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On the local variation of the crack driving force in a double mismatched weld
Jožef Predan, Nenad Gubeljak, Otmar Kolednik, 2007, izvirni znanstveni članek

Opis: A material inhomogeneity in the direction of crack extension causes a difference between the near-tip crack driving force, Jtip, and the nominally applied far-field crack driving force, Jfar. This difference can be quantified by a material inhomogeneity term, Cinh, which is evaluated by a post-processing procedure to a conventional finite element stress analysis. The magnitude of the material inhomogeneity term is evaluated for cracks in an inhomogeneous welded joint made of a high-strength low-alloy steel. Both a crack proceeding from the under-matched (UM) to the over-matched (OM) and from the OM to the UM weld metal are treated. The effects of the inhomogeneity of the different material parameters (modulus of elasticity, yield strength, and strain hardening exponent) on Cinh and Jtip are systematically studied. The results demonstrate that the material inhomogeneity term is primarily influenced by the inhomogeneity of the yield strength. A crack growing towards an OM/UM interface experiences an accelerated crack growth rate or a pop-in, an UM/OM interface leads to a reduced crack growth rate or a crack arrest. The application of global assessment methods of the mismatch effect which are included in the Engineering Treatment Model (ETM) or in the Structural Integrity Assessment Procedures for European Industry (SINTAP) is discussed.
Ključne besede: crack driving force, material inhomogeneity, mismatched weld, interface, J-integral, finite element modeling
Objavljeno: 31.05.2012; Ogledov: 933; Prenosov: 54
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Charpy toughness and microstructure of vibrated weld metal
Bogdan 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: 1439; Prenosov: 68
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The effect of small flaws on the fatigue strength of HAZ at the weld toe
Vladimir 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: 1069; Prenosov: 47
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Fuzzy logic based quality monitoring in short-circuit gas metal arc welding
Marjan Golob, Arpad Köveš, 2007, izvirni znanstveni članek

Opis: Conventional methods, e.g. destructive and non-destructive testing methods, are expensive and time-consuming; therefore, possibilities of online and automated quality control of a welding process during welding as such are investigated. The paper deals with the possibilities of application of fuzzy logic to the analysis of weld quality, particularly assessment of the weld surface condition by means of measurable electric signals emitted during welding. A simple fuzzy inference system was realised which could relatively efficiently assess the weld quality on the basis of time variations of the welding voltage and short-circuit time in a certain time window.
Ključne besede: fuzzy systems, fuzzy logic, gas metal arc welding, GMAW, quality monitoring, statistical analysis, quality control, weld quality
Objavljeno: 31.05.2012; Ogledov: 1026; Prenosov: 72
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Weldability investigation of TStE 420 after weld thermal cycle simulation
Marko Dunđer, Tomaž Vuherer, Ivica Kladarić, 2010, izvirni znanstveni članek

Opis: The paper presents a short description and overview of the weld thermal cycle simulation methods. An experimental investigation on the weld cycle simulated specimens is predicted by a plan of experiment using single cycle simulation. The obtained hardness results are compared with double cycle simulated specimens results of individual points in the heat-affected zone. Beside that, the characteristic microstructures of points in the heat-affected zone after a single weld thermal cycle simulation are given. A thermal simulation method is suitable for different weldability examinations. In this paper, the authors would like to investigate the maximal hardness of individual points in the weld joint what is important when considering cold cracking sensitivity.
Ključne besede: HSLA steels, weldability, weld thermal cycle simulation, hardness
Objavljeno: 31.05.2012; Ogledov: 840; Prenosov: 18
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Some factors affecting fatigue resistance of welds
Vladimir Gliha, Zijah Burzić, Tomaž Vuherer, 2010, objavljeni znanstveni prispevek na konferenci

Opis: The fatigue strength of polycrystalline metals is hardness and defect size dependent. The fatigue strength of welds depends on the size of grains and actual local mean stress, too. Local mean stress is influenced by the global R-ratio and static preloading. The welds in the as-welded condition are pre-stressed due to the existence of welding residual stresses. The sign and magnitude of residual stresses depend on the welding conditions. Changes of residual stresses, hardness and defect size are experimentally determined in the present work. Samples of studied metals are prepared by using either thermal cycle simulator or laboratory furnace and water quenching. The methods of microstructure preparation and type of load result in actual local R-ratio at the used weld toe models. Specimens are cyclic loaded in the similar way as steel at the weld toe. The bottom of the notch is either defect-free or defected by Vickers indentation. Final results show that coarser grain and higher local R-ratios lower the fatigue strength of welds.
Ključne besede: CrNiMo steels, weld, fatigue resistance, heat-affected zone, small weld defects
Objavljeno: 31.05.2012; Ogledov: 881; Prenosov: 10
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Determination of lower bound fracture toughness of a high strength low alloy steel welded joint
Nenad 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: 1220; Prenosov: 65
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Vloga in nastanek mikrostrukturnih sestavin M-A v zvarnih spojih maloogljičnih visokotrdnostnih konstrukcijskih jekel
Zdravko Praunseis, Masao Toyoda, Alojz Križman, Mitsuru Ohata, 2001, pregledni znanstveni članek

Opis: The existence of martensite-austenite constituents in the weld metal and heat-affected zone seriously reduces the fracture toughness of the welded joint. Therefore, we have investigated the formation of the martensite-austenite constituents when high-strength low-alloy steel is welded with a high heat input or using multi-pass welding. This paper deals with the effects of martensite-austenite constituents on the fracture toughness, the metallurgical features of the martensite-austenite constituents, and the prevention and elimination of the martensite-austenite constituents in the welded joints.
Ključne besede: arc welding, high-strength low-alloy steels, microstructure, martensite-austenite constituents, weld metal, heat-affected zone, cooling rate
Objavljeno: 01.06.2012; Ogledov: 1124; Prenosov: 59
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Wear resistance of coke-crushing hammers
Franc Zupanič, 2013, izvirni znanstveni članek

Opis: This work investigated several methods for improving the wear resistance of the 42CrMo4 tool steel used for coke-crushing hammers. The methods embraced heat treatment for obtaining different combinations of strength and ductility of the bulk steel, and surface-engineering processes comprising TiN thin-film deposition using magnetron-sputtering, nitriding and WC/Ni weld deposit surfaced by gas metal arc (GMA) method. The wear-resistance was tested under industrial conditions. Oil quenching and tempering significantly increased the wear resistance in comparison to the as-received condition. Conversely, nitriding and TiN-hard coating did not show any significant improvement. WC/Ni weld deposit provided the best protection against wear, which was caused predominantly by hard abrasive particles present in the coke.
Ključne besede: zvarni spoj, mikrostrukture, obrabna odpornost, toplotna obdelava, obraba, kladivo, weld deposition, microstructure, impact coke crusher, heat treatment, wear, hammer
Objavljeno: 10.07.2015; Ogledov: 682; Prenosov: 40
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Role of residual stresses on fracture properties of under-matched butt weld
Inoslav Rak, Vladimir Gliha, 1999, izvirni znanstveni članek

Opis: Under-matched weld joints were adopted for producing a penstock of 47 mm wall thickness and 4200 mm diameter, to improve the weldability. The isothermal Robertson wide plate test on the under-matched weld joints, were the allowable stress level was 0.6 of the yield stress, proved that the longitudinal stresses caused the crack propagation alog the weld joint to deviate into tough base material where it was arrested. The arrest temperature was between -10 and -20°C. The behaviour of the crack propagation was the reason to omit the thermal stress relieving of the pipe section and on circumferential weld joints executed on this side. One can always take into consideration the role of axial residual stresses when assessing whether to use, or not, under-matched weld joints in severer loaded structures.
Ključne besede: strjeni zvar z nižjo mejo tečenja, zaostale napetosti, potek loma, lastnosti ujetja, lomna žilavost, under-matched weld joint, residual stresses, fracture path, arrest properties, fracture toughness
Objavljeno: 10.07.2015; Ogledov: 683; Prenosov: 17
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