Characteristics Of Non-metallic Inclusions Research Articles

The Effect of Ca Pretreatment on the Characteristics of Rare‐Earth Nonmetallic Inclusions in Super‐Duplex Stainless Steel

Rare‐earth treatment enhances steel properties. Nevertheless, this beneficial effect can be counterbalanced by the formation of large‐sized rare‐earth inclusions. To refine and spheroidize these inclusions, Ca pretreatment is employed between Al deoxidation and Ce treatment stages for super‐duplex stainless steel 2507 at 1580 °C, using a vertical tube furnace. A systematic analysis is performed to compare the characteristics of nonmetallic inclusions between the Ca‐untreated and Ca‐treated tests. Prior to Ce addition, Ca pretreatment transforms irregular and clustered MgO–Al2O3 spinel inclusions into spherical CaO–MgO–Al2O3 inclusions. Additionally, Ca pretreatment significantly reduces mean diameter of the inclusions from 2.36 ± 0.54 to 1.27 ± 0.47 μm. After Ce addition, rare‐earth inclusions appear near‐spherical with few clustered inclusions in Ca pretreatment case. Conversely, the absence of Ca treatment results in numerous clustered inclusions, thereby yielding significantly larger rare‐earth inclusions, measuring 2.75 ± 1.66 μm compared to 1.99 ± 1.09 μm. CaO component in CaO–MgO–Al2O3 inclusions is largely reduced by Ce. Consequently, Ca pretreatment only yields slight alterations in chemical composition of rare‐earth inclusions. Ce in situ reduces parent inclusions, initiating from the periphery and progressing toward the center. Therefore, the size and morphology of final rare‐earth inclusions are largely inherited from parent inclusions.

Relation of nonmetallic inclusions to the cyclic properties of steel 42CrMo4 after steel melt cleaning by filtration and related processes

Nonmetallic inclusions (NMIs) detrimentally influence the steel properties. The application of metal melt filter systems promises to reduce NMIs in steel. In the current work, NMIs are analyzed from metallographic sections after metal melt filtration or cleaning by a related process of the steel 42CrMo4 using immersion filtration, flow-through filtration as well as filters floating on top of the melt. Analyses by automated scanning electron microscopy (ASPEX) as well as light optical microcopy (LOM) revealed the chemistry, size distribution, and morphology of NMIs. The filter systems had a significant effect on these characteristics of NMIs. The threshold value ΔKth against fatigue crack propagation was found to depend on the amount of NMIs within the steel. A higher amount of NMIs caused less crack deflection leading to less roughness-induced crack closure and in turn resulted in lower threshold values at a stress ratio of R = 0.1. The comparison of the present analysis of NMIs with fatigue data from previous work indicated that the analysis of NMIs from metallographic sections can be used to approximate the fatigue strength of a batch in the same way as the analysis of NMIs from fatigue fracture surfaces for single specimens, as suggested by Murakami.

An experimental and statistical study on the characteristics of non-metallic inclusions that serve as hydrogen-induced crack nucleation sites in pipeline steel

This study consists of a statistical study to identify spatial distribution parameters of non-metallic inclusions (NMIs) at hydrogen-induced cracking (HIC) nucleation sites in both low-strength and high-strength steel pipes. The electrochemical cathodic charging method was used to induce HIC in pipeline steel plates, and the nucleation of the HIC was monitored using straight beam ultrasonic testing. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to evaluate the shape, size, area fraction, and separation distance of NMIs. The hydrogen microprint technique (HMT), electron backscattered diffraction (EBSD) analysis, and finite element (FE) stress analysis were performed to characterize HIC nucleation sites. The findings showed that cubical and spinal NMIs, characterized by strong hydrogen trapping capacity due to high misfit strain and von Mises stress, are favored sites for HIC nucleation. The main finding of this study is that the shape and sharpness of NMIs are the factors that determine when NMIs will be a HIC nucleation site, rather than size, as generally accepted.

Novel applications of ferroalloys for manufacturing of ultra-clean CoCrFeMnNi high-entropy alloy by slagging method

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.

Characteristics of various non-metallic inclusions in ancient cast iron and solid-state decarburized iron products from a government-run iron workshop during the Han Dynasty in Dongpingling Ruins, Shandong Province

With low content of impurity elements and the near absence of large slag inclusions, ancient Chinese cast iron used to be viewed as a kind of pure iron material. Thus, some crucial non-metallic inclusions in cast iron had been missed in previous studies. Here, we dedicated to analyze a series of pig iron and solid-state decarburized iron implements excavated from a Han dynasty official iron workshop at the Dongpingling site, and proposed to reveal the characteristics of non-metallic inclusions in these iron products. Judging by low impurity elements, the iron samples were identified with 5 categories of non-metallic inclusions, including silicate inclusions, sulfide inclusions, silica inclusions, endogenous oxide inclusions (agglomeration of multiple oxide) and those recombined by them. In addition, formation mechanism of each inclusion has been discussed as well.

In-depth analysis of crack area characteristics of fisheye failures influenced by the multiaxial stress condition in the tooth root fillet of high-strength gears

Modern high-strength gears have to satisfy many requirements, such as improved tooth root bending strength. The process of shot-peening is correlated to the introduction of compressive residual stresses in the surface layer of a gear to achieve a higher tooth root bending strength. However, due to the compressive residual stresses fisheye failures can occur and can have a determining effect on the endurance of high-strength gears. By preventing such failures, it should be possible to increase further the tooth root bending strength of high-strength gears. However, this requires a deeper understanding of the crack initiation and propagation processes. Especially the unique multiaxial stress condition in the tooth root fillet of a gear could influence the crack area characteristics significantly. Though, in the literature there is no proper characterization of crack area characteristics in the tooth root fillet of gears in detail, so far. Furthermore, in previous work a model approach for the evaluation of the tooth root bending strength of gears was presented, which is based on the results of Murakami. A first comparison with experimental data showed a basic applicability of the model approach on gears. However, the derived model approach showed some room for improvement. Questions arose as to whether the approach is really fully applicable to gears, whether further modifications are needed, or whether further extension is even practical, since the fisheye fracture characteristics of gears might differ significantly from those of standard specimens. The aim of this paper is therefore to present an extensive in-depth analysis of the crack area characteristics in the event of tooth root fracture damages caused by a fisheye failure in high-strength gears. Furthermore, a case study is used to verify whether a detailed evaluation of the characteristics of non-metallic inclusions leads to more accurate results of the model approach.

Characterization of Nonmetallic Inclusions in Low‐Alloyed Steels Using Pulse Distribution Analysis Optical Emission Spectroscopy and Offline Investigation Methods

The characteristics of nonmetallic inclusions (NMIs) in low‐alloyed steel samples taken during ladle treatment before and after Ca treatment are evaluated using the pulse distribution analysis optical emission spectroscopy (PDA/OES) method, INCA‐Feature investigations of inclusions on a polished surface of steel samples, and 3D investigations of NMIs after electrolytic extraction (EE) of steel samples. The investigation results of NMIs using the different methods were compared. The PDA/OES results show a clear tendency of a change in the average composition and quantity of NMIs during ladle treatment, which correlates well with the results obtained from the other two methods. Overall, it is found that the application of the PDA/OES method is appropriate to enable a fast online evaluation of inclusion compositions and their behaviors during steelmaking. This, in turn, provides the means for establishing an online control and correction of technological operations of the ladle treatment to implement necessary modification of NMIs to improve the cleanliness of steels and avoid clogging problems during casting.

Effect of characteristics of non-metallic inclusions on fatigue life of high-speed rail bearings based on multiple linear regression

In the process of smelting and manufacturing, bearing steel inevitably have defects such as non-metallic inclusions. Moreover, inclusion geometry (type, quantity, size, distribution, and shape) has a significant effect on the fatigue failure of bearing steel. In order to study effect extent of the size and distribution of inclusion on the fatigue life of bearings, this paper uses the test high-speed rail bearings LY-Z028 to research and analysis. The micro-model of contact between single inclusion and the matrix is established, and then performs ANSYS simulation on stress concentration which caused by size of single inclusion and relative surface depth. Further, multiple linear regression method is used to compare the effect extent of these two factors on the fatigue life. The results show that size of single inclusion has a greater impact on the fatigue life than relative surface depth, which can provide an effective reference for smelting and production requirements.

Melting route effects on the rotatory bending fatigue and rolling contact fatigue properties of high carbon bearing steel SAE52100

The rotatory bending fatigue (RBF) behavior, the rolling contact fatigue (RCF) behavior and inclusion characteristics are examined on SAE52100 steels fabricated by three different melting routes. It is found that both RCF-life and RBF-strength are strongly determined by the inclusion size and no RCF could be found when the maximum inclusion size is smaller than 16 μm, which is strongly dependent on the melting routes. This conclusion is not only very important to control of the characteristics of non-metallic inclusions but to the adjustment and improvement of processing techniques of SAE52100 bearing steels in the steel industry.

Effect of Cerium Content on the Evolution of Inclusions and Formation of Acicular Ferrite in Ti-Mg-Killed EH36 Steel

Ce has been widely used in oxide metallurgy for modifying inclusions and refining microstructure. Effect of Ce contents on the evolution and characteristics of non-metallic inclusions and the formation of acicular ferrite (AF) in Ce-treated Ti-Mg-killed EH36 steel was investigated. The results showed that the main type of inclusions in Ti-Mg deoxidized steel was MgO·Al2O3-MnS. After 0.014%, 0.024% and 0.037% Ce were added into the steels, dominant inclusions became CeAlO3-MgO-MnS, Ce2O2S-MgO-MnS and Ce2O2S-MnS, respectively. The precipitation of pure MnS was suppressed in the steel with 0.024% Ce, while the number density of total inclusions increased significantly in the steel with 0.037% Ce, causing the inclusions distributing densely. Ce addition refined the microstructure of tested steels by promoting AF formation and polygonal ferrite distributing evenly. Dispersive distribution, low lattice mismatch against α-Fe, as well as lower number density of pure MnS caused the highest number density of effective inclusions (nucleus of AF formation) in 0.024% Ce-treated steel.

Characterization of Inclusions in 3rd Generation Advanced High-Strength Steels

Samples taken from laboratory-produced 3rd generation advanced high-strength steels, solidified at a low cooling rate, have been investigated to study the characteristics of non-metallic inclusions. Two steels, containing 2 and 5 pct Mn content, were produced for this purpose. A higher number of total inclusions were observed in 5 pct Mn steel. The four main types of inclusions observed were Al2O3, AlN, MnS, and AlSiMn-oxide. These classes were divided into subclasses according to variations in their chemistry. The major subclasses of AlN inclusions are either plate-like or regular in shape and have different size distributions. Thermodynamic calculations suggest that plate-like AlN inclusions are formed at the initial stage of solidification, while faceted/regular-shaped inclusions are precipitated toward the end of solidification. Moreover, it was found that the size of nitride inclusions is related to their N content. This phenomenon is discussed from the viewpoint of nucleation theory.

Oxygen transport behavior and characteristics of nonmetallic inclusions during vacuum electroslag remelting

The oxygen transport behavior and characteristics of nonmetallic inclusions during electroslag remelting (ESR) under vacuum condition were investigated by comparison with ESR under argon atmosphere. The improvement of VSR on oxygen control and refinement of inclusions was further understood. The results show that oxygen content increases from 13 ppm to 24 ppm and 18 ppm after ESR process under argon atmosphere (101325 Pa) and vacuum condition (10000 Pa), respectively. The carbon deoxidization contributes to the smaller oxygen increase when vacuum condition was applied. The complex inclusions with two-layer structure are found after ESR process, and less oxide inclusions are observed in the ingot under vacuum condition. The inclusion amount per mm2 increases after ESR process, but the maximum equivalent diameter of inclusions decreases. Due to the lower oxygen content and faster solidification rate in ESR under vacuum condition, the total area of inclusions per mm2 in the ingot are smallest and the inclusions is much finer and more uniform. The results confirm that ESR under vacuum condition is more beneficial to improve steel cleanliness.

Characteristics and Formation Mechanism of Inclusions in 304L Stainless Steel during the VOD Refining Process

The formation and characteristics of non-metallic inclusions in 304L stainless steel during the vacuum oxygen decarburization (VOD) refining process were investigated using industrial experiments and thermodynamic calculations. The compositional characteristics indicated that two types of inclusions with different sizes (from 1 μm to 30 μm) existed in 304L stainless steel during the VOD refining process, i.e., CaO-SiO2-Al2O3-MgO external inclusions, and CaO-SiO2-Al2O3-MgO-MnO endogenous inclusions. The calculation results obtained using the FactSage 7.1 software confirmed that the inclusions that were larger than 5 μm were mostly CaO-SiO2-Al2O3-MgO; the similarity in composition to the slag indicated that these inclusions originated from the slag entrapment. The CaO-SiO2-Al2O3-MgO-MnO inclusions that were smaller than 5 μm originated mainly from the oxidation reaction with Ca, Al, Mg, Si, and Mn. The changes in the inclusion composition resulting from changes in the Ca, Al, and O contents, and the temperature during the VOD refining process was larger for the smaller inclusions. Generating mechanisms for the CaO-SiO2-Al2O3-MgO-MnO inclusions in the 304L stainless steel were proposed.

Formation of Plastic Inclusions in U71Mnk High-Speed Heavy-Rail Steel Refined by CaO-SiO2-Al2O3-MgO Slag

To precisely control the characteristics of nonmetallic inclusions with high plasticity in U71Mnk high-speed heavy-rail steel deoxidized by Si and Mn, the chemical composition of CaO-SiO2-Al2O3-MgO slag in the ladle furnace (LF) refining process was optimized by thermodynamic calculations. Relationships among the refining slag, nonmetallic inclusions, and elemental concentrations in the steels were analyzed to obtain the conditions for determining the optimal composition of refining slag. The optimal compositions and the component activities of CaO-SiO2-Al2O3-MgO refining slag were determined and discussed according to calculations using FactSage 7.0. Finally, ten industrial experiments were performed. Variations in the compositions of LF refining slag, inclusions, and steel, as well as the sizes of the inclusions, and metallographic classification assessment of the U71Mnk steel product, were investigated and discussed to verify the feasibility and effectiveness of the optimized LF refining slag. Kinetics of steel–slag and steel-inclusion reactions during LF refining was studied and compared. The results show that to obtain low [O], [Al], and [S] contents and inclusions with high plasticity in molten steel, the optimal composition of the LF refining slag should be CaO: 44.2 to 51.7 wt pct, SiO2: 41.4 to 47.0 wt pct, Al2O3: 0 to 4.7 wt pct and MgO 5.0 to 7.0 wt pct, where the basicity R is above 1.00 and the C/A ratio is above 9.00. After the LF refining process, the contents of total oxygen (T.O.), [Al], and [S] gradually stabilized in the range of 0.0008 to 0.0012 wt pct, 0.002 to 0.0028 wt pct and 0.0026 to 0.0037 wt pct, respectively. The densities of the three kinds of typical inclusions (CaO-SiO2-Al2O3-MgO, MnS, and CaO-SiO2-Al2O3-MgO-MnS) also decreased and reached approximately 0.25/mm2, and no inclusion larger than 6.0 μm was found. The composition of the inclusions gradually changed into the optimal range in the CaO-SiO2-Al2O3-MgO slag system during the LF refining process.

Characterization of non-metallic inclusions and their influence on the mechanical properties of a FCC single-phase high-entropy alloy

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.

Characteristics of Non-metallic Inclusions in Steel Obtained from Different-Sized Samplers

To determine the effect of sampler size on the detection of inclusions in liquid steel, four different-sized samplers were designed for industrial sampling from a continuous casting tundish. The samples were evaluated via scanning electron microscopy and energy-dispersive spectroscopy. With increasing sample size, the cooling rate of the sample and velocity for particle engulfment decreased. The extent of encapsulation of complex inclusions increased from 0 pct with the smallest sampler to 48.2 pct with the largest sampler. In the samplers that cooled at 41.2 and 26.7 K s−1, the major inclusions were calcium aluminates; in the larger samplers, which cooled more slowly at 8.2 and 1.0 K s−1, the complex inclusions predominately comprised Al2O3 + (Ca, Mn)S and Al2O3 + CaS, respectively. The sizes of the sulfides gradually increased with increasing sampler size; for example, the sulfide average diameter increased from 2.5 to 3.5 μm when the sampler inner diameter increased from 50 to 70 mm. The main direction of movement of the inclusions during the sampling process was upward.

Inclusion characteristics of stainless steel castings

The characteristics of non-metallic inclusions in 347 stainless steel samples taken during a net-shape casting process after different holding times and melting temperatures were investigated using electrolytic extraction and polished cross-sections. The inclusions can be classified into three groups: oxides, composite inclusions and sulphides. Within the first two groups the inclusions were subdivided based on their morphology into the following categories: spherical, irregular and clusters. As the temperature decreased, sulphide and NbC precipitated within the steel melt as well as on the surface of existing oxides. The number density and area fraction of inclusions as well as the total oxygen contents increased with higher melt temperatures and longer holding times. The results obtained from thermodynamic calculations carried out using the FactSage™ commercial software agreed well with the experimental observations. In addition, it has been concluded that turbulent collisions are the dominant mode for the growth of inclusion clusters.

Inclusion evolution during refining and continuous casting of 316L stainless steel

The formation and characteristics of non-metallic inclusions in 316L stainless steel produced by the AOD (argon oxygen decarburization)–ladle furnace–continuous casting process were investigated. The morphology and composition of inclusions changed significantly during the refining and casting processes. After de-oxidation with Si/Mn additions, spherical complex inclusions mainly consisting of calcium silicates were observed. The contents of MgO and Al2O3 in these inclusions continuously increased as the steel moved from the AOD through ladle processing to the tundish. As the temperature decreased from the tundish through to solidification, harmful crystals of MgO/Al2O3 spinel were precipitated within the steel melt as well as within the calcium silicate matrix of existing inclusions. The results obtained from thermodynamic calculations carried out using FactSage™ commercial software agreed well with the information derived from evaluation of the industrial samples enabling recommendations to be made for the avoidance of detrimental spinel inclusions.

Effect of Non-metallic Inclusions in Fe-Al-Ti-O-N-S Alloy on Grain Size

The effect of characteristics of non-metallic inclusions in Fe-Al-Ti-O-N-S alloys with various compositions at 1473 K (1200 °C) on the microstructure was studied. The ASTM grain size number was determined in as-cast and heated samples by the optical microscopy, and the inclusion types in each sample were determined from composition analysis by field-emission scanning electron microscope with energy-dispersive spectroscopy. The TiN-based inclusions certainly had a positive effect on the grain refinement. On the other hand, TiS-based inclusions exhibited no influence on the decrease of grain size. In addition, the formation and evolution behavior of inclusions by heating solid-state Fe-Al-Ti-O-N-S alloys with those locations were clarified. A different change of inclusions in alloys was observed depending on the distribution and composition of inclusions.

Formation of Mn-depleted zone in Ti-containing weld metals

Characteristics of non-metallic inclusions are studied using three shielded metal arc (SMA) weld metals which differ greatly in Ti content and are correlated with the acicular ferrite contents measured in the as-deposited weld microstructure. Correlation is found between the inclusion phases present and the amount of acicular ferrite formed in each weld. In the 5 ppm Ti weld, the weld microstructure is extremely low in acicular ferrite content and this microstructure can be correlated with the amorphous Mn-silicate phase that formed abundantly in the inclusions of this weld. At 410 and 580 ppm Ti, the weld microstructure contains very high amount of acicular ferrite, and this microstructure can be explained by the presence of a Mn-depleted zone (MDZ), likely formed associating with the formation of inclusions dominant with Ti2O3 and MnTiO3. The possible mechanisms for MDZ formation are discussed.