Research On Deformation And Fracture Mechanism Of Casting Austenitic Stainless Steel For Nuclear Power Plant

For evaluating the performances of homebred static casting, homebred centrifugal casting and French-made centrifugal casting Z3CN20-09M austenitic stainless steels, providing technical support for improving the safety of nuclear power station and prolonging the nuclear power plant operation life, the mechanisms of plastic deformation and fracture of Z3CN20-09M casting austenitic stainless steels treated at 400℃for 300h,3000h and 10000h were studied by in-situ tension under Optical Microscopy (OM) and scanning electron microscope (SEM).XueYuting and ZhangLinlin etc had studied conventional mechanical properties and thermal aging mechanism of different casting austenitic stainless steels by different thermal aging. The research shows that the strength of three kinds of casting austenitic stainless steel increased, and the plastic decreased, and the hardness of ferrite phase increased and the hardness of austenitic phase virtually unchanged with the increase of aging time. At room temperature, Thermal aging time has significantly impact on the strength and plasticity of domestic steel. Long-term ageing has significantly impact on the strength of French steel. Based on the conventional mechanical properties, the microcosmic mechanisms of casting austenitic stainless steel were investigated in greater depth in this thesis.The studies of OM in situ tension and SEM in situ tension show that sliding always happen first in austenite phase, initial slip line direction was roughly parallel with the ribbon ferrite phase distribution direction. As load increases, slide line in austenitic become closer at the same time from uniserial slipping to multi-slip systems. Most of slip line intersection into 60°or 90°in the same austenitic grain. Austenite around larger board strips or the reticular ferrite the deformation is small; Austenite surrounding small, fine strip or island shape ferrite the deformation is large. When the slip line of austenite was full of whole sample deformation area, the slip line began to appear in ferrite. Slip line appeared in ferrite was less, spacing was large and direction was unit, with load increases developed into sliding step. The distribution, size, and shape of ferrite phase determine the mechanical deformation of casting austenitic stainless steel.The results of the study first show that the slip line can pass through coherent boundary of y/a, and can't cross incoherent boundary of y/a. The conjugacy of austenitic and ferritic phase interface can decide the strength of y/a phase interface for the casting austenitic stainless. The higher level of the conjugate phase interface, the higher intensity is. Compared with static casting austenitic stainless steel, the centrifugal casting austenitic stainless steel has better the phase relationship and more coherent phase boundary, so y/a interfacial has stronger ability of plastic deformation in the centrifugal casting austenitic stainless steel. The study about the rules of plastically deform the phase interface do not see in any research.The crack of Z3CN20-09M casting austenitic stainless steel comes from three kinds of ways:phase interface, casting shrinkage or nearby impurity particles. When the angle contained by y/a interface and tensile stress direction is 90°, crack initiation will occurred in phase interface and along the interface to expansion, otherwise expand to ferritic phase. The extension direction of the crack initiation induced by casting shrinkage and foreign particle is decided by its location. The crack initiation induced by casting shrinkage and foreign particle in ferritic more likely to occur expansion, generally the extension direction perpendicular to the tensile stress direction, this is because there are so many microcrack which perpendicular to tensile stress direction gathered around the hole.Short thermal aging has no significant influence for the strength and deformation for y/a interfacial. As thermal aging time increases, the intensity of y/a interfacial decreases, The crack initiation will easily occur on y/a interfacial and expand to ferritic phase. Thermal aging time has biggest significant effect on the homebred static casting austenitic stainless steel and embrittle ferritic in it. Spinodal transformation is obviously occurred during the homebred static casting austenitic stainless steel after 3000h thermal aging in SEM.The OM in situ tension deformation processes of three kinds of steel had no significant differences. The OM dynamic tensile deformation process has no significant differences between original state and different thermal aging time of the three materials. As the aging time increased, the interface of a/y of homebred static casting austenitic stainless steel was smooth at first, and then became increasingly sharp, this trend could not be found in domestic centrifugal casting steel and French-made centrifugal casting steel.