Papers by Author: S.I. Kwun

Abstract: The effect of thermo-mechanical treatment (TMT) process on the strength and precipitation behavior of an 11%Cr-0.3Mo-1.6W steel during long term aging was investigated. The major precipitates in as-tempered specimen were identified as M23C6 (M=Fe,Cr) and MX (M=Ta,V;X=C,N). The M23C6 precipitate in TMT sample was more finely distributed due to the increased heterogeneous nucleation sites at dislocations. The tensile strength of TMT sample is higher than that of the conventional heat treatment (CHT) sample in as-tempered condition. However, Laves phase starts to precipitate additionally in the aged condition. The growth rate of Laves phase in TMT sample is much faster than that in CHT sample. Therefore, the difference in tensile strength between CHT and TMT sample decreases as the aging time increases.

Abstract: Acoustic emission (AE) technique was applied to evaluate both tensile deformation of 316L stainless steel and bending-fatigue damage of socket-welded pipe. AE activity was monitored during tensile deformation of plate-type specimens subjected to various heat treatments. Variation in RMS voltage of AE signal was correlated with the amount of strain-induced martensite phase. Secondly, actual size of socket-welded pipe specimen was bending-fatigued under various stresses. Crack initiation was determined by observing an abrupt increase in AE count, and confirmed by radiographic examination before and after the crack initiation cycle. Potentials of AE technique for monitoring fatigue crack initiation were discussed.

Abstract: In order to enhance the corrosion and wear resistance of the STVAX alloy, the CrN film has been deposited by UBM(unbalanced magnetron) sputtering. In this study, the influence of process parameters, such as N2 gas concentration and Cr interlayer insertion, on the adhesive strength was investigated. By increasing the concentration of N2 gas from 30% to 33%, the microstructure of the coating layer was changed from hexagonal Cr2N to cubic CrN. By increasing N2 gas concentration to 38%, a CrN coating layer with a preferred orientation of <111> plane was formed. Nanoindentation testing showed that the mechanical properties of coatings varied according to the coating type and crystallographic orientation. Superior adhesive strength was obtained at the of N2 gas concentration of 38%, and higher hardness at 33% concentration. The adhesive strength of the coating film was higher with a Cr interlayer than that without. These findings showed that the Cr interlayer between the CrN coating and the STAVAX substrate significantly improved the adhesive strength of coating to the substrate.

Abstract: The phases formed in the wide-gap region brazed with BNi-3 filler metal powder in IN738 superalloy were investigated by electron backscatter diffraction (EBSD). The morphology and chemical composition of the phase were investigated by field emission scanning electron microscopy (FESEM) and Auger electron spectroscopy (AES). The wide-gap region brazed with BNi-3 filler metal had a microstructure consisting of primary Ni3B, binary eutectic of Ni3B-Ni solid solution and ternary eutectic of Ni3B-Ni-Ni3Si structure. EBSD pattern analysis revealed that the Ni3B had orthorhombic structure with lattice parameter of a=0.439, b=0.522 and c=0.662 nm, and the Ni3Si phase had cubic structure with lattice parameter of a=0.350 nm.

Abstract: The precipitation behaviors of γ″(Ni3Nb) in four Ni-base alloys were investigated. The four alloys were forged Ni20Cr20Fe5Nb alloy, mechanically alloyed Ni20Cr20Fe5Nb alloy, IN 718 alloy and ECAPed(equal channel angular pressing) IN 718 alloy. Aging treatment was employed at either 600°C or 720°C for 20 hrs. The TEM observation and hardness test were performed to identify the formation of γ″. The precipitation of γ″ was noticed after aging at 600°C for 20 hrs in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy, while it was observed after aging at 720°C for 20 hrs in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP. It seemed that the lower aging temperature for γ″ precipitation in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy than in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP appeared to be due to the severe plastic deformation which occurred during mechanical alloying or ECAP.

Abstract: This study investigated the microstructure and mechanical properties of the wide-gap region brazed with various powder mixing ratios of additive powder (IN738) to filler metal powder (DF4B). The wide-gap brazing process was carried out in a vacuum of 2×10-5 torr at 1230°C for 1 hr. The microstructure of the brazed region was analyzed by FESEM and AES. The wide-gap region brazed with 60wt.% IN738 additive powder and 40 wt.% DF 4B filler metal powder had a microstructure consisting of Ni solid solution + γ' and (Cr, W)2B. The fracture strength of the wide-gap region brazed with 60 wt.% IN738 additive and 40 wt.% DF 4B powder was as high as 832 MPa at room temperature. It was found that the (Cr, W)2B and pores in the brazed region are important microstructural factors affecting the mechanical properties of the wide-gap brazed region.

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 formation behavior of γ″(Ni3Nb) precipitates in IN 718 alloy before and after ECAP(equal channel angular pressing) was investigated by microstructural observation and the hardness test. For the alloy examined before ECAP, the γ″ precipitates were formed only after aging treatment at 720
, whereas after ECAP, the γ″ precipitates were formed at the aging temperatures of both 600
and 720
. 600
is normally too low a temperature for γ″ precipitates to be formed in commercial IN 718 alloy, however, they were able to be formed due to severe plastic deformation by ECAP. It was found that the ECAP process changed the formation behavior of the γ″ precipitates in IN 718 alloy.

Abstract: Microstructural parameters (kinds of phases, inter-lamellar spacing of pearlite, precipitate size, number of precipitates), mechanical properties(UTS, Vickers hardness) and non-destructive parameters(ultrasonic velocity, ultrasonic attenuation coefficient, electrical resistivity, magnetic coercivity, magnetic remanence, Barkhausen noise) in various metallic materials were measured in order to investigate the mutual relationship among these parameters. The optimum non-destructive parameters were selected for particular purposes in order to evaluate the level of damage to the metallic structures, to differentiate the microstructures and to predict the mechanical properties of superalloy, low and eutectoid steel and Cu alloys non-destructively.

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.