Papers by Keyword: Hot-Working

Abstract: With the aim of increasing the volume fraction and stability of the retained austenite characteristics in a transformation-induced plasticity (TRIP)-aided steel with wider lath-martensite structure matrix, the effects of varying the post-hot-working cooling rate of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (mass%) steel on the retained austenite characteristics were investigated. When, after hot-working at 950°C, the steel was cooled to room temperature from 430°C above the martensite-start temperature using cooling rates lower than 3°C/s, the steel attained a higher volume fraction of metastable retained austenite and lower volume fractions of a finely dispersed martensite-austenite complex phase, carbide, and pro-eutectoid ferrite, although the volume fraction of bainitic ferrite increased. This was associated with a marked carbon-enrichment in the untransformed austenite, which was mainly due to the promoted bainitic ferrite, the initial lath martensite, and the refined prior austenitic grain.

Abstract: The high-manganese austenitic steels are an answer for new demands of automotive industry concerning the safety of passengers by the use of materials absorbing high values of energy during collisions. The chemical compositions of two high-manganese austenitic steels containing various Al and Si concentrations were developed. Additionally, the steels were microalloyed by Nb and Ti in order to control the grain growth under hot-working conditions. The influence of hot-working conditions on a recrystallization behaviour was investigated. Flow stresses during the multistage compression test were measured using the Gleeble 3800 thermo-mechanical simulator. To describe the hot-working behaviour, the steel was compressed to the various amount of deformation (4x0.29, 4x0.23 and 4x0.19). The microstructure evolution in successive stages of deformation was determined in metallographic investigations using light microscopy. The flow stresses are much higher in comparison with austenitic Cr-Ni and Cr-Mn steels and slightly higher compared to Fe-(15-25) Mn alloys. Making use of dynamic and metadynamic recrystallization, it is possible to refine the microstructure and to decrease the flow stress during the last deformation at 850°C. Applying the true strains of 0.23 and 0.19 requires the microstructure refinement by static recrystallization. The obtained microstructure – hot-working relationships can be useful in the determination of powerful parameters of hot-rolling and to design a rolling schedule for high-manganese steel sheets with fine-grained austenitic structures.

Abstract: Forged and extruded TiAl products suffer from structural and chemical inhomogeneities that reduce the reliability of components. In an attempt to improve the homogeneity of the material, the feasibility of cyclic axial deformation and cyclic torsional deformation superimposed with compression, where much higher strains can be imparted into the material than during forging and extrusion, were investigated. Accordingly, during torsion superimposed with compression pronounced shear localization and cracking occurs. These difficulties can largely be overcome by cyclic axial deformation.

Abstract: Alloying high-chromium steels with Nitrogen leads to increase in strength, fatigue life and corrosion resistance, but reduce ductility and could induce cracks formation during forging. In order to address these problems, the hot working response of a high Nitrogen martensitic stainless steel (Fe-16.2%Cr-1.1%Mo-0.33%N-0.34%C) has been investigated by means of hot torsion tests up to rupture, in the temperature and strain rate ranges of 900-1200°C and 0.005-5 s-1 respectively. The peak stresses of the flow curves were related to strain rate (e&) and temperature (T) by the well known sinh equation. The ductility and the safe working conditions were presented in terms of processing map. The microstructure of the steel in the quenched state after deformation was analyzed by means of optical microscopy; the differences in term of morphology and distribution of the various constituents were discussed.