This thesis aims to investigate both the two-dimensional and three-dimensional (sub)structure of the butterfly martensite (α’BF) morphology. In order to do this, multiple different types of heat treatments were applied to a Fe-25Ni alloy to find the optimal morphology consisting of a low density of α’BF that is surrounded by austenite(γ). Two dimensional analyses were performed by applying a combination of optical microscopy, scanning electron microscopy and Electron Backscatter Diffraction (EBSD). The α’BF morphology was found to nucleate and grow as the first martensite (α’) morphology just below the martensite start temperature (Ms) within this 25Ni alloy. A decrease in austenisation time resulted in smaller γ grain sizes. This reduction in γ grain size resulted in a reduction of the Ms of the alloy which in its turn reduced the amount of undercooling applied to the material below Ms upon quenching towards room temperature, causing a decrease in the freshly formed α’ fraction. Upon observation of the freshly formed α’ it was found that the α’BF seemed to prefer formation near the centre of γ grains instead of near γ grain boundaries. When α’BF was formed near a γ grain boundaries, one wing tends to aligned itself with this boundary. Through trace analysis, it was found that the habit planes of α’BF were close to {557}γ, {225}γ and {3 10 15}γ, which are characteristic habit planes of lath, butterfly and lenticular α’, respectively. The orientation relationship (OR) between the γ and α’BF is found be a combination of both the Greninger-Troiano (G-T) and Nishiyama-Wasserman (N-W) OR. Three dimensional analysis was performed using both serial sectioning and 3D-EBSD. Serial sectioning showed that α’BF within this alloy was sensitive to formation upon mechanical polishing. 3D-EBSD gave insight on the three-dimensional morphology and substructure of the α’BF grains. It was observed that the junction plane could be non-continuous. Moreover, it is shown that the apparent wing angle greatly depends on the angle that the α’BF grain makes perpendicular to the sample surface and that the tail of the the α’BF grain can run along the entire length of the grain.

Deliberately mutilated weapons and other objects are repeatedly discovered in ancient burials from the Iron Age. This research is focused on the Early Iron Age bent swords from the Hallstatt C period (800-600 BC) found in archaeological sites in the Netherlands. Metallographic research methods are used to investigate how these swords were bent, i.e., using a blacksmith’s fire or brute force. This elaborates on the Early Iron Age culture as it infers what kind of knowledge and skills were required for the bending process. With the help of a blacksmith, we created a replica to analyse the effect of different types of bending on the microstructure. This is compared with museum samples. Using optical microscopy and SEM(-EBSD) the microstructure of the museum sample and the replica are analysed for signs of deformation. Elemental analysis (SEM-EDS) is used on slag inclusion to estimate the initial iron and sword production processes. EPMA analysis was used to determine the carbon concentration throughout the samples, suggesting the use of wrought iron and hardening techniques. Results show that the Heythuysen sword contains multiple microstructure phases with various carbon concentrations. Most probably a combination of piling techniques and carburisation was applied during the production of the Heythuysen sword. The several bending methods of the replica show a distinction in the microstructure on the level of local misorientation. This is sensitive to the presence of inclusions and the changes in phase and grain size, which complicated the evaluation of the Heythuysen sword. The Heythuysen sword does not show strong evidence of bending by brute force and is most likely bent by a blacksmith with a fire.

Tailored Universal Feedstock for Forming (TuFF) is a highly aligned discontinuous fiber (ADF) composite, developed to create composite laminates with metal-like formability. This material has shown fiber directional stretch while retaining fiber volume fractions up to 57%, demonstrating the ability to create complex geometry components. Conventional stretchforming of ADF laminates results in an undesirable high number density of localized deformations, and non-uniformity in cure ply thickness and areal weight throughout the material. Emphasis is placed on achieving a consistently repeatable behavior of the material during stretch forming, aiming to minimize variability in material characteristics throughout the forming process steps. This objective responds to challenges faced in conventional stretchforming of ADF laminates, which tends to yield undesirable outcomes despite the material’s demonstrated capabilities. The study explores advancements in enhancing the formability of ADF thermoplastic materials, particularly through the application of confinement pressure and a comprehensive analysis of the materials’ microstructural behavior during stretchforming. Composite blanks consist of 8 ply thick cross-layup laminates, composed of 3 mm long IM7 carbon fibers with a semi-crystalline thermoplastic resin of Low-Melting Polyaryletherketone (LMPAEK). The report discusses crucial aspects associated with forming ADF composites, including their unique characteristics, processing considerations, and limitations. Additionally, it presents a case study of double-diaphragm forming process applied to the novel composite material TuFF. The upgradation presented by this work shows how relatively small amounts of confinement pressure allow for a significant improvement in part quality at aerospace-grade components.

Roller bearings are one of the most important industrial components with a large variety of challenging environmental applications. Under water-contaminated lubricant conditions, a pro- tective layer of oxide forms on the surface of the most used 100Cr6 bearing steel, which helps suppress the corrosion rate, while adding corrosion-resistant alloying elements can improve this corrosion resistance further. Recent research suggests that the Cu alloying element in high- carbon steels contains numerous carbide precipitates, which can help to form a stronger passive layer and, thus, more effective in protecting the steel against corrosion. Meanwhile, the accu- mulation of Cu as a tramp element can degrade the quality and performance of scrap-based steel. This limits the proportion of scrap that can be used in new products and may require adding ore-based steel to dilute the copper content and maintain the required properties. This study investigates the effect of incorporating copper in high-carbon bearing steel manufactured from scrap on enhancing long-term atmospheric corrosion resistance. The effective usage of this tramp element will contribute to boosting the circular economy of steel.
In this project, high-carbon bearing steel containing a high amount of Cu (0.5 wt %) was hardened using martensitic or bainitic hardening and compared with the baseline hardened steel with no excessive Cu. The microstructures of the steel were characterized by LOM, SEM (EDS), XRD and XPS to study the retention/segregation of Cu. Electrochemical corrosion tests were used to validate the prediction. Finally, LOM, SEM (EDS), XRD and XPS were utilised to characterise the corroded surfaces.

This study investigates the microstructural evolution of high-carbon quench and partitioning (Q&P) steels and the effect of the chemical composition using techniques such as dilatometry, optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). The applied Q&P treatments, starting from full austenitization, were applied to the developed steel, leading to microstructures containing volume fractions of retained austenite between up 0.15-0.30. During the partitioning step, a large fraction of the austenite was sufficiently carbon enriched to be retained at room temperature. In some alloys, depending on chemical composition and fraction of austenite at quench, bainite formation occurred during isothermal holding. Notably, no fresh martensite was formed. The microstructure showed pronounced microstructural banding due to the segregation of alloying elements such as Mn and Si. Solute-rich bands show a coarser microstructure of tempered martensite (TM), relatively large austenite islands and bainite. While solute-lean regions show a finer microstructure of TM and fine film-like retained austenite. Increasing C content resulted in a decrease of the martensite start temperature (𝑀𝑠 ) leading to a higher fraction of untransformed austenite at similar quenching temperatures as well as tetragonal martensite. At higher fractions of untransformed austenite the microstructural difference between solute-rich and solute-lean bands is more pronounced. Alloying elements, such as Mn and Ni, play a critical role in refining the microstructure and stabilising austenite.

The presence of retained austenite (RA) has been proven beneficial for the formability of Advanced High Strength Steels. Therefore, multiple thermal austenite stabilization methods have been researched, such as Mn-partitioning during intercritical annealing in medium Mn steels. The Cyclic Partial Phase Transformation (CPPT) approach has been successful in obtaining localized Mn-enrichment at the ferrite/austenite interface through cyclic annealings at intercritical temperatures in a medium Mn steel. In this thesis, the possible application of CPPT heat treatments towards interfacial austenite stabilization at room temperature in medium Mn Dual Phase (DP) steels is studied. ThermoCalc modelling and dilatometry measurements were used to determine the optimal CPPT parameters for fully martensitic Fe-Mn-C-Si samples with varying concentrations of manganese. Scanning Electron Microscopy acquisitions of the resulting microstructure showed a strong influence from the initial microstructure. Samples which went through a full austenitization formed coarse martensitic islands and equiaxed ferrite grains. However, samples which were intercritically annealed from a fully martensitic state resulted in fine and elongated ferrite grains surrounded by martensite. X-Ray Diffraction analysis revealed that CPPT treated samples with 2wt.%Mn and 4wt.%Mn had a maximum RA volume fraction of 1wt.% and 4wt.% respectively. Up to 16wt.% of RA was obtained at room temperature through CPPT heat treatments for samples with 6wt.% Mn. This difference in results has been attributed to the concentration spike of Mn at the ferrite/austenite interface characterizing the NPLE austenite growth, as well as to the widening of the Mn-enriched zone through repeated isothermal intercritical annealing cycles. While a relatively high volume fraction of RA has been successfully obtained at room temperature in 6wt.% Mn DP steels, its interfacial morphology could not be confirmed due to the interference of the fine ferrite/martensite microstructure with the Electron Back-Scattered Diffraction analysis.

The presence of bainite and retained austenite in advanced high-strength steels (AHSS) has grown a great interest in the automotive industry as simultaneously provides strength and the transformation-induced plasticity (TRIP) effect. This type of microstructure is present in modern trailing arms. A trailing arm is part of the air suspension system in heavy vehicles. In spring steels, such as the ones used to produce trailing arms, this combination of bainite and retained austenite is obtained by a heat treatment named austempering.

In this work, the microstructural distribution of a silicon spring steel (Fe-0.6C-1.63Si- 0.97Mn-0.48Cr) was investigated after austempering for 1 hour at 300o C. Microstruc- tural heterogeneity was investigated at different scales using optical microscopy, scan- ning electron microscopy (SEM), electron probe micro analysis (EPMA), X-ray diffraction (XRD), and hardness measurements. Observed heterogeneities were related to the pres- ence of thermal gradient, chemical segregation, and carbon gradient.

The results gave insights into the effect of the processing route on the microstruc- ture. After austempering, bainitic ferrite, martensite-austenite islands, and carbide par- ticles were observed. The thermal gradient was confirmed by the identification of auto- tempered M-A islands due to the slower cooling rate in the bulk of the material. More- over, chemical segregation of Si, Mn, and Cr parallel to the rolling direction was observed and confirmed from optical microscopy, SEM, and EPMA results. In regions with high concentrations of substitutional elements, the Bs temperature was locally reduced and the bainitic ferrite formation kinetics was slow. Carbon gradient in M-A islands was ob- served as carbide precipitation occurred at the center of the island indicating that the carbon content was lower compared to at the edges of the constituent. Furthermore, in areas of the trailing arm with lower fractions of retained austenite (≺ 8%), bainite forma- tion was ceased based on the To curve (austenite reached a specific carbon content over which growth of bainite based on the diffusionless theory is not possible). These results indicate that bainitic ferrite advances non-uniformly through the trailing arm.

Based on these indications, the carbon gradient had a more significant effect on the bainitic ferrite formation in the spring steel during austempering compared to the ther-mal gradient and chemical segregation.

Quenching and Partitioning (Q&P) is a novel steel heat treatment to create microstructures containing martensite and retained austenite. In recent years, there has been a growing interest in applying Q&P treatments to martensitic stainless steels with the aim to create stainless steels with the mechanical properties of Advanced High-Strength Steels (AHSS). The development of Q&P stainless steels for lightweight structural applications could be a game changer for the automotive industry. The application of Q&P stainless steels can increase the service life of a car, reduce maintenance cost and contribute to lowering CO2 emissions. In this thesis, Q&P treatments with varying quenching temperatures were applied to two novel stainless steel alloys with composition 0.2C-0.35Si-0.7Mn-12.5Cr and 0.2C-0.35Si-3.0Mn-12.5Cr. The effect of quenching temperature and Mn content on the microstructure development was investigated using dilatometry, X-ray diffraction (XRD), optical microscopy, electron backscatter diffraction (EBSD), electron probe micro analysis (EPMA) and phase field simulations. It was found that Mn addition lowers the optimal quenching temperature and increases the maximum retained austenite fraction that can be obtained by Q&P treatment. Austenite phase fractions of approximately 0.22 and 0.3 were stabilized in the microstructures of the low-Mn and the high-Mn alloy, respectively. The carbon concentration of retained austenite increases with decreasing quenching temperature but is much lower than expected from the full partitioning assumption. The non-uniform distribution of primary martensite and untransformed austenite as well as carbide precipitation was seen as a means by which austenite enrichment and retention are reduced. Additionally, results from this study indicate that segregation of Mn and Cr affects the local Ms temperature and causes microstructural banding of primary martensite which leads to a non-uniform retained austenite distribution in the final microstructure.

The strength-ductility trade-off has been a long standing dilemma in material science. With the use of laser surface treatments, effort has been made in order to obtain a heterogeneous material with a well defined architectured microstructure to optimize this trade-off. In the present study, it is investigated how the deformation behaviour of Martensite/Austenite steel microstructures in a Fe-25Ni-0.2C alloy can be tailored by the creation of patterned microstructures with localized laser treatment. Due to these treatments strong variations in microstructure are observed. Two different patterns are created (i) a dotted diagonal pattern and (ii) a dotted horizontal pattern which are both evaluated using as-quenched and tempered martensite as base material. The deformation behaviour of the patterned microstructures is investigated using both experiments and simulations. In the experimental approach characterization of the laser treated specimens is carried out using Optical Microscopy (OM) and micro-hardness measurements. The local deformation of the patterned microstructure is investigated using Digital Image Correlation (DIC). During deformation, strain partitioning is observed in the austenitic areas. In these areas mechanically induced martensitic transformation takes place, influencing the hardening of the material. The phase strength does not seem to influence the austenite stability in the analysed patterns, however it does show difference in hardness of the freshly formed martensite. For the simulation, Crystal Plasticity Finite Element Modelling (CPFEM) is used to describe the plastic deformation of the patterned material. Both the local and overall behaviour are investigated and validated with the experimentally obtained results. The simulation can serve as a tool to identify patterns which show promising deformation behaviour.

In this work the effect of ferrite on the formation of bainite is studied. Different ferrite fraction of up to 25% were obtained through the application of three different intercritical annealing treatments to an Fe-0.2C-3Mn-2Si (wt%) steel. Two of the heat treatments consisted of an isothermal hold at temperatures between 835°C and 950°C, which was followed by slow cooling to a bainite formation temperature of 400°C. A third heat treatment included fast cooling from top temperatures between 810°C and 950°C to the same bainite formation temperature of 400°C. The heat treated material has been studied by means of dilatometry, optical microscopy, scanning electron microscopy and Vickers micro-hardness tests. All of the involved ferrite fractions had an accelerating effect on bainite formation, when compared to samples where no prior ferrite was present. The accelerating effect was larger for smaller prior ferrite fractions. It is proposed that the amount of preferential nucleation sites for bainite is increased by the formation of prior ferrite and leads to an acceleration of bainite formation. The presence of a significant amount of silicon (2 wt%) prevented the formation of cementite and resulted in carbon enrichment of austenite during ferrite formation. For the evaluated ferrite fractions, the accelerating effect due to ferrite presence is greater than the decelerating effect caused by the carbon enrichment of austenite, the latter becoming stronger with increasing ferrite fraction. Additionally, it is proposed that, due to the increase in preferential nucleation sites, a large amount of bainite sheaves start to grow simultaneously. Together with the autocatalytic nucleation of new bainite sub-units, this could account for the high bainite formation rates that were observed in the presence of ferrite. A change in bainite morphology from degenerated upper bainite to granular bainite with increasing prior ferrite fractions was also noticed. No significant difference in hardness was observed between the samples that contained prior ferrite, but the fully austenitised samples were slightly harder. It is expected that increasing ferrite fractions will result in a decrease in overall hardness, when macro-hardness tests would be applied.

Quenching and partitioning (Q&P) is a novel processing route that can create microstructures that combine high strength, ductility and easy formability without the excess use of alloying elements by also keeping the production cost low. Therefore, this method is ideal to be applied to steels that are used in the automotive industry. The material is initially annealed at a high temperature to form austenite. Then it is quenched to a temperature below the martensite start temperature to form a controlled fraction of martensite. Second annealing follows, at a lower temperature than the first one, that stabilises the austenitic phase at room temperature. It has been found that the austenite grain size at the end of the first annealing (prior austenite grain size) has a significant impact on the processing method, the final microstructure and consequently on the final properties of the material. In this thesis, phase-field modelling was employed to investigate the role of prior austenite grain size on the quenching and partitioning process. Simulations of the processing were performed for three different prior austenite grain sizes (6, 25 and 67 μm). Microstructures consisting of about 10.5% austenite and 89.5% martensite were created by simulating the first quenching of the Q&P process. Then, the evolution of the microstructure and carbon distribution were investigated for partitioning at 400 ◦C for times up to 4000 s, and quenching to room temperature. The results gave insights into the effect of the prior austenite grain size on the morphology of the microstructure and the kinetics of carbon during partitioning. It was found that a finer microstructure was derived with decreasing PAGS. Moreover, partitioning becomes more efficient because of the shorter distance that carbon has to diffuse, and the even spatial distribution of austenite. Finally, wide grain size distributions of the austenite that is present during partitioning can lead to significant variations in the carbon content among the austenite grains of the final microstructure, especially for short partitioning times. This affects the efficiency of the partitioning process and has an impact on the mechanical stability of the austenite in the final microstructure and consequently on the TRIP phenomenon.

Laser surface treatments offer interesting prospects for the creation of architectured microstructures formed by distinct phases in metastable austenitic steels. In the present study, a laser-based localised heat treatment was developed to locally create an austenitic region in a quenched Fe25Ni0.2C martensitic microstructure. The highly localised laser heat flux gives rise to high spatial gradients in peak temperature and heating rate. This results in strong variations in the microstructures observed over short distances, which are related to local changes in the martensite to austenite phase transformation temperatures and formation mechanisms, the occurrence of grain growth and recrystallization in the newly formed austenite, and the tempering of the initial martensite. Moreover, thermal stresses and surface effects influence the final microstructure. In this work, effects of heating rates and peak temperatures are studied by dilatometry whereas Electron Backscatter Diffraction (EBSD) and optical microscopy are used to assess austenite grain size and morphology. This information is linked to a Finite Element Model of the local thermal history to investigate the evolution of the local microstructure throughout the zone affected by the laser heat source. This research provides insight into localised microstructural control of steels with laser surface treatment and provides a thermal model for detailed understanding of the mechanisms controlling the microstructural changes taking place during these treatments.

Microstructural evolutions during heat treatment of steel have been generally studied employing post-processed (ex-situ) characterisation methods. However, the data obtained cannot describe the kinetics of the transformation processes (in-situ) itself. With several microstructural processes happening simultaneously during an isothermal treatment below Ms, an in-situ characterisation method which can provide information on the microstructure development is needed. In this regard, Synchrotron X-ray Diffraction (SXRD) method is found to be a suited method. This thesis aims to study the microstructural developments during isothermal treatments below the Ms temperature by in-situ SXRD measurements in a model alloy of composition Fe–0.2C–3.51Mn–1.52Si–0.25Mo (wt.%). Results from this study show that during the isothermal treatment below Ms, bainite was formed right after the isothermal treatment started, which supports previous dilatometry experiments. During the isothermal treatment below Ms, four processes are suggested to occur simultaneously: (i) carbon redistribution processes from martensite, (ii) formation of bainite at the martensite/austenite interface, (iii) formation of bainite at the austenite/austenite interface, and (iv) carbon redistribution processes associated with bainite formation.

Abstract
The Quenching and Partitioning (Q&P) process starts with an austenitization process followed by rapid cooling to a temperature named quenching temperature ranged between the martensite start (M_s) and martensite finish (M_f) temperatures to establish initial fractions of martensite and austenite. Afterwards, the material is reheated to a particular partitioning temperature and for a given time to allow the carbon diffusion from martensite to austenite. Finally, the steel is quenched to room temperature and a fraction of fresh untempered martensite may form from the least stable austenite. The whole process determines the phase fraction of the constituent phases, namely: tempered martensite (M₁), retained austenite, and un-tempered martensite (M₂). This combination of phases has the potential to simultaneously improve strength and elongation of steels due to their small grain sizes and the transformation-induced plasticity resulting of the transformation of the retained austenite into martensite during a deformation. 
It is known that the fraction of retained austenite of Q&P steels depends on the quenching temperature, showing a bell-like relation. The volume fraction of retained austenite shows a maximum at a particular quenching temperature somewhat intermediate between the M_s and M_f temperatures, known as the optimum quenching temperature, below and upper which the volume fraction of retained austenite decreases in either way. 
Recently, it is reported [1] that retained austenite fraction after reheating up to 700^oC followed by quench losses its dependency of quenching temperature and reaches a constant value. This can be a manner to further control phase fractions and consequently the mechanical properties of Q&P steels. The mechanisms by which the volume fraction of austenite changes to reach that constant value are not well understood and are the focus of the present study. 
In the present work, Q&P samples with chemical composition 0.31C-4.58Mn-1.52Si (wt.%) were reheated up to 700^oC and quenched directly to compare the retained austenite fractions of reheated and un-reheated (Q&P) samples. It was found that the retained austenite fractions tend to have a constant value after reheating to 700^oC followed by quenching, whereas the retained austenite fraction of the Q&P samples showed an optimum value and usual dependency with quenching temperature. It was also observed that the highest and lowest drop of retained austenite occurred in the reheated Q&P samples at 160^oC and 80^oC of quenching temperature. These samples were reheated to 900^oC to have a complete observation of microstructural events by means of dilatometry, optical and electron scanning microscopy, hardness, and X-Ray diffraction measurement. 
Interrupted reheating was applied to understand the microstructural changes during reheating. The two samples were interruptedly reheated and directly quenched at 450^oC, 530^oC, 610^oC, 720^oC, and 740^oC to measure the retained austenite fraction and to capture the corresponding microstructure around the observed changes in slope. The retained austenite fractions evolution of both samples were measured and compared. It was found that the decreasing of retained austenite fraction started around the critical temperature of 600^oC and remained up to 700^oC of reheating temperature. Further reheating process increased the retained austenite fraction. 
In the specimen showing the lowest drop of retained austenite fraction, retained austenite decomposed by two mechanisms. The first one was the formation of discrete particles of cementite, which originally came from film type austenite. The second was the formation of globular carbides within tempered martensite. On the other hand, river-like patterns were observed surrounding the martensite region. The river-like patterns mainly consist of martensite from new austenite that formed between 720^oC and 750^oC.
In the highest drop of retained austenite fraction sample, retained austenite decomposed by three mechanisms. The first two mechanisms were similar to the previous sample. The last mechanism was the formation of substructure within Martensite-Austenite islands. River-like patterns were also observed surrounding the martensite region, consisting of martensite which originally came from new austenite as a product of (martensite and) carbide transformation that formed between 720^oC and 745C and lath martensite which originally came from the unstable austenite forming upon reheating to 740^oC.

The availability of low cost, efficient and wearable glucose sensors is one of the prerequisites for managing diabetes, one of the most diffuse chronic disease worldwide. Efficient monitoring of glucose levels in diabetics patients is essential to control symptoms and prevent severe complications. Starting from this principle, wet metallization and low cost inkjet printing were employed in the present work to manufacture non-enzymatic electrochemical sensors.
CuO nanoparticles were inkjet printed on platinum, which was electrodeposited on stainless steel. The active layer obtained in this way showed an acceptable linear range for glucose detection and a good sensitivity when used as sensor. The influence on performances of interfering species and curvature were investigated, demonstrating a negligible effect for the first and a decrease in linearity of the response and sensitivity for the latter. A second CuO based
sensor was realized using a PET flexible substrate. Electroless deposition was employed for the formation of a conductive NiP layer on top of which the CuO nanoparticles were inkjet printed. Glucose sensing was demonstrated by amperometric measurement, but the manufacturing process still has to be optimized to achieve a linear response and acceptable values of sensitivity. Finally, the effectiveness of CuO nanoparticles towards glucose sensing was
studied by printing CuO based inks on a Cu/graphene electrochemical sensor.

Martensitic and bainitic steels are two types of widely used steels with excellent mechanical behaviors in automatic industry. It’s universally acknowledged that as- quenched martensite possesses poor ductility and impact toughness, which should be tempered before putting into application. During tempering, as-quenched martensite changes from a hard and brittle microstructure to more ductile and softer microstructure, leading to improved mechanical properties for manufacturing. Bainitic microstructures is less sensitive to tempering in the improvement of mechanical properties, while the explicit tempering effects are still unknown.
The primary goal of the current research is to study the effect of tempering on the microstructure and properties of martensitic and bainitic microstructures developed in the same low carbon steel with high content of silicon and manganese (0.2wt.%C- 2wt.%Si-3wt.%Mn). Results show that for tempered martensite, the hardness, yield strength and tensile strength have a decreasing tendency with higher tempering temperatures. Through tempering, martensite became more ductile at the expense of strength. Based on the literatures, transitional carbide precipitation is expected to take place instead of cementite at early stage of tempering due to the retarding effect of high silicon content. Investigations also show that bainite is less sensitive to tempering compared to martensite. The yield strength and elongation firstly increase due to the TRIP effect of retained austenite and then decrease with higher tempering temperature. Hardness and tensile strength have a similar decreasing tendency as martensite during tempering. The fracture modes of tempered martensite for all temperatures show a typical ductile fracture, while for bainite, a morphology of quasi-cleavage fracture is observed.

Bainitic steels have a right combination of strength and toughness when compared to that of martensitic and pearlitic steels. For the same reason, it is becoming a popular kind of steel among the automobile industry. It is important to study the bainite transformations to have an explicit knowledge about the same and to design bainitic steels accordingly based on the application which has to be satisfied. This requirement has led to an immense research on the bainitic steels in the recent past. One of the aspects which is studied recently is the behaviour of the transformation kinetics of bainite during the isothermal treatments. Factors like, austenite grain size, chemical composition (Si, Mn and C) and the presence of prior athermal martensite have been found to affect the transformation kinetics of bainite. The presence of prior athermal martensite (PM) has been reported to have an accelerating effect on the transformation kinetics of bainite by few of the authors. The two reasons given for the acceleration by two different school of thoughts are: 1.) due to the creation of additional potential nucleation sites from the martensite-austenite (’-) interfaces with the presence of prior athermal martensite (PM) and 2.) due to the strain induced (through dislocations) in the austenite matrix with the presence of PM. Therefore, further investigations are required to concrete the mechanism responsible for the acceleration. On the other hand, carbon content has been reported to have a decelerating effect on the kinetics of bainite formation due to the reduction in the driving force (based on the thermal stability of austenite). Thus, the effect due to the presence of PM and the effect due to the carbon content have opposite influence on the kinetics. It is important to study the effect of PM on the bainite transformation kinetics for steels with different compositions in order to understand if the alloying elements also have an effect on the kinetics. Having said that the carbon content has an effect on the kinetics, it will be interesting to study the effect of carbon content on the kinetics along with the presence of PM.

The primary goal of the current research is to examine the effect of prior athermal martensite (PM) on the kinetics of bainite transformation in a medium carbon steel (0.57 wt% C). In order to investigate the effect of carbon content, the current study will be compared with the research by A. Navarro-López et al. 1 which was on a low carbon steel (0.2 wt% C). There is clear evidence from the results of the current study that there is a competition between the decelerating effect of carbon content and the accelerating effect of PM at the beginning of the isothermal transformation. The results show that the carbon content has a significant role on the intensity of the acceleration effect due to the presence of prior athermal martensite.

The role of carbon in the strengthening mechanisms of martensitic steels has been studied for decades. However, uncertainties still exist regarding how the distribution of carbon to various locations inside martensite contributes to the development of its observed microstructure and high strength. The strengthening mechanisms depend on carbon content and process parameters,but are also inter-related. The current work is an attempt to address the existing uncertainties regarding the relation between martensite dislocation density, prior austenite grain size (PAGS) and the manner in which total carbon is distributed into martensite interstitial sites and segregations near dislocations.Two steel alloys with different carbon content were selected to study, having following composition in wt.%: 0.3C-3.6Mn-1.5Si and 0.6C-3.5Mn-1.5Si. Alloys were heat treated in dilatometer to obtain martensite with different prior austenite grain sizes (PAGS). Microstructure characterization was performed using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) & Electron back-scatter diffraction (EBSD) and strength was evaluated by hardness measurements. It was found that the main factor influencing the carbon distribution in martensite is the martensite dislocation density, but the magnitude of its influence depends upon PAGS and alloy composition. In 0.3 wt.% C alloy, dislocation density decreases with increasing PAGS, as a result carbon atoms migrating towards dislocations during cooling also decrease, leaving higher number of carbon atoms at interstitial sites. However in 0.6 wt.% C alloy, such trend is not observed. On increasing the total carbon content in the alloy, very small increase in interstitial carbon content of martensite is observed if the PAGs are large. An increase in hardness was observed when the samples were introduced in liquid nitrogen, even in those cases where contribution from all strengthening mechanisms remained almost the same. This was unexpected and is most probably due to the relaxation of residual stresses and formations of carbon clusters at cryogenic temperature. In the end, an extension to already existing model is proposed to connect several microstructural features together in order to explain how they interact and evolve to give rise to observed strength of martensite.