In Sup Kim

Evaluation of thermal aging embrittlement in CF8 duplex stainless steel by small punch test

Abstract Small punch test was performed on CF8 duplex stainless steel aged at 370 and 400°C for up to 5000 h to characterize thermal aging embrittlement. At room temperature, the small punch (SP) load–displacement curve was similar in shape to those of ferritic steels and exhibited a good reproducibility in spite of ferrite–austenite structure. As the test temperature was lowered to a certain temperature depending on the degree of aging, the SP load showed a sudden drop followed by curve serration before the SP specimen fractured, resulting from the cracking of ferrite phase. While the aging heat treatment led to a slight increase of the yield strength, the transition appearing in the SP energy versus temperature curves shifted to higher temperature due to the hardening of ferrite phase. Additionally, phase boundary separation was an important factor in the degradation of the steel aged at 400°C.

Dynamic strain aging in the high-temperature low-cycle fatigue of SA508 Cl. 3 forging steel

Abstract The effect of dynamic strain aging on cyclic stress response and fatigue resistance of ASME SA508 Cl.3 forging steel for nuclear reactor pressure vessels has been evaluated in the temperature range of room temperature to 500°C. Total strain ranges and strain rates were varied from 0.7 to 2.0% and from 4 × 10 −4 to 1 × 10 −2 s −1 , respectively. The cyclic stress response depended on the testing temperature, strain rate, and range. Generally, the initial cyclic hardening was immediately followed by cyclic softening at all strain rates. However, at 300°C, the operating temperature of nuclear reactor pressure vessels, the variation of cyclic stress amplitude showed the primary and secondary hardening stages dependent on the strain rate and strain range. Dynamic strain aging was manifested by enhanced cyclic hardening, distinguished secondary hardening, and negative strain rate sensitivity. A modified cell shutting model was described for the onset of the secondary hardening due to the dynamic strain aging and it was in good agreement with the experimental results. Fatigue life increased in strain rate at all testing temperatures. Specifically the fatigue life was longer at the dynamic strain aging temperature. Further, the dynamic strain aging was easy to initiate the crack, while crack propagation was retarded by crack branching and suppression of plastic zone, hence the dynamic strain aging caused the improvement of fatigue resistance.

Effects of the accumulated annealing parameter on the corrosion characteristics of a Zr–0.5Nb–1.0Sn–0.5Fe–0.25Cr alloy

Corrosion behavior, hydrogen pickup, oxide microstructure, and precipitate characterization have been studied in order to investigate the effect of the accumulated annealing parameter on the corrosion characteristics in a Zr–Nb–Sn–Fe–Cr alloy. An autoclave corrosion test was carried out in 400°C steam for 300 days on the Zr–0.5Nb–1.0Sn–0.5Fe–0.25Cr alloy, which had been given 18 different accumulated annealing parameters. The corrosion rate increased with increasing the accumulated annealing parameter. To investigate the crystal structure of oxide layer, the corroded specimens were prepared to have an equal oxide thickness (∼1.6 μm) by controlling exposure time. The relative fraction of tetragonal ZrO2 also decreased gradually with increasing accumulated annealing parameter. From the hydrogen analysis of the corroded samples for 300 days, it was observed that, with increasing the size of precipitates, the hydrogen pickup was enhanced. It was revealed from transmission electron microscope (TEM) observation of the oxide that the larger precipitates still remained to be oxidized in the oxide layer and had undergone a reduction of Fe/Cr ratio from 2.1 to 1.5. The oxidation of the precipitates in the oxide gave rise to a volume expansion at the precipitate–oxide interface. This volume change could lead to the transformation in the oxide phase from tetragonal ZrO2 to monoclinic ZrO2 and in oxide structure from columnar grain to equiaxed grain. The precipitate in a Zr–0.5Nb–1.0Sn–0.5Fe–0.25Cr alloy is composed of Nb, Fe, and Cr and the Nb content in the precipitate increase with increasing accumulated annealing parameter. Thus, it can be thought that Nb within precipitates plays a key role in the microstructural change of oxide.

Initial Deformation During Small Punch Testing

Initial deformation behavior during small punch (SP) testing was investigated in order to estimate yield stress using two sizes of SP specimens, 10 by 10 by 0.5 mm3 and 10 by 10 by 0.25 mm3 by finite element (FE) analysis and by experiments with heattreated SA508 and 12Cr steels. The increase of thickness resulted in a deviation from the initial linearity of the load-displacement curve at higher loads. The deviation was attributed to different causes, namely loss of the constraint by the surrounding material and radial propagation of plastic bending deformation for 0.5 and 0.25-mm specimens, respectively. The causes for the transition of deformation mode were identical irrespective of material strength and work-hardening rate in the range of yield stress about 400 to 900 MPa. Based on the invariance of the deformation mode, a distinct linear relationship between the load at the breakaway from initial linearity and the yield stress was predicted by the FE analysis and compared with the experimental results.

Estimation of fracture toughness transition curves of RPV steels from ball indentation and tensile test data

Abstract It was attempted to estimate the fracture toughness transition curves of reactor pressure vessel (RPV) steels from the ball indentation and tensile test data using the indentation energy to fracture (IEF) model and the relationships describing the effect of stress state on fracture. In the IEF model the fracture toughness is expressed as a function of the ball indentation test parameters and critical mean contact pressure. From the relationships among the stress components the fracture stress is derived as a function of stress triaxiality and flow property. In this approach the fracture stress calculated for the stress triaxiality of indentation deformation is assumed as the critical mean contact pressure. Indentation and tensile tests were performed on six RPV steels at transition temperatures of −160–25°C. The values of critical mean contact pressure were in the range 2500–2800 MPa. The temperature dependence of the estimated fracture toughness, K JC , agreed well with that obtained by the master curve method of ASTM E 1921 using three-point bend (TPB) specimens. In addition, the critical fracture stresses were obtained by considering the stress triaxiality for the crack tip. All test materials revealed the values of the critical fracture stress ranging from 2100 to 2500 MPa.

Effect of (α + β) heat treatment on the mechanical properties of Zircaloy-4

Abstract The mechanical properties of Zircaloy-4 were investigated after quenched from the (α + β) phase temperature. The microstructural observation was made and the α → β transformation kinetics was analyzed. The activation energy for α → β transformation was found to be 33 kcal/mol. Microhardness of constituent phases revealed that the strength of transformed β phase was almost constant, while that of primary α increased due to smaller grain size and oxygen enrichment. The yield strength was dependent on the strength and amount of primary α phase and on the interactions between the phases. Two stages of work hardening existed. Up to 0.015 strain, work hardening increased when matrix was the hard primary α, but decreased as the soft transformed β phase became continuous. Above the 0.015 strain, the work hardening was similar to that of α-annealed Zircaloy-4. Uniform elongation decreased remarkably for small amount of transformed β phase, and then decreased gradually with increasing transformed β.

Thermal recovery characteristics of neutron-irradiated high-copper weld (Linde 80)

Abstract The recovery activation energy, the order of reaction and the characteristic recovery rate constant were determined by isochronal (573–823 K) and isothermal (723 K and 775 K) annealing experiments on specimens made from a broken half of a surveillance weld specimen (fluence: 1.21 × 10 23 n/m 2 , E ≥ 1 MeV, Cu: 0.29 wt%) to investigate the recovery characteristics of a high copper weld of neutron-irradiated reactor pressure vessel (RPV). Vickers microhardness tests were conducted to trace the recovery behavior after heat treatments. The results were analyzed in terms of recovery stages, behavior of responsible defects and recovery kinetics. It was shown that recovery occurred through two annealing stages (stage I: 673–753 K, stage II: 753–823 K) with recovery activation energies of 2.68 eV and 2.83 eV for stage I and II, respectively. The isothermal hardness recovery at 723 K and 775 K coincided with the ratio of the characteristic rate constant for each recovery stage. The order of reaction was 2 for both recovery stages. The recovery activation energies of present specimens are approximately equal to that of copper diffusion in α-iron in the presence of vacancies, suggesting that recovery may occur through the diffusion of copper atoms. The present results strongly support the copper precipitate coarsening model.