What surface defects are commonly inspected for on Q355B I-beams, and which are acceptable?

Surface quality inspection is a vital part of the final inspection before shipment. While some surface imperfections are inherent to the hot-rolling process, standards set limits on their severity. Common defects and their acceptability are: Acceptable (Within Limits): 1. Mill Scale: The thin, adherent layer of iron oxides (FeO, Fe2O3, Fe3O4) formed during rolling is normal and acceptable. It may be bluish-grey or black. It provides some temporary corrosion protection but is often removed before painting by blast cleaning. 2. Slight Roll Marks: Minor, periodic impressions from the rolls are acceptable if they do not affect dimensions beyond tolerance. 3. Light Surface Rusting: Slight, uniform rust staining after storage is generally acceptable as it does not signify a material defect. Not Acceptable (Rejectable Defects): 1. Cracks: Any surface cracks, regardless of length or depth, are unacceptable as they are stress concentrators. This includes cracks in the web, flanges, or fillet areas. 2. Laps/Seams: These are folds of metal that have been rolled into the surface but are not welded. They appear as linear defects, often on the edges of flanges, and can harbor impurities. 3. Deep Scratches or Gouges: Mechanical damage from handling that significantly reduces the cross-section or creates a sharp notch. 4. Pitting and Heavy Rust Scale: Deep, localized corrosion or thick, flaky rust that indicates poor storage and may mask other defects or lead to section loss. 5. Excessive Roller Marks or Bruises: Deep, non-uniform depressions. 6. Slivers: Elongated, loose or partially attached pieces of metal rolled into the surface. 7. Laminations (detected visually at edges): Separation within the steel, usually seen as a crack or layering at the cut end, indicating internal non-metallic inclusions. Inspection is usually visual, aided by wire brushing to remove loose scale. For critical applications, more sensitive methods like dye penetrant testing (PT) might be used on weld preparation areas. Any questionable defect is typically measured for depth; if it exceeds a specified fraction of the nominal thickness (e.g., 0.5mm or 5% of thickness), the section may be rejected or require repair by grinding (provided the ground area is smooth and the remaining thickness is within tolerance).