S355K2G1W Interpretation of the European Standard Weathering Steel Grade

European steel grades look like a secret code at first. But once you learn the pattern, the name tells you almost everything you need to know. S355K2G1W is no exception. Let's decode it piece by piece.

S – Structural steel. This isn't a tool steel, a pressure vessel grade, or a free-cutting alloy. It's meant for load-bearing structures: bridges, buildings, towers, railcars.

355 – Minimum yield strength in Megapascals (MPa) for thicknesses up to 16mm. That's roughly 51,000 psi in imperial units. Higher than common structural grades like S235, comparable to many ASTM A572 grades.

K2 – Impact toughness rating. The "K" indicates Charpy V-notch testing. The "2" specifies testing at -20°C with a minimum of 27 Joules average (no single value below 20J). A "K1" rating would test at 0°C instead.

G1 – Weathering steel (G stands for "guan" or corrosion-resistant in some contexts, though EN 10025-5 simply uses W for weather resistance). The "1" indicates a specific phosphorus range. G1 keeps phosphorus at standard low levels (≤0.035%). The alternative, G2, allows higher phosphorus (up to 0.12%) for enhanced corrosion performance.

W – Weathering steel. This is the family designation. Without the W, you'd have ordinary structural steel. The W tells you the steel contains copper, chromium, and nickel to form that protective patina.

Put it all together: A structural steel with 355 MPa minimum yield strength, guaranteed impact toughness of 27J at -20°C, from the G1 phosphorus-controlled weathering steel family, designed for atmospheric corrosion resistance.