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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.
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.
Kim, In (Dongkuk Steel R&D Center, Pusan National University) | Jo, Seung-Jae (Dongkuk Steel R&D Center) | Jo, Soo-Chal (Dongkuk Steel R&D Center) | Kang, Ki-Bong (Dongkuk Steel R&D Center) | Kang, Nam-Hyun (Pusan National University)
Most offshore structural projects require multilayer welding owing to the use of thick plates and the maximum heat input limit (<50 kJ/cm in offshore fields). It is generally known that toughness decreases in the following two regions of the welded heat-affected zone (HAZ): coarsegrained heat-affected zone (CGHAZ) and intercritically reheated coarse-grained heat-affected zone (IRCGHAZ). Moreover, the formation of the martensite–austenite (M–A) constituent in the welded HAZ is an important phenomenon that greatly decreases the toughness in this zone. Therefore, in order to improve toughness, it is very important to control the fraction and shape of the M–A constituent.
In this study, a test plate (specified minimum yield strength, SMYS: 500 MPa) was fabricated by direct quenching and tempering process. Differently simulated weld thermal cycles for CGHAZ and IRCGHAZ were carried out using a Gleeble tester. In order to determine the relationship between simulated and real HAZ, real welded joints were also fabricated. Microstructure observations and phase fraction measurements were carried out for these joints. The obtained results showed that low-temperature toughness in welded HAZ increased in proportion to that in the fine-grained heat-affected zone (FGHAZ) owing to the passes employed during the multi-pass welding; in contrast, the toughness in welded HAZ decreased in proportion to that in IRCGHAZ fraction.