•Pit-shaped defects in Cf53 rolled products due to large composite CaS-inclusions.•Large composite CaS-inclusions accumulate on the inner wall to form SEN clogging.•T.O, T.Ca, T.S content, and steel ...temperature affect the formation of CaS-inclusions.•Reducing Ca wire amount fed in Cf53 steel has significant industrial application.
Aiming at the frequent issue of pit-shaped surface defects in S-bearing and Al-killed steel products and the related poor castability due to submerged entry nozzle (SEN) clogging, the melting and casting process of a popular auto steel Cf53 has been studied based on in-situ investigation and experimental analysis. To clarify the generation of the pit-shaped defects, the local surface chemistry, evolution of steel inclusions throughout the melting process, SEN clogging substances, and the internal relationship among them were systematically analyzed. The results show that this type of defect in the Cf53 rolled products is caused by the large-sized composite inclusions rich in CaS, which originate from the molten steel during the refining process, gradually accumulate on the inner wall of the SEN during casting, and finally lead to SEN clogging. The loose structure of clogging inclusions makes them prone to fall off into the mold and get trapped by solidifying steel shell. The formation mechanism of the inclusions was studied through FactSage software calculation. The results show that the increase in total sulfur and total calcium content, and the decrease in total oxygen content and temperature in steel will lead to CaS inclusion formation, which agrees reasonably well with experimental results. An optimized refining process to reduce calcium addition into molten steel is proposed accordingly, which achieves a remarkably alleviation in casting SEN clogging and an elimination of the pit-shaped defects on the final rolled products as well.
Hydrogen as an energy carrier plays an important role in achieving the ambitious climate targets associated with decarbonization. The main challenge is to ensure safe transport of gaseous ...respectively liquid hydrogen through the already existing natural gas pipelines. Since experimental methods, such as hydrogen-induced-cracking tests, fail to provide quantitative answers to the question of which local effects/mechanisms trigger failure under hydrogen loading, complex numerical multi-scale approaches are needed. In this work, a phenomenological crystal plasticity model based on dislocation slip mechanisms was implemented in Abaqus/Implicit and coupled with a hydrogen diffusion model to investigate the influence of microstructural characteristics in terms of grain size, texture, phase fraction but also lattice defects such as dislocations, grain boundaries, carbides, and non-metallic inclusions on hydrogen-induced-damage behavior. As a driving force for hydrogen diffusion, both hydrostatic stresses and plastic strains associated with mechanically and thermally induced residual stresses under various stress states were investigated. It was found that the impurity degree of the steel in terms of non-metallic inclusions plays an important role in hydrogen susceptibility in two aspects, namely geometrically (notch-effect) and in the generation of residual stresses. The effect of matrix shrinkage on the inclusions and the resulting stress field after cooling leads to the accumulation of hydrogen atoms around the inclusions, resulting in locally high critical concentrations. The simulations performed on microstructures with and without non-metallic inclusions under different stress states demonstrated the notched effect that the presence of non-metallic inclusions increases the local hydrogen concentration up to 28%.
•Implementation of a coupled crystal plasticity model with hydrogen diffusion.•3d RVEs containing substructures, texture, and non-metallic inclusions.•Non-metallic inclusions lead to higher local hydrogen concentrations.•Local hydrogen concentration in the microstructure depends on local stress state.
Dissolution of Al2O3-based inclusions are paramount during production of special steel and slag engineering is key to enhance this phenomenon. This work evaluates the influence of thermodynamic ...driving force (ΔC) and effective viscosity (ηe) on steel cleanliness of three distinct steel grades and slag composition (Steel/Slag A, Steel/Slag B and Steel/Slag C). Initial and final steel and slag samples were withdrawal on an electric steelmaking facility. The first samples after addition of aluminum as deoxidizer and the second after vacuum degassing stage. X-ray fluorescence and optical spectrometry obtained initial and final samples chemical composition. An ASPEX Explorer acquired inclusions data and FactSage v.7.2 performed thermodynamic calculations. Analysis of steel Al and O content highlighted an inefficient dissolution of Al2O3-based inclusions for Steel/Slag A, deteriorating steel cleanliness. Furthermore, this phenomenon is supported by average inclusions composition on initial and final samples plotted in pseudo-ternary diagram. Steel/Slag B and C provided improved results regarding the dissolution of Al2O3-based inclusions with a final inclusion density below 0.50 mm-2. These two slags composition achieved values of ΔC above 25 and ηe close to 0.10 Pa·s. In addition, their combined effect (ΔC/ηe) presented values above 250 and had the highest linear fit among these analyses. These properties are influenced by slags chemical composition and can be improved controlling parameters as binary basicity and CaO/Al2O3 ratio. Slags B and C were selected to define an optimal range for these parameters, with binary basicity between 3.00–3.50 and CaO/Al2O3 ratio in a range from 2.50–3.50.
In the present article, the performance and the efficiency of ceramic filters for continuous steel filtration in an induction crucible furnace, which is part of the steel casting simulator facility ...located at Technische Universität Bergakademie Freiberg, is investigated numerically. In order to achieve this objective, a macro‐scale simulation for the melt flow in the crucible is coupled with a pore‐scale simulation for the flow inside the ceramic filter that is adequately resolved by its detailed filter geometry, obtained from computed tomography scan images. The considerable influence of the filter on the flow field is indicated from the present results. Moreover, the efficiency of the employed filter is also determined and compared for two pore densities.
Numerical simulations are performed in order to determine the filtration efficiency of ceramic filters in an induction crucible furnace. Both pore‐scale and macro‐scale simulations are coupled in order to achieve this task. The considerable influence of the filter on the flow field is shown in the present article. Moreover, it is indicated that the filtration efficiency of the filters depends on the pore density of the filters.
Nb is an important microalloying element in steelmaking. Its interaction with liquid Fe during an early stage of the alloying process has a considerable influence on the Nb recovery. In the present ...work, the inclusions in FeNb alloys were characterized using the electrolytic extraction method combined with SEM-EDS. The interfacial reactions between FeNb alloy and liquid Fe, as well as inclusion formations, were studied during an early stage of an alloy addition using a liquid-metal-suction method. The results revealed that a diffusion zone consisting of different regions of Fe–Nb phases was formed and that the thickness of the zone increased with time. Based on the experimental findings, the mechanism of the early dissolution process of FeNb alloys in liquid Fe was discussed. Moreover, the Nb rich regions formed after the alloy contacted with liquid Fe could modify the existing inclusions in the alloy, also their evolution mechanisms were studied. The addition of FeNb alloys can introduce inclusions, such as Al–O and Al–Ti–Nb–O inclusions to the liquid steel. Overall, this study has contributed to the understanding the behaviour of impurities from the FeNb source at the early dissolution process during the microalloying process of steels containing Nb.
Display omitted
•Predicating subsurface butterfly-wing cracking damage due to non-metallic inclusions.•Developing Continuum Damage Mechanics FE modelling under rolling contact fatigue.•Integrating ...localised plastic deformation and kinematic hardening in damage modelling.•Investigating effect of inclusion boundary separation and internal cracking on damage evolution.
Wind turbine (WT) gearbox bearings experience premature failures. Damage characterisation of failed bearings has shown that subsurface micro cracks and butterfly-wing cracks are associated with non-metallic inclusions in the bearing raceways. The existing studies are unable to predict crack propagation under rolling contact fatigue considering shakedown and ratchetting when the material around an inclusion experiences sufficiently high levels of stresses. In this study, a finite element (FE) damage model based on the Continuum Damage Mechanics (CDM) is developed, integrated with the modelling of plastic deformation and kinematic hardening when the material around inclusion is subjected to alternating tension and compression. Two damage types of manganese-sulphide inclusions are investigated, showing significant effects of inclusion boundary separation and internal cracking on the subsurface RCF crack evolution. The modelling results also show that higher surface traction, overloads and varied loading sequences, commonly experienced by WTs in operation, have significant effects on the increase of the subsurface butterfly-wing crack lengths, because of early crack initiation and accelerated crack propagation. The developed CDM FE model has shown its effectiveness in predicting the damage evolution to gain new insights of complex interactions of a number of critical factors that can lead to the premature failure of bearings.
Nb is an important microalloying element in steelmaking. Its interaction with liquid Fe during an early stage of the alloying process has a considerable influence on the Nb recovery. In the present ...work, the inclusions in FeNb alloys were characterized using the electrolytic extraction method combined with SEM-EDS. The interfacial reactions between FeNb alloy and liquid Fe, as well as inclusion formations, were studied during an early stage of an alloy addition using a liquid-metal-suction method. The results revealed that a diffusion zone consisting of different regions of Fe–Nb phases was formed and that the thickness of the zone increased with time. Based on the experimental findings, the mechanism of the early dissolution process of FeNb alloys in liquid Fe was discussed. Moreover, the Nb rich regions formed after the alloy contacted with liquid Fe could modify the existing inclusions in the alloy, also their evolution mechanisms were studied. The addition of FeNb alloys can introduce inclusions, such as Al–O and Al–Ti–Nb–O inclusions to the liquid steel. Overall, this study has contributed to the understanding the behaviour of impurities from the FeNb source at the early dissolution process during the microalloying process of steels containing Nb.
•White Etching Crack is a premature failure in wind turbine rolling bearings.•Hydrogen, sliding contact and inclusions could act as main WEC drivers.•Electrical currents, temperature and high ...stresses act as accelerating WEC drivers.•DER, WEB and butterflies are microstructural changes different from and close to WEC.
Subsurface crack networks surrounded by microstructural alterations, named as “White Etching Cracks” (WEC), lead to wind turbine gearbox bearing (WTGB) premature failures. This phenomenon has been studied for decades without yet reaching a sturdy and justified cause of its occurrence. This paper reviews the most recent literature regarding WEC discussing steel microstructural alterations, WEC initiation modes, main WEC drivers, experimental procedures, as well as the influence of coatings and heat treatments on the occurrence of this phenomenon. A broad insight of the effect of each driver in WEC formation is presented and an undisclosed hypothesis related to WEC initiation is suggested. From this work it may be concluded that in the majority of the tests where WEC has been generated, three drivers usually appear and act synergistically: non-metallic inclusions, sliding and hydrogen.
•Pitting corrosion of the equiatomic CoCrFeMnNi alloy was initiated at the S-containing segments of the Cr–Mn–O inclusions.•Pitting potential of 0.1 M NaCl did not increase significantly after the ...removal of the S-containing inclusions.•Polarization in passive region in 1 M H2SO4 significantly improved the pitting corrosion resistance.
The pitting initiation site of an arc-melted equimolar CoCrFeMnNi alloy was investigated, and a method to improve the pitting corrosion resistance of the alloy was proposed. Cr–Mn–O inclusions with S-containing segments on the outer sections were observed; these S-containing segments served as the pitting initiation sites. The S-containing inclusions were removed by potentiostatic polarization and immersion in H2SO4, but pitting corrosion resistance after treatment did not improve. By potentiodynamic polarization in H2SO4, the S-containing inclusions were removed and the passive film on the alloy matrix became Cr enriched, resulting in a significant improvement in the pitting corrosion resistance.
The evolution of non-metallic inclusions in liquid steel involves a series of processes, including nucleation, growth by diffusion and Ostwald ripening, growth by collisions, floating up and removal ...of relatively large inclusions, as well as pushing and engulfment of remaining inclusions during solidification. All the evolution processes occur uniquely at the interface between inclusions and liquid steel. Therefore, interfacial properties between inclusions and liquid steel, such as interfacial energy and contact angle, play a crucial role in the evolution of inclusions, thus determining the inclusion characteristics. To effectively control the inclusion characteristics, the role of interfacial properties in the evolution processes of non-metallic inclusions is systematically reviewed in this work, based on theoretical analysis and published experimental results. In the early and middle stages of deoxidation, inclusions should have as high interfacial energy or contact angle as possible to enhance inclusion removal. In the later stage, however, the interfacial energy should be decreased as much as possible to weaken the clustering and pushing of inclusions, favoring the formation of small-sized and uniformly-distributed inclusions. To optimize the characteristics of Al2O3 inclusions, which are the most common in steel, several control strategies are proposed.
PDF
