Rules of Nippon Kaiji Kyokai

Nippon Kaiji Kyokai (NK), also known as ClassNK, is a prominent member of the International Association of Classification Societies (IACS) and one of the most influential classification societies globally. Its "Rules for the Survey and Construction of Steel Ships" provide comprehensive technical standards for ship design, construction, and inspection, with strict and systematic requirements for hull structural steel plates to ensure structural integrity, safety, and durability under various sea conditions.

In terms of steel plate specifications, ClassNK classifies hull steel into ordinary strength grades (A, B, D, E) and higher strength grades (such as AH32, AH36, AH40, DH32, DH36, DH40, EH32, EH36, EH40), based on the importance of the structural component, stress levels, and environmental temperature. Each grade has clearly defined requirements for chemical composition, mechanical properties, and impact toughness. To ensure good weldability and resistance to brittle fracture, NK standards strictly control the carbon equivalent (Ceq) and limit harmful elements such as sulfur (S) and phosphorus (P), with even stricter limits for high-strength steels and those intended for cold-service environments.

Mechanical properties are central to NK’s plate requirements. The rules specify yield strength, tensile strength, elongation, and Charpy V-notch impact energy (KV2) at ambient and low temperatures (e.g., 0°C, -20°C, -40°C). For vessels operating in Arctic or cold waters, steel plates used in critical structural areas—such as the bow, side shell plating, and strength deck—must pass impact tests at -40°C or lower to verify their toughness in low-temperature conditions and prevent brittle fracture.

Additionally, ClassNK sets detailed requirements for manufacturing processes, surface quality, dimensional tolerances, non-destructive testing (e.g., ultrasonic testing), and traceability. Steel production must be subject to inspection by NK surveyors (WITNESS), with supervision throughout the entire process, including steelmaking, rolling, heat treatment, and testing. All plates used in primary load-bearing structures must be accompanied by complete material certification and maintain full traceability from slab to final product.

In summary, ClassNK ensures high reliability and long-term safety of marine steel plates in demanding maritime environments through a scientific material grading system, rigorous performance verification, and comprehensive quality control, providing high-standard technical support and quality assurance for the global shipbuilding industry.

Official Website: https://www.classnk.or.jp