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2. Charpy toughness and microstructure of vibrated weld metalBogdan Pučko, Vladimir Gliha, 2006, izvirni znanstveni članek Opis: Vibration during welding can be used to obtain certain changes in mechanical properties of weld metal. Research work on the influence of vibration on the secondary microstructure of welds and hence on the Charpy toughness was performed. Vibration during welding exhibits positive effects on the microstructure constituent formation. Multipass welding was simulated with reheating of the original single pass weld in order to obtain similar microstructure to multipass welds. Microstructures were examined with an optical microscope. Additionally, fractographic examination of the rupture of Charpy specimens was performed. Changes in the microstructure according to vibration were observed which affect toughness of the weld metal. Vibration during welding was rated more effective in the case of reheating the weld metal, which is the case in multipass welding.
Ključne besede: welding, welded joints, mechanical properties of metals, ferrite, vibration, microstructure, toughness, notched bar testing, weld metal
Objavljeno: 31.05.2012; Ogledov: 1618; Prenosov: 77
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3. The effect of small flaws on the fatigue strength of HAZ at the weld toeVladimir Gliha, 2007, izvirni znanstveni članek Opis: The effects of small artificial surface flaws on the fatigue strength of HAZ material are studied. The samples of coarse grain HAZ material were prepared using a welding thermal-cycle simulator. The artificial flaws were produced byindenting with a Vickers pyramid at different loads as either single indentations or as a series. The size of the flaws did not exceed the primary austenitic grain size of the material, which is the most relevant microstructural unit of carbon steels. The dependence of the experimentally determined bending fatigue strength of treated coarse grain HAZ materials on the properly evaluated size of the artificial flaws was compared with the law of long-crack propagation. The biggest flaws evaluated with the flaw-size parameter ?area ? 100 m are still small for the studied coarse grain HAZ. In further experimental work, the effects of flaw size and the effects of residual stresses should be analysed separately.
Ključne besede: welding, welded joints, coarse grained HAZ, crack initiation, flaw-size parameter, small surface flaws, stress intensity range, Vickers indentation, weld toe
Objavljeno: 31.05.2012; Ogledov: 1237; Prenosov: 77
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4. Determination of lower bound fracture toughness of a high strength low alloy steel welded jointNenad Gubeljak, 2005, izvirni znanstveni članek Opis: The use of high-strength low-alloy steels for high performance structures (e.g. pressure vessels and pipelines) requires high strength consumables to produce an overmatched welded joint. This globally overmatched multipass welded joint contains two significantly different microstructures, as welded and reheated. In this paper, the influence of weld metal microstructure on fracture behaviour is estimated in comparison with the fracture behaviour of composite microstructures (as welded and reheated). The lower bound of fracture toughness for different microstructures was evaluated by using the modified Weibull distortion. The results, obtained using specimens with crack front through the thickness, indicated low fracture toughness, caused by strength mismatching interaction along thevcrck front. In the case of through thickness specimens, at least one local brittle microstructure is incorporatedin the process zone at the vicinity of the crck tip. Hence, unstable fracture occurred with small or without stable crack propagation. Despite the fact that the differences between the impact toughness of a weld metal and that of base metal are insignificant, the future toughness of a weld metal can be significantly lower.
Ključne besede: welded joints, fracture mechanics, Crack tip opening displacement, high strength seeel weld, mismatch effects, fracture toughness, resistance curves
Objavljeno: 01.06.2012; Ogledov: 1423; Prenosov: 78
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5. Effect of vibration on weld metal hardness and toughnessBogdan Pučko, Vladimir Gliha, 2005, izvirni znanstveni članek Opis: Vibration of welded parts is usually applied to achieve effects similar to thermal stress relief. With vibration, it is not only residual stresses that are affected. Using two different welding processes, the influence of vibration on hardness and toughness of the weld was measured. For each weldingprocess, two series of Charpy specimens were made over the temperature range from -60 to +20°C. The only difference between the two series was in performing welding with or without vibration. Slight differences in weld metalhardness were observed. Toughness measurements show an increase in impacttoughness and a significant increase in fracture toughness in samples which were vibrated during welding.
Ključne besede: welding, welded joints, residual stresses, vibrational stress relief, hardness, toughness, steel
Objavljeno: 01.06.2012; Ogledov: 1529; Prenosov: 70
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6. Suspectibility to hydrogen induced cracking of weld beads on high-strength structural steelJadranka Malina, Ivan Samardžić, Vladimir Gliha, 2005, izvirni znanstveni članek Opis: We study the hydrogen embrittlement and rupture of specimens of high-strength low-alloy steel with weld beads subjected to hydrogenation under the conditions of cathodic polarization. The specimens fail in the heat-affected zone on the boundary between the main metal and the weld bead in the zone of application of the latter and, especially, near its end. This explained by a higher hardness of the metal in this zone caused by the specific features of the temperature field formed in the process of welding.
Ključne besede: welding, welded joints, materials, structural steel, cracking, hydrogen
Objavljeno: 01.06.2012; Ogledov: 1038; Prenosov: 20
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7. Assessment of the load-bearing capacity of a primary pipelineNenad Gubeljak, Jelena Vojvodič-Tuma, Borivoj Šuštaršič, Jožef Predan, Maks Oblak, 2007, izvirni znanstveni članek Opis: High-alloyed Cr-Ni-based two-phase stainless steel (SS) cast alloys are commonly used in nuclear power plants. The mechanical equipment in these facilities can contribute to a reduction in its resistance to stable crack growth as a result of extended operating times and high temperatures. The toughness of these materials strongly depends on their delta (▫$/delta$▫) ferrite content, which spinodally decomposes into two phases with different ratios of Cr and Ni at a relatively low (slightly above 300 °C) temperature. This temperature is similar to the operating temperature of the vital parts, for example, the coolant system. The formation of two phases with the same crystalstructure but different lattice parameters causes internal elastic stresses that result in a hardness increase and an impact-toughness decrease. The result is an increased risk of crack formation in the stress-concentration zones such as the critical regions of different welded joints (e.g. "L, T, K and X" shapes). The values of the critical stress intensity factor change according to its position along the crack contour. Therefore, the aim of our study was to assess the influence of the materials' changes on the crack extension and the decrease of the primary pipeline's bearing capacity by taking account of the increased temperature and time of operation for the given loading conditions. The SINTAP (European Structural Integrity Assessment Procedure) was used for this assessment.
Ključne besede: welding, welded joints, pipelines, fracture toughness testing, structure integrity assessment procedure
Objavljeno: 01.06.2012; Ogledov: 1217; Prenosov: 63
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8. Micromechanical analysis of constraint effect on fracture initiation in strength mis-matched welded jointsMiloš Dobrojević, Marko Rakin, Nenad Gubeljak, I. Cvijović, Milorad Zrilić, N. Krunich, Aleksandar Sedmak, 2007, izvirni znanstveni članek Opis: In this paper the micromechanical approach to ductile fracture was applied in a study of constraint effect crack growth initiation in mismatched welded joints. The single edged- notched bend specimens (precrack length ▫$a_o/W=0.32$▫) were experimentally and numerically analysed. The coupled micromechanical model prposed by Gurson, Tvergaard and Needleman was used. Constraint effect was tested by varying widths of the welded joints (6, 12 and 18 mm). HSLA steel was used as the base metal in a quenched and tempered condition. The flux-cored arx-welding process in shielding gas was used. Two different fillers were selected to obtain over- and undermatched weld metal. The micromechanical parameters used in prediction of the crack growth initiation on precracked specimen were calibrated on a round smmoth specimen. The difference in fracture behaviour between over- and undermatched welded joints obtained in experimental results was followed by numerical computationsof void volume fraction in front of the crack tip.
Ključne besede: fracture mechanics, ductile fracture, high strength low alloyed steel, cracks, micromechanical model, mismatched welded joints, constraint effect, void growth
Objavljeno: 01.06.2012; Ogledov: 1260; Prenosov: 72
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9. Strength mismatch effect on yield load in X-shaped weldment with centre crackDražan Kozak, Nenad Gubeljak, Jožef Predan, Franjo Matejiček, 2004, objavljeni znanstveni prispevek na konferenci Opis: This paper provides yield load solutions for single edged fracture toughness specimen subjected to bending SE (B) with present X-shaped weld joint. The weld centre crack is located in the overmatch weld part. The corresponding fully plastic yield loads were obtained directly by plain strain FEM analysis for five characteristic a/W ratios: 0,1;0,2;0,3;0,4 and 0,5. Also, the influence of the sistematically varied weld root width 2H on the fracture behaviour has been evaluated. It was found that yield load decreases with the increasing weld root, because the undermatched region ahead the crack tip plays the dominant role.
Ključne besede: fracture mechanics, X-shaped welded joints, centre crack, strength mismatching, plastic yield load, constraint effects
Objavljeno: 01.06.2012; Ogledov: 1088; Prenosov: 28
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10. The fracture behaviour of global/local mis-matched weld joints provided on HSLA steelsInoslav Rak, Nenad Gubeljak, Zdravko Praunseis, 2001, izvirni znanstveni članek Opis: The fracture behaviour of thick section high strength steel weldments, that contain soft root passes, has been studied. Two different weld consumables with different strength mismatch (1>M>1) and fracture toughness properties in the WM have significantly increased the complexity of the mis-match effect and failure behaviour of weld joints, depending on the crack location and the thickness of the soft root layer. The aim was to explain the effect of strength heterogeneity between BM and WM and between different regions in WM (global/local mismatching). R-curves of WM and HAZ regions were also discussed. The conclusion is that the application of a welding procedure with a two-pass soft root layer, introduced for the purpose of reducing or even omitting preheating, can be recommended in mis-matched weld joints on steel grade HT80. Nevertheless, the alloying from BM, and the tempering effect of subsequent weld passes, have to be taken into account. They can cause reduction of the root region ductility and affect the local mismatch in WM and HAZ. The deterioration by providing a soft root layer can probably be reduced by a selected consumable and proper welding procedure. The final conclusion is that the application of a mismatched weld joint with a soft root layer can be recommended only if high root toughness can be provided.
Ključne besede: welding, welded joints, strength mismatching, fracture toughness, CTOD, hardness, impact toughness, HSLA steels
Objavljeno: 01.06.2012; Ogledov: 1318; Prenosov: 79
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