Cette thèse étudie l'effet de la ségrégation des solutés d’étain sur la formation de la texture de recristallisation dans les alliages ferritiques. La diffraction d’électrons rétrodiffusés (EBSD) et la sonde atomique tomographique ont été utilisées pour étudier respectivement le développement de la texture et la ségrégation locale des atomes de soluté. Des mesures d’EBSD in situ révèlent que l'hétérogénéité de la déformation dans la microstructure laminée à froid est un facteur crucial pour l’évolution au cours du recuit ultérieur, en particulier dans les alliages ternaires Fe-Si-Sn. L’ajout d’étain s'est avéré avoir un effet profond sur la texture de recuit. Il a été montré que Sn affecte principalement les phénomènes de recuit par interaction soluté-dislocation et ségrégation aux joints de grains. Des observations directes par sonde atomique tomographique à chaque étape de la recristallisation est discuté et un effet fort au stade de la germination de la recristallisation est mis en évidence. La sonde atomique tomographique combinée à la modélisation atomistique de la ségrégation à l’équilibre a permis de conclure que la ségrégation dépend de la désorientation. Néanmoins, la ségrégation du soluté dans les joints de grains à grand angle (joints de grains spéciaux et généraux) s'est avérée indépendante de leurs caractéristiques géométriques. Enfin, le développement de la texture peut s’expliquer par la théorie de la nucléation orientée de la recristallisation, alors que la présence de certaines interfaces mobiles pourrait également contribuer à la croissance orientée de certains grains recristallisés / This Ph.D. thesis investigates the effect of Sn solute segregation on the formation of recrystallisation texture in ferritic alloys. Both electron back-scatter diffraction and atom probe tomography were used to investigate the texture development and the local solute segregation respectively. In-situ electron back-scatter diffraction reveals that the strain heterogeneity in the deformed microstructure is a crucial factor for subsequent annealing, especially in the solute added alloys. Solute was found to have a profound effect on the annealing texture. Mainly, Sn was shown to affect the annealing phenomena via solute-dislocation interaction and grain boundary segregation. Direct observations with atom probe tomography reveal and quantify the levels of segregation at grain boundaries during the development of the recrystallised microstructure. The role of segregation at each stage of recrystallisation is discussed and a strong effect at the recrystallisation nucleation stage is suggested. Atom probe tomography results in combination with atomistic modelling of equilibrium segregation, concluded that the segregation depends on the misorientation. Nonetheless, the solute segregation in high-angle grain boundaries was found to be independent of their geometric characteristics (i.e. general, special grain boundaries). Finally, texture development could be explained in terms of the oriented nucleation theory of recrystallisation, while the presence of some mobile interfaces may subsequently also contribute in the oriented growth of some recrystallised grains

The feasibility of in-situ alloying of AISI 410L martensitic stainless steel with nitrogen during Nd-YAG laser cladding was investigated with the aim of achieving a nitrogen content of at least 0.08 wt% and fully martensitic microstructures in the final clad deposit. Two in-situ nitrogen alloying techniques were studied. In the first set of experiments, the absorption of nitrogen from nitrogen-rich gas atmospheres was studied. Laser cladding with commercially available AISI 410L powder was performed using nitrogen-rich shielding and carrier gas. A marginal increase in deposit nitrogen content was observed, with the clad deposit displaying low hardness and mostly ferritic microstructures. Poor nitrogen absorption from nitrogen-containing atmospheres during Nd-YAG laser cladding is generally attributed to the short thermal cycle and to suppression of plasma formation above the weld pool. In the remaining experiments, Si3N4 powder was investigated as an alternative source for nitrogen during cladding. The addition of Si3N4 to the AISI 410L powder feed resulted in clad microstructures consisted of columnar -ferrite grains with martensite on the grain boundaries, higher hardness and an increase in deposit nitrogen content (to a maximum of 0.064 wt% nitrogen). Higher nitrogen contents in the clad deposit, however, significantly increased the volume percentage porosity in the clad layer. This prompted an investigation into the feasibility of raising the nitrogen solubility of the alloy through additions of manganese and nickel to the powder feed. Thermodynamic modelling revealed that the addition of manganese to AISI 410L powder increases the nitrogen solubility limit due to its negative interaction parameter with nitrogen. The addition of up to 3.5 wt% manganese to AISI 410L powder containing Si3N4 significantly increased the nitrogen solubility in the deposit. A martensitic microstructure with 0.12 wt% nitrogen and a peak hardness of 410 HV was achieved without any adverse increase in porosity in the clad layer. The clad nitrogen content easily exceeded the minimum requirement of 0.08 wt%. High nickel concentrations in AISI 410L stainless steel expand the austenite phase field at the expense of -ferrite and alter the solidification mode from ferritic to austenitic-ferritic. The addition of up to 5.5 wt% nickel, or combinations of nickel and manganese, to the nitrogen-alloyed AISI 410L powder feed raised the deposit nitrogen content, but not to the same extent as those deposits alloyed with manganese only. Since more austenite is present on cooling in nickel-alloyed AISI 410L deposits, less nitrogen is rejected to the liquid phase on solidification, resulting in higher nitrogen contents and less porosity in the room temperature microstructures. The amount of dilution during single-track laser cladding is mainly influenced by the specific energy per unit mass delivered by the laser beam. The clad height is strongly influenced by the powder deposition rate, whereas the bead width is influenced by the wettability of the deposits during laser cladding. During multi-track cladding, the observed percentage porosity is a function of the aspect ratio of the individual beads making up the clad layer, the deposition rate and the clad height. High deposition rates result in thicker layers, increasing the distance that N2 gas bubbles have to travel to escape to the atmosphere, while a high aspect ratio favours interbead porosity. The results suggest that in-situ nitrogen alloying during laser cladding should preferably be performed at low deposition rates to ensure higher clad nitrogen contents and hardness, lower clad heights, less dilution and less porosity. / Dissertation (MEng)--University of Pretoria, 2016. / Materials Science and Metallurgical Engineering / MEng / Unrestricted

Die Anwendbarkeit des Master Curve-(MC-)Konzepts zur Charakterisierung des Zähigkeitszustandes bestrahlter Reaktordruck¬behälter-(RDB-)Stähle wurde an drei RDB-Stählen überprüft: IAEA-Referenzstahl 3JRQ57, 1JFL11 (vergleichbar mit 22NiMoCr3-7) sowie russischer WWER-440 Grundwerkstoff KAB-B. In Zugversuchen, Charpy-V-Tests, Risswiderstandskurven nach ASTM E1820 und Master Curve Tests zur Bestimmung der Referenztemperatur T0 nach ASTM E1921 wurden der unbestrahlte Ausgangszustand, je drei Bestrahlungszustände bis hin zu Neutronenfluenzen von 100∙10^18 n/cm² (E>1MeV) sowie bei 475°C/100h thermisch ausgeheilte Zustände untersucht. Mit zusätzlichen auf dem MC-Konzept basierenden Auswerteverfahren nach SINTAP, multimodalem MC-Ansatz (MML) sowie der Unified Curve erfolgte die Bewertung des Einflusses von Materialinhomogenitäten und möglicher MC-Formänderung bei hohen Fluenzen. Wie erwartet geht Neutronenbestrahlung mit Verfestigung und Duktilitätsabnahme einher, d.h. Härte, Festigkeitskennwerte, Charpy-V-Übergangstemperaturen T28J und T41J sowie T0 steigen mit der Neutronenfluenz, während die Bruchdehnung und Hochlagenzähigkeit abnehmen. Am bestrahlungsempfindlichsten reagiert der Stahl 3JRQ57, gefolgt von KAB-B und 1JFL11. Durch die Ausheilbehandlung von 475°C/100h erholen sich die Werkstoffkennwerte der Zugversuche, Charpy-V-Tests und MC-Versuche auf den jeweiligen unbestrahlten Ausgangszustand. Die technischen Ersatzkennwerte für duktile Rissinitiierung bleiben relativ unbeeinflusst von der Neutronenbestrahlung. Die MC nach ASTM E1921 beschreibt die Bruchzähigkeits-Temperaturverläufe für alle drei RDB-Stähle in allen Bestrahlungs- und Ausheilzuständen gut. Bei den niedrig und mittel bestrahlten Zuständen liegen meist mehr als 5% der KJc(1T)-Werte unterhalb der MC-Kurve für 5% Versagenswahrscheinlichkeit. Die MC beschreibt den hoch bestrahlte Zustand (ca. 100∙10^18 n/cm², E>1MeV) aller drei RDB-Stähle sehr gut, auch für Daten außerhalb des Gültigkeitsbereiches T0±50K, und auch für den bestrahlungsempfindlichen 3JRQ57 mit inhomogenem Gefüge. Die Unified Curve überbewertet den Einfluss der Neutronenbestrahlung auf die MC-Kurvenform. Eine mögliche Formänderung der MC durch Neutronenbestrahlung konnte bei keinem der drei untersuchten RDB-Stähle nachgewiesen werden.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21434号 / エネ博第376号 / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 今谷 勝次 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

Modified rolled homogeneous armor (RHA) steels were designed and produced to characterize the effects of niobium additions and accompanying heat treatments on microstructure and mechanical properties. This study combines in-house steel production and property analysis to advance the understanding of niobium on enhanced hardenability and weldability in a chemistry-process-structure-property relationship paradigm. For steel production, designed alloys were cast in a vacuum induction melting furnace and thermo-mechanically processed. Optimal heat treatment conditions were determined by utilizing a thermo-mechanics calculation software. Microstructures were investigated by optical and electron microscopy while hardenability was characterized by Jominy end-quench tests. Mechanical tests were performed at various stress states, strain rates, and temperatures to understand deformation behavior under complex loading conditions. Encouraging results in performance were observed in the micro-alloyed armor steels as compared to reference materials from earlier studies.

A seven-element Modified Embedded Atom Method (MEAM) potential comprising Fe, Mn, Si, C, Al, Zn, and O is developed by employing a hierarchical multiscale modeling paradigm to simulate low-alloy steels, inhibition layer, and galvanized coatings. Experimental information alongside first-principles calculations based on Density Functional Theory served as calibration data to upscale and develop the MEAM potential. For calibrating the single element potentials, the cohesive energy, lattice parameters, elastic constants, and vacancy and interstitial formation energies are used as target data. The heat of formation and elastic constants of binary compounds along with substitutional and interstitial formation energies serve as binary potential calibration data, while substitutional and interstitial pair binding energies aid in developing the ternary potential. Molecular dynamics simulations employing the developed potentials predict the thermal expansion coefficient, heat capacity, self-diffusion coefficients, thermomechanical stress-strain behavior, and solid-solution strengthening mechanisms for steel alloys comparable to those reported in the literature. Interfacial energies between the steel substrate, inhibition layer, and surface oxides shed light on the interfacial nanostructures observed in the galvanizing process.

No / M3/2 sintered high speed steel and composite materials processed by initial admixing of 5 wt.% TiC (to decrease wear) and 5 wt.% MnS (to minimise friction) powders, singly and in combination, were assessed in pin-on-disc tribometers specially constructed to simulate use in the automotive valve train. Pins were of the sintered materials and the mating tribological material discs of T1 high speed steel. For comparison with existing conventional materials, identical tests were performed with discs and pins of two types of spheroidal cast iron. Testing at 110 °C, employing a few drops of fresh Shell Helix Standard SAE: 15W-40, API: SJ/CF oil, in daily segments of 5000 m of sliding distance was carried out until the break of the boundary film and appearance of the early signs of seizure. Accordingly the conditions were initially elastohydrodynamic, then mixed lubrication, then boundary, and finally decaying boundary. In comparison with the baseline cast iron system, the friction, wear and lifetime performance of all the high speed steel systems was markedly superior. MnS further lowered the coefficient of friction and TiC increased the load carrying capacity of M3/2 steel. The lifetime, test distance until seizure, was the most discriminating parameter between the high speed steel systems, being 1.5¿3 times longer for the unmodified M3/2 than the composites and 10 times longer than that of the cast irons system.

No / To interpret nitrogen sintering of chromium and manganese steels without the formation of deleterious oxides, but with manganese and carbon the formation of deleterious oxides, but with manganese and carbon modifying the local microclimate, the role of the volatile Mn and modifying the local microclimate, the role of the volatile and Mn carbothermic reactions were considered. carbothermic reactions were considered. Reduction of Cr Reduction of Cr 2 2 O About 3 3 by Mn vapour by Mn vapor is always favourable. is always favorable. CO is an effective reducing agent, however, whereas CO is an effective reducing agent, however, whereas at atmospheric pressure it will reduce FeO at730°C, temperatures some at atmospheric pressure it will reduce FeO at730 ° C, temperatures some 500 and 700°C higher, ie above those for conventional sintering, are 500 and 700 ° C higher, ie above those for conventional sintering, are necessary for reducing Cr necessary for reducing Cr 2 2 O About 3 3 and MnO, respectively. and MNO, respectively. Accordingly partial Accordingly partial pressures must be considered and the sintering process is modelled at a pressures must be considered and the sintering process is modeled at a conglomerate of several surface oxidised alloy particles surrounding a pore conglomerate of several surface oxidised alloy particles surrounding a pore with graphite present and a tortuous access to the nitrogen-rich atmosphere with graphite and present a tortuous access to the nitrogen-rich atmosphere containing some water vapour and oxygen. containing some water vapor and oxygen. The relevant partial pressures The relevant partial pressures were calculated and reduction reactions become thermodynamically were calculated and reduction reactions become thermodynamically favourable from 200°C. favorable from 200 ° C. Kinetics, however, dictates availability of CO and Kinetics, however, dictates availability of CO and the relevant reactions are the water-gas, C + H the relevant reactions are the water-gas, C + H 2 2 O = CO + H O = CO + H 2 2 from 500°C from 500 ° C and the Boudouard, C + CO and the Boudouard C + CO 2 2 = 2CO, from 700°C. = 2CO, from 700 ° C. Discussion of sintering Discussion of sintering mechanisms is extended to processing in semi-closed containers, also mechanisms is extended to processing in a semi-closed containers, also possessing specific microclimates.

<p> The advantages of Thin-Slab Cast Direct-Rolling (TSCDR) process include reduced capital, energy, labour and inventory costs, as well as the ability to roll thinner strip compared to the conventional process of thick slab casting, reheating and hot rolling. There is great interest in utilizing this technology to produce microalloyed steels which can meet American Petroleum Institute (API) standards. However, whereas the conventional approach can produce APIX80, APIXlOO, and even APIX120 steels; the TSCDR process can only produce APIX70 and APIX80. The main obstacles in the way of achieving high API grades are the non-uniform initial as-cast microstructure and the large grains that result from grain growth at high temperature. The production of APIX80 and higher grade steels can only be achieved through a comprehensive research initiative that combines careful control of solidification, homogenization, thermomechanical-processing, cooling and coiling. </p> <p> This contribution examines the solid state microstructure evolution of microalloyed steels under simulated TSCDR conditions. The grain growth kinetics in delta-ferrite and austenite were studied separately using two model alloys. At high temperatures and in the absence of precipitation, the growth kinetics in both delta-ferrite and austenite appeared to follow a simple parabolic growth law. The measured grain growth kinetics was then applied to the problem of grain-size control during the process of TSCDR. Several strategies of controlling and refining the grain size were examined. The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer; the results showed that the austenite phase formed along the original delta grain boundaries, and that the precipitation of austenite at the delta-ferrite grain boundaries effectively pins delta grain growth. The kinetics of the phase transformation was modeled using a local equilibrium model that captures the partitioning of the substitutional elements during the transformation. </p> <p> A novel delta-ferrite/austenite duplex microstructure is proposed to achieve fine and uniform high-temperature microstructure. The grain growth of the matrix phase (delta-ferrite) is controlled by the coarsening mechanism of pinning phase (austenite). The effectiveness of this delta/austenite duplex microstructure was validated experimentally and analyzed in details using a physically-based model. </p> / Thesis / Doctor of Philosophy (PhD)

The unique combination of high specific strength and ductility in third generation advanced high-strength steels (3G-AHSSs) has garnered significant attention from top automotive steel industries. These materials are being considered as potential options for making lighter body components due to their strength and ability to tolerate thinner material cross-sections. However, galvanizing these steels through the continuous hot-dip galvanizing process is challenging, because the main alloying elements such as Mn, Si, Al, and Cr tend to selectively oxidize on the steel surface during the annealing process before being immersed in the galvanizing bath containing Zn(Al, Fe). The presence of these oxides extensively covering the substrate surface can negatively impact reactive wetting, coating adhesion, and overall coating quality. In this study, the selective oxidation kinetics and reactive wetting of a series of Fe-(2-10)Mn-(0.00/0.01/0.03)Sb (at. pct) were determined and a model was proposed for analyzing oxide growth during intercritical annealing prior to galvanizing. Annealing heat treatments were carried out at 676, 725, 775, and 825 ˚C for 60-480s holding time in a N2-5vol pct H2 process with a dew point of –10 ˚C. MnO was formed on all samples after annealing. It was determined that the annealing conditions (temperature and isothermal holding time) affected the external oxide thickness and depth of the oxidation zone, which in turn influenced the MnO growth rate. With increasing the bulk Mn content of the alloy, the Mn elemental flux to the external surface increased, resulting in an increase in the oxidation parabolic rate constant. The average activation energy of internal oxidation for the Fe-2Mn, Fe-6Mn and Fe-10Mn alloys were determined to be 216±15 kJ/mol, 178 ± 18 kJ/mol and 152 ±10 kJ/mol, respectively, which are consistent with the activation energy of oxygen diffusion through MnO interfaces and the bulk diffusion of oxygen in austenite. Moreover, the average activation energy for external oxide growth was ~113±18 kJ/mol, which was attributed to the diffusion of Mn cations along the grain boundaries of the external Mn oxides. It was determined that micro addition of Sb to the Fe-Mn alloys led to a reduction in the oxidation rate constant, external oxide thickness, and internal oxidation zone, which was attributed to Sb segregation at both the external and internal oxide interface, resulting in the reduction of oxygen permeability. The reduction was more significant in the Fe-10Mn alloys, primarily attributable to the increased Sb segregation at the interfaces. The research showed that when the bulk Mn content increased, more antimony (Sb) segregated at both the internal and external oxide/substrate interface. As a result, the oxygen present at these interfaces decreased. This is attributed to the reduction of Sb solubility in α-Fe with increasing Mn and positive interactions between Sb and Mn. Advanced Atom Probe Tomography (APT) analysis confirmed that as more Sb segregated at the interfaces, the excess oxygen reduced due to site competition between O and Sb. Additionally, Sb surface segregation kinetics for Fe-(0.01/0.03)Sb and Fe-2Mn-(0.01/0.03)Sb at.% were determined based on the modified Darken model and linear heating followed by isothermal annealing. After the annealing, Sb segregation was detected on the surface of both the Fe-xSb and Fe-2Mn-xSb alloys, which increase with increasing temperature and holding time. The segregation rate, as determined from the Darken curves, was higher in Fe-Sb alloys compared to Fe-2Mn-Sb alloys, which can be attributed to variations in the crystal structure and the density of defects within the metal matrix. Additionally, the activation energy for Sb diffusion in both Fe-Sb and Fe-2Mn-xSb alloys were determined to be approximately 193±18 kJ/mol closely aligns with the activation energy of Sb bulk diffusion in α-Fe. Simulated galvanizing treatments were conducted on Fe-(2-10)Mn-(0.00/0.03)Sb at.% alloys. It was found that Sb segregation at the external/oxide interface resulted in a decrease in the size and thickness of the external oxide particles, which can facilitate better contact between the zinc bath and the substrate. Furthermore, it was found that Sb segregation at the interface between the external oxide and substrate led to a decrease in the stability of the interfacial region. This effect was attributed to an increase in the local atomic spacing near the interface, caused by Sb segregation. As a result, a local strain was observed near the interface. This localized strain significantly reduced the energy needed to separate the oxide from the metal matrix, contributing to decreased stability of the interfacial region. The higher bulk manganese (Mn) content led to increased segregation of antimony (Sb), resulting in a greater local strain within the interfacial region. These effects, in turn, enhanced the kinetics of the aluminothermic reduction reaction and assisted oxide lift-off. Furthermore, the closely packed Fe-Al intermetallics at the coating/steel interface increased as a result of adding Sb to the steel. In addition, no Sb segregation was observed at interfacial layer/metal interface. This absence of segregation can be attributed to the dissolution of segregated Sb into the liquid zinc. It was determined that Sb, which segregated at the external oxide/substrate interface during annealing, dissolved into the zinc bath and disrupted its bond with iron. This disruption occurred due to the higher electronegativity of Sb compared to Fe with Zinc, as well as the sufficient solubility of Sb in liquid zinc. / Thesis / Doctor of Science (PhD) / The unique combination of high specific strength and ductility exhibited by third-generation advanced high-strength steels has captured the attention of automotive industries. However, challenges arise when attempting to galvanize these steels through continuous hot-dip galvanizing processes. The selective oxidation of alloying elements during annealing can have detrimental effects on reactive wetting and coating adhesion. The objective of this research was to improve the coating quality of Mn-containing steels by introducing micro-additions of Sb. Sb segregation to the surface and interfaces began to occur during annealing. Segregated Sb resulted in a reduction of the oxidation rate. Sb segregation at oxide interfaces also contributed to decreased oxygen permeability. Upon immersion in the liquid zinc bath, both Sb and Fe dissolved into the zinc, leading to the formation of an interfacial layer on the surface, which indicates successful reactive wetting. The findings of this research provide valuable insights for improving galvanizing processes and enhancing coating quality, specifically in the context of third-generation advanced high-strength steels.