What is the difference between S355H and S355NH?

The primary difference between S355H and S355NH lies in their material specifications, which are primarily related to their intended use, production process, and specific mechanical properties.

Here's a clear breakdown based on the available information:

Standard: Typically refers to a structural steel grade used for hollow sections (e.g., circular, square, or rectangular hollow sections) as per standards like EN 10210 (hot-finished structural hollow sections) or EN 10219 (cold-formed welded structural hollow sections).

Purpose: Designed specifically for structural hollow sections used in construction, such as columns, beams, or other structural components.

Production: Hot-finished (EN 10210): Formed at high temperatures, resulting in better structural integrity and fewer internal stresses.

Cold-formed (EN 10219): Formed at ambient temperatures, often followed by welding, which may introduce residual stresses.

Mechanical Properties:

• Minimum yield strength: 355 MPa (hence the "S355" designation, where "S" stands for structural steel and "355" indicates the yield strength in MPa).
• Tensile strength: Typically 470–630 MPa (varies by standard and thickness).
• Impact toughness: Not always specified, but when required (e.g., S355J2H), it is tested at specific temperatures (e.g., -20°C for J2H).

Designation: The "H" typically denotes hollow sections. Variants like S355J2H or S355K2H indicate specific impact toughness requirements (e.g., J2 = 27 J at -20°C, K2 = 40 J at -20°C).

Applications: Commonly used in construction for structural components like pipes, tubes, and hollow profiles.

Standard: This likely refers to S355NH, which is specified under EN 10210 (hot-finished structural hollow sections) or EN 10219 (cold-formed welded hollow sections), with additional requirements for normalization or normalization rolling, as per EN 10025-3 for normalized fine-grain steels.

Purpose: Similar to S355H, it is used for structural hollow sections, but the "N" indicates that the steel is normalized or normalized rolled, which improves its toughness and weldability by refining the grain structure.

Production: Normalized (N): The steel undergoes a heat treatment process (normalizing) to achieve a uniform, fine-grain microstructure, enhancing toughness and ductility, especially at low temperatures.

Hot-finished or cold-formed: Like S355H, it can be produced as hot-finished (EN 10210) or cold-formed (EN 10219) hollow sections.

Mechanical Properties:

• Minimum yield strength: 355 MPa, same as S355H.
• Tensile strength: Typically 470–630 MPa, similar to S355H.
• Impact toughness: S355NH requires guaranteed impact toughness at low temperatures, typically 40 J at -20°C (as per EN 10210 or EN 10219 for NH grades).
• Fine-grain structure: Due to normalization, S355NH has better toughness and is more suitable for demanding applications in cold environments.

Designation: The "N" stands for normalized or normalized rolled, and the "H" indicates hollow sections. For example, S355NH is a normalized grade for hollow sections.

Applications: Used in structural applications requiring higher toughness, such as in cold climates, offshore structures, or where improved weldability is needed.

S355H is a general structural steel grade for hollow sections, with optional impact toughness requirements (e.g., S355J2H).

S355NH is a normalized version of S355H, offering enhanced toughness and weldability due to its fine-grain structure, making it suitable for more demanding applications, such as in low-temperature environments or critical structural components.

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