The transition towards more sustainable steel production and increased use of electric arc furnace (EAF) steelmaking has heightened interest in the influence of tramp elements. Copper (Cu) is of particular concern because it accumulates in recycled steel feedstocks and cannot readily be removed through conventional refining techniques. This study investigates the effects of Cu on the thermodynamics, transformation kinetics, microstructure and hardness of bainitic 100Cr6 bearing steel. Dilatometry is used to study austenitisation, martensite formation and isothermal bainite formation between 220 °C and 280 °C. Bainite formation is analysed using a shifted Johnson–Mehl–Avrami-Kolmogorov (JMAK) model. Thermo-Calc calculations, light optical and scanning electron microscopy, X-ray diffraction, electron backscatter diffraction and Vickers hardness testing support the analysis.

Cu is found to systematically retard bainite formation throughout the investigated temperature range. The TTT diagram is shifted towards longer transformation times, reflecting both a delay in the detectable onset of transformation and slower overall kinetics. The measurements do not separate individual contributions of nucleation and subunit growth.

Thermo-Calc calculations predict mild stabilisation of austenite by Cu through a shift of the equilibrium phase boundaries towards lower temperatures. This effect is not resolved experimentally. T_onset and T_completion measurements show no measurable difference within their uncertainty and M_S measurements show no consistent difference between the steels. Refined retained austenite fractions are slightly higher in the Cu-containing specimens, although no replicate XRD measurements were performed. The results therefore do not independently indicate increased austenite stability or a reproducible effect of Cu on retained austenite fraction.

No statistically significant effect of Cu on cementite dissolution or prior austenite grain size is resolved and SED micrographs show no clear Cu-related difference in final bainitic morphology. EBSD measurements on 100Cr6 qualitatively show a finer microstructure and more FCC-indexed regions following transformation at 220 °C than at 280 °C. Transformation temperature has a substantially greater influence on bainitic hardness than Cu addition. Hardness clearly decreases with increasing temperature, whereas differences between Cu-containing and reference 100Cr6 are modest and do not follow a consistent direction.

It is concluded from this study that the principal measurable effect of 0.5 wt.% Cu is a retardation of bainite formation, while the underlying local mechanism remains to be studied in further detail. Under the investigated conditions, Cu does not produce a clear change in hardness, but its influence on transformation time should be considered in heat treatment design. Since only the isolated effect of Cu is investigated, the results do not establish the suitability of recycled 100Cr6 containing multiple residual elements for bearing applications. ... Read more

The present study investigates the microstructural evolution of Sn-Ag-Cu (SAC) solder joints subjected to thermal cycling, with emphasis on recrystallization and grain orientation. Thermal cycling is the main failure source in reliability tests of electronic devices, yet its microstructural effects remain insufficiently understood. Although numerous studies have examined solder joint fatigue using numerical modeling, the early microstructural changes are often overlooked. To address this gap, the present work characterizes experimentally stages of thermal-cycling-induced microstructural evolution.
The evolution of SAC solder joints was characterized using Electron Backscatter Diffraction (EBSD) on two sets of samples. (i) joints thermally cycled for an increasing number of cycles and subsequently cross sectioned, and (ii) pre-cross-sectioned joints analyzed after consecutive rounds of cycling. Two distinct recrystallization mechanisms, namely primary and continuous dynamic, were identified, stabilizing after three thermal cycling stages. Grain rotation toward the [001]Sn-substrate angle and activation of slip systems 4, 6, and 10 suggest a strong link to the initial reflow texture. Moreover, IMC coarsening which facilitates grain boundary unpinning and promotes crack initiation, was also observed. Recrystallized grains exhibited a decrease in Young's Modulus, likely associated with orientation effects.
Calculation of Geometrically Necessary Dislocation (GND) density and Stored Energy Density (SED), with the aim to link EBSD and numerical modeling, while adequate for a first estimation, showcases the need for improved integration methods. Finally, the use of pre-cross-sectioned samples for in situ study of microstructural evolution is not recommended due to stress relaxation effects. ... Read more

Studies on the effect of the prior austenite grain size on the phase transformation kinetics of bainite are contradictory. Literature explains these contradictions by the presence of two different types of nucleation sites, the prior austenite grain boundaries and the tips of previously formed bainitic ferrite sub-units. The difference in their activation energies for nucleation, ΔQ, is known to determine whether the phase transformation kinetics of bainite are accelerated by prior austenite grain refinement or coarsening. However, the factors that influence ΔQ are not entirely understood, which is the reason why the contradictory results regarding the effect of the prior austenite grain size on the phase transformation kinetics of bainite observed in the different studies cannot be explained yet.

This master thesis investigates the effect of the prior austenite grain size on the phase transformation kinetics of bainite at different transformation temperatures in a low-carbon high-silicon steel. The experiments were divided into two groups, with one group consisting of specimens with finer prior austenite grains and one with coarser prior austenite grains. Specimens were transformed to bainite at three different isothermal transformation temperatures, 440 °C, 410 °C and 380 °C. Bainite formation was investigated by in-situ synchrotron XRD experiments performed at DESY to study the evolution of the phase fractions, lattice parameters and microstrains of bainitic ferrite and austenite. Furthermore, microstructure investigations on the specimens transformed at DESY and additional interrupted quenching experiments were conducted to understand the effect of the prior austenite grain size and the isothermal transformation temperature on the microstructure and the sheaf morphology formed in the early stages of the phase transformation. Finally, simulations were performed to determine the effect of the prior austenite grain size and the transformation temperature on ΔQ.

The experiments showed that, by decreasing the transformation temperature, the phase transformation kinetics of the group with fine prior austenite grains were decelerated, whereas the phase transformation kinetics of the group with coarse prior austenite grains were accelerated. The simulations exhibited an increase in ΔQ as the isothermal transformation temperature was decreased, indicating that sheaf growth by successive nucleation events at the tips of previously formed sub-units becomes increasingly prevalent. While the specimens with coarse prior austenite grains provide more potential nucleation sites at the tips of previously formed sub-units, the specimens with fine prior austenite grains provide more nucleation sites for grain boundary nucleation, which explains the reverse effect of the transformation temperature on the phase transformation kinetics of the two experiment groups. Microstructure observations have shown that the effect of the transformation temperature on processes, such as carbon partitioning and the transition from upper to lower bainite, could play an important role in explaining the observed effect of the prior austenite grain size on the phase transformation kinetics of bainite. ... Read more

An effective way for the automotive industry to tackle the growing concern of increasing CO2 emissions is to reduce the vehicle's overall weight, without compromising its performance and passenger safety. With the increasing demand for steels with enhanced properties in the last decades, the development of advanced high-strength steels (AHSSs) has been focused on the design of complex microstructures, leading to exceptional combinations of strength and ductility. On the other hand, stainless steels also offer significant potential in automotive lightweight applications to complement carbon steel AHSS grades. Stainless steels can offer additional advantages, including better corrosion resistance (no need for galvanising) and increased strength and fatigue resistance. However, a significant barrier to their application is the alloying cost (nickel in austenitic stainless steels) or low ductility and formability (standard martensitic stainless steels).... ... Read more

Traveling by train is becoming increasingly important, and, especially for long-distance travels, the current societal pressure for decarbonization is accelerating a modal shift from air travel to train travel.
With the introduction of faster and more comfortable trains, rolling contact fatigue (RCF) damage in rails started to spread over the networks, already in the 1990s. The RCF is controllable with newly introduced railhead profiles and with rails made of steels with improved wear resistant properties. Rails nevertheless need frequent maintenance, and for that purpose, high-powered precision grinding trains were developed. In Chapter 1 the backgrounds and motivation to understand the mechanisms for damage development, in particular the relationship between rail grinding and the durability of the resulting rail contact surface, are presented.
Despite the proven rail life extension due to grinding maintenance, grinding-related rail damage may occur. The fixed mutual distance between initiations is a specific characteristic of this damage type. A second characteristic is the complete removal of the running surface during ongoing crack propagation. Grinding-related damage is mainly detected in the newly introduced rail steels with improved wear resistance.
Limited empirical evidence is available to explain the initiation of damage from the surface condition of the ground rail. The empirical evidence that is available points to an initiation mechanism involving deep grinding grooves and surface heating and rapid cooling during grinding, which produces hard and brittle white etching layers (WEL).
With grinding being an irreplaceable maintenance activity, the objective of the research is to obtain metallurgical understanding of the relation between the characteristic features of the freshly ground surface, the formation of the contact surface, and damage initiation. This understanding will support decisions on rail grinding design and further research into rail grinding and the development of applications, thereby avoiding the introduction of preferential locations for damage initiation.
Chapter 2 presents the results of a track test. In this test the processes acting on the freshly ground rail are studied in a medium-wide curve of a live track. This experiment is designed as follows. A new rail is installed before the start of the experiment and subsequently in-situ ground according to the ProRail specifications for preventive rail grinding. Inspections are performed during eight months to evaluate the contact surface formation and four rail samples were extracted for detailed tribological and metallographic evaluation.
First, the freshly ground rail surface is studied, and the surface condition is characterized. The grinding introduced facets on the rail head, and grinding roughness is present on these facets. This roughness has a normally distributed variation in height profile. Friction between abrasives and the rail surface caused deformation beneath the surface and surface temperatures to rise to above AC3 leading to fresh WEL formation. Ploughing, typical for grinding processes, has resulted in the formation of ridges and slivers, which partially cover the freshly ground surface.
Inspection results and micrographs have revealed that roughness asperities are deformed and extruded under recurring wheel contacts, a process that proceeds fastest at the facet transitions. Most of the grinding-related WEL is fractured during the deformation processes and removed from the surface and fresh WEL is formed due to frictional heat generation during deformation.
The observed roughness-reduction mechanism contributes to the fast formation of the contact surface but carries the risk that detrimental particles cannot escape and become trapped. The results further show that especially deep grinding grooves containing WEL facilitate the initiation of damage. The characteristic ground surface features, successive steps in contact surface formation and damage initiation are captured in a novel schematic 5-stage wear model for ground rail surfaces.
The case study in Chapter 3 is performed on rail samples extracted from the railway line Zutphen-Hengelo in The Netherlands. The objective of the study is to evaluate damage formation in rails subjected to light bi-directional traffic after grinding. For that purpose rail surface conditions, subsurface deformation patterns, and representative surface breaking damages are studied.
The rail surface shows, three years after the grinding maintenance, that grinding roughness is still present on the running surface, while on the gauge corner the roughness has worn away. These differences in wear are explained by the bi-directional use of the railway line. Tangential shear stresses acting on the gauge corner are independent of the direction of travel, causing locally a ‘normal’ wear rate whereas tractive shear stress reversal reduces the wear rate on the running surface.
The S-shaped deformation patterns of the lamellae of the pearlitic rail steel beneath the contact surface are explained by the tractive shear stress reversal and cracks propagate symmetrically in both running directions.
Finally, the damage initiation is explained by two, line-specific conditions that occur
simultaneously. First, when traffic is light, rail wear is low and this wear rate is further reduced by the rolling direction reversal. Observations of the rail surface showed that these specific loading conditions prevent the complete removal of the ground surface features and thus the formation of a durable load-carrying surface. Second, rail corrugation is not fully removed by grinding, causing variations in the wheel contact conditions.
The case study shows that the sustainable maintenance of rails in lightly loaded railway lines requires a distinct specification, with special attention to the surface roughness
after grinding and to the removal of short-pitch corrugations.
Chapter 4 presents the results of an experiment using a twin-disc test setup in which five representative surface conditions that may occur after rail maintenance, are studied. The objective of the experimental design was to evaluate the contact surface formation, the number of load cycles until the coefficient of friction between the two surfaces increases, and the level that is reached.
Periodic rail maintenance removes damage in a timely manner, but all characteristic surface features must also be worn away again and again by the passing wheels in order to obtain a smooth rail surface.
The experimental results show that the initial coefficient of friction is low. This is a known characteristic of freshly machined surfaces, but it is not reflected in the considerations in rail maintenance as found in the literature.
The results also reveal that when the contact surface is formed by asperity deformation, the coefficient of friction rises already after a low number of load cycles and a high level is reached. When deformation is insufficient and wear have to take place, the process takes more load cycles.
The third observation worth mentioning is that when rail maintenance leads to subsurface deformation and strain hardening, it can take longer for breaking-in processes to be completed.
The experimental results contribute to the determination of specific requirements for, for example, tool management during the performance of rail maintenance.
The railway industry is constantly striving for performance improvements, for example by developing rail steels with improved wear resistance, which can extend its service life. The increased wear resistance may result from controlled accelerated cooling after hot rolling. Another method is to alter the chemical composition. Chapter 5 presents the results of an experimental study to determine the mechanical properties of a novel air-cooled, vanadium-alloyed, hypereutectoid rail steel. Fractographic evaluations are made to study the crack paths. The experimental results are compared with the performance of standard rail steels and a controlled accelerated cooled rail steel.
The air-cooled hypereutectoid rail steel and the hypo-eutectoid controlled accelerated cooled rail steel that are compared, exhibit comparable hardness, the material property that is traditionally used for the classification of rail steel grades. The mechanical behaviors of both rail steels are different. For example, the strain-hardening capacity and the crack growth rate of the air-cooled steel is higher compared to the controlled accelerated cooled rail steel.
The results of the linear elastic fracture mechanics tests are explained by means of a detailed characterization of the microstructure. Therefore the investigation contributes to the development of knowledge on the microstructure-mechanical relationships of pearlitic rail steels. In addition, the results also contribute to considerations in the selection of railway steel for specific applications.
Finally, Chapter 6 discusses the main conclusions of the research presented in this thesis and the consequences for the performance of rail maintenance. It is concluded during the project that research on damage that may initiate on the freshly ground surface is a green field and, as a result, extensive research is necessary into various aspects.
Recommendations for further research comprise the quantification of detrimental features of the freshly ground surface condition that were identified, the determination of limits of acceptable presence of these features, and the development of maintenance applications that are robust against the variations encountered in track.

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Advancing our understanding of the mechanisms of phase transformations is an efficient pathway to designing new steels with enhanced, tailor-made mechanical properties at minimal experimental cost. This dissertation investigates the bainitic transformation in steels. The mechanism of bainite formation is still not well understood, as there is no consensus on the role of carbon diffusion during the growth of bainite. Thus, it is difficult to understand and predict chemical and microstructural effects on the kinetics of bainite formation, such as the effect of prior austenite grain size and the presence of prior martensite. A better understanding of such effects may allow the design of new high performance bainitic steels with lean chemical composition and energy efficient heat treatments. In this dissertation, a new analytical model of the kinetics of bainite formation based on the displacive-diffusionless theory is proposed. This new model can reproduce and offer insights into the effect of prior austenite grain size and prior martensite on the kinetics of bainite formation. Following the understanding achieved with the model, the accelerating effect that martensite has on bainite formation kinetics is used to develop novel advanced high strength steels for the automotive industry that can be manufactured in existing continuous annealing lines thanks to fast bainite formation. Finally, to better understand the nucleation and growth of bainite, a real time observation of bainite formation using in situ transmission electron microscopy (TEM) is shown and discussed. ... Read more

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. ... Read more

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. ... Read more

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.
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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. ... Read more

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. ... Read more

WIND turbines play a crucial role in the global transition towards a sustainable energy future. Maximizing energy production and ensuring a reliable operation is essential to harnessing the full potential of wind energy. Among the critical components, the main shaft bearings have for several years been a focal point due to their significant downtime. In this context, a tribochemical treatment called case-carburization has gained notable attention for enhancing the microstructure of these bearings, to improve their reliability. Case-carburization is a surface treatment technique capable of modifying steel to exhibit a combination of properties such as high fatigue strength, toughness, and wear resistance, that are essential for these bearings as they operate in high-load-bearing environments. In a multi-stage heat treatment process involving case-carburization as the initial stage, the microstructure development at each stage is affected by the final microstructure of the preceding stage. Therefore, a comprehensive understanding of the microstructure at every stage is crucial for assessing its impact on the final microstructure and its properties. This Ph.D. research investigates the microstructure evolution throughout a four-stage heat treatment: carburization, sub-critical isothermal treatment, hardening, and tempering. The second stage is where the sole difference lies with regard to the heat treatment parameters, and is performed along two different routes, also in industrial practise, called the "bainitic route" and "pearlitic route". One of the primary goals of this research is to understand the microstructure development during the different stages of the two heat treatment routes and to provide an understanding of the microstructural features that can potentially affect the properties/performance of bearings. Additionally, this research also aims to identify the specific stage at which these features form and to provide insight into their formation mechanisms to explore strategies to rectify or mitigate the formation of detrimental features in the microstructure.... ... Read more

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. ... Read more

Dual-phase (DP) steels are an important class of advanced high-strength steels (AHSS) and constitute a major share of steels for the automotive industry. A microstructure consisting of hard martensite embedded in a soft ferritic matrix gives them a good combination of strength and ductility. The martensite formation in the microstructure from austenite involves a shape and volume change, which is accommodated by the deformation of the surrounding ferritic matrix. This accommodation is known to impart typical characteristics in DP steels such as the absence of a yield point, continuous yielding and high initial work hardening rate. This thesis is an attempt to understand and model the aforementioned accommodation process in the ferritic matrix of DP steels. Traditionally, in predictive modelling of DP steel mechanical behaviour, the region of ferrite which undergoes deformation to accommodate martensitic transformation is taken into consideration as a constant thin layer of strain-hardened ferrite at the ferrite/martensite interface. This approach is shown to be inadequate for capturing local variations in ferrite deformation. Hence, electron backscatter diffraction (EBSD) experiments were carried out to study in detail the influence of various microstructural features on local variations in the transformation-induced deformation of ferrite. It was found that the crystallographic orientation of ferrite grains, martensite variant and its prior austenite grain (PAG) play an important role in determining the extent of transformation-induced deformation of ferrite. Taking a cue from this, a novel methodology comprising sequential experimental and numerical research on DP steels is developed which combines the results of PAG reconstruction, phenomenological theory of martensite crystallography (PTMC) and EBSD orientation data to estimate ferrite deformation due to every martensitic variant formed, via full-field micromechanical calculations on a virtual DP steel microstructure. Furthermore, the influence of self-accommodation during martensite variant formation on transformation-induced deformation of ferrite was also investigated. It is shown that the higher the number of variants which form from a PAG, the less the deformation caused by that PAG in the surrounding ferritic matrix. This is because of a decrease in the effective magnitude of the shear component of martensitic transformation during multi-variant transformation. The scientific findings presented in this work can be used for developing predictive models for the mechanical behaviour of not only DP steels but any multiphase steels which exhibit plastic accommodation and residual stresses in their microstructure due to martensitic phase transformation. ... Read more

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.
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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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more

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. ... Read more