The objective of this investigation was to provide a detailed evaluation of the heat-affected zone (HAZ) toughness of a high-strength TMCP steel designed for low-temperature applications. The results from both Charpy-vee notch (CVN) and cracktip-opening displacement (CTOD) tests conducted on two straight-walled narrow groove welds, produced at energy inputs of 1.5 and 3.0 kJ/mm, show that significantly lower toughness was exhibited by the grain-coarsened HAZ (GCHAZ) compared with the intercritical HAZ (ICHAZ) region. This is explained based on the overall GCHAZ microstructure, and the initiation mechanism which caused failure. For the particular TMCP steel investigated in this study very good ICHAZ toughness properties were recorded using both HAZ Charpy and CTOD tests. In general, this was attributable to the low hardness, relatively fine ferrite microstructure, and the formation of secondary microphases that were not overly detrimental to the toughness. The lower-bound GCHAZ CTOD results obtained for both welds (KA W-L and KA W-H) did not meet the targeted requirement of δ = 0.07 mm at −50°C. It was found in both welds that low CTOD toughness was associated with the initiation of fracture from nonmetallic inclusions, which were complex oxides containing Ce, La, and S. The sites were located in the subcritical GCHAZ (SCGCHAZ) region in the case of the 1.5 kJ/mm weld and in the GCHAZ for the 3.0 kJ/mm weld. Some variation in CVN toughness was observed at different through-thickness locations. Toughness was lowest for the GCHAZ of the weld deposited at 3.0 kJ/mm and was related to the proportion of GCHAZ being sampled, which was ~55 percent for the bottom compared to 25–30 percent for that of the top location. Recommendations are proposed on the preferred practices and criteria that should be used in establishing guidelines and specifications for evaluating the HAZ toughness of candidate steels for construction of Arctic class ships.
Two hydrostatic tests were carried out on an X-52 (Grade 359) pipe containing sixteen cracks of depths up to 55% wall thickness. Stress corrosion cracking (SCC) growth rates were measured in full-scale tests performed before and after the first hydrotest in order to demonstrate the effects of hydrostatic testing on subsequent crack growth rates. The effects on crack tip deformation were investigated by metallographic examination of crack cross-sections immediately following the second hydrotest. The SCC tests were performed using a saw-tooth type load spectrum with the maximum stress set at 95% of the actual (as opposed to specified minimum) yield strength of the linepipe and R = 0.8. During the first hydrotest, the maximum applied stress was 108% of the yield stress; the total hoop strain in the pipe body reached about 0.2%, which is less than would have been reached in a uniaxial tensile test at this stress level because of the effect of the biaxial stress state in the pipe. The highest SCC growth rate measured before the first hydrotest was about 0.88 mm per year (2.4 * 10−3 mm/day), and the growth rate of the same crack after this hydrotest was about 0.37 mm per year (0.79 * 10−3 mm/day). The other cracks all showed varying degrees of reduction in growth rate. Post-mortem examination indicated that the hydrotests did not cause significant crack blunting. The beneficial effects of hydrotests are attributed primarily to the presence of compressive residual stresses in the heavily deformed region in front of the crack tip. The majority of cracks showed some growth during the first hydrotest. SCC growth and growth during the hydrotests are associated with distinctly different microscopic features on the fracture surface.
A primary consideration in the welding of structures for service in Canadian offshore and arctic regions is the toughness of weld metals required at very low ambient temperatures (−30°C to −60°C). To assess the suitability of cored wires for applications in these environments, some currently available commercial consumables for the flux-cored arc welding (FCAW) process were evaluated. Cored wires belonging to four different categories: basic, rutile, metal-cored and innershield, were used to prepare welds with similar welding procedures. Weld metal Charpy V-notch (CVN) and crack tip opening displacement (CTOD) tests were carried out and the effect of weld metal composition, microstructure and inclusion content in the weld metal toughness was examined. The Charpy transition temperatures and the CTOD toughness results indicated that, of the 16 wires tested, there were only seven that would be suitable for critical applications.
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