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Cavitation resistance of welded joints of the three-component aluminum alloy AlMg4.5Mn was examined in this paper. Tests were performed on samples of welded joints as well as on samples of the base material. Welding was performed by TIG and MIG procedures in a protective atmosphere of pure argon. After the welding, metallographic tests were performed. The ultrasonic cavitation test method with a stationary sample was used to test the resistance to cavitation according standard ASTM G32 [1]. During the test, the change in mass of the samples was monitored in order to determine the cavitation rate. The least squares method was applied, where the slope of the straight line determines the cavitation rate (Fig.1). The base material of the AlMg4.5Mn alloy has the highest resistance to the effect of cavitation (the lowest total mass loss and the lowest cavitation rate). The welded joint obtained by the MIG process has a higher resistance to the effect of cavitation compared to the sample obtained by the TIG process. This behavior is a consequence of the smaller porosity and finer microstructure of the sample obtained by the MIG process (Fig. 1). The morphology of damage to the surface of samples exposed to the effect of cavitation during testing was monitored using scanning microscopy. It was shown that ductile fracture mechanism was identified for all studied samples together with the presence of small fatigue like craters.

The effect of the shielding gas arc welding process on the cavitation resistance of the three-component aluminum alloy AlMg4.5Mn and its welded joints was investigated. Welding was performed using the GTAW and GMAW processes in a shielded atmosphere of pure argon. After the welding, metallographic tests were performed, and the hardness distribution in the welded joints was determined. The ultrasonic vibration method was used to evaluate the base metal’s and weld metal’s resistance to cavitation. The change in mass was monitored to determine the cavitation rates. The morphology of the surface damage of the base metal and weld metal due to cavitation was monitored using scanning electron microscopy to explain the effect of the shielding gas arc welding process on their resistance to cavitation.

This paper deals with a three-component aluminum alloy AlMg4.5Mn that was welded using a GTAW process in the shielded atmosphere of Ar+70%He+0.015%N2. The weld-metal toughness was evaluated at three different temperatures using instrumental Charpy pendulum impact testing to measure not only the total energy, but also the crack initiation energy and the crack growth energy. Fractographic analysis of the fracture surfaces and EDS analysis of large second-phase particles on fractured surfaces at each temperature were also carried out. Fractographic analysis at different temperatures indicated a clearly distinguishable fracture mechanism. It was inferred that the absorbed energy was closely correlated with the fracturing of surfaces. Moreover, it was concluded that with decrease in the amount of microscopic voids and dimples, the total energy absorbed also decreased.

Predgovor Ovaj udžbenik je namenjen studentima koji slušaju predmet Mašinski materijali 1, na osnovnim akademskim studijama u drugom semestru sa fondom časova 1+1, a napisan je prema nastavnom planu i programu. Osnovna namena ovog udžbenika je da pomogne studentima u savladavanju osnovnih znanja iz nauke o materija- lima, koja su im potrebna u toku daljeg studiranja. Pred autorima je bilo velikih dilema kako, da na jasan način izlože veoma komplikovanu i raznovrsnu materiju, imajući u vidu da prethodno stečeno srednjoškolsko znanje studenata, iz fizike i hemije, nije ujednačeno. Nakon uvodnih razmatranja o istorijskom razvoju i podeli materijala, prelazi se na strukturu atoma i veze između atoma, kao i na kristalnu strukturu materijala i greške u kristalnoj strukturi. Objašnjen je mehanizam difuzije u čvrstom stanju, mehanička svojstva materijala, kao i oštećenja i vrste lomova u materijalima. Takođe je razmatrana teorija legura i osnovne vrste dvokomponentnih dijagrama stanja. Nastavnim planom i programom je predviđeno da se osim metalnih materijala razmatraju, u manjoj meri, polimeri, keramike i kompozitni materijali. Imajući u vidu stalni razvoj materijala kao i pojavu novih inženjerskih materijala, svakako se vodilo računa da se i njima posveti odgovarajuća pažnja. Na kraju knjige navedena je i literatura koja može da se koristi u cilju proširenja znanja iz oblasti nauke o materijalima. Tokom pisanja ove knjige ideja je bila da se materija izloži u što jasnijem, a donekle, i popularnom stilu da bi bila dostupna i razumljiva studentima sa manjim obimom predznanja iz ove oblasti. Zbog toga, strogo se vodilo računa da se, gde god je to moguće, reči zamene dobrom slikom ili dijagramom. Ukoliko uspešno savladaju ovo gradivo studenti mogu biti sigurni da će imati solidnu osnovu za savladavanje gradiva iz određenih predmeta tokom studiranja, kao i da će u praksi primeniti stečeno znanje. Autori se zahvaljuju recenzentima i kolegama na korisnim primedbama i sugestijama.

This paper presents a review of different approaches through history concerning DTB transition phenomenon of ferritic steels and their characterization using FM concept, from the earliest studies based on LEFM to the application of the EPFM concept. The large scattering of the experimental fracture toughness data, characteristic of all ferritic steels in the transition temperature region, has imposed the need of including statistical methods for data processing. Such approach that began in the 1970s can be encountered even nowadays as the base of fracture toughness data interpretation in DTB problems. An overview of studies with statistical interpretation of experimental data in the transition temperature region is also given. Aforementioned provides a foundation for novel approaches in DTB transition problems, which include size effects and scaling of geometrically similar specimens.

Tema ove doktorske disertacije je feritni reaktorski čelik 20MnMoNi 55, konkretno njegova žilavost u području prelaznog temperaturnog režima najviše zbog njegove primene u ekstremnim uslovima, odnosno niskim temperaturama. Ispitivanja su urađena na C(T) uzorcima različitih dimenzija i tempraturama -60 i -90℃, sa ciljem dobijanja krivih verovatnoća loma cepanjem ispitivanih manjih C(T) epruveta i predikcije ponašanja većih uzoraka na ispitnim temperaturama statističkim metodama (konkretno, Vejbulovom raspodelom) obrade dobijenih rezultata. Kumulativnim raspodelama verovatnoće loma cepanjem i predikcijama ponašanja većih uzoraka u funkciji parametra mehanike loma Jc direktno je prikazan uticaj veličine uzoraka na integritet konstrukcije izrađene od ovog materijala i njegovu sposobnost ili težnju ka žilavom ili krtom lomu usled prisustva prsline. Pored šire slike ponašanja feritnog reaktorskog čelika 20MnMoNi 55 na ispitnim temperaturama -60℃ i -90℃ preko parametara Jc i drugih analiziranih relavantnih parametara, ovom disertacijom pružen je dodatni uvid uticaja dinamičkog opterećenja pripremljenih uzoraka na dobijene rezultate preko plastične zone oko vrha prsline i analizom rasipanja dobijenih rezultata. Dobijene su i analizirane određene zakonitosti uočenih pojava, poput potencijalne temperaturne zavisnosti Vejbulovih parametara kumulativne raspodele za C(T)50 uzorke na dvema ispitnim temperaturama. Skaliranjem određenih eksperimentalnih rezultata ispitivanja dobijena je empirijska zakonitost u cilju analize uticaja veličine geometrijski sličnih C(T) uzoraka na temperaturi -60℃. Cilj skaliranja je sveobuhvatno predviđanje ponašanja uzoraka većih dimenzija na osnovu ispitivanja manjih., The topic of this doctoral thesis is ferritic reactor steel 20MnMonNi 55, i.e. its toughness in transition-temperature region, tipically used in application for structures working at low temperature. Tests were performed on the C(T) specimens with different geometry and on -60 and -90℃ in order to get cleavage fracture probabilities curves of small specimens and to predict behavior of the larger ones at specific temperatures. Method used for probability prediction is Weibull statistic method of results processing. The cumulative distributions of cleavage fracture probability, as well as obtained behaviour predictions of the larger specimens in the function of fracture mechanics parameters Jc, show the influence of specimen size effect on the integrity of real structure made of this steel, and its ability and tendency towards ductile or brittle fracture in the presence of the crack. In addition, experimental testing in this thesis provided a broad picture of ferritic reactor steel 20MnMoNi 55 behavior at temperatures -60℃ and -90℃ by analyzing Jc and other relevant parameters. This dissertation provides additional insight into the influence of dynamic loading via specimens pre-cracking and plastic strain around the specimen crack tip by analyzing the obtained scattered results and parameters. Certain relations of observed phenomena were analyzed and discussed, such as the potential temperature dependence of the Weibull parameters of the cumulative distribution for C(T)50 specimes at two temperatures. By scaling of certain experimental results with the aim of determining of size effect of geometrical similar C(T) specimens, the empirical relation for temperature of -60℃ was obtained. The goal of scaling is to comprehensively predict the behavior of larger specimens based on the examination of smaller ones.

Priručnik za lab.ispitivanja iz Mašinskih materijala 1 i Mašinskih materijala 2

Predgovor Predmeti koji nas okružuju izrađeni su od različitih vrsta materijala, pa je poznavanje njihovih osobina, načina proizvodnje, postojanosti u radnom okruženju, mogućnosti nabavke i cene od velikog značaja pri izboru materijala za konkretnu namenu. Tendencije savremenog načina života su usmerene ka smanjenju potrošnje energije i zaštite životne sredine, pa se pred inženjere, projektante i konstruktore postavljaju zahtevi da izaberu materijal koji ima što manju cenu i masu uz istovremeno zadovoljavajuće fizičke, hemijske, mehaničke, tehnološke ili neke specifične osobine. U svetu se dosta novca ulaže u istraživačke i razvojne projekte za razvoj novih materijala ili modifikaciju osobina postojećih materijala. Ovaj udžbenik je namenjen studentima Osnovnih akademskih studija koji slušaju predmet Mašinski materijali 2 sa fondom časova 5 u trećem semestru na Mašinskom fakultetu Univerziteta u Beogradu. Gradivo u ovom udžbeniku prati kontinuitet gradiva izloženog u knjizi Mašinski materijali 1, pa je za praćenje nastave iz predmeta Mašinski materijali 2 neophodno predznanje iz tog predmeta. Cilj autora je bio da ovako složenu problematiku što jednostavnije prikažu, a da se pri tome održi odgovarajući akademski nivo. Uvek, kada je to bilo moguće, autori su se potrudili da odgovarajuće objašnjenje ilustruju dobrom slikom. Plan i program predmeta Mašinski materijali 2 se sastoji iz dve celine i to: metalni materijali i postupci spajanja materijala zavarivanjem i lemljenjem. U prvom delu udžbenika opisani su metalni materijali, njihov način dobijanja, kao i fizička, hemijska i mehanička svojstva. Od metalnih materijala najviše pažnje je posvećeno železu i njegovim legurama jer su to još uvek najzastupljeniji konstrukcioni materijali u mašinstvu. Železo gradi dve grupe legura: čelike i gvožđa; pa je opisan način njihovog dobijanja, mehaničke i fizičke osobine, kao i njihovo označavanje po najnovijem standardu. Takođe su opisane neravnotežne transformacije pri hlađenju č

Predmet ove doktorske disertacije je istraživanje ponašanja zavarenih konstrukcija sa smanjenim stepenom sigurnosti prema krtom lomu i otpornost na prsline u cilju ocene njenog integriteta i veka. U tom slučaju posebna pažnja se posvećuje mestima koncentracije napona, jer se krti lomovi ovih konstrukcija po pravilu događaju upravo na takvim mestima. Radi detaljnog proučavanja nakstanka i rasta prsline, neophodna je primena principa mehanike loma, jer klasični pristup konstruisanju (projektovanju) preko faktora koncentracije napona i čvrstoće u tom slučaju nije dovoljan. Primenom parametra mehanike loma, kao što je faktor inteziteta napona, čiji opseg definiše brzinu rasta prline, a čija kritična veličina (žilavost loma) određuje kritičnu dužinu prsline. Time se obezbeđuje pouzdana procena integriteta konstrukcije (ako se dokaže da je faktor intenziteta napona manji od žilavosti loma), odnosno procena veka konstrukcije (primenom zakona mehanike loma na izračuvavanje krritičnih parametara za poznatu ili pretpostavljenu početnu dužinu prsline, njenu kritičnu vrednost, kao i izračunati opseg faktora intenzitata napona). Konzervativni pristup u svim koracima ove procedure obezbeđuje pouzdanu eksploataciju konstrukcije sa smanjenim stepenom sigurnosti., The subject of this doctoral dissertation is the research of the behavior of welded structures with a reduced degree of safety on brittle fracture and crack resistance in order to assess its integrity and age. In this case, special attention is paid to the stress concentration points, since the fractures of these structures tend to occur exactly in such places. For the detailed study of the crack and growth of the crack, it is necessary to apply the principle of fracture mechanics, since the classical approach to design (design) through the factor of stress concentration and strength is not sufficient in that case. Using the fracture mechanics parameter, such as the voltage intensity factor, whose range defines the rate of crack growth, and whose critical size (fracture toughness) determines the critical crack length. This ensures a reliable assessment of the integrity of the structure (if it is proven that the stress intensity factor is less than the tensile strength of the fracture), or the assessment of the century of construction (by applying laws of fracture mechanics to calculate critical parameters for known or assumed initial crack length, its critical value, as well as calculate the range voltage intensities factor). A conservative approach in all steps of this procedure ensures reliable exploitation of a structure with a reduced degree of safety

Surface welding of rail steel with self-shielded wire was conducted with different heat inputs, and the influence of welding heat input on the total impact energy and its components, crack growth rate and fracture mechanism was systematically investigated. It is shown that toughness decreases as heat input increases, but, with a temperature decrease, these differences are not so marked. An increase of heat input leads to increasing the share of transgranular brittle fracture, what is in complete accordance with the obtained energy values. It is established that fatigue life increases when increasing the welding heat input, while resistance to crack growth decreases in the final deposit layer up to the HAZ at all heat inputs. Based on obtained results, the optimal value of heat input is defined for the selected welding procedure.

Zavarivanje se široko primenjuje u industriji pri izradi različite opreme, mostova, kranova, transportnih sredstava i sl. Ono ima odlučujući uticaj na kvalitet, pouzdanost, vek i cenu izrade navedenih konstrukcija. Međutim, kvalitet izvedenih zavarenih spojeva je teško ili je često gotovo nemoguće utvrditi ispitivanjima na samim konstrukcijama. Zbog toga je potrebno praćenje toka izrade zavarenih konstrukcija od ugovaranja i projektovanja, preko kvalifikacije i sertifikacije osoblja i opreme, kvalifikacije procedura zavarivanja i kontrole tokom i nakon završene izrade konstrukcije. Zahtevi za kvalitet koje treba ispuniti u pojedinim fazama izrade konstrukcija definisani su u standardima serije EN ISO 3834. Odstupanja izvedenog kvaliteta zavarenih spojeva od potrebnog najčešće nastaju u fazi zavarivanja. Do sada nije postojala mogućnost za kontinualno praćenje, beleženje i skladištenje podataka o parametrima zavarivanja. U radu je prikazan sistem zasnovan na informacionim tehnologijama, koji omogućava prikupljanje i beleženje podataka o parametrima zavarivanja, upotrebljenim materijalima i angažovanim zavarivačima, njihovo poređenje sa zadatim vrednostima i obustavljanje zavarivanja u slučajevima utvrđenih odstupanja. Na taj način se znatno poboljšava kvalitet i pouzdanost zavarenih konstrukcija.

Stabilnost procesa zavarivanja ima značajan uticaj na postizanje propisanog nivoa kvaliteta zavarenog spoja. Povećanje stabilnosti utiče na smanjenje mogućnosti pojave nesavršenosti, odnosno grešaka u samom spoju. Stabilnost procesa zavarivanja se ogleda u stabilnosti električnog luka. Promene dužine luka tokom zavarivanja utiču na varijaciju parametara zavarivanja, tačnije na jačinu struje i napon. Na osnovu praćenja parametara zavarivanja u realnom vremenu može se izvršiti statistička analiza dobijenih rezultata. U radu je prikazan uticaj brzine dovođenja žice i vrste zaštitne atmosfere na stabilnost MIG/MAG postupka zavarivanja pri upotrebi metalne punjene elektrodne žice.

Smanjenje otvora žljeba može značajno doprineti efikasnosti zavarivanja i produktivnosti teških metalnih konstrukcija Uobičajena je upotreba uglova žljeba od 45° i 60°. Smanjenje ugla otvora žljeba smanjuje potreban broj zavara, čime se smanjuje trajanje zavarivanja, potrošnja materijala za ispunu i zaostali naponi i deformacije. Međutim, smanjivanje ugla žljeba dovodi do određenih poteškoća. Teško je kontrolisati luk u uskom žljebu, što može dovesti do problema sa kvalitetom spoja. U ovom radu je prikazano zavarivanje sučeonih spojeva od čelika debljine 22 mm sa uglom žljeba od 20°. Zavarivanje je izvršeno korišćenjem MAG postupkom zavarivanja, uz softversku kontrolu luka., Reduction in groove opening can significantly contribute to welding efficiency and productivity of heavy metal structures. It is common practice to use welded joint groove angles of 45° and 60°. Reduction of the groove opening angle decreases the necessary number of welding passes, thus decreasing the welding duration, filler material consumption and residual stresses and strain. However, groove angle reduction leads to certain difficulties. It is hard to control the arc in a narrow groove, which could lead to welded joint quality issues. In this paper, welding of butt joints made of 22 mm thick steel with the groove angle of 20° is shown. Welding was performed using the MAG procedure, with software controlled ar.

In this paper is presented behavior of microalloyed steel weld metal with niobium during impact testing, as well as microstructural characterization. Impact tests were done on instrumented Charpy pendulum at room temperature, at -40 degrees C and -55 degrees C. At room temperature, crack propagation energy is much higher than crack initiation energy, while at -40 degrees C and -55 degrees C crack growth energy is lower than crack initiation energy. Fractographic investigation of the fracture surfaces has shown that at room temperature ductile trans-granular fracture is dominant, with a small amount of brittle trans-granular fracture. At lower temperature, the share of brittle fracture is increased, and inter-granular brittle fracture becomes dominant

Surface welding is a way to extend the exploitation life of damaged parts and constructions and the heat input has a major influence on the weldment properties. In this paper is shown the influence of the heat input on the toughness and the fracture mechanism of the surface welded joint. Surface welding of high carbon steel with self shielded wire was conducted with three different heat inputs (6kJ/cm, 10 kJ/cm and 16 kJ/cm). Total impact energy, crack initiation and crack propagation energy were estimated at room temperature, -20 degrees C and -40 degrees C. Fracture analysis of fractured surfaces was also conducted and it has been found that increasing of heat input leads to an increase of share of transgranular brittle fracture, what is in complete accordance with the obtained energy values. Based on all obtained results, the optimum value of heat input for welding procedure applied was defined.

Последњих година спроводе се значајна истраживања, која се односе на продужење животног века радних делова термоенергетских постројења. Опрема у термоелектранама је изложена дуготрајном дејству високих температура и притисака, хабању и цикличном оптерећењу. У наведеним условима експлоатације различити узроци могу довести до оштећења, појединих компонената као што је хабање радних делова вентилационог млина. Последице су смањење производног капацитета и вентилационог дејства млина у односу на пројектовану вредност, као и чести застоји због замене делова, што значајно утиче на продуктивност, економичност и енергетску ефикасност термоелектране. Уколико се констатује оштећење компоненте, које може довести до поремећаја у раду постројења, у пракси постоје три могућности: да се она остави у погону уз редовну и чешћу контролу, да се изврши ревитализација компоненте или да се замени потпуно новом компонентом. Одлука се доноси на основу техничких, технолошких и економских критеријума у зависности од могућности набавке новог дела, цене замене или ревитализације, могућности заустављања постројења и аспеката сигурности и заштите околине. У овиру ове докторске тезе пажња је усмерена на анализу радних услова и израду процедуре за продужење радног века делова вентилационог млина у Термоелектрани Костолац Б, поступцима наваривања и метализације. Тачније, предмет ових истраживања ће бити да се на основу техничко – технолошке анализе рада и стања компоненти вентилационог млина, које су доминантно изложене абразивном и ерозивном хабању, изврши избор радног дела, у овом случају усисних плоча, као и карактеризација основног материјала. Први део дисертације је усмерен и на примену савремених метода нумеричке симулације вишефазног струјања у вентилационом млину у циљу детаљније анализе услова хабања (проблема) везаних за појаву абразије и ерозије изабране компоненте. На овај начин, применом софтверског пакета ANSYSFLUENT 12, добиће се детаљнији приказ мултифазног струјања и брзине мешавин, In recent years implemented significant research, concerning the extension of the life of the working parts of a thermal energy plant. Equipment in thermal power plants exposed to long-term effect of high temperatures and pressure, abrasion and load cycle counting. In these circumstances the exploitation of different causes can lead to damages, the individual components such as the wear and tear of working parts of an air mill. The consequences of the reduction in production capacity and air operations of the mill compared to the projected value, as well as frequent delays due to the replacement of parts, which significantly affects the productivity, profitability and energy efficiency of thermal power plants. If is detected damage to components, which can lead to disturbances in the plant, in practice there are three possibilities: that it be left in the drive with a regular and frequent control, to carry out a revitalization component or to replace entirely new component. The decision is made on the basis of technical, technological and economic criteria, depending on the possibilities for acquiring new works, replacement rates or revitalization, the possibilities of stopping the plant and aspects of safety and environmental protection. As part of this doctoral thes is in focus on the analysis of working conditions and create procedures for the extension of the service life of parts of the ventilation mill in thermal power plant Kostolac B, surfacing and metallization processes. More specifically, the subject of this research will be that based on the technical - technological analysis of the work and the state of the components of the ventilation mills, which are predominantly exposed to abrasive and erosive wear, make the selection of the working part, in this case the suction plate and the characterization of the base material. The first part of the dissertation will focus on the use of modern methods of numerical simulation of multiphase flow in ventilation mi

The problems related to welding of the high strength steel, aimed for manufacturing of welded structures operating in the prominent wear conditions, is considered in this paper. The paper presents an analysis of possibilities and prescribing the technology for welding the high strength steel HARDOX 450. The methodology for estimate the weldability of this steel was established in the theoretical part of the paper, as well as calculations of the welding parameters, while the hardness was investigated in details and the macro and micro structures of the individual welded joint zones were estimated in the experimental part. Obtained experimental results could be usefully applied for selecting and establishing the optimal welding technology for structures made of this steel. The technology thus established and the GMAW welding regime can enable successful execution of the welded joints and reduce the possibility for appearance of flaws and cracks to a minimum.