Overview
Grade ST35 is a low-carbon steel pipe primarily used for low-pressure applications. The "ST" stands for "Steel Tube," and the "35" indicates a minimum tensile strength of approximately 350 MPa (or 35 kg/mm²).
It is a general-purpose pipe known for its good weldability, formability, and cost-effectiveness. It is not designed for high-pressure or high-temperature services.
1. Key Specifications and Standards
ST35 is not a standalone ASTM standard like A106 or A53. Instead, it is most commonly associated with the German standard DIN 2440 / DIN 1626 and is often considered the European equivalent of ASTM A53 Grade A or API 5L Grade A.
Primary Standard: DIN 2440 - Seamless Tubes for General Purpose
Material Standard: DIN 1626, Part 1 / DIN 17100 - This defines the chemical and mechanical properties for the ST35 grade.
Common Equivalents:
ASTM A53 Grade A (USA)
API 5L Grade A (Line Pipe)
EN 10255 / EN 10216-1 (European Standards)
2. Chemical Composition
The composition is characterized by low carbon and alloy content, making it soft and ductile.
| Element | Composition (%) |
|---|---|
| Carbon (C) | ≤ 0.17 % |
| Manganese (Mn) | ≤ 0.70 % |
| Phosphorus (P) | ≤ 0.040 % |
| Sulfur (S) | ≤ 0.040 % |
| Silicon (Si) | Typically low, not a key controlled element |
3. Mechanical Properties
| Property | Value |
|---|---|
| Tensile Strength (min) | 350 MPa (or 35 kg/mm²) |
| Yield Strength (min) | 235 MPa (or 24 kg/mm²) |
| Elongation (min) | Typically 25% or more (depends on wall thickness) |
4. Common Characteristics
Excellent Weldability: Due to its low carbon content, it can be easily welded using all common methods without pre-heating or post-weld heat treatment (under normal conditions).
Good Ductility and Formability: Easy to bend, flange, and shape without cracking.
Low Cost: One of the most economical steel pipe grades available.
Low Corrosion Resistance: Like all carbon steels, it will rust when exposed to moisture and air. It often requires protective coatings (paint, galvanization) for outdoor or corrosive environments.
Not for High-Temperature/Pressure: Its strength decreases significantly at elevated temperatures.
5. Applications
ST35 pipes are used in a wide range of low-stress applications:
Low-Pressure Plumbing and Water Lines: For transporting water, sewage, and other non-corrosive fluids.
Structural Applications: As handrails, scaffolding, fence posts, and general structural frames where high strength is not critical.
HVAC Systems: For ductwork, air handling units, and low-pressure heating systems.
Automotive Exhaust Systems: (Often in a coated form to resist corrosion).
General Engineering and Machinery: For fluid power cylinders, compressed air lines, and hydraulic systems at low to moderate pressures.
Furniture and Decorative Uses: For tables, chairs, and retail displays.
6. ST35 vs. Other Common Grades
| Feature | ST35 | ASTM A53 Grade B | ASTM A106 Grade B |
|---|---|---|---|
| Tensile Strength | 350 MPa (min) | 415 MPa (min) | 415 MPa (min) |
| Yield Strength | 235 MPa (min) | 240 MPa (min) | 240 MPa (min) |
| Carbon Content | ~0.17% (max) | ~0.30% (max) | ~0.30% (max) |
| Primary Use | General Purpose, Low Pressure | General Purpose, Higher Pressure | High-Temperature Service |
| Key Difference | Lowest strength, most ductile | Stronger than ST35/A53A | Made for high temps (e.g., steam) |
Key Takeaway: ST35 is the "basic" or "mild steel" option. If you need higher strength or service at elevated temperatures, grades like A53 B or A106 B are required.
7. Important Considerations When Ordering or Using ST35
Seamless vs. Welded: ST35 is available in both seamless (DIN 2440) and welded (DIN 2441) forms. The seamless version is more uniform and can handle slightly higher pressures.
Surface Finish: It can be black (mill finish), galvanized (zinc-coated for corrosion resistance), or painted.
Certification: For critical applications, ensure the supplier provides a mill test certificate (MTC) or 3.1 material certificate to verify the pipe meets the required chemical and mechanical properties.
Corrosion: Always account for corrosion. In environments with moisture or chemicals, galvanized ST35 or a different material (like stainless steel) should be considered.
In summary, Grade ST35 Carbon Steel Pipe is a versatile, economical, and easy-to-work-with material ideal for a vast range of low-stress structural and fluid conveyance applications. Its limitations are its strength and corrosion resistance, which must be considered during the design and material selection process.
