| Abstract Scope |
In pressure vessels, cracks are often found outside the heat-affected zone (HAZ) of repair welds or welded joints after about 13-15 years of service. In this study we did both welding and artificial aging to simulate the microstructure of joining and repairing of the 1Cr-0.5Mo pressure vessel. Following the physical simulation, low-cycle fatigue tests were conducted at both room temperature and elevated temperatures. Cracking of all of the low-cycle fatigue tested samples was found to initiate at a distance
of 9-12 mm from the fusion line, outside the HAZ, indicating a potential over-temperated, embrittled zone. Detailed characterization, including x-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), and Auger microscope, was used to characterize the microstructure close to the fatigue crack path.
In this over-tempered zone, the pearlite region was found to have degenerated, leading to the grain boundary carbides. An advanced finite element model is under development to gain a deeper understanding of the thermal history that contributes to the embrittlement mechanism of 1Cr-0.5Mo steel under the welding thermal cycle. Additionally, the caribide dissolution kinetics model will be built to correlate the precipitation diameter changes. Furthermore, a mesoscale microstructure-based crystal
plasticity model is developed to provide insights into the effect of this overtempered microstructure on the fatigue cracking initiation behavior. |