Stainless Steel Bridge Anti-collision Guardrail Processing: Bending Angle (90°-135°) Control and Surface Treatment of Hot-rolled 304 Steel Plate

Bridge anti-collision guardrails serve as the last line of safety for vehicles and pedestrians, requiring both robust impact resistance and long-term corrosion resistance. Hot-rolled 304 stainless steel plate, with its 18% chromium and 8% nickel composition, has become the preferred material for such guardrails. It offers excellent ductility, tensile strength of over 520MPa, and superior corrosion resistance against rain, snow, and road deicing salts. The core challenges in processing lie in two aspects: precisely controlling the 90°-135° bending angles (critical for guardrail structure assembly) and implementing effective surface treatment (to enhance weather resistance). This article details the key techniques for these two processes, providing practical guidance for high-quality guardrail production.

Core Logic: Why Hot-rolled 304 Steel Stands Out for Bridge Guardrails

Bridge guardrails operate in harsh outdoor environments, and material selection directly affects safety and maintenance costs. Hot-rolled 304 steel plate addresses the limitations of traditional materials like carbon steel and ordinary stainless steel.

Mechanical Performance Advantage Hot-rolled 304 steel has a yield strength of ≥205MPa, which is 30% higher than that of cold-rolled 304 steel in the same thickness. This ensures the guardrail can absorb impact energy without deformation during collisions.

Corrosion Resistance Guarantee The chromium-nickel alloy forms a dense oxide film on the surface, preventing rust even in coastal or high-humidity areas. Its service life exceeds 30 years, far longer than the 5-8 years of galvanized carbon steel guardrails.

Formability for Bending Requirements The hot-rolling process improves the steel’s ductility, making it easy to achieve stable 90°-135° bends without cracking—essential for the connection between guardrail posts and crossbeams.

Pre-processing Preparation: Laying the Foundation for Precision Bending

Poor pre-processing can lead to bending defects such as angle deviation and surface scratches. A standardized preparation process is crucial for subsequent operations.

1. Material Cutting: Ensuring Dimensional Accuracy

Use CNC plasma cutting machines for hot-rolled 304 steel plates (common thickness: 3-8mm). Set the cutting speed to 300-500mm/min and the plasma arc current to 80-120A. After cutting, remove burrs with a grinding wheel (grain size 120#) to ensure the edge flatness error is ≤0.2mm—uneven edges will cause uneven stress during bending.

2. Stress Relief: Preventing Bending Deformation

Hot-rolled steel may retain residual stress from the rolling process. Anneal the cut plates at 1050℃ for 30 minutes, then cool them in air. This reduces internal stress by 60%, avoiding angle rebound after bending.

3. Surface Cleaning: Removing Contaminants

Clean the steel surface with a 5% sodium hydroxide solution at 50℃ for 10 minutes to remove oil and oxide scale. Rinse thoroughly with deionized water and dry with compressed air. A clean surface ensures uniform force transmission during bending and improves the adhesion of subsequent surface treatments.

Key Process: 90°-135° Bending Angle Control Techniques

The 90° angle is used for guardrail post bases and crossbeam connections, while the 135° angle applies to curved bridge sections. Precise control relies on reasonable equipment selection and parameter setting.

1. Equipment Selection and Die Matching

Adopt CNC press brakes with a tonnage of 100-300 tons, depending on plate thickness: 100 tons for 3-5mm plates and 300 tons for 6-8mm plates. Match the die radius to the plate thickness: use a 5mm radius die for 3-4mm plates and an 8mm radius die for 5-8mm plates. This prevents cracking at the bending point.

2. Parameter Setting for Different Angles

Bending parameters vary with angles and plate thicknesses. The following table provides optimized settings for common specifications:

3. Angle Correction and Quality Inspection

304 steel has a certain rebound after bending—usually 2°-5° for 90° bends and 1°-3° for 135° bends. Compensate by setting the press brake angle 3°-6° smaller than the target angle. After bending, use a digital protractor (accuracy ±0.1°) to inspect 5 points per workpiece. If the deviation exceeds 1°, re-bend with adjusted parameters.

Surface Treatment: Enhancing Durability and Aesthetics

Bridge guardrails face long-term exposure to sunlight, rain, and road pollutants. Proper surface treatment extends service life and maintains appearance.

1. Mechanical Polishing: Improving Surface Smoothness

Use a belt polishing machine with progressive grit sizes: 240# for initial grinding (removing bending scratches), 400# for intermediate polishing, and 800# for fine polishing. The polishing speed is 1.5-2m/min, and the pressure is controlled at 0.3-0.5MPa. The final surface roughness should be ≤Ra0.4μm, reducing dust and water accumulation.

2. Passivation Treatment: Strengthening Corrosion Resistance

After polishing, immerse the guardrail in a 20% nitric acid solution at 40℃ for 20 minutes. This thickens the chromium oxide film from 3nm to 15nm. Rinse with deionized water and dry at 80℃. Passivation can improve the guardrail’s salt spray resistance from 200 hours to over 1000 hours.

3. Optional Coating: For Extreme Environments

In coastal areas with high salt content, apply a 5-10μm thick fluorocarbon coating after passivation. Use electrostatic spraying with a curing temperature of 200℃. The coating has a glossiness of 60-80 and a weather resistance of over 15 years, preventing seawater corrosion.

Common Processing Issues and Solutions

Practical production often encounters problems like bending cracks and uneven surface color. Targeted solutions are essential for efficiency improvement.

Bending Cracks Caused by excessive bending speed or inappropriate die radius. Solution: Reduce the bending speed by 30%, replace with a larger radius die (e.g., 10mm radius for 8mm plates), and ensure the bending direction is consistent with the steel’s rolling direction.

Angle Rebound Exceeding Standard Resulting from insufficient dwell time or unrelieved stress. Optimize by extending the dwell time at the bottom of the press by 2 seconds and re-performing stress relief annealing for the steel plate.

Uneven Surface Color Due to uneven polishing pressure or contaminated passivation solution. Adjust the polishing pressure to keep it consistent, and replace the passivation solution every 500 workpieces.

Application Case: Urban Bridge Guardrail Project

A municipal engineering project adopted hot-rolled 304 steel plates (6mm thickness) for 2km of bridge guardrails, requiring 90° bends for straight sections and 135° bends for curved sections, with a service life guarantee of 25 years.

Processing process: CNC plasma cutting → stress relief annealing → surface cleaning → CNC press brake bending (parameters: 200kN force, 3mm/s speed, 5s dwell time) → angle correction → 240#-800# polishing → nitric acid passivation.

Inspection results: Bending angle deviation was ≤0.5°, surface roughness Ra0.3μm, salt spray resistance 1200 hours. After 2 years of operation, the guardrails showed no rust or color fading, meeting the design requirements. Compared with galvanized carbon steel guardrails, the maintenance cost was reduced by 80%.

Conclusion: Precision and Protection Ensure Guardrail Quality

The processing of 304 stainless steel bridge guardrails is a systematic project, where 90°-135° bending angle control determines structural safety, and surface treatment affects service life. By optimizing pre-processing preparation, setting scientific bending parameters, and implementing multi-layer surface protection, the guardrails can achieve both mechanical performance and corrosion resistance. As infrastructure construction pursues higher safety and durability standards, the application of hot-rolled 304 steel in bridge guardrails will become more widespread. Continuous optimization of processing techniques will further reduce costs and promote the development of high-quality bridge safety facilities.