Selecting hardfacing technologies for ventilation mill suction plates and extending its working life

This paper presents results of the suction plate surface modification by hardfacing, of the ventilation mill in the Kostolac B power plant. Before applying hardfaced suction plates to exploitation conditions, it is necessary to study model hardfaced samples for determining the optimal tech-nology and filler material for revitalization. Experimental tests of revitalized mill suction plates in real exploitation conditions show that the proposed modification, hardfacing technologies and filler materials give good results. The relative weight loss of the suction plates after 1440 h period of exploitation in real conditions is 1-3%. The application of this approach can reduce the number of possible repairs and extends the period between them, resulting in significant economic effects

Selection of the optimal filler material for on-site repair welding of the turbine shaft at the hydropower plant

Najznačajniji delovi hidroelektrana, vratila turbina izložena su tokom rada visokim naprezanjima, puzamju, zamoru i koroziji. Zbog toga je korisno preduprediti oštećenja tokom rada, povećati pouzdanost i produžiti radni vek postrojenja. Eksploatacioni uslovi mogu da dovedu do prslina i loma u homogenom materijalu, tako da postoji potreba za reparaturom dela postrojenja. A ona može biti skupa ili dugotrajna. U okolnostima kada se reparatura izvodi na licu mesta, posebno pažljivo treba da se definiše tehnologija reparaturnog zavarivanja, parametri postupka, i izbor dodatnog materijala. U ovom istraživanju izbor dodatnog materijala je bio tema i dati su rezultati. Vratilo je izrađeno od čeličnog liva 20Mn5 i na njemu su metodama IBR, otkrivena oštećenja tipa prslina dužine do 400 mm i dubine do 20 mm. Kako je planirano da se reparatura izvede bez demontaže vratila, primena termičke obrade nije bila izvodljiva. Analiza zavarljivosti osnovnoog materijala urađena je korišćenjem analitičkih jednačina. Rezultati su pokazali da je zavarljivost ovog čelika ograničena. Za reparaturno zavarivanje izabran je postupak REL (111) i dve obložene austenitne elektrode sa ciljem određivanja pogodnosti za upotrebu kao potencijalnih dodatnih materijala za reparaturu. U ovom istraživanju ispitane su Fe-Ni-Cr-Mo elektroda i elektroda na bazi Ni i njihove osobine su upoređene. Ispitivanje zatezanjem, ispitivanje apsorbovane energije, ispitivanje savijanjem, merenje tvrdoće i ispiivanje osetljivosti kao što su 'CTS' i 'Y' probe, izvedeni su na ispitnim spojevima dobijenim zavarivanjem osnovnog materijala, 20Mn5, ispitivanje savijanjem, merenje tvrdoće i ispiivanje osetljivosti kao što su 'CTS' i 'Y' probe izvedeni su na ispitnim spojevima dobijenim zavarivanjem osnovnog materijala, čelika 20Mn5, sa ispitivanim dodatnim materijalima. Dobijeni rezultati su analizirani i na osnovu toga, odabrana je elektroda na bazi Ni kao pogodnija za reparaturno zavarivanje.The most significant components of hydropower plants are turbine shafts and generator rotors which undergo time-dependent processes such as high stresses, creep, fatigue and corrosion. It is therefore desirable to prevent in-service damages, improve reliability and extend the operational life of the plant. Plant operation can lead to cracking and failures in homogeneous materials, therefore a need for repair welding on plant components, which can be expensive and time-consuming, exists. In the circumstances when repair welding has to be carried out on site, special care has to be taken in defining repair welding technology, process parameters and selection of filler materials. In this research a selection of filler materials for repair welding on site of hydropower turbine shaft was performed and results were presented. The shaft was made of the cast steel 20Mn5 and presence of damages in the form of cracks which were up to 400 mm long and up to 20 mm deep was detected through the use of NDT methods. As the repairing was planned to be carried out without disassembling of the shaft, application of heat treatment procedures was not feasible. Weldability analysis of the base material was performed through the use of analytical equations. Results have shown that weldability of this steel is limited. For the repair welding a MMA (111) welding process was selected and two covered austenitic electrodes were analyzed in order to establish the feasibility of their use as potential filler material for repairing. In this research a Fe-Ni-Cr-Mo electrode and Ni based electrode were tested and their properties were compared. Tensile testing, absorbed energies tests, bending tests, hardness measurements and sensitivity tests such as 'CTS' and 'Y' were performed on test joints obtained by welding of base material, 20Mn5 steel, with investigated filler materials. Obtained results were analyzed and a Ni based electrode was selected as most suitable for repair welding

Selecting hardfacing technologies for ventilation mill suction plates and extending its working life

This paper presents results of the suction plate surface modification by hardfacing, of the ventilation mill in the Kostolac B power plant. Before applying hardfaced suction plates to exploitation conditions, it is necessary to study model hardfaced samples for determining the optimal tech-nology and filler material for revitalization. Experimental tests of revitalized mill suction plates in real exploitation conditions show that the proposed modification, hardfacing technologies and filler materials give good results. The relative weight loss of the suction plates after 1440 h period of exploitation in real conditions is 1-3%. The application of this approach can reduce the number of possible repairs and extends the period between them, resulting in significant economic effects

The tribological performance of hardfaced/ thermal sprayed coatings for increasing the wear resistance of ventilation mill working parts

During the coal pulverizing, the working parts of the ventilation mill are being worn by the sand particles. For this reason, the working parts are usually protected with materials resistant to wear (hardfaced/thermal sprayed coatings). The aim of this study was to evaluate the tribological performance of four different types of coatings as candidates for wear protection of the mill’s working parts. The coatings were produced by using the filler materials with the following nominal chemical composition: NiFeBSi-WC, NiCrBSiC, FeCrCTiSi, and FeCrNiCSiBMn, and by using the plasma arc welding and flame and electric arc spraying processes. The results showed that Ni-based coatings exhibited higher wear resistance than Fe-based coatings. The highest wear resistance showed coating produced by using the NiFeBSi-WC filler material and plasma transferred arc welding deposition process. The hardness was not the only characteristic that affected the wear resistance. In this context, the wear rate of NiFeBSi-WC coating was not in correlation with its hardness, in contrast to other coatings. The different wear performance of NiFeBSi-WC coating was attributed to the different type and morphological features of the reinforcing particles (WC)

Development of welding technology for high strength dual hardness Cr-Mo-Ni steel

Višeslojni metalni materijali su nastali kao rezultat specifičnih potreba ili sa ciljem dobijanja kvalitetnog, a jeftinijeg materijala. Posebnu grupu čine višeslojni čelici, koji su našli primenu za izradu sečiva u grafičkoj industriji, u hemijskoj industriji za izradu delova postrojenja, kao i u vojnoj industriji. O problematici zavarivanja višeslojnih čelika ima relativno malo podataka i oni se uglavnom odnose na zavarivanje kombinacije višeslojnih niskolegiranih i nerđajućih čelika, dok o zavarivanju višeslojnih čelika visoke čvrstoće skoro i da nema raspoloživih podataka. U radu su prikazani rezultati istraživanja sa ciljem osvajanja tehnologije zavarivanja dvoslojnih čelika visoke čvrstoće i dobijanja kvalitetnog zavarenog spoja sa optimalnim mehaničkim i strukturnim osobinama. Ispitivanja su vršena na dvoslojnim čelicima, debljina 8 i 11mm, tvrdoća 60 i 45 HRc, sa odnosom debljina tvrdog i plastičnog dela 40:60. U okviru obavljenih istraživanja definisani su optimalni parametri elektrolučnog zavarivanja i dodatni materijal i izvršena je strukturno mehanička karakterizacija zavarenih spojeva.Multi layer metal materials, are developed as a result of specific requirements or/and to obtain lower price metal material with high performances. Special group of multilayer materials are multilayer steels which are applied for cutters in graphical industry, in chemical industry for process equipment and for military purposes. Process data about technologies for welding of multilayer steels, are very exceptional in literature. Most data are treated welding of clad carbon steels with stainless steel. For welding high strength dual hardness steels, technology data practicality not exists in literature. In the paper are presented results of investigation of welding technologies development for high strength dual hardness steels with aim to obtain high quality welded joint with optimal mechanical and structural properties. Investigations were performed on two layer steel plates of 8 and 11 mm in thickness, with dual hardness of 60 and 45 HRc and thickness ratio of 40:60. The optimal welding parameters for arc welding process were defined. Testing of mechanical and structural properties of welded joint was performed

Microstructural analysis as the indicator for suitability of weld repairing of the heat resistant Cr - Ni steel

Metallurgical evaluation was performed on a fractured column tube of the reformer furnace in an ammonia plant. The tubes were manufactured from centrifugally cast heat resistant steel HK 40. Optical and scanning electron microscope (SEM) were used for microstructural and fracture analysis. For composition determination of the microconstituents energy dispersive X ray spectroscopy (EDS) was used. To evaluate mechanical properties, hardness and microhardness measurements were performed. Investigations based on the microstructural features with the idea to indicate suitability of weld repair of the column were performed in this study. It was observed that the crack initiation, caused by oxidation/corrosion and thermal stresses induced by temperature gradient, appeared in the inner side of the tube wall and propagation occurred along grain boundaries. The results clearly showed the presence of an irregular microstructure which contributed to crack propagation through the tube wall. An occurrence of precipitated needle-shaped carbides/carbonitrides and brittle a phase was also identified in the microstructure. Results of the microstructural and fracture analysis clearly indicate that reformer furnace columns made of heat resistant steel HK 40 were unsuitable for weld repair

Stavovi knjižničara o ulozi narodnih knjižnica u informacijskom poremećaju

Cilj. Cilj je ovoga istraživanja utvrditi stavove i iskustva knjižničara zaposlenih u narodnim knjižnicama o ulozi narodnih knjižnica u suočavanju s informacijskim poremećajem te s dezinfodemijom vezanom uz bolest COVID-19. Pristup/metodologija/dizajn. Za potrebe istraživanja u radu korišten je mrežni anketni upitnik koja je sadržavao pitanja zatvorenog i otvorenog tipa. Upitnik je distribuiran svim narodnim knjižnicama u Republici Hrvatskoj. Rezultati. Rezultati istraživanja pokazuju da su ispitanici, knjižničari zaposleni u narodnim knjižnicama, u velikoj većini zainteresirani za problematiku informacijskog poremećaja te za dodatnu edukaciju o istom. Gotovo polovica ispitanika svoju je pripremljenost za organiziranje i provođenje programa edukacije korisnika o dezinformacijama ocijenila niskom. Velika većina ispitanika slaže se s tvrdnjom da narodne knjižnice trebaju sudjelovati u suočavanju s informacijskim poremećajem te da u istom mogu pridonijeti, međutim stavovi o konkretnim načinima i odgovornostima u suočavanju s informacijskim poremećajem pokazuju razjedinjenost. Mišljenja ispitanika o tome koje su ključne prepreke uključivanju narodnih knjižnica u suočavanje s informacijskim poremećajem ukazuju na petnaest specifičnih problema među kojima se ističu needuciranost knjižničara, čuvanje percepcije knjižnice kao ideološki neutralnog mjesta, nezainteresiranost korisnika te različiti infrastrukturni problemi. Originalnost/vrijednost. U ovom se radu prvi put iznose stavovi knjižničarske zajednice u Republici Hrvatskoj o ulozi narodnih knjižnica u informacijskom poremećaju. Rad otvara pitanja o izazovima aktivnog uključivanja narodnih knjižnica u suočavanje s informacijskim poremećajem, među kojima se ističu stručne kompetencije knjižničara te etički principi pružanja informacijskih usluga.   Rad je napisan na temelju diplomskog rada Antonele Čepčar „Stavovi knjižničara o ulozi narodnih knjižnica u (dez)infodemiji“ obranjenog na Sveučilište J. J. Strossmayera u Osijeku pod mentorstvom doc. dr. sc. Snježane Stanarević Katavić i izv. prof. dr. sc. Borisa Badurine. Zbog znatnih izmjena koje su učinjene u odnosu na diplomski rad u pregledu literature, dijelu analize rezultata te raspravi i zaključku, koautori su usuglasili raspored autorstva kako stoji

Stavovi knjižničara o ulozi narodnih knjižnica u informacijskom poremećaju

Cilj. Cilj je ovoga istraživanja utvrditi stavove i iskustva knjižničara zaposlenih u narodnim knjižnicama o ulozi narodnih knjižnica u suočavanju s informacijskim poremećajem te s dezinfodemijom vezanom uz bolest COVID-19. Pristup/metodologija/dizajn. Za potrebe istraživanja u radu korišten je mrežni anketni upitnik koja je sadržavao pitanja zatvorenog i otvorenog tipa. Upitnik je distribuiran svim narodnim knjižnicama u Republici Hrvatskoj. Rezultati. Rezultati istraživanja pokazuju da su ispitanici, knjižničari zaposleni u narodnim knjižnicama, u velikoj većini zainteresirani za problematiku informacijskog poremećaja te za dodatnu edukaciju o istom. Gotovo polovica ispitanika svoju je pripremljenost za organiziranje i provođenje programa edukacije korisnika o dezinformacijama ocijenila niskom. Velika većina ispitanika slaže se s tvrdnjom da narodne knjižnice trebaju sudjelovati u suočavanju s informacijskim poremećajem te da u istom mogu pridonijeti, međutim stavovi o konkretnim načinima i odgovornostima u suočavanju s informacijskim poremećajem pokazuju razjedinjenost. Mišljenja ispitanika o tome koje su ključne prepreke uključivanju narodnih knjižnica u suočavanje s informacijskim poremećajem ukazuju na petnaest specifičnih problema među kojima se ističu needuciranost knjižničara, čuvanje percepcije knjižnice kao ideološki neutralnog mjesta, nezainteresiranost korisnika te različiti infrastrukturni problemi. Originalnost/vrijednost. U ovom se radu prvi put iznose stavovi knjižničarske zajednice u Republici Hrvatskoj o ulozi narodnih knjižnica u informacijskom poremećaju. Rad otvara pitanja o izazovima aktivnog uključivanja narodnih knjižnica u suočavanje s informacijskim poremećajem, među kojima se ističu stručne kompetencije knjižničara te etički principi pružanja informacijskih usluga.   Rad je napisan na temelju diplomskog rada Antonele Čepčar „Stavovi knjižničara o ulozi narodnih knjižnica u (dez)infodemiji“ obranjenog na Sveučilište J. J. Strossmayera u Osijeku pod mentorstvom doc. dr. sc. Snježane Stanarević Katavić i izv. prof. dr. sc. Borisa Badurine. Zbog znatnih izmjena koje su učinjene u odnosu na diplomski rad u pregledu literature, dijelu analize rezultata te raspravi i zaključku, koautori su usuglasili raspored autorstva kako stoji

Numerical flow simulation in ventilation mill-duct system of lignite power plant Kostolac B

U radu su izloženi rezultati numeričke simulacije višefaznog strujanja u ventilacijskom mlinu i kanalima aerosmješe termoelektrane Kostolac B. Ovo strujanje razrijeđene mješavine plina i čvrstih djelića ugljenog praha i pijeska, modelirano je pomoću softvera ANSYS FLUENT 12, sa modelom mješavine u pristupu Ojler-Ojler. Proces simulacije obuhvata modeliranje geometrije, generiranje mreže i praćenje putanja djelića mješavine. Izvršeno je poređenje dobivenih numeričkih rezultata s rezultatima mjerenja za raspodjelu plinske mješavine i brzinu aerosmješe na ulasku u plamenike.This paper presents the results of numerical simulations of multiphase flow in ventilation mill-ducts system of the power plant Kostolac B. This flow of diluted gas and solid piece of coal and sand mixture, was modeled using software ANSYS FLUENT 12, with a mixture model in the Euler-Euler approach. Process simulation involves geometry modeling, mesh generation and tracking trajectory of mixture pieces. A comparison of numerical and measurement results for the distribution and the speed of the mixture, at the entrance to the burner, was made

Wear resistance improvement of thermal plant fan mill impact plates based on numerical flow simulation

Numerical simulation of 3D multiphase flow is performed to determine the velocity field in the fan mill. The mixture model of the Euler-Euler approach is used. Recirculating gas is a primary phase that carries granular phases including the coal powder and sand. The latter causes heavy wear of mill impact plates. The optimal form of hardfacing geometry and technology has an impact both on velocity magnitude and direction of flow in such a way as to reduce plate wear. This paper presents results of surface modification (based on numerical simulation), implemented on impact plates of the fan mill in the Kostolac B power plant in Serbia. Fishbone hardfacing is made in order to increase the lifetime of impact plates and extend the period between overhauls of fan mills. The experimental tests of fishbone surfacing plates, in exploitation conditions, show that the applied modification, hardfacing technologies and coating materials, give expected results. The application of this hardfacing in the form of a fishbone is simpler, faster and more economical compared to complete overlaying and honeycomb impact plate surfacing. The relative weight loss of base plate after an 1440-hour period of exploitation is 8%, while the weight losses for the hard-faced plate is below 7%