P355NH steel plate is a high-strength, non-alloy, fine-grain structural steel produced by hot rolling, specifically designed for pressure equipment operating safely in low-temperature environments. Its technical requirements are primarily governed by the European standard EN 10028-3:2022 "Flat products made of steels for pressure purposes – Part 3: Non-alloy and alloy steels with specified low-temperature properties". This standard is the core international specification widely adopted for steel plates used in low-temperature pressure equipment, clearly defining the chemical composition, mechanical properties, low-temperature impact toughness, dimensional tolerances, test methods, and quality assurance requirements for P355NH. As a key grade within the EN 10028-3 standard, P355NH is extensively used in Europe and in countries adopting the European Pressure Equipment Directive (PED), particularly for pressure vessels and load-bearing structures that must operate safely under sub-zero conditions over long periods. It is an important structural material in the field of cryogenic engineering.
The designation "P355NH" consists of four parts: "P" stands for "Pressure equipment", indicating the steel is intended for pressure vessels; "355" denotes the specified minimum yield strength at room temperature of 355 MPa, placing it in the medium-high strength category. This provides excellent load-bearing capacity and is significantly higher than grades such as P275NH; "N" means "supplied in normalized or normalized-rolled condition", a heat treatment process that produces a uniform and fine-grained microstructure, significantly enhancing the steel’s toughness and low-temperature performance; "H" in this standard does not indicate high-temperature properties, but rather serves as an identifier within the EN 10028 series. Combined with "N", the "NH" suffix specifically denotes a pressure vessel steel with guaranteed low-temperature impact properties supplied in the normalized condition. P355NH ensures good ductility and resistance to brittle fracture at low temperatures through strict control of carbon equivalent (typically no more than 0.44%) and the addition of elements such as aluminum (Al), niobium (Nb), or vanadium (V) for deoxidation and grain refinement.
The primary applications of P355NH steel plate are in the oil and gas, chemical, refrigeration, and energy sectors, suitable for manufacturing medium- and high-pressure equipment operating in low-temperature environments, including:
Low-Temperature Pressure Vessels: Used for storage tanks, separators, reactors, and spherical tanks for cryogenic media such as liquefied natural gas (LNG), liquefied petroleum gas (LPG), ethylene, liquid oxygen, and liquid nitrogen.
Refrigeration Systems: Used in large industrial freezing units, low-temperature heat exchanger shells, and piping.
Offshore Platforms and Ships: Used for pressure components in cold regions or vessels operating in polar waters.
Chemical Plants: Used in low-temperature gas separation, liquefaction, and storage systems.
Key characteristics include: excellent low-temperature toughness (standard requires Charpy V-notch impact testing at -50°C, with minimum average impact energy typically 27 J or 31 J), high strength (yield strength ≥ 355 MPa), good weldability (requiring preheating of 150–200°C and controlled heat input, with post-weld stress relief annealing recommended), and excellent cold-forming capability. As a pressure material combining high strength with outstanding low-temperature toughness, P355NH plays a critical role in ensuring the structural integrity and safety of cryogenic equipment and is an indispensable mainstream steel grade in modern low-temperature pressure vessel manufacturing.
Ultrasonic Testing (UT)
A key non-destructive testing technique that uses high-frequency sound waves to detect internal flaws in steel plates. The probe emits sound waves, which reflect when encountering defects such as cracks or inclusions. The receiver captures the echoes, enabling precise determination of defect location and size. With high sensitivity, strong penetration, and fast inspection speed, UT effectively ensures internal quality, widely used in the production of heavy plates, pressure vessel plates, and other high-end products to guarantee safety and reliability.
Magnetic Particle Testing (MT)
A common surface inspection method that magnetizes the workpiece, causing leakage magnetic fields at surface or near-surface defects like cracks or inclusions, which attract magnetic particles to form visible indications. Simple to operate and highly sensitive, MT is suitable for rapid inspection of surface and near-surface flaws in ferromagnetic materials, widely used for online or offline inspection of plate edges, ends, and welds, ensuring product quality and safety.
Penetrant Testing (PT)
A non-destructive method for detecting surface-breaking flaws. A penetrant liquid is applied to the cleaned steel surface, allowing it to seep into defects such as cracks or pores. After removing excess penetrant, a developer is applied, causing the trapped penetrant to bleed out and form visible indications. Simple and cost-effective, PT is suitable for inspecting surface defects in various non-porous materials, commonly used for welds, castings, and complex components, effectively ensuring surface quality of steel plates.
