Reading the Q355GNH Grade Designation
The designation follows the Chinese structural steel naming system used in GB/T 4171, the standard for high atmospheric corrosion resisting structural steel. Q means that the grade is classified by yield strength, 355 is the minimum specified yield strength in megapascals for the standard thickness range, and GNH indicates the high atmospheric corrosion resistance class. A related designation, NH alone, marks the ordinary corrosion resistance class, so the letter G is the key difference when the two are compared.
What Makes the Steel Weather Resistant
Weathering steel owes its performance to deliberate alloying rather than to coatings. Small additions of copper, chromium and nickel, and in some grades phosphorus, change the chemistry of the rust that forms on a bare surface. Instead of a loose, flaking oxide that keeps exposing fresh metal, the surface develops a dense, tightly adherent patina that slows the ingress of oxygen and moisture and reduces the corrosion rate to a fraction of that of plain carbon steel. This is why the material is often used unpainted, and why it performs best where the surface is alternately wetted and dried rather than permanently wet or buried.
Mechanical and Delivery Properties
| Property | Specified value |
|---|---|
| Grade designation | Q355GNH |
| Minimum yield strength | 355 MPa for the standard thickness range of GB/T 4171 |
| Tensile strength and elongation | Specified by GB/T 4171 for the thickness and delivery condition ordered |
| Impact toughness | Quality grades A to E differ by the impact test temperature specified |
| Corrosion resistance class | GNH, high atmospheric corrosion resistance |
The quality grades A, B, C, D and E are not strength classes. They denote progressively lower impact test temperatures, so a structure that must stay tough in cold weather is ordered in a higher quality grade while the strength class stays the same.
Applications
Q355GNH plate and section are used where a structure is exposed to the weather for decades and maintenance painting is difficult or uneconomic. Typical applications include steel bridges and bridge girders, container frames, railway wagons and rolling stock, transmission towers and poles, industrial chimney and stack shells, coal and ore hopper linings, and exposed architectural structures where the stable patina is also an aesthetic choice. In each case the design assumes a small corrosion allowance during the period before the patina stabilises, and detailing avoids water traps, crevices and ponding that would keep the surface permanently wet.
Fabrication and Welding Notes
Q355GNH is welded with the same general practice as a 355 MPa structural steel, using low-hydrogen consumables that produce a weld metal of at least matching strength and toughness. Because the alloying elements that give corrosion resistance are diluted in the weld pool, the weld and its heat-affected zone weather slightly differently from the parent plate unless a matching weathering consumable is used, and this is normally specified for exposed welds. Cutting, drilling and cold forming follow normal structural steel practice, but flame-cut edges on exposed surfaces are dressed where the patina quality matters. The material should not be painted over an unstable, still-flaking surface, and it should not be used in continuously wet, buried or heavily chloride-contaminated conditions without protection.
FAQ
Q: What does Q355GNH mean?
Q stands for yield strength, 355 is the minimum yield strength in MPa and GNH denotes the high atmospheric corrosion resistance class, so the grade is a 355 MPa weathering structural steel.
Q: Which standard covers Q355GNH?
It is specified in GB/T 4171, the Chinese standard for high atmospheric corrosion resisting structural steel, which also defines the quality grades from A to E.
Q: Can Q355GNH be used without painting?
Yes. That is its main advantage, because a bare surface develops a dense protective rust layer in a well-ventilated, alternately wet and dry environment, although a small corrosion allowance is still taken into account in design.
Q: What is the difference between Q355GNH and Q355NH?
Both are weathering steels, but GNH marks the high corrosion resistance class and NH the ordinary class, so the two differ in the alloying additions and in the corrosion resistance they deliver.
Q: Is Q355GNH the same as a 355 MPa carbon steel?
The yield strength class is comparable, but the alloying elements that provide atmospheric corrosion resistance change the welding consumable recommendation and the surface protection strategy, so the two should not be treated as interchangeable.
Q: Where should Q355GNH not be used?
It is unsuitable for permanently wet, buried or chloride-rich environments such as immersion in seawater, because the protective patina cannot form and stabilise under those conditions.
