A coupled numerical simulation between thermal-mechanical and microstructure evolution was realized through embedding the developed user subroutines into the FEM software DEFORM-3D system. Then the dynamic recrystallization fraction and average grain size of In718 alloy in cylindrical cup backward extrusion with different parameters was solved and analyzed. The complete dynamic recrystallization occurs in the middle of cylinder wall and the grain size is the finest. However, the grain size of top of cylinder wall changes less because of the less plastic deformation. Furthermore, higher speed of punch is useful to the DRX but it is not enough time to occur dynamic recrystallization completely with much higher speed of punch. In spite of more recrystallization occurring in the bottom, the grains grow in the cylinder wall so that much higher temperature goes against improving finer and uniform of grain size. Therefore, it is better for obtaining finer and uniform grain size with 1000(°C)-1050(°C) and 5(mm/s) in In718 alloy cylindrical cup backward extrusion according to the research.
In this study, the effects of isothermal bainite treatment (IBT) holding time on the microstructure of transformation induced plasticity (TRIP) seamless steel tube were studied via optical microscopy, TEM and XRD. Its mechanical properties and hydroformability were evaluated by tensile test and flaring test, respectively. The results revealed that the volume fraction of retained austenite (RA) increased at first then decreased with IBT holding time for a particular set of intercritical annealing (IA) temperature, IA holding time and IBT temperature. It was also demonstrated that high tensile strength of 618MPa, total elongation of 35.5%, n-value of 0.23 and better hydroformability could be successfully produced in this TRIP steel tube at IA temperature of 800°C, holding for 10 min, and IBT of 410°C for 4 min holding time.
Two types of steels which are different in Ni content were tested in three groups of different quenching temperatures and same tempering temperature respectively under the same conditions, and then tested the mechanical properties and observated the microstructure, comparing with the effects of different Ni content on the microstructure and properties. When 1Ni-steel is under the different quenching temperature, its tensile strength range is 1269-1290MPa and changes subtlely. The yield strength and impact energy reach the highest at 910°C, and the mircostructure is fine and uniform. With the quenching temperature increasing, the strength of 3Ni-steel decreases ,but the toughness increases comparing with both kinds of the steels, the microstructure of 3Ni-steel has much more hard phases in its tempered martensite microstructure. Its tensile strength, yield strength and toughness are higher than 1Ni-steel at the lower quenching temperature. The inclusions of 3 Ni-steel are characterized by spheroidization and refinement. In the NACE-A test, the SSC resistance of 3Ni-steel is better than that of 1Ni-steel, which indicates that the high Ni steel has better comprehensive properties.
The effects of direct quenching and tempering (DQ-T) process and conventional reheat quenching and tempering (RQ-T) processes on the microstructure and mechanical properties of a high strength low alloy steel were investigated. In the as-quenched DQ steel, prior austenite grains are elongated parallel to the rolling direction, whereas the as-quenched RQ steel mainly consists of equiaxed grains; The DQ process was found to enhance the hardenability of steel effectively. The tensile strength and yield strength of DQ specimen, were higher than that of RQ specimen. In contrast, low temperature toughness of DQ-T specimen was generally inferior to that of RQ-T specimen.
The continues cooling transformation (CCT) of a low carbon Mn-Nb-B steel in the undeformed and deformed conditions were investigated, respectively. The CCT diagrams of the steel were constructed. The microstructures and microhardness were analysized. The results showed that the microsructures contains ferrite, pearlite, granular bainite, acicular ferrite and lath bainite depending on cooling rate; Deformation moved the CCT curve to the top left corner, increased the transformation start temperatures slightly, and promoted the formation of ferrite and pearlite. Furthmore deformation also fined the transformed microstructures.
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