What is the yield strength of P355NL2 Steel Plates?

P355NL2 is a low-temperature pressure vessel steel manufactured in accordance with EN 10028-3. It is a non-alloy, fine-grain steel supplied in the normalized condition, specifically developed for equipment operating under high pressure and very low ambient temperatures.

355 – Minimum yield strength of 355 MPa (depending on thickness)

NL2 – Normalized steel with guaranteed notch toughness at −50℃

P355NL2 is designed for reliable service in environments where temperatures may drop to −50℃ or below, while still maintaining good strength up to approximately 400℃.

P355NL2 is primarily supplied as flat products, with additional processing options available.

These properties ensure high resistance to brittle fracture and reliable load-bearing capacity in cold service conditions.

The chemical composition of P355NL2 is carefully controlled to achieve low-temperature toughness and good weldability.

Low phosphorus and sulfur contents are particularly important for preventing brittle fracture at low temperatures.

P355NL2 is widely used in industries where low-temperature toughness and pressure resistance are critical.

P355NL2 steel offers a combination of properties that make it suitable for demanding low-temperature applications:

Excellent resistance to brittle fracture at −50℃

P355NL2 is a high-quality low-temperature pressure vessel steel designed for use in harsh and cold operating environments. With guaranteed toughness at −50℃, good weldability, and stable mechanical performance, it is an ideal material for pressure vessels, boilers, pipelines, and cryogenic equipment used in the oil & gas, chemical, and power generation industries.

1. Q: What is the fundamental difference between P275N, P275NH, P275NL1, and P275NL2?
A: The core distinction lies in their guaranteed impact toughness temperatures and application temperature ranges, which result from differences in chemical composition control and delivery condition requirements.
* P275N: Guarantees impact toughness at room temperature and down to -20℃. Suitable for general pressure vessels and boilers (-20℃ to +350℃).
* P275NH: Builds upon P275N with stricter chemical controls (e.g., lower S, P content) to ensure mechanical property stability at elevated temperatures. Suitable for higher temperature service (up to +400℃), such as steam headers and high-temperature vessels.
* P275NL1: Guarantees impact toughness down to -50℃ (longitudinal specimens). Designed for low-temperature environments (e.g., -40℃ to +350℃).
* P275NL2: Guarantees impact toughness down to -60℃ (longitudinal specimens) with even tighter control of S and P. Intended for more severe cryogenic applications (e.g., -50℃ to +350℃), such as LNG-related equipment.

2. Q: Why is controlling heat input and performing PWHT particularly important when welding P275NH plate?
A: P275NH is designed for high-temperature service.
* Controlling Heat Input (typically recommended <=35 kJ/cm): Prevents excessive grain growth in the Heat-
* Mandatory PWHT Requirement: Primarily aims to eliminate welding residual stresses. Under high-temperature and high-pressure conditions, residual stresses can promote stress corrosion cracking or creep damage. PWHT significantly reduces these stresses, enhancing the long-term safety and dimensional stability of the structure at high temperatures.

3. Q: How is the allowable stress for P275N steel plate determined at different design temperatures?
A: The room temperature yield strength cannot be used directly. It is essential to consult the allowable stress values at the design temperature provided in the relevant pressure vessel design codes (e.g., ASME BPVC Section II-D, EN 13445-2). These values are derived from the material's strength properties at elevated temperatures, considering factors like creep and oxidation, and then divided by a safety factor. For example, the allowable stress for P275N at 350℃ is significantly lower than its value at room temperature.

4. Q: When purchasing P275NL1/NL2 plate, what is the most critical acceptance test besides room temperature mechanical properties?
A: The most critical test is the Low-Temperature Impact Toughness (Charpy V-notch, CVN). Strict adherence to contract and standard (EN 10028-3) requirements is necessary:
* Verify Test Temperature: Is it -40℃, -50℃, or another specified temperature?
* **Verify SpecVerify Specimen Orientation: Transverse (T) or Longitudinal (L). Standard requirements for longitudinal specimens are typically higher than for transverse. The guaranteed values for NL1 and NL2 primarily apply to longitudinal specimens.
* Verify Acceptance Value: Confirm that the minimum impact energy (J) meets the requirement. This is the key indicator to prevent brittle fracture at low temperatures.

5. Q: What does the "Normalized" condition of P275N steel plate imply, and how does it affect fabrication?
A: "Normalized" is the standard delivery condition. It means the plate is reheated above its austenitizing temperature after rolling and then cooled uniformly in air.
* Purpose: To refine grain structure, homogenize the microstructure, improve mechanical properties and toughness, and relieve internal stresses.
* Impact on Fabrication: Subsequent hot forming (e.g., hot rolling into shells) or welding (equivalent to localized reheating and cooling) alters the material's condition. If the hot working temperature enters the normalizing range and is followed by air cooling, it may have a "normalizing effect." However, excessive temperatures or improper cooling can degrade properties. Therefore, for critical pressure parts subjected to significant hot work, re-normalizing heat treatment may be required to restore the specified properties.

6. Q: The yield strength of P275N plate varies with thickness. How is this considered in design?
A: EN 10028-3 clearly specifies the minimum yield strength (ReH) values for different thickness ranges (e.g., decreasing from 275 MPa for ≤16mm to 235 MPa for >100mm). This reflects the "thickness effect" where thicker sections cool slower, leading to a slight reduction in strength.
* In Design: The corresponding minimum yield strength value for the actual thickness range of the plate being used must be selected for calculations.
* In Procurement and Certification: The manufacturer's test certificate must provide the actual measured yield strength for the batch, corresponding to its thickness. This value must be >= the standard's specified minimum for that thickness.

7. Q: Can P275N be used interchangeably with common carbon steels (e.g., Q235B, Q345R) or American standard materials (e.g., SA516 Gr.60)?
A: They are not directly interchangeable. So-called "equivalent" grades are for approximate reference only.
* Different Standard Systems: P275N conforms to the European standard (EN). Its chemical composition, mechanical properties, test methods, and acceptance criteria differ from Chinese (GB) or American (ASTM/ASME) standards.
* Differences in Key Properties: Even if the strength levels are similar, there can be variations in the alloying system, impact toughness requirements, and applicable temperature ranges.
* Correct Procedure: Any material substitution must be re-calculated and approved by the design authority, ensuring the new material fully complies with all requirements of the original design code. Compatibility with the welding procedure must also be re-evaluated.