The aim of the work presented was to investigate the effects of upper bainite on impact toughness in quench and tempered low alloy plate steels. The experimental research included construction of CCT diagrams by dilatometry, verification of phases by optical microscopy (OM), Vickers hardness, scanning electron microscopy (SEM), transmission electron microscopy (TEM) on precipitates extracted by carbon replica and by electrolytic means and finally impact testing of Charpy specimens with mixed bainite:martensite microstructures. Bainite was formed in High Chromium Low Molybdenum (HCrLMo) and in High Molybdenum Low Chromium (HMoLCr) steel samples by isothermal annealing within the bainite C-curve of the respective CCT diagrams. The isothermal kinetics of the upper bainite transformation was modelled with the Johnson Mehl Avrami Kolmogorov (JMAK) model. Avrami exponents of 1.4 and 1.3 were obtained for the HCrLMo and HMoLCr steels respectively which indicated linear growth with a considerable lengthening rate of laths and negligible thickening. The measurably slower growth kinetics in the HMoLCr steel as observed in the JMAK model and the higher hardenability with reference to its CCT diagram, suggested a strong Mo alloying element effect. The stronger effect of Mo compared to Cr was attributed to a solute drag like effect. The effect of upper bainite in a tempered martensitic matrix was investigated for the following amounts of bainite; 0%, 10%, 25%, 60%, 75%, 90% and 100%. The impact toughness of the mixed bainite:martensite samples was evaluated against the toughness of 100% bainite and 100% martensite. It was demonstrated that upper bainite reduces the total absorbed impact energy by an adverse effect on crack nucleation energy and crack propagation energy. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Materials Science and Metallurgical Engineering / Unrestricted

The aim of the work presented in this thesis is to clarify one of the most fundamental aspects of fatigue damage in bearings steels through critical experiments, in particular whether damage in the form of cracks precedes hard “white-etching matter" formation, which is carbon supersaturated nanoscaled ferrite. Heat treatments have been designed to create four different crack types and distributions: scarce martensite plate cracks, fine grain boundary cracks, abundant martensite plate cracks, and surface cracks. Subsequent rolling contact fatigue experiments showed that the amount of hard white-etching matter is higher in pre-cracked samples compared to those without prior damage and that its formation mechanism is the frictional contact of disconnected surfaces within the bulk that elevate the temperature and localise deformation. These key experiments indicate that hard white-etching matter is the consequence, not the cause, of damage. Therefore, one way to avoid white-etching matter is by increasing the toughness of the material. The macroscopically homogenous distribution of microcracks proved also to be a useful rolling contact fatigue life enhancer due to damage deflection via crack branching and a powerful trap for diffusible hydrogen. Successful trapping was corroborated by the inability of hydrogen to cause crack propagation via embrittlement or accelerate white-etching matter generation during rolling contact fatigue. By also studying the behaviour of a nanostructured bainitic steel under rolling contact fatigue, it was found that its degradation mechanism is ductile void formation at bainitic ferrite/stress-induced martensite interfaces, followed by growth and coalescence into larger voids that lead to fracture along the direction of the softer phase as opposed to the conventional damage mechanism in 52100 steel of crack initiation at inclusions and propagation. Given the relevance of phase quantification in nanobainite and the possible surface artefacts introduced by preparation, alternative methods to X-ray diffraction such as magnetic measurements were also investigated. The lack of hard white-etching matter obtained in the carbide-free nanostructured bainite led to conclude that an alternative route to mitigate hard white-etching matter could be by eliminating pre-eutectoid carbides from the microstructure, therefore restricting their dissolution and ultimate carbon supersaturation of the mechanically deformed and homogenised nanoferrite.

The current work contains experimental and theoretical work about the formation of martensite from the austenitic state of the steel Hardox 450. Simulation of rolling and subsequent quenching of martensitic steel plates requires a model that can account for previous deformation, current stresses and the temperature history, therefore dilatometry experiments were performed, with and without deformation. Two austenitization schedules were used and in the standard dilatometry the cooling rates varied between 5-100 °C/s, in order to find the minimum cooling rate that gives a fully martensitic microstructure. Cooling rates larger than 40°C/s gave a fully martensitic microstructure. The cooling rate of 100 °C/s was used in the deformation dilatometry tests where the uniaxial deformation varied from 5-50 %. The theoretical work involved modelling of the martensite formation and the thermal/transformation strains they cause in the steel. Characterizations were done using light optical microscopy, hardness tests and electron backscatter diffraction technique. The parent austenite grains of the martensitic structure were reconstructed using the orientation relationship between the parent austenite and the martensite. Kurdjumov-Sachs orientation relationships have previously been proven to work well for low-carbon steels and was therefore selected. The standard implementation of the Koistinen-Marburger equation for martensite formation and a more convenient approach were compared. The latter approach does not require the storage of initial austenite fraction at start of martensite formation. The comparison shows that the latter model works equally well for the martensite formation. The results showed that the use of martensite start and finish temperatures calibrated versus experiments for Hardox 450 works better when computing thermal expansion than use of general relations based on the chemistry of the steel. The results from deformation dilatometry showed that deformation by compressive uniaxial stresses impedes the martensite transformation. The simplified incremental model works well for deformation with 5 % and 10 %. However, the waviness in the experimental curve for deformation 50 % does not fit the model due vi to large barrelling effect and the large relative expansion for the material that the sample holders are made of. Crystallographic reconstruction of parent austenite grains were performed on a hot-rolled as-received reference sample and dilatometry samples cooled with 60 °C/s and 100 °C/s. The misorientation results showed that the samples match with the Kurdjumov-Sachs orientation relationship in both hot rolled product and dilatometry samples. When misorientation between adjacent pixels are between 15° and 48°, then the boundary between them was considered as a parent austenite grain. The austenitic grain boundaries of the sample cooled at 100 °C/s is in general identical with the hot rolled sample when considering high angle boundaries (15°-48°). The results from the hardness tests showed that the rolled product exhibits higher hardness as compared to samples cooled by 100 °C/s and 60 °C/s. This can be attributed to the formation of transition-iron-carbides in the hot rolled product due to longer exposure of coiling temperature.

The current work contains experimental and theoretical work about the formation of martensite from the austenitic state of the steel Hardox 450. Simulation of rolling and subsequent quenching of martensitic steel plates requires a model that can account for previous deformation, current stresses and the temperature history, therefore dilatometry experiments were performed, with and without deformation. Two austenitization schedules were used and in the standard dilatometry the cooling rates varied between 5-100 °C/s, in order to find the minimum cooling rate that gives a fully martensitic microstructure. Cooling rates larger than 40°C/s gave a fully martensitic microstructure. The cooling rate of 100 °C/s was used in the deformation dilatometry tests where the uniaxial deformation varied from 5-50 %. The theoretical work involved modelling of the martensite formation and the thermal/transformation strains they cause in the steel. Characterizations were done using light optical microscopy, hardness tests and electron backscatter diffraction technique. The parent austenite grains of the martensitic structure were reconstructed using the orientation relationship between the parent austenite and the martensite. Kurdjumov-Sachs orientation relationships have previously been proven to work well for low-carbon steels and was therefore selected. The standard implementation of the Koistinen-Marburger equation for martensite formation and a more convenient approach were compared. The latter approach does not require the storage of initial austenite fraction at start of martensite formation. The comparison shows that the latter model works equally well for the martensite formation. The results showed that the use of martensite start and finish temperatures calibrated versus experiments for Hardox 450 works better when computing thermal expansion than use of general relations based on the chemistry of the steel. The results from deformation dilatometry showed that deformation by compressive uniaxial stresses impedes the martensite transformation. The simplified incremental model works well for deformation with 5 % and 10 %. However, the waviness in the experimental curve for deformation 50 % does not fit the model due vi to large barrelling effect and the large relative expansion for the material that the sample holders are made of. Crystallographic reconstruction of parent austenite grains were performed on a hot-rolled as-received reference sample and dilatometry samples cooled with 60 °C/s and 100 °C/s. The misorientation results showed that the samples match with the Kurdjumov-Sachs orientation relationship in both hot rolled product and dilatometry samples. When misorientation between adjacent pixels are between 15° and 48°, then the boundary between them was considered as a parent austenite grain. The austenitic grain boundaries of the sample cooled at 100 °C/s is in general identical with the hot rolled sample when considering high angle boundaries (15°-48°). The results from the hardness tests showed that the rolled product exhibits higher hardness as compared to samples cooled by 100 °C/s and 60 °C/s. This can be attributed to the formation of transition-iron-carbides in the hot rolled product due to longer exposure of coiling temperature.

Ni-Mn-Ga alloys close to the stoichiometric composition Ni2MnGa belong to the quite new family of ferromagnetic shape memory alloys. These alloys are characterized by the magnetic field induced strain (MFIS) based on the comparably easy motion of twin boundaries under a magnetic field. They are mostly chosen as a potential candidate for practical application especially promising for actuators and sensors because they are showing the largest MFIS so far. Depending on the chemical composition and heat treatment, at least three martensitic structures can be distinguished in the Ni-Mn-Ga system. However, the effect mentioned above only exists in two modulated structures. Since for the intended application of MFIS in technology polycrystalline materials seem to be more appropriate in contrast to single crystals, the specific polycrystalline aspects are considered. Factors important for decreasing the twinning stress and increasing the twinning strain of polycrystalline Ni-Mn-Ga alloys are texturing, adjusting the structure by annealing and training by thermomechanical treatments. To achieve pronounced MFIS in polycrystals, fabrication processes are needed to produce specific strong textures. The material texturing has been obtained by directional solidification and plastic deformation by hot rolling and hot extrusion as well as high pressure torsion (HPT). To examine the texture of coarse-grained Ni-Mn-Ga alloys (due to a solidification process or dynamic recrystallization), diffraction of synchrotron radiation and neutrons was applied. The texture results show that the texture of Ni-Mn-Ga subjected to directional solidification, hot rolling and hot extrusion is a fibre or weak biaxial texture. However, local synchrotron measurements reveal that the global fibre texture of the hot extruded sample is a ”cyclic” fibre texture, i.e. it is composed of components related to the radial direction rotating around the extrusion axis. This allows finding regions with a strong texture component. The texture after HPT is characterized by a strong cube with the cube favourably oriented. The initial microstructure of the Ni-Mn-Ga alloys is a typical self-accommodated microstructure of martensite. High resolution EBSD mappings show macro, micro twins and two types of microstructure. The twin plane is determined to be {110). In a typical martensitic transformation the high-temperature phase has a higher crystallographic symmetry than the low-temperature phase. Consequently, austenite may transform to several martensitic variants, the number of which depends on the change of symmetry during transformation. Generally, in a cubic-to-tetragonal transformation (5M case) three variants can form with the c-axis oriented close to the three main cubic axes of austenite. However, close examination of the high resolution EBSD mapping reveals that more than just three orientations, as expected from the Bain model, exist in Ni50Mn29Ga21. Each of three Bain variants may be split in some twin relations in different regions of the sample which differ from each other by about few degrees creating a much higher number of variants. The training process, as the last step in the preparation procedure of Ni-Mn-Ga alloys, consists of multi-axis compression finally leading to a single-variant state. Compression of polycrystalline samples leads to motion of those twin boundaries changing the volume fraction of particular martensitic variants in such a way that the shortest axis (c-axis) becomes preferentially aligned parallel to the compression axis. It allows reducing the twinning stress and maximizing the twinning strain. To understand the training process in more detail, the interaction of the twin variants with the neighbourhood of parent austenite grains was investigated.

The background to this bachelor thesis is an on-going project within the VINN Excellence Center Hero-m. The task in this thesis is to perform a literature survey about the martensite transformation and investigate how the resulting fraction depends on cooling below the Ms-temperature. Instead of calculating the undercooling for each of the known fractions of martensite the driving force will be evaluated. Several efforts have been made through the years to describe the relationships between fraction transformed austenite and temperature. The approaches to the first models were empirical and derived from collections of data regarding the amount of retained austenite at different quenching temperatures. Lately, studies have been made to derive a thermodynamical relationship using how the Gibbs energy is affected by increments in volume transformed austenite. Two equations are derived by calculating the resulting driving force at different known quenching temperatures and the respective percentage transformed martensite found in previous works. The data for the steels used show a characteristic slope when linearised. A trend for the steels which have a high characteristic slope is that they also have a high Ms temperature, and the steels which have a low characteristic slope tend to have a low Ms. Previous relationships which describe the martensitic transformation have considered the importance of the Ms temperature only in it being a starting temperature for the transformation. To further incorporate the Ms temperature in the equations presented, further research of the martensitic transformation is required. The approach in this thesis of using thermodynamically calculated data is a base for further investigation of the range of the martensite transformation.

Thesis: B.S., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 1980 / Includes bibliographical references. / by Evan Joseph Hammerman. / B.S. / B.S. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Multiple studies on the microstructures of advanced high-strength steels are presented here that seek to add to the already substantial body of knowledge on martensite in steel. These studies seek to gain additional insight into the role that the martensite transformation has on the observed mechanical properties of modern steels. Crystallographic Reconstruction of Parent Austenite Twin Boundaries in a Lath Martensitic Steel The study of post-transformation microstructures and their properties can be greatly enhanced by studying their dependence on the grain boundary content of parent microstructures. Recent work has extended the crystallographic reconstruction of parent austenite in steels to include the reconstruction of special boundaries, such as annealing twins. These reconstructions present unique challenges, as twinned austenite grains share a subset of possible daughter variant orientations. This gives rise to regions of ambiguity in a reconstruction. A technique for the reconstruction of twin boundaries is presented here that is capable of reconstructing 60 degree twins, even in the case where twin regions are comprised entirely of variants that are common between the twin and the parent. This technique is demonstrated in the reconstruction of lath martensitic steels. The reconstruction method utilizes a delayed decision-making approach, where a chosen orientation relationship is used to define all possible groupings of daughter grains into possible parents before divisive decisions are made. These overlapping, inclusive groupings (called clusters) are compared to each other individually using their calculated parent austenite orientations and the topographical nature of the overlapping region. These comparisons are used to uncover possible locations of twin boundaries present in the parent austenite. This technique can be applied to future studies on the dependence of post-transformation microstructures on the special grain boundary content of parent microstructures. Coupling Kinetic Monte Carlo and Implicit Finite Element Methods for Predicting the Strain Path Sensitivity of the Mechanically Induced Martensite Transformation The kinetic Monte Carlo method is coupled with a finite-element solver to simulate the nucleation of martensite inside the retained austenite regions of a TRIP (transformation induced plasticity) assisted steel. Nucleation kinetics are expressed as a function of load path and kinematic coupling between retained austenite regions. The model for martensite nucleation incorporates known elements of the kinetics and crystallography of martensite. The dependence of martensite transformation on load path is simulated and compared to published experimental results. The differences in transformation rates of retained austenite are shown to depend on load path through the Magee effect. The effects of average nearest neighbor distance between austenite grains is shown to affect the rate at which martensite nucleates differently depending on load path. Ductility and Strain Localization of Advanced High-Strength Steel in the Presence of a Sheared Edge The localization of strain in the microstructures of DP 980 and TBF 980 is quantified and compared. Of particular interest is the difference in final elongation observed for both materials in the presence of a sheared edge. Scanning electron micrographs of etched microstructures near the sheared edge are gathered for both materials at varying amounts of macroscopic strain. These micrographs are used to generate strain maps using digital image correlation. A two point statistical measure for strain localization is developed that utilizes strain map data to quantify the degree to which strain localizes around the hard phase of both materials. The DP steel exhibits higher strain localization around the martensite phase. Reasons for differences in strain localization and shear banding between the two materials are suggested, and the role played by the mechanically induced martensite transformation is speculated.

This short communication is devoted to the room temperature processes of diffusion and redistribution of dissolved carbon atoms in martensite to the nanosegregation regions at dislocations and grain boundaries. It is related to the DF7 contribution of M. Lavrskyi et al. on the carbon kinetics in martensite [1] and to the DF7 contribution of Yu. Nechaev on the compound-like nanosegregation at dislocations and grain boundaries in metallic materials.

The Latin American steel making company Ternium S.A. aims to develop and produce a new type of advanced high strength steel (AHSS) in which the main alloying elements are carbon, aluminium, manganese, and silicon. The present work is the first phase of the development project and it involves casting and characterization of four steel compositions with varying amounts of the aforementioned elements. The results revealed that the Mn-content had a large impact on the development of hard phases during solidification. A steel with a Mn-content of 2 %wt. had almost completely transformed to pearlite during cooling, while a steel with a 4 %wt. Mn-content consisted of primarily martensite and retained austenite. Only the impact of the Mn-content is evaluated. The columnar grain size for two of the four steel compositions were in the range of 20-30 mm, which is similar to those observed from continuous casting. This indicate that the heat transfer rate was slow enough to allow these grains to grow. Measurements during casting showed an initial cooling rate of 10-20°C/min at a distance of 10 mm inside the ingot, which is much slower than the surface cooling rate during continuous casting (100-150°C/min). It was assumed that the cooling rate was similar for all castings since the methodology was identical. However, the steel used for cooling rate measurements was not characterized, why a correlation between cooling rate and composition could not be obtained. A heat transfer model was developed to gain further knowledge of the solidification process. As a reference to the heat transfer model, a eutectic Bi-42Sn alloy was cast with temperaturemonitoring using a casting setup identical to that of the steel castings. A similar cooling rate tothe Bi-42Sn reference casting was obtained where the cooling was faster from above of the ingot than below. Thus, the last part of the metal to solidify during the simulation was situated in the lower half of the ingot. This provides a model for testing future steel compositions.