| Popis: |
In this thesis, the effects of strain path reversal on microstructure evolution, in terms of austenite grain subdivision and its recrystallisation as well as continuous cooling phase transformations, are investigated using an API X-70 grade microalloyed steel and two non-transforming austenitic model alloys, namely 316L and Fe-30%wtNi with low and medium stacking fault energies (SFE), respectively. The flow stress-strain and static recrystallisation behaviours of the X-70 steel were studied by monotonic and interrupted hot plane strain compression (PSC) tests. A fraction softening parameter calculated from "double-hit" PSC tests has been found to be capable of quick outlining the partial recrystallisation temperature region without complicated microstructure analysis involved. When hot deformed to medium strains (Evm=O.3~0.5) in austenite non-recrystallisation temperature region by forward-reverse torsion tests, simple strain path changes do not impose significant influence on the microstructural evolution of this X-70 steel through continuous cooling phase transformations. The post-deformation cooling rate is the more influential factor which determines the transformation mechanisms and microstructure, therefore mechanical properties of the final products. However, when subjected to large accumulative strains (Evm2.0), the effects of multiple strain path reversals combined with small amplitude of each forward strain are found to be very profound on microstructural evolutions in austenite studied by hot cyclical torsion using two non-transforming model alloys. The formation of high angle boundaries (HABs) by austenite grain subdivision was significantly delayed by multiple strain path reversals in the Fe-30wt%Ni model alloy with medium SFE. Dynamic recrystallisation (DRX) of austenite in low SFE 316L stainless steel was almost completely suppressed by multiple strain path reversals. Dynamic strain induced transformation (DSIT) in the X-70 steel was also found to be inhibited by multiple strain path reversals when deformed to large strains between the Ac3 and Ar3 temperatures by similar hot cyclical torsions which were applied to model austenitic alloys. Interpretations of the observations were made by correlating the behaviour of the austenite in X-70 steel, which could not be directly observed easily, to the observations on microstructural evolution made in the two model austenitic alloys. This provides better insights into the transformation mechanisms of DSIT process. |
