Difference between S355G7+N and S355G9+N

S355G7+N and S355G9+N are normalized fine-grain structural steel plates conforming to the European standard EN 10025-4. The designation indicates a minimum yield strength of 355 MPa, with the "+N" suffix specifying the Normalized delivery condition. The critical "G" component designates these grades for use at Low (sub-zero) temperatures, where "G7" and "G9" specify the guaranteed minimum Charpy V-notch impact energy (in Joules) at a specific test temperature. These grades are engineered for critical welded structures operating in Arctic and extreme cold environments, such as offshore platforms, LNG storage tanks, icebreaker hulls, and bridges in cold regions.

Key Differences:

The core distinction lies in their low-temperature toughness performance. S355G7+N is impact tested at -50°C with a guaranteed minimum energy of 27 Joules, whereas S355G9+N is tested at the significantly lower temperature of -60°C with the same 27 Joules minimum. This makes S355G9+N the tougher material, designed for more severe Arctic or deep-cryogenic conditions where S355G7+N would not be suitable.

Achieving this enhanced toughness at a lower temperature requires a more stringent metallurgical process for S355G9+N. While both grades utilize fine-grain practice and normalization, S355G9+N typically involves stricter control over chemical composition (particularly lower phosphorus and sulfur levels for improved cleanliness) and processing parameters to ensure a more homogeneous and defect-resistant microstructure capable of resisting brittle fracture at extreme cold. Consequently, the fabrication and welding of S355G9+N demand more stringent procedures to preserve its superior low-temperature toughness in the weld heat-affected zone, often requiring specially qualified welding consumables and controlled heat input. 

S355G7+N Steel plate Chemical Composition

S355G9+N Steel plate Chemical Composition

S355G7+N Steel plate strength property

S355G9+N Steel plate Mechanical Properties