Normalizing Heat Treatment Process and Benefits
The “+N” in S355J2+N signifies that the steel plate has undergone a normalizing heat treatment after rolling. The process involves heating the plate to approximately 880-920°C (above the upper critical temperature), holding for a sufficient time to achieve full austenitization, then cooling in still air. This refines the grain structure, transforming any irregular as-rolled grains into uniform equiaxed ferrite and pearlite. Normalizing eliminates internal stresses from rolling, improves ductility, and enhances impact toughness, especially at low temperatures.
For S355J2+N, the normalized condition is critical to achieving the specified Charpy V-notch impact value of 27J at -20°C. Without normalization, the same chemical composition might only meet S355JR or S355J0 grades. The process also reduces anisotropic properties, meaning the plate behaves similarly in longitudinal and transverse directions. Suppliers must document time-temperature cycles to ensure compliance. In practice, normalizing may be performed by the steel mill or as a separate treatment. Always request heat treatment certificates to verify that the +N condition was properly executed.
Impact Test Requirements According to EN 10025-2
According to EN 10025-2:2019, S355J2+N steel plates must pass Charpy V-notch impact tests with specific parameters. The standard requires a minimum average absorbed energy of 27 Joules at -20°C. Test specimens are taken transverse to the rolling direction, with standard dimensions of 10mm x 10mm x 55mm. For plates thinner than 10mm, sub-size specimens (e.g., 7.5mm or 5mm) are permitted, with proportional energy reduction. Three specimens are tested per heat and per thickness, and all three must individually meet the minimum value (no single value below 70% of 27J, i.e., 19J).
The impact test is performed on a pendulum impact tester. The test temperature must be controlled within ±2°C using a cooling bath. For plates with thickness ≥ 6mm, testing is mandatory. Unlike S355JR (tested at 20°C) or S355J0 (0°C), S355J2+N’s -20°C requirement makes it suitable for European winter conditions. Third-party inspection agencies (e.g., DNV, TÜV) often witness these tests. Ensure your material certificate explicitly states “KV 27J at -20°C” to confirm compliance.
Practical Quality Control and Verification on Site
On a construction site or fabrication shop, verifying that S355J2+N meets heat treatment and impact requirements requires checking documentation and performing supplemental tests. First, review the mill test certificate (MTC) per EN 10204 Type 3.1 or 3.2. Look for explicit mention of “normalized” or “+N” in the delivery condition, and the Charpy test results at -20°C. Many counterfeit plates are labeled “J2+N” but are actually as-rolled; they may show acceptable tensile strength but fail impact tests.
For critical projects, you can commission independent lab testing. Cut a sample coupon from the plate, machine Charpy specimens, and test at -20°C. Also, perform a simple hardness check; normalized plates typically have a more uniform hardness profile (e.g., 150-190 HBW) compared to as-rolled (which may vary by 30 HBW). Microstructural examination (etching with 2% nital) reveals fine, uniform ferrite-pearlite grains – if you see banding or elongated grains, normalization was inadequate. Always reject plates without full traceability, and consult the project specification before accepting alternate heat treatment conditions like controlled rolling (which is not equivalent to +N).
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