| 其他摘要 | Large semi built-up crankshaft is one of the most important components of a low speed diesel engine, which acts as the heart of a large ship. The manufacturing process of a marine crankshaft with a length of over 20 meters, a weight of over 200 tons usually includes: ingot vacuum metallurgy, vacuum pouring, bend forging, heat treatment, rough machining, shrink fitting and finish machining. The ability of manufacturing large marine crankshaft is a significant advantage in ship building and hot working for a country. In this study, the bend forging, the heat treatment, and the shrink fitting processes were investigated by FEM simulation and experiment, and the optimized designs for these processes were proposed and applied successfully in several forging plants. The main contents and results in this study include:
1. The constitutive equations of steel S34MnV were established based on measured stress-strain curves. The traditional forging process was simulated by a coupled thermal-mechanical FEM, the deformation mechanism of crankthrow and the safety of the mould were analyzed, the forging defects including the necking in the root of crankweb, the “horn mouth” and the folded crack on the crankthrow were predicted and validated by a industrial trial. A reverse deformation method was proposed to optimize the shapes of the preformed blank, and the key dimensional factors of the mould that affecting the forging quality were also investigated. In order to avoid the forging defects, we proposed a preformed blank with a “V” shape dent in the middle, combined with a top die with a value of 0.4 for the thickness coefficient, and a lower die with a value of 1.7 for the opening width coefficient and a value of 12° for the sloping degree of the lower die cavity. Based on the simulated results, a series of new designs on moulds and the near net-shape forging procedure were proposed. As a result, the volume of forging material can be saved for about 15% and the resistance of bend forging was 72% that of by conventional method. Finally, the new design was applied in actual forging process, and qualified crankthrow blanks with reasonable shape and dimensions were obtained.
2. An empirical model predicting microstructure evolution and mechanical properites in crankthrow forging and heat treatment process was established. The austenite recrystallization and grain coarsening in the forging process, and the austenite transformation in the heat treatment process for the K90MC-C crankthrow were simulated, and the heat treatment distortion of the crankthrow was also investigated. It is found that metadynamic and static recrystallization are two main microstructural evolution mechanism in the bend forging process, and fully dynamic recryllization merely occurs in the connected region between crankweb and crankpin. In the heating process, the time difference of occuring austenization for different regions on crankthrow reaches 7 hours, and the difference between the grain sizes on crankthrow reaches 40 μm. The final distribution of microstructure and mechanical properties on the crankthrow are as follows: in the crankpin region, the percentage of ferrite is richer, and the grain size and the perlite interlamellar spacing is relatively larger than that in the crankweb region; the mechanical strength is relatively higher in the surface of crankweb than that in the center of the crankpin. The difference of tensile stress between the inner and outer surface of the crankthrow leads to nearly 10 mm increasement for the distance between two crankwebs.
3. An experimental forging and heat treatment process was performed, a piece of crankthrow forging with a weight of 38 tons was cutted, and the microstructure and the mechanical strength were tested. The experimental data confirms the good prediction on the microstructure and mechanical properties distribution of the crankthrow, and the prediction error of mechanical strength is within 5%. Mixed crystal and segregation were found in the specimen with low impact toughness, which were considered to be the main factors that affecting the impact toughness of crankthrow. The fatigue strength of the material that sustained largest stress on crankthrow in working condition was tested, the results show that the fatigue strength in this region reaches 250 MPa, the aluminium and magnesium oxides were considered to be the key factors degrading the fatigue strength of crankshaft material.
4. An equation to calculate the shrink fit allowance was derived. The shrink fitting process was investigated through 3D FEM simulation. Series of heating tests were performed to verify the FEM results. The expansion and shrinkage of the bore were tracked. It is found that increasing the temperature gradient between the area near pin side and the area near round side can obtain a satisfied bore roundness. The contact behavior between the bore and the journal during the shrink fit process was studied. It is found that the contact pressure on the bore/journal interface reaches a stable value after cooling for 320 minutes, and the whole part can be lifted out of the operating pit safely, as a result, the waiting time is reduced to 16% that of the original operation. The distortion of the crankshaft in shrink fitting process was investigated, it is found that the crankweb bends upward and forward in heating process, then bend downward after final shrink fitting, which leads to a trapezoidal shape of the bore cross section. The journal’s centerline offsets away from the crankpin centerline for about 0.5 mm and rotates in clockwise for about 0.044°. According to the FEM results and analysis, optimized suggestions were proposed and applied in actual shrink fitting processing to eliminate the distortion. As a result, the machining time was reduced for 33% compared with the original process. |
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