1. Limitations of Thermal Surface Treatment (e.g., Bright Annealing, Bright Quenching/Tempering)
Alloy Sensitivity to Atmosphere: S355J0WP contains trace Cu, Cr, and P. During bright heat treatment, an impure protective atmosphere (e.g., insufficient inert gas, residual oxygen) can cause surface oxidation or decarburization. This not only reduces brightness but also weakens the corrosion resistance of the weathering steel.
Temperature Control Constraints: Its optimal heat treatment temperature range is narrow (typically 850–950℃ for annealing). Exceeding 980℃ may lead to grain coarsening, reducing the steel's toughness and weldability. Conversely, low temperatures result in incomplete annealing, failing to achieve the desired bright finish.
Workpiece Size/Thickness Limits: Large or thick S355J0WP workpieces (e.g., ≥50mm thick plates) face uneven heating in vacuum/inert gas furnaces. This causes inconsistent brightness and potential thermal deformation, especially for components with complex shapes.
High Cost and Equipment Dependence: Bright thermal treatment requires professional vacuum or controlled-atmosphere furnaces. Small-batch production is cost-inefficient, and maintenance of furnace atmosphere purity (e.g., regular replacement of inert gas) increases operational complexity.
2. Limitations of Chemical Surface Treatment (e.g., Chemical Polishing, Pickling-Brightening)
Corrosion Risk from Chemical Agents: Chemical polishing relies on acidic electrolytes (e.g., phosphoric acid-nitric acid mixtures). Prolonged immersion or improper concentration can over-etch the surface, causing pitting or unevenness. For S355J0WP, excessive acid exposure may dissolve the passive film formed by Cr/Cu, compromising its inherent weather resistance.
Environmental and Safety Hazards: Waste liquids from chemical treatment contain heavy metals (e.g., Fe, Cu ions) and residual acids. Unauthorized discharge violates environmental regulations, requiring specialized treatment equipment that increases costs. Additionally, acidic fumes pose health risks to operators.
Limited Brightness Durability: The bright finish from chemical polishing is less wear-resistant than mechanical polishing. Without post-treatment (e.g., anti-corrosion coating), the surface is prone to tarnishing when exposed to moisture or industrial fumes, especially in harsh outdoor environments.
Incompatibility with Welded Zones: Welded S355J0WP workpieces have uneven alloy distribution in the heat-affected zone (HAZ). Chemical polishing may cause the HAZ to etch faster, resulting in visible color differences between the weld and base metal, affecting surface uniformity.
3. Core Precautions for Both Processes
Preserve Corrosion Resistance as a Priority: Avoid processes that damage the weathering steel's passive film. After thermal/chemical treatment, conduct a salt spray test (per EN ISO 9227) to verify that corrosion resistance meets the original S355J0WP standard (≥48 hours without red rust).
Strict Process Parameter Validation:
For thermal treatment: Use high-purity inert gases (Ar, N₂) with oxygen content ≤50ppm. Preheat workpieces to 300–400℃ to remove moisture, preventing atmosphere contamination.
For chemical treatment: Test the polishing solution concentration (e.g., 60–70% phosphoric acid + 5–10% nitric acid) and temperature (40–60℃) on small samples first. Control immersion time to 3–8 minutes to avoid over-etching.
Post-Treatment Protection:
After thermal treatment: Cool workpieces slowly in the furnace (≤50℃/hour cooling rate) to reduce internal stress and maintain brightness.
After chemical treatment: Rinse thoroughly with deionized water, neutralize with 5–10% sodium carbonate solution, and dry immediately. Apply a thin layer of anti-corrosion oil or transparent passivation film to extend brightness retention.
Avoid Conflicts with Subsequent Processing: If S355J0WP requires welding or bending after surface treatment, prioritize mechanical polishing (lower impact on core performance) over chemical/thermal processes. Thermal treatment post-welding may alter the weld's mechanical properties.
Test Before Mass Production: Conduct pilot tests on 1–2 workpieces to confirm brightness, corrosion resistance, and dimensional stability (e.g., no deformation, no surface defects) before full-scale processing.
