Pursuing sustainable and efficient energy storage technologies has led to advancements in iron-air batteries. Understanding the intricate relationship between the microstructural features of iron electrodes and their oxidation and reduction behaviour is crucial for optimizing battery performance and lifespan. This thesis aims to investigate the impact of microstructural characteristics, such as phases, grain size, and defect density, on the formation of stable iron oxide/hydroxide compounds and the evolution of hydrogen gas (HER), using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments in 6M KOH. The influence of an electrolyte additive, sodium stannate trihydrate, and an iron foam functioning as alternative electrode material are also examined. Hot-rolled, pure iron samples were subjected to annealing heat treatments, resulting in different grain-sized specimens. A dual-phase steel, DP1000 steel composed of ferrite and martensite phases as well as hot-rolled and cold-rolled iron electrodes, completed the materials that formed this study’s basis. Initial surface identification via optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), atomic force microscopy (AFM) and X-ray diffraction (XRD) have been performed. Results indicate a relatively lower formation of FeOOH for a DP1000 steel anode compared to cold-rolled alpha iron over 12 cycles. A marginally larger decline in HER kinetics is observed for a hot-rolled small grained anode compared to a coarse grained anode, while a clear effect of grain size on the development of Fe3O4 and FeOOH could not be established. An iron foam electrode showcases greatly enhanced anodic and cathodic current densities in comparison to solid sheet iron electrodes, due to its cellular structure. The effect of 0.01M sodium stannate added to the electrolyte illustrates a significant reduction in HER intensity for both foam and solid iron samples.

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.

Master thesis about machine learning in materials science

An effective way for the automotive industry to tackle the growing concern of CO2 emissions from automobiles is to reduce the overall weight of the vehicle, without compromising its performance and passenger safety. With the increasing demand of steels with enhanced properties in the last decade, 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. One such steel is Quenching and Partitioning (Q&P) steel, which is typically composed of a high strength phase, martensite, and a softer phase, austenite, which contributes to the ductility of the material. The main strategy in developing Q&P steels involves partitioning of carbon, an interstitial alloying element, from supersaturated martensite (α׀, formed in an initial quenching step from the austenitisation temperature) into austenite (γ) during an isothermal holding (partitioning stage) to enhance the thermal and mechanical stability of austenite. If the partitioning step is subjected at higher temperatures, substitutional austenite-stabilising alloying elements, such as manganese, may partition to the austenite and significantly enhance the stability of austenite in the final microstructure. Keeping this in mind, experimental and modelling approaches are employed in this Ph.D. thesis to investigate the microstructural evolution and the mechanisms involved during the quenching and high-temperature partitioning process in five different medium manganese steels. @en

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.

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.

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.

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.

A long standing research field in material science has been the trade-off between strength and ductility of steels. Advanced High Strength Steels (AHSS) aim to combine these two properties and provide steels that can increase safety and lower CO2 consumption in automotive applications, by decreasing weight. To further decrease the environmental impact, novel quenched and partioned (Q&P) martensitic stainless steel is considered to prevent corrosion. Quenching and partitioning is a heat treatment that aims for a martensite/retained austenite microstructure with specific phase fractions to optimize strength and ductility. While mechanical properties are well researched for Q&P treated martensitic stainless steel, the corrosion properties are not that widely documented. This thesis aims to discover the influence of the microstructural development in terms of phase fractions due to the Q&P treatment, and the manganese content on the corrosion properties of martensitic stainless steel. Two alloys with chemical composition 0.2C-12.5Cr-0.35Si-XMn, where X stands for 0.7 for the low manganese alloy, and 3 for high manganese alloy, are considered for this study, and are compared to a commercial AISI 420 martensitic stainless steel. The microstructure was investigated by the use of optical microscopy, energy dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), scanning Kelvin probe force microscopy (SKPFM). This was combined with electrochemical experiments including Open-circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). When compared to commercially available AISI 420 martensitic stainless steel, the novel Q&P MSS showed improved corrosion properties. This can mainly be attributed to the lack of chromium rich carbides (Cr23C6 or Cr7C3) in the Q&P treated MSS. However, this improvement in corrosion properties was only observed for the samples with low manganese content. The increase of fraction retained austenite did show an increase in corrosion potential, making the steel more cathodic, but the improvements in terms of pitting potential and passive film properties are limited. Samples with high fresh martensite fractions also showed an increase in corrosion potential, but only marginally increased in terms of length of the passive region. EIS data shows that the addition of fresh martensite reduces the passivity properties of the material. Volta-potential measurements done by SKPFM showed a clear difference between primary martensite and fresh martensite of up to 20mV and were overlapping with the topography maps. This can be an indication of increased tendency to form micro-galvanic cells between martensite phases. Manganese was found to be detrimental for the corrosion properties of the Q&P treated MSS. A clear drop in the length of the passive region shows that the high manganese samples are more susceptible to pitting. EDS measurements and EPMA elemental distribution maps showed local zones of decreased chromium and increased manganese and silicon. This indicates that there are secondary phase particles present in the material in the form of manganese silicide or manganese sulfide. Volta-potential measurements done by SKPFM showed these particles behave more anodic in comparison to the matrix of the material. The number of particles per unit area was increased for the samples with high manganese content. These inclusions greatly reduce corrosion performance, showing that the addition of manganese is detrimental for corrosion performance. Several other microstructural features are discussed in terms of influence on the corrosion performance of the material. Which include prior austenite grain boundaries, elemental banding of chromium and manganese and Volta-potential differences between martensite laths.

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.

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.

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 (Ms) and martensite finish (Mf) 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 Ms and Mf 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 700oC 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 700oC 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 700oC 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 160oC and 80oC of quenching temperature. These samples were reheated to 900oC 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 450oC, 530oC, 610oC, 720oC, and 740oC 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 600oC and remained up to 700oC 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 720oC and 750oC. 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 720oC and 745C and lath martensite which originally came from the unstable austenite forming upon reheating to 740oC.

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.

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.

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.