1. Decoding the Name "St35.8"
The designation "St35.8" follows the German/DIN standard (DIN 17175), which is widely used internationally for boiler and pressure vessel tubes. Let's dissect it:
St: Stands for "Stahl" (German for "Steel").
35: Represents the minimum yield strength (Rp0.2) of the material at room temperature. In this case, 35 translates to approximately 355 MPa (or 35.5 kgf/mm²). This is a key mechanical property indicating the stress at which the material begins to deform plastically.
8: This is a quality index number that relates to the long-term behavior of the steel at elevated temperatures. It indicates the minimum creep rupture strength over 100,000 hours (over 11 years) at a specific temperature. For St35.8, this temperature is typically around 450-500°C.
So, in simple terms, St35.8 is a carbon steel grade guaranteed to have a minimum yield strength of ~355 MPa and is certified to perform under high pressure and temperature for long durations.
2. Material Composition (Chemical Properties)
St35.8 is a low-carbon steel. A typical chemical composition is:
| Element | Content (%) |
|---|---|
| Carbon (C) | ≤ 0.17 % |
| Manganese (Mn) | 0.40 - 0.80 % |
| Phosphorus (P) | ≤ 0.025 % |
| Sulfur (S) | ≤ 0.025 % |
| Silicon (Si) | 0.10 - 0.35 % |
Key Points on Composition:
Low Carbon Content: This provides good weldability and formability but limits the maximum hardness and strength that can be achieved.
Low Phosphorus and Sulfur: These are controlled to be low to improve toughness and weldability.
No significant Alloying: It does not contain significant amounts of chromium, molybdenum, or nickel. This makes it unsuitable for very high temperatures or corrosive environments but keeps the cost lower.
3. Key Characteristics & Why It's Used for Boilers
High-Temperature Strength: Its primary feature is the ability to maintain strength at the operating temperatures of boilers and superheaters (typically up to ~450°C).
Good Creep Resistance: The "8" quality index ensures it can withstand constant stress at high temperatures without excessive deformation over time (creep).
Excellent Weldability: The low carbon content makes it very easy to weld using all common methods without pre- or post-weld heat treatment in most cases.
Good Formability: It can be bent and formed into various shapes required for boiler tubing systems.
Cost-Effective: As a carbon steel, it is more economical than alloy steels like T11, T12, T22, or T91, which are used for higher temperature/pressure sections.
4. Common Applications
Boiler Tubes: For both water-tube and fire-tube boilers.
Superheater Tubes (in lower-temperature sections).
Economizer Tubes.
Steam Piping (for saturated and low-temperature superheated steam).
Pressure Vessel Piping.
Heat Exchanger Tubes.
5. Equivalent Grades in Other Standards
St35.8 is functionally equivalent to several other internationally recognized grades. The most common equivalents are:
| Standard | Equivalent Grade | Note |
|---|---|---|
| ISO | P235GH | The most common international equivalent. |
| EN (European) | 1.0345 | The modern European material number. |
| ASTM (USA) | A192 / A210 Grade A1 | A192 is for high-pressure boiler tubes, A210 for seamless medium-carbon. |
| JIS (Japan) | STB410 | |
| GB (China) | 20G | A very common Chinese equivalent. |
Note: While these grades are very similar, there can be slight variations in chemical composition and mechanical properties. Always consult the specific standard when specifying materials for a critical application.
6. Available Forms and Specifications
St35.8 boiler pipes are almost always supplied as seamless pipes (SMLS) to ensure integrity under pressure. They are manufactured to standards like:
DIN 17175 (the native standard)
EN 10216-2 (the modern European standard for pressure purposes)
They are available in a wide range of outside diameters (OD) and wall thicknesses (WT), defined by schedules (e.g., SCH 40, SCH 80) or direct dimensions.
Summary
St35.8 Carbon Steel Boiler Pipe is a reliable, cost-effective, and widely used material for constructing pressure parts in boiler systems where temperatures do not exceed approximately 450°C. Its key strengths are its guaranteed long-term high-temperature performance and excellent weldability, making it a fundamental building block in thermal power plants and industrial heating systems.
