Papers by Author: Tadeusz Siwecki

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Authors: Philippe Maugis, M. Gouné, P. Barges, D. Dougnac, D. Ravaine, M. Lamberigts, Tadeusz Siwecki, Y. Bi
Authors: Ulrika Borggren, Göran Engberg, Tadeusz Siwecki
Abstract: Precipitation of carbonitrides has been studied in as-cast slabs of one Nb and one Nb and Ti containing HSLA steel. The precipitates have been quantified using LOM and TEM. The measured size and number distributions was then compared to model calculations of precipitate nucleation and growth using estimates of the cooling rates in the austenitic range (1490oC to 800oC) during casting. Both average size and number distributions could be modelled with good agreement using identical model parameters (except for individual diffusion coefficients for the participating species). The model is based on classic nucleation rate theory and a quasistationary approximation for growth of spherical particles. Local equilibrium is assumed at the phase boundary.
Authors: Tadeusz Siwecki, T. Koziel, W. Bevis Hutchinson, Per Hansson
Authors: Ludwik Błaż, Andrzej Nowotnik, Tadeusz Siwecki
Abstract: The effect of hot deformation process on the structure within the temperature range corresponding to the g→a+P transformation was tested for commercial carbon steel containing of 0.158% C. Hot compression tests were performed at computer controlled constant cooling rate. Before testing CCT diagrams were carefully determined in order to fit hot deformation conditions to the g→a+P phase transformation range. The material microstructure was examined by means of optical and electron transmission microscopy. It was found that ferrite, pearlite or bainite were typical microstructural components observed at hot deformed samples. Localized distribution of pearlite or bainite that resulted from flow localization was observed rarely. Precipitation of carbides on ferrite grain boundaries is the most typical microstructural effect of the hot deformation/precipitation interaction that was noted for the most of hot deformed samples.
Authors: Tadeusz Siwecki, Johan Eliasson
Abstract: Improving the steel properties and production processes with reduced energy demand for high strength steels requires improved process control in close relation to the steel composition. Hot rolling of steel is an energy-intensive process, especially in respect of preheating the steel slabs. The present work was carried out with the aim of reducing the initial slab temperature while at the same time improving properties by optimization of the steel composition and process parameters. Optimization of slab reheating and hot rolling parameters in connection with plate and strip rolling was carried out on low C-Mn high-strength steels microalloyed with Mo-Ti-Nb-B, both in laboratory and full scales processing. The effects of a low slab reheating temperature, high finish rolling temperature (FRT) during thermo-mechanical controlled processing (TMCP) and accelerated cooling rates following hot rolling to RT or to the coiling temperature have been investigated. Improvement of yield strength of the plate has been obtained by lowering the slab reheating temperature, especially with high cooling rates (>20°C/s) to room temperature. The results obtained for strip steels also show that a reduced reheating temperature combined with high finish rolling temperatures and cooling rates (>20°C/s) to a coiling temperature of 450°C produces very positive microstructures and mechanical properties in the present steels. Lowering the slab reheat temperature reduces energy consumption and accordingly releases less CO2 into the atmosphere during the thermo-mechanical processing of the present steels.
Authors: X.T. Wang, Z.L. Yu, Tadeusz Siwecki, Göran Engberg, Zu Qing Sun
Abstract: A physical model for austenite recrystallization of steel concerning TMCP is developed. Dislocation density plays a key role as recrystallization driving force. The dislocation density change is a result of competition between dislocation generation and dynamic recovery. Recrystallization is described as a nucleation-growth process. An abnormal subgrain growth mechanism is introduced for nucleation. A few subgrains fulfilling abnormal growth conditions will stand out and become nuclei of recrystallization. The recrystallized grain grows to the deformed materials driven by the stored energy. Oswald ripening occurs for grains surrounded by recrystallized grains. The models were verified by laboratory simulation results for selected austenite stainless steels. It showed good agreement between predicted and experimental results.
Authors: X.T. Wang, Tadeusz Siwecki
Abstract: Susceptibility of hydrogen embrittlement of a super grade AISI 420 tool steel was studied. Tensile samples were cathodically charged to different hydrogen level. Hydrogen induced mechanical property degradation was measured by tensile tests at a low strain rate. Fractography of broken surfaces was observed using SEM. Relationship between hydrogen content and tensile strength and elongation were studied. Critical hydrogen contents were obtained for different heat treatment states. It was found that for annealed materials could stand for a 3.5ppm hydrogen for keeping 80% of original ductility, and the effect of hydrogen on strength was unobvious. However, for material quenched and tempered at 250°C, only 0.3ppm hydrogen could lead the ductility drop to 80% of original. The material quenched and tempered at 500°C was more sensitive on hydrogen, less than 1ppm hydrogen could lead the strength drop to 80% of original.
Authors: Stanislaw Zajac, Tadeusz Siwecki, W. Bevis Hutchinson, R. Lagneborg
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