Internal Defects and Non-Destructive Testing Control of Rail Welded Joints
What are the most common internal defects in rail welded joints?
These mainly include: incomplete penetration, slag inclusions, porosity, cracks, gray spots, coarse microstructure, overheating, and segregation. These defects significantly reduce the joint's strength, toughness, and fatigue life, and are prone to rapid propagation under repeated train loads, leading to sudden fracture.
Why must flash welded joints undergo full-section ultrasonic testing?
Flash welds have large weld surfaces, short welding times, and large temperature gradients, making them highly susceptible to planar defects (gray spots, incomplete penetration) at the weld center and near the fusion line. Ordinary inspections are difficult to detect these defects; only full-section ultrasonic scanning can cover the entire weld cross-section, ensuring no defects are missed. This is a mandatory requirement for high-speed rail and heavy-haul lines.
What are the defect characteristics and key points of inspection for aluminothermic welded joints?
Aluminothermic welding has a cast structure, and defects are mostly volumetric: porosity, slag inclusions, shrinkage cavities, and cracks. The key inspection areas are the rail head and rail base triangular regions, as these areas experience the greatest stress and are also high-risk areas for defects. Simultaneously, attention should be paid to crystallization cracks, which are small but extremely dangerous.
What are the criteria and handling principles for different defects?
Both national and railway standards clearly state that cracks, incomplete penetration, and overheating are fatal defects that must be scrapped and re-welded. Porosity, slag inclusions, and gray spots are graded according to size, quantity, and location; exceeding the limits results in disqualification. Stricter criteria apply to critical load-bearing areas such as rail heads and rail bases, allowing no defects exceeding the standards.
How to improve the first-pass yield of welded joints?
Strictly control end-face treatment, welding process parameters, upsetting amount, and heat treatment curves; perform rust removal, water removal, and oil removal before welding to ensure clean end faces; use automated welding equipment to reduce fluctuations from manual operation; implement "inspect immediately after welding, rework immediately if unqualified," and prevent defective joints from being put into production.