Selective corrosion of type 316L austenitic stainless steel welds during the production of organic acids resulted in losses in production due to unscheduled downtimes to perform repairs. Estimated corrosion rates of type 316L filler material welds were an order of magnitude higher than that of the base material. Alternative higher alloyed commercial filler materials were evaluated under actual production conditions. The evaluated filler materials were types 316L, 317L, 309L, 309MoL, 2205, 2507, 625, 825 and 904L. The effect of nitrogen on the corrosion properties of type 309L filler material was evaluated by manipulating the nitrogen concentration of the shielding gas during MIG welding. These changes in nitrogen concentration did not influence the corrosion resistance of the type 309L filler material. No correlation could be established between the corrosion rates, analysed chemical composition of the product and operating temperature during production. In almost all the cases where the chemical composition of the filler material was comparable with that of the base material the corrosion rates of the filler materials were higher than that base material. It might be expected that the ferrite phase with higher molybdenum and chromium should be more corrosion resistant while the austenite should be less resistant. This was, however, not the case with the corrosion of type 309L filler material. It would thus appear that in this case nickel enrichment of the austenite phase had a larger influence on the corrosion resistance of the austenite phase than the chromium and molybdenum had on the corrosion resistance of the ferrite phase. It appears that nickel and molybdenum had the largest contribution to the corrosion resistance of stainless steels welds under these operating conditions. It is, however, believed that a certain minimum concentration of chromium is also required to provide corrosion resistance to these alloys in hot organic acid environments. In contrast with the fact that a substantial alloying content is required to improve corrosion resistance of the filler material, the small difference in composition between ferrite and austenite phases, due to micro segregation, appeared to affect the corrosion resistance on micro scale. This is illustrated by the micrographs, which show corrosion to etch out the dendrite structure. Since the morphology of the austenite and ferrite phases is so similar, it could not always be conclusively established which one of the two phases corroded selectively. Analyses performed on the austenite and ferrite phases did not indicate a concentration difference within the phases itself. However, there were significant differences in the concentration of elements between the phases, with the austenite stabilising elements reporting to the austenite phase and the ferrite stabilizing elements reporting to the ferrite phase, in line with thermodynamic predictions. In the case of the filler materials following the austenite mode of solidification, no significant concentration differences were detected within the matrix. Although all highly alloyed high nickel alloyed filler materials (types 904L, 825 and 625) corroded at a lower rate than the type 316L base material, type 625 filler material was the filler material of choice due to the lack of any pitting of the weld. Pitting was detected in both the 825 and 904L filler materials. Galvanic corrosion was not noted at any of the weld/HAZ interfaces and in no case did the type 316L parent metal adjacent to the weld corrode preferentially to the material further away from the weld. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted

No description available.

A good understanding of the solidification process of a cast material is essential to understand how the combination of alloy composition and the casting process variables combines into the solid cast component and its performance. The wrong combination may result in poor performance or casting defects. Spheroidal graphite has been well researched in ductile irons (SGI) where it is predominant. Spheroidal graphite is also present in smaller amounts in compacted graphite irons (CGI), but its nature and role in this material is less understood. Recent associations of spheroidal graphite in CGI with shrinkage porosity problems highlights the need for better understanding in this area. The importance of the dendritic austenite structure to the properties and solidification behaviour of cast irons has been highlighted in recent research. However, progress is to a degree limited by lack of practical means to characterize the structure. In the present work, the transition of a cast iron from SGI to CGI though remelting was studied. As the fraction of SG dropped, the tips of the compacted graphite tended to lose contact with the melt at a later stage. After this occurred, solidification appeared to be dominated by spheroidal graphite. Compacted and spheroidal graphite was found to solidify mostly segregated, and the increased recallescence induced by a higher fraction of compacted graphite displayed small influence on the size distribution of spheroidal graphite apart from the total number and fraction. The partitioning of Si, Mn and Cu in SGI and CGI was found to agree well with each other, as well as with theoretical predictions under the assumptions of zero diffusion of the elements in the solid. This shows that the proportions of spheroidal and compacted graphite has small or no influence on the evolution of these elements in the melt during solidification. A method for characterization of the dendritic austenite in quenched cast irons was introduced and evaluated. The method includes a technique for producing a visual contrast between the ledeburite matrix and the dendritic austenite, and a scheme for producing binary images from the resulting micrographs which are suitable for automatic image analysis. Measurements of the volume fraction and surface area per unit volume of the dendritic austenite structure using the introduced method was found to agree reasonably with traditional point counting and line intercept techniques. The difficulty in finding the exact boundary was proposed to be the major source of systematic disagreement. / En god förståelse för stelningsprocessen av ett gjutet material är väsentligt för att förstå hur kombinationen av legeringens kemiska sammansättning och gjutprocessens variabler resulterar i den stelnade komponenten och dess prestanda. Fel kombination kan resultera i sänkt prestanda eller gjutdefekter. Sfärisk grafit är väl studerad i segjärn (SGI) där grafitmorfologin är dominant. Mindre mängder sfärisk grafit är dock närvarande även i kompaktgrafit, där dess karaktär och roll är mindre känd. Samband mellan sfärisk grafit i kompaktgrafitjärn och krympporositet understryker behovet av bättre förståelse inom detta område. Dessutom har betydelsen av den tidiga dendritiska austenitstrukturen för senare delen av stelningen uppmärksammats. Utveckling inom detta område är dessvärre till en grad begränsad av bristen på kunskap om praktiska metoder för att karaktärisera dess struktur. I detta arbete studerades övergången från segjärn till kompaktgrafit genom omsmältning. Vid sänkt fraktion sfärisk grafit visade sig kompaktgrafiten tappa kontakten med smältan vid senare stadie av stelningen. Efter detta inträffade, dominerades stelningen till synes av tillväxt av sfärisk grafit. Kompaktgrafit och sfärisk grafit bildades i huvudsak segregerade. Ökad rekallesens till följd av ökad fraktion kompaktgrafit visade sig ha låg inverkan på storleksfördelningen av eutektisk sfärisk grafit bortsett från dess totala antal och fraktion. Omfördelningen av Si, Mn och Cu mellan stelnad matris och smälta i segjärn och kompaktgrafitjärn fanns stämma bra överens med varandra, samt med teoretiska värden med antagande om försumbar diffusion i stelnad matris. Detta visar att proportionerna av sfärisk och kompaktgrafit har liten eller ingen inverkan på halten av dessa ämnen i smältan under stelningen. En metod för karaktärisering av den dendritiska austenitstrukturen i släckt gjutjärn introducerades och utvärderades. Metoden inkluderar en teknik för att åstadkomma kontrast mellan ledeburitmatrisen och den dendritiska austeniten, och en teknik för att producera binära bilder från resulterande mikroskopbilder som är lämpliga för automatisk bildanalys. Mätningar av volymfraktion och yta per volymenhet av dendritstrukturen genom tillämpning av den introducerade metoden visade rimlig överensstämmelse med traditionella punktfraktion‐ och linjetekniker. Svårigheten att hitta den exakta gränslinjen mellan dendritisk struktur och ledeburit föreslogs vara den huvudsakliga källan till systematisk oöverensstämmelse.

The thesis deals with the influence of solidification conditions on structure and mechanical properties of austenitic stainless steels. The first part involves the distribution of stainless steels and a basic understanding of the issue of achieving the desired structure and mechanical properties. The second part is focused on austenitic steels and their basic characteristics. This section also describes the experiment and the results achieved. Finally, these results were evaluated and further research steps were recommended.

This work deals with the use of solidification technology for the disposal of hazardous waste with the aim of its material recovery. The goal of this Master's Thesis is to find and verify a new solidification way that will allow efficient transformation of hazardous waste into new material and its further use in civil engineering.

This diploma thesis focuses on the causes of surface shrinkages at tur-bochargers wheels castings made from TiAl alloy. On the basis of simulation of solidification and cooling, the defect is being repaired by the simulation software Procast.

<p>Predmet izučavanja ove disertacije je ispitivanje mogućnosti imobilizacije&nbsp;toksičnih metala u sedimentu primenom imobilizacione tehnike&nbsp;solidifikacije/stabilizacije portland cementom, kalcijum-oksidom, prirodnim zeolitom,&nbsp;letećim pepelom, kaolinitom, montmorilonitom, i različitim sme&scaron;ama ovih agenasa&nbsp;kao i određivanje njihove efikasnosti u zavisnosti od brojnih faktora. Efikasnost&nbsp;imobilizacionih postupaka praćena je kori&scaron;ćenjem tzv. testova izluživanja, odnosno&nbsp;određivanjem koncentracije metala koji su u toku određenog vremena iz imobilisane&nbsp;faze pre&scaron;li u mobilnu fazu gde se smatraju potencijalno opasnim i biodostupnim.&nbsp;Krajnji cilj svakog testa izluživanja je mogućnost procene adekvatnosti primene&nbsp;određene imobilizacione, odnosno remedijacione tehnike. Efikasnost imobilizacionih&nbsp;postupaka zavisi i od određenih osobina: pH rastvora, dodatka imobilizacionog agensa&nbsp;&scaron;to je istraživanjem ispitano. Na osnovu ovog testa, odnosno &nbsp; odnosa kumulativne&nbsp;frakcije metala koja je oslobođena iz imobilizacione sme&scaron;e u mobilnu fazu i vremena&nbsp;određen je tip mehanizma koji omogućava transport metala &nbsp;(spiranje, difuzija,&nbsp;rastvaranje). Takođe su određeni i i parametri (koeficijenti difuzije, indeksi&nbsp;izlužljivosti) koji&nbsp; će poslužiti za ocenu efikasnosti prethodno primenjenih&nbsp;imobilizacionih tehnika. Primenjeni su i modifikovani testovi izluživanja sa ciljem &scaron;to&nbsp;bolje simulacije realnih uslova. U modifikovanim testovima izluživanja kori&scaron;ćen je&nbsp;rastvor sa pH 3,25 da bi se simulirali uslovi koji opona&scaron;aju kisele ki&scaron;e, odnosno uslovi&nbsp;u realnom sistemu. Takođe je kor&scaron;ćen i rastvor huminskih materija sa ciljem&nbsp;simulacije uslova velikog organskog opterećenja do kojeg bi moglo doći u slučaju&nbsp;akcidentnih situacija (na primer poplava) i generalno uslova koji se stvaraju u &nbsp;prirodi&nbsp;pri raspadanju organskog materijala (li&scaron;će, trava, itd.). Rezultati dobijeni simulacijom&nbsp;ovih uslova će dalje omogućiti modelovanje pona&scaron;anja metala u smislu dugoročnog&nbsp;&quot;izluživanja&quot; iz tretiranog sedimenta kao i procenu najefikasnijih agenasa &nbsp; za&nbsp;imobilizaciju različitih metala u sedimentu. Na osnovu dobijenih rezultata, zaključeno&nbsp;je da su optimalni agensi za imobilizaciju metala u sedimentu sme&scaron;a cementa i kreča&nbsp;(5% cementa i 10% kreča), sme&scaron;a montmorilonita i kreča (30% montmorilonita i 10%&nbsp;kreča), leteći pepeo (30%) i zeolit (30%). U ovim sme&scaron;ama dominantan mehanizam&nbsp;izluživanja je difuzija, a ove sme&scaron;e se mogu smatrati inertnim otpadom po svim&nbsp;ispitivanim kriterijumima. Ovi rezultati se mogu upotrebiti za projektovanje i&nbsp;izgradnju pilot postrojenja na kome bi se ispitala efikasnost ovih agenasa za&nbsp;remedijaciju sedimenta u realnim uslovima. Dobijeni podaci su neprocenjivi sa&nbsp;aspekta ekonomski i ekolo&scaron;ki prihvatljivog upravljanja sedimentom.</p> / <p>This work is concerned with exploring the possibilities of immobilization of toxic&nbsp;metals in sediments using solidification/stabilization as imobilization technique and&nbsp;using Portland cement, calcium oxide, natural zeolite, flying ash, kaolinite, montmorilonite, and various mixtures of these agents and determine their&nbsp;effectiveness depending on many factors. Performance of imobilization procedures&nbsp;was followed by the use of so-called leaching tests and determination of the metals&nbsp;concentration that are within a specified interval of time leached from the immobile phase and as such can be considered potentially hazardous and bioavailable. The ultimate goal of every leaching test is to assess the adequacy of the possibility of applying certain imobilization or remediation technique. Performance of imobilizaction procedures depends on certain characteristics: pH of solution, additon of imobilization agent, etc. Based on this test, ie relation between the cumulative fraction of metal leached from imobilization mixture and time, the type of leaching mechanism that allows the transport of metals (wash-off, diffusion, dissolution) was determined. Other parameters which will serve for evaluating the efficiency of the previously applied imobilization techniques (diffusion coefficients, leaching indices) were also determined. Modified leaching tests were applied with the aim of better simulation of real conditions on the field. In the modified leaching tests the solution with pH 3.25 was used to simulate conditions that mimic acid rain and conditions in the real system. The humic acid solution was also used with the aim to simulate high organic loads which could occur in the case accident situation (eg floods) and the general conditions that are created in naturewhen decaying organic material (leaves, grass, etc.). The results obtained by simulating these conditions will enable the modeling of behavior of metals in terms of long-term leaching period from the treated sediment and assess the most effective agents for the immobilization of various metals in the sediments. Based on these results, it was concluded that the optimal agents for immobilization of metals in the sediment mixture of cement and lime (5% cement and 10% lime), a mixture montmorilonite and&nbsp; lime (30% montmorilonite and 10% lime), fly ash (30%) and zeolite (30%). In these mixtures dominant leaching mechanism is diffusion, and these mixtures can be considered as inert waste by all tested criteria. These results can be used to design and builda pilot plant in which order to evaluate the effectiveness of these agents for remediation of sediment in real &nbsp;terms. The obtained data are invaluable from the aspect of economic and ecologically acceptable management of sediment.</p>

<p>Predmet&nbsp; izučavanja&nbsp; ove&nbsp; disertacije&nbsp; bio&nbsp; je&nbsp; ispitivanje&nbsp; mogućnosti&nbsp; kori&scaron;ćenja&nbsp; piritne&nbsp;izgoretine&nbsp; u&nbsp; tretmanu&nbsp; otpadnih&nbsp; voda&nbsp; tekstilne&nbsp; industrije&nbsp; kao&nbsp; i&nbsp; dalji&nbsp; tretman&nbsp; mulja&nbsp; nastalog&nbsp;nakon ovog procesa. Naime, nakon iskori&scaron;ćenja ovog otpada &nbsp;u Fenton procesima, iskori&scaron;ćena&nbsp;piritna izgoretina u najvećem procentu čini mulj koji zaostaje nakon tretmana. Kako se piritna&nbsp;ruda jo&scaron; u procesu proizvodnje sumporne kiseline&nbsp; koristi nepreči&scaron;ćena, zaostali&nbsp; mulj koji je&nbsp;potencijalno&nbsp; toksičan&nbsp; i&nbsp; ne&nbsp; može&nbsp; se&nbsp; direktno&nbsp; odlagati&nbsp; bez&nbsp; prethodnog&nbsp; tretmana.&nbsp; Stoga,&nbsp;primenjena&nbsp; je&nbsp; tehnika&nbsp; stabilizacija&nbsp; i&nbsp; solidifikacija&nbsp; (S/S)&nbsp; uz&nbsp; kori&scaron;ćenje&nbsp; portland&nbsp; cementa,&nbsp;kalcijum-oksida, prirodnog zeolita, letećeg pepela, kaolinita,&nbsp; bentonita i&nbsp; autohtone&nbsp; gline &nbsp;za&nbsp;imobilizaciju metala. Istraživanja u ovoj disertaciji vr&scaron;ena su u dve faze. Prva faza imala je za&nbsp;cilj&nbsp; optimizaciju&nbsp; operativnih&nbsp; uslova&nbsp; Fenton&nbsp; tretmana&nbsp; pri&nbsp; kojima&nbsp; se&nbsp; postiže&nbsp; najvi&scaron;i&nbsp; stepen&nbsp;obezbojavanja&nbsp; i&nbsp; mineralizacije&nbsp; ispitivanih&nbsp; efluenata.&nbsp; Takođe&nbsp; ispitivanje&nbsp; je&nbsp; uključilo&nbsp; i&nbsp;karakterizaciju nastalih efluenata kako bi se procenila efikasnost primenjenog, i mogućnost&nbsp;daljeg&nbsp; tretmana.&nbsp; Druga&nbsp; faza&nbsp; uključivala&nbsp; je&nbsp; karakterizaciju&nbsp; nastalog&nbsp; mulja&nbsp; nakon&nbsp; tretmana&nbsp;obojenih&nbsp; efluenata&nbsp; i&nbsp; njegov&nbsp; tretman&nbsp; solidifikacijom&nbsp; i&nbsp; stabilizacijom.&nbsp; Vr&scaron;eno&nbsp; je&nbsp; određivanje&nbsp;efikasnosti&nbsp; primenjenih&nbsp; imobilizacionih&nbsp; agenasa&nbsp; u&nbsp; S/S tretmanu&nbsp; i&nbsp; dominantnog&nbsp; mehanizma&nbsp;<br />izluživanja&nbsp; metala&nbsp; koji&nbsp; opisuju&nbsp; njihov&nbsp; transport&nbsp; iz&nbsp; dobijenih&nbsp; solidifikata.&nbsp; Primenjeni&nbsp; su&nbsp; i&nbsp;modifikovani&nbsp; testovi&nbsp; izluživanja&nbsp; sa&nbsp; ciljem&nbsp; &scaron;to&nbsp; bolje&nbsp; simulacije&nbsp; realnih&nbsp; uslova.&nbsp; Rezultati&nbsp;dobijeni&nbsp; simulacijom&nbsp; uslova&nbsp; će&nbsp; dalje&nbsp; omogućiti&nbsp; modelovanje&nbsp; pona&scaron;anja&nbsp; metala&nbsp; u&nbsp; smislu&nbsp;dugoročnog&nbsp; &quot;izluživanja&quot;&nbsp; iz&nbsp; tretiranog&nbsp; otpada&nbsp; kao&nbsp; i&nbsp; procenu&nbsp; najefikasnijih&nbsp; agenasa&nbsp; za&nbsp;imobilizaciju različitih metala u ovom tipu otpada. TakoĎe primenjeni su testovi izluživanja&nbsp;sa jednom ekstrakcijom kako bi se osiguralo&nbsp; slaganje rezultata sa maksimalno dozvoljenim&nbsp;koncentracijama&nbsp; aktuelnih&nbsp; pravilnika.&nbsp; Na&nbsp; osnovu&nbsp; dobijenih&nbsp; rezultata&nbsp; zaključeno&nbsp; je&nbsp; da&nbsp; se&nbsp;piritna izgoretina može uspe&scaron;no koristiti kao izvor katalitičkog gvožđa u Fenton tretmanima&nbsp;obojenih&nbsp; efluenata&nbsp; i&nbsp; može&nbsp; se&nbsp; smatrati&nbsp; efektivnom&nbsp; metodom &nbsp; predtretmana&nbsp; za&nbsp;nebiodegradabilne&nbsp; otpadne&nbsp; vode,&nbsp; čineći&nbsp; ih&nbsp; pogodnijim&nbsp; za&nbsp; konvencionalni&nbsp; biolo&scaron;ki&nbsp; tretman.&nbsp;Takođe&nbsp; u&nbsp; drugoj&nbsp; fazi&nbsp; istraživanja,&nbsp; na&nbsp; osnovu&nbsp; dobijenih&nbsp; rezultata,&nbsp; zaključeno&nbsp; je&nbsp; da&nbsp; je&nbsp; mulj&nbsp;tertian&nbsp; sa&nbsp; navedenim&nbsp; imobilizacionim&nbsp; agensima&nbsp; uspe&scaron;no&nbsp; solidifikovan&nbsp; i&nbsp; stabilizovan&nbsp;materijal.&nbsp; Optimalane&nbsp; su&nbsp; sme&scaron;e&nbsp; koje&nbsp; kao&nbsp; jedan&nbsp; od&nbsp; imobilizacionih&nbsp; agenasa&nbsp; sadrže&nbsp; leteći&nbsp;<br />pepeo, prevashodno u kombinaciji sa cementom i krečom. Dobijeni podaci su &nbsp;neprocenjivi sa&nbsp;aspekta ekonomski i ekolo&scaron;ki prihvatljivog upravljanja opasnim industrijskim otpadom.&nbsp;</p> / <p>This work is concerned with exploring the possibility of using pyrite cinders in the treatment&nbsp;of&nbsp; textile&nbsp; industry&nbsp; effluents&nbsp; and&nbsp; further&nbsp; treatment&nbsp; of&nbsp; sludge&nbsp; generated&nbsp; after&nbsp; this&nbsp; process.&nbsp;Namely,&nbsp; after&nbsp; the&nbsp; utilization&nbsp; of&nbsp; this&nbsp; waste&nbsp; in&nbsp; Fenton&nbsp; processes&nbsp; slag&nbsp; that&nbsp; remains&nbsp; after&nbsp;treatment is mostly composed of used pyrite cinder. As the pyrite ore, in the production of&nbsp;sulfuric&nbsp; acid,&nbsp; is&nbsp; used&nbsp; untreated,&nbsp; sludge&nbsp; that&nbsp; remains&nbsp; is&nbsp; potentially&nbsp; toxic&nbsp; and&nbsp; cannot&nbsp; be&nbsp;disposed&nbsp; directly&nbsp; without&nbsp; treatment.&nbsp; Therefore,&nbsp; the&nbsp; stabilization&nbsp; and&nbsp; solidification&nbsp; (S/S)&nbsp;technique&nbsp; with&nbsp; the&nbsp; usage&nbsp; of&nbsp; portland&nbsp; cement,&nbsp; calcium&nbsp; oxide,&nbsp; zeolite,&nbsp; fly&nbsp; ash,&nbsp; kaolinite,&nbsp;bentonite and native clay was used for metal immobilization. The research in this dissertation&nbsp;was carried out in two phases. The first phase aimed to optimize the operational conditions of&nbsp;Fenton&nbsp; treatment&nbsp; in&nbsp; which&nbsp; the&nbsp; highest&nbsp; degree&nbsp; of&nbsp; decolourization&nbsp; and&nbsp; mineralization&nbsp; was&nbsp;achieved in tested effluents. Also the research has included the characterization of generated&nbsp;effluents in order to evaluate the effectiveness of the treatment applied, and the possibility of&nbsp;further&nbsp; treatment.&nbsp; The&nbsp; second&nbsp; phase&nbsp; involved&nbsp; the&nbsp; characterization&nbsp; of&nbsp; the&nbsp; generated&nbsp; sludge&nbsp;after the treatment of effluents that contained dyes and its treatment with solidification and&nbsp;stabilization technique.&nbsp; The determination of used immobilization agents effectiveness was&nbsp;Conducted&nbsp; to&nbsp; determine&nbsp; the&nbsp; efficacy&nbsp; of&nbsp; immobilization&nbsp; agents&nbsp; in&nbsp; S&nbsp; /&nbsp; S&nbsp; treatment&nbsp; and&nbsp; the&nbsp;dominant&nbsp; mechanism&nbsp; of&nbsp; leaching&nbsp; of&nbsp; metals&nbsp; that&nbsp; describe&nbsp; their&nbsp; transport&nbsp; from&nbsp; the&nbsp; received&nbsp;solidificate . Were applied and modified leaching tests with the aim of better simulation of&nbsp;real&nbsp; conditions.&nbsp; The&nbsp; results&nbsp; obtained&nbsp; by&nbsp; condition&nbsp; simulation&nbsp; will&nbsp; enable&nbsp; the&nbsp; modeling&nbsp; of&nbsp;behavior of metals in terms of long-term leaching period from the treated sludge and assess&nbsp;the most effective agents for the immobilization of various metals in this type of waste. Also,&nbsp;single-extraction leaching tests were applied to ensure the accordance of obtained results &nbsp;with&nbsp;the maximum permissible concentrations of current regulations. Based on these results, it was&nbsp;concluded&nbsp; that&nbsp; the&nbsp; pyrite&nbsp; cinders&nbsp; can&nbsp; be&nbsp; successfully&nbsp; used&nbsp; as&nbsp; a&nbsp; source&nbsp; of&nbsp; catalytic&nbsp; iron&nbsp; in&nbsp;Fenton&nbsp; treatment&nbsp; of&nbsp; effluents&nbsp; containing&nbsp; dyes,&nbsp; and&nbsp; can&nbsp; be&nbsp; considered&nbsp; an&nbsp; effective&nbsp;pretreatment&nbsp; method&nbsp; for&nbsp; non-biodegradable&nbsp; waste&nbsp; waters,&nbsp; making&nbsp; them&nbsp; amenable&nbsp; to&nbsp;conventional biological treatment. Also in the second phase, based on the results obtained, it&nbsp;was concluded that the sludge treated with&nbsp; aforementioned&nbsp; immobilization agents represents&nbsp;a successfully solidified and stabilized material. The optimal mixtures are those that contain&nbsp;fly ash as one of the used immobilization agents, primarily in combination with cement and&nbsp;lime.&nbsp; The&nbsp; obtained&nbsp; data&nbsp; are&nbsp; invaluable&nbsp; in&nbsp; terms&nbsp; of&nbsp; economic&nbsp; and&nbsp; environmentally&nbsp; sound&nbsp;management of hazardous industrial waste.</p>

<p><a>Direct chill (DC) casting is the main production method for wrought aluminum alloys. In this semi-continuous process, significant heat is extracted through a narrow, solidified shell by impinging water jets. A combination of rapid cooling and inoculation of the liquid metal with heterogenous nucleation sites (grain refiner) produces the proper conditions for equiaxed solidification. As equiaxed grains nucleate and grow in the slurry, they are transported by natural convection until their eventual coalescence into a rigid mush. The preferential accumulation of these solute-depleted grains in localized regions of the casting can lead to long range composition differences known as macrosegregation. Because macrosegregation cannot be mitigated by subsequent processing, it is critical to understand and prevent its development during casting. </a></p> <p>Numerical models are often used to gain insight into the interplay of the different transport phenomena that cause macrosegregation. The formation of mobile equiaxed grains creates a multiphase system with many moving interfaces, causing several modeling challenges. In principle, a model could be formulated in terms of local instantaneous variables describing the evolution of these interfaces, however the associated computational cost prohibits its extension to the length scale of industrial castings. For this reason, macroscopic transport equations are mathematically formulated using volume averaging methods. Two different volume-averaged model formulations can be distinguished in the solidification literature. The first approach is the multiphase formulation, which solves separate sets of governing equations for each phase that are coupled using microscale interfacial balances. While this approach retains closure models to describe the behavior of the sub-grid interfaces, these interfacial models introduce significant uncertainty that is propagated through the model. The second approach is the mixture formulation, which solves a single set of governing equations for the mixture and utilizes more pragmatic closure relationships. While this approach significantly reduces the complexity and computational cost of the model, previous formulations have oversimplified the microscale transport. Recognizing the advantages and disadvantages of both formulations, a mixture model is rigorously derived, retaining appropriate relationships for the grain structure and microsegregation behavior in equiaxed solidification </p> <p>Implementation of this model into a 3-D finite volume method (FVM) code using a co-located grid is discussed along with appropriate treatment of the discontinuous body forces and phase mass fluxes across the interface between the slurry and rigid mush. More specifically, body forces in the momentum equation are treated at the face-centers of a control volume to prevent erroneous velocity oscillations near this interface, and a diffuse phase flux method is proposed to reduce the sensitivity of composition predictions to the numerical grid. The proposed methods are verified across a wide range of conditions present in equiaxed solidification. </p> <p>This model is then used to investigate the role of grain motion on macrosegregation development in equiaxed solidification, specifically in horizontal and vertical DC casting. In horizontal DC casting, the casting axis is perpendicular to gravity and there is a tendency for grains to accumulate along the bottom of the ingot. Feeding liquid metal through a constrained inlet near the bottom suspends grains in the slurry, both reducing the overall macrosegregation and improving the macrosegregation symmetry in the ingot. In vertical DC casting, the casting axis is parallel to gravity and there is a tendency for grains to accumulate in the center of the ingot. It is determined by a combination of simulations in the current work and previous experimental results that a strong localized jet at the centerline can suspend grains in the slurry and reduce negative centerline segregation. The change in segregation is attributed to a combination of reducing the accumulation of solute-depleted grains near the centerline and thinning the rigid mush where solidification shrinkage pulls enriched liquid away from the centerline. The strong localized jet also causes significant refinement and homogenization of the grain structure, which improves the mechanical properties of the ingot. These studies indicate that it is beneficial for DC casting practices to move towards agitated or stirred melts, and away from conventional practices which promote thermal stratification and localized accumulation of equiaxed grains.</p>

In dieser Arbeit wurde der Einfluss der Schmelzkonvektion auf das erstarrende Gefüge von peritektischen Nd-Fe-B – und TiAl-Legierungen mit Hilfe neuartiger Methoden untersucht. Da die magnetischen und mechanischen Eigenschaften dieser technisch relevanten Legierungen stark vom Gefüge und insbesondere vom Volumenanteil der properitektischen Phase abhängen, sind diese Untersuchungen von großem Interesse. Auf der Basis der numerischen Simulationen der Schmelzkonvektionsmoden und des elektromagnetischen Problems in einer induktiv beheizten Schmelze, die am Forschungszentrum Dresden-Rossendorf durchgeführt wurden, wurden am IFW Dresden neuartige Versuchsaufbauten entwickelt, die die Modifizierung der Konvektion in einer Metallschmelze ermöglichen. Dies sind ein Aufbau zur erzwungenen Schmelzrotation in einem Tiegel und eine modifizierte Floating-Zone-Anlage. Die erzwungene Schmelzrotation, bei der der Schmelztiegel mit einer definierten Frequenz rotiert, führt in Übereinstimmung mit der Simulation zu einer starken Reduzierung der Konvektion in Abhängigkeit von der Frequenz. Diese Methode wurde auf Nd-Fe-B-Legierungen angewendet mit dem Ziel, die Bildung der unerwünschten weichmagnetischen Eisenphase zu unterdrücken bzw. deren Volumenanteil zu reduzieren. Im Ergebnis konnte der Volumenanteil der properitektischen Phase mit diesem Verfahren um 38.5 % reduziert werden. Das dendritische Gefüge wurde einer ausführlichen statistischen Analyse unterzogen, bei der die Abstände der sekundären Dendritenarme (SDAS) gemessen wurden. Es konnte gezeigt werden, dass die SDAS sich mit steigender Frequenz der Tiegelrotation, was einer reduzierten Schmelzkonvektion entspricht, verringern. Die Verringerung des Volumenanteils der properitektischen Eisenphase und der SDAS wird mit dem reduzierten konvektiven Massentransport unter reduzierter Schmelzkonvektion erklärt. Starke interdendritische Strömung reduziert die Dicke der Diffusionsgrenzschicht um die properitektische Phase. Dadurch wird der Stofftransport durch die Grenzschicht erleichtert. Kleinere Dendritenarme werden in die Schmelze zurückgeschmolzen, wodurch sich der Abstand zwischen den verbleibenden Dendritenarmen vergrößert. Eine Floating-Zone-Anlage, die das tiegelfreie Prozessieren von Metallschmelzen erlaubt wurde so modifiziert, dass mit Hilfe eines Doppelspulensystems eine zusätzliche wohl definierte elektromagnetische Kraft eingebracht wird, über die eine sehr intensive (Zweiphasenrührer in Parallelschaltung) bzw. stark verringerte Strömung (Doppelspule in Reihenschaltung) in der Schmelze eingestellt werden kann. Die experimentellen Ergebnisse der Untersuchungen am Nd-Fe-B-System mit der Doppelspule in Reihenschaltung zeigten, dass sich bei einem optimalen Spulenabstand von 5,1 mm die geringste Schmelzkonvektion ergab, wobei der Anteil des a-Eisen-Volumenanteils weiter verringert werden konnte. Im Gegensatz dazu wurde mit dem Zweiphasenrührer in Parallelschaltung eine sehr starke Schmelzkonvektion mit einem maximalen Volumenanteil der a-Eisen-Phase eingestellt, wobei durch die starke Rührung ein Wechsel der Morphologie von dendritisch zu globular zu beobachten war. Die Untersuchungen zum Einfluss der starken Schmelzkonvektion wurden auf ein weiteres peritektisch erstarrendes System ausgedehnt, um eine generalisierte Aussage zum Einfluss der Konvektion auf Gefüge und Eigenschaften peritektisch erstarrender Legierungen zu erhalten. Die ausgewählte Ti45Al55 - Legierung erstarrte unter starker Schmelzkonvektion ebenfalls globulitisch, wobei Reste dendritisch erstarrter properitektischer Phase gefunden wurden. Der Volumenanteil der properitektischen Phase steigt dabei mit zunehmender Rührwirkung an. Der Wechsel der Morphologie von dendritisch zu globular/dendritisch kann mit sphärischem Wachstum oder Fragmentierung der Dendritenarme erklärt werden. Die mechanischen Eigenschaften unter unterschiedlicher Schmelzkonvektion erstarrter Ti45Al55 – Legierung wurden bei Druckversuchen untersucht. Es wurde eine signifikant höhere plastische Verformbarkeit an der unter starker Schmelzkonvektion erstarrten Ti45Al55 – Legierung gefunden. Dies wird der isotropen spherischen Morphologie der lamellaren a2/g-Phase zugeordnet, während die anisotrope Orientierung der dendritisch- lamellaren Phase unerwünschte plastische Eigenschaften zeigt. Die Untersuchungen des Einflusses der Schmelzkonvektion auf das Gefüge peritektisch erstarrender Legierungen zeigten, dass ein maßgeschneidertes Gefüge durch optimale Wahl der Schmelzkonvektion möglich ist und damit magnetische bzw. mechanische Eigenschaften verbessert werden können. Die Kontrolle der Schmelzkonvektion ist daher ein geeignetes Mittel gewünschte Gefüge und Eigenschaften in Abhängigkeit von den Prozessabläufen einzustellen. / In this work, the effect of melt convection on the microstructure evolution of peritectic Nd-Fe-B and Ti-Al alloy systems was studied using novel techniques. The microstructural formation including the change in volume fraction and morphology of the properitectic phase influences the magnetic and mechanical properties for the Nd-Fe-B and Ti-Al alloy systems, respectively. On the basis of numerical simulations by the research group of Dr. Gunter Gerbeth from Department of Magnetohydrodynamics, Forschungszentrum Dresden-Rossendorf, two types of specially designed facilities were developed where melt convection can be altered by changing a number of parameters. These are: forced rotation facility and modified floating zone facility. According to the numerical simulation, an additional crucible rotation suppresses the internal melt motion significantly during forced rotation experiments, where the molten alloy is rotated at a well-defined frequency. This method was applied during the solidification of Nd-Fe-B alloys with the aim to suppress the volume fraction of undesired soft magnetic a-Fe phase. As a result, the volume fraction of properitectic phase with this method can be reduced up to 38 %. A detailed statistical analysis of secondary dendritic arm spacing (SDAS) measurements of a-Fe showed that the SDAS decreases as the rotational frequency increases and melt convection decreases. The reduction in the phase fraction and SDAS of properitectic phase is attributed to the reduced convective mass transfer under reduced melt motion. At high fluid velocity and low rotational frequency, the stronger interdendritic flow reduces the solute boundary layer and increases the transfer of solute through the interface. The smaller dendrite arms dissolve into the melt and thus the SDAS becomes higher than that of the samples solidified at higher rotational frequencies with reduced melt convection. Floating zone facility, which allows contactless heating without any contamination for highly reactive melts, was modified with a double coil system so that an additional electromagnetic force is introduced inside the melt. This induces either very intensive (two-phase stirrer in parallel connection coil system) or very reduced flow (series connection coil system) inside the melt The experimental results of series connection coil system showed that a reduced melt convection state is achieved near 5.1 mm coil distance where a-Fe volume fraction becomes minimum. On the contrary, the parallel coil system experiments showed that a-Fe volume fraction becomes maximum when the phase shift between the coils is close to 90°. The morphology of the a-Fe becomes globular due to spherical growth under strong convection. The study on the effect of strong stirring was extended to another alloy to get a generalized idea about the influence of melt convection on the microstructure development and resulting properties of peritectic alloys. Peritectic Ti45Al55 alloys were investigated by the two-phase stirrer using the coils connected in parallel to study the effect of enhanced melt convection. The increase in the properitectic phase fraction together with a strong change in the morphology from dendritic to spherical were observed in the stirred samples. The increase in the properitectic phase fraction occurs due to the enhanced effective mass transfer under strong melt convection. The change in morphology of the properitectic phase is attributed to spherical growth or fragmentation of dendrite arms under strong convection. The mechanical properties of Ti45Al55 alloys, which are solidified at different convection states, were studied. There was a significantly higher plastic deformability of stirred samples compared to unstirred samples. The coarse anisotropic orientation of the dendritic lamellar phase is detrimental for the plastic deformability, which is absent in the stirred samples due to the spherical and discrete morphology of the properitectic phase. This study indicates that tailored microstructure can be obtained either by decreasing (e.g. for Nd-Fe-B alloy) or increasing (e.g. for Ti-Al alloy) the convection state using effective techniques inside the melt to improve the magnetic and mechanical properties, respectively. Thus, controlling convection is a useful way to get favorable microstructure according to the process need.