This thesis focuses on experimental characterization, understanding and modelling of nickel-based alloy 718, for a large range of loading conditions. Alloy 718 is the most widely used nickel-based superalloy, due to its high strength, high corrosion resistance and excellent mechanical properties at high temperatures. In this work, the mechanical behavior and microstructure evolution of this alloy during high strain rate deformation is investigated. Compression tests using a Split-Hopkinson pressure bar (SHPB) device were performed and the microstructure of the deformed sample was observed using optical microscope (OM) and scanning-electron microscope (SEM) coupled with electron back-scattered diffraction (EBSD) technique. The microstructural evolution according to the deformation conditions was characterized. For high deformation temperatures (1000 C and above), recrystallisation is identifed as the main deformation mechanism. A physics-based model was employed to simulate the deformation behavior of alloy 718. This type of models accounts for the microstructural mechanisms taking place during deformation. Knowledge about the deformation mechanisms of alloy 718, acquired experimentally and from literature, enables to formulate mathematically the microstructural phenomena governing the deformation behavior of the alloy. The proposed model includes the effects of strain hardening, grain boundary strengthening (Hall-Petch), solid solution strengthening, phonon and electron drag and recovery by dislocation glide and cross-slip. It is calibrated and validated using data obtained from mechanical tests, as well as values captured by the microstructural analysis. / H2020-MSCA-ITN-2017 grant agreement Nº764979 - ENABLE project

Understanding the precipitation reaction is very important since precipitation hardening is one of the most effective strengthening mechanisms in metallic alloys. In martensitic steels, a tempering heat treatment is often performed. During tempering various new phases are precipitated and the spatial and temporal evolution of these precipitates strongly influences the properties of the steel, such as strength/ductility, creep, fatigue and hot corrosion resistance. Therefore, the possibility of quantitative modelling of the precipitation process will provide many opportunities for advanced materials and process design and optimization as well as service life assessments. The Fe-C-Cr system forms the basis for tool steels and is consequently used in many applications such as e.g. metal forming operations. They are characterized by a high hardness and good toughness, even at elevated temperatures.In the present work, the as-quenched martensitic microstructures of four Fe-C-Cr alloys with varying Cr and C contents were characterized by Light Optical Microscopy (LOM) and Electron Microscopy. The effects of Cr and C on the morphology of martensite were investigated. It was found that Cr addition had a similar effect as C on the martensitic morphology and on the ratio of high-angle grain boundary (HAGB) to low-angle grain boundary (LAGB). However, the micro-hardness was unaffected by the Cr addition whilst it was strongly influenced by the C addition.In addition, a quantitative experimental characterization of the precipitates formed during tempering of the martensite was performed. The Langer-Schwartz theory combined with the Kampmann-Wagner-Numerical (KWN) method, as implemented in the software TC-PRISMA, was used to predict the precipitation of carbides after tempering in one of the model alloys: Fe-0.15C-4.0Cr (mass%). The microstructure characterization of the as-quenched material provided vital input parameters for the modelling work and a comparison was made between the modelling predictions and the experimental results. The effect of parameters such as dislocation density, grain size and interfacial energy on the precipitation of carbides was discussed. / <p>QC 20151105</p>

This thesis work is focused on the influence of process parameters during gas atomization on the thermal spraying properties of a Ni-Cr-B-Si hardfacing alloy. The metal powder alloy, known as 1-60-20, is produced by Höganäs AB. There have been problems with insufficient fusing during flame spraying of this particular alloy sometimes, even though the chemical composition is always within spec. This has lead to a theory that the difference in performance is caused by differences in parameters during gas atomization. Several gas-gas and gas-water atomizations with varying parameters were performed at the Höganäs Pilot Centre. The powder samples were then analyzed by sieving, scanning electron microscopy, x-ray diffraction and finally tested by powder welding. The results show that by increasing the cooling rate during gas atomization the formation of unstable Ni-borides is possible for this alloy. If these Ni-borides will enhance the fusing properties of the alloy is unknown. According to the literature studied, it should however improve the fusing properties.

Duplex Stainless Steels (DSS) are used extensively in various industrial applications where the properties of both austenite and ferrite steels are required. Higher mechanical strength and superior corrosion resistance are the advantages of DSS. One of the main drawbacks for Duplex steels is precipitation of sigma phase and other intermetallic phases adversely affecting the mechanical strength and the corrosion behavior of the steels. The precipitation of these secondary phases and the associated brittleness can be due to improper heat treatment. The instability in the microstructure of Duplex stainless steels can be studied by understanding the phase transformations especially the ones involving sigma phase. To reduce the time and effort to be put in for experimental work, computational simulations are used to get an initial understanding on the phase transformations. The present thesis work is on the phase transformations involving sigma phase for Fe-Cr system and Fe-Cr-Ni system using theoretical approach in 1D and 2D geometries. A phase field model is implemented for the microstructural evolution in DSS in combination with thermodynamic data collected from the Thermo-Calc software. The Wheeler Boettinger McFadden (WBM) model is used for Gibbs energy interpolation of the system. FiPy- Finite volume PDE solver written in python is used to simulate the phase transformation conditions first in Fe-Cr system for ferrite-austenite and ferrite-sigma phase transformations. It is then repeated for Fe-Cr-Ni ternary system. In the present study a model was developed for deriving Gibbs energy expression for sigma phase based on the common tangent condition. This model can be used to describe composition constrained phases and stoichiometric phases using the WBM model in phase field modeling. Cogswell’s theory of using phase order variable instead of an interpolating polynomial in the expression for Gibbs energy of whole system is also tried.

One main reason for the use of Fe-Cu-C system in PM industry is the presence of liquid phase (copper) at the start of sintering (1120oC). The diffusion of liquid copper into iron causes swelling in the structure. This in turn can cause high dimensional change and, if not controlled properly, may cause distortion. So it is of paramount importance to control the copper diffusion. Carbon, added as graphite, reduces the swelling of copper by changing the dihedral angle. The affect of graphite on copper diffusion depends on the graphite type, particle size of graphite and heating rate. The aim of this work was to find the influence of graphite type and particle size of graphite on copper diffusion. Water Atomized iron (ASC100.29) produced in Höganäs AB was taken as the base powder.  Two types of graphite were used each with two different particle sizes. Two different graphite quantities (0.2% &amp; 0.8%) for each type was taken. Natural fine graphite (UF4), Natural coarse graphite (PG44), Synthetic fine graphite (F10) and  Synthetic coarse graphite (KS44) were the graphites used in this work. Powders were compacted at 600 Mpa and the sintering was done at 1120oC for 30 minutes in 90/10 N2/H2. Dilatometry and metallographic investigation of the samples sintered in the production furnace were used to understand the graphite influence.   The investigation showed that at low graphite levels (0.2%), the affect of graphite type or graphite size was not significant on copper diffusion. At high graphite levels (0.8%),  synthetic graphites were more effective in reducing the swelling of copper. Influence of  particle size of synthetic graphites on Cu diffusion was not significant compared to the influence of particle size of natural graphite. There was also a considerable affect of heating rate on graphite dissolution and copper swelling.

When cold rolling thin steel strips with high tensile strength, reversible cluster mills are used. The customer’s demands are high on the products flatness, which is controlled by the different flatness actuators that the mill is equipped with. The objective of this thesis was to find an optimal CVC shape on the shifting inner second intermediate roll in the 20-high cluster mill KV96 at Sandvik Materials Technology in Sandviken. The project consisted of a theoretical calculation of the roll shape that can give the desired change of the strips flatness and practical development and testing of these rolls in the mill. The results showed that the flatness of the strips can be altered with up to 30 I-units at the edges and 20 I-units in the center with the use of an inner second intermediate roll of the CVC type as this study has concluded. In this work, three CVC shapes was developed using the calculation software Cluster. The rolls was then ground and verified in the mill. / Vid kallvalsning av tunna band med hög hållfasthet används reversibla mångvalsarsverk. Kundkraven är höga på produkternas planhet, vilken styrs av de planhetsställdon verket är utrustat med. Målsättningen med detta examensarbete var att finna en optimal CVC-form på den förskjutningsbara inre mellanvalsen i 20-valsarsverket KV96 vid Sandvik Materials Technology AB i Sandviken. Projektet har bestått av att teoretiskt beräkna den valsform som kan ge den önskade förändringen i planhetsmätarbilden samt att praktiskt ta fram och prova dessa valsar i verket. Resultaten visar att man kan påverka planheten på banden med upp till 30 I-enheter i kanten och 20 I-enheter i mitten med användandet av en inre mellanvals av CVC-typ som denna studie kommit fram till. I det här arbetet utvecklades, med hjälp av beräkningsprogrammet Cluster, tre CVC-valsgeometrier som sedan slipades fram och verifierades i valsverket.

Due to the complex nature of phase interfaces, acquiring precise interfacial energies is usually a big challenge for both experimental measurements and computational modelings. In this thesis, we put forward an efficient route for assessing the temperature dependence of the interfacial energy using density functional theory (DFT). For our investigations, we select the Cu-Co binary system as a model with large miscibility gap. Most of the first-principles calculations presented here are carried out using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA), but other alternative DFT methods are also included in the various stages of the project.The first step is to acquire an accurate thermodynamical description of the Cu-Co binary system. We assess the quality of the predicted thermodynamic properties by an effort to reproduce the phase diagram for the entire range of composition using first-principles calculations and alloy theory. The calculations are performed for the random Cu-Co alloys with face-centered cubic (fcc) structure at both ferromagnetic (FM) and paramagnetic (PM) states, depending on the composition. We demonstrate that the equilibrium volumes and magnetic states are crucial for the proper description of the magnetic entropy of the Cu-Co system at elevated temperatures. More specifically, the contribution of magnetic entropy to the free energy in the Cu-rich region obtained for the PM state turns out to be critical. Furthermore, the adopted equilibrium volumes strongly affect the contribution of the vibrational entropy to the free energy. When all effects are properly accounted for, we find that the ab initio phase diagram of the Cu-Co system agrees well with the Thermo-Calc phase diagram and the experimental observations.The Cu-Co system has a large miscibility gap. The interface between the decomposed Cu-rich and Co-rich phases plays critical roles in the precipitation nucleation and growth, therefore having huge effects on the physical and mechanical properties of the alloys. Therefore, adopting the thermodynamical properties of the bulk Cu-Co alloys successfully obtained by our ab initio calculations, we go further and investigate the interfacial properties of the Cu-Co alloys using a coherent interface model. The chemical, magnetic, and strain energy contributions to the formation energy of the interfaces are analyzed separately. We find that the chemical interfacial energies generally decrease with increasing concentrations, namely when the compositions accross the interface become more homogenous. We identify a sizable contribution to the interfacial energies from the magnetic effects. The temperature dependence of the interfacial energy is estimated, to the first-order approximation, through considering how the equilibrium compositions of the two phases vary at different temperatures. Our results show that the temperature dependence of the interfacial energy originates primarily from the temperature-induced increase of the mutual solubility of the alloy constituents and the loss of the magnetic long range order near the Curie temperature. Our ab initio results are compared with the experimental data as well as with those extracted from Thermo-Calc modeling. The present thesis provides an atomic-level description of the bulk and interfacial properties of the Cu-Co binary system using quantum mechanics simulations. This approach is believed to be useful for a complete thermodynamical description of other similar immiscible alloy systems as well from first-principles. / På grund av fasgränssnittens komplexa karaktär är det vanligtvis en stor utmaning att få exakta gränssnittsenergier för både experimentella mätningar och beräkningsmodeller. I denna avhandling presenterar vi en effektiv väg för att bedöma temperaturberoendet för gränssnittsenergin med hjälp av densitetsfunktionell teori (DFT) i ett modellsystem, Cu-Co-legeringar. Våra första principberäkningar är baserade på den exakta muffins-tennorbitalmetoden (EMTO) i kombination med den koherenta potential-approximationen (CPA).Det första steget är att skaffa en noggrann termodynamisk beskrivning för det binära systemet. Vi bedömer kvaliteten på de förutsagda termodynamiska egenskaperna genom ett försök att reproducera fasdiagrammet för hela kompositionen med hjälp av första principberäkningar och legeringsteori. Beräkningarna utförs för de slumpmässiga Cu-Co-legeringarna med ansiktscentrerad kubisk (fcc) struktur vid både ferromagnetiska (FM) och paramagnetiska (PM) tillstånd, beroende på sammansättningen. Vi visar att jämviktsvolymer och magnetiska tillstånd är avgörande för en korrekt beskrivning av den magnetiska entropin i Cu-Co-systemet vid förhöjda temperaturer. Närmare bestämt visar sig den magnetiska entropins bidrag till den fria energin i den Cu-rika regionen som erhålls vid PM-tillståndet vara kritisk. Vidare påverkar de antagna jämviktsvolymerna starkt vibrationsentropins bidrag till den fria energin. När alla effekter är korrekt redovisade kommer vi fram till att ab initio fasdiagrammet för Cu-Co-systemet överensstämmer väl med experimentellt resultat.Cu-Co-systemet har ett stort blandningsgap. Gränssnittet mellan de sönderdelade Cu-rika och Co-rika faserna spelar en avgörande roll för nederbördskärnbildning och tillväxt och har därför enorma effekter på legeringarnas fysiska och mekaniska egenskaper. Här, med de termodynamiska egenskaperna hos bulk-Cu-Co-legeringarna framgångsrikt erhållna med våra ab initio-tillvägagångssätt, går vi vidare och undersöker gränssnittsegenskaperna för Cu-Co-legeringarna med en koherent gränssnittsmodell. De kemiska, magnetiska och stamenergibidragen till gränssnittets bildningsenergi analyseras separat. Vi finner att de kemiska gränssnittsenergierna generellt minskar med ökande koncentrationer, nämligen när kompositionerna över gränssnittet blir mer homogena. Vi identifierar ett betydande bidrag till gränssnittsenergierna från de magnetiska effekterna. Temperaturberoendet för gränssnittsenergin uppskattas, till första ordningens approximation, genom att överväga hur jämviktskompositionerna i de två faserna varierar vid olika temperaturer. Våra resultat visar att temperaturberoendet för gränssnittsenergin främst härrör från den temperaturinducerade ökningen av legeringskomponenternas ömsesidiga löslighet och förlusten av magnetisk långdistansordning nära Curie-temperaturen. Våra ab initio resultat jämförs med experimentella data såväl som med de som extraherats från Thermo-Calc-modellering.Föreliggande avhandling ger en atomnivåbeskrivning av bulk- och gränssnittsegenskaper hos Cu-Co-binära systemet med hjälp av kvantemekaniska simuleringar, vilket antas vara användbart för en fullständig termodynamisk beskrivning av liknande icke-blandbara legeringssystem med exakta initieringsmetoder.

This thesis focus on investigations of the influence of non-metallic inclusions on corrosion in steel samples taken at different stages of the steel production when producing pipeline steels. The electrolytic extraction technique was used to extract inclusions from these steel samples and thereafter studying them using scanning electron microscopy in combination with energy dispersive microscopy. This approach enabled three-dimensional evaluations of different non-metallic inclusions present in two metal samples of low-carbon Ca-treated pipeline steels. The modification of the inclusion characteristics was investigated and compared for steel samples taken from various stages of the smelting production process. Thereafter, the corrosion resistance of these steels was discussed depending on the characteristics of non-metallic inclusions present in the steel. In addition, this study also presents a new method of soft chemical extraction for qualitative and quantitative evaluations of the initial corrosion processes of a steel matrix surrounding various non-metallic inclusions. The results showed that the initial dissolution of the steel matrix started in areas surrounding CaS inclusions or inclusion phases containing a CaS phase. Furthermore, the results showed that no dissolution of the metal matrix surrounding CaO·Al2O3 and TiN inclusions could be detected.     Key words: pipeline steels, Ca-treatment, non-metallic inclusions, electrolytic extraction, corrosion, chemical extraction. / Sammanfattning Denna avhandling fokuserar på påverkan av icke-metalliska inneslutningar på korrosion i stålprover tagna under olika delar av ståltillverkningsprocessen för tillverkning av rör. Den ektrolytiska extraktionstekniken användes för att extrahera dessa inneslutningar från stålprover och därefter undersöka dem med användande av svepelektronmikroskopi i kombination med energidispersiv spektroskopi. Detta angreppssätt möjliggjorde tredimensionella studier av olika icke-metalliska inneslutningar i två lågkolhaltiga kalciumbehandlade metallprover som används för tillverkning av rör. Modifieringen av inneslutningars karakteristik undersöktes i prover tagna från olika delar av ståltillverkningsprocessen. Därefter så undersöktes motståndet mot korrosion baserat på karakteristiken hos de icke-metalliska inneslutningar som återfanns i stålproverna. Denna studie presenterar också en ny metod baserad på en försiktig kemisk extraktion för kvalitativa såväl som kvantitativa utvärderingar av den initiala korrosionsprocessen av stålmatrisen som omger olika typer av icke-metalliska inneslutningar. Resultaten visade att den initiala upplösningen av stålmatrisen började runt CaS inneslutningar eller komplexa inneslutningar innehållande en CaS fas. Samtidigt så visade resultaten att metallmatriser som omger CaO·Al2O3 och TiN inneslutningar eller faser innehållande dessa inte löstes upp.   Nyckelord: stål för tillverkning av rör, kalciumbehandling, icke-metalliska inneslutningar, elektrolytisk extraktion, korrosion, kemisk extraktion.

Sampling of liquid steel to control the steel making process is very important in the steel industry. However, there are numerous types of disposable samplers and no united standard for sampling. The goal in this study is to investigate the effect of slag protection type and sample geometry on sampling parameters and sample homogeneity. Three sample geometries were selected: i) Björneborg ii) Lollipop with a 6 mm thickness and iii) Lollipop with a 12 mm thickness. These sample geometries have been tested with two types of slag protection: metal-cap-protection and argon-protection. The filling velocity and solidification rate of steel samples have been experimentally measured during plant trials. The sample homogeneity with respect to total oxygen content and inclusion size distribution has been determined in different parts of the samples. The study shows that argon-protected samplers have lower, more even, filling velocities (0.19±0.09 m/s) compared to metal-cap-protected samplers (1.28±2.23 m/s). The solidification rate measurements of the different samplers show that the 6 mm thick Lollipop has the highest solidification rate (99~105 °C/s).  Measurements of total oxygen content in argon-protected samples showed little variation between different zones of the samples. However, metal-cap-protected samples contained much higher total oxygen contents. Light optical microscope studies showed that the increase in total oxygen content was probably caused by entrapment of top slag during sampling. Furthermore, it was found that the contamination of top slag in the metal samples increased with a decreased sample weight. Determination of inclusion size distribution in argon-protected Lollipop samples showed that a larger number of primary inclusions are found in the top part compared to the middle and the bottom part of the samples.

In the present work, the thermodynamic activities of chromium and vanadium oxide in CaO-SiO2-MgO-Al2O3 slags were measured using gas-slag equilibration technique. The slag was equilibrated with a gas mixture of CO, CO2 and Ar gases enabling well-defined oxygen partial pressures in the gas mixture (PO2=10-3,10-4,10-5 Pa) at temperatures 1803, 1823K, 1873, 1923 K. The slags were kept in Pt crucibles during the equilibration and the duration of which was 20 h. From a knowledge of the thermodynamic activity of chromium and vanadium in Cr or V in Pt alloy, obtained from literature, and the oxygen partial pressure in the gas stream calculated by Thermo Calc software, the thermodynamic activity of chromium, vanadium oxide in the slags could be observed.An assessment of the experimental studies in earlier works reveal that, the activities of chromium at low chromium contents and vanadium in their respective alloys in platinum exhibits a strong negative deviation from ideality, the logarithms of activity coefficient of these elements were found to increase with increasing mole fractions of these metals in the Pt-alloys.Regarding the slag phase, all the chromium in the slags was assumed to be present in the divalent state in view of the low Cr contents and the low oxygen potentials employed in the present studies. Analogously, vanadium in the slag was assumed to be in the trivalent state in view of the low vanadium contents in the slag and the low oxygen partial pressure in the gas phase. Activity of chromium oxide, CrO decreases with increasing temperature and decreasing content of chromium oxide in slag and oxygen partial pressure in the gas phase. Activity of vanadium oxide, VO1.5 in slag phase shows a negative deviation from ideality. Activity coefficient of vanadium oxide shows a decrease with basicity of slag and the “break point” occurs at about slag basicity of 1 under the oxygen partial pressure of 10-3 Pa and temperature of 1873 K.A relationship for estimating the actual content of chromium, vanadium in slag as a function of activities of chromium or vanadium, temperature, oxygen partial pressure and slag basicity were developed from the present results, the agreement between the estimated and experimental values is satisfactory, especially at lower oxygen partial pressure.