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A study has been made to investigate the high temperature deformation behavior of Ti-6Al-4V alloy and to predict the final microstructure under given forming conditions. Equiaxed microstructure of Ti6Al-4V alloy was prepared as starting microstructures. By performing the compression tests at high temperatures (700~1100℃) and a wide range of strain rates (10^(-4) ~ 10²/s), flow stress-strain curves obtained and various parameters such as strain rate sensitivity (m) and activation energy (Q) were obtained and used to established constitutive equations. When the specimens were deformed up to strain of 0.6, equiaxed microstructure did not show any significant changes in morphology, while the average α grain size was varied with variation of volume fraction at different temperatures and strain rates. Accordingly, constitutive equations were developed by use of internal state variables, and applied to predict the final microstructure after forming. Finally, the finite element analysis was performed considering the microstructural evolution during the hot deformation, and the grain size and the volume fraction of α phase of deformed body were predicted and compared with the experimental results through the actual forging process.

keyword : Ti-6Al-4V alloy, Equiaxed, Hot Workability, Instability, Constitutive Equation, Processing Map

Microstructure and texture development of Mg alloys during equal channel angular pressing (ECAP) was investigated and correlated with the mechanical properties. The microstructure was effectively refined by ECAP and the original fiber texture of extruded Mg alloy was disintegrated and new texture components were gradually developed by repetitive ECAP pressing.

keyword : Mg Alloy, AZ31, AZ61, Texture, Pole Figure, ECAP, Mechanical Properties, HCP

Microstructural evolution and degradation of tensile properties during thermal exposure in Ni-base superalloy CM247LC have been investigated. Two different morphology of carbides, blocky MC and scriptal MC, observed in aged specimen had different compositions due to the segregation during casting process. The morphology of MC depends on its composition in aged specimen. Coarsening of γ` and decomposition of MC occurred during thermal exposure. M6C formed from scriptal MC decomposition and M23C6 formed from blocky MC near eutectic. Grain boundary had both M6C and M23C6 after MC decomposition. Strengths decreased during thermal exposure due to coarsening of gamma prime. Effect of thermal exposure on ductility is not significant at both room temperature and 760℃.

keyword : CM247LC, Thermal Exposure, MC decomposition, Tensile properties

Effects of alloying elements and tempering conditions on mechanical properties in Cu bearing 9 to 12% Cr ferritic heat resistant steels have been studied by the tensile tests, the Charpy impact test, the creep rupture test, and the microstructural analysis of crept specimen. Alloys studied met sufficiently the minimum requirements for tensile strength (TS) of 800 MPa and FATT50 of 60℃ of high pressure turbine rotor steel using up to 593℃. Yield strength (YS) was more than the minimum criterion of 700 MPa by a slim margin. It was difficult to meet the Charpy energy requirements of minimum 3 kgf-m in alloys containing about 3.5% W or 1.8% Mo equivalent. Strength and toughness were improved simultaneously by additions of C, N, Cu, Ni, and V elements. TS and YS of BCu alloy had the relationships with Larson-Miller parameter (LMP) by TS (±14.0 MPa) = 2637-0.07606*LMP and YS (±12.5 MPa) = 2359-0.05917*LMP, respectively. Cu element in alloys studied caused the precipitation of fine Cu-rich precipitates of nanometer size acting on a nucleation site for Laves phase and M23C6 carbide. In Cu bearing alloys, the needle type Laves phase existing simultaneously with the globular type Laves phase contributed to the improvement of creep rupture strength (CRS) of alloys studied. The CRS at 650℃ were similar to one another regardless of minor modifications of Mo, Cr, and Cu contents in Cu bearing alloys. Tempering treatments at higher temperatures were more advantageous for the long-term CRS due to the formation of the more stable microstructure.

keyword : 9-12Cr heat resistant steels, Creep rupture strength, Tensile properties, Toughness, Tempering, Turbine rotor

Instrumented indentation technique (IIT), because it is fast, precise, and nondestructive, has been widely used to determine such mechanical properties as tensile properties, residual stress, fracture properties and hardness. The IIT has been applied to evaluate properties and to diagnose safety and integrity of materials and structural units such as gas/steam pipeline, railway and bridge. In particular, in US, new DOT (Department of Transportation) regulations stipulated to evaluate tensile properties of gas pipeline, and IIT has been researched as the most serviceable method to determine the properties of in-service system without damage and shutdown. Owing to great interest in the technique, standardization for testing procedure and property evaluation using IIT has been progressed in many-sidedness. For indentation hardness, ISO 14577 standard, FDIS in present, be scheduled for international standard in a short time. For indentation tensile property, international working group has been operated to make international standard since 2004, and domestic standard, KS standard, was established as KS B 0950, prior to IS, in 2002. In addition, for indentation residual stress, domestic standard has been registered as KS B 0951 since 2003.

keyword : Standardization, Instrumented indentation, Flow properties, Flesidual stress, Fracture tougnness

The Creep Rupture data are the criteria to select the high temperature structural materials for the different operating conditions (Temperature, Stress, Design Life). With these Creep Rupture data set, the design life can be calculated with the safety margin. In case of Creep Rupture data, the uncertainty is difficult to calculate since the testing time is relatively long and the testing temperature is very difficult to be hold as constant along the gage section. With these uncertainties, the safety margin for creep conditions are too large for the components to be very sound beyond the design life. even triple of the design life. To reduce the uncertainty level of Creep Rupture data, the number of good data must be larger than a certain values, which can be used to calculate the reliability range. Since the data of Creep Rupture data time are converted to Creep Rupture parameter, that reliability must be evaluated for the regression curves of the Creep Rupture data parameters. In this paper, there will be mentioned how to get the analysis on the Creep Rupture data time and parameters.

keyword : Creep rupture parameter, Data base, Reliability

In using the annual data of outer diameters in reformer tube, a statistical treatment was accomplished in order to predict expansion rate and reliability of the tube by probabilistic assessment. The distribution of outer diameters detected by ultrasonic test was identified as the 3-parameter Lognormal distribution deviated from the normal distribution by severe increasing of dispersion and its fluctuation as annually using the tubes. In the assumption of critical value (CV) determination for the outer diameter, the probabilistic assessment was proceeded into deriving the failure probability from the statistical information. Finally, the safety index was also derived from the failure probability, making the failure estimation available for each annual using of the reformer tubes.

keyword : Reformer tube, Life prediction, 3-parameter Lognormal, Failure probability, Safety index