Difference between S355G7+N and S355G8+N - Knowledge

S355G7+N and S355G8+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 "G8" 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. Both grades are tested at -50°C. However, S355G7+N guarantees a minimum impact energy of 27 Joules, whereas S355G8+N guarantees a higher minimum of 40 Joules at the same temperature. This makes S355G8+N the tougher material, providing a greater safety margin against brittle fracture in the most severe low-temperature service.

Achieving this enhanced toughness requires a more refined metallurgical process for S355G8+N. While both grades utilize fine-grain practice and normalization, S355G8+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. Consequently, the fabrication and welding of S355G8+N may demand slightly more stringent procedures to preserve its superior toughness in the weld heat-affected zone, often requiring qualified welding consumables and controlled heat input. The selection between G7 and G8 is a direct response to specific design codes and safety factors: S355G8+N is specified for the most critical applications where the highest guaranteed toughness at -50°C is non-negotiable, while S355G7+N provides excellent performance for demanding but slightly less critical low-temperature service.

Chemical Composition of S355G7+N

S355G7+N Chemical Composition
Grade	The Element Max (%)
Grade	C	Si	Mn	P	S	Al	N
S355G7+N	0.14	0.15-0.55	1.0-0.01	0.02	0.01	0.0055	0.01
	Nb	V	Ti	Cu	Cr	Ni	Mo
	0.04	0.06	0.025	0.3	0.25	0.5	0.08

Chemical Composition of S355G8+N

S355G8+N Chemical Composition
Grade	The Element Max (%)
Grade	C	Si	Mn	P	S	Al	N
S355G8+N	0.14	0.15-0.55	1.0-0.01	0.02	0.01	0.0055	0.01
	Nb	V	Ti	Cu	Cr	Ni	Mo
	0.04	0.06	0.025	0.3	0.25	0.5	0.08

Mechanical Property of S355G7+N

S355G7+N high strength property
Grade		Mechanical Property			Charpy V Impact Test
Grade	Thickness	Yield	Tensile	Elongation	Degree	Energy 1	Energy 2
S355G7+N	mm	Min Mpa	Mpa	Min %	-40	J	J
	t≤50	355	470-630	22%		50	50
	50＜t≤63	355	470-630	22%		50	50
	63＜t≤100	325	490-630	22%		50	50
Note: Energy 1 is transverse impact test, Energy 2 is longitudinal

Chemical Composition of S355G8+N

Grade	S355G8+N Mechanical Properties (min)
	Tensile Strength		Yield Strength						Elongation A
	Thickness (mm)		Thickness t - mm						Elongation A
	≤100	>100	≤16	16<t≤25	25<t≤40	40<t≤63	63<t≤100	100<t≤150	%
S355G8+N	470/630	460/620	355	355	345	335	325	320	22