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34CrMo4 Seamless High Pressure Gas Cylinder Pipe

1. Material: 34CrMo4 (EN 1.7220 / AISI 4130 approx.)

Composition: Low-alloy chromium-molybdenum steel with high strength.

Key Properties:

High Tensile Strength: 700-950 MPa (depends on heat treatment).

Hardness: Typically 230-270 HB after QT (Quenched & Tempered).

Impact Toughness: Excellent even at low temperatures (Charpy V-notch required).

Fatigue Resistance: Critical for pressure cycling.

Weldability & Machinability: Good with proper pre/post-weld heat treatment.

2. Manufacturing Process: Seamless Pipe

Mandatory Seamless Construction: Eliminates weld seams (potential failure points under cyclic pressure).

Production Method: Piercing → Extrusion → Rotary Stretching/Pilgering → Heat Treatment.

Advantages: Uniform microstructure, consistent wall thickness, superior pressure containment reliability.

3. Critical Design & Performance Factors

Working Pressure: Typically designed for 200-300 bar (e.g., CNG/LPG cylinders).

Wall Thickness: Precisely controlled (e.g., 5-12mm) to withstand yield pressure (≥1.6x working pressure).

Heat Treatment: Quenched & Tempered (QT) state mandatory for gas cylinders per ISO 9809-3:

Austenitization (900°C) → Quenching (accelerated cooling) → Tempering (500-700°C).

Achieves optimal strength-toughness balance.

Pressure Testing: All cylinders undergo hydraulic test to ≥1.5x working pressure.

4. Key Standards & Compliance

Primary Standard: ISO 9809-3 (Seamless steel gas cylinders – Design, construction and testing – Part 3: CrMo steel).

Other Relevant Standards:

EN ISO 11120 (Large seamless steel cylinders for gas transport).

DOT-SPEC/TP-Series (US transport regulations).

PED 2014/68/EU (EU pressure equipment directive).

5. Applications

CNG/LPG Cylinders: Fuel storage for vehicles/residential use.

Industrial Gas Storage: Oxygen, nitrogen, argon for medical/industrial use.

Special Gases: High-pressure hydrogen, helium (requires strict embrittlement control).

6. Quality Control Requirements

Mandatory Tests:

Test	Requirement	Standard
Hydrostatic Test	≥1.5x WP, no leakage/permanent strain	ISO 9809-3
Tensile Test	Validate yield/tensile strength	ISO 6892-1
Charpy Impact Test	≥27J @ -20°C (min)	ISO 148-1
Ultrasonic Testing	Full-body for inclusions/defects	ISO 10332
Hardness Test	Verify QT uniformity	ISO 6506

Traceability: Full material certificates (EN 10204 3.1 or 3.2).

7. Critical Failure Considerations

Hydrogen Embrittlement: 34CrMo4 is susceptible; avoid use with wet H₂S environments.

Stress Corrosion Cracking: Requires protective coatings/internal liners.

Fatigue Life: Design must exceed 15,000+ pressure cycles (ISO 9809-3 minimum).

Vendor Selection Advice:

Certification: Demand ISO 9809-3 accreditation + PED/ASME certifications.

Testing Documentation: Ensure batch-specific test reports are provided.

Manufacturer Experience: Prioritize suppliers specializing in gas cylinders (e.g., Tenaris, Vallourec, TPCO).

For procurement, always request:

Material Test Report (MTR) with heat number traceability.

Third-party inspection certificates (e.g., SGS/BV).

This material is engineered for life-critical applications. Compliance with international standards, stringent QC, and proper handling during cylinder fabrication are non-negotiable. Never compromise on heat treatment validation or impact toughness certifications.

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