• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 추계학술대회논문집
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• pp.319-324
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• 2004

Cross wedge rolling process is utilized to manufacture multi-stepped axis symmetrical parts. This process is generally performed under high temperature conditions in order to induce serious deformation. But cold cross wedge rolling process has been rarely studied due to the limits of deformation. Recently, the cold cross wedge rolling process has been utilized to enhance the material strength in specified parts of manufactured products. In this paper, experimental researches were carried out with various forming conditions of cold cross wedge rolling process in order to suggest the design guidance to make preform for cold cross wedge rolling. The tensile strength and the surface hardness of specified region were compared to that of initial material with the variation of the area reduction and the rotational speed of rolling die. With respect to the area reduction, the maximum tensile strength was linearly increased and the surface hardness was rapidly increased within lower percent of area reduction. The surface hardness was saturated over the rotational die speed of 0.8 RPM.

• Journal of Welding and Joining
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• 제12권2호
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• pp.87-96
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• 1994

Correlation between microstructural features and segregation of elements (Si, Mn, P and S) near the mid of thickness in the base metal and the synthetic HAZ was investigated. Furthermore, the relationship between the degree of center segregation and weld cold cracking susceptibility in the thickness direction was also conducted by evaluating the effect of P concentration on the critical applied stress. The results obtained are as follows: 1) Pearlite band, containing the MnS type inclusion and a locally transformed structure with a higher hardness, was observed in the center segregation region. 2) By the weld thermal cycle, center segregation region was transformed to the white band which had a higher hardness than that of base metal due to a greater hardenability of concentrated Mn, P etc.. 3) Weld cold cracking susceptibility in the thickness direction was mainly dependent on the concentration of impurity elements rather than on the number of the segregated particles near the mid of thickness. 4) During welding, the higher concentrated region was easily changed into white band. Therefore, it could be predicted that the initiation and propagation of a cold crack would be promoted by increasing the restraint stress and hydrogen content.

• 한국레이저가공학회지
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• 제11권2호
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• pp.15-19
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• 2008

Electron beam (EB) weldability of pure grade Nb sheet was studied. One of Nb sheets was as-annealed and the other was cold rolled. Microstructures, Vickers hardness, and transverse weld tensile test were carried out for the base metal, the heat affected zone (HAZ) and weld metal. In the case of the EB welds made using the annealed Nb sheeet, fine equiaxed grains and coarse grains were dominant at the base metal and the HAZ, respectively, and columnar grains were observed at the weld metal. For the EB welds made using the cold rolled Nb sheet, elongated grains in the rolling direction at the base metal, and the microstructures of the weld metal and the HAZ are similar to those of the EB welds made using the annealed Nb sheet, respectively. For both annealed and cold rolled Nb sheet, the width of the HAZs are unusually wide in spite of using high density heat source, i.e. electron beam, and the grain sizes of both HAZs are similar. When tensile test was carried out using the transverse weld specimens, the failure occurred at the HAZ for both EB welds made using Nb sheets annealed and cold rolled, respectively and the tensile strengths of both specimens were 161MPa. Vickers hardness of EB welds made using annealed Nb was 56-57 Hv at both base metal and weld metal, 52-53 Hv at the HAZ. On the other hand, Vickers hardness of EB welds made using cold rolled Nb was 97-99 Hv at the base metal, but the hardness values of weld metal were similar to those obtained at the weld metal of annealed Nb.

• 열처리공학회지
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• 제25권5호
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• pp.233-238
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• 2012

The effect of subzero treatment on the mechanical properties of cold rolled high manganese austenitic stainless steel was investagated. ${\alpha}$'-martensite was formed by cold rolling, and it was formed with surface relief and specific direction or crossing each other. The volume fraction of martensite increased by subzero treatment, and it was increased with longer time of subzero treatment and higher temperature of subzero treatment. The hardness and strength increased by subzero treatment, while the elongation decreased. With the increase of volume fraction of martensite, the hardness and strength was increased steeply with proportional relationship, elongation was decreased slowly. The results show that the hardness and strength was strongly controlled by the volume fraction of martensite, and the elongation was affected by transformation behavior of deformation induced martensite in the initial stage of deformation.

• 소성∙가공
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• 제32권6호
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• pp.329-334
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• 2023

The process of heat treatment in cold forging is an essential role in enhancing mechanical properties. However, it relies heavily on the experience and skill of individuals. The aim of this study is to predict hardness using machine learning to optimize production efficiency in cold forging manufacturing. Random Forest (RF), Gradient Boosting Regressor (GBR), Extra Trees (ET), and ADAboosting (ADA) models were utilized. In the result, the RF, GBR, and ET models show the excellent performance. However, it was observed that GBR and ET models leaned significantly towards the influence of temperature, unlike the RF model. We suggest that RF model demonstrates greater reliability in predicting hardness due to its ability to consider various variables that occur during the cold forging process.

• Journal of Advanced Marine Engineering and Technology
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• 제31권6호
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• pp.729-735
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• 2007

AC cold arc welding process with AWS E Ni-CI and NiFe-CI is sometimes used to repair damaged cast iron parts in diesel engine room. But if some difference in hardness on welding zones, repaired parts would be cracked in a short. To overcome this default. this study is performed on varying preheating temperature of welding parts, selecting welding rod etc. Experimental results showed that difference in hardness on welding zones at $200^{\circ}C$ was less than $100^{\circ}C$ and less low current than high current. From this study we could conclude that repair welding at $200^{\circ}C$ preheating and low current as possible as welding in damaged cast iron parts was a little difference in hardness on welding zones.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2004년도 춘계 학술발표대회 개요집
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• pp.311-313
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• 2004

WC-12-17%Co powders with nano- and micro-structures were deposited by cold spray process using nitrogen and helium gases. The results show that there is no detrimental phase transformation and/or decarburization of WC by cold spray deposition as expected. It is also observed that nano-sized WC in the feedstock powder is maintained in the cold spray deposition. It is demonstrated that it is possible to fabricate the nano-structured WC-Co coating with low porosity and very high hardness (-2050 HV) by cold spray deposition with reasonable powder preheating.

• 한국산림과학회지
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• 제95권6호
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• pp.672-679
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• 2006

밤의 이용가치를 높이기 위하여 밤 과실의 등급화 및 규격화에 필요한 정확한 품질평가 기준을 확립하고자 대보 등 13개 주요 재배품종을 대상으로 수확직후의 과실품질 및 저온저장 기간에 따른 밤 과실의 당도와 과육정도의 변화를 조사하였다. 2001년부터 2005년까지 수확 직후의 밤 과실을 대상으로 밤의 품질을 크게 좌우하는 입중, 당도, 과육경도, 열과율, 다배율을 조사한 결과, 각 형질에 있어서 품종간에 큰 차이가 있었으며 동일한 품종에서도 해마다 상당한 변이가 관찰되어 유전적인 요인 외에도 환경적인 영향을 많이 받는 것으로 나타났다. $2^{\circ}C$에서 16주간 저장하면서 4주 간격으로 당도변화를 조사한 결과 저장기간에 따라 상당한 차이가 나타났으며, 출운 품종의 경우 당도가 저장 16주째에 수확직후보다 8%나 증가되었다. 조생종을 제외한 대부분의 품종에 있어 저온저장 12주까지는 당도가 증가하였으나 그 이후에는 점차 감소하였다. 저온저장 중 과육정도는 석추품종을 제외하고는 기간이 경과함에 따라 약간 증가하였으나 유의성은 인정되지 않았다. 이상의 연구결과로부터 얻어진 주요 재배품종에 대한 과실품질 자료는 밤 재배지는 물론 육종가 등에게 유용한 정보가 될 수 있으리라 생각된다.

• 소성∙가공
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• 제6권5호
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• pp.395-399
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• 1997

There exists a certain functional relation between Vickers hardness and flow stress in the strain-hardened material. Using this relation, the Vickers hardness values in the strain-hardened material can be converted into the flow stress values in good approximation. Therefore, the information about the flow stress distribution in the material can be easily acquired through a forming experiment. That makes it possible to simulate the process state for a critical moment of cold forging under the given boundary conditions very quickly without calculating the foregoing history of the actual deformation from the beginning.

• 한국재료학회지
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• 제10권11호
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• pp.725-731
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• 2000

냉간가공과 베타급냉된 Zr-Sn 합금의 재결정 거동을 미소경도 시험과 미세 조직 관찰 방법에 의해서 조사하였다. 베타급냉 처리된 합금의 재결정은 냉간가공된 합금의 재결정보다 늦게 일어났는데, 이는 냉간 가공에 의해 도입된 축적에너지가 더 높다는 것을 의미한다. 냉간가공재와 베타급냉재의 초기 경도는 동일하지라도 재결정거동은 아주 다르게 나타났다. TEM 조직관찰 결과를 근거로 할 때, 냉간가공재는 subgrain합체에 의해서 재결정이 일어나며 베타급냉재는 응력유기 입계이동에 의해서 재결정이 일어나는 것으로 밝혀졌다.