After the 50×50 crane rail square steel is welded and the module is removed, the welded joint should be heat treated in time to slow down the cooling rate, improve the structure and performance of the joint, and simultaneously make hydrogen easily diffuse and discharge to reduce the occurrence of cold cracks. Considering the conditions at the construction site, the 50×50 crane rail square steel joints are treated with medium temperature tempering with oxyacetylene flame. The torch flame is adjusted to a neutral flame, and the welding zone is heated simultaneously on both sides of the 50×50 crane rail square steel by two welding torches, from top to bottom, and repeated until the temperature of the welded joint reaches about 350°C. A torch can be used for insulation. Reduce the jet velocity of the mixed gas and the temperature of the flame, and use the welding torch to continuously heat the weld on both sides of the 50×50 crane rail square steel for about 30 minutes. After heating, cover the weld around with the high temperature material such as asbestos, and let it cool naturally to the ambient temperature. After the joint temperature drops to room temperature, the top and sides of the 50×50 crane rail square steel are polished.
The slag inclusion is due to the fact that the weld slag is not completely discharged or cleaned during welding and remains inside the weld. During the welding operation, the arc blow force and the gap between the side mold and the 50×50 crane rail square steel can be appropriately increased, and the welding slag of the previous layer is cleaned and then welded to the next layer. The crack generated near the fusion line is a cold crack, which is due to the hardening tendency of the base material, the restraining stress, and the action of hydrogen. The effective process to reduce cold cracking is to further improve the pre-weld preheating temperature (above 300°C) and the post-weld tempering temperature (above 400°C). Preheating is necessary after installation of the side mold and before welding the waist. Local unfused is mainly because the high temperature residence time of the arc at the fusion line is too short, and the arc is not in place. This requires the welding operator to control the heat input during the welding, so that the weld metal and the base material can achieve mutual fusion on atomic level.
Practice has proved that although the welding performance of the QU crane rail is poor, as long as a reasonable welding process is adopted, the internal structure and mechanical properties of the rail welded joint can fully meet the design and specification requirements.