Papers by Author: Won Jong Nam

Abstract: The effects of annealing temperature and silicon content on mechanical properties on cold drawn pearlitic steel wires were investigated. Cold drawn steel wires, containing Si, 0.99 ~ 1.4%, were annealed at the temperature of 200 ~ 450°C with different annealing time. The variation of microstructural evolution with annealing temperature was not affected by silicon content. For steels containing high silicon content above 1.0%, the increase of silicon content did not cause the changes of peak temperature showing age hardening and age softening, except for the increase of tensile strength due to solid solution hardening.

Abstract: The effects of deformation strains and annealing temperatures on microstructures and mechanical properties of martensitic steels were examined. The amount of cold deformation was changed as 30%, 50% and 60%, and annealing temperatures varied from 500°C to 600°C. In samples cold rolled 30%, the dominant microstructure for an annealing at 500°C was dislocation substructures with uniformly distributed rod-shaped carbide particles. For an annealing at 600°C, the microstructure consisted of equiaxed ultrafine grains, spherical carbide particles and elongated dislocation substructures. A proper annealing temperature for martensitic steels received 30% reduction, showing a good combination of a high strength, 1230MPa, and an adequate total elongation. 9.4%, was found as 500°C.

Abstract: The microstructural evolution during thermal annealing of a cryogenic rolled 5052 Al alloy was investigated. The activation energy for annealing behavior was calculated using DSC data. For the heating rate of 16°C/min, the precipitation occurred at the annealing temperature of 150~230°C due to Mg self diffusion, recovery occurred at the annealing temperature of 230~260°C, and recrystallization proceeded at a higher temperature up to about 370°C. Both recovery and recrystallization gave rise to non-uniform, bimodal grain-size distributions, which may result from heterogeneous nanostructures. In addition, the activation energy for the precipitation was found as ~115kJ/mol, indicating the process was diffusion-controlled (Mg in Al), and the activation energy for recovery was found to be ~140kJ/mol, representing self-diffusion in pure Al.

Abstract: The effects of annealing temperature and silicon content on mechanical properties on cold drawn pearlitic steel wires were investigated. Cold drawn steel wires, containing Si, 0.99 ~ 1.4%, were annealed at the temperature of 200 ~ 450°C with different annealing time. The variation of microstructural evolution with annealing temperature was not affected by silicon content. For steels containing high silicon content above 1.0%, the increase of silicon content did not cause the changes of peak temperature showing age hardening and age softening, except for the increase of tensile strength due to solid solution hardening.

Abstract: The activation energy for recovery and recrystallization was calculated using DSC data. The annealing below 250°C resulted in the bimodal grain size distribution, while that above 300°C resulted in the uniform distribution of coarse grains. The formation of a bimodal microstructure would be responsible for the good combination of uniform elongation and tensile strength. Additionally, the little variation of hardness for different annealing time at 300°C also indicated that mechanical properties of deformed and annealed 5052 Al alloy were significantly influenced by the volume fraction of recrystallized grains rather than the coarsening of recrystallized grains.

Abstract: Two commercial Al alloys having different second phase particle distributions were subjected to severe plastic deformation (SPD) via equal channel angular pressing with or without subsequent cold rolling, and the effect of such SPD on the particle size distribution of the alloys was investigated. The particles larger than ∼ 3 μm were fragmented into several smaller ones by SPD. Contrarily, those smaller than ∼ 3 μm were hardly broken up by SPD but their distribution became more uniform. Along with these findings and the theoretical models for cavity nucleation at second phase particles, the cavitation behavior of ultrafine grained Al alloys during low temperature or high strain rate superplastic deformation was discussed.

Abstract: The effects of annealing temperature and annealing time on mechanical properties of cold drawn pearlitic steel wires containing 0.84wt% of silicon were investigated. Annealing treatment was performed on cold drawn steel wires for the temperature range of 200°C to 450°C with the different annealing time of 30sec, 1min, 15min and 1hr. The increase of tensile strength at the low annealing temperatures would be related with strain ageing behavior, while the decrease of tensile strength at the high annealing temperature is due to the spheroidization of cementite plates and the occurrence of recovery of the lamellar ferrite in the pearlite.

Abstract: The large deformation at cryogenic temperature would be one of the effective methods to produce large bulk UFG materials. The effects of annealing temperature on microstructure and mechanical properties of the sheets received 85% reduction at cryogenic temperature were investigated for the annealing temperature of 150 ~ 300°C , in comparison with those at room temperature. Annealing of 5083 Al alloy deformed 85%, at 200°C for an hour, results in the considerable increase of tensile elongation without the great loss of strength and the occurrence of equiaxed grains less than 300nm in diameter.