• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2012.05a
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• pp.309-310
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• 2012

스테인레스강의 내식성을 향상시키기 위한 고농도 침입형 질화층(S-phase)처리를 하는 논문들이 발표되고 있지만 테스트 시편이 아닌 실제 제품이나 가공품등의 질화 처리 하였을 경우 내식성향상이 아닌 저하되는 것을 볼 수 있다. 스테인레스강의 제품화 과정에서 압연 및 기계가공에서 발생되는 각종 문제, 가공경화로 인한 현상 및 질화 처리후 표면개질의 석출상이나 입계로부터 시작되는 크랙이 내식성을 저감시키는 경향을 살펴보았다. 이러한 경향은 봉재나 두꺼운 소재보다는 가공소재인 판재나 형상이 있는 제품에 더 심하게 나타나는 것을 보여준다. 소재의 관리와 질화 온도 및 여러가지 변수에 의해 최적화 되어 S-phase 질화층을 형성해야 내식특성을 유지 할 수 있다.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.3
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• pp.380-388
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• 2016

Commercially pure titanium (CP Ti) has been actively used in plate heat exchangers due to its light weight, high specific strength, and excellent corrosion resistance. However, compared with automotive steels and aluminum alloys, there has not been much research on the plastic deformation characteristics and press formability of CP Ti sheet. In this study, the mechanical properties of CP Ti sheet are clarified in relation to press formability, including anisotropic properties and the stress-strain relation. The flow curve of the true stress-true strain relation is fitted well by the Kim-Tuan hardening equation rather than the Voce and Swift models. The forming limit curve (FLC) of CP Ti sheet was experimentally evaluated as a criterion for press formability by punch stretching tests. Analytical predictions were also made via Hora's modified maximum force criterion. The predicted FLC with the Kim-Tuan hardening model and an appropriate yield function shows good correlation with the experimental results of the punch stretching test.

• Composites Research
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• v.17 no.5
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• pp.54-60
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• 2004

A new three-dimensional model for predicting the relationship between the prestrain of the composite and the amount of phase transformation of shape memory alloy inducing shape memory effect has been proposed by using Eshelby's equivalent inclusion method with Mori-Tanaka's mean field theory. The model composite is aluminum matrix reinforced with short TiNi fiber shape memory alloy, where the matrix is work-hardening material of power-law type. The analytical results predicted by the current model show that most of the prestrain is induced by the plastic deformation of the matrix, except the small prestrain region. The strengthening mechanism of the composite by the shape memory effect should be explained by excluding its increase of yield stress due to the work-hardening effect of the matrix.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.17 no.4
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• pp.718-725
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• 2016

In this study, the plastic flow curve of commercially pure titanium sheet (CP Ti) actively used in the plate heat exchanger etc., was evaluated. The plastic flow curve known as hardening curve is a key factor needed in conducting finite element analyses (FEA) for the forming process of a sheet material. A hydraulic bulge test was performed on the CP Ti sheet and the strain in this test was measured using the DIC method and ARAMIS system. The measured true stress-true strain curve from the hydraulic bulge test (HBT) was compared with that from the tensile test. The measured true stress-true strain curve from the hydraulic bulge test showed stable plastic flow curve over the strain range of 0.7 which cannot be obtained in the case of the uniaxial tensile test. The measured true stress-true strain curve from the hydraulic bulge test can be fitted well by the hardening equation known as the Kim-Tuan model.

• Nuclear Engineering and Technology
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• v.10 no.2
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• pp.73-78
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• 1978

The effect of temperature and strain rate on the deformation behavior of $\alpha$-uranium was investigated in the temperature ranged 300$^{\circ}$ to 55$0^{\circ}C$ by strain, rate change test. Strain rate sensitivity, activation volume, strain rate sensitivity exponent and dislocation velocity exponent were determined. The strain rate sensitivity exponent and dislocation velocity exponent were determined. The strain rate sensitivity exponent increases with strain below 40$0^{\circ}C$, while the exponent decreases with strain above 50$0^{\circ}C$. It is believed that the increase of strain rate sensitivity exponent with strain below 40$0^{\circ}C$ can be attributed to an increase in internal stress as a result of work hardening while decrease of the exponent with strain above 50$0^{\circ}C$ is due to predominance of thermal softening over work hardening because more slip, system are active in deformation above about 50$0^{\circ}C$.

• Journal of the Korea Institute of Building Construction
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• v.15 no.5
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• pp.451-456
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• 2015

This study aimed to evaluate of the hardened properties (mortar consistency, setting time, absorption properties, drying shrinkage, and bond strength) of the basalt powder sludge mortar recycling a basalt powder sludge occurred during the manufacture process of basalt stone as a replacing material for the sea-sand used to cement filling compound for surface preparation. The hardened mortar made of the basalt powder sludge showed an enhanced performance or similar with the properties of normal mortar used to cement filling compound for surface preparation. But, the drying shrinkage was increased more than a normal cement mortar in the hardened mortar made of the basalt powder sludge since curing 8 - 9days. And the bond strength is low in the hardened mortar used the basalt powder sludge. On the whole, properties of the hardened mortar used the basalt powder sludge correspond to the required minimum quality criterion in the KS F 4716 'cement filling compound for surface preparation'.

• Journal of Advanced Marine Engineering and Technology
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• v.36 no.1
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• pp.78-84
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• 2012

Mold may experience deterioration of molding quality as the abrasion of mold occurring due to friction between mold and molded product works as allowance in the course of press process. Therefore, to improve the wear-resistance of mold, methods like nitriding, carburizing, flame and induction surface hardening treatment etc have been applied. However, as such methods are accompanied by problems like shape limitation or product deformation etc, laser surface treatment technology is under review as surface treatment method that can solve such problems. Therefore, in this study, mold material cast iron was surface-treated by using high power diode laser. In previous report 1 and report 2, the heat treatment characteristics were compared by the differences of die materials and shapes, then this paper observed microstructure by using optical microscope and scanning electronic microscope to analyze the structural difference of hardened zone, interface area and base metal after heat treatment. And the structural condition was grasped through EDS. As a result of microstructure, hardened zone showed formation of acicular martensite.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.95-95
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• 2010

자동차 산업에서 차체를 성형하는 프레스 금형 산업은 꾸준히 증가하고 있는 자동차 생산대수와 함께 성장해가고 있으며, 자동차 산업의 국제 경쟁이 심해지고 소비자들의 요구가 다양해짐에 따라 신제품 개발주기에 발맞추어 금형의 제작에도 단납기 및 비용절감을 위한 노력과 제품의 품질 향상을 위해 신기술, 신공법이 적용되고 있다. 한편 자동차 차체를 제작하는 프레스 금형가공은 박판소재를 원하는 형상으로 제작하는 공정으로써, 프레스의 상 하 운동을 이용하여 강판을 성형한다. 이러한 금형의 형태는 곧 자동차 차체 제품의 형태를 완성하므로 제품을 성형하는 도중에 금형과 소재의 마찰에 의해 금형의 마모나 마멸이 발생하여 제품의 품질을 저하시킬 우려가 있다. 따라서 금형의 내마모성 및 수명을 향상시키기 위한 방안들 중 표면경화처리가 행해지고 있으며, 그중 공정 속도가 빠르고 국부적인 열처리가 가능한 레이저 표면처리 방법이 많은 관심을 받고 있다. 본 연구에서는 이러한 금형의 성질을 향상시키기 위해 고출력 다이오드 레이저를 이용하여 프레스 금형공정 중 드로잉(drawing) 공정에의 적용을 위한 표면경화처리를 실시하였다. 최대출력 4.0kW의 다이오드 레이저를 사용하였으며, 6축 외팔보 로봇에 열처리용 광학계를 장착하여 열처리를 실시하였다. 또한 광학계 부근에는 적외선 온도센서가 부착되어있어 열처리시 시험편의 표면온도를 실시간으로 측정할 수 있도록 구성되어져있다. 시험편은 금형재료용 구상흑연 주철인 FCD550 소재를 사용하였으며, 공정변수에 따른 열처리 특성을 파악하고, 그 경화특성을 평가하였다. 실험 결과, FCD550 소재의 표면 열처리시 레이저 출력 3.5kW, 빔 이송속도 3mm/sec에서 최적의 열처리 특성을 나타내었으며, 이때의 최고 경도는 930Hv을 나타내며 모재에 비해 경도가 3배 정도 상승하는 우수한 경화특성을 보였다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.10
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• pp.1239-1249
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• 2013

In this study we establish a process to predict hardening behavior considering the Bauschinger effect for zircaloy-4 sheets. When a metal is compressed after tension in forming, the yield strength decreases. For this reason, the Bauschinger effect should be considered in FE simulations of spring-back. We suggested a suitable specimen size and a method for determining the optimum tightening torque for simple shear tests. Shear stress-strain curves are obtained for five materials. We developed a method to convert the shear load-displacement curve to the effective stress-strain curve with FEA. We simulated the simple shear forward/reverse test using the combined isotropic/kinematic hardening model. We also investigated the change of the load-displacement curve by varying the hardening coefficients. We determined the hardening coefficients so that they follow the hardening behavior of zircaloy-4 in experiments.

• Elastomers and Composites
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• v.44 no.3
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• pp.315-322
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• 2009

Influence of blend mode of extender oil on the properties of thermoplastic vulcanizates (TPVs), based on an ethylene-propylene-diene copolymer (EPDM) and a polypropylene (PP), was studied. The EPDM/PP TPVs were prepared in an open roll mill using two different modes in blending sequence of paraffinic oil and phenolic curative, i.e., Oil-Cure and Cure-Oil modes. Degree of cross-linking by gel fraction and properties such as hardness, tensile strength, elongation at break, and melt flow rate were investigated as a function of extender oil content for the two modes. Little influence of the blend mode of extender oil on the degree of cross-linking and mechanical behaviors was observed. However, the use of Cure-Oil mode in the preparation of EPDM/PP TPVs resulted in a marked increase in the level of processability as reflected by melt flow index, as compared to the use of Oil-Cure mode.