The material of the QU120 crane rail is U71Mn. It can be seen from the chemical composition that U71Mn is a kind of high carbon steel with high carbon, phosphorus and sulfur contents, insufficient weldability and high crack sensitivity. It must be preheated with arc welding. In order to avoid martensite in the heat affected zone of the weld and ensure the quality of the final rail weld, preheating should be carried out at 350 °C – 400 °C during welding. QU120 crane rail joint should be hoisted in place and a piece of 8mmx50mmx200mm steel plate should be placed directly under the interface. The corner of the interface section is polished with a sander. The alignment tolerance of the top and side faces is ≤0.5mm, and it’s better to make the reserved gap of the butt joint kept at 15mm-20mm. Since the base material of QU120 crane rail has high carbon content, large rigidity, and large thickness, it is necessary to select an alkaline low-hydrogen electrode with high resistance to cold cracking and thermal cracking resistance and similar strength level. Before welding, the electrode must be baked with oxyacetylene flame. The baking temperature is controlled at 350 °C-400 °C. After the temperature is reached, it is placed in the previously prepared heat preservation tube for heat preservation. During welding process, if the DC welding machine is used, the polarity should be reversed. An AC welder can also be used, but the current is large.
Before the welding operation, the 100 mm area on both sides of the rail interface and the interface was repeatedly baked with an oxyacetylene flame, and the temperature was controlled at 350 ° C – 400 ° C for 2 hours, and then welding was started from the joint of the QU120 crane rail section and the bottom steel plate. During the welding process, the welding process must always ensure that the QU120 crane rail and the bottom steel plate are completely fused together. The weld seam of each layer is kept at 3mm-4 mm. After each welding, the slag is cleaned and then the next layer is welded. This layer-by-layer welding, layer by layer, until the entire welding process is completed. It should be noted that during the welding process, the rail interface and the 100mm area on both sides of the interface should be baked and kept in time to ensure that the temperature is not lower than 180 °C during the welding process to ensure the quality of the rail welding.