| Crawler crane is a kind of important machinery in the engineering crane industry, and is the main product of a company. The boom of crane is the most important part of the equipment, and boom is made of high strength low alloy steel to reduce the weight and bearing capacity of unit weight. However, the high strength low alloy steel is easy to produce a series of welding problems, such as embrittlement and cracking of welded joints, which often lead to failure of the structure.The main and abdominal chord of boom often appear tear of welded joint. In order to optimize the original welding process technology of the main and abdominal chord and improve the quality of welded joint, BG770 QL and 20Mn2 high strength low alloy steels using in the crane boom are the subject investigated. In this thesis, evaluation of appropriate preheating temperature through numerical simulation, select the appropriate welding materials through test and explore new welding method. Finally, a series of welding studies are conducted to evaluate the new welding process plan and achieve the good performance of welded joints.In view of the BG770QL+20Mn2 dissimilar steels which are used in the crane boom, the two kinds of high strength low alloy steels need to be preheated by analyzing the weldability of steels. The FEM(finite element method)software ABAQUS is used to simulate the dynamic welding temperature and stress fields in Y-Slit type cracking test on steel board, analysis the effect of different preheating temperature on the distribution of temperature field and stress field, and guess appropriate preheating temperature, it is concluded that the high preheat temperature should not be used for welding the dissimilar steels, and 100℃ is suitable. The dissimilar steels are welded through the mixed Ar+CO2 gas shielded welding method, preheating temperature of 100℃, T Union GM90 and T Union GM100 welding wires. The welded joints with different welding materials are analyzed by microstructure observation, mechanical property test, fracture morphology observation, and analysis of the strength and toughness of welded joints by microstructure. Analysis the influences of welding materials with different strength matching on the mechanical property and microstructure of welded joints. The comparative analysis shows that the comprehensive performance of the welded joint using Union T GM100 is better. In view of the dissimilar steels, using the laser welding, to explore the feasibility of laser welding of and study whether the laser welding can be applied to the actual production of boom. Laser welding test shows that and formability of the welding seam is well under different welding heat input conditions, and weld joints don’t have porosity, crack and so on. The chemical composition of base metals in the weld pool are uniform when welding the dissimilar steels, the microstructure and properties of welded joints are evenly distributed, and dissimilar steel joints don’t appear softeneing phenomenon in the heat affected zone. Preliminary determination of the dissimilar steels between BG770 QL and 20Mn2 can be welded by laser welding.To sum up, in order to optimize the original welding process technology and obtain better welded joint, the boom with BG770 QL and 20Mn2 dissimilar steels need to be preheated, and selection of preheating temperature 100℃ is suitable. The comprehensive performance of the welded joint using Union T GM100 is better. Laser welding with advanced welding method can weld the material of the dissimilar steels. |
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