New materials are always being developed to get the best properties possible where needed. The way to create these materials and test them is also developing. When it comes to high-strength steels, a martensitic microstructure is a common choice. Martensite is a diffusionless phase transformation that generates the brittle martensitic microstructure. Tempering is a process where the brittle martensite is heat treated to make it more ductile and tough while simultaneously precipitating secondary phase particles that could help to strengthen the material. This study focuses on a novel dual-hardening martensitic steel that combines two different precipitates: carbides and intermetallics. The investigations are performed using simulations with the Thermo-Calc module TC-PRISMA to analyse the precipitation. The precipitation modelling is also compared to experimental data from the literature to evaluate the accuracy of the modelling. Out of the six alloys in this study, five were supposed to have NiAl precipitates. What was found was that two alloys, Alloy B and Alloy E, had NiAl precipitates that showed in PRISMA. In the three alloys that did not show NiAl precipitates, two of them did not have the phase stable at respective ageing temperatures. In the last alloy, that only had carbides, both of the precipitates showed up in PRISMA. More work needs to be done on co-precipitation, with comparison between simulations and experiments to confirm that the databases are reliable enough to be used to develop the materials of the future. / Nya material utvecklas hela tiden för att få de bästa möjliga egenskaperna där det behövs. Sättet att skapa dessa material, och testa dem, håller också på att utvecklas. När det gäller höghållfastastål är en martensitisk mikrostruktur ett vanligt val. Martensit är en diffusionsfri fasomvandling som genererar denna spröda martensitiska mikrostruktur. Härdning är en process där den spröda martensiten värmebehandlas för att göra den mer seg och duktil samtidigt som den skiljer ut sekundärfas-partiklar som kan hjälpa till att stärka materialet. Denna studie fokuserar på ett nytt dubbelhärdat martensitiskt stål som kombinerar två olika utskiljningar: karbider och intermetalliska utskiljningar. Undersökningarna utförs med hjälp av simuleringar med Thermo-Calc-modulen TC-PRISMA för att analysera utskiljningarna. Utskiljningsmodelleringen jämförs också med experimentella data från litteraturen för att utvärdera modellens noggrannhet. Av de sex legeringarna i denna studie skulle fem ha NiAl-utskiljningar. Det som konstaterade svar att endast två legeringar, legering B och legering E, hade NiAl-utskiljningar som visades i PRISMA. I de tre där det inte visades hade två av dem inte den fasen stabil vid respektiveåldringstemperaturer. Den sista legeringen hade bara karbider och i PRISMA dök de båda två upp. Mer arbete måste göras med samhärdning, med jämförelser mellan simuleringar och experiment för att bekräfta att databaserna är tillförlitliga nog för att kunna användas för att utveckla framtidens material.

L’acier Ferrium® M54® présente une composition chimique optimisée, basée sur 40 ans d’évolution et de développement des aciers à durcissement secondaire à précipitation de carbures M2C. Le compromis de propriétés Rm/KIC/KISCC obtenu par la nuance M54® permet d’envisager son utilisation dans les trains d’atterrissage d’avions gros porteurs. Cependant, les premiers essais mécaniques, réalisés par l’utilisateur pour la montée en maturité de la nuance, ont montré une variabilité des propriétés mécaniques suivant le traitement thermique appliqué. Ce travail de thèse s’applique donc à décrire les évolutions microstructurales au cours du traitement thermique de la nuance M54® et les impacts sur les propriétés mécaniques en se concentrant notamment sur le traitement par le froid. Les différentes conditions de mise en solution et de revenu testées ont montré une certaine stabilité de la précipitation au revenu et des propriétés mécaniques qui en découlent. La précipitation a été caractérisée à différentes échelles afin de la comparer avec celle issue des nuances de la même famille. En revanche, selon les conditions de traitement par le froid réalisées, la limite d’élasticité varie de manière significative sans qu’aucun des paramètres liés à la précipitation ne soient modifiés. Le taux d’austénite est en revanche un paramètre déterminant pour la limite d’élasticité et est très sensible aux conditions de traitement par le froid : temps et température entre la trempe à l’huile et le traitement cryogénique et température de traitement cryogénique. Un traitement thermique amélioré a ainsi été proposé pour obtenir un taux d’austénite réduit et constant et limiter ainsi les variations de limite d’élasticité. / Ferrium® M54® steel presents an optimized composition, based on 40 years of research and development on secondary hardening steels. This alloy exhibits an excellent Rm/KIC/KISCC balance that allows considering its use in landing gears applications of wide-body aircrafts in the future. However, initial mechanical tests performed by the end-user have shown variability in mechanical properties depending on the applied heat treatment. The main goal of this work is to describe the microstructural evolutions of the alloy M54® during heat treatment and their impact on the resulting mechanical properties with a specific focus on the effect of the cryogenic treatment.The different austenitizing and tempering conditions investigated have shown a stability of the tempering precipitation and mechanical properties. This precipitation has been characterized at different scales and compared with other grades of the same family. On the other hand, depending on cryogenic treatment conditions, a significant variation of the mechanical properties and in particular of the yield strength is observed without any modification in the precipitation distribution and volume fraction or size. Austenite content is critical for the yield strength and is very sensitive to the cryogenic treatment conditions: time and temperature before cryogenic treatment and temperature of cryogenic treatment. An improved heat treatment to obtain reduced and constant austenite content is proposed.

Atualmente, há a necessidade de se desenvolver aços com alta resistência à propagação de trincas, especialmente em condições de carregamentos cíclicos, ou seja, resistentes à fadiga, na qual sua aplicabilidade se da em função de suas propriedades mecânicas. No presente trabalho estudou-se o efeito do preaquecimento na zona afetada pelo calor do aço AISI/SAE 4130 com composição química modificada, com altos teores de Mo, comparando as propriedades mecânicas e microestruturais nas condições como soldada, tratada termicamente e aplicando a técnica de Metodologia do Preaquecimento Combinado (MPC) com otimização de ciclos térmicos através da combinação do preaquecimento entre o 1º e 3º passe da 1º e 2º camada, respectivamente. A fim de avaliar as propriedades mecânicas, foram realizados ensaios de microdureza, mapeamento de dureza e ensaio de tenacidade ao impacto charpy. Para análise microestrutural, foi realizado microscopia ótica e microscópio eletrônico de varredura (MEV) para analisar as regiões de grãos grosseiros, fino e as intersecções entre as regiões da Zona Afetada pelo Calor (ZAC), quando aplicado o MPC. Como resultado, observa-se que preaquecimento é uma forma efetiva de redução de dureza, chegando a uma redução máxima de 71 HV0,1, quando comparado às temperaturas de preaquecimento entre 150 e 400 ºC. No entanto há um severo efeito deletério na tenacidade, podendo chegar a uma queda de 71% da energia absorvida. O tratamento térmico pós soldagem (TTPS) se mostrou eficiente apenas para amostra soldada com temperaturas de preaquecimento de 150 ºC, para as demais temperaturas não houve benefício, tanto em redução de dureza, quanto na restauração da tenacidade. No entanto, para temperatura de preaquecimento de 230 ºC também foi observado o acréscimo de dureza após o TTPS devido ao efeito de endurecimento secundário por precipitação de carbonetos metálicos (MC). A técnica MPC se mostrou muito eficiente em redução da dureza e restauração da tenacidade, e este fenômeno está associado à capacidade de solubilizar os carbonetos que precipitam durante a soldagem, fenômeno que não ocorre com a aplicação do TTPS. / Currently, it is necessary to develop materials with high resistance to crack propagation, especially under conditions of cyclic loading condition such as fatigue resistant, in which its applicability is due to its mechanical properties. In the present work the effect of preheating in the heat-affected zone of the AISI / SAE 4130 steel with modified chemical composition (High Mo) was compared, regarding its mechanical and microstructural properties of each welding condition, As weld, post weld heat treated and applying the Methodology of combined preheating (MCP) with optimization of thermal cycles by combining the preheating between the 1st pass of 1st layer and the 3rd pass of 2nd layer. In order to evaluate the mechanical properties, microhardness tests, hardness mapping and charpy V notch tests were performed. For microstructural analysis, optical and scanning electron microscopy (SEM) were used to analyze the coarse grained regions and the intersections between the Heat Affected Zones (HAZ) regions, when applied to the MPC. As a result, it is observed that preheating is an effective form of reduction of hardness, reaching a maximum reduction of 71 HV0,1, when compared to the preheating temperatures between 150 and 400ºC, however there is a severe deleterious effect in the toughness, dropping up to 71% of the absorbed energy. The post weld heat treatment (PWHT) is efficient only for welded sample with preheating temperatures of 150 ºC, for the other temperatures there was no benefit, either in reduction of hardness or restoration of toughness. However, for the preheating temperature of 230 °C it was observed the increase of hardness after the PWHT due to the effect of secondary hardening by precipitation of metal carbides (MC). The MPC technique proved to be very efficient in decreasing hardness and restoring toughness, and this phenomenon is associated with the ability to solubilize the carbides that precipitate during welding, which is not observed while PWHT is applied.

Secondary hardening ultra-high strength (SHUHS) steels possess a unique combination of strength, fracture toughness and stress corrosion cracking resistance, which makes them candidate materials for aircraft landing gear and armour applications. There is a sustained drive to develop stronger and tougher materials for such applications. The objectives of this thesis are two-fold: first, to develop a new SHUHS alloy that is stronger than the existing SHUHS steel developed at Defence Metallurgical Research Laboratory (DMRL), Hyderabad and second, to establish processing-structure-property correlations for the new alloy. Empirical design and development of these complex steels involves enormous effort, cost, time and materials resources. To avoid this, a semi-empirical approach was espoused in this thesis wherein thermodynamic calculations using ThermoCalc were conducted to computationally design a series of alloys with varying levels of Cr and Mo. The design space was constrained by two objectives related to M2C carbides which are the primary cause of secondary hardening in these alloys. The first objective was to increase the amount of M2C to increase the peak strength, while the second objective was to lower the Cr/Mo ratio of the M2C to control its over-ageing behavior. Two new alloys C23 (with 2Cr-3Mo, wt. %) and C55 (with 5Cr-5Mo, wt. %) and a base alloy akin to the DMRL SHUHS steel, C21 were selected for experimental validation. These alloys were melted, rolled and subjected to a battery of heat treatments. Austenitization studies revealed that the new alloys required higher austenitization temperatures to dissolve primary carbides. However such a treatment also resulted in an austenite composition that was not conducive for obtaining a fully martensitic microstructure on quenching. Based on these studies, the design space was modified to include additional criteria related to the Ms and precipitate dissolution temperatures. C55 failed to clear either criteria, while C23 cleared both, and so tempering studies were limited to C23. Isochronal tempering studies revealed that C23 in the peak aged condition was >10% stronger than C21 indicating that the alloy design objective of strength enhancement was achieved successfully. Microstructural characterization revealed that the strength enhancement was due to the higher number density and volume fraction of the M2C-like solute clusters in C23, which resist shearing in the under-aged condition and strengthen by Orowan mechanism in the over-aged condition. This thesis has successfully demonstrated that the design paradigm of enhancing strength by increasing the amount of M2C is justified and that ThermoCalc can be used to as an objective-oriented alloy design tool in this class of the steels.

One of the most important additives in High Speed Steels (HSS) is cobalt, mainly for its effect on the hot properties. Based on statistic data about the increased price of cobalt and its negative effect on human health, an ethical and financial barrier in the steel industry have occurred. In order to solve the problem, it is of great importance to examine the future cobalt price and accessibility, as well as examine the possibility of finding alternative substitutes to cobalt. The purpose of this project was therefore to examine alternatives to cobalt as an alloying element in HSS. A qualitative literature study was performed by analyzing the economy of cobalt, studying the main reasons for cobalts tendency to improve the hot properties of the steel and finding alternative elements to replace, or at least reduce, cobalt in HSS without degrading the hot properties. Cobalt is used both in the chemical and metallurgical business. But the demand of cobalt is largely driven by chemical purposes with the focus on its rechargeable battery applications. The analysis shows that there is nothing pointing at a significant decrease of the price of cobalt. Lithium ion batteries stands for about 50% of current cobalt supply, which is why the price has surged the recent years. The market for electric vehicles and rechargeable batteries has skyrocketed. To decrease the price of cobalt, a substitute for cobalt in rechargeable batteries would need to be found, which is not very likely for the time being. The effect of cobalt in HSS is mainly on the red hardness and tempering resistance. Cobalt increases the bonding strength in the steel matrix and changes the microstructure of the finer secondary carbides. Also the growth rate and coalescence rate of the carbides decreases. This causes the red hardness and the tempering resistance to increase. To replace cobalt, several alternative alloying elements have been researched. Among the most promising are niobium, nitrogen and aluminium, where niobium were found to be of most interest, due to the broad support of relevant articles in the field of powder metallurgical processing. The positive effect of niobium could be regarded as three-fold. The first contribution is the refinement of grain size and homogeneity of the primary carbides, which increases the overall hardness. The second effect is that the addition of niobium shifts the phase equilibria in such a way that the precipitation of primary carbides mainly will be in the form of hard and stable NbC. The majority of the other alloying elements will hence be precipitated as secondary carbides during tempering. The final effect is an increase in secondary hardness, as a consequence of the large amounts of vanadium and smaller amounts of niobium that is being precipitated during tempering to the secondary carbides. This enables a high matrix hardening potential in the optimal state of tempering.

The development of tool steels for making and shaping other materials requires a better understanding of the material's properties during manufacture. These high-quality steels include many alloying elements, which give increased hardness during tempering. For producing hardened microstructures, austenite generation is essential. The martensite formed by rapid quenching of austenite followed by tempering helps develop high strength steels. Studying carbide precipitation is a challenge as they are very small in size, present only in small volume fractions and high number densities. The carbide reactions are complicated due to so-called metastable carbides, which are only present as part of the precipitation process. This work focuses on model alloys with two main elements in addition to iron and carbon, molybdenum, and vanadium, to clarify and simplify the carbide characterization. This is done to determine the effect of molybdenum and vanadium carbides on the overall hardness. In this work, two model alloys, A and B, are tempered at 550°C and 600°C with the same vanadium content but different molybdenum contents. The hardness of the materials is evaluated and compared at these temperatures. A more detailed characterization work is done for material A with Scanning Transmission Electron Microscopy-Energy Dispersive Spectroscopy (STEM-EDS) to understand the microstructure and analyze the precipitates. Simulations are performed with Thermo-Calc Prisma (TC-Prisma) to support the experimental work, which includes the simulation of the secondary carbide precipitation, mainly molybdenum carbides in material A tempered for 24h at 600°C, and predicts the carbide precipitation behavior in this steel. The results from STEM-EDS and TC-Prisma for material A, show that the small secondary carbides in the martensite contribute to the increased strength of material A. Due to the overaging of the carbides at 600°C, the hardness at 550°C is higher than at 600°C for material A. The given thesis work is an attempt to interpret the development of secondary carbides of Mo and V in the martensitic matrix and their role in the overall hardness. / Den ständiga utvecklingen av högpresterande stål för transport, konstruktion och energisektorn kräver bättre förståelse för materialets egenskaper vid tillverkning. Dessa martensitiska stål inkluderar många legeringselement vilket ger ökad hårdhet vid härdning och anlöpning. Att studera utskiljning av karbider är en utmaning eftersom de är närvarande endast i liten volymsfraktion. Karbidreaktionerna är komplexa till följd av så kallade metastabila karbider vilka endast är närvarande vid en del av utskiljningsförloppet. För att tydliggöra och förenkla karbidkarakteriseringen fokuserar detta arbete på modellegeringar med två huvudelement utöver järn och kol, molybden och vanadin. Detta görs för att fastställa effekten av molybden och vanadinkarbider på den totala hårdheten. I detta arbete studeras två modellegeringar, A och B, härdade och anlöpta vid 550 °C och 600 °C med samma vanadininnehåll men olika molybdeninnehåll. Materialens hårdhet utvärderas och jämförs vid dessa temperaturer. Ett mer detaljerat karaktäriseringsarbete görs för material A med hjälp av STEM-EDS för att förstå mikrostrukturen och analysera utskiljningarna. Simuleringar görs med TC-PRISMA för att stödja det experimentella arbetet, vilket inkluderar simulering av den sekundära karbidutskiljningen och predikterar karbidstrukturen i dessa stål. Resultaten visar att de små sekundärkarbiderna i martensiten bidrar till den ökade styrkan hos material A. Hårdheten vid 550 °C är högre än vid 600 °C för material A eftersom både utskiljningen av karbider är sker långsammare och även dislokationsåterhämtning.