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In evaluation of materials for arctic applications, their low temperature properties are addressed. The heat affected zone toughness has been shown to be critical with respect to satisfactory fracture toughness. Less attention has been given to the weld metal. Therefore, the present study was initiated with the objective to assess the fracture toughness of weld metals deposited with different welding wires. Both impact and fracture toughness testing were included; the latter one considered testing of full sized single edge notch bending specimens with through thickness notch in the weld metal and sub-sized specimens with surface notch in primary weld metal and in re-heated weld metal. The testing was performed at -60°C and three parallels were run for all configurations.
The results showed that both the Charpy V notch and fracture toughness varied substantially between the different welding wires employed. For the Charpy case, impact properties scattered from about 20 J for Weld 3 to 75-115 J for Weld 5. This ranking changed when it comes to full size CTOD specimens. Still Weld 3 had lowest values, while Welds 1 and 2 appeared with best toughness. The behaviour of Welds 1 and 2 was also different from the other welds regarding sub-sized samples with notches in the primary and reheated weld metals. Here, Welds 1 and 2 had similar toughness for the two weld metal regions, while Welds 3, 4 and 5 had higher CTOD values for the reheated weld metal. These results are discussed in terms of the weld metal microstructure observations.
In the North Sea oil and gas installations, steel castings have been used for many decades. Here, high strength steel castings offer the chance to manufacture complex heavy-lift and fatigue-critical components for larger offshore structures without increasing the weight of the components or platforms. However, when the activities are moving north to colder climates, current existing castings may fail to meet the toughness requirements, and there is very limited information available on behaviour of weldments of castings under such extreme conditions. Therefore, the present investigation was carried out addressing the low temperature toughness of high nickel (~1.5% Ni) steel casting with 460 MPa yield strength. Preliminary welding trials were performed with flux-cored arc welding (FCAW) with an overmatch in weld metal strength. Both Charpy V notch impact and CTOD fracture mechanical testing were included at ?60°C. The results show that the Charpy V notch toughness is excellent at -60°C (> 100 J). The fusion line CTOD fracture toughness showed low values for the SENB05 samples, while SENB02 gave higher values. For both geometries, the lowest values were connected with pop-in events. The weld metal fracture toughness was satisfactory with the lowest value of 0.28 mm.
Akselsen, Odd M. (SINTEF Materials and Chemistry) | Lange, Hans I. (Norwegian University of Science and Technology (NTNU)) | Ren, Xiaobo (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry)
For steel structures to be installed in the Arctic region, the risk of brittle fracture represents a primary concern due to the ductile to brittle usually transition taking place at sub-zero temperatures. Therefore, the present investigation addressed the heat affected zone and weld metal toughness of two extra low carbon steels of 420 MPa yield strength grade, supplied in 20 and 50 mm thickness. The testing included tensile, Charpy V and CTOD. The results obtained showed that the Charpy V toughness was relatively high at -600C, but that some low values may occur for the fusion line position. The fracture toughness at -600C, based on SENB05 (a/t=0.5) geometry, appeared to be low for both weld metal and fusion line positions. More specific measures may be taken into account in welding procedure qualification of the current steels, such as using lower crack length (e.g., a/t=0.2), tension instead of bending (SENT testing) or a full engineering critical assessment.
The oil and gas industry has been gradually moving towards the north. In Norwegian waters, the Goliat field was recently set in production by ENI. The design temperature for this field was -200C, which is somewhat lower than previously experienced, and below the lowest design temperature in the NORSOK standard (2014), which is currently -140C. Not far from the Goliat, Johan Castberg may be the next field of exploration, and is now under evaluation by Statoil. When going further north and east, the ice edge is approached, and the design temperature may fall down to -300C, or even below. This represents huge challenges to the materials which are to be used. Normally, e.g. structural steels and pipelines may easily satisfy toughness requirements at such low temperature. However, welding tends to be very harmful to low temperature fracture toughness. Recent results have demonstrated that the toughness may be on the borderline for both the heat affected zone and the weld metal (e.g., Akselsen et al, 2015; Akselsen & Østby, 2014; Akselsen et al, 2012; Akselsen et al, 2011), indicating that required robust solutions are not yet available for the most challenging part of the Arctic region, unless some constraint loss corrections are applicable.