Abstract In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route). In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted. When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work. The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.

Os aços de ultra resistência com médio carbono e baixa liga apresentam excelentes propriedades de resistência mecânica e à fadiga além de uma elevada temperabilidade. No entanto, tais aços apresentam o fenômeno da têmpera ou de tratamentos superficiais. O aço 300M foi desenvolvido visando a ampliação desta faixa, permitindo tratamentos em temperaturas em torno de 350ºC viabilizando o emprego da técnica de nitretação do mesmo por plasma sem a consequente fragilização. Neste trabalho comparou-se as características e propriedades do aço 300M nitretado por plasma à 350ºC e 550ºC com os aços de alta-resistência AISI 4340, 4140 e 5140 nitretados à 550ºC. Variou-se as condições de tratamento de forma a verificar a influência do tempo de nitretação e de frequência de pulso nas camadas nitretadas. Verificou-se que a nitretação por plasma demostrou ser bastante efetiva para conferir resistência ao desgaste abrasivo para todos os aços nitretados. O aço 300M nitretado em 350ºC apresentou no ensaio de desgaste abrasivo comportamento similar aos demais aços nitretados em 550ºC, enquanto que o melhor resultado geral foi alcançado pelo 300M nitretado à 550ºC em CC durante 6 horas, devido à formação de uma camada dupla de levada dureza e espessura. / The ultrahigh-strength steels with medium carbon and low alloy, like AISI 4340, show excellent tensile strength and fatigue properties further higher hardenability. However, these steels have the phenomenon of tempering embrittlement, that restrict the temperature of treatments after quench or surfaces treatments. The 300M steel was developed aiming the enlargement this area of application, this steel permits treatments in temperatures about 350°C what allowed to use the plasma nitriding without problems with the embrittlement. In this production were compared the properties of the nitrited layers of the 300M nitriding by plasma in 350°C and 550°C with the steels AISI 4340, AISI 4140, AISI 5140 nitriding in 550°C. Plasma nitriding conditions like time and pulse frequency were varied. It was checked that the plasma nitriding showed good performance to improve the resistance to the abrasive wear in ali the steels tested. The 300M steel nitrited in 350°C showed results similar to the others in the pin-on-disk test and the better result occurred to the 300M steel nitrited in 550°C in dC plasma during 6 hours, because of a duble compound lawyer that arouse with a high thickness and high hardness.

Microalloyed steels are widely used in oil and gas pipelines. They are a class of high strength, low carbon steels containing small additions (in amounts less than 0.1 wt%) of Nb, Ti and/or V. The steels may contain other alloying elements, such as Mo, in amounts exceeding 0.1wt%. Microalloyed steels have good strength, good toughness and excellent weldability, which are attributed in part to the presence of precipitates, especially nano-precipitates with sizes less than 10nm. Nano-precipitates have an important strengthening contribution, i.e. precipitation strengthening. In order to fully understand steel strengthening mechanisms, it is necessary to determine the precipitation strengthening contribution. Because of the fine sizes and low volume fraction, conventional microscopic methods are not satisfactory for quantifying the nano-precipitates. Matrix dissolution is a promising alternative to extract the precipitates for quantification. Relatively large volumes of material can be analyzed, so that statistically significant quantities of precipitates of different sizes are collected. In this thesis, the microstructure features of a series of microalloyed steels are characterized using optical microscopy (OM) and scanning electron microscopy (SEM). Matrix dissolution techniques have been developed to extract the precipitates from the above microalloyed steels. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) are combined to analyze the chemical speciation of these precipitates. Rietveld refinement of the XRD pattern is used to fully quantify the relative amounts of the precipitates. The size distribution of the nano-precipitates (mostly 10 nm) is quantified using dark field imaging (DF) in the TEM. The effects of steel chemistry and processing parameters on grain microstructure and the amount of nano-precipitates are discussed. Individual strengthening contributions due to grain size effect, solid solution strengthening and precipitation strengthening are quantified to fully understand the strengthening mechanisms of the steels. / Materials Engineering

Martensitic steels have become very important engineering materials in modern society. Crucial parts of everyday products are made of martensitic steels, from surgical needles and razor blades to car components and large-scale excavators. Martensite, which results from a rapid diffusionless phase transformation, has a complex nature that is challenging to characterize and to classify. Moreover the possibilities for modeling of this phase transformation have been limited, since its thermodynamics and kinetics are only reasonably well understood. However, the recent development of characterization capabilities and computational techniques, such as CALPHAD, and its applicability to ferrous martensite has not been fully explored yet. In the present work, a thermodynamic method for predicting the martensite start temperature (Ms) of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X systems obtained from experiments using very rapid cooling, and Ms values for lath and plate martensite are treated separately. Comparison with the experimental Ms of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of Ms from commercial steels. A major part of the present work is dedicated to the effect of carbon content on the morphological transition from lath- to plate martensite in steels. A range of metallographic techniques were employed: (1) Optical microscopy to study the apparent morphology; (2) Transmission electron microscopy to study high-carbon plate martensite; (3) Electron backscattered diffraction to study the variant pairing tendency of martensite. The results indicate that a good understanding of the martensitic microstructure can be achieved by combining qualitative metallography with quantitative analysis, such as variant pairing analysis. This type of characterization methodology could easily be extended to any alloying system and may thus facilitate martensite characterization in general. Finally, a minor part addresses inverse bainite, which may form in high-carbon alloys. Its coupling to regular bainite is discussed on the basis of symmetry in the Fe-C phase diagram. / <p>QC 20120824</p> / Hero-m

Stainless steels are high-alloyed, usually with multiple components and often also dual matrix phases, as for duplex stainless steels. This make predictions and calculations of alloying effects on equilibria and transformations complicated. Computational thermodynamics has emerged as an indispensable tool for calculations within these complex systems with predictions of equilibria and precipitation of phases. This thesis offers examples illustrating how computational methods can be applied both to thermodynamics, kinetics and coarsening of stainless steels in order to predict microstructure and, to some extent, also properties. The performance of a current state-of-the-art commercial thermodynamic database was also explored and strengths and weaknesses highlighted. / <p>QC 20130429</p>

Concrete is strong in compression; however, it is quite fragile in tension. To overcome this flaw, concrete is frequently reinforced with bars typically made of low grade, low carbon steel. The environment inside of concrete is favorable for steel; unfortunately when passive steel is exposed to chlorides, active corrosion can initiate, resulting in damage to the structure. One source of chloride contamination is through anti-icing agents which are used to inhibit the formation of ice on roadways, ensuring safe driving conditions. This represents a serious concern from both the cost associated with rehabilitation (Canadian infrastructure deficit in 2003 was $125 billion [1]) and as a safety concern to the public. In Canada, 5 million tonnes of road salts are used each year [2], of which Ontario uses 500 to 600 thousand tonnes [3]. As a result, the Ministry of Transportation Ontario (MTO) has requested a study of four frequently used anti-icing agents: 25.5% NaCl, 31.5% MgCl2, 37.9% CaCl2 and 32.6% multi Cl- (12% NaCl, 4% MgCl2 and 16% CaCl2). The objective of the study is two-fold, the first is comparing the effects of the solutions on steel embedded in concrete (high pH environment) and the second is to compare the effects of the anti-icing agents to a variety of construction steels in atmospheric conditions (neutral pH). Macro-cell and micro-cell corrosion in concrete were tested using both modified ASTM G109 prisms and concrete beams with 6 embedded black steel bars. Unfortunately, these tests proved inconclusive; all of the steel remained passive. This was a result of casting a high quality concrete in laboratory conditions which ultimately lead to minimal diffusion of the anti-icing solutions. Therefore, it is recommended that for short term corrosion testing (<2 years), poor quality concrete or cement paste should be used. Micro-cell testing in synthetic concrete pore solution contaminated with the anti-icing solutions was conducted in order to obtain results in the period of the M.A.Sc. program and to directly observe the corrosion. The initial concentration of Cl- in each solution was 0.00% Cl-; this was incrementally increased by 0.005% Cl-/week. Potentiostatic linear polarization to resistance measurements and pH measurements were used to monitor the corrosion on a weekly basis. The results of this test showed that MgCl2 has the most detrimental effects due to the drop in pH (from 13.5 to 9.1) caused by Mg replacing Ca in Ca(OH)2 to form the less soluble Mg(OH)2. The transition from passive to active corrosion initiated at 0.7, 0.4-0.9, 0.6 and 0.6% Cl- for NaCl, MgCl2, CaCl2 and multi Cl-, respectively. The active corrosion current densities were 11mA/m2 for NaCl, CaCl2 and multi Cl-, whereas MgCl2 had active corrosion rates of ~100 mA/m2. One bar exposed to CaCl2 showed corrosion rates as high as 600 mA/m2. This was a result of crevice corrosion between the shrink fitting and the rebar. Once the expansive corrosion products broke through the shrink fitting and ample supply of oxygen became available, allowing the corrosion rates to spike dramatically. The following steels were tested directly in the diluted solution in a cyclic corrosion chamber: stainless steels: 304L, 316LM, 2101, 2205, 2304, XM28; corrosion resistant steel reinforcing bars (rebar): galvanized rebar, guard rail (galvanized plate steel) and MMFX; carbon steels: black steel rebar, box girder, drain, weathering steel. The reinforcing bars were virgin steels whereas the remaining steels were components from the field. The testing regime followed SAE J2334 using the anti-icing solutions diluted to 3% by wt. Cl- as the immersion liquid. Unfortunately, the mutli Cl- solution was not tested due to time constraints. The mass change per unit area was measured every five cycles. All stainless steels exposed to all anti-icing solutions exhibited similar changes in mass per unit area, less than 10 g/m2. All plain carbon steels including weathering steel exhibited mass changes per unit area of more than 1000 g/m2 with some variability between the various anti-icing solutions and steel types, although the black steel rebar typically outperformed the other carbon steels. The corrosion products of MMFX were non-adherent, resulting in inconclusive results. The galvanized layer on the guard rail, which had been exposed to the environment in service, proved to be more protective than the fresh zinc coating on the galvanized rebar. When exposed to the MgCl2 solution, the mass change of both new and used galvanized steels was comparable to that found in the stainless steels. When exposed to NaCl solutions, the galvanized guard rail also exhibited this trend, whereas the new galvanic coating did not, suggesting that with exposure to the atmosphere a galvanic coating will protect the steel against NaCl. In all cases galvanized steel exposed to CaCl2 solutions exhibited mass changes per unit area of less than 100 g/m2 this is considered moderate, as this value is one order of magnitude higher than the stainless steels and one order of magnitude lower than the carbon steels exposed to the same test. It is recommended that galvanic coatings be utilized in areas heavily exposed to anti-icing solutions. The weathering steel offers no advantages over carbon steels when directly exposed to anti-icing solutions. Furthermore, in areas with high amounts of exposed galvanized steel, CaCl2 should be avoided. Between the four solutions tested, NaCl solutions are recommended as the anti-icing agents that, overall, causes the least amount of damage to both the reinforcing steel in concrete and to exposed metallic components. NaCl is followed by multi Cl- and CaCl2. Even though MgCl2 causes less damage when directly exposed to carbon steels and galvanized steels than CaCl2, it is much easier to repair external components than internal components. Therefore, MgCl2 is not recommended.

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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.

In this study a new method is introduced to produce bimodal grain structures in low carbon steels. This method is based on cold rolling of dual phase structures and appropriate annealing treatments. The difference in the recrystallization behaviour of ferrite and martensite yields a heterogeneous microstructure with a distribution of coarse and fine grains. These types of microstructures are of interest for optimizing the balance of strength and uniform elongation in ultra-fine grained low carbon steels.

Concrete is strong in compression; however, it is quite fragile in tension. To overcome this flaw, concrete is frequently reinforced with bars typically made of low grade, low carbon steel. The environment inside of concrete is favorable for steel; unfortunately when passive steel is exposed to chlorides, active corrosion can initiate, resulting in damage to the structure. One source of chloride contamination is through anti-icing agents which are used to inhibit the formation of ice on roadways, ensuring safe driving conditions. This represents a serious concern from both the cost associated with rehabilitation (Canadian infrastructure deficit in 2003 was $125 billion [1]) and as a safety concern to the public. In Canada, 5 million tonnes of road salts are used each year [2], of which Ontario uses 500 to 600 thousand tonnes [3]. As a result, the Ministry of Transportation Ontario (MTO) has requested a study of four frequently used anti-icing agents: 25.5% NaCl, 31.5% MgCl2, 37.9% CaCl2 and 32.6% multi Cl- (12% NaCl, 4% MgCl2 and 16% CaCl2). The objective of the study is two-fold, the first is comparing the effects of the solutions on steel embedded in concrete (high pH environment) and the second is to compare the effects of the anti-icing agents to a variety of construction steels in atmospheric conditions (neutral pH). Macro-cell and micro-cell corrosion in concrete were tested using both modified ASTM G109 prisms and concrete beams with 6 embedded black steel bars. Unfortunately, these tests proved inconclusive; all of the steel remained passive. This was a result of casting a high quality concrete in laboratory conditions which ultimately lead to minimal diffusion of the anti-icing solutions. Therefore, it is recommended that for short term corrosion testing (<2 years), poor quality concrete or cement paste should be used. Micro-cell testing in synthetic concrete pore solution contaminated with the anti-icing solutions was conducted in order to obtain results in the period of the M.A.Sc. program and to directly observe the corrosion. The initial concentration of Cl- in each solution was 0.00% Cl-; this was incrementally increased by 0.005% Cl-/week. Potentiostatic linear polarization to resistance measurements and pH measurements were used to monitor the corrosion on a weekly basis. The results of this test showed that MgCl2 has the most detrimental effects due to the drop in pH (from 13.5 to 9.1) caused by Mg replacing Ca in Ca(OH)2 to form the less soluble Mg(OH)2. The transition from passive to active corrosion initiated at 0.7, 0.4-0.9, 0.6 and 0.6% Cl- for NaCl, MgCl2, CaCl2 and multi Cl-, respectively. The active corrosion current densities were 11mA/m2 for NaCl, CaCl2 and multi Cl-, whereas MgCl2 had active corrosion rates of ~100 mA/m2. One bar exposed to CaCl2 showed corrosion rates as high as 600 mA/m2. This was a result of crevice corrosion between the shrink fitting and the rebar. Once the expansive corrosion products broke through the shrink fitting and ample supply of oxygen became available, allowing the corrosion rates to spike dramatically. The following steels were tested directly in the diluted solution in a cyclic corrosion chamber: stainless steels: 304L, 316LM, 2101, 2205, 2304, XM28; corrosion resistant steel reinforcing bars (rebar): galvanized rebar, guard rail (galvanized plate steel) and MMFX; carbon steels: black steel rebar, box girder, drain, weathering steel. The reinforcing bars were virgin steels whereas the remaining steels were components from the field. The testing regime followed SAE J2334 using the anti-icing solutions diluted to 3% by wt. Cl- as the immersion liquid. Unfortunately, the mutli Cl- solution was not tested due to time constraints. The mass change per unit area was measured every five cycles. All stainless steels exposed to all anti-icing solutions exhibited similar changes in mass per unit area, less than 10 g/m2. All plain carbon steels including weathering steel exhibited mass changes per unit area of more than 1000 g/m2 with some variability between the various anti-icing solutions and steel types, although the black steel rebar typically outperformed the other carbon steels. The corrosion products of MMFX were non-adherent, resulting in inconclusive results. The galvanized layer on the guard rail, which had been exposed to the environment in service, proved to be more protective than the fresh zinc coating on the galvanized rebar. When exposed to the MgCl2 solution, the mass change of both new and used galvanized steels was comparable to that found in the stainless steels. When exposed to NaCl solutions, the galvanized guard rail also exhibited this trend, whereas the new galvanic coating did not, suggesting that with exposure to the atmosphere a galvanic coating will protect the steel against NaCl. In all cases galvanized steel exposed to CaCl2 solutions exhibited mass changes per unit area of less than 100 g/m2 this is considered moderate, as this value is one order of magnitude higher than the stainless steels and one order of magnitude lower than the carbon steels exposed to the same test. It is recommended that galvanic coatings be utilized in areas heavily exposed to anti-icing solutions. The weathering steel offers no advantages over carbon steels when directly exposed to anti-icing solutions. Furthermore, in areas with high amounts of exposed galvanized steel, CaCl2 should be avoided. Between the four solutions tested, NaCl solutions are recommended as the anti-icing agents that, overall, causes the least amount of damage to both the reinforcing steel in concrete and to exposed metallic components. NaCl is followed by multi Cl- and CaCl2. Even though MgCl2 causes less damage when directly exposed to carbon steels and galvanized steels than CaCl2, it is much easier to repair external components than internal components. Therefore, MgCl2 is not recommended.