【作者】 何建丽；

【导师】 崔振山； 刘娟；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2014， 博士

【摘要】 韧性断裂是金属材料在塑性变形过程中发生剧烈塑性变形后，由于材料承载能力超过了其极限值而导致破裂的现象。在热成形过程中，材料在高温、大成形力作用下易发生韧性断裂，尤其以大型自由锻件的高温锻制最为典型。断裂的发生导致产品质量不合格、生产周期延长及生产成本剧增，从而阻碍了热加工制造的发展。因此预防热成形过程中开裂的发生是一亟待解决的问题。目前，国内外对于韧性断裂的研究主要集中在冷成形的断裂行为及断裂的预测模型，鲜有热成形的断裂行为及断裂判据的研究。本文以核、火电装备结构钢为主，研究了材料热成形过程中韧性断裂以及影响机制，提出了核、火电装备结构钢高温韧性断裂判据，并与大变形热力耦合有限元方法集成，预报了核、火电装备结构钢在热成形过程中裂纹的发生。通过镦粗试验及数值模拟，验证了高温韧性断裂判据的有效性和准确性。研究成果应用于大锻件生产中预测裂纹的发生，并可为在实际生产中制定合理的工艺参数提供指导。主要研究内容和成果如下：通过SA5083低合金钢、30Cr2Ni4MoV低压转子钢，X12CrMoWVNbN.10.1.1铁素体耐热钢和316LN奥氏体不锈钢四种常用于火电和核电装备结构钢的Gleeble热模拟高温拉伸实验及断口和显微组织的观察，获得了温度和应变速率对断裂应变的影响规律，建立了相应材料高温临界断裂应变模型，并提出了一种新的测量拉伸后断裂应变的等效体积法，以剔除非均匀颈缩变形的影响。所建立的临界断裂应变模型获得的预测值与实验值的吻合度较好。通过对SA5083低合金钢、30Cr2Ni4MoV低压转子钢，X12CrMoWVNbN.10.1.1铁素体耐热钢和316LN奥氏体不锈钢四种材料的高温断裂行为的研究，发现这四种材料的临界断裂应变随温度的变化曲线中都存在一个温度峰值。经不同温度下的微观组织观察，认为碳化物及第二相是诱发空洞缺陷的主要因素，解释了温度峰值与裂纹萌生的成因。针对预防材料在锻造成形过程中开裂的问题，基于临界断裂应变模型和损伤力学理论模型，提出了SA5083低合金钢、30Cr2Ni4MoV低压转子钢，X12CrMoWVNbN.10.1.1铁素体耐热钢和316LN奥氏体不锈钢的高温韧性断裂判据。判据认为当破坏累积值达到了临界值1时，断裂发生。考虑了应力三轴度、温度和应变速率对韧性断裂的影响。判据中参数物理意义明确，并易确定、便于工程计算。以SA5083低合金钢为例，比较和评价了工程中常用的五种非耦合韧性断裂判据(COCKCROFT&LATHAM判据、Normalized COCKCROFT&LATHAM判据、BROZZO判据、RICE&TRACEY判据和MCCLINTOCK判据)与提出的韧性断裂判据的准确性。结果表明，利用所建立的韧性断裂判据预测断裂临界压下率(裂纹开裂时试样被压下的高度与试样原始高度的比率)与试验结果吻合较好。相比于其它几种判据，具有更高的预测精度。通过高温拉伸实验和镦粗实验及数值模拟，验证了提出的高温韧性断裂判据的有效性和准确性。以30Cr2Ni4MoV低压转子钢和316LN奥氏体不锈钢为例进行颈缩实验的数值模拟，并与拉伸实验及实验后的断口形貌相对比。结果表明，裂纹萌生于颈缩中心处，这与扫描电镜下观察的微裂纹产生的位置相吻合；另外，以X12CrMoWVNbN.10.1.1铁素体耐热钢和316LN奥氏体不锈钢为例，设计了镦粗实验，并进行了与实验相一致的内嵌韧性断裂判据的数值模拟。数值模拟结果与实验结果吻合较好。将建立的韧性断裂判据应用于预测316LN奥氏体不锈钢直管锻坯和SA5083低合金钢蒸汽发生器椭圆封头在锻造过程中裂纹的发生，并分析了在多道次镦拔过程中，砧宽比、初始压下量、拔长初始温度和形变速度对开裂的影响，确定了最优的砧宽比、压下量及拔长初始温度等工艺参数。研究表明，建立的韧性断裂判据不仅能够预测在热成形过程中裂纹的发生，而且能够结合数值模拟分析和工艺优化，指导热锻成形工艺的制定，从而获得更好的成形质量。更多还原

【Abstract】 Ductile fracture occurs when the loading capacity of the material is beyond its limit load afterextensive plastic deformation during metal forming, which results in material rupture. In general, thematerial failure caused by cracking during hot deformation is considered as ductile fracture,especially in large free forging processes at high temperature. The occurrence of rupture leads tounqualified products, long production cycle and high cost, which hinders the development ofmanufacture during hot forming. So, it is urgent to prevent cracking during hot formings. Presently,for ductile fracture during metal forming, the studies mainly focus on cold forming, while lessresearch on hot forming is conducted. In this paper, researches on the behaviors of ductile fractureand the influence factors of ductile fracture of materials used in the nuclear and thermal power werecarried out. And the ductile criteria of different materials at elevated temperature were put forward.In conjunction with the numerical simulations by integration the proposed ductile fracture criteria(DFC) and finite element method (FEM), the occurrences of cracks were predicted in the hot forgingprocesses. In order to validate the efficiency and accuracy of the criteria, the upsetting tests with thecorresponding simulations were conducted. The achieved results were applied in predicting the onsetof cracks, instructing the actual production and making the optimal process parameter s in forgingprocesses. The main research contents are as follows:Based on the Gleeble thermo mechanical simulation tests of four types of steels widely used inthe nuclear and thermal power―SA5083,30Cr2Ni4MoV, X12CrMoWVNbN.10.1.1and316LN andthe observations of the tensile fracture morphology and microscopic structure, the influences of thetemperature and strain rate on ductile fracture were investigated and the models of the criticalfracture strain of four steels were put forward. And a new measurement method was proposedaccording to the method of the equivalent volume. Using this method, the non uniform deformationwas converted into the uniform one. Subsequently, the predicted values of the critical fracture strainare in good agreement with the experimental results.Based on the characteristics of fracture behaviors at high temperature of SA5083,30Cr2Ni4MoV, X12CrMoWVNbN.10.1.1and316LN, it is found that there exists a peak temperaturein each curve of the critical strain and temperature. In order to investigate the influence of the peaktemperature on the occurrence of cracks, the microstructures at different temperatures were observed. The results show that the carbides and the second phase particles are the mainly influence factors,which induce the generation of voids and micro-crack.Based on the model of critical fracture strain at the elevated temperatures and the theory ofdamage mechanics, the ductile criteria of SA5083,30Cr2Ni4MoV, X12CrMoWVNbN.10.1.1and316LN were established by considering the influences of stress triaxiality, temperature and strain rateon ductile fracture. In the each criterion, the fracture occurs when the accumulative damage factor isgreater than or equal to1. the parameters in the criterion have clear physical meaning and are easy tobe determined.Taking SA5083steel as an example, the accuracy of five uncoupled ductile fracture criteria(COCKCROFT&LATHAM, Normalized COCKCROFT&LATHAM, BROZZO, RICE&TRACEYand MCCLINTOCK) often used in engineering and the proposed criterion was compared andevaluated. The result shows that there is the better agreement with the predicted critical heightreduction ratio and experimental one by using the new criterion, in which indicates the proposedcriterion has better predictive capability. In here, the critical height reduction ratio is defined as theratio of the height reduction to the original height of specimen when the cracks occur.By the tensile tests, upsetting tests at high temperatures and the corresponding numericalsimulations, the availability and accuracy of the proposed fracture criteria were verified. Taking30Cr2Ni4MoV and316LN steels as examples, the simulations of tensile test were carried out andcompared to the actual tests. The results show the cracks initiate and propagate from the centerregion of the necked specimen, which are consistent with the microscopic observation from thescanning electron microscope (SEM) pictures. Additionally, taking X12CrMoWVNbN.10.1.1and316LN steels as examples, the upsetting tests and the corresponding numerical simulations byimplanting the ductile criterion were performed. It was found that the experimental critical heightreduction ratio of each type of material is consistent with the predicted one, which indicates all of thecriteria of four types of steels can be used to predict the onset of ductile fracture at elevatedtemperatures.The new ductile fracture criteria of316LN and SA5083steel were used in predicting the onsetof cracks in the stretching process of main pipe of316LN billet and in the forging stretching processof the billet of SA5083steam generator head, respectively. By analyzing the influence of the anvilwidth ratio, the initial reduction, the initial temperature and the deformation speed on cracking inmulti stages forging stretching processes, the optimization parameters were determined. These results show that the new ductile fracture criteria not only predict the onset of cracks during hotforming, but also instruct the actual production and make the actual process parameter and the bestforging quality can be achieved through the numerical simulation analysis and the processingoptimization.更多还原