• 열처리공학회지
• /
• 제12권2호
• /
• pp.166-173
• /
• 1999

Commercial carbon steels containing 0.2~0.55 wt.----C were plasma-nitrided or plasma nitrocarburized at $550^{\circ}C$ for 21.6Ks using $H_2-N_2$ or $H_2-N_2$-CO mixed gas respectively. The characteristics of hardening and wear-resistance of each treatment were studied and compared. And also microstructure of nitrided layer and nitrides formed in compound layer near surface were studied. All plasma-nitrided steels investigated showed remarkable increase of surface hardness with the increase of carbon content. But nitrocarburized steels resulted in higher surface-hardness than plasma-nitrided steels, which means that nitrocarburized has higher surface-hardening effect. Plasma-nitrided steels showed hardness increase in through-thickness direction near surface. And also nitrocarburized steels showed similar hardness distribution in through-thickness direction to that of plasma-nitrided steel. However, nitrocarburized steels had higher cross-sectional maximum-hardness than plasma-nitrided steels as much as 100Hv. Wear test showed that the amount of specific wear was reduced by both plasma-nitriding and nitrocarburized, showing that the amount of specific wear was not related to the hardness. But non-treated specimen showed that the amount of specific wear was related to the hardness.

• 한국작물학회지
• /
• 제24권3호
• /
• pp.21-25
• /
• 1979

수도 유묘의 내냉성을 증대시키는 방법으로써 저온경화의 가능성과 한계를 구명하고저 품종통일과 진흥을 공시하여 주야간 $30^{\circ}/20^{\circ}$에서 생장한 제3 엽기묘를 6일간 저온경화를 한후 주야간 $8^{\circ}C$ 항온의 저온에 0, 2, 4, 6, 또는 8일간 처리하고 $30^{\circ}$20^{\circ}C 조건에서 냉해를 발현시키므로써 저온경화를 평가하였다. 저온경화방법은 주야간 $30^{\circ}/20^{\circ}C\;생장묘\rightarrow24^{\circ}14^{\circ}C(3일간)\rightarrow18^{\circ}/12^{\circ}C(3일간)\rightarrow8^{\circ}C$ 냉해유발처리의 순서를 갖는 점진적 경화와 $306{\circ}/20^{\circ}C\;생장묘\rightarrow30^{\circ}/20^{\circ}C$, 단 1일 4시간 $8^{\circ}C(2일간)\rightarrow30^{\circ}/20^{\circ}C$, 단 1일 6시간 $8^{\circ}C(2일간)\rightarrow30^{\circ}/20^{\circ}C$, 단 $8^{\circ}C\; 16시간 1회\rightarrow8^{\circ}C$ 냉해유발처리의 단시간 저온의 반복적 경화 2가지 방법이었으며 그 결과는 다음과 같다. 1. 내냉성이 약한 품종 통일은 $8^{\circ}C$ 저온을 4일간 처리하면 경화를 하지 않은 유묘는 묘생존율이 25%에 지나지 않았으나 점전적 경화률은 묘생존율이 59%, 반족적 단시간 경화묘는 묘생존율이 89% 정도로써 저온경화의 효과가 뚜렷하였으나 $8^{\circ}C$ 저온이 6일간 이상이 되면 경화묘들도 묘생존율이 모두 20%이하로 떨어졌다. 2. 내냉성이 강한 품종 진흥은 비경화묘도 $8^{\circ}C$ 저온처리 8일간에서도 묘생존율은 90%이상이어서 묘생존율면에서는 저온경화 효과를 분화시키지 못하였다. 3. $8^{\circ}C$ 저온처리 및 냉해발현후 엽신의 변색고사현상은 품종 통일과 진흥 모두 비경화묘보다는 점진적 경화묘, 그보다는 반복적 단시간 경화묘가 뚜렷하게 정상엽면적비율이 높고 저온경화의 효과가 뚜렷하였다.

• 한국소성가공학회:학술대회논문집
• /
• 한국소성가공학회 2007년도 춘계학술대회 논문집
• /
• pp.421-424
• /
• 2007

Dry sliding wear behavior of ultra-fine grained (UFG) plain low carbon dual phase steel, of which microstructure consists of hard martensite in a ductile ferrite matrix, has been investigated. The wear characteristics of the UFG dual phase steel was compared with that of a coarse grained dual phase steel under various applied load conditions. Dry sliding wear test were carried out using a pin-on-disk type tester at various loads of 1N to 100N under a constant sliding speed condition of 0.20m/s against an AISI 52100 bearing steel ball at room temperature. The sliding distance was fixed as 1000m for all wear tests. The wear rate was calculated by dividing the weight loss, measured to the accuracy of 10-5g by the specific gravity and sliding distance. The worn surfaces and wear debris were analyzed by SEM, EDS and profilometer. Micro-vickers hardness of the cross section of worn surfaces were conducted to analyze strain hardening underneath the contact surfaces. The wear mechanism of the UFG dual phase steel was investigated with emphasis on the unstable nature of the grain boundaries of the UFG microstructure.

• 동력기계공학회지
• /
• 제21권3호
• /
• pp.37-44
• /
• 2017

In this study, failure analysis of the angular pin for molding machines to aluminum component molding was carried out. SM45C steel was used for the angular pin, it was surface hardened by the induction surface hardening heat treatment. The cross section of damaged angular pin was observed, and micro Vickers hardness value from the fractured part was measured. Brittle fracture was occurred from the fracture surface of angular pin, therefore, impact toughness value was evaluated by V-notch Charpy impact test. It was confirmed that the impact absorption energy was high when was tempered at a high temperature for a long time, and the toughness was slightly increased. Also, 2-parameter Weibull statistical analysis was investigated in order to evaluate the reliability of the measured micro Vickers hardness values and absorbed energy. The micro Vickers hardness and absorbed energy well followed a two-parameter Weibull probability distribution, respectively. The reverse design against angular pin was proposed as possible by using test results.

• 소성∙가공
• /
• 제14권1호
• /
• pp.71-77
• /
• 2005

Thermal stability and dry sliding wear behavior of ultra-fine grained 6061 Al alloy fabricated by an accumulative roll-bonding (ARB) process have been investigated. After 4 ARB cycles, an ultra-fine grained microstructure of the 6061 Al alloy composed of grains with average size of 500nm, and separated mostly by high-angle boundaries was obtained. Though hardness and tensile strength of the ARB processed Al alloy increased with ARB cycles up to 4 cycles, the processed alloy exhibited decreased ductility and little strain hardening. Thermal stability of the ARB-processed microstructure was studied by annealing of the severely deformed alloy at $423K{\sim}573K$. The refined microstructure of the alloy remained stable up to 473K, and the peak aging treatment of the alloy at 450K for 8 hrs increased the thermal stability of the alloy. Sliding-wear rates of the alloy increased with the number of ARB cycles in spite of the increased hardness with the cycles. Wear mechanisms of the ultra-fine grained alloy were investigated by examining worn surfaces, wear debris, and cross-sections by a scanning electron microscopy (SEM).

• 소성∙가공
• /
• 제14권7호
• /
• pp.607-612
• /
• 2005

In this paper, warm deep drawing process with local heating and cooling technique was attempted to improve the formability of AZ31 magnesium alloy which is impossibly to form by conventional methods at room temperature by finite element method and experiment. For FE analysis, in first model with considering heat transfer, both die and blankholder were heated to 573K while the punch was kept at room temperature by cooling water. Also distribution of thickness and von Mises stress at room temperature and 498k for warm deep drawing were compared by FEM. Uniaxial tension tests at elevated temperature were done in order to obtain the temperature dependence of material constant under temperature of $293K\~573K$ and cross head velocity of $5\~500mm/min$. The phenomenological model for warm deep drawing process in this work was based on the hardening law and power law strain rate dependency. Deep drawing experiment were conducted at temperatures of room temperature, 373K, 423K, 473K, 498K, 523K, and 573K for the blank and deep drawing tools(holder and die) and at a punch speed of 10mm/min.

• Geomechanics and Engineering
• /
• 제15권4호
• /
• pp.973-986
• /
• 2018

The objective of this paper is to present abacuses obtained from a parametric study of deep-lined tunnels using a numerical finite element model. This numerical model was implemented in software GEOMEC91, which is a two-dimensional axisymmetric model that considers the progress of excavation and the placing of the lining through the activation and deactivation of elements. It is adopted a step of excavation constant (1/3 of radius), constant velocity and circular cross section along the tunnel axis. It is used for rock mass a viscoplastic constitutive law with von-Mises criterion of viscoplasticity without hardening whose deformation rate over time is given by the Bingham model. The lining uses a linear elastic constitutive law. In total are 1716 analysis presented in 60 abacuses that show the value of ultimate convergence ($U_{eq}$) due to tunneling speed. In addition, it is shown an example of the use of the abacuses to determine the ultimate convergence ($U_{eq}$) of the tunnel and pressure ($P_{eq}$) on the lining.

• 소성∙가공
• /
• 제16권4호
• /
• pp.299-303
• /
• 2007

Dry sliding wear behavior of ultra-fine grained(UFG) plain low carbon dual phase steel, of which microstructure consists of hard martensite in a ductile ferrite matrix, has been investigated. The wear characteristics of the UFG dual phase steel was compared with that of a coarse grained dual phase steel under various applied load conditions. Dry sliding wear test were carried out using a pin-on-disk type tester at various loads of 1N to 100N under a constant sliding speed condition of 0.20m/s against an AISI 52100 bearing steel ball at room temperature. The sliding distance was fixed as 1000m for all wear tests. The wear rate was calculated by dividing the weight loss, measured to the accuracy of 10-5g by the specific gravity and sliding distance. The worn surfaces and wear debris were analyzed by SEM, EDS and profilometer. Micro-vickers hardness of the cross section of worn surfaces were conducted to analyze strain hardening underneath the contact surfaces. The wear mechanism of the UFG dual phase steel was investigated with emphasis on the unstable nature of the grain boundaries of the UFG microstructure.

• 한국기계가공학회지
• /
• 제11권6호
• /
• pp.82-87
• /
• 2012

The purpose of this study is to produce a side impact beam with high tensile steel using a roll forming process. The door side impact beam plays an important roll in a car because it protects passengers from external crash. The roll forming process is a continuous bending process wherein a long metal sheet is bended as it continuously passes several rolls. The characteristic of this study is that an impact beam is produced by a continuous process using a ultra high strength steel without a hardening heat treatment. A model was determined by analysing plasticity of a cross section shape considering high strength. Design parameters of the impact beam was determined by crash-analysing the model. Workpiece products were manufactured by designing dies for roll forming and setting them up in a following process line. Results of a bending test and a FEM analysis was considered and reviewed.

• International Journal of Automotive Technology
• /
• 제8권2호
• /
• pp.193-201
• /
• 2007

The present paper deals with the application of the explicit finite element code, PAM-CRASH, to simulate the crash behavior of steel thin-walled tubes with various cross-sections subjected to axial loading. An isotropic elastic, linear strain-hardening material model was used in the finite element analysis and the strain-rate sensitivity of mild steel was modeled by using the Cowper-Symonds constitutive equation with modified coefficients. The modified coefficients were applied in numerical collapse simulations of 11 types of thin-walled polygon tubes: 7 convex polygon tubes and 4 concave polygon tubes. The results show that the thin hexagonal tube and the thick octagonal tube showed relatively good performance within the convex polygon tubes. The crush strengths of the hexagonal and octagonal tubes increased by about 20% and 25% from the crush strength of the square tube, respectively. Among the concave tubes, the I-type tube showed the best performance. Its crush strength was about 50% higher than the crush strength of the square tube.