Products Description
HP325 steel (GB/T 6653 standard) is a specialized high-strength carbon steel designed exclusively for welded gas cylinders and pressure vessels. To ensure safety, compliance, and performance in pressure-containing applications, HP325 must meet a stringent hierarchy of material standards, pressure vessel design/fabrication codes, mechanical property mandates, chemical composition limits, and testing/certification requirements. Below is a complete, detailed breakdown of all mandatory safety standards and specifications.
At GNEE, we assist our clients in selecting the optimal grade based on their specific vessel design and safety requirements. Our technical team has decades of experience in the steel industry, ensuring you receive the most reliable advice.
Core Material Standard
GB/T 6653: Steel Plates and Strips for Welded Gas Cylinders
This is the mandatory national standard defining HP325's chemical, mechanical, and quality requirements (replaces GB/T 6653-2008).
Scope: Applies to hot-rolled (2.5–12.0mm) and cold-rolled (1.5–4.0mm) HP325 steel for welded gas cylinders.
Key Mandates: All clauses except 6.1.5, 6.6, 6.7.4 are compulsory (non-negotiable) for safety.
Equivalent International Standards (for Export/Global Compliance)
- EN 10120: European standard for gas cylinder steels (equivalent grade: P310NB)
- JIS G 3116: Japanese standard (equivalent grade: SG325)
- ISO 4978: International standard for welded gas cylinder flat steel products
- ASTM/ASME: Recognized via material equivalency for North American pressure vessel codes
Hot-rolled steel plate for welding gas cylinders
Mandatory Mechanical Property Requirements
HP325 must strictly meet/exceed these minimum mechanical properties (GB/T 6653):
| Property | Requirement (Min/Range) | Safety Significance |
|---|---|---|
| Yield Strength (Rp0.2) | ≥325 MPa | Defines minimum pressure resistance; prevents permanent cylinder deformation under working pressure. |
| Tensile Strength (Rm) | 490–600 MPa | Ensures resistance to rupture/bursting; balances strength and ductility. |
| Elongation (A50) |
- ≥18% (t < 3mm) - ≥22% (t ≥ 3mm) |
Critical for formability (deep stamping) and safety; prevents cracking during manufacturing/use. |
| Yield Ratio (Rp0.2/Rm) | ≤0.80 (Typical: ~0.78) | Limits plastic deformation risk; maintains elastic safety buffer for overpressure events. |
| 180° Cold Bend Test | d=2a (no cracking) | Verifies formability/toughness; ensures no failure during cylinder rolling/stamping. |
| Charpy Impact Test | ≥27 J (at 20°C, V-notch) | Ensures low-temperature toughness; prevents brittle fracture in cold environments. |
Strict Chemical Composition Limits
HP325 has tight controls to ensure weldability, toughness, and resistance to brittleness/cracking (GB/T 6653):
| Element | Max Limit (%) | Safety Purpose |
|---|---|---|
| Carbon (C) | 0.20 | Prevents hardening, cold cracking, and poor weldability. |
| Silicon (Si) | 0.35 | Controls strength without reducing ductility. |
| Manganese (Mn) | 1.00 | Enhances strength/toughness; balances carbon effects. |
| Phosphorus (P) | 0.025 | Minimizes brittleness (especially at grain boundaries). |
| Sulfur (S) | 0.015 | Reduces hot cracking and improves weld quality. |
| Aluminum (Alt, total) | ≥0.020 | Grain refinement; boosts toughness and formability. |
| Carbon Equivalent (Ceq) | ≤0.25 | Critical for weldability; limits preheat requirements and cracking risk. |
Pressure Vessel Design & Fabrication Safety Codes
HP325 cylinders/vessels must comply with design, manufacturing, inspection, and testing standards:
Chinese National Standards (Domestic Use)
- GB/T 150 (All Parts, 2024 Edition): Pressure Vessels
- GB/T 150.1: General requirements (safety, design principles, scope)
- GB/T 150.2: Materials (approves HP325 for pressure vessel use; defines allowable stress)
- GB/T 150.3/4: Design, fabrication, inspection, testing
- Allowable Stress: HP325 = 163 MPa (at room temp) (per GB/T 150.2)
GB 5099: Steel Seamless Gas Cylinders / GB 5100: Welded Gas Cylinders
Mandates hydrostatic pressure testing (1.5× working pressure)
Defines service life (8–12 years) and periodic inspection rules
NB/T 47014: Welding Procedure Qualification
Requires certified welding procedures (WPS/PQR) for HP325
Mandates qualified welders for pressure vessel joints
International Standards (Export Markets)
- ASME BPVC Section VIII: U.S. pressure vessel code (Div. 1 for standard pressure, Div. 2/3 for high pressure)
- DOT Specification 4BW: U.S. Department of Transportation for LPG/compressed gas cylinders
- EN 1442: European welded gas cylinders standard
- ISO 11439: Gas cylinders - Refillable welded steel cylinders
Manufacturing & Quality Control Safety Requirements
Material Production Controls
Hot Rolling Process: Controlled finish rolling temperature for grain refinement; ensures uniform yield strength/toughness.
Heat Treatment: Normalized (N) delivery condition mandatory; optimizes mechanical properties.
Non-Destructive Testing (NDT): 100% ultrasonic testing (UT) for internal flaws (laminations, inclusions).
Cylinder Fabrication Mandates
Welding: Preheat (80–120°C) required; post-weld heat treatment (PWHT) for thick sections to relieve stress.
Forming: No cold forming cracks; strict stamping parameters to avoid stress concentrations.
Surface Quality: No scars, cracks, or inclusions >0.2mm depth; prevents failure initiation.
Application-Specific Safety Restrictions
HP325 is not universal - safety standards limit its use to:
- Design Pressure: ≤4.0 MPa (medium-pressure vessels; not for ultra-high-pressure industrial gases)
- Temperature: -20°C to +120°C (avoid low-temp brittle fracture; no high-temperature creep)
- Media: Non-corrosive gases (LPG, air, nitrogen, acetylene); not for acidic/alkaline/corrosive gases
- Wall Thickness: 1.5–12.0mm (standard range; outside requires special approval)
Why Choose GNEE as Your Supplier?
- 18+ years manufacturing experience
- Full certification support (EN, JIS, GB standards)
- Custom sizes, thickness, and surface treatment
- Fast delivery and global logistics
- Strict quality inspection (chemical + mechanical testing)
We don't just sell steel-we provide complete gas cylinder material solutions.
Are you still unsure which grade is best for your upcoming project? Don't leave your material selection to chance. Contact GNEE today for a detailed technical consultation and a competitive quote on HP325 steel plates. Our team of experts is ready to help you optimize your production and ensure the highest safety standards for your products.
Can HP325 be used without meeting GB/T 6653?
No. GB/T 6653 is legally mandatory in China; non-compliant material is unsafe and uncertifiable for pressure vessels.
Why is yield ratio limited to ≤0.80?
A lower yield ratio provides a safety buffer - the steel deforms elastically longer before permanent plastic deformation, preventing sudden failure.
Is preheating mandatory for welding HP325?
Yes (80–120°C) per GB/T 150/NB/T 47014 - prevents cold cracking in the heat-affected zone (HAZ).
Does HP325 meet international export standards?
Yes - when produced to EN 10120/JIS G 3116 equivalency and certified via DOT/CE/PED, it complies with EU/U.S./global safety rules.
What happens if HP325 fails impact testing?
The material is rejected - insufficient toughness causes brittle fracture (catastrophic cylinder failure) in cold conditions.
