Kyoung Joon Choi

THREE DIMENSIONAL ATOM PROBE STUDY OF NI-BASE ALLOY/LOW ALLOY STEEL DISSIMILAR METAL WELD INTERFACES

Three dimensional atom probe tomography (3D APT) is applied to characterize the dissimilar metal joint which was welded between the Ni-based alloy, Alloy 690 and the low alloy steel, A533 Gr. B, with Alloy 152 filler metal. While there is some difficulty in preparing the specimen for the analysis, the 3D APT has a truly quantitative analytical capability to characterize nanometer scale particles in metallic materials, thus its application to the microstructural analysis in multicomponent metallic materials provides critical information on the mechanism of nanoscale microstructural evolution. In this study, the procedure for 3D APT specimen preparation was established, and those for dissimilar metal weld interface were prepared near the fusion boundary by a focused ion beam. The result of the analysis in this study showed the precipitation of chromium carbides near the fusion boundary between A533 Gr. B and Alloy 152.

NANO-STRUCTURAL AND NANO-CHEMICAL ANALYSIS OF NI-BASE ALLOY/LOW ALLOY STEEL DISSIMILAR METAL WELD INTERFACES

The dissimilar metal joints welded between Ni-based alloy, Alloy 690 and low alloy steel, A533 Gr. B with Alloy 152 filler metal were characterized by using optical microscope, scanning electron microscope, transmission electron microscope, secondary ion mass spectrometry and 3-dimensional atom probe tomography. It was found that in the weld root region, the weld was divided into several regions including unmixed zone in Ni-base alloy, fusion boundary, and heat-affected zone in the low alloy steel. The result of nanostructural and nanochemical analyses in this study showed the non-homogeneous distribution of elements with higher Fe but lower Mn, Ni and Cr in A533 Gr. B compared with Alloy 152, and the precipitation of carbides near the fusion boundary.

Effects of Dissolved Hydrogen on Crack Growth Rate of Warm-Rolled 316L Austenitic Stainless Steel in Primary Water Condition

With the extension of pressurized water reactor’s design life or continued operation, more careful study on the integrity of the internal structures needs to be pursued. In this study, warm-rolling and heat-treatment were applied to 316L stainless steel, in order to simulate the effect of radiation damage such as hardening and radiation-induced grain boundary segregation. And, the crack growth rate testing under constant load condition was performed in the primary water conditions of a pressurized water reactor. Also, in order to investigate the effect of dissolved hydrogen on the crack growth, the dissolved hydrogen concentration was varied between 30 to 50 cc/kg in simulated primary water condition of a pressurized water reactor. The warm-rolled specimens showed the higher crack growth rate than as-received one. Also, the crack growth rate increased as the dissolved hydrogen concentration increases.Copyright

PWSCC Initiation of Alloy 600: Effect of Long-Term Thermal Aging and Triaxial Stress

Thermally aged nickel based Alloy 600 was investigated to evaluate the effects of long-term thermal aging and triaxial stress on primary water stress corrosion crack initiation behavior. Long-term thermal aging was simulated by heat treatment at 400 °C, a temperature that does not cause excessive formation of second phases that cannot form in nuclear power plant service conditions. Triaxial stress was applied by a round notch in the gauge length of some test specimen; other specimens were smooth. Slow strain rate tests (SSRT) monitored by the direct current potential drop method were conducted to evaluate stress corrosion crack initiation susceptibility of the thermally aged specimens in the primary water environment. For smooth specimens (which experience uniaxial stress), the susceptibility of those thermally aged for the equivalent of 10-years was the highest, while the susceptibility of the as-received specimens was the lowest. However, for the notched specimens (which experience triaxial stress), the specimens thermally aged for the equivalent of 20-years showed the highest susceptibility, while the as-received specimens showed the lowest.

Investigation of Microstructural Changes due to Thermal Aging in Dissimilar Metal Welds

To investigate the effect of long-term thermal aging on the fusion boundary region between low-alloy steel A533 Gr. B and weld metal Alloy 152, a representative dissimilar weld mockup composed of Alloy 690/Alloy 152/A533 Gr. B was aged in laboratory furnaces under accelerated temperature conditions. The aging time was determined using the diffusion equation. The heat treatment was performed at 450°C for 60-y equivalent time (5,500 h) to simulate thermal aging effects. An additional aging heat treatment was also performed at 400°C for 15- and 30-y equivalent times (6,450 and 12,911 h, respectively) to determine the effects of temperature on aged microstructures. The characterization was mainly conducted in the microstructure of the fusion boundary region in the weld root region using scanning electron microscopy, transmission electron microscopy, and three-dimensional atom probe tomography. It was determined that the region near the fusion boundary was generally divided into several regions, such as a dilution zone (that included a chemical gradient in the weld side), fusion boundary, and heat-affected zone in the low-alloy steel. The results of this study showed that heat treatment increased Cr content in the dilution zone, but the chemical gradient in the weld side near the fusion boundary persisted. For the microstructure, it was observed that treatment induced the formation and growth of Cr precipitates in the fusion boundary region of the dissimilar metal joints due to the thermodynamic driving force. At two heat treatment conditions (400 and 450°C), although the extent of the results described above differed, the trend in the results appeared to be the same. This microstructure information can improve the understanding of cracking-resistant change when structural changes occur. Furthermore, such data will be important for assessing the effects of aging on structural components and for evaluating the long-term operation of nuclear power plants.Copyright

Thermal Aging Effects on Microstructures of Type-II Boundary in Dissimilar Metal Weld Joints

In order to investigate the long-term thermal aging effects on the type-II boundary region in Alloy 152 weld metal, a representative dissimilar weld mock-up made of Alloy 690–Alloy 152–A533 Gr. B has been fabricated and heat treated under accelerated temperature conditions. To simulate the thermal aging effects, the heat treatment was performed at 450°C for 15, 30 and 60-yr equivalent times (1,375, 2,750 and 5,500 h). The aging time was determined by the diffusion equation based on the activation energy for chromium diffusion. The microstructure characterization was primarily conducted in the type-II boundary region of the weld root, which is a boundary parallel to fusion boundary existing within 100um from the fusion boundary and is known to be less resistant to stress corrosion cracking than other regions in the weld. The investigations were performed by scanning electron microscope, electron backscatter diffraction, and nanoindentation test. In this study, the dilution zone of the chromium content was observed at the weld metal region within a 1.5-mm range from the fusion boundary. Ferrites and high angle grain boundaries are found at the type-II boundary region of weld metal. In the narrow zone between the type-II boundary and fusion boundary, the hardness is relatively higher than that of other regions. These results show that the chromium content in the dilution zone increases with heat treatment, but the stiff chemical gradient still exists in the weld region at the narrow zone between the type-II boundary and fusion boundary.Copyright

A Study of Thermal Aging Effects on Microstructures of Dissimilar Metal Weldment

Characterizations of microstructure and mechanical property were conducted to investigate the effect of long-term thermal aging on the microstructural evolution in the fusion boundary region between weld metal and low alloy steel in dissimilar metal welds. A representative dissimilar weld mock-up made of Alloy 690-Alloy 152-A533 Gr. B was fabricated and aged at 450°C for 5,500 hours. It was observed that the region near the fusion boundary was generally divided into several regions including dilution zone in the Ni-base alloy weld metal, fusion boundary, and heat-affected zone in the low alloy steel. A steep gradient was shown in the chemical composition profile across the interface between A533 Gr. B and Alloy 152. The precipitation of carbides was also observed along and near the fusion boundary of aged dissimilar metal joints. It was also found that the precipitation of Cr carbides was enhanced by the thermal aging near the fusion boundary.Copyright

Nanostructural Characterization of Ni-Base Alloy/Low Alloy Steel Dissimilar Metal Weld (DMW) Interfaces

Dissimilar Metal Welds (DMWs) are generally applied to structural components such as reactor pressure vessel and pressurizer nozzles in nuclear power plants. A filler metal is used in the manufacture of DMWs in light water reactors (LWR) to join the low alloy steel pressure vessel nozzles and steam generator nozzles to nickel-based wrought alloy or austenitic stainless steel components. In recent years crackings have been observed in the welded joints of DMWs. Since there is the high susceptibility of heat affected zone (HAZ) and fusion zone (FZ) to stress corrosion cracking (SCC), a concern has been raised about the integrity and reliability in the joint transition zone. In this study, the dissimilar metal joints welded between Alloy 690, Ni-based alloy and Alloy 533 Gr. B (A533B), low alloy steel with Alloy 152 filler metal were investigated. Detail nano-structural and nano-chemical analysis were performed between Alloy 152 and A533B by using optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), secondary ion mass spectrometry (SIMS) and 3-dimensional atom probe tomography (3D APT). It was found that in the weld root region, the weld was divided into different regions including unmixed zone in Ni-alloy, fusion boundary (FB), and the HAZ in the low alloy steel. The result of TEM, EDS and 3D APT analyses showed the non-homogeneous distribution of elements with higher Fe but lower Mn, Ni and Cr in A533B compared with Alloy 152, and the precipitation of carbides near the FB.Copyright