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Welding repair of shearing beam of gantry shearing machine
The cutting edge beam of a large gantry shear machine imported from the former Soviet Union had suffered a serious fracture after years of operation. The beam, made of gray cast iron (equivalent to China's QT400-15 inoculated cast iron), was difficult to repair due to poor weldability. A previous attempt using the wire planting method resulted in worsened microstructure and reduced weldability, leading to the machine being taken out of service, despite its high value.
Later, a combination of riveting and cerium block welding was successfully applied. This method not only restored the structural integrity of the beam but also allowed the machine to be reused after years of inactivity, bringing significant economic benefits. The beam had a flat cross-section with a wall thickness of 95 mm, and the crack extended over 2 meters. Due to the large size of the welding area, post-weld machining was required.
To ensure quality, the welds and heat-affected zones needed to be free of white cracks and hardened structures. Additionally, the welding workload had to be minimized. By using 235 steel strips of varying lengths in the grooves, the welding process became more efficient, reducing both time and stress. Eight electrodes were used for welding and riveting, resulting in higher strength, better plasticity, and lower thermal expansion, which improved crack resistance.
Welding rods of type 422 were used for the shovel blocks and seams due to their good workability and low cost, making slag removal easier. Before welding, the area was cleaned with a torch to remove oil and moisture. The fracture surface was ground with an angle grinder to eliminate any unfavorable structures from previous repairs. To prevent oxidation, the polished area was covered with plastic wrap.
Each fractured part was machined separately, and the groove angle after assembly was approximately 70 degrees. Steel bars with a side length of 10 mm were used. The welding process began by preheating the round hole with a welding torch until it turned dark red, followed by layer-by-layer welding and hammering with a round-headed air gun to relieve stress. For continuous riveting, the area between two rivets was preheated to 450°C before welding, and each weld was tapped immediately with a small hammer to reduce residual stress.
After the transition layer was riveted, a grid of 408 welds was formed, and 8 welding rods were used to fill the gaps between the grids. Each grid was hammered after welding to minimize stress. The 235 steel bars were positioned to hold the beams in place, while angle steel bars were welded layer by layer in the seam. When placing the angle steel bars, they were arranged in a latitude-longitude pattern. The 422 electrode was used for solid welding.
The steel bar 2408 cladding was filled with the 422 electrode, ensuring that all gaps were properly filled. Slag removal was carefully performed. After welding, the weld was tapped with a round hammer until it was fully filled. Using this method, the beam was successfully repaired and has been functioning normally ever since.