Miodrag Arsić

API J55 Steel Casing Pipe with an Initial Surface Crack under Internal Pressure - Determination of Fracture Parameters

Seam casing pipe used in an oil drilling rig, manufactured by high frequency (HF) contact welding of API J55 steel, is tested. The influence of an initial defect (machined surface crack) is analysed, by performing pressure test of a pipe segment closed at both ends. Besides the damages at the internal surface, casing pipes are exposed to damage at the external surface, which is why such configuration is analysed here. Measurement of strains and crack mouth opening displacement (CMOD) enabled the application of direct method for J integral evaluation. This procedure is based on the path independence of the J integral and can be applied both in laboratory conditions (on specimens) and on structures. However, it requires a demanding experimental - computational procedure, which is accomplished here using the developed routine. Additionally, the behaviour of the pipe under internal pressure, including fracture mechanics parameters determination, is modelled numerically (by finite element method) in software package Abaqus. The pressure is applied as distributed load acting on the inner surface of the three-dimensional model, and axial tension is applied at the end of the pipe to simulate the closed end. J integral values determined numerically and using direct method are used for estimation of the critical pressure corresponding to the crack growth initiation. Additionally, plastic limit load, i.e. pressure which causes yielding of the ligament, is determined. Based on the results, criteria for pipe integrity assessment are discussed

Ductile fracture prediction of steam pipeline steel

Considering the conditions to which steels used for the manufacture of steam pipelines are exposed, the micromechanism of their destruction in exploitation is exclusively the ductile one. In order to make an estimation of the level of the damage that occurs in exploitation, in this paper a combined experimental and numerical procedure has been developed based on micromechanical or local approach to the fracture mechanics of metallic materials. After the analysis of the results obtained for micromechanical criterion of failure for virgin steel and that used in the steam pipeline, a proposal for prolongation of the working life of tested steel for steam pipelines until the next overhaul is given.

Influence of Crack Length on Ductile Fracture Initiation in Welded Joints with one and Two Weld Metals

Among various micromechanical models for ductile fracture analysis using local approach, those based on the Gurson plastic flow criterion have been intensively developed in the past three decades. However, their application is still subject of many researches, with the aim to improve the assessment of ductile fracture in various, mainly metallic, materials. In this work, Gurson criterion is applied to welded joints produced with one or two different weld metals (one of these is overmatched, while the other is undermatched). Welded single-edge notched bend (SENB) specimens are examined, with an initial pre-crack located in the symmetry plane of each joint. Local approach is chosen for assessment of behaviour of the joints under the external loading, to capture the effect of material inhomogeneity and initial crack length on deformation and ductile fracture initiation in analysed specimens.

Analysis of current state and strength evaluation of the pipeline at hydro power plant 'Pirot'

Hidroelektrana Pirot, koja je izgrađena 1990 godine, je akumulaciona elektrana derivacionog tipa, koja se sastoji od dva nadzemna vertikalna agregata sa Francis turbinama nominalne snage 41,5 MW, proizvedene u Češkoj Republici, tunela i ukopanog cevovoda ukupne dužine 2030 m i prečnika od Ø 3000 mm do Ø 3500 mm. Cevovod debljine lima 22 mm je izrađen od čelika kvaliteta S275J2G3 (stara oznaka Č.0462). Maksimalni pritisak u njemu ispred turbinskog zatvarača je 2,5 MP. Na cevovodu se u kontinuitetu od završetka izgradnje i puštanja u rad sprovode geodetska merenja kada je cevovod prazan i neopterećen hidrostatickim pritiskom i kada je cevovod pun. Pregledom i analizom dobijenih podataka o pomeranjima duz trase cevovoda utvrđeno je da od 2003-2004 godine postoje znatno veće razlike u pomeranjima za pun i prazan cevovod, nego u prethodnom periodu. U radu je izvršena analiza trenutnog stanja i ocena čvrstoče cevovoda kao celine na osnovu rezultata ispitivanja sučeonih zavarenih spojeva i ukrsnih masta metodama bez razaranja u zoni preloma na stacionaži dužini cevovoda 1+263m (vizuelno ispitivanje, ispitivanje magnetnim česticama, ispitivanje penetrantima, ispitivanje ultrazvukom, radiografsko ispitivanje, metalografsko ispitivanje metodom replika). Abstract

Experimental examination of fatigue life of welded joint with stress concentration

This paper presents results of experimental examinations of stress concentration influence to fatigue life of butt welded joints with K-groove, produced from the most frequently used structural steel S355J2+N. One group of experiments comprised examinations carried out on the K-groove specimens with stress concentrators of edged notch type. Specimens with short cracks (limited length of initial crack), defined on the basis of the experience from fracture mechanics by the three points bending examinations, have been examined according to standard for the determination of S-N curve, and aimed to determine fatigue strengths for different lengths of initial crack and Relationship between fatigue strength and crack length. Other group of experiments comprised examinations of specimens with edge notch, prepared in accordance with ASTM E 399 for three points bending, in order to establish regularity between crack growth and range of exerted stress intensity factor aimed to determine resistance of welded joint to initial crack growth, namely fatigue threshold

Integrity and life estimation of turbine runner cover in a hydro power plant

This paper presents integrity and life estimation of turbine runner cover in a vertical pipe turbines, Kaplan 200 MW nominal output power, produced in Russia, and built in six hydro-generation units of hydroelectric power plant „Đerdap 1” in Serbia. Fatigue and corrosion-fatigue interaction have been taken into account using experimentally obtained material properties, as well as analytical and numerical calculations of stress state, to estimate appropriate safety factors. Fatigue crack growth rate, da/dN, was also calculated, indicated that internal defects of circular or elliptical shape, found out by ultrasonic testing, do not affect reliable operation of runner cover.

Assessment of the integrity of welded pipes

The subject of the paper is analysis of the integrity of welded pipes made of API J55 steel by high frequency contact welding (HF). Experimental research on the mechanical properties of the base material was conducted on pipes withdrawn from exploatation after 70 000 hours at service. Defect influence of the surface crack on the integrity of pipes was tested using hydrostatic pressure of pipes with axial surface crack in the base material. Fracture behaviour was tested using modified compact specimen (CT), with the initial crack in the base material, welded joint and heat affected zone (HAZ). Critical value of the tensile strength factors KIc was determined based on the critical value J of the integral JIc. Apart from the experimental research, based on the derived values of Kr and Sr and by applying fracture analysis diagram (FAD) an assessment of the integrity of welded pipes with axial surface crack on the outer surface area was conducted.

Investigation of the Repair Welding Technology Using Ni Base Electrode

Metal materials are subjected to innumerable time-dependent degradation mechanisms when operate in power, petrochemical and refinery plant. These materials are subjected to multiaxial stresses, creep, fatigue, corrosion and abrasion. As a result of service especially at high temperatures and high pressures, can lead to forming cracks, damages or failures. In situation of breakdown in such systems there is a need for weld repair on plant components and repair work can be expensive and time-consuming. Most weld repairs of low alloy steels require high-temperature post weld heat treatment (PWHT); but in certain repairs, however, this is not always possible. Expenses of the repair work could be reduced if the weld repairing is performed on site. Application of the nickel based filler metal can be alternative to performing PWHT. These repair welding procedures with Ni based filler metal could be categorized as cold repair welding. Purpose of presented investigation was to compare a repair welding technology with filler austenite material based on Ni and without application of the PWHT, with a classical repair welding procedure with preheating and PWHT and using a filler metal with chemical composition similar to parent metal. Properties comparison of the welded joints obtained by these two repair welding technologies was performed for the Cr-Mo steel (13CrMo4-5) by the metal arc welding procedure with covered electrode (MMA - 111). Weldability analysis by the analytical equations and technological tests for determination of the sensitivity to crack forming for cold and hot cracks by the CTS and Y tests, were performed for both repair welding technologies. Tensile tests, absorbed energies tests, banding tests and hardness measurements were performed on trial joins. Light optical microscopy (LOM) was applied for microstructure analysis. The fracture toughness for both technologies, were estimated by the calculated stress intensity factor KIc and dynamic stress intensity factor KId for weld metal and heat affected zone. All of the obtained results were analyzed and discussed. It was concluded that repair welding technology with Ni base filler material without PWHT, enables welded joints without the appearance of cracks, with a good mechanical properties, slightly higher hardness in the HAZ, but with lower expenses compared to standard repair welding technology. In applying this technology in emergency welding repairing on-site, on the equipment and industrial facilities with high security requirements, inspection using non destructive technique has to be frequently applied compared to standard procedures.

State Analysis and Integrity of Welded Structures of the Upper Ring of the Turbine Runner Guide Vane Apparatus of Hydroelectric Generating Set A6 on Hydro Power Plant Djerdap 1

During the refurbishment of the hydroelectric generating set A6 on HPP Djerdap 1, for the state analysis and assessment of the level and causes of degradation of the structure of the vertical Kaplan turbine runner guide vane apparatus non-destructive tests and inspections, as well as destructive tests of base material and welded joints, have been carried out after 40 years of service. Results of non-destructive tests performed on welded joints are presented in this paper (magnetic particle and ultrasonic inspection), as well as results of destructive tests of the base material and welded joints (testing of tensile properties, impact testing, metallographic analyses of the structure). A large number of crack type line indications were detected by magnetic particle inspections, while the occurrence of partial penetration in roots of welded joints was detected by ultrasonic inspections, as well as lamellar tearing of the base material. Tensile properties of tested samples taken in longitudinal and transverse direction fulfilled the requirements of the standard with certain deviations, which does not stand for samples taken in z-direction with significantly lower values of contraction of the cross-section than minimum prescribed values, which proves that base material is not resistant to the occurrence of lamellar cracks. Metallographic tests performed on specimens taken from the base material in longitudinal and transverse direction showed that the microstructure is stripe-shaped and ferrite-pearlite with non-metallic inclusions of oxide, sulphide, silicate and aluminate type, while metallographic tests performed on specimens taken from welded joints showed that their microstructure is stripe-shaped and ferrite-pearlite with non-metallic inclusions of oxide type. A large number of micro-and macro-cracks were detected in the microstructure as well. Experimental tests enabled the determination of the causes of occurrence of lamellar tearing in base material and crack type defects in welded joints, while analytical calculation that refers to the stress state enabled integrity of welded structure of the upper ring of the turbine runner guide vane apparatus.