Welding Process and Quality Control for National Standard Rails
- What are the differences in welding principles and application scenarios among flash butt welding, thermit welding, and gas pressure welding?
Flash butt welding applies voltage to both ends of the rail, causing an arc (flash) at the rail contact. After the contact surface is melted, a forging force is applied to fuse the rails. The tensile strength of the welded joint is ≥800MPa, which is suitable for in-plant long rail welding (such as welding 25m short rails into 500m long rails). It has high welding efficiency (about 5 minutes per joint) but requires special large-scale equipment and cannot be operated on-site. Thermit welding uses the high temperature (2500-3000℃) generated by the combustion of thermit (aluminum powder and iron oxide mixed in proportion) to melt the rail joint and flux to form a weld. The tensile strength of the joint is ≥700MPa, which is suitable for on-site rail joint welding (such as seamless track rail break repair). The equipment is lightweight (only requiring a crucible and mold) and easy to operate, but the welding time is long (about 30 minutes per joint) and the joint toughness is poor. Gas pressure welding heats the rail joint to a plastic state (about 1200℃) through a heating torch (acetylene-oxygen flame) and applies a forging force to make the rails fit and fuse. The tensile strength of the joint is ≥750MPa, which is suitable for on-site long rail welding (such as section seamless track laying). The joint has good toughness (elongation ≥12%), but it has high technical requirements for operators, who need to accurately control the heating temperature and forging force.
- What are the appearance quality requirements for GB standard rail welded joints, and how to judge the welding quality through appearance inspection?
Appearance requirements: The weld surface is flat, with no obvious protrusions (height ≤0.5mm) or depressions (depth ≤0.3mm); the weld edge has no cracks, slag inclusions, or air holes (air holes with a diameter >1mm are not allowed); the rail head, rail web, and rail bottom of the rail joint transition smoothly, with no steps (step height ≤0.2mm). During inspection, first, the naked eye is used to observe whether there are defects such as cracks and air holes on the weld surface, then a straightedge (accuracy 0.02mm) is used to measure the weld flatness and step height, and a magnifying glass (10x) is used to check for fine cracks. If the protrusion exceeds the standard, an angle grinder is used to grind it flat; if there are cracks or air holes, the defects must be removed and re-welded to ensure the appearance meets the standard.
- Which defects do the non-destructive testing methods (ultrasonic, magnetic particle) of welded joints detect respectively, and what are the testing standards?
Ultrasonic testing mainly detects internal defects of the joint (such as slag inclusions, incomplete fusion, internal cracks). A longitudinal wave probe is used to scan along the longitudinal direction of the rail, and a transverse wave probe is used to detect the rail head and rail bottom corner areas. The standard requires: no slag inclusions with an area >5mm² are allowed, the incomplete fusion length ≤2mm, and internal cracks are not allowed; if excessive defects are found, the defect location and size must be determined to formulate a repair plan. Magnetic particle testing mainly detects surface and near-surface defects of the joint (such as surface cracks, undercuts). The rail joint is magnetized and magnetic powder is applied, and magnetic marks will form at the defects. The standard requires: surface crack length ≤1mm, undercut depth ≤0.2mm; if the magnetic marks indicate excessive defects, the defects must be ground and removed, and re-welded before testing again.
- How do welding process parameters (such as heating temperature, forging force) affect the quality of welded joints, and how to control these parameters?
If the heating temperature is too low, the rail joint does not reach a plastic or molten state, which will lead to incomplete fusion of the joint and a decrease in tensile strength (may be <600MPa); if the temperature is too high, the rail grains will become coarse, the joint toughness will decrease (elongation <8%), and cracks are prone to occur. Control measures: Flash butt welding uses an infrared thermometer to monitor the temperature in real time (controlled at 1300-1350℃), thermit welding controls the thermit formula and combustion time (ensuring the temperature ≥2500℃), and gas pressure welding adjusts the heating torch firepower through a flame temperature meter (controlled at 1200-1250℃). Insufficient forging force will lead to loose fusion of the joint, gaps, and easy fatigue cracks; excessive forging force will cause excessive extrusion of the joint metal, forming folding defects. Control measures: Flash butt welding sets the forging force according to the rail specification (forging force ≥300kN for 60kg/m rails), and gas pressure welding accurately controls the forging force through a hydraulic system (calculated by the rail cross-sectional area, ≥15N per square millimeter) and records the forging displacement (ensuring ≥5mm).
- When cracks or insufficient strength occur in welded joints during use, how to analyze the causes and deal with them?
Cause analysis: Cracks may be caused by coarse grains due to excessive welding temperature, or gaps in the joint due to insufficient forging force, resulting in fatigue cracks under repeated train loads; insufficient strength may be caused by improper welding parameters (such as low heating temperature, small forging force) or defects such as slag inclusions and incomplete fusion in the joint. Treatment methods: First, determine the crack location and depth through non-destructive testing. If the crack depth is <3mm, it can be ground and removed for repair welding (manual arc welding is used, and the electrode is E5015); if the crack depth is ≥3mm or the strength is insufficient (tensile strength <700MPa), the joint must be cut off and re-welded according to the standard process. At the same time, record the welding parameters and test results, analyze the causes of parameter deviations (such as equipment failure, operation error), conduct trial welding after adjusting the process parameters, and ensure that the subsequent welding quality meets the standard.