Rolling and annealing is a crucial technology to produce electrical steel sheets. This technology is not just aimed to control the geometry of steel sheets but more importantly to enhance the magnetic properties of the final products via appropriate microstructure and crystallographic texture. In this study, the evolution of microstructures and textures of an Fe-1.2 wt.% Si alloy through the entire processing route (from reheating, warm rolling to annealing) is monitored by electron back-scatter diffraction. Plastic flows of the material during conventional and asymmetric rolling are analyzed in detail based on geometric parameters of the rolling gaps. Deformation textures are accurately predicted by the full-constraint Taylor and advanced Lamel (ALAMEL) crystal plasticity models. The development of recrystallization textures is accounted for by the plastically stored energy in deformed crystals, which in turn is approximated by the plastically dissipated power (i.e., the Taylor factor) as predicted by the full constraint Taylor model. Although asymmetric warm rolling does not produce an improved texture or microstructure for electrical steels, the present study provides useful information on the evolution of the recrystallization microstructure and texture in steels with a complex strain history after asymmetric warm rolling.

The austenitization of an initial pearlitic microstructure is simulated using the phase field model to achieve insight into White Etching Layer (WEL) formation in pearlitic railway steels. The simulations take into account the resolution of the cementite lamellae within a pearlite colony as well as the presence of pro-eutectoid ferrite. The austenite growth kinetics and morphology obtained via simulations are compared with dilatometry and microscopy observations. The influence of γ/θ and γ/α mobilities on the austenite growth morphology are studied. The simulations reproduce the microstructural features as well as the experimentally observed kinetics behavior of austenite formation, involving the correlation between mobilities and nucleation behavior.

A novel double-step intercritical annealing (DSIA) process was conducted on Fe-0.16C-4.7Mn-1.6Al-0.2Si transformation induced plasticity (TRIP) steel. In a present study, an effect of different heating and cooling rates applied to the second intercritical annealing (IA) step on the microstructure was studied in detail by using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The quantitative analysis of the retained austenite fractions for particular heat treatment variants was carried out via X-ray diffraction (XRD). The formation of complex microstructures containing a small fraction of ferrite, retained austenite and two types of martensite was observed regardless on the applied heating and cooling rates. It was found that the soft ferrite can be replaced by martensite through the incomplete stabilization of austenite with C and Mn during short second intercritical annealing step. The heating rate influences significantly the kinetics of austenite formation during the second heat treatment step. Low heating rates ensure more time for formation and homogenization of austenite during heating improving its thermal stability. Thus, the best balance of martensite and austenite fractions in combination with homogenous morphology of the microstructure was noted for the specimen heated at a rate of 3 °C/s, which was the lowest one in the reported experiment.

The phase transformations, microstructure and properties of two Medium-Mn processed via quenching and partitioning steels were compared in this contribution and a new strategy for controlling mechanical properties by introducing and controlling cold-worked ferrite prior to heat treatment is proposed. It was found that during heating, the recovery and recrystallization of cold-worked ferrite compete with austenitization, thereby inhibiting the coarsening of austenite. The cold-worked ferrite interface will significantly delay the austenitization kinetics during the partitioning local equilibrium stage compared to martensite. These results lead to a diverse parent austenite, as well as a refined martensite substructure. As a result, the randomly distributed variants increase the number of effective grain boundaries, thus enhancing yield strength. The intercritical annealing process at a temperature of 860 °C resulted in the formation of fresh martensite-retained austenite (M/RA) constituents exhibiting a remarkably fine (<2 μm) and uniform grain morphology. Such microstructure yielded substantial improvement in both the strength and ductility of the steel. The proposed treatment led to excellent elongation (24%) at fracture, combined with very high ultimate tensile strength and yield strength of 1345 MPa and 1163 MPa, respectively, of the steel, resulting in a product of strength and elongation that exceed 32 GPa%.

The strengthening via precipitation of nano-sized κ-carbides leads to exceptional strength-ductility balance in low-density steels. During aging, such nanocarbides form through spinodal decomposition by fluctuations in the aluminum and carbon content in the austenite, followed by short-range ordering. At lower aging temperatures and short aging times, the κ-carbides are very fine, coherent with the matrix, and homogeneously distributed. When the aging temperature increases, heterogeneous nucleation initiates on the grain boundaries, and the κ-carbides become coarse and lose coherency with the austenite matrix, leading to the deterioration of the mechanical properties. This work studies a fully austenitic hot rolled Fe-29Mn-8.7Al-0.9C alloy after different aging treatments. Two aging treatments were selected for a detailed study of the microstructure based on the exceptional strength-ductility balance demonstrated by these conditions. The samples aged at 550 °C for 8 h exhibited an ultimate tensile strength of 1141 MPa and strain at failure around 49%. The second aging treatment selected was aging at 600 °C for 1 h, and these samples exhibited an ultimate tensile strength of 1084 MPa, with strain at failure 62%. The size and morphology of the austenite grains and the annealing twins were studied through EBSD. Additionally, the size, morphology, and volume fraction of the nano-sized κ-carbides were studied using TEM. Both aging conditions led to microstructures consisting of a matrix formed by equiaxed austenite grains with homogeneously distributed intragranular κ-carbides. The κ-carbides were coherent with the matrix and showed globular morphology with a diameter between 3 and 6 nm and coherent with the austenite matrix. The interaction between gliding dislocations and κ-carbides was analyzed. It was shown that the mechanical behavior of the studied material is characterized by very high sensitivity to the size of κ-carbides and, therefore, can be tailored by appropriate aging treatment.

Roller element bearings present in the intermediate and high-speed stages of wind turbine gearboxes operate in dynamic working conditions and in some cases may fail within 30% or less of their designed lifetime. Upon investigation, it has been identified that these premature failures happen due to a peculiar failure mode associated with formation of white etching cracks (WEC). This continues to be a great challenge for the wind energy operators as it leads to an increase of maintenance and operation costs in addition to long wind turbine downtime. Therefore, the industry is in dire need of a lifetime prediction methodology that could take in multi-scale inputs ranging from bearing loads at the system level down to the level of bearing material properties at the microscopic level. This work summarizes the overall approach of a project that aims towards an integrated framework which links load data from the bearings and microstructure related non-metallic inclusion statistics from bearing steels, to predict a material based probability of failure. The interlink between both aspects is a numerical rolling contact fatigue (RCF) framework based on finite element analysis, which includes multi-scale data as an input to calculate rolling contact fatigue damage. The outcome will help the wind industry to better predict bearing failures.

Additive manufacturing technologies in general and laser powder bed fusion (L-PBF) in particular have been on the rise in different applications, including biomedical implants. The effects of the various L-PBF process parameters on the microstructure and properties of Ti6Al4V lattice structures have been studied before. However, the relationship between the different modes of laser scanning and the resulting microstructure, internal imperfections, and surface morphology is still underexplored. In this study, the aforementioned parameters and their effect on the compressive mechanical properties and fatigue behaviour of lattice titanium have been studied for both continuous and pulsed laser scanning modes. Moreover, the influence of various combinations of post-processing treatments, such as hot isostatic pressing (HIP), sandblasting, and chemical etching, on the quasi-static mechanical properties and fatigue endurance of the resulting materials were investigated. It was found that continuous laser strategy results in fewer imperfections and higher fatigue resistance, while pulsed laser showed a more homogenous microstructure; likely leading to a more isotropic behaviour. Furthermore, the continuous mode showed larger prior β grains preferentially oriented in the building direction, while pulsed specimens exhibited finer equiaxed grains with no preferred orientations. The highest level of fatigue life was obtained by using an optimized combination of HIP, sandblasting, and chemical etching.

The effect of heating rate on the formation and decomposition of austenite was investigated on cold-rolled low carbon steel. Experiments were performed at two heating rates, 150 ˚C/s and 1500 ˚C/s, respectively. The microstructures were characterized by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Experimental evidence of nucleation of austenite in α/θ, as well as in α/α boundaries is analyzed from the thermodynamic point of view. The increase in the heating rates from 150 ˚C/s to 1500 ˚C/s has an impact on the morphology of austenite in the intercritical range. The effect of heating rate on the austenite formation mechanism is analyzed combining thermodynamic calculations and experimental data. The results provide indirect evidence of a transition in the mechanism of austenite formation, from carbon diffusion control to interface control mode. The resulting microstructure after the application of ultrafast heating rates is complex and consists of a mixture of ferrite with different morphologies, undissolved cementite, martensite, and retained austenite.

The microstructure and texture evolution of cold-rolled low carbon steel after ultrafast heating and quenching is investigated. Experiments were carried out at heating rates of 150 °C/s and 1500 °C/s. The recrystallization of ferrite is studied by scanning electron microscopy and electron backscattered diffraction techniques. The texture evolution of cold rolled steel during ultrafast heating was studied, making it possible to estimate the precise effect of heating rate on the orientations of newly formed grains. The experimental results showed that the recrystallization of ferrite was not completed before the full transformation of austenite. The noticeable increase in the fraction of recrystallized grains of diameter less than 1 µm, when the heating rate is increased from 150 °C/s to 1500 °C/s suggests that the increase of the heating rate enhances the nucleation of ferrite. The crystallographic orientations in recrystallized ferrite are strongly influenced by the heating rates. The effect of heating rate in the releasing of stored energy, carbon diffusion and spheroidization of cementite might explain some differences in textures observed in recrystallized ferrite.