S355G8+N is a high-strength, ultra-high-toughness structural steel plate specifically designed for extreme cryogenic environments. Its designation follows the European standard (EN) steel nomenclature system and represents one of the highest levels of low-temperature toughness requirements for structural steels in current international standards. The "S" stands for "Structural steel," indicating its use in critical load-bearing and welded structures; "355" denotes the specified minimum yield strength of 355 MPa at room temperature, classifying it as a high-strength grade suitable for components subjected to complex loads and dynamic stresses. "G8" is the latest and highest quality grade in the series, specifically developed for structural safety under the most severe polar and deep-sea conditions. According to the latest standard requirements, the G8 grade must meet Charpy V-notch impact testing at -120°C, with an average absorbed energy of no less than 47 J for three specimens and no single value below 35 J, demonstrating unparalleled resistance to brittle fracture. This performance makes S355G8+N the ideal material for future Arctic offshore platforms, ultra-low-temperature storage and transportation systems for liquefied natural gas (LNG) and liquid hydrogen (LH2), deep-sea manned research vessels, and core structural components of polar icebreakers. The suffix "+N" indicates the delivery condition: normalized or normalized rolling. In this process, the steel is heated above its upper critical temperature (austenitizing temperature) and then cooled uniformly in air, forming a fine, homogeneous microstructure—typically ferrite-pearlite or bainite. Normalizing effectively refines grain size, eliminates internal stresses, improves microstructural uniformity and isotropy, and significantly enhances low-temperature toughness, thickness-direction properties (Z-direction properties), and weld joint stability. It is particularly suitable for thick-section plates and complex welded structures.
The main characteristics of S355G8+N steel plate include ultra-high strength, exceptional cryogenic toughness, outstanding weldability, and superior thickness-direction performance. Its chemical composition is based on carbon and manganese, with additions of microalloying elements such as niobium, vanadium, and titanium. Combined with the normalizing process, these elements promote grain refinement and precipitation strengthening, significantly improving the steel’s comprehensive mechanical properties. The tensile strength ranges from 470 to 630 MPa, with elongation (A5) ≥20%, providing good ductility and fatigue resistance. Its high impact toughness at -120°C makes it an indispensable key material for the most demanding service conditions globally, effectively preventing brittle fracture in extreme cold and ensuring the long-term safety and reliability of offshore installations and energy equipment. Additionally, S355G8+N exhibits excellent resistance to lamellar tearing, making it suitable for thick-section welded joints and complex connections in high-reliability welded structures.
S355G8+N is widely used in cutting-edge engineering fields such as future polar energy development, liquid hydrogen storage and transport systems, pressure hulls for deep-sea explorers, and next-generation polar offshore wind support systems.
The current standard for S355G8+N steel plate is the European standard EN 10225:2023 "Weldable structural steels for use in offshore structures — Technical delivery conditions for fabricated or rolled products". Specifically tailored for offshore and marine engineering, this standard includes the G8 grade for the first time, providing detailed specifications for chemical composition, mechanical properties, impact toughness, manufacturing processes, non-destructive testing, and inspection procedures. It serves as a core technical reference for modern high-end offshore and energy engineering design and construction. Due to its exceptional performance, S355G8+N has become a strategic key material for future extreme-environment engineering projects.
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.
