20°C), or Q345E (-40°C) instead. For new projects, consider the newer Q355 series under GB/T 1591-2018 . Grade A should only be used for non-critical, indoor applications where impact properties are not a concern .
