Research On Ductile Fracture Criteria Of Structural Steels Used In Nuclear And Thermal Power During Hot Forming At High Temperature And In Their Applications

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.