Refined Processing Technology for Rail Welded Joints and Smoothness Adaptation Solutions for Different Track Lines
What is the core technology of profile grinding for rail welded joints in high-speed railways?
The core of profile grinding for rail welded joints in high-speed railways is to achieve seamless connection between the joint and the base metal profile of the rail. First, a CNC profile grinding machine is adopted, which is equipped with a laser profile scanner that can collect the profile data of the rail base metal in real time and generate an accurate grinding path. Grinding is divided into three stages: rough grinding, fine grinding and polishing. In the rough grinding stage, a 120-mesh grinding wheel is used to remove the excess height of the welded joint, and the excess height is controlled within 0.2mm; in the fine grinding stage, a 240-mesh grinding wheel is used to correct the profile deviation of the joint, with the profile deviation ≤0.1mm/m; in the polishing stage, a 360-mesh grinding wheel is used to reduce the surface roughness of the joint, with the roughness Ra≤0.8μm. During grinding, it is necessary to control the grinding temperature, adopt air cooling for cooling, avoid secondary quenching of the joint part due to high temperature, which may lead to uneven hardness. After grinding, profile detection is carried out, using a rail profiler to detect the joint profile point by point to ensure that the coincidence degree with the base metal profile is ≥99%. At the same time, wheel-rail contact stress test is carried out, with the contact stress peak value ≤110% of the base metal, meeting the smoothness requirements of high-speed railways.
What are the key points of heat treatment strengthening design for rail welded joints in heavy-haul railways?
The core of heat treatment strengthening design for rail welded joints in heavy-haul railways is to improve the fatigue resistance and wear resistance of the joint. First, medium-frequency induction heating equipment is used for normalizing treatment of the welded joint, the heating temperature is controlled at 850-900℃, and the holding time is 10-15 minutes, so that the metallographic structure of the joint is transformed into uniform pearlite and the welding stress is eliminated. After normalizing treatment, tempering treatment is carried out, the heating temperature is 350-400℃, and the holding time is 20 minutes, reducing the hardness gradient of the joint, with the hardness difference ≤HRC3, avoiding stress concentration. After heat treatment, hardness testing is carried out on the joint, and the hardness of the joint must be consistent with the base metal, controlled at HRC28-32, ensuring that the wear resistance of the joint is equivalent to that of the base metal. In addition, flame quenching treatment is carried out on the tread part of the joint, the quenching depth is 2-3mm, and the surface hardness is increased to HRC58-62, enhancing the wear resistance of the joint to adapt to the high-frequency impact of heavy-haul trains. After heat treatment, ultrasonic flaw detection shall be carried out to ensure that there are no cracks, slag inclusions and other defects inside the joint, and the defect detection rate reaches 100%.
What are the low-cost smoothness treatment measures for rail welded joints in ordinary-speed railways?
The low-cost smoothness treatment of rail welded joints in ordinary-speed railways needs to balance effect and economy. First, a manual profile grinding machine is adopted, and grinding is carried out with a rail profile template, the profile accuracy of the template is ±0.1mm, which can ensure that the ground joint profile meets the standard. During grinding, the wet grinding process is adopted, adding water to cool the grinding wheel and the joint part, avoiding high-temperature damage to the joint material caused by dry grinding, and reducing dust pollution at the same time. The excess height of the ground joint is controlled within 0.5mm, and the surface roughness Ra≤1.6μm, meeting the smoothness requirements of ordinary-speed railways. For the undercut defect of the welded joint, it is treated by repair welding and grinding, the repair welding material selects electrodes matching the rail base metal, and grinding is carried out after repair welding to ensure that the joint surface has no protrusions. In addition, a double-layer protection process of anti-rust primer + topcoat is adopted for anti-corrosion treatment of the welded joint. The primer is epoxy zinc-rich primer, the topcoat is polyurethane topcoat, the coating thickness ≥80μm, and the salt spray test corrosion resistance time ≥800 hours, adapting to the outdoor environment of ordinary-speed railways.
What are the non-destructive testing technologies and defect judgment standards for rail welded joints?
The non-destructive testing of rail welded joints mainly adopts three technologies: ultrasonic flaw detection, magnetic particle flaw detection and penetrant flaw detection. Ultrasonic flaw detection is used to detect volumetric defects such as cracks and incomplete penetration inside the joint. Combined detection with dual-crystal straight probes and angle probes is adopted, the detection depth can reach the full section of the rail, and when the defect equivalent ≥φ2mm, it shall be judged as unqualified. Magnetic particle flaw detection is used to detect crack defects on the surface and near-surface of the joint, adopting the wet continuous magnetization method, the concentration of the magnetic suspension is controlled at 10-20g/L, and when the defect display length ≥2mm, it is judged as unqualified. Penetrant flaw detection is used to detect open defects on the joint surface, adopting the dye penetrant method, the penetration time ≥10 minutes, and when the defect display width ≥0.1mm, it is judged as unqualified. The defect judgment standard is divided according to the line type. The internal defect equivalent of high-speed railway joints ≤φ1mm, and the surface defect length ≤1mm; the internal defect equivalent of heavy-haul railway joints ≤φ1.5mm, and the surface defect length ≤1.5mm; the internal defect equivalent of ordinary-speed railway joints ≤φ2mm, and the surface defect length ≤2mm. Unqualified joints need to be repaired by welding or cut and re-welded to ensure that the joint quality meets the standard.
What are the quality acceptance and maintenance strategies for rail welded joints of different lines?
The quality acceptance of rail welded joints of different lines needs to formulate differentiated standards. The acceptance focus of high-speed railway joints is profile accuracy and surface roughness, with profile coincidence degree ≥99%, roughness Ra≤0.8μm, and fatigue life ≥2×10⁷ times; the acceptance focus of heavy-haul railway joints is hardness and internal quality, with hardness difference ≤HRC3 and ultrasonic flaw detection Ⅰ-level qualified; the acceptance focus of ordinary-speed railway joints is appearance quality and excess height, with excess height ≤0.5mm and no obvious surface defects. The maintenance strategy should be formulated according to the line type. The profile of high-speed railway joints is tested every quarter, and ultrasonic flaw detection is carried out every year. When the profile deviation exceeds the limit, grinding is carried out in time; the hardness of heavy-haul railway joints is tested every month, and magnetic particle flaw detection is carried out every six months. When the hardness is uneven, supplementary heat treatment is carried out; the appearance of ordinary-speed railway joints is inspected every six months, and the anti-corrosion coating is maintained every year. When the coating falls off, it is touched up in time. In addition, establish a full life cycle file of welded joints, record welding process parameters, test data and maintenance situation, and realize traceable management of joint quality.