HOT-DEFORMATION BEHAVIOR OF HOT ISOSTATIC PRESSED Ti-6Al-4V ALLOY DURING HOT COMPRESSION

A single-pass and multi-pass compression experiments with a Ti-6Al-4V alloy fabricated with hot isostatic pressing (HIP) were carried out on a Gleeble-1500D simulator. The true stress-strain curve, microstructure and microhardness of the alloy deformed at a total strain of about 70 % were studied. The results show that the stress value increased as the strain rate increased during each deformation pass and the same was true for the single pass. At the initial deformation stage, the flow stress increased rapidly with the increase in the strain until it reached the peak; then it showed a different softening extent, which was more obvious for the three-pass deformation than for one-pass and two-pass deformation. During the multi-pass deformation, the lamellar α phase was mainly distorted, but during the one-pass and two-pass deformation, it was bent; the spheroidisation of the lamellar α phase mainly occurred during the three-pass deformation, significantly increasing the extent of softening. Meanwhile, the degree of the spheroidized α phase and the volume fraction of β-transformed phase (βt) increased as the strain rates increased. Finally, a tri-modal microstructure, including the β-transformed phase (βt), lamellar α phase and equiaxed α phase was obtained during three-pass deformation. In the process of multi-pass deformation, the microhardness increased with the increase in deformation passes. When the strain rate was 1 s–1, the microhardness increased from 299.5 HV and 309.1 HV to 342.6 HV with the increase in deformation passes. It was also found that the microhardness increased with an increase in the strain rate under a certain amount of deformation.

Influence of Inter-Pass Cooling on Microstructural Evolution and Plastic Deformation of Heavy EH47 Plates

Herein, the influence of inter-pass cooling (IC) and conventional two-stage rolling (CTR), on microstructural evolution and plastic deformation behavior of ultra-heavy EH47 plates, is demonstrated. It is reported that the deformation amount and deformation rate, in every deformation pass during rough rolling, at 1/4- and 1/2-thickness of IC steel were higher than the CTR steel. The volume fraction of ferrite and acicular ferrite was 45% and 18%, at 1/4-thickness, and 35% and 50% at 1/2-thickness of IC steel, respectively, whereas the sum of both ferrite phases was smaller than 25% in the CTR steel. The austenite grain boundary area and high-angle grain boundary fraction in the IC steel were higher than the CTR steel. The high density of fine and shapeless pearlite has been observed in IC steel, whereas large-size carbides, with hexagonal structure, have been observed in CTR steel. Compared to the CTR steel, the density of precipitates was apparently lower in IC steel. Two kinds of Nb containing precipitates, such as (Ti, Nb)(C, N) and (Nb, Ti)C, were observed in the tested steels. Total ductility and uniform elongation of the IC steel were higher than the CTR steel. During the tensile process, the crack initiation energy and crack propagation energy of the IC steel were higher than the CTR steel. Moreover, the volume fraction of retained austenite (FCC) was reduced from 7.71% to 0.42% near the tensile fracture in IC steel at 1/4-thickness. In additon, the strain of synergetic plastic deformation of the IC steel was higher than the CTR steel. Meanwhile, compared to the CTR steel, the synergetic plastic deformation of the IC steel occurred at low stress after the yield point, which can be ascribed to the presence of fewer microcracks in the IC steel. Hence, a delayed fracture has been observed in the IC steel plate.

Effect of the Time between Last Deformation Pass and Accelerated Cooling on the Mechanical Properties in Nb and Nb-Mo Microalloyed Steels

The microstructural refinement induced when the holding time between last deformation pass and accelerated cooling is reduced, affects the mechanical properties in low carbon Nb and Nb-Mo microalloyed steels. Plane strain compression tests were performed and mechanical property samples machined in order to quantify this effect using tensile and Charpy impact tests. A complete microstructural characterization was carried out using electron backscattered diffraction (EBSD) measuring unit size distributions and homogeneity of complex microstructures. The synergetic combination of Nb and Mo elements modifies the final microstructures and, therefore, affects the contribution of different strengthening mechanisms, such as substructure, precipitation hardening and dislocation density. Even though strength is not clearly affected by the reduction of the holding time after the last deformation pass, Charpy properties are considerably improved in the case of the Nb steel. The presence of MA islands in the Nb-Mo steel limits the beneficial effect of the microstructural refinement and toughness remains unmodified.

Analysis of Recrystallization and Strain-Induced Precipitation on High Nb- and N-Bearing Austenitic Stainless Steel

The mechanical properties and corrosion resistance of stainless steels are due to the combined effect of chemical composition and thermomechanical processing. The objective of this study was to investigate the interaction precipitation-recrystallization of an austenitic steel with high additions of nitrogen and niobium through continuous-cooling multiple deformation hot-torsion tests. Samples were heated up to a soaking temperature of 1250oC and kept at this temperature for 5 minutes, and then deformed during cooling. The deformation pass was 0.3 with a strain rate of 1 s-1and interpass times of 20 or 50 s. The evolution of the microstructure was investigated by optical, EBSD and transmission electron microscopy, using thin foils and carbon extraction replica samples. The results showed that some precipitates were not dissolved after reheating and the presence of niobium-and chromium-rich particles after processing was confirmed. The strain accumulation with the interpass time of 20 s yielded finer precipitation and improved grain refinement than observed after 50 s. Some interaction of the precipitates with dislocations and grain boundary could be evidenced.