Un Bong Baek

EVALUATION OF IMPACT PROPERTIES TO FORGED 3CR STEEL BY BARKHAUSEN NOISE

This paper reports that the Barkhausen noise method can be used to accurately characterize forged reactor vessels. The Charpy V-notch impact tests were conducted on the respective specimens with three different types of heat history. Various test results including fracture appearance transition temperature (FATT) were obtained. The Barkhausen noise voltage changed with heat treatment temperature (870~1000°C) and conditions (Tempered, PWHT). The fracture appearance transition temperature can be predicted using the Barkhausen noise voltage.

CHARACTERIZATION OF TRIS (8-HYDROXYQUINOLINE) ALUMINUM (Alq3) FILM USING SPECTROSCOPIC ELLIPSOMETRY

We have applied spectroscopic ellipsometry to obtain the optical constants, i.e., refractive indices (n) and extinction coefficients (k) of a thin tris (8-hydroxyquinoline) aluminum (Alq3) film evaporated on the top of 300-nm-thick SiO2 layer on a silicon substrate. Using a spectroscopic ellipsometer we collected the ellipsometric data of Ψ and Δ at 3 incident angles of 65°, 70° and 75° and 226 energies of 1.5 eV to 6.0 eV. For the organic sample, we adopted a simple two-film model consisting of ambient/organic film/SiO2 film/silicon substrate. Using the multiple oscillators of Tauc-Lorentz (TL) dispersion function and the two-film model, we obtained the thickness of the Alq3 film. By data inversion technique, the spectra of n and k for the Alq3 film are calculated from measured Ψ and Δ at each photon energy when the film thickness is fixed to a value obtained by the TL dispersion fitting. The extrapolation of optical function near band gap yields the band gap of 2.64 eV for Alq3 film.

Nondestructive Evaluation for Remanent Life of 1Cr-0.5Mo Steel by Reversible Permeability

Peak interval for reversible permeability is presented for nondestructively evaluating the remanent life of 1Cr-0.5Mo steel. The method to measure the peak interval of reversible permeability is based on the value of reversible permeability is the same as the differential value of the hysteresis loop. The measurement principle is based on the first harmonics voltage induced in a sensing coil using a lock-in amplifier tuned to a frequency of the exciting voltage. Results obtained for the peak interval of reversible permeability and Rockwell hardness on the aged samples decrease as aging time and the Larson-Miller parameter increase. We could estimate the remanent life of 1Cr-0.5Mo steel by using the relationship between the peak interval of reversible permeability and the Larson-Miller parameter, nondestructively.

A CORRELATION BETWEEN THE AGING PARAMETER AND ELECTRICAL RESISTIVITY FOR THE DEGRADED 9 CR STEEL

It is advantageous to use a non-destructive evaluation (NDE) method to assess the mechanical properties of materials since conventional methods are time-consuming and usually require sampling from the material or component. In this study, an electrical resistivity method was attempted for the estimation of the creep damage of degraded 9Cr steel. The specimens with six different kinds of aging periods were prepared by an isothermal heat treatment at 690 °C. The electrical resistivity was determined by a standard DC four-point probe method at 24±0.5 °C. The home-made probe station and Burster 2304 were utilized for the resistance measurement. A correlation between the measured electrical resistivity and the tensile properties was studied. The electrical resistivity showed a declining tendency with the increase of aging time because the electrical resistivity is strongly dependent on the microstructural changes. A correlation between the electrical resistivity and aging parameter was established, which allow...

Hydrogen Embrittlement for X-70 Pipeline Steel in High Pressure Hydrogen Gas

The tensile properties of API 5L X70 pipeline steels have been measured in a high-pressure (10 MPa) hydrogen gas environment. Significant decreases in elongation at failure and reduction of area were observed when testing in hydrogen as compared with air, and those changes were accompanied by noticeable changes in fracture morphology. The present paper exposes the changes in mechanical properties of a grade API 5L X70 steel through numerous mechanical tests, i.e. tensile tests, notch tensile tests, fracture toughness and fatigue crack growth measurements, performed either in atmosphere or in 10 MPa pressure of hydrogen gas.Copyright

Magnetic Shielding Effect on Halbach Cylinder used in Magnetic Refrigerators

The system for producing magnetic field constitutes an important component of magnetic refrigerator. Many researchers have directed significant effort to increase the magnetic field intensity, because the magnetocaloric effect at the Curie temperature increases with the power of 2/3 of the magnetic field. In this study, we report the simulation of the magnetic field intensity at polar axis of a Halbach cylinder (HC) by i) changing the length and thickness of the HC, ii) having with or without gap of the HC, and iii) surrounding the HC with a soft magnet shell, acting as a shielding. We simulated the field distribution of a HC with a finite size. Furthermore, the detailed numerical results of the magnetic field distribution and its dependence on shielding are presented in this study.

Improved Reliability of Small Punch Testing by Correction of Testing System Stiffness and Measurement of Hydrogen Sensitivity

Conventionally embrittlement of components for petrochemical equipment has been determined by small punch (SP) testing, which involves carrying out mechanical testing of specimens sampled from practical structures to estimate the integrity and residual life of the equipment. The application range of SP testing has been expanded to cryogenic environments, and attempts have recently been made to expand it further to high-pressure hydrogen environments. To examine the effect of hydrogen on pipeline materials before SP testing, the amount of hydrogen permeation was measured under several testing conditions. The test specimens were prepared as follow. First, after obtaining a section (10 mm in width, 10 mm in height, and 0.7 mm in thickness) from a steel pipe made of X70, X65, or X42 as the base metal by wire cut electric discharge processing. Subsequently, grinding both surfaces, the test specimen was finished to maximum surface roughness of less than 6 μm, with a final thickness of 0.5 ± 0.005 mm. The aim of this study was to determine whether sensitivity to hydrogen could be measured by SP testing; hence, the test was conducted at a punch speed of 4 × 10−3 mm/min, which is much slower than the 0.2 ∼ 2 mm/min adopted for conventional SP testing, like CWA 15627. The maximum load measured for all three steels under 10 MPa was not influenced by machine type (servo hydrodynamic or electromechanical), holding time after charging hydrogen, or bolt fastening torque. Different means for correcting SP curves depending on the stiffness of the testing machines were suggested, and their effectiveness throughout testing was demonstrated.Copyright

Tensile properties of carbon nanotube with different growth methods

We have attempted to observe straining responses of a multi-walled carbon nanotube (MWCNT) with different growth methods by performing an in situ tensile testing in a scanning electron microscope. Linear deformation and fracture behaviors of MWNT were successfully observed and its force-displacement curve was also measured from the bending stiffness and displacement of the force sensor and manipulator, respectively. We obtained different tensile strength of MWCNTs with different growth methods and the elastic modulus of MWCNTs was also calculated and compared with each other.

A Study on the Temperature Calibration for a Small Electrical Resistance Furnace

A severe thermal stress occurs during start up/shutdown transients in thick walled components of high temperature power plants. Thus, a precise consideration of this issue is very important. Many researchers have studied low-cycle fatigue at high temperatures and small box-type electrical resistance furnaces have been developed for small-sized fatigue specimens. However, these small-scale electrical resistance furnaces need precise temperature calibrations because temperature control is difficult in a small space. Thus, a method for the temperature calibration of a box-type electrical resistance furnace is investigated and calibration procedures are proposed in this study.Copyright

Crack Growth Behavior in the Interface of Repair Welded 1Cr-0.5Mo Steel

Considerable amount of creep-fatigue crack growth (CFCG) data with characterizing fracture parameter C* or Ct have been generated for the last 25 years. On the contrary similar efforts for weld interface crack started only a few years ago. In this article the CFCG behavior of lCr-0.5Mo steel particularly for weld interface cracks is intensively studied by experimental methods.