발간논문

Home > KJMM 논문 > 발간논문

The influence of group-1 alkali hydroxides on different Zr-based alloys have been carried out in static autoclaves at 350℃ in pressurized water, conditioned in low(0.32 mmol), medium(4.3 mmol) and high(31.5 mmol) equimolar concentration of Li-, Na-, K-, Rb- and Cs-hydroxide. Two types of alloys have been investigated: Zr-Sn-(TRM, Transition metal) and Zr-Sn-Nb-(TRM, Transition metal). From the experiments the cation could be identified as the responsible species for corrosion of Zr alloy in alkalized water. The radius of the cation governs the accelerated corrosion in the pre-transition region of Zr alloy. Incorporation of alkali cation into the zirconium oxide lattice is probably the mechanism which allows the corrosion enhancement for Li and Na and the significant lower effect for the other bases. Nb containing alloys showed lower corrosion resistance than Zr-Sn-TRM alloys in all alkali solutions. Both types of alloys were corroded significantly more in LiOH and NaOH than in the other alkali environments. Lowest corrosive aggressiveness has been found for CsOH followed by KOH. Concluding from the corrosion behavior in the different alkali environments and taking into account the tendency to accelerate the corrosion of Zr alloys, CsOH and KOH are possible alternate alkali for PWR(Pressurized Water Reactor) application.

keyword :

Stress rupture behavior of Mar-M247LC alloy was investigated in the temperature range of 760℃-980℃. The amount of residual eutectic γ` was varied by controlling the solution treating time at 1260℃. As the solution treating time at 1260℃ was increased, the amount of residual eutectic γ` was reduced and the stress-rupture life was increased. Especially the increase of rupture lives are more pronounced in the 760℃/587MPa condition than in the 982℃/152MPa condition. In the 760℃/587MPa rupture is occured by both transgranular and intergranular mechanism. It was observed that the small grains were disappeared during solution treatment because of the reduced amount of residual eutectic γ` associated with the increased solution treating time, which was considered as one of the barriers for the migration of grain boundaries. The improved stress-rupture life was attributed to the disappearance of the isolated small grains as well as the reduction of the amount of residual eutectic γ` which acted as early crack initiation sites.

keyword :

Abstract The objective of the present study is to investigate thermal fatigue properties in three work rolls, i.e., Ni-grain cast iron roll, high Cr (Hi-Cr) cast iron roll, and high speed steel (HSS) roll. These three rolls ware manufactured by a centrifugal casting method. The basic microstructures of their shell regions were observed to be composed mainly of coarse primary carbides and tempered martensite matrix, and the amount of primary carbides was increased in the order of the HSS roll, the Hi-Cr roll, and the Ni-grain roll. The thermal fatigue test results indicated that thermal fatigue properties were the better in the order of the HSS roll, the Hi-Cr roll, and the Nigrain roll, and that the thermal fatigue life of each roll was decreased with increasing the mean temperature and with increasing the temperature range of the thermal fatigue cycle. These results were interpreted by the amount of primary carbides and by cyclic softening phenomena associated with the exposed time at elevated temperatures during the thermal fatigue test.

keyword :

This study aims at clarifying microstructural factors such as tempered martensite matrix, coarse primary carbides, and fine secondary carbides, which influence thermal fatigue properties of Ni-grain cast iron roll, high Cr(Hi-Cr) cast iron roll, and high speed steel(HSS) roll. The micromechanism of fatigue process involved in crack initiation and propagation was identified by the observation of the sectioned region beneath the fracture surface and by the direct observation of fracture process using in situ SEM technique. The thermal fatigue properties were analyzed by the microstructural difference of the rolls and by the increase in tensile stress which might cause fracture when the tensile stress reached to the tensile strength of the roll. The microstructural observations indicated that coarse primary carbides acted as fatigue crack initiation sites on the specimen surface since they cleaved first to form cracks at very low stress levels. Thus, the HSS roll having the higher tensile strength and the smaller amount of primary carbides showed the better thermal fatigue properties than the Hi-Cr roll or the Ni-grain roll. In order to improve thermal fatigue properties of the rolls, it is also suggested that the homogeneous distribution of primary carbides must be achieved by the decrease in carbide segregation along the solidification cell boundary and by the optimization of the roll casting process.

keyword :

Temperature response is tested for a Ni-Ti shape memory spring with prestrain or a constant load. An apparatus for measuring variation in deflection of the spring with temperature and DSC are employed to evaluate the characteristics. Localized elastic stress, accumulating on thermo-mechanical cycling, give rise to excess recovery, i.e., continuance of shape change at temperatures well above Af. The accumulation of even apparently elastic stress concentration can initiate dislocation slip, causing a SMA spring to lose its precision. Thus it is strongly advised that SMA thermo-actuator be cycled until no further permanent is induced, before its parameters are evaluated for designing.

keyword :

Based on the thermodynamic model between TiN-BN-AlN in austenite, the effect of the boron content of direct-quenched low-carbon type Ti-B containing steels on the tensile strength and impact toughness was investigated. Variation of tensile strength showed a good relationship with the calculated soluble boron content at the start cooling temperature, showing that the thermodynamic model was applicable to the prediction of the hardenability of direct-quenched Ti-B containing steels. Impact toughness was varied form 20J to 250J at -40℃, depending on microstructure which was determined by the soluble boron content and the cooling rate. The highest absorbed energy of 200∼250J was obtained at martensite and lower-bainite mixed microstructure, followed by 100∼200J at ferrite and bainite mixed microstructure and 20∼40J at bainite with martensite-austenite constituent microstructure.

keyword :

It has been known that lead causes hot shortness of copper. Recent continuous casting process of copper scrap for electrical purpose needs to maintain lead content of copper rod above 400 ppm to avoid hot shortness and casting defects. In this study, to discover the effect of lead on the high-temperature mechanical properties of copper, five kinds of copper test samples with different lead content were made and tested at 850℃ with various cross head speeds. At low cross head speed, lead caused hot shortness of copper and mechanical properties of specimens containing lead were worse than those of pure copper, but at high cross head speed, 600 ppm of lead improved mechanical properties of copper. And mechanical properties of specimen containing 600 ppm of lead were comparable to those of pure copper.

keyword :

The present study aims at investigating powder metallurgy (P/M) fabrication processes of high performance 2009 Al composites reinforced with SiC particulates. In particular, the effect of consolidation temperature during the composite processing on microstructure and mechanical properties has been focused on. Rapidly solidified 2009 Al powders produced by commercial atomization technique were mixed with SiC particulates using a fluidized zone mixer. Composite billets were consolidated using a vacuum hot press at the temperature range from 560℃ to 620℃ under the pressure of 50 MPa, and then extruded at 450℃. Using the hot pressing temperature of 560℃, good quality Al-SiCp composite billets having relatively homogeneous microstructure and sound Al/SiC interfacial bonding were obtained, while the composite billets consolidated at the higher temperatures contained SiC depleted regions because of the larger amount of liquid. These SiC depleted regions were not removed during the extrusion process, resulting in the unhomogeneous distribution of SiC particulates. The better mechanical properties of the extruded bars were achieved in the composites fabricated with the lower consolidation temperature since they had the uniform distribution of SiC reinforcements together with full densification. Microstructure and mechanical properties were compared with those of the composites processed by Advanced Composite Materials Corp. or the composites reinforced with SiC whiskers so as to obtain a proper perspective of the Al-SiC composites.

keyword :

This study was aimed to investigate influence of microstructure, abrasive heat affected zone(AHAZ), primary silicon and eutectic silicon on abrasive characteristics of AC8A/SiC_p composite fabricated by the squeeze casting under various conditions. The abrasive characteristics was improved by addition of Mg and Ca elements because of improvement of SiC_p wettability and hardness in matrix. It is general theory that abrasive amount is decreased with increase of squeezing pressure. In this study, however, as squeezing pressure applies over a certain limit value(220MPa), the abrasive resistance under the condition of the high abrasive speed(3.67 ㎧) is not increased with increase of squeezing pressure. This is due to the crack by the generation of abrasive heat and fatigue below abrasive surface under the condition of high abrasive speed(3.67 ㎧). In abrasive heat affected zones of AC8A and AC8A/SiC_p composite fabricated by same squeezing pressure(220MPa), eutectic Si of directly below abrasive surface is finer than that of matrix, and abrasive heat affected zone of AC8A/SiC_p composite is thinner than that of AC8A. These reasons are that abrasive heat makes α dendrite and eutectic Si fine in AC8A/SiC_p composite. Therefore, the hardness of heat affected zone of directly below abrasive surface was increased sharply owing to abrasive heat.

keyword :

Aluminum and chromium were simultaneously co-deposited on the austenitic stainless steel substrates by pack cementation process. The effects of various combinations(masteralloy-activator-diffusion process) on the characteristics of coating layer on 310S and 316 were investigated. The most important factor controlling the simultaneous Cr-Al coating was composition of the masteralloy. The 80Cr-20Al(wt%) masteralloy exhibited better coating characteristics than the 95Cr-5Al(wt%) masteralloy. The 2-step coating cycle showed more desirable coating layer than the 1-step coating cycle. The coating morphology of each specimen consisted of two distinct layers, an outer layer of a brittle and continuous β-NiAl intermetallic phase and an underlying diffusion zone. In the same coating cycle with the same masteralloy composition, the more uniform coated layer was formed on 316 than on 310S due to the austenite stabilization of the substrate.

keyword :