The section of the crane track is I-shaped, the top of the track is convex, and the bottom has a certain width to increase the contact surface with the foundation, with good bending strength, although the production standards and specifications are different, but the crane track is usually rolled with U71Mn steel with a high content of C and Mn. Considering the smoothness of the crane operation, the continuity of the crane track and the smooth transition of the joint are essential to reduce the impact load of the operation, so the crane track usually needs to be welded. Because U71Mn steel belongs to high carbon medium manganese steel, the welding performance is poor, and the particularity of the track section, the welding of the track is more difficult.
U71Mn is a high carbon medium manganese steel. With the increase of C content, it is easy to get a tough martensitic structure during welding cooling, and the strength, hardness and wear resistance are also improved. Generally, manganese steel is wear-resistant, but it is easy to produce low-temperature martensite in the welding process, and its strength and impact toughness increase with the increase of manganese content. Carbon equivalent Ce is calculated as follows:
Welding construction steps of rail joints
The rail joint is hoisted into place and a steel plate with the size of 8mm×50mm×200mm is placed directly under the interface. The edge and corner of the interface section are polished with a polishing machine. The amount of wrong edge on the top and side is less than 0.5mm, and the reserved gap of the butt is maintained at 15-20mm. The straightness of both ends of the track should be checked with a straight ruler during assembly, and the straightness should not be greater than ±1mm within the 2m range. The deviation between the left and right side of the track interface and the height of the rail top is ±0.5mm. After alignment, baffles should be positioned on both sides of the track, and the track should have enough freedom in heating expansion and contracting after welding cooling. The joint needs to do reverse deformation, generally add a backing plate under the joint, add a height of L/1000(L is the total length of the docking track), and generally put 6mm reverse deformation of the 6m track.
Selection of welding material
Because the carbon content of the base material is high, and the rigidity is large, and the thickness is large, it is necessary to choose the alkaline low hydrogen type electrode with high resistance to cold cracking and hot cracking and the strength level is comparable. In line with the principle of simple material selection on the basis of application, J507 welding rod is selected as the welding material. J507 is a low hydrogen sodium type alkaline electrode, its tensile strength relative to E4315 and other ordinary electrode is much larger, generally used for large force or dynamic load of steel structure welding, and can be all-position welding.
Rust, paint, moisture and other impurities must be removed from both ends of the track within 150mm before welding. Before welding, it is necessary to bake at 350 ~ 400℃ for 1 hour, and then put it in 100 ~ 150℃ constant temperature bucket with use, and the welding rod must be used up within 4 hours of receiving; If the electrode is damp, it can only be re-dried once; Close the lid immediately after removing each electrode from the electrode holder to protect the electrode from moisture. When welding, if the DC welder is selected, the polarity should be reversed; Ac welding machine can also be used, but the current is larger. Dc welding machine can be used, the selection of 4mm diameter electrode, the current is adjusted to 210 ~ 220A. If there is wind when the track is welded, the wind baffle should be used to block the wind source to avoid porosity and cracks in the joint.
Before the welding operation, the rail interface and the 200mm area on both sides of the interface were repeatedly baked with oxyacetylene flame, the temperature was controlled at 300℃ ~ 350℃, and the preheating time was 20min. The preheating temperature can fluctuate according to the air temperature, such as the temperature is preheating above 10℃, the temperature is the lower limit. Each joint principle of the last welding is completed, the welding layer should not be lower than the minimum preheating temperature, such as forced to interrupt, and in the middle of a long time to stop welding, must be re-preheated before welding. The welding starts from the joint of the rail section and the bottom plate. In the welding process, the initial welding process must always ensure that the rail and the bottom steel plate are fully integrated together, each layer of weld is maintained at 3 ~ 4mm, and the slag removal treatment is carried out after each layer of welding, and then continue to weld the next layer. So layer by layer welding, layer by layer cleaning 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 insulated in a timely manner to ensure that the temperature is always maintained at not less than 180 ° C during the welding process, so as to ultimately ensure the quality of rail welding.
After the rail welding is completed, both sides of the rail joint should be heated within 200mm on each side of the track welding, and heated to 650 ~ 750℃ with oxygen-acetylene neutral flame for 20 ~ 30min. Finally, the joint and the 100mm area on both sides should be piled with lime for 2h to keep warm, so as to facilitate the escape of diffused hydrogen in the weld. To eliminate welding stress and heat affected zone hard brittle phenomenon. After the weld is cooled by heat treatment, the track is laid flat, the rail surface is smoothed by a rail grinding machine, and both sides and the padding are smoothed by wind or electric grinding, which should be smooth and consistent with the shape of the track, and can be inspected by a template or ruler. 100%MT inspection shall be carried out 24h after finishing the track joint grinding. If defects are found on the surface or inside, they need to be repaired in time. If the sanding is not completely removed, you can use carbon planing to polish and clean up. Before and after the carbon grinding process, it is necessary to preheat the operation, and it is necessary to do a good job of heat preservation after welding.