• Journal of the Korean Society for Precision Engineering
• /
• v.30 no.8
• /
• pp.809-814
• /
• 2013

The relationship between the small-punch creep test and the conventional creep test was investigated experimentally using a method similar to that of the Monkman-Grant relationship. Uniaxial and small-punch creep rupture tests were carried out on 9Cr-2W ferritic steel (Commercial Grade 92 steel: X10CrWMoVNb 9-2) at elevated temperatures. From the relation derived in the same manner as the Monkman-Grant relation, a correlation between the displacement rate in response to the small-punch creep test and the strain rate in the uniaxial creep test was found, and the creep life was calculated using this relation. Furthermore, the failure modes of the small punch creep test specimens were investigated to show that the fracture was caused by creep.

• Journal of Mechanical Science and Technology
• /
• v.18 no.12
• /
• pp.2158-2173
• /
• 2004

In this paper, the higher order terms in the crack tip stress fields are investigated macroscopically for more realistic assessment of structural material behaviors. For reactor pressure vessel material of A533B ferritic steel, effects of crack size and temperature have been evaluated using 3-point SENB specimens through a series of finite element analyses, tensile tests and fracture toughness tests. The T-stress, Q-parameter and q-parameter as well as the K and J-integral are calculated and mutual relationships are investigated also. Based on the evaluation, it has proven that the effect of crack size from standard length (a/W=0.53) to shallow length (a/W=0.11) is remarkable whilst the effect of temperature from -20$^{\circ}C$ to -60$^{\circ}C$ is negligible. Finally, the cleavage fracture toughness loci as a function of the promising Q-parameter or q-parameter are developed using specific test results as well as finite element analysis results, which can be applicable for structural integrity evaluation considering constraint effects.

• Proceedings of the KSME Conference
• /
• 2008.11a
• /
• pp.98-103
• /
• 2008

This paper is to discuss distribution of welding residual stresses of a ferritic low alloy steel nozzle with dissimilar metal weld using Alloy 82/182. Two dimensional (2D) thermo-mechanical finite element analyses are carried out to simulate multi-pass welding process on the basis of the detailed and fabrication data. On performing the welding analysis generally, the characteristics on the heat input and heat transfer of weld are affected on the weld residual stress analyses. Thermal analyses in the welding heat cycle process is very important process in weld residual stress analyses. Therefore, heat is rapidly input to the weld pass material, using internal volumetric heat generation, at a rate which raises the peak weld metal temperature to $2200^{\circ}C$ and the base metal adjacent to the weld to about $1400^{\circ}C$. These are approximately the temperature that the weld metal and surrounding base materials reach during welding. Also, According to the various ways of appling the weld heat source, the predicted residual stress results are compared with measured axial, hoop and radial through-wall profiles in the heat affected zone of test component. Also, those results are compared with those of full 3-dimensional simulation.

• Proceedings of the KSME Conference
• /
• 2008.11a
• /
• pp.83-84
• /
• 2008

The primary water stress corrosion cracking (PWSCC) of dissimilar metal weld based on Alloy 82/182 is one of major issues in material degradation of nuclear components. It is well known that the crack initiation and growth due to PWSCC is influenced by material's susceptibility to PWSCC and distribution of welding residual stress. Therefore, modeling the welding residual stress is of interest in understanding crack formation and growth in dissimilar metal weld. Currently in Korea, a numerical round robin study is undertaken to provide guidance on the welding residual stress analysis of dissimilar metal weld. As a part of this effort, the present paper investigates distribution of welding resisual stress of a ferritic low alloy steel nozzle with dissimilar metal weld using Alloy 82/182. Two-dimensional thermo-mechanical finite element analyses are carried out to simulate multi-pass welding process on the basis of the detailed design and fabrication data. The present results are compared with those from other participants, and more works incorporating physical measurements are going to be performed to quantify the uncertainties relating to modelling assumptions.

• Journal of Mechanical Science and Technology
• /
• v.20 no.3
• /
• pp.335-344
• /
• 2006

A comprehensive review of through thickness transverse residual stress distributions in a range of as-welded and mechanically bent components made up of a range of steels has been carried out, and simplified generic transverse residual stress profiles for a plate and pipe components have been proposed. The geometries consisted of welded pipe butt joints, T-plate joints, tubular T-joints, tubular Y-joints and a pipe on plate joints as well as cold bent tubes and pipes. The collected data covered a range of engineering steels including ferritic, austenitic, C-Mn and Cr-Mo steels. Measured residual stress data, normalised with respect to the parent material yield stress, has shown a good linear correlation versus the normalised depth of the region containing the residual stress resulting from the welding or cold-bending process. The proposed simplified generic residual stress profiles based on the mean statistical linear fit of all the data provides a reasonably conservative prediction of the stress intensity factors. Whereas the profiles for the assessment procedures are fixed and case specific, the simple bilinear profiles for the residual stresses obtained by shifting the mean and bending stress from the mean regression line have been proposed and validated.

• Korean Journal of Materials Research
• /
• v.25 no.11
• /
• pp.607-611
• /
• 2015

The microstructural evolution of Grade 91 tempered martensite ferritic steels heat treated at $760{\sim}1000^{\circ}C$ for two hours was investigated using scanning electron microscopy(SEM), energy disperse spectroscopy(EDS), electron backscattered diffraction (EBSD), and transmission electron microscopy(TEM); a microhardness tester was also employed, with a focus on the grain and precipitate evolution process as well as on the main hardening element. It was found that an evolution of tempered martensite to ferrite($760{\sim}850^{\circ}C$), and to fresh martensite($900{\sim}1000^{\circ}C$), occurred with the increase of temperature. Simultaneously, the parabolic evolution characteristics of the low angle grain boundary(LAGB) increased with the increase of the heating temperature(highest fraction of LAGB at $925^{\circ}C$), indicating grain recovery upon intercritical heating. The main precipitate, $M_{23}C_6$, was found to be coarsened slightly at $760{\sim}850^{\circ}C$; it then dissolved at $850{\sim}1000^{\circ}C$. Besides this, $M_3C$ cementite was formed at $900{\sim}1000^{\circ}C$. Finally, the experimental results show that the hardness of the steel depended largely on the matrix structure, rather than on the precipitates, with the fresh martensite showing the highest hardness value.

• Journal of the Korean Society for Heat Treatment
• /
• v.13 no.3
• /
• pp.143-150
• /
• 2000

Ferritic plasma nitrocarburising was performed on pure iron using a modified DC plasma unit. This investigation was carried out with various gas compositions which consisted of nitrogen, hydrogen and carbon monoxide gases, and various gas pressures for 3 hours at $570^{\circ}C$. After treatment, the different cooling rates(slow cooling and fast cooling) were used to investigate its effect on the structure of the compound layer. The ${\varepsilon}$ phase occupied the outer part of the compound layer and ${\gamma}^{\prime}$ phase existed between the ${\varepsilon}$ phase and the diffusion zone. The gas composition of the atmosphere influenced the constitution of the compound layer produced, i.e. high nitrogen contents were essential for the production of ${\varepsilon}$ phase compound layer. It was found that with increasing carbon content in the gas mixture the compound layer thickness increased up to 10%. In the gas pressure around 3 mbar, the compound layer characteristics were slightly effected by gas pressure. However, in the low gas pressure and high gas pressure, the compound layer characteristics were significantly changed. The constitution of the compound layer was altered by varying the cooling rate. A large amount of ${\gamma}^{\prime}$ phase was transformed from the ${\varepsilon}$ phase during slow cooling.

• Journal of the Korean Society for Heat Treatment
• /
• v.11 no.4
• /
• pp.241-246
• /
• 1998

It is well known for 410L ferritic stainless steel powder to applicate a sensor ring in anti-lock brake system of automobile, several studies, because of its excellent magnetic properties. This study was carried out to investigate the magnetic properties such as the maximum magnetic induction, coercivity and maximum permeability of the materials with functions of sintering density, time and temperature, and concluded as follows: 1. Sintering under the circumstances of Ar gas and the temperature of $1250^{\circ}C$ for 60min, showed that nitrogen was increased, whereas carbon and oxygen decreased in quantities. 2. Both maximum magnetic induction value of 4700Gauss and permeability of 200 were obtained at the maximum sintering density of $6.89g/cm^2$. Here, the properties showed a linear increasement with increasing the sintering density. 3. Coercivity sharply decreased with incresing the sintering density and reached to 7.6Oe at the maximum sintering density of $6.89g/cm^2$.

• Journal of the Korean Ceramic Society
• /
• v.42 no.12 s.283
• /
• pp.833-841
• /
• 2005

The oxidation resistance and electrical conductivity of various commercial ferritic stainless steels including STS444 were tested in an air atmosphere at $700^{\circ}C$. Crofer22 developed specially for SOFC interconnect was also examined for the aim of comparing with the test results of STS444. Although STS444 exhibited higher oxidation resistance than Corfer22, the electrical conductivity of the scale formed on Crofer22 was higher, indicating that the resistivity of scale formed on Crofer22 is much lower than that of STS444. To gain a better understanding of the relation between oxidation behavior and electrical conductivity, the oxide scales formed on STS444 and Crofer22 were analyzed in terms of the structure, composition, and phase. Consequently, the influence of alloying elements on electrical conductivity of Fe-Cr alloys was discussed.

• Corrosion Science and Technology
• /
• v.16 no.1
• /
• pp.8-14
• /
• 2017

The corrosion behavior of a ferritic/martensitic steel T91 exposed to an aqueous solution containing chloride and sulfate ions is investigated depending on the stimulated all-volatile treatment (AVT) and under oxygenated treatment (OT) conditions. The corrosion of T91 steel under OT condition is severe, while the corrosion under AVT condition is not. The co-existence of chloride and sulfate ions has antagonistic effect on the corrosion of T91 steel in both AVT and OT conditions. Unlike to corrosion resistance in the aqueous solution, OT pretreatment provides T91 steel lower oxidation-resistance than VAT pretreatment. From scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis, the lower corrosion resistance in the aqueous solution by VAT conditions possibly is due to the formation of pits. In addition, the lower oxidation resistance of T91 steel pretreated by OT conditions is explained as follows: the cracks formed during the immersion under OT conditions accelerated peeling-off rate of the oxide film.