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A Study On Grain Evolution Characteristics Of P91Steel And Numerical Simulation Of Hot Extrusion Of Thick Wall Tube

Posted on:2014-01-31Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y W DiFull Text:PDF
GTID:1221330398478021Subject:Mechanical design and theory
Abstract/Summary:PDF Full Text Request
In order to simulate both "’forming" and "property improvement" during hot forging, the macroscopic mechanical behaviors and grain evolution under the condition of hot forging are studied experimentally for P91alloy steel. A constitutive model, which considers macro-microscopic coupling and corresponds to whole processes of hot forging, is constructed. A set of user subroutines of commercial software is redeveloped for grain size evolution prediction. The redeveloped software is used to simulate deformation and grain evolution of a thick wall pipe during hot extrusion process. The following innovative results are obtained in this paper.The quantitative relationship between mechanical behaviors and Deform parameters is obtained by means of hot compression experiment for P91alloy steel. An upward warping of true stress-true strain curves appear on larger strain positions. The contribution of both the friction between specimen and anvil and continuous rise of strain rate during constant velocity compression on the curves upward warping is analyzed. A method which can obtain the true stress-true strain curves with constant strain rate based on the data from the constant velocity compression or tension test, is put forward. The method is effective in eliminating curve upward warping.The static and dynamic grain evolutions and their relationship with Deformation parameters are obtained by means of metallographic observations of P91alloy steel. It is found from the experiment that both grain evolution and ratio of initial grain size Do and static grain size Ds are dependent to the parameter Z. Such as, the case of Do/Ds>1and higher Z value corresponds to the dynamic recrystallization with single peak stress-strain curves, and the case of Do/Ds<0.5, lower Z value corresponds to the dynamic grain growth with multiple peak stress-strain curves. The case of0.5<Do/Ds<1and lower Z value corresponds to a phenomena of firstly grain refinement and then grain growth, where the multiple peak character of the stress-strain curves is not evidence.Based on experimental observation and existing knowledge, a model of dynamic grain growth is developed. A relative grain size evolution equation is derived in the irreversible thermodynamics frame. The evolution equation illustrates the contribution of dislocation energy and grain boundary energy to grain growth and their variation during grain growth. The grain growth model is used to analyze grain growth process of P91alloy steel, and the calculated grain size and experiment result are basically in agreement. A macro-microscopic coupling constitutive model is constructed here. The constitutive model includes dynamic recrystallization, dynamic grain growth and static grain growth and can be used for numerical simulation and grain size prediction during whole hot extrusion process. All material parameters in the constitutive model are measured and verified for P91alloy steel. The user routine for the commercial software DEFORM is redeveloped in basis of the constitutive model in order to predict grain evolution during whole hot extrusion process.The scaled hot extrusion experiment of thick-wall pipe of P91alloy steel is carried out. The whole hot extrusion process includes heating in furnace, extrusion and air cooling. By using the constructed constitutive model, the redeveloped user routines of DEFORM and all measured constitutive parameters of P91alloy steel, the whole process of the scaled experiment is simulated and the grain size during the whole process is predicted. The major microstructure variables, such as various grain sizes, volume fraction of microstructure evolution and etc., are in a good agreement. It shows that the constitutive model for whole hot extrusion process, the redeveloped user routine and the measured material parameters of P91alloy steel are reasonable and reliable.
Keywords/Search Tags:whole process of hot forging, constitutive model, grain evolution equation, dynamic grain growth, P91alloy steel, hot extrusion, numericalsimulation upward warping of stress-strain curves
PDF Full Text Request
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