Dominant factors affecting fatigue failure from non‐metallic inclusions in the very‐high‐cycle fatigue (VHCF) regime are reviewed, and the mechanism for the disappearance of the conventional fatigue ...limit is discussed. Specifically, this paper focuses on the following: (i) the crucial role of internal hydrogen trapped by non‐metallic inclusions for the growth of the optically dark area (around the non‐metallic inclusion at fracture origin), (ii) the behaviour of the crack growth from a non‐metallic inclusion as a small crack and (iii) the statistical aspects of the VHCF strength, in consideration of the maximum inclusion size, using statistics of extremes. In addition, on the basis of the aforementioned findings, a new fatigue design method is proposed for the VHCF regime. The design method gives the allowable stress, σallowable, for a determined design life, NfD, as the lower bound of scatter of fatigue strength, which depends on the amount of components produced.
The modification of MgO·Al2O3 spinel inclusions into less detrimental mixture phases of CaO–MgO–Al2O3 plays an essential role in refining calcium-treated aluminium killed steels. This study uses ...Raman spectroscopy for the characterisation of binary phase samples that contain MgO·Al2O3 spinel and calcium aluminate (CaO)x–(Al2O3)y phases. Samples were synthesised from MgO·Al2O3 spinel (MA), Al2O3 and calcium aluminate phases to achieve binary samples of CA–MA, C3A–MA, C12A7–MA and Al2O3–MA with varying phase fractions. The study also examined the possibility of a slight variation for non-stoichiometric spinel samples below the 1600°C region in an MgO–Al2O3 binary system. The relative intensities of the Raman band were used for the quantification of the phase fractions. For a quantitative prediction, linear regression calibration models were identified for each of the studied systems. This work demonstrates the use of Raman spectroscopy for the characterisation of calcium aluminate phases of CA, C3A, C12A7 and magnesium aluminate spinel phases along with Al2O3 and its potential application in inclusion characterisation.
The present study provides a benchmark of a sustainable solution utilizing ferroalloys to prepare ultra-clean CoCrFeMnNi high-entropy alloy (HEA). The designed CaO-MgO-Al2O3 slags saturated with ...CaAl2O4-MgAl2O4 (CA-MA, Slag A) and CaO-MgO (C-M, Slag M) were used to refine the HEA in Al2O3 and MgO refractory in an induction furnace under high-purity Ar atmosphere at 1773 K. The characteristics of non-metallic inclusions in the sampled HEA at different time intervals were quantitatively investigated. The results showed that three types of inclusions, i.e., sulfide (MnS), oxide, and complex type (oxide+sulfide), were found in the HEA regardless of refractory and slag types. The oxide inclusions such as MnAl2O4 and MgAl2O4 spinel particles can exist stably in the HEA melted in Al2O3 and MgO refractories with slag A and slag M, respectively. This fact is also confirmed not only by the electrolytic extraction method with elimination of the alloy matrix affection but also by the thermodynamic stability diagram for the HEA. For the structure of the complex inclusions, the core of oxide inclusions usually can act as the subsequent nucleation site for MnS since the precipitation temperature of the oxide inclusions (above the liquidus temperature of the HEA, TL ≅ 1623 K) is higher than that of MnS (below the solidus temperature of the HEA, TS ≅ 1573 K). The HEA melted in the MgO refractory with slag M had a higher cleanliness compared with that melted in the Al2O3 refractory with slag A, indicating that the MgO refractory with C-M saturated CaO-MgO-Al2O3 slag is suitable for producing an ultra-clean CoCrFeMnNi HEA prepared by the ferroalloys feedstock as the raw materials.
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•Novel and low-cost procedure was developed for manufacturing ultra-clean HEA using commercial ferroalloys feedstock.•Two kinds of crucibles and designed slag compositions were used to compare the cleanliness of the produced HEA.•MgO crucible with the CaO-MgO-Al2O3 slag saturated with CaO-MgO is suitable for refining the HEA at 1773 K.
High temperature confocal scanning laser microscopy (HT‐CSLM) is used to study the dissolution behavior of Al2O3 inclusions in various slag compositions in the system CaO‐Al2O3‐SiO2‐MgO. This method ...enables the in situ observation of the dissolution at steelmaking temperatures. The change of the diameter of the spherical inclusion is measured by image analysis of pictures obtained from the HT‐CSLM. Subsequently, dissolution rates and normalized dissolution curves are determined, and the governing dissolution mechanism is identified by the use of a modified approach of the diffusion equation introduced by Feichtinger et al. and compared with the dissolution of SiO2 previously reported by the same authors. Finally, effective binary diffusion coefficients are calculated. Slag viscosity is shown to essentially affect the dissolution behavior, changing the normalized dissolution pattern from rather S‐shaped (high slag viscosity) to a parabolic form (low slag viscosity).
The dissolution behavior of SiO2 and Al2O3 particles in CaO‐Al2O3‐SiO2‐MgO slags is studied in situ by means of high temperature confocal scanning laser microscopy. Absolute dissolution rates and the governing dissolution mechanisms are evaluated. The influence of slag viscosity on the dissolution mechanism is discussed in detail and effective binary diffusion coefficients are calculated.
The characteristics of non-metallic inclusions (NMI) that precipitated in an equiatomic CoCrFeMnNi high-entropy alloy (HEA) were investigated in order to understand their effect on the mechanical ...properties of the HEA. As the existence of NMI could degrade the mechanical properties, improved information concerning NMI could hold key importance in controlling the promising applications of HEA. An equiatomic HEA composed of CoCrFeMnNi was manufactured using vacuum induction melting (VIM) method. A thermodynamic computation program (FactSage™7.0) was used to investigate the solidification process of the HEA at both equilibrium and non-equilibrium states. Furthermore, the computational program also predicted the type of inclusions that would precipitate. Through an electrolytic extraction process and scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS) observations, the actual compositions of the precipitated inclusions were observed and classified as (a) Mn-Cr-Al oxide, (b) Mn(S,Se), and (c) mixed type; a Mn-Cr-Al oxide core with a Mn(S,Se) shell. Mn-Cr-Al oxide, in a brittle spinel-structured phase with high melting temperature, was also observed in dimples on the fracture surface. The relationship between the tensile properties of HEA and the characteristics of NMI were discussed by comparing two CoCrFeMnNi specimens with the same structure and composition. Overall, the present results indicate that the tensile properties of the HEA were significantly degraded as the area fraction (AF) and number density (ND) of NMI increased.
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•Qualitative and quantitative characterization of non-metallic particles in HEA was performed.•Effect of impurity on non-metallic particle in HEA was thermodynamically simulated.•Fracture mechanics of HEA with crack initiation under load condition was revealed.•Importance of manufacturing (impurity, casting) conditions of HEA was highlighted.•Necessity of refining non-metallic particles in HEA was highlighted.
Laboratory experiments are carried out to investigate the effect of cooling rate on oxide inclusions during the solidification of Si‐Mn killed 304 stainless steel. The oxide inclusions in the 304 ...stainless steel were Al2O3‐SiO2‐CaO‐MnO‐MgO system. During the cooling process, the change of inclusions is supposed to consist of two steps. The first step is the precipitation and growth of SiO2‐MnO‐Al2O3 type inclusions during the solidification process of steel. The second step is the transformation of inclusions into the MnO‐Cr2O3‐Al2O3 system in solid steel by the interaction between inclusions and the steel matrix. Smaller cooling rate led to the more complete growth and transformation of inclusions. With decreasing the cooling rate, the number of large inclusions increased while that of small inclusions decreased. Meanwhile, the diameter of the inclusions increased. The main reason for the size difference of inclusions at different cooling rates is the difference of inclusion growth during solidification.
The change of inclusions in 304 stainless steel during the cooling process is estimated to consist of two steps: 1) The precipitation and growth of SiO2‐MnO‐Al2O3 type inclusions during the solidification process and 2) the transformation of inclusions into the MnO‐Cr2O3‐Al2O3 system in solid steel by the interaction between inclusions and the steel matrix when the temperature decreased to the formation temperature of MnO‐Cr2O3.
Al-killed hot rolled steels are mainly produced with calcium treatment for the purpose either to minimize nozzle clogging in continuous casting or to modify plastic MnS inclusions. There are some ...negative effects of calcium treatment. In order to solve the problems, the possibility was explored to applying medium basicity refining slag for the production of hot rolled steel without calcium treatment. In this study, effect of medium basicity refining slag on desulphurization and inclusions in Al-killed steel was investigated by thermodynamic calculation and laboratory experiment. Results showed that some medium basicity slags of CaO–Al2O3–SiO2–MgO system, such as basicity of 4 and Al2O3 content of 20%, had relatively high aCaO and low aAl2O3, as well as satisfactory desulphurization efficiency, which indicated that they had strong desulphurization ability. After reacted with such slags, the surrounding part of most inclusions transformed from MgO–Al2O3, which was the main type in master steel, to MgO–Al2O3–SiO2–CaO(–MnO), the shape changes from irregular to near-spherical, and a large number of inclusions go into or near the region with relatively low melting temperature. It indicated that reaction with such slags could lower melting temperature of inclusions. In addition, steel cleanliness was improved. Based on the results, medium basicity slag may be used for production of some Al-killed hot rolled steel in which calcium treatment could be cancelled.
Behavior of collision and agglomeration between solid inclusion particles MgO · Al2O3 and Al2O3 on H13 molten steel surfaces is observed in situ through a confocal scanning laser microscope (CSLM) ...equipped with a gold image furnace, and the attractive force between these solid inclusions is calculated by using Newton's second law. Results and analysis show that the attractive force between alumina particles in H13 steel without magnesium is stronger than that between MgO · Al2O3 particles in H13 steel containing magnesium, and the action radius of attractive force between alumina particles is larger than that between MgO · Al2O3 particles. MgO · Al2O3 particles have a much weaker tendency to collide, agglomerate, grow, and form clusters than alumina particles in H13 steel. Therefore, the collision, agglomeration and growth of inclusions, as well as the formation of clusters in H13 steel can be effectively impeded by the adding of magnesium.
The formation sequence of MgO · Al2O3 cluster is the same as that of alumina cluster, but the MgO · Al2O3 clusters cannot form the outside branch and attract more distant particles. The collision, agglomeration and growth of inclusions, as well as the formation of clusters in H13 steel can be effectively impeded by the adding of magnesium.
Investigating non-metallic inclusions within ultra-high-strength-steel via conventional methods is a known: however, the challenge is to obtain chemical information of such inclusions at the ...sub-micrometer level. In this context, probing Fe-based oxide in inclusions is a vital aspect for guiding steel’ performance. The vibrational properties of sub micrometer size Fe-based oxides were investigated by Raman mapping along with chemometric analysis with the aim of probing their chemical composition. Highly contrasted Raman spectra were recorded from several inclusions embedded at different spatial locations. The observed spectral features were identified as specific markers of hematite (α-Fe2O3) and magnetite (Fe3O4). Principal Component Analysis was used to confirm the presence of these markers and potentially revealing additional patterns. Their unambiguous assignment has been inferred by comparing our experimental findings with the literature data recorded either in single crystals of iron oxides or oxyhydroxides. Micro-Raman spectroscopy is proven to be a reliable, cost-effective, and non-invasive tool for the unambiguous identification of subsurface regions of steel.
•Raman mapping deciphers Fe-based oxide in inclusions in UHSS.•Distinctive Raman spectra unveil spatial distribution and chemical identity of hematite and magnetite inclusions.•Chemometric analysis validates spectral markers, advancing understanding of Fe-based oxide vibrational properties.•Practical utility of Raman mapping demonstrated, informing engineering decisions for enhanced UHSS performance.
Calcium aluminate (CaO–Al2O3) phases play a critical role in the study of non-metallic inclusions in aluminium killed, and calcium treated steels. In this study, the Raman spectroscopy technique, a ...versatile and non-destructive approach, was used to characterise binary calcium aluminate phases qualitatively and quantitatively. Calcium aluminate samples with varying CaO/Al2O3 ratios were synthesised to produce a binary phase samples mixture of C12A7–C3A and C12A7–CA. Quantitative estimation was based on plotting a linear regression calibration model between the ratio of Raman band intensities and the phase fraction in the samples. With the linear regression, the phase fraction of C12A7–C3A and C12A7–CA was estimated with average absolute errors of 2.97 and 2.55 percentage points. This work demonstrates the potential suitability of using Raman spectroscopy technique for evaluating whether calcium aluminate phases in oxide inclusions fall within the liquidus region at steelmaking temperatures.
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