Abstract: Tensile creep tests were conducted at 980°C under a constant stress on a single crystal
nickel base superalloy. Some tests were interrupted at different stages during the creep process. The
strain-time curves indicated that this alloy exhibited conventional primary, steady-state and tertiary
stages at this temperature. The transmission electron microscope (TEM) observations of foils taken
from the gauge sections of specimens were made to interpret the microstructural evolution that
occurred during the creep process. It was found that the γ′ particles were rafted in the direction
perpendicular to the applied stress. The acceleration of the creep rate was related to the change of the
Abstract: Hot compression tests at different temperatures and strain rates were performed on
superalloy 718 annealed at 1100 °C for 30 min as well as 950 °C for 30 min in order to investigate
the effect of the original microstructure on the hot deformation behavior. The results show that the δ
phase precipitated on the grain boundaries of superalloy 718 prior to hot deformation leads to a
decrease of the flow stress, which can be attributed to the drop of the matrix strength. The activation
energy (Q) for hot deformation of the fine-grained material with δ phase on the grain boundaries is
determined to be 381 kJ/mol, which is much lower than that of the coarse-grained material without
δ phase (472 kJ/mol). It is suggested that the initial δ phase precipitated on the grain boundaries of
the annealed material affect the thermal activation process of hot deformation for superalloy 718,
resulting in the decrease of activation energy.
Abstract: The advancement in superalloys permits the hot gas path components to operate for
thousands of hours under severe centrifugal, thermal and vibratory stresses. The blade of a gas turbine
must withstand the most severe condition combined of temperature, stress, and environment. After a
long operation, the damaged blades of a gas turbine used are welded for build-up and repaired. We
analyzed and compared the mechanical properties of GMAW(Gas Metal Arc Welding), a manual
welding method, a laser cladding method, and an automatic welding method under research and
Abstract: The Ni-based superalloy Inconel 718 (IN-718) was developed in the 1950s and is currently
used for several critical gas turbine components due to its good balance of mechanical properties.
Because of its stable mechanical properties at high temperature, the alloy is used for turbine bolts. The
mechanical properties of in-service facilities are required to maintain safety operation in power
plants. Ultrasonic tests, tensile tests and hardness tests were performed to evaluate the degree of the
material degradation of Inconel 718. The mechanical properties decreased as degraded, but the
longitudinal velocity of the ultrasonic signal increased. Also, the microstructure of the degraded
Inconel 718 was to study the relationship between the result from ultrasonic test and the results from
Abstract: TMCP steels produced by thermo-mechanical control process are now spot lighted due to
the excellent combinations of strength, toughness and weldability. Recently, in Korea, high strength
SM570-TMCP steel whose tensile strength is 600MPa has been developed and applied to steel
structures due to its excellent nature. But, for the application of the TMCP steel to steel structures, it is
necessary to elucidate not only the material characteristics but the mechanical characteristics of
welded joints. In this study, high temperature tensile properties of the SM570-TMCP steel were
investigated through the elevated temperature tensile test and the characteristics of residual stresses in
welded joints of the TMCP steel were studied through the three-dimensional (3-D) thermal
elastic-plastic finite element (FE) analysis on the basis of mechanical properties at high temperatures
obtained from the experiment. The results are then compared with the conventional quenched and
tempered high strength SM570 steel.
Abstract: Creep testing of Alloy 718 has been carried out at various loads in the temperature range
near 650°C in constant load control mode in order to understand how to predict the creep
behavior including tertiary creep. The test results have been used for evaluating the existed
models, such as Theta projection and Omega method that have been widely used for predicting long
term creep strain and rupture time. After determining variables and material parameters of each
method with the test results, estimated creep data from each model have been compared with the
each measured creep data from the creep tests. The root cause of the discrepancy between estimated
and measured data has been analyzed in order to improve the existed methods. The reliability of
the improved model has been evaluated in relation to creep data.
Abstract: The mechanism of the degradation process of nickel based superalloy CM247LC under
creep loading was analyzed by considering the microscopic chemical composition of the material
such as phase boundary of γ and γ’ phases.
In this study, a Directionally Solidified (DS) nickel-based superalloys CM247LC was used as test
specimens. The creep test was performed at 900oC under an uni-axial stress of 216 MPa. Each
specimen was creep ruptured and interrupted at different hours and then analyzed.
The change of chemical composition around the interface analyzed precisely by used Auger
Electron Spectroscopy (AES). It was found that the local enrichment or lack of cobalt and
chromium was found at the interface. This tendency was not found in the stress free area. Since
chromium is the well-known element that dominates the degradation of this material, such an
enrichment or lack of cobalt and chromium may play an important role for forming a crack
propagation path near the interface. This local segregation should be analyzed further to make clear
degradation mechanism of this material.
Abstract: The effects of hydrogen contents and processing parameters of hot deformation on a
Ti-6Al-2Zr-1Mo-1V alloy were investigated. Hot compressive tests were conducted at different
temperatures and strain rates with various hydrogen contents. Based on these experimental data, the
simulation models for predicting flow stress and microstructure evolution have been built by back
propagation (BP) neural network. The numerical results gained via the networks were compared
with the experimental results.
Abstract: There exists strong environmental and economic pressure to increase the thermal efficiency
of fossil fuel power stations and this has led to a steady increase in operating temperature and pressure
resulting in the world wide construction plans for ultra super-critical power plants. Consequently, in
order to improve the thermal efficiency of power plant, there has been a strong drive to develop more
advanced heat resistant steels with excellent creep, high temperature fatigue and thermal fatigue
resistant properties as well as superior oxidation and corrosion resistant properties. In this study, the
test material was P122 alloy which was developed for ultra super-critical power plant. To measure the
fatigue crack growth rate in low #K range, fatigue tests were performed on the P122 alloy welds by
#K decreasing method at three different microstructure (Base metal, HAZ, Weld metal) regions.
Microstructure observation and micro-hardness tests performed for all three regions to find the
relationship among the crack growth rate, microstructure and hardness. Fatigue tests were performed
with compact tension specimens at 600°C, 650°C and 700°C at the loading frequency of 20Hz.