Papers by Author: Sung Joon Kim

Abstract: Low alloy transformation induced plasticity (TRIP) steels have a complex microstructure consisting of ferrite, bainite and retained austenite. Their excellent mechanical properties are ascribed to the martensitic transformation of retained austenite during plastic deformation. In the present contribution, the crystallographic texture of fcc and bcc phases in TRIP steels was measured by means of orientation mapping. The austenite texture was close to a typical rolling texture of fcc metals. For bcc phase, the effects of orientation and grain size on the distribution of pattern quality were investigated. The texture of transformation product phase was separated by grain size. The transformation texture showed stronger α fiber including {113}<110> component than the recrystallization texture. It showed a good agreement with a transformation texture predicted by Kurdjmov-Sachs (KS) relationship without any variant selection.

Abstract: In general, plastic strain occurs over a certain stress, called yield stress. However, it has been reported that the permanent strain could happen during boundary migrating even under the extremely slight externally applied stress. In this study, we performed dilatometry experiments under the various compressive stresses and measured the amount of recrystallization and growth induced permanent strain. A new empirical constitutive equation was suggested to describe the recrystallization and growth induced plasticity. This equation was verified by comparing the calculated values with dilatometric experimental data under the various compressive stresses.

Abstract: Formability and mechanical property of Al sheets whose surface was locally modified by the concept of SFJ (Surface Friction Joining) were analyzed. It is noteworthy that the formability of the surface-modified sheets is greatly improved compared with as-received sheets. The formability is improved as the tool diameter is increased. It is found that more plastic deformation is accommodated at modified region during LDH test.

Abstract: Sliding wear mechanisms of the solution treated 18Cr-18Mn-2Mo-0.9N high nitrogen steel (HNS) were studied. Room-temperature dry sliding wear tests of the steel were carried out at various applied loads to explore the mechanism as a function of the load. The wear rate of the steel increased with the increase of the load; however, the increase rate was not constant. The rate increased slowly at low loads, rapidly at intermediate loads, and finally, the increase-rate became low again at high loads. Worn surfaces, their cross sections, and wear debris were examined, and phases of the heat-treated HNS as well as the wear debris were identified to find out the mechanism. The wear of the steel was found to be controlled by the tribo-oxidation, strain-induced phase transformation, and reverse transformation due to temperature rise on a wearing surface. The influence of each mechanism on the rate varied depending on the magnitude of the applied load.

Abstract: A novel process for butt welding of thin metal sheets was developed and named as surface friction welding (SFW), which utilizes friction heat and severe plastic deformation like friction stir welding (FSW). The joining mechanism of the SFW is based on not stirring by the pin tool but surface friction between tool shoulder and joining metals. The developed method was successfully applied to butt welding of various metal sheets thinner than 1.5 mm thick. This paper deals with the principle of SFW, the difference between FSW and SFW, the effect of welding parameters, and the microstructure and mechanical properties of welded sheets.

Abstract: The effect of Cr2N precipitation on deformed microstructure in high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N steel was investigated with a particular emphasis on deformation twinning. Based on the crystallographic analysis in the stereographic projection, the orientation relationship between austenite (γ) matrix and Cr2N was determined to be Cr2N [110]γ //[1100] and Cr2N (111) γ //(0001) . The deformation twinning had {111} < 112 > crystallographic component similar to that of cellular Cr2N. The cellular Cr2N precipitates caused a different orientation dependence of deformation twinning: only one twinning system in the <111 > grain was activated almost parallel to the growth direction of Cr2N.

Abstract: Tensile deformation behavior of the high-nitrogen austenitic Fe-18Cr-14Mn-4Ni-3MoxN steel with various nitrogen contents has been studied. The nitrogen content of the steel varied from 0.28 to 0.88 wt. %. Nitrogen atoms in high nitrogen steel (HNS) make an interstitial solid solution by being scattered in the steel constituting a short-range order. They strengthen the austenite matrix without deteriorating ductility of the steel. The present investigation was carried out to elucidate the hardening and plasticizing role of the nitrogen in the HNS by analyzing tensile deformation behavior of the steel containing various nitrogen contents. Tensile tests of the steel specimens were performed at room temperature with a constant strain rate of 5x10-5/sec. Microstructure of the tested specimens was analyzed to explore the deformation mechanism of the HNS as a function of nitrogen contents. The flow stress of the steel increased with the increase of the nitrogen content; however, the specimen with the highest nitrogen content (0.88 wt. %) showed saturated strength and reduced ductility. The superior mechanical property of the HNS was explained by the low stacking fault energy and the twin-induced plasticity provoked by the nitrogen.

Abstract: The effect of prior T4 heat treatment and subsequent microstructural evolution on tensile behavior of friction stir welded (FSWed) AZ31B-H24 alloy was examined in this study. Selected AZ31B-H24 plates were prior T4 heat treated at 400
for 24 hours and subsequently friction stir welded. The tensile properties, optical micrographs and SEM fractographs for FSWed AZ31B-T4 specimens were compared with those for the H24 counterparts. Prior T4 heat treatment tended to decrease the tensile ductility reduction in FSWed AZ31B-H24 specimen. The tensile ductility reduction mechanism as associated with prior T4 heat treatment is discussed based on detailed micrographic and fractographic observations.

Abstract: Deformation behavior of high nitrogen austenitic Fe-18Cr-18Mn-2Mo-0.9N stainless steel was investigated utilizing electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). During deformation, the <110> grains rotated and the trace of these grains moved towards into the dodecahedral plane (the line connecting (001) and (111) planes). Misorientation mapping in EBSD showed that the special boundaries (almost Σ3) gradually diminished whereas the low-angle boundaries were developed. TEM observation showed that (i) the low-angle boundaries developed corresponded to the deformation twinning with {111}<112> component, (ii) the deformation twinning showed the strong orientation dependence relative to tensile axis, and (iii) the deformed microstructure was characterized by extended stacking faults, planar dislocation array in low strain regime, and by well-developed deformation twinning in high strain region, respectively.

Abstract: Fatigue crack propagation (FCP) behavior of friction stir welded (FSWed) 6061-T651 Al alloy was examined with the fatigue crack growing either along the dynamically recrystallized zone (DXZ) at variable
K or perpendicular to the DXZ at a constant
K value of 7.5, 10 and 15 MPa√m, respectively. The FCP rates in the DXZ tended to be significantly lower than those in the PM particularly in low
K regime. Compressive residual stress reducing effective
K cannot be solely responsible for the enhanced FCP resistance. The fine recrystallized grains in the DXZ causing intergranular fatigue failure appeared to be detrimental to the FCP behavior of FSWed 6061-T651 specimen. The constant
K fatigue test across the weld zone showed that substantial crack retardation occurred far beyond the heat affected zone (HAZ) at low
K regime. The FCP behavior of FSWed 6061-T651 is discussed based on residual stress measurement and fractographic observation.