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A research report submitted in fulfilment of the requirements for the degree of Master of Science in Engineering (50/50) to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, 2018 / Metal dusting (MD) is a severe form of corrosion in which iron, steels, and nickel (Ni) and cobalt (Co) based alloys disintegrate into a metal or carbide powder with a coke deposit when exposed to strongly carburising gases (carbon activity, ac>1) at elevated temperatures (400800°C). Temperature affects both the driving force and rate of the reaction, represented by gas phase supersaturation with carbon, and the rates of the various processes involved in converting that energy difference into the dusting process. Therefore, process streams such as reformer gas can be benign when hot, but becomes aggressive below critical temperatures. There are different views in literature about the effect of temperature on metal dusting of different materials and alloys. Alloy 800 experiences metal dusting (MD) at 525°C, which is the temperature of the tube sheet of reformers in petrochemical industry. This alloy is specifically used for tube ferrules in the reformers. The reformer trains can reach a critical (highest) internal temperature of 650°C. Therefore, these two temperatures were compared. The effect of temperature and exposure time on the metal dusting of Alloy 800 were investigated in terms of the form of attack and the degradation mechanism. From the results obtained, it was observed that the longer exposure periods result in more carbon deposition and the carbon filaments in the coke become finer as compared to the nanotubes obtained after shorter exposure periods. The alloy suffered metal dusting attack after a relatively short exposure period of three days (72 hours) at both temperatures of 525°C and 650°C, with very little coking. / TL2019

The increased copper price during the first world war forced the Swedish master founder OttoMeyer to invent a new alloy in order to continue casting artworks at a reasonable cost. The recipe of the alloy was a secret that Meyer kept to himself even unto death. The name of the alloy became Fritzi, but the number of objects cast in this alloy was never published.However, among the art pieces is G. Lindberg’s statuette “Vågen”. In this project, the composition of Fritzi was determined using SEM and EDS analysis on a sample from “Vågen”, and some thermodynamic, physical, and casting properties were evaluated usingsimulations and calculations. In addition to this, a 3D-replica of “Vågen” was created. / Det ökade kopparpriset under första världskriget tvingade gjutmästaren Otto Meyer attuppfinna en ny legering för att kunna fortsätta driva sitt konstgjuteri. Legeringen fick namnet Fritzi och blev tillverkad i ett okänt antal föremål. Receptet skrevs aldrig ned och fick följa med sin skapare ned i graven. Bland konstverken som göts i Fritzi finns G. Lindbergs statyett “Vågen”, från vilken prover har tagits i analyssyfte. Som en del av projektet har sammansättningen av Fritzi bestämts med SEM- och EDS-analys och några termodynamiska, fysikaliska och gjuttekniska egenskaper har bestämts genom simuleringar och beräkningar.Utöver detta har en 3D-replik av “Vågen” tillverkats.

This project that has been executed in collaboration with Saab aerostructures deals with how three materials, aluminum, titanium and corrosion resistant steel react when they are affected by different energy levels. These energy levels serve the purpose of representing damages that can happen in storage and handling to components used in production. A literature study was made to gather information regarding the treated materials as well as some methods that can be used to test a materials property. The goal of the project is to create a tool for Saab to use in their production in order to first estimate the energy levels that created a mark on a material and secondly evaluate if a crack can be suspected, by consequence evaluate if a NDT (non-destructive test) is required. The experiment was executed as such that all of the materials were put through drop-tests simulating different energy levels, with two different strikers, blunt and sharp, and then the materials were examined. Non-destructive testing was made on all of the marks made on the different materials using penetrant- or eddy current-method to see if there were any cracks formed in the material after impact. The experiment did not show signs of any cracks from the tested energy levels. However, it showed that crack propagation is highly dependent on the shape of the striker. To make the experiment more reliable and give a better result more energy levels would have needed to be tested. Also, a large number of different strikers and more non-destructive testing methods would have to be used. / Detta projekt som har utförts i samarbete med Saab aerostructures behandlar hur tre material, aluminium, titan och rostfritt stål reagerar när de blir utsatta för olika energinivåer. Dessa energinivåer ska simulera skador som kan uppkomma under hantering och förvaring av komponenter i Saabs produktion. En litteraturstudie har utförts för att samla information om materialen samt om några testmetoder som kan användas för att utvärdera ett materials egenskaper. Målet med projektet är att få fram ett tillvägagångssätt för Saab att i första hand uppskatta energinivåerna bakom ett slagmärke i ett material, därefter bedöma ifall en sprickinitiering kan misstänkas och därmed avgöra huruvida komponenter som utsatts för energinivåer behöver undergå oförstörande provning eller om de kan fortsätta användas i produktionen utan någon form av testning. Ett experiment utfördes genom att utsätta samtliga av materialen för dropp-tester med olika energinivåer med två olika slagdon, trubbigt och spetsigt, för att sedan kunna undersöka om sprickor hade propagerats. Oförstörande provning gjordes på alla simulerade skador antingen via penetrantmetoden eller virvelströmsprovning, för att se om det bildats sprickor i de olika materialen efter slaget. Experimentet visade inte på några sprickor vid de testade energinivåerna. Däremot såg man att sprickinitiering beror bland annat på slagdonets utformning, där en skarpare form ökade risken för sprickor. För att experimentet ska bli mer pålitligt och ge ett bättre resultat hade fler energinivåer behövt testas, en större variation av slagdon hade behövt användas samt att man hade behövt använda fler typer av oförstörande provning.

During the time we live in, climate change is more important than ever. HYBRIT, formed bySSAB, LKAB, and Vattenfall, is an initiative set out to make the steel production fossil free, by developing a novel process that produce direct reduced iron (DRI) using hydrogen as reagent. Electric arc furnaces (EAF) will then be employed to melt the DRI. For SSAB, it is important to obtain a satisfied dephosphorisation in the EAFs for the production of high strength steel. The composition of the DRI, has a big impact on the dephosphorisation and it is important that the slag has an optimal basicity and FeO content. To better understand the impact that FeO has on dephosphorisation, experimental data has to be collected and the FeO activity calculated in different slag systems. Activity measurements of FeO is rather complex. In this thesis, three methods were looked at: experimental determination, electrochemical methods and equilibrating the slag. Equilibrating the slag turned out to be the best suitable method forcalculating the FeO activity and is recommended to be evaluated further. / I den tid vi lever i nu är frågan om klimatförändringarna viktigare än någonsin. HYBRIT, bildatav SSAB, LKAB och Vattenfall, är ett initiativ som syftar till att göra stålproduktionen fossilfri genom att utveckla en ny process som producerar direkt reducerat järn (DRI) med väte som reagens. Elektriska bågugnar (EAF) kommer sedan att användas för att smälta DRI. För SSAB är det viktigt att få en bra fosforrening i EAF:erna för produktion av höghållfast stål. DRI:s sammansättning har stor inverkan på fosforreningen och det är viktigt att slaggen har en optimal basicitet och FeO-innehåll. För att bättre förstå vilken inverkan FeO har på fosforreningen måste experimentella data samlas in och FeO-aktiviteten beräknas i olika slaggsystem. Aktivitetsmätningar av FeO är ganska komplicerade. I denna avhandling undersöktes tre metoder: experimentell bestämning, elektrokemiska metoder och jämvikt av slaggen. Jämvikt av slaggen visade sig vara den mest lämpliga metoden för beräkning av FeO-aktiviteten och rekommenderas att utvärderas ytterligare.

Abstract Hot ductility during continuous casting of duplex stainless steels In the continuous casting of the duplex stainless steel, longitudinal corner cracks in the first casted bloom of SAF 2507 have been discovered by Sandvik. SAF 2205 contains the same alloying substances as SAF 2507, but with some difference in the concentration of the elements. Despite this similarity, the occurrence of corner cracks in SAF 2205 is minimal and thus is not considered to be a significant problem. The reason for the appearance of hot cracks in SAF 2507 may be due to reduced ductility and due to stress variations in the material that occurs during casting. An earlier research for mapping longitudinal corner cracks in SAF 2507 and SAF 2205 proved to occur due to thermal stresses created by shrinkage of the shell in the mold. A reduced ductility can be obtained by embrittlement of grain boundaries that may arise with a brittle phase. The high amount of alloy elements in duplex stainless steels can result in precipitation of intermetallic phase, which is a brittle phase. Through a mapping of microstructure of continuous casted SAF 2507 and SAF 2205, intermetallic phase was observed in samples from SAF 2507.In order to gain an understanding of the appearance of hot cracks in SAF 2507, in situ hot tensile tests were performed. The hot tensile tests were performed in order to compare and analyze the ductile behavior of SAF 2507 with SAF 2205. The purpose of the tensile tests was to simulate continuous casting process at Sandvik.The results from the tensile tests show that SAF 2507 and SAF 2205 are not sensitive to hot cracking at temperatures near solidus. However, intermetallic phases were found in SAF 2507 during hot tensile testing at 800 ° C. But no intermetallic phase could be found in SAF 2205. Temperature measurements and calculations by CFD during continuously casting of SAF 2507 showed that intermetallic phases could possibly precipitate in the mold. In a previous research done by Sandvik Materials Technology, shows how thermal stresses can occur on the corners of a cast inside the mold, during continuous casting of SAF 2205 and SAF 2507.The conclusion of this work is that longitudinal corner crackers in SAF 2507 can be initiated in the mold, because of a combination of thermal stresses and intermetallic phases. Keywords ∙ Duplex rostfritt stål ∙ Varmsprickkänslighet ∙ SAF 2507 ∙ SAF 2205 ∙ Stränggjutning ∙ Duktilitet ∙ Sigma ∙ Kromnitrider ∙

The fundamental aspects of rising argon bubbles in molten metal flow were investigated by numerical simulations. The process of a bubble rising in the molten metal includes two steps, one is the bubble rising inside the liquid, and the other one is the bubble rising across the liquid surface. The bubbling dynamics inside the liquid phase was studied in terms of the bubble’s trajectory, shape and terminal velocity over a wide range of bubble diameters. The flow was assumed to be laminar. The results show that 3~10 mm bubbles rise in a spiral way with strong instabilities which cause them to change their instantaneous shapes. In addition, 10~20 mm bubbles rise rectilinearly and their shapes are kept almost steady. All these bubbles’ terminal velocities are around 0.3 m/s, which are in accordance with literature data. For a bubble with a specific size, small metal droplets can be formed due to the bubble bursting which takes place at the free surface. In a situation when the top surface of the bubble is ruptured, the remains of the bubble will collapse and jet droplets may be formed. Therefore, the simulations of jet droplets were qualitatively analyzed. The results show that when the surface tension is 1.4 N/m, the critical bubble size is 9.3 mm. Also, the ejection is found to increase with an increased surface tension value, unless a critical bubble size is reached. The bubble formation during gas injection into liquids was studied by using a water model and a three-dimensional numerical model. In the experiment, a high-speed camera was used to record the bubble formation processes. Nozzle diameters of 0.5 mm, 1 mm and 2 mm were investigated under both wetting and non-wetting conditions. The bubble sizes and formation frequencies as well as the bubbling regimes were identified for each nozzle size and for different wettabilities. The results show that the upper limits of the bubbling regime were 7.35 L/h, 12.05 L/h and 15.22 L/h under wetting conditions for the 0.5 mm, 1 mm and 2 mm nozzle diameters, respectively. Meanwhile, the limits were 12.66 L/h, 13.64 L/h and 15.33 L/h for the non-wetting conditions. In the numerical model, the Volume-of-Fluid (VOF) method was used to track the interface between the gas and liquid. The simulation results were compared with the experimental observations in the air-water system. The comparisons show a satisfactory good agreement between the two methods. The mathematical model was then applied to simulate the argon-steel system. Simulation results show that the effect of nozzle size is insignificant for the current studied metallurgical conditions. The upper limits of the bubbling regime were approximate 60 L/h and 80 L/h for a 2 mm nozzle and for wetting and non-wetting conditions, respectively. In addition, a poor wettability leads to a bigger bubble size and a lower frequency compared to a good wettability, for the same gas flow rate. / <p>QC 20150504</p>

Shape rolling is a common manufacturing process used to produce long products i.e. bars and wire. One of the problems that might occur during rolling is defect formation leading to rejection of the finished product. This work is a step towards a better understanding of the evolution of some of these defects. The evolution and reduction of cracks during shape rolling is studied in this thesis. To accomplish this, artificial longitudinal cracks are machined along bars of high speed steel. The cracks are positioned at different sites evenly distributed along the periphery in intervals of 45°. Some of the cracks are left open and some are filled with carbon or stainless steel welds. FE simulations are performed using the commercial code MSC.Marc and the results from the simulations are compared with experimental ones. Generally, simulations predict less reduction than observed experimentally. For most positions, the cracks tend to reduce most effectively followed by carbon steel welds and stainless steel welds. To evaluate the inner deformation of a cross section during shape rolling in an oval-round-oval-round series, sample bars of M2 high speed steel are prepared with grids made up by stainless steel wires. After collecting samples after each pass, they are X-rayed to create an image of the grid. The deformation of the wires can favorably be described by FE simulations of a bar originally rotated 10° when entering the first pass. The results suggest that the simulations describe the deformation during shape rolling well. / QC 20101115