In engineering components, high-strength stainless steels are well known for their corrosion resistance and excellent combination of strength and toughness. Heat treatment can change their properties and make them suitable for a variety of applications such as offshore platforms for oil extraction, cutting tools, steam generators, pressure vessels, and blades. Manufacturing of these large components starts with the sequence of casting, forging, quenching, and tempering operations. The main aim of forging the as-cast ingot is to eliminate the casting defects that occur during solidification including shrinkage, segregation, porosity, non-metallic inclusion, and cracks. These defects are initiated during the teeming of the liquid steel and/or during solidification in the mold. Although casting process optimization has been used to reduce the incidence of these defects, preventing them is very difficult. A detrimental defect causes heterogeneous properties in the final cast ingot, which significantly affects the performance and suitability of the final products. However, if the defects are too big, it is very difficult to heal all the porosity just by the forward forging process. As a result, the appropriate and accurate forging schedule is significant for heavy manufacturing industries. It is the main purpose of this work to examine the mechanisms involved in the microstructural evolution of a martensitic stainless steel used in oil and gas industries as a turbine shaft during the breakdown of the as-cast structure. In addition, the hot deformation behavior of the investigated steel was modeled using the best predictable material model. Using a finite element code, the constitutive equations and microstructure evolution models are implemented, and then thermomechanical and microstructure evolution are coupled.
The first part of this study provides the microstructure characterization of casting structures during solidification. Advanced characterization techniques were used to determine the phase distribution, chemical composition, and phase identification. All determining factors were considered to investigate the occurrence of phase precipitation and segregation during the casting process.
The second part of this study is focused on isothermal hot compression tests using the thermomechanical simulator machine, Gleeble TM 3800, in order to determine the flow behavior of the investigated martensitic stainless steel under a wide range of temperatures and strain rates corresponding to the industrial forging process for large size ingots. The measured flow stresses were corrected by considering the effect of friction and adiabatic heating and then used to develop a model in order to predict the flow behavior of the material. The mathematical equations that best predict the flow curves model were established and the accuracy of the developed material model was verified with experimental measurements.
Heavy forging is a highly nonlinear process in which plastic deformation and recrystallization change both the microstructure and the boundary conditions. In hot forging, microstructure evolution impacts deformation behaviors due to its effects on dynamic recovery, dynamic recrystallization, and grain growth. At the same time, forging parameters such as strain, strain rate, temperature, and pass sequence control microstructure evolution. Therefore, the third part of this project was allocated to investigating the effect of working parameters on microstructure evolution during the hot forging of martensitic stainless steel. After experimental analysis of microstructure evolution, the mathematical equations which describe the microstructure changes were developed.
The next step was integrating the constitutive equations and microstructure models into a 3D finite element code to predict the variations of strain, dynamic recrystallization fraction, DRX grain size, and damage. The most important criterion in the forging industry is to determine the deformation conditions at which the ingot is susceptible to cracking and failure of the component. Therefore, the prediction of the risky deformation conditions or areas of the component is of great importance.
After the validation of all models, the forging process of a 13.5-ton ingot was simulated by the developed system based on industrial operational practice. The influence of working parameters including temperature, strain, and strain rate on microstructure evolution was analyzed. Simulation of the forging process has been used to analyze strain, temperature, dynamic recrystallization volume fraction, and grain size variations. Based on the developed simulation system, an optimum hot forging process was designed.
| Date | 27 Dec 2022 |
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| Original language | American English |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Mohammad Jahazi (Supervisor) & Gholamreza Ebrahimi (Co-supervisor) |
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