PSI - Issue 42

Tadeu Messias Donizete Borba et al. / Procedia Structural Integrity 42 (2022) 276–283 Tadeu Messias Donizete Borba / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction Due to extreme application conditions such as low temperatures, corrosion, cyclic loading and long service life, offshore operations are critical, complex, and dangerous. Because of these, one of the main concerns during the construction of the fixed or floating structures, both for oil platforms and wind towers, or submerged structures, like pipelines and Oil Country Tubular Goods (OCTG), is related to structural integrity, especially due to disasters that occurred in the second half of the 20th century, which were associated to material quality problems. In this sense, the offshore structure steels for the construction of this equipment must have the properties of high-strength, good fatigue resistance, lamellar tear resistance, good weldability, and high-toughness (resistance to crack) (Anderson, T. L. (2005), Easterling, K. (1983), Miao, Z, Wu, H. (2008)). In this context, with the concern of minimizing the failures in welded regions of offshore structures, the main committees, institutes and international normative societies have developed codes and standards to evaluate the effect of the welding thermal cycle on the base metal properties and procedures for each type of application and created programs for professional training and inspection of welded components. In addition to a series of base metal qualification tests, there are tests to assess the fracture toughness of the heat-affected zone (HAZ). Prequalifications such as DNV-OS-C401(2018), BSI-EN10225(2019) and API RP 2Z(2005) were created to ensure that different regions of the HAZ present acceptable values of fracture toughness, determined through the CTOD parameter, after completion of different standardized welding operations. According to the API RP 2Z standard, for steel qualification, it is necessary to carry out several weldability tests and the HAZ of welded joints under different conditions shall be evaluated by CTOD and Charpy impact tests. In this work, it has been summarized the results of CTOD tests obtained in the coarse-grained region of the HAZ (CGHAZ) of welded joints of API X80MO steel, according to the requirements of the API RP 2Z standard specification. 2. Experimental Procedures 2.1. Base Material The base material assessed was API X80MO structural steel, of 37 mm thickness, produced on an industrial scale by Usiminas company according to the API Specification 2W (2006). The material was processed by controlled rolling followed by accelerated cooling in the CLC® equipment. The chemical composition and mechanical properties of the base material are given in Table 1. The base metal microstructure was very refined, with an effective average grain size of 2.1 ± 0.2 µm. These results were obtained using the EBSD technique, considering the degree of disorientation between grains greater than 15°. These grains showed a certain density of subgrains, with a boundary fraction peak with an orientation lower than 5° and a considerable fraction of high-angle boundaries ( ≥ 15°), which offer an important resistance to the propagation of cleavage crack (Borba, TMD et al (2020)). Apparently there is no preferential orientation of the grains, characterizing a low anisotropy material. 1.1. Production of the welded joints In accordance with the API RP 2Z standard, to assess the heat affected zone toughness for the steel plate, two butt joints were welded with heat inputs at 0.8 kJ/mm and 4.5 kJ/mm. For the lowest and highest heat input, the joints were welded using gas shielded flux cored arc welding process (FCAW) and submerged arc welding (SAW), respectively. The welding direction was parallel to the main rolling direction using single-bevel joints. The consumable used in the SAW process consisted of a wire-flux combination OK Autrod 13.40 (Ø = 4.0 mm) with OK Flux 10.62, complying with AWS A5.23 F10A8-EG-F3 and F9P8-EG-F3. For FCAW process it was used a externally supplied C25 shielding gas and the welding wire Dual Shield 55 (Ø = 1.2 mm), an AWS A5.29 E91T1-Ni1M consumable. These consumables are commonly used in applications that require high toughness values. The chemical compositions of the weld metal, for the FCAW and SAW processes are shown in Table 2. The detailed welding conditions used in the FCAW and SAW processes, including heat input, amperage, voltage, welding speed, preheating temperature, and groove configuration are shown in Table 3. After welding, an ultrasound test was performed to assess the integrity of the weld beads. For each joint, the longitudinal-short transverse plane was polished and etched with 4% Nital etchant to observe the macrostructures of the welded specimen by using an optical microscope.