Papers by Author: C.S. Kim

Abstract: The evolution of the strain-induced α′ martensite during the cyclic deformation of austenitic AISI 316L stainless steel (SS) was investigated. The low-cycle fatigue (LCF) test was conducted at various strain amplitudes in air. The amount of α′ martensite was determined for the fatigue-failed specimens as well as for the specimens interrupted after a specific number of cycles. The volume fraction of α′ martensite increased with increasing strain amplitude and number of fatigue cycles. The cyclic hardening behavior was discussed in terms of the dislocation density and strain-induced α′ martensite transformation.

Abstract: The fatigue crack growth (FCG) in dissimilar weld metal joints between SA 508 Cl.3 low-alloy steel and AISI 316L stainless steel (SS) was investigated. The dissimilar weld metal joint was made after buttering alloy 82 on the SA 508 Cl.3 side by gas tungsten arc welding (GTAW). Alloy 82 welding consumable was selected to join these two metals. The fatigue crack growth rate (FCGR) in each material in the dissimilar weld metal joint increased in the order: weldment, AISI 316L SS and SA 508 Cl.3, at the same stress intensity factor range, /K. As the crack propagated across the AISI 316L SS and heat affected zone (HAZ) into the weldment or across the SA 508 Cl.3 and HAZ, into the weldment, the FCGR in the HAZ region did not change or decrease, in spite of the increase in /K. The retardation in the FCGR in the HAZ region was discussed in terms of the welding residual stress.

Abstract: The effects of the precipitate and martensite lath on the softening behavior have been investigated for 11Cr-3.5W-3Co steel during creep at 700
. During creep, the precipitate on the PAG (prior austenite grain) boundaries and martensite lath boundaries coarsened. The recovery of dislocation density and an increase of martensite lath width took place. It is shown that the inverse of the size of the precipitates and the inverse of the square root of the martensite lath width have a linear relation with the Vickers hardness, which corresponds to the Hall-Petch relation and particle looping mechanism.

Abstract: The magnetic coercivity of martensitic 12Cr steel was measured in order to evaluate its degree of isothermal aging. As the aging time increased, the lath width increased and the dislocations were recovered. Aging resulted in the coarsening of the as-tempered carbides (M23C6 and MX) followed by additional precipitation of Fe2W. The magnetic coercivity rapidly decreased during the initial aging period of about 1,000 hours and then decreased slightly thereafter. The decrease in the coercivity with increasing aging time was related to the decrease in the number of pinning sites, those associated with the dislocations, fine precipitates and coarsening of the martensite lath/subgrain.

Abstract: In the present work, the strain induced martensite in 316L stainless steel was quantitatively characterized by X-ray diffraction, the measurement of the magnetic coercivity and the AE technique during the monotonic tensile deformation of plate specimens. Plate specimens subjected to different heat treatments (i.e. having different initial microstructures) were tensile-deformed and the AE counts obtained during tensile deformation were correlated with the microstructural development. The AE count was observed to increase with increasing amount of strain induced martensite phase, as determined by X-ray diffraction analysis. The potential of the AE technique and the measurement of the magnetic coercivity to be used for the evaluation of the tensile deformation was discussed in relation to the existence of strain-induced martensite.

Abstract: The mechanical alloying processes was employed to fabricate Al-4at.%Zr alloy with nano-sized grains and very fine Al3Zr compounds. The phase transformations and the stability of the phases formed during mechanical alloying and heat treatment processes were investigated. The grain sizes of the alloys immediately after milling and following the subsequent heat treatment at 550°C were 54.2nm and 106.4nm, respectively. Some of Zr atoms were dissolved into the Al matrix and most of them reacted with hydrogen produced by decomposition of PCA(process control agent) to form ZrH2 during mechanical alloying process. These ZrH2 hydrides decomposed gradually after the heat treatment. Stable Al3Zr with a DO23 structure was formed by heat treatment at temperature of more than 4500C. The hardness of the Al-4at.%Zr alloy was more than two times higher than those of other Al-based alloys.

Abstract: The magnetic coercivity of ferritic 12Cr steel was experimentally studied in order to characterize its microstructures and mechanical properties during isothermal aging. As the aging time increased, the M23C6 carbide coarsened and additional precipitation of Fe2W phase was induced. The width of martensite lath increased to about 0.4μm after 4000 hrs of aging. The coercivity decreased as the number of precipitate decreased and the width of martensite lath increased. The hardness was proportional to the magnetic coercivity. These empirical linear relations suggested that the change in the microstructures and strength of ferritic 12Cr steel during thermal aging could be evaluated by monitoring the magnetic coercivity.

Abstract: We attempted to estimate the residual stress which evolved during the shot peening of Al 7075 alloy using leaky surface acoustic wave (LSAW). Shot peening was conducted to produce a variation in the compressive residual stress with the depth from the surface at a shot velocity of 30m/s. The LSAW velocity was measured using a scanning acoustic microscopy (SAM). The Vickers hardness profile obtained inwards from the surface showed significant work hardening of the near surface layer with a thickness of about 0.3mm. The variation in the LSAW velocity through the shot peened surface layer was in good agreement with the distribution of the residual stress measured by X-ray diffraction.

Abstract: The cyclic deformation in Cu and Cu-35Zn alloy were studied using ultrasonic nondestructive evaluation (NDE) technique to measure the ultrasonic velocity, attenuation coefficient and a nonlinear parameter. These materials were cyclically deformed under total strain amplitude control to investigate their ultrasonic reaction to different dislocation substructures, depending on the stacking fault energy difference. The microstructure evolution was observed using a transmission electron microscope, and the ultrasonic NDE parameter was measured after several cycles of fatigue deformation, in order to clarify the relationship between them. In both materials, the ultrasonic velocity was observed to decrease as the fatigue life fraction increased, which was attributed to the increasing dislocation density caused by the cyclic deformation. In the case of Cu, with its cell structure evolving during cyclic deformation, the rate of increase of the ultrasonic attenuation coefficient and the ultrasonic nonlinear parameter was higher than that observed in the case of the Cu-35Zn alloy with its planar array structure. This result implies that the dislocation cell structure is more sensitive to the ultrasonic parameter changes than the planar array structure formed during cyclic deformation.

Abstract: This study investigated the microstructure and mechanical properties of a wide-gap region brazed with various process variables. The IN738 and DF 4B alloy powders were used as additive and filler metal powder for the brazing process. The wide-gap brazing process was carried out in a vacuum of 2×10-5 torr. The wide-gap region brazed with 60wt.% IN738 additive powder had a microstructure consisting of IN738 additive and (Cr, W)2B. The fracture strength of the wide-gap region (60 wt.% additive and 40 wt.% filler metal powder) brazed at 1230°C for 30hr was as high as 862MPa (93% of base material strength). It was observed that the brazing temperature was the main process variable affecting the mechanical properties of the wide-gap brazed region. The creep rupture life of the region brazed with 60wt.% additive and 40 wt.% was longer than that of other brazed samples. The Cracks in the wide-gap brazed region initiated at the (Cr, W)2B and propagated through them. It was found that the (Cr, W)2B and the pore in the brazed region are important microstructural factors affecting the mechanical properties of the wide-gap brazed region.