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ABSTRACT: M.E.F. dual phase steel was made from structural steel (SS41), and corrosion fatigue tests With rotated bending test machine have been performed in 35% NaCl solution The variable was temperature (278, 288, 303 and 318ยฐK). The effects of solution temperature on the corrosion fatigue strengths, corrosion fatigue crack propagation rates, fracture appearances and corrosion morphologies are described. 1 INTRODUCTION Extensive studies on the corrosion fatigue have been performed for the purpose of safe designs and life predictions of the structures in the corrosive environments The structures, used particularly under the sea-water corrosive condition, can undergo the earlier fatigue crack initiation and the faster crack propagation to fracture by corrosion activity. The fracture of the steels can be influenced by the mechanical, metallurgical properties, and also the service conditions or environments. In aqueous solution, the environmental variables are purity, composition, pH, temperature, dissolved oxygen, and flow rate (Scott and Turnbull, 1985), which can significantly affect the stress corrosion cracking (SCC), the hydrogen embrittlement cracking (HEC), and corrosion fatigue behaviors of the structure. But It is inevitable to use the high tensile strength steel because of the increasing needs of design, cost and safety. Hence, in case that the dual phase steel should be used for the offshore structures, It IS necessary to investigate its corrosion fatigue characteristics m sea-water environment. Effects of pH(Oh et aI, 1987), solution concentration (Oh et al, 1988) and ferrite grain size(Oh et al, 1987) on the corrosion fatigue behavior of the dual phase steel In sea-water environment have been reported, but few on the effect of the solution temperature In this study, rotated bending corrosion fatigue tests have been carried out With structural steel (SS41) and dual phase steel that was produced from SS41 by a series of heat treatment.
ABSTRACT: Transportation of wet sour gas, produced offshore, to onshore terminals, poses severe corrosion problems. Formations enveloping 25% H2S & C02 at high pressures and temperatures of 20,000 psi & 200ยฐC respectively are not uncommon. Treatment of sour gas being prohibitive offshore due to unfavourable economics, the pipeline option IS indispensable. Presence of water vapour In sour gas results In Internal pipeline corrosion mainly In the form of Hydrogen Embrittlement & Sulphide S tress Cracking and also General weightloss, Hydrogen Blistering and Hydrogen - Induced Cracking. Material selection for such environments IS determined by trade-off In economics between greater strength and thicker walled pipelines. However, recent developments In coating technology, cathodic protection systems and new design concepts like Pipe-within- Pipe type transportation a designer. INTRODUCTION As shallow reserves are depleted, the worldwide search for new sources of hydrocarbon IS turning to deeper reservoirs. Drilling and producing these reservoirs require the use of new materials not used previously. Deeper drilling means that the higher pressures will be encountered and that, the reservoir fluids Will be at a higher temperature than normal. The like hood of acid gases (CO2 and H2S) as a reservoir component IS also Increased. The transportation of these sour gases to onshore terminals poses some interesting pipeline design problems, particularly those pertaining to corrosion prevention. Pipeline transportation being the only answer, the presence of water vapour In the sour gas results In the formation of atomic hydrogen In the pipeline from the oxygen accelerated dissociation of hydrogen sulphide gas molecules in presence of water. This ultimately leads to hydrogen Induced corrosion (HIC) In the pipeline. Typically, hydrates In the sour gas form at 70-80ยฐ F and hence design considerations demand some form of insulation or heating to prevent blocking.
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Corrosion inhibition and management (including H2S and CO2) (1.00)
- Health, Safety, Environment & Sustainability > Health > Noise, chemicals, and other workplace hazards (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers (1.00)
ABSTRACT: Currently adopted modified-WOL specimen has difficulty in producing fatigue precrack. And Ktscc evaluation using the analytic compliance formula has many problems. Hence, Oh and Kim(1989) have designed such a specimen geometry as mixed modified-WOL with CT types. and proposed the experimental evaluation techruque for bolt-load(consequently KrscC>. With this specimen, stress corrosion cracking and subsequent Jic test has been performed in 20% HCI solution saturated with as gas. 1. INTRODUCTION Among the test methods to evaluate stress corrosion cracking on the basis of fracture mechanics, constant-displacement (bolt) loading method using modified-WOL specimen (Novak and Rolfe, 1969; Lisagor, 1984) is practically convenient test procedure because it needs no external stressing equipment. Besides, it facilitates exposure to operating service environments. As stress corrosion (or corrosion/corrosion fatigue) might show very different behaviors by the delicate change of the environment, the above facility can be considered as important merit for Industrial application. Fell this method meanwhile compliance formula is generally required to calculate bolt-load (consequently Kiscc) if Instrumented bolt (Gilbreath and Adamson 1976) is not used. By the way, the analytic compliance values have to be transformed according to the state-of-stress. And the exact values of material constants (E, v) are requisite to calculate the bolt-load. And also the possibility of stress relaxation and/or creep for long-term test cannot be considered. Furthermore, Oh and Kim(1989) have reported that the analytic compliance formula had difficulty in being, applied to the case of ductile materials. Therefore, it is necessary to develop new experimental evaluation method for bolt-load. In this study, we have adopted the altered configuration of modified- WOL specimen (that is, mixed modified-WOL With CT specimen geometries) in order to produce fatigue precrack easily and also to propose the experimental evaluation technique for boltload (or Krscc) instead of using the analytic compliance formula.
ABSTRACT: Between the weld metal which reaches its melting point and above, and the cold parent metal, there are zones, within which all intermediate temperatures are reached promoting localized changes in the microstructure due to which pitting and severe corrosion were located. These are due to the difference in thermal expansion coefficients of the involved materials. In the present research, the effect of some of the heat treatment operation that are commonly used have been investigated. Also, an attempt was made to calculate residual thermal stresses at weldments due to the difference in thermal expansion coefficients at the involved materials. SELECTION OF THE STEEL UNDER INVESTIGATION Steels are selected on a performance basis rather than purely on strength. Where strength is a criterion however, the ferritic steels will be less strong and ductile than austenitic, and martensitic and precipitation hardening steels will be the strongest. In the present work, however, the environmental factors (corrosivity, operational temperature, resistance to pick-up of elements, e.g., sulphur, carbon from service atmospheres) were of considerable importance in selection of the material used in the present investigation, i.e., 316L authentic stainless steel. The chemical composition of which is given in Table I. The use of 316L austenitic stainless steel was encouraged by the knowledge that it offers much better welding characteristics than the other stainless steels (Baker, 1972). Metallographic examination of the 316L stainless steel showed that the structure consisted of homogeneous stable austenite with an assembly of more or less polygonal grains, see Fig. 1. The present steel belongs to a group of steels which constitutes the more highly alloyed austenitic steels, which contain chromium (12-30%) plus nickel (7-25%) and in most cases, smaller proportions of other elements. These steels are structurally unaffected by heating and cannot be hardened by quenching.
ABSTRACT: The local corrosion of high-strength steel of 440N/mm class manufactured by the thermo-mechanical control process was examined. INTRODUCTION The local corrosion of welded joints of icebreakers resulting from damage of paint and cathodic protection by ice collision is a problem to be discussed [2] [l] [5]. The steels for icebreakers are required to have (i) high resistance to local corrosion in seawater, (ii) high toughness at low temperature and (iii) good weldability. In this paper, the local corrosion of high strength steel of Y.P. 440 N/mm class manufactured by a thermo-mechanical control process (TMCP) is studied. And the properties of the steel, and welding materials developed on the basis of the results of the local corrosion resistance are also discussed. 2. LOCAL CORROSION OF THE TMCP STEELS 2-1. Test Specimens 2-1-1. Tested Steels Table I shows the chemical compositions of the steels tested. PI and CT are high-strength steels having yield strength of 440 N/mrn2 class produced by the TMCP. Both belong to the Cu-Ni series. A small amount of Nb was added to improve the notch toughness of the base metal, and a similar amount of Ti was added to improve the notch toughness of the weld heat affected zone (HAZ) [6]. CF and TB are the reference steels. Both belong to the Si-Mn series; CF is low Mn steel and TB is low C steel. Photo. 1 shows the basemetal microstructures. PI and CT have fine proeutectoid ferrite (F) and bainite (Bu) microstructures. CF and TB have ferrite and pearlite (F+P) microstructures. 2-1-2. Test Welding Materials Table 2 lists the test welding materials. N100, N100Cu, N10, N13, ASl5 and NUSN were used for shielded metal are welding (SHAW) and YE-NB55E, IN-NB55E and YDN-NB55L for submerged are welding (SAW).
- Asia (0.69)
- North America (0.47)
ABSTRACT: With 14 samples of various chemical compositions of 12% Cr steel and one 1% CrMoV steel, the effect of back stress on creep rate was studied in relation with activation energy and stress exponent. Back stress is depended on applied stress and temperature. The equation can be formulated. Effective stress exponent, ฮทฮฟ is not fixed value for all materials as reported but strong dependence of temperature and microstructure ฮทฮฟ value represents the stability of a material at the elevated temperature and no value is expected to be a criterion of stability of material in the other atmospheres. Elements of Nb, Zr, Ti, W, La, Pd and Ru are found to be effective to form very stable microstructure in 12% Cr steel and the elements are expected to improve creep rupture strength. INTRODUCTION In order to improve the thermal efficiency of a steam turbine in the power generation system, there is a trend towards increasing the operating temperature and pressure. 1% Cr-Mo-V steel which has high creep rupture strength and good toughness up to 550" C of steam temperature has been successfully applied for the material of turbine rotor. However, to meet higher creep rupture strength above 600t, 12% Cr steel is considered a promising material to replace the conventional 1% Cr-Mo-V steel. An intensive study of 12% Cr steel for the application of turbine rotor was made by General Electric, Climax Molybdenum Company" in U. S. A in 1960s and Mitshubishi, Tokyo Electric PowerCo", and Tokyo University in Japan in 1980s. Hitachi studied" mould design for ingot making to reduce segreagation in ingot. The high creep rupture strength of 12% Cr steel was reported due to the secondary hardening by increasing coherent strains or increasing the volume fraction of precipitates such as M2X or MX carbides.
- Materials > Metals & Mining (0.88)
- Energy (0.68)
- Well Drilling (0.38)
- Reservoir Description and Dynamics (0.34)
ABSTRACT: While recently developed platform steels have excellent low temperature toughness properties they may be nevertheless, susceptible to cleavage fracture initiation at focally brittle zones (LBZ" s) developed in coarse grain heat affected zones (CGHAZ" s) of welds. Current offshore material specifications such as EEMUA and EPRCO require prequalification of plate in regard to LBZ such that a COD notchtip intersecting 15 percent or more of the CGHAZ in a weld sample meets lower bound toughness values. In the present work the significance of the 15 percent intersection requirement has been assessed using a roll bonded sandwich technique with a 0.45 percent carbon steel plate bonded between outer plates of offshore steel. It was observed that at this percentage intersection cleavage fracture, initiated as microvoid cleavage, readily propagated into the off shore plate resulting in crack opening displacement toughness values of less than O.lmm at -30ยฐC. The significance of the pre-qualification requirement and the roll bonding technique are discussed. 1. INTRODUCTION Owing to section size, offshore platforms are fabricated using multi-pass welds, with the heat affected zone (HAZ) adjacent to the welds comprising a complex array of individual zones where the HAZ of one weld is influenced by the heat of subsequent passes. Multi-pass weldments as a whole have improved toughness as compared with single run welds due to the refining and tempering effect subsequent weld runs have on previous deposits. However" there are small, zones generated in multi-pass welds that suffer poor toughness, properties, similar to those experienced in single weld HAZ" s. Of particular importance are coarse grained heat affected zones (CGHAZ) from a previous run that constitute tile inter critical- heat affected zone (ICHAZ) or CGHAZ as shown in Figure: I. The ICHAZ is that region exposed to temperatures between AC3 and AC3 of approximately 850ยฐC.
ABSTRACT: Influence of the local brittle zone on HAZ CTOD was discussed. From weld HAZ CTOD tests and simulated HAZ CTOD tests, it was clarified that the local brittle zone of a multi-pass weld HAZ is intercritically reheated coarse-grained zone, where high-carbon martensitic island was formed. Factors influencing the formation and decomposition of high-carbon martensitic island (M*) in the intercritically reheated coarsegrained HAZ was investigated. Based on a simple estimation model of volume fraction of the M*, it became possible to estimate quantitatively the influencing factors on the formation and decomposition of the M*. It was clarified that combination of steel with low carbon, low microa110y chemical composition and suitable welding condition, such as application of preheating and thin weld bead placement, is important for reducing or eliminating the H*. 1. INTRODUCTION Since low CTOD (Crack-Tip Opening Displacement) value was obtained in a HAZ (Heat-Affected Zone) CTOD test of offshore structural steel in the early 1980s, extensive studies have been made to clarify the cause of the low CTOD value. It has been made clear that the low value was caused by the LBZ (Local Brittle Zone) in the HAZ. Factors influencing the microscopic toughness of the LBZ and measures to prevent low CTOD value have been studied (OMAE, 1988),(OMAE, 1989) and new steels which have high HAZ CTOD value have been developed (e.g. Chijiiwa et aI, 1988). For improving the LBZ toughness, selection of chemical composition which has low susceptibility to cleavage fracture is primarily important. In addition, selection of a suitable welding condition (heat input, preheating and bead placement etc.) is also important. For this purpose, it is of urgent necessity to elucidate these factors quantitatively. From this standpoint, the present paper will discuss the factors controlling the LBZ toughness.
ABSTRACT: Three different types of steel, normalized, normalized-z and TMCP-z steels, have been studied for the cracking susceptibility in the thickness direction using the implant tests and the thermal simulator. The results obtained from the implant tests indicated that the TMCP-z steel has the highest critical cold cracking rupture stress even at high diffusible hydrogen content (24.5ml/100gr weld metal) in comparison with the normalized and the normalized-z steels which show a substantial decrease at increased diffusible hydrogen levels. Electron metallography showed that fracture surfaces of the TMCP-z implant specimen are ductile mode, while those of the normalized and the normalized-z are intergranular. Simulated heat-affected zone studies using the induction heating type simulator demonstrated that liquation hot cracking occurs in the normalized steel but the normalized-z and the TMCP-z steels are rather insensitive to liquation hot cracking_ 1. INTRODUCTION TMCP(Thermo-Mechanical Controlled Process) steels provide benefits of low manufacturing costs and good weldability both to steel makers and fabricators. respectively. Cold cracking. hot cracking in the HAZ (Heat Affected Zone) and lamellar tearing still can occur in TMCP steels unless adequate welding parameters are maintained. These defects are related with amounts of alloying elements and the shape of nonmetallic inclusions and microstructures. Both a normalized-z and a TMCP-z quality steels of which ductility in the thickness direction is improved and a normalized steel of T.S.50kg/mm have been investigated regarding cracking susceptibilities in the thickness direction using the implant test machine and the thermal simulator. 2. EXPERIMENTAL Three different types of steel whose chemical compositions are shown in Table 1 were tested. The normalized steel has the highest carbon equivalent and sulfur content while the normalized- z and the TMCP-z have the lowest sulfur contents because of Ca wire treatment in the ladle before pouring.
ABSTRACT: The higher strength steels often show HEAT-AFFECTED ZONE (HAZ) softening. In the previous study, it was shown that CTOD toughness is much more important than strength matching to get higher welded joint performance in a stress concentrated region on the basis of the FEM analysis and wide plate test with through-thickness notch. This paper describes the further test results of the surface notched wide plate test with and without a stress concentrator. In conclusion, it shows that HAZ undermatching does not influence the deformability at the fracture of a welded joint in offshore structures. Moreover, it was shown that the local strain based CTOD approach can be applied to assess the fracture behavior of strength in an unevenly matched joint. INTRODUCTION Offshore structures have been successfully used in cold and harsh environments such as the North Sea. This is partly due to the fact that high fracture toughness, especially CTOD toughness has been demanded in offshore structural steels as well as in welding consumables based on the fracture mechanics evaluation. Stringent CTOD testing is now carried out for weld HAZ of the steel. As offshore structures are going to be installed deeper and deeper in the sea, structures will become bigger and bigger. In order to save the weight of the topside structures from an economical viewpoint, there has been a trend toward using higher strength steels. Ih higher strength steels manufactured by either the thermo-mechanical control process (TMCP) or the quenching and tempering process (Q&T), HAZ softening (HAZ undermatching of strength) may occur, unlike normalized steel in which strength in the HAZ region is usually higher than the base plate, although the level and the width of HAZ softening depends on the chemistry of the base plate and the welding condition, especially heat input.
- Europe > United Kingdom > North Sea (0.24)
- Europe > Norway > North Sea (0.24)
- Europe > North Sea (0.24)
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- Well Completion > Hydraulic Fracturing (1.00)
- Well Drilling > Drillstring Design > Drill pipe selection (0.40)