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The relationships between flow stress curve and microstructure evolution of strain induced dynamic phase transformation (SIDT) using low carbon steel (0.22wt.%) were quantitatively investigated. The deformation was carried out at just above Ar3 temperature (710℃) as a function of strain rate (0.0l-5/sec). The softening process of SIDT was well modeled based on Avrami and constitutive equations similar to the existing dynamic recrystallization process. However, the calculated results differed from the experimental curve as decreased the strain rate under 0.2/ sec, this is due to that the dynamic transformation from austenite to ferrite is not completed by lower deformation stored energy.

keyword : Hot deformation, Strain induced dynamic, Transformation, Prediction of flow stress, Critical strain

In the present research work, it was studied whether magnetic approaches can be used for the life prediction evaluation and defect measurement in heat-resistance E911 ferritic steel. Destructive and non-destructive test methods were used. Transmission electron microscopy, coercivity measurements and Barkhausen-noise(BN) measurements were also used. This research aims to clarify the dependency how the thermal fatigue process influences the magnetic properties of E911 steel, which are often used in european power plant applications. It is also attempted to investigate, whether the BN measurement can be applied to monitor the microstructural changes caused by thermal fatigue of the E911 ferritic steel.

keyword : E911 ferritic steel, Barkgausen noise, Thermal fatigue

Irradiation effect on hardness and corrosion behavior of aluminum for thermal insulator and zirconium alloy for nuclear cladding tube of a nuclear power plant were studied. The specimens were irradiated in CT hole of HANARO after installed in an instrumented capsule with a specimen case. The irradiation test was carried out at the temperature of 300℃ and neutron fluence a of 4.8 × 10^20 n/cm^20/sec. The average hardness determined by nanoindentor of the aluminum was increased from 0.05 GPa to 0.55 due to irradiation hardening, that was decreased to 0.23 GPa by 6 hour thermal annealing. Zirconium hydride (ZrH) were well formed in the zirconium alloy by the infiltration mechanism of hydrogen ion that was thermodynamically explained by correspondence principle. Corrosion potential and rate of the zirconium alloy in 40wt.% HCl-500 ppm FeCl3 aqueous solution were about 72.0 mVSHE and 6.070 × 10^(-5)/㎠, respectively Addition of 1500 ppm of FeCl3 changed those values up, to about 31.0 mVSHE and 1.809 × 10^(-4)A/㎠, respectively. Corrosion resistance of the alloy was decreased with Fe^(3+) and Cl- content in solution and hydride formation in the alloy, which can be well described by the mechanism of passive film dissolution.

keyword : Irradiation, Reactor materials, HANARO, Hardness, Corrosion

Structural integrity assessment or regulatory procedures of cracked components are based on the use of fracture toughness parameters. ASTM E1921 presents a direct method to obtain the fracture toughness, the Master Curve, using small specimens in the transition range. Fracture toughness can be affected by the size and thickness of specimen in the transition range. These effects can be explained by the plastic constraint loss and the statistical weakest-link theory in general. ASTM E1921 presents a statistical model to correct these effects. In this work, a numerical procedure based on micro-mechanical model is described in detail and applied to adjust the plastic constraint loss in 1/3PCVN and PCVN specimens. Plastic constraint loss can be effectively adjusted by comparing the J-integral values calculated in the 2-D finite element analysis for small-scale yielding and 3-D finite element analysis for large scale yielding in the same size of crack tip plastic area. After KJC is adjusted by using the corrected J-values, the statistical size effect are also corrected according to the standard ASTM E 1921 procedure. The results showed a good correction effect for the small size specimens.

keyword : Fracture toughness, Ductile-to-brittle transition, Finite element analysis, Master curve, Reference temperature T0, PCVN, ASTM E1921

Commercial purity titanium of a hexagonal close packed a phase was plastic- deformed through equal channel angular pressing (ECAP). ECAP was conducted on specimens of different initial microstructure so that the effect of the initial grain size and texture on the subsequent evolution characteristics could be studied. Specimens of equiaxed microstructure were prepared with the initial grain size ranging from 4 μm to 60 μm. During deformation, low angle boundaries as well as twin boundaries were produced. Formation of twin boundaries was promoted by the large initial grain size whereas small angle boundaries were more readily generated in the initially smaller grains due to the difficulty of twinning. Through the tensile test, it was confirmed that the low angle boundaries and twin boundaries both increased the yield strength. Particularly, specimens of large grain size showed pronounced increment of the yield strength although the elongation was reduced due to the twin boundaries produced during the deformation.

keyword : Titainium, Equal channel Angular pressing, Texture, Mechanical property

Individual flow behavior of a and β, phases in Ti-6Al-4V alloy was analyzed in the context of self-consistent approach. For this purpose, high temperature compression tests were conducted at various temperature ranges using single-phase α(Ti-7.0Al-1.5V), and two-phase Ti-6Al-4V alloy with an equiaxed microstructure. The individual flow behavior of a phase was deduced from the experimental results of single-phase alloy whose chemical composition is close to that of phase in Ti-6Al-4V. On the other hand, the flow behavior of β phase was calculated using self-consistent approach. From the results, it was found that the flow stress of α phase was higher than that of β, phase above 750℃, but the former revealed lower strength than the latter below 650℃. It was found that the relative strength and strain rate ratio between α and β, phase significantly varied with temperature. From this approach, the mode for grain matrix deformation was proposed as a mixed type of both iso-stress and iso-strain rate modes. The results of present approach were applied to predicted the flow behavior of Ti-alloys on the basis of phase volume fraction. Besides, the source of strength softening caused by temperature rise was examined. The increase of soft β, phase volume fraction plays significant role in flow stress decrease as well as easy slip transmission due to temperature rise.

keyword : Ti-6Al-4V, Self-consistent, Flow behanvior, Phase volume fraction, Dislocation glide

In a Mod.9Cr1Mo steel widely applied to boiler components, low hardness problem related with manufacturing and fabrication processes has become a critical issue recently. In this study, hardness, microstructure, tensile and creep rupture tests have been performed using specimens experienced various thermal cycles to investigate the detailed mechanism causing low hardness values of 150 to 170 Hv and the minimum hardness requirement from a standpoint of tensile and allowable stresses. Low hardness values were mainly attributed to the formation of ferrite structure on cooling after heating at intercritical temperature just above the Ac1, 850℃. Ferrite transformation on cooling after intercritical heating occurred even at the relatively faster cooling compared to normal heating above the Ac3 since the nose of ferrite transformation in the CCT curve moved to the left due to the very low carbon content in austenite phase formed at intercritical temperature. Low hardness value of 160`s Hv occurred occasionally in practical applications was observed at a cooling rate of below 0.167℃/sec (10℃/min) after intercritical heating. At least 190 Hv of hardness values or more were needed to satisfy tensile properties and maximum allowable stresses specified in ASME B&PV code.

keyword : Mod. 9Cr1Mo steel, Low hardness, Creep, Dilatation