| 大型船用曲轴热加工工艺模拟和组织性能控制 | |
| Alternative Title | Simulation and Microstructure-Mechanical Properties Control of Large Marine Crankshaft in Hot Working Process |
| 孙明月 | |
| Subtype | 博士 |
| Thesis Advisor | 李殿中 |
| 2009-05-22 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料加工工程 |
| Keyword | 船用曲轴 锻造 热处理 红套 组织演化 解剖检测 有限元模拟 |
| Abstract | 大型半组合式船用曲轴是低速大马力柴油发动机最核心的部件之一，是大型船舶的“心脏”。大型船用曲轴长度可达20余米，重量可达200余吨，制备过程包括钢锭真空冶炼、真空浇注、锻造、热处理、粗加工、红套和整轴精加工等工序，制造难度大，能否制造大型船用曲轴是衡量一个国家造船水平和热加工能力的重要标志之一。开展大型船用曲轴制造工艺研究，对于解决长期困扰我国造船业发展的瓶颈问题具有十分重要的意义。本文采用有限元模拟和实验相结合的方法，对船用曲轴曲拐的弯锻、热处理、红套过程进行了研究，提出了具体的工艺优化设计方案，并在多家企业获得应用。论文的主要研究内容和结论包括： 1. 建立了曲轴用钢S34MnV的高温本构关系模型。对传统工艺下曲拐的弯锻过程进行了热力耦合模拟，系统地分析了锻件的变形机制和模具安全性，预测并分析了采用传统工艺形成的细腰、减薄、喇叭口和折叠裂纹等锻造缺陷。提出反变形法优化预制坯形状，研究了模具的关键尺寸参数对成形质量的影响。模拟发现，在预制坯中间设置V形凹槽，并选择上模前端宽度系数为0.4、下模开口宽度系数为1.7、下模内模腔倾斜角度为12°时，可消除喇叭口和折叠裂纹等锻造缺陷。在此基础上，提出了一套完整的曲拐近终形锻造的预制坯和模具设计方案，与传统工艺相比，节约材料15%，变形抗力降为原来的72%。将新工艺应用到实际锻造过程，成功获得了无成形缺陷、加工余量均匀的曲拐锻件。 2. 建立了曲轴用钢S34MnV的高温组织演化模型，系统地模拟了曲拐弯锻和热处理过程的再结晶和奥氏体相变现象，预测了最终锻件的组织和力学性能，预报了热处理变形情况。模拟发现：曲拐弯锻时的组织演化以伪动态和静态再结晶为主，仅在曲臂与曲柄销联接区域发生了完全动态再结晶；热处理加热时锻件表层和心部奥氏体化开始时间相差7 h，加热结束后锻件表层晶粒比心部大40 μm。热处理后锻件的组织和力学性能分布规律为：曲柄销区域铁素体含量高、晶粒较粗大，珠光体含量低、片层间距较大，曲臂区域的组织分布与之相反；曲臂表层强度较高，曲柄销中心环形区域强度较低。正火冷却时曲拐内外表面形成的拉应力差造成两曲臂之间开口距离增大10 mm。 3. 试制了38 t重的K90MC-C型号曲拐锻件，并进行了解剖实验研究，获得了锻件上不同位置的组织和力学性能数据。实测数据与模拟结果吻合良好，力学性能预报误差率在5%以下。分析了锻件上个别区域材料冲击韧性偏低的原因，发现该区域存在混晶和偏析。对曲轴工作状态受力最大区域的材料疲劳强度进行了检测，结果表明材料的拉压疲劳强度为250 MPa，研究发现Mg、Al等氧化性夹杂物是导致个别试样疲劳强度偏低的主要因素。 4. 推导了红套过盈量的计算公式，建立了曲轴红套过程的有限元模型并获得了加热实验验证。通过分析加热时红套孔的膨胀和收缩规律发现，增大曲柄销和曲臂圆头之间的温差可获得具有高圆整度的红套孔。红套界面的变形和传热行为的研究结果表明，界面处的接触压力在插入主轴并冷却320 min后达到稳定值，此时可安全将红套件吊离地坑，使等待时间缩短为原来的16%。通过追踪红套孔的变形历程发现，加热时曲臂向前上方弯曲，冷至室温后转为向前下方弯曲，导致最终红套孔为上大下小的圆台形。研究发现红套后零件的结构变形规律为：主轴中心线向远离曲柄销一侧偏移0.5 mm，并顺时针偏转0.044°。在此基础上，提出了预防曲轴红套变形的措施并应用到生产中，使整轴加工机时缩短33%。 |
| Other Abstract | 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. |
| Pages | 149 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17145 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 孙明月. 大型船用曲轴热加工工艺模拟和组织性能控制[D]. 金属研究所. 中国科学院金属研究所,2009. |
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