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최병학(B. H. Choe),이상호(S. H. Lee),김영표(Y. P. Kim),김우식(W. S. Kim),고영태(Y. T. Ko) 대한기계학회 2004 대한기계학회 춘추학술대회 Vol.2004 No.4
A study on the probabilistic methodology for the estimation of the remaining life of Pressurized pipelines containing active corrosion defects is presented. This reliability assessment is carried out using extream value distribution of the corroded defects instead of already published failure perssure moded like NG18 or ASME B31G. The failure probability of pipelines depends on the number of corroded defects, and it could be calculated directly as the area exceeded a defined LV(Limited Value of corrosion depth). The remaining life of pressurized pipelines can also be estimated by the PDF of extream value distribution as calculating the exceeded area with a defined failure probability.
초내열합금 CM 247LC 에서 변형률속도가 고온 저주기 피로성질에 미치는 영향
최병학,이후철,김학민 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.7
The effect of strain rate on the low cycle fatigue (LCF) properties of CM 247LC superalloys were studied. Two different strain rates of 0.05 ㎐ and 0.003 ㎐ were used for polycrystalline of Cm247LC. The microstructural changes and dislocation structures were investigated by electron microscopy. It was found strain rate does not affect cyclic dislocation structures, but dislocation recovery was believed to be affected by strain rates. At lower strain rate, i.e. 0.003 ㎐, the plastic strain was larger at constant total strains but the applied stress was smaller at constant plastic strain range. The increase in LCF life was resulted from the lower applied stresses at constant plastic strain range due to the recovery of dislocations at the lower strain rate.
최병학,이후철 대한금속재료학회(대한금속학회) 1988 대한금속·재료학회지 Vol.26 No.1
The hot embrittlement of KS SPS 5 spring steel in austenite region was studied in order to clarify the cause and mechanism of surface cracking during preheating, hot forming and cooling operations in spring making with rolled steel. The loss of hot ductility was attributed to precipitation of nitrides which were identified as AIN by TEM. Sulfur did not influence the hot embrittlement in this steel because of its decreased solubility due to high Mn content. Through the TEM observation of the size and distribution of the precipitated particles the mechanism of hot embrittlement in spring steel was thought to be relative weakening of the grain boundaries caused by the matrix which is strengthened by AIN precipitates. The effects of the strain rate and the grain size on the hot ductility were discussed.
Ti3Al-Nb 합금에서 미세조직과 상변화가 기계적 성질에 미치는 영향
최병학,이용태 대한금속재료학회(대한금속학회) 1991 대한금속·재료학회지 Vol.29 No.11
Various thermomechanical treatments were performed in order to optimize the ductility and strength of Ti₃Al-Nb alloys, called Alpha 2 and Super Alpha 2. The mechanical properties such as strength, elongation and hardness can be improved by the thermomechanical treatment. Especially Super Alpha 2 which has more β-stabilizing alloy elements than Alpha 2 exhibited gradually improved mechanical strength as the solution heat treatment temperatures are approached to the β-transus temperature. Further improvement of the hardness could be achived depending upon aging time and temperature. This is due to the phase transformation happened during thermomechanical treatment. Formation and quantity of the orthorhombic martensite phase is the most important factor on mechanical properties. The martensite is formed in the matrix after solution heat treatment and quenching. During aging the martensite transforms to stable secondary α₂+$quot;0$quot;+β/B2 phases which are distributed finely as lamellar type. These microstructural refinement. together with proper amount of primary α₂ Phase increases strength and hardness in Super Alpha 2. While in Alpha 2 the martensitic transformation is suppressed and strength improvement can not be achived.