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Results
Initiation of Cleavage Fracture In a Weld Simuled Low Carbon Low Alloy Arctic Steel
Brandt, Kristin (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Solberg, Jan Ketil (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Akselsen, Odd Magne (SINTEF Materials and Chemistry, SINTEF) | Østby, Erling (SINTEF Materials and Chemistry, SINTEF)
ABSTRACT The risk of brittle fracture is an important aspect when oil and gas exploration move to Arctic regions. Therefore, the present work addresses examination of fracture surfaces in scanning electron microscope to identify brittle fracture mechanisms. The study is based on Charpy and CTOD bend testing at -60°C of weld thermal simulated specimens corresponding to both coarse grained HAZ (CGHAZ) and intercritically reheated coarse grained HAZ (ICCGHAZ). Brittle fracture was obtained by both test methods, and SEM investigation of the fracture surfaces revealed that slag particles like sulphides and oxides had initiated cleavage in the CGHAZ specimens, and that martensite-austenite (MA) particles probably were responsible for cleavage in the ICCGHAZ specimens. In both types of weld simulated structures, the crack initiation points were located close to the pre-crack tip. INTRODUCTION The increased focus on oil and gas exploration in the high north will inevitably give renewed attention to the low temperature embrittlement of structural steels. The ductile to brittle fracture transition temperature that occurs in such steels will consequently be very critical due to low service temperature. There are low alloy steels available with sufficiently low transition temperature, but they tend to loose their excellent properties after welding, primarily the toughness. If steels are to be made with the substantial improvements in weldability, further developments in steel processing technology, combined with optimization in alloying practice, must be made. In parallel, a better understanding of brittle fracture mechanisms of different weld microstructures must be sought. Such information has been collected from scanning electron microscopy (SEM) examination of fracture surfaces of Charpy V or CTOD (crack tip opening displacement) tests taken from either real weldments or weld thermal simulation. However, this may not be straight forward because of difficulties in finding the first point of fracture initiation (first microcrack to form).
- Europe (0.29)
- North America > United States (0.29)
- Energy > Oil & Gas > Upstream (0.89)
- Materials > Metals & Mining > Steel (0.88)
Comparison of Fracture Toughness In Real Weld And Thermally Simulated CGHAZ of a 420 MPa Rolled Plate
Østby, Erling (SINTEF Materials and Chemistry) | Kolstad, Gaute T. (NTNU (Norwegian University of Science and Technology)) | Thaulow, Christian (NTNU (Norwegian University of Science and Technology)) | Akselsen, Odd M. (SINTEF Materials and Chemistry, and NTNU (Norwegian University of Science and Technology)) | Hauge, Mons (NTNU (Norwegian University of Science and Technology), and Statoil ASA)
ABSTRACT Establishing the fracture toughness of real HAZ can be challenging due to the small extension of the potentially brittle zones. Application of testing of weld thermal simulated HAZ microstructures can provide an alternative approach to establish fracture properties of the more brittle microstructures. In this paper CTOD values measured in real and simulated CGHAZ of a 420 MPa steel are compared at -60 and -30ºC. It is shown that for the lower temperature, where the fracture toughness is very low, similar CTOD values are obtained in both the real and simulated HAZ. At the higher temperature the fracture toughness increases more in the real HAZ compared to the simulated one. FE analyses are used to propose an explanation for this behaviour. The results present an interesting first step in the direction of use of weld thermal simulated samples for obtaining relevant CTOD values for used in assessment of real HAZ. INTRODUCTION The low temperatures encountered in the Arctic poses a special challenge with regards to the use of steel structures. The risk of brittle behaviour in steels and their weldments must be controlled in order to have acceptable structural integrity. Especially weld metal and heat affected zone (HAZ) could be subject to low toughness. These are also areas where defects are most likely to occur. A challenge associated with the characterization of brittle fracture is the large scatter in fracture toughness test results. Application of statistics is necessary to obtain reliable estimates of the toughness. General approaches in this respect have been developed by Beremin (1983) and Wallin (2002), and examples of statistical methods applied to characterize scatter in HAZ fracture can be found in Hauge (1990). Determination of fracture toughness in the HAZ represents a special challenge. The brittle zones may be of limited size and there could be a large gradient of toughness acting over small distances in the material.
- Energy > Oil & Gas (1.00)
- Materials > Metals & Mining > Steel (0.48)
ABSTRACT The present work was initiated to study the low temperature brittle behaviour of welded 42"" API X80 pipe for applications in arctic areas. The pipe was girth welded using GMAW of root and hot pass, and PGMAW of the fillers. The testing comprised of both mechanical testing and fracture toughness testing. The fracture mechanics SENT test results (-60°C) revealed quite good toughness with CTOD values from 0.3 to 5.5 mm for the fusion line, and from 2 to 5.5 mm for the weld metal. For the fusion line, the large scatter observed seems to be related to the fatigue pre-cracking, meaning that the fatigue pre-crack did not always grow to hit the fusion line. In fact, in some samples it grew towards the base metal. Therefore, the CTOD values do not necessarily be representative the coarse grained HAZ fracture toughness. Metallographic inspection confirmed very narrow HAZ due to the low heat input employed, i.e., visible HAZ width of ~ 300 μm. INTRODUCTION The huge foreseen oil and gas resources in the Arctic areas (Gauthier, 2009) may throw new impetus on steel development due to the harsh environments like very low temperature, strong polar winds, ice/iceberg, thaw settlement and frost heave. In addition, requirements may also be set to low weight due to long transport distances. With respect to climate conditions, steels with improved base metal toughness are sought to give sufficient robustness after welding, while the low weight focus points to higher strength. This combination of strength and toughness has always been a challenge, but considering the severe service conditions in the high north, the situation approaches the borderline for applications of "affordable" steels, containing reasonable alloying levels; i.e., not special alloys such a 9% Ni steel or stainless steel. Onshore pipelines may suffer from similar phenomena due to welding degradation of the initial low temperature base metal toughness.
- North America (0.46)
- Europe > Norway (0.29)
- Materials > Metals & Mining > Steel (1.00)
- Energy > Oil & Gas > Upstream (0.66)
On Local Properties of Microstructures In Welding
Haugen, Veronica (NTNU (Norwegian University of Science and Technology)) | Rogne, Bjørn Rune Søraas (NTNU (Norwegian University of Science and Technology)) | Akselsen, Odd M. (NTNU (Norwegian University of Science and Technology)) | Thaulow, Christian (NTNU (Norwegian University of Science and Technology) ,and SINTEF) | Østby, Erling (SINTEF)
ABSTRACT Steels that are used in arctic climates are introduced to extreme environmental weather conditions, where, e.g., the temperature may fall as low as -60°C. This phenomenon will inevitably affect the material properties since structural steels undergo the ductile to brittle transition. Hence, the fracture toughness may be substantially reduced after welding. In order to establish reliable models to predict the behavior of steels under such extreme conditions, the knowledge of the local properties would represent a vital contribution. In the present investigation, local properties are measured in microstructures typically present in the intercritically reheated coarse grained HAZ (ICCGHAZ) of a 490 MPa forging. Focused ion beam (FIB) is used to fabricate pillars from weld thermally simulated ICCGHAZ with cooling rate corresponding to a cooling time between 800 and 500°C (Δt8/5) of 5 and 10 s, followed by nanomechanical testing by compression. The results achieved showed significant scatter in the stress-strain data, depending on the actual local microstructure being tested. Bulk martensite and upper bainite, as well as blocky martensite-austenite (MA) particles present at prior austenite grain boundaries, were examined. INTRODUCTION The phenomenon of low toughness in certain regions in welding of structural steels is well known (Thaulow, 1985, 1987). These are typically located in the coarse grained heat affected zone (CGHAZ, Thaulow, 1985, 1987; Grong, 1986) and the intercritically reheated coarse grained HAZ (Akselsen, 1987, Davis, 1994). The embrittlement has been attributed to the formation of brittle microstructures. In the former case, a brittle coarse upper bainite is usually present in a certain critical volume fraction (Grong, 1986; Zhang, 2009). For the ICCGHAZ, the formation of hard, brittle martensite-austenite (MA) islands adds to the problem (Tanoue, 1980; Akselsen, 1987, 1988). The macroscopic behaviour of such microstructures have been studied by extensive impact and fracture toughness testing, which will bring forward some average of the properties of the microstructure present in the actual test sample.
- Energy > Oil & Gas (1.00)
- Materials > Metals & Mining > Steel (0.66)
Determination of Ccrystallographic Facet Orientations On Fracture Surfaces of an Arctic Steel By Using EBSD
Mohseni, Peyman (Norwegian University of Science and Technology (NTNU)) | Solberg, Jan Ketil (Norwegian University of Science and Technology (NTNU)) | Karlsen, Morten (Norwegian University of Science and Technology (NTNU), and Statoil ASA) | Akselsen, Odd M. (Norwegian University of Science and Technology (NTNU), and SINTEF) | Østby, Erling (SINTEF)
ABSTRACT Electron backscatter diffraction (EBSD) has been increasingly used to identify the crystallographic planes and orientation of cleavage facets with respect to the rolling direction in fracture surfaces. The crystallographic indices of cleavage planes can be determined either directly from the fracture surface or indirectly from metallographic sections perpendicular to the plane of the fracture surface. The commonly observed crystallographic mode (i.e. the micromechanism) of brittle fracture in ferritic steels is cleavage, which involves the separation of atomic bonds along low-index {100} crystallographic planes. The main purpose of this work was to identify the crystallographic orientations of cleavage facets in fracture surfaces with respect to the rolling direction of a well known steel at low temperatures. The material used for the work was a steel (API X80 grade) that has been developed for applications at low temperatures. The crystallographic indices of the cleavage crack planes were identified to be {100}, {110}, {211} and {310} at all temperatures. INTRODUCTION High strength and toughness pipeline steels have been used at high operation pressures to improve the transport efficiency of oil and gas. The major motivation behind the development of such steels has been to obtain the best combination of strength and toughness (Bose-Filho, Carvalho, Strangwood, 2007). Weldability, high fracture toughness and fatigue resistance at low temperatures are additional requirements for transmission of oil and gas through pipelines (Das, Sivaprasad, Dasa, Chatterjee and Tarafder, 2006). The balance of high strength and high toughness can be deteriorated by welding thermal cycles, producing local poor toughness in the welded joints (Lambert-Perlade, Gourgues, Besson, Sturel, and Pineau, 2004, Davis and King, 1996). The heat affected zone (HAZ) is in many cases considered to be the most critical part of a weld. The region of lowest toughness after single pass welding is the coarse grained heat affected zone (CGHAZ) (Qiu, Mori, Enoki and Kishi (2000) and Akselsen, Solberg and Grong, 1988).
- Energy > Oil & Gas (1.00)
- Materials > Metals & Mining > Steel (0.48)
ABSTRACT For steel application at low temperatures their brittle fracture resistance is of vital importance. The present investigation addresses the susceptibility to brittle fracture of forgings of F70 grade. To evaluate the mechanical properties after welding, weld thermal simulation tests with different cooling times were performed. Both CTOD testing with shallow and deep cracks was carried out for coarse grained (CGHAZ) and intercritically reheated coarse grained heat affected zone (ICCGHAZ) to identify possible influence of microstructure and eventual constraint effects. In addition Charpy-V notch and hardness tests of CGHAZ and ICCGHAZ were carried out. The results are discussed in terms of crack position, microstructure and constraint and compared to the base material results. INTRODUCTION Forgings are widely used in offshore applications such as piping components (e. g. flanges or T-pieces) or structural parts in oil and gas exploration. They can be found in both, welded and non-welded conditions. With the expansion of the oil and gas recovering area in the arctic region, resistance to brittle fracture at low temperatures is of great importance for all components, regardless of their manufacturing process. Since forgings generally have larger wall thicknesses than e.g. tubes, it is more difficult to realize the required properties over the entire cross-section. At the same time a good weldability of the parts has to be provided. In a former paper (Welsch, Bruch and Østby, 2011) the base material properties of forged materials for arctic application were addressed. From the two investigated material grades, namely ASTM A694 F65 and F70, the F70 showed improved properties in all types of tests, especially in fracture toughness (CTOD) testing. In terms of arctic application not only the base materials properties but furthermore the weldability of the grades is of high importance. The higher alloying content of the F70 compared to the F65 leads to a higher carbon equivalent (CE-L).
- Energy > Oil & Gas > Upstream (0.55)
- Materials > Metals & Mining > Steel (0.49)