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Article

Tracing the Origin of Oxide Inclusions in Vacuum Arc Remelted Steel Ingots Using Trace Element Profiles and Strontium Isotope Ratios

by
Christoph Walkner
1,*,
Gulnaz Mukhametzianova
1,
Stefan Wagner
1,
Jörg C. Korp
2,
Andreas Graf
3,
Johanna Irrgeher
1,
Thomas C. Meisel
1 and
Thomas Prohaska
1
1
Chair of General and Analytical Chemistry, Montanuniversität Leoben, Franz Josef-Straße 18, 8700 Leoben, Austria
2
Dr. Korp Technological-Consulting, Buchengasse 17, 8793 Trofaiach, Austria
3
Breitenfeld Edelstahl AG, Breitenfeldstraße 22, 8662 St. Barbara im Mürztal, Austria
*
Author to whom correspondence should be addressed.
Metals 2025, 15(1), 67; https://doi.org/10.3390/met15010067
Submission received: 12 December 2024 / Revised: 2 January 2025 / Accepted: 10 January 2025 / Published: 14 January 2025
(This article belongs to the Special Issue Recent Advances in Microstructure and Mechanical Properties of High-Strength Steels)
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Versions Notes

Abstract

:
Non-metallic inclusions (NMIs) in steel have a detrimental effect on the processing, mechanical properties, and corrosion resistance of the finished product. This is particularly evident in the case of macroscopic inclusions (>100 µm), which are rarely observed in steel castings produced using state-of-the-art technologies, whereby casting parameters are optimized towards steel cleanliness, and post-treatment steps such as vacuum arc remelting (VAR) are used, but frequently result in the rejection of the affected product. To improve production processes and develop effective countermeasures, it is essential to gain a deeper understanding of the origin and formation of NMIs. In this study, the potential of elemental and isotopic fingerprinting to trace the sources of macroscopic oxide NMIs found in VAR-treated steel ingots using SEM-EDX, inductively coupled plasma mass spectrometry (ICP-MS), laser ablation ICP-MS (LA-ICP-MS), and laser ablation multicollector ICP-MS (LA-MC-ICP-MS) were exploited. Following this approach, main and trace element content and 87Sr/86Sr isotope ratios were determined in two specimens of macroscopic NMIs, as well as in samples of potential source materials. The combination of the data allowed the drawing of conclusions about the processes leading to the formation of these inclusions. For both specimens, very similar results were obtained, indicating a common mechanism of formation. The inclusions were likely exogenous in origin and were primarily composed of calcium–aluminum oxides. They appeared to have undergone chemical modification during the casting and remelting process. The results indicate that particles from the refractory lining of the casting system most likely formed the macroscopic inclusions, possibly in conjunction with a second, calcium-rich material.

1. Introduction

Non-metallic inclusions (NMIs) are virtually ubiquitous in all types of steel, exhibiting considerable variation in composition, size, and shape. Such inclusions can have a detrimental effect on several steel parameters, including ductility, fracture toughness, fatigue life, and corrosion resistance [1,2]. The severity of these effects is believed to increase with the frequency and size of the inclusions [3].
NMIs are classified with respect to their diameter into sub-micro- (<1 µm), micro- (1–15 µm), meso- (15–100 µm), and macro- (>100 µm) inclusions [4,5]. Although macro-inclusions are only sporadically observed in state-of-the-art casting technologies, they nevertheless present a significant challenge. The presence of macro-inclusions is not in compliance with the requirements for different steel grades, which typically results in the rejection of the affected product. This is particularly undesirable in ingot casting, since in a worst-case scenario, the entire ingot requires remelting [6]. Consequently, the formation of NMIs in high-performance steel must be monitored and controlled.
The assessment of sources and mechanisms leading to the formation of NMIs in steel production is still elusive. In principle, NMIs can be either endogenous or exogenous. Indigenous NMIs are formed during deoxidation processes in the steel melt, when dissolved oxygen reacts with added oxygen-affine elements such as aluminum or silicon, typically in a temperature range of 1550 °C to 1650 °C. Exogenous NMIs originate from other sources, such as entrained slag, fragments of refractory lining, or particles of auxiliary materials, tend to be larger than indigenous NMIs, and are typically located near the ingot surface [3,7]. Given this great diversity of potential source materials, tracing of NMI formation currently relies on assessing various factors such as their total amount, morphology, size distribution, spatial distribution, and chemical composition [8].
In a comprehensive review [9] over 20 different methods for assessing NMIs were cited, encompassing both indirect and direct approaches. In indirect methods, the presence of NMIs is inferred from parameters serving as relative indicators for NMIs. For example, the total oxygen content in steel is the sum of dissolved oxygen, which can relatively easily be measured using oxygen sensors, and oxygen present in NMIs, giving an estimation of their total amount. In general, these methods are fast and inexpensive and are therefore widely applied in industry. However, the information they provide on NMI formation is limited [9,10]. Direct approaches, in contrast, allow for a more detailed investigation of NMI characteristics. Amongst them, optical metallography [11,12,13] is the most widely used technique for determining the presence and two-dimensional morphology of NMIs due to its simplicity and widespread availability. In addition, ultrasonic inspection [14] and methods for X-ray detection of NMIs, including X-ray computed tomography [15], have been also applied for direct NMI analysis. While these techniques allow for non-destructive and even online detection of NMIs during steel production, they lack information about the NMIs’ chemical composition [8], which is considered critical for the investigation of their formation mechanisms.
For this purpose, targeted chemical analysis techniques are required. A traditional approach is to isolate NMIs from a certain volume of steel by melting or (electro)chemical dissolution of the matrix, followed by their ex situ chemical analysis using spectrometric methods [9,16]. Disadvantages of this approach include the high expenditure of time for dissolution of the steel matrix and the lack of specificity, as other inclusions (particularly sulfides) are attacked by the reagents as well [8,16]. In contrast, scanning electron microscopy (SEM) [11,12,13] can be used to directly measure the 2D morphology of NMIs at the microlevel, and in combination with energy-dispersive X-ray spectroscopy (EDX), to simultaneously determine the spatial distribution of multiple elements in NMIs. However, the relatively low sensitivity of SEM/EDX limits the analysis to major elemental constituents, while assessing larger numbers of NMIs across extensive regions of interest remains time-consuming, even though automated particle analysis routines have been implemented recently [8].
An alternative approach is to use laser ablation combined with inductively coupled plasma mass spectrometry (LA-ICP-MS), which allows for spatially resolved quantification of elemental mass fractions, including trace elements, in single NMIs [17,18,19]. In LA-ICP-MS, a focused laser beam ablates the material surface as a spot, line, or raster scan, and the produced aerosol is then transferred into a coupled ICP-MS to record the isotopes of elements according to their mass-to-charge ratio (m/z). The ICP-MS can also be operated in liquid mode (solution-based), allowing for elemental analysis in, for example, digests of potential source materials. This approach has been recently used for the first time for rare earth element (REE) fingerprinting in inclusion metallurgy, demonstrating its effectiveness for linking NMIs to potential source materials [20]. Auspiciously, (LA-)ICP-MS also allows for isotopic analysis with high precision, especially when using multicollector (MC-)ICP-MS instrumentation. In the context of NMI tracing, the isotopic system of strontium (Sr) is especially promising, as the ubiquitous nature and the relatively large radiogenic isotopic variations of Sr (87Sr/86Sr) between different materials may provide an additional parameter for identifying distinct sources of NMIs. While this capability of the Sr isotopic fingerprinting method is well established in geological, archeological, and forensic contexts [21,22,23], its potential for advancing source apportionment in NMI tracing has not been explored until now.
Thus, at present, there is no single ideal method for assessing impurities in steel. Therefore, it is preferable to use a combination of methods to obtain meaningful results [10,24]. The objective of this study was to employ a multi-method approach comprising SEM/EDX, solution-based (MC-)ICP-MS, and LA-(MC-)ICP-MS to provide a comprehensive characterization of two specimens of macroscopic inclusions and to compare the data with those of potential source materials. By applying this comprehensive combination of state-of-the-art analytical techniques, we aimed to narrow down the field of possible source materials and draw conclusions regarding the origin of the inclusions. In addition to elucidating the provenance of the inclusions in this specific case, the objective was also to expand the set of tools available for extracting a maximum amount of information from macroscopic inclusions in steel.

2. Materials and Methods

2.1. Inclusion Specimens

In the present study, two macroscopic inclusions, hereinafter referred to as inclusion 1 and inclusion 2, were investigated (Figure 1). They were found in ingots consisting of steel grades 1.4545 (ingot 1) and 1.2738 (ingot 2) (Table 1). Selected specifications of these steel grades in terms of element content are summarized in Table 2 [25]. In addition, slag and refractory samples were taken after the casting of an additional ingot (ingot 3, see below). All ingots were bottom-poured using argon as protective gas, annealed, and subsequently subjected to vacuum arc remelting (VAR), forging, and again annealing. Aluminum metal was used as a deoxidizing agent in casting ingots 1, 2, and 3. In addition, calcium treatment using CaSi wire was applied in casting ingots 1 and 3, but not ingot 2. Figure 2 shows a schematic representation of the steps involved. A more detailed schematic representation of the ingot casting step, indicating some of the components potentially acting as source materials for inclusions, is given in Figure 3. Element content determined in ingot 1 and ingot 2 before and after VAR are summarized in Supplementary Table S1 (refer to electronic Supplementary Material).

2.2. Potential Source Materials

Potential source materials for exogenous inclusions were analyzed. Five auxiliary materials used during the casting process were sampled: two casting powders, two blower powders, and one covering powder.
In order to account for possible chemical changes in the applied auxiliary materials during the process, a sample of the slag formed on top of an additional ingot (ingot 3) during the casting process was taken. Ingot 3 was cast under similar conditions to ingot 1, except that a different blower powder (blower powder 2) was used.
Then, two specimens of the refractory lining used in the casting of the same ingot were taken: one from the mold entrance and one from the runner brick (cf. Figure 3). Another part of the same runner brick was sampled for LA-ICP-MS analysis (“runner brick used”). Finally, for comparison, an unused sample of an identical runner brick (“runner brick unused”) was analyzed (Figure 4). An overview of the auxiliary materials used and the samples that were taken for analysis is provided in Table 1, and the analytical methods applied to all samples are summarized in Table 3.

2.3. Sample Preparation

Samples for analysis by means of solution-based ICP-MS were ground using a laboratory mixer mill (MM400, Retsch GmbH, Haan, Germany) and subjected to sintering digestion using sodium peroxide (Na2O2, analytical grade, Merck KGaA, Darmstadt, Germany) following a published procedure [26] that is known to allow complete digestion of silicate-rich materials that might include refractory minerals such as chromite or zircon. For strontium isotope ratio measurements of these sample digests, matrix separation was performed with manual column-type ion exchange chromatography using a Sr-selective resin (SR resin; TrisKem, Bruz, France) following established procedures [27].

2.4. SEM/EDX Analysis

The major element composition of the inclusion specimens was measured using scanning electron microscopy (SEM/EVO MA 25, Carl Zeiss SMT, Oberkochen, Germany) coupled with energy-dispersive X-ray spectroscopy (EDX/X-ACT10, Oxford Instruments Nano Analysis, Bucks, UK), with a working distance of 9.7–9.8 mm, an acceleration voltage of 15 kV, a tilt of 0°, and a magnification of 1000–1500 times.

2.5. ICP-MS Analysis

Detailed parameters for all ICP-MS measurements are summarized in Supplementary Tables S2 and S3.

2.5.1. Solution-Based ICP-MS

Trace elements in sample digests were measured using an Agilent 7500ce ICP-MS (Agilent Technologies, Santa Clara, CA, USA). External calibration of the recorded m/z signal intensities was carried out using matrix-matched reference material calibration standards as described previously [26]. In addition, International Association of Geoanalysts (IAG) reference material GeoPT 45 (siliceous siltstone, GONV-1 [28]) was prepared and analyzed along the samples for quality control, with a median recovery (determined as the ratio between the measured and certified values for each measurand) of 101%. Detailed results and recovery rates for all measurands are listed in Supplementary Table S4.
For strontium isotope ratio analysis, 87Sr/86Sr ratios in purified sample digests were measured using a Nu Plasma HR MC-ICP-MS (NP048, Nu Instruments, Wrexham, UK) equipped with a membrane desolvation unit (Aridus II, Teledyne, Omaha, NE, USA) following a published method [29]. Combined external and internal interelemental correction of the instrumental isotopic fractionation (IIF) was accomplished by standard-sample bracketing (SSB) using a solution of standard reference material (SRM) NIST 987 (SrCO3; National Institute of Standards and Technology, Gaithersburg, MD, USA) as isotopic reference and Zr as internal standard [30]. Data acquisition was performed in static mode. Correction for blanks, outliers, and residual Rb was accomplished as reported previously [30]. All samples and standards were measured in diluted HNO3 (Roth, Karlsruhe, Germany, w = 2%).

2.5.2. Laser Ablation ICP-MS

All laser ablation experiments in this study were performed using an NWR 213 laser ablation system (ESI-NWR, Omaha, NE, USA). Detailed experimental parameters are summarized in Supplementary Table S5.
For trace element analysis, the laser ablation system was coupled with an Agilent 8800 ICP-MS/MS (Agilent Technologies, Santa Clara, CA, USA). For quantification, a one-point calibration using one of SRM NIST 612 or NIST 610 (trace elements in glass; National Institute of Standards and Technology, Gaithersburg, MD, USA) was performed. The signal at m/z = 27 (Al+) was used as an internal standard, with Al mass fractions for the inclusions obtained from SEM/EDX analysis.
The NWR 213 laser ablation system was further coupled with a Nu Sapphire MC-ICP-MS (SP017, Nu Instruments, Wrexham, UK) to directly assess 87Sr/86Sr ratios in inclusions as well as the unused and used runner brick. Raw signal intensities at m/z 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, and 92 were acquired simultaneously in time-resolved analysis mode with an integration time of 0.1 s. A membrane desolvation unit (Aridus 3, Teledyne, Omaha, NE, USA) was connected to the line from the laser to the MC-ICP-MS injector via a Y-piece, enabling introduction of a solution of SRM 987 for internal intraelemental IIF correction of the 87Sr/86Sr ratios using the fractionation factor of 88Sr/86Sr and applying Russell’s law [31,32]. Raw signal intensities were on-peak baseline-corrected by subtracting a corresponding gas blank.

3. Results

Table 4 contains a selection of analytical results for all samples that are most relevant for the discussion (see below). A complete list of results in terms of main constituents, trace elements, and 87Sr/86Sr ratios is given in Supplementary Table S4.

3.1. Morphology and Main Constituents

As determined by SEM (Figure 5), the morphology of both inclusions is similarly angular and edged, and both appear elongated and deformed, presumably due to forging of the ingots.
Element mappings of the inclusions carried out using SEM/EDX (Figure 6) show that the two inclusions are also similar in composition, consisting mainly of aluminum, calcium, and oxygen. Silicon content, on the other hand, is rather low. Results of SEM/EDX and (LA-)ICP-MS analyses (Table 4) also show that mass fractions of aluminum and calcium oxides in both inclusions are very similar, but differ from all reference samples.

3.2. REE Fingerprinting

The comparison of the content and distribution of trace elements, and especially the rare earth elements (REE), so-called trace element fingerprinting, has been shown to be a valuable method for classifying and correlating geological materials [33]. Recently, this method has also been applied to link mesoscopic NMIs found in Ti-stabilized ULC steel to source materials, in this case mold slag and casting powder [20]. Therefore, REE fingerprinting was also applied to the present problem of determining the origin of the inclusions. All mass fractions obtained by means of ICP-MS and LA-ICP-MS in all samples analyzed are summarized in Supplementary Table S4. La mass fractions and anomalies calculated for Eu, Er and Yb (see below) are presented in Table 4.
In natural samples, REEs are distributed according to the Oddo–Harkins rule, with even-numbered elements being more abundant than odd-numbered. Since the resulting “sawtooth” pattern makes identification of differences in REE patterns difficult, it is customary to normalize REE mass fractions by division by a suitable reference dataset [34]. Most frequently, a dataset obtained for chondritic meteorites is used, whose composition is thought to approximate that of Earth as a whole [35]. Therefore, relatively smooth curves are achieved, allowing for straightforward visual comparison.
Figure 7a contains chondrite-normalized REE profiles for inclusions 1 and 2 and reference samples. Both inclusions show a very distinctive pattern: a slightly concave curve descending from La to Lu, from which normalized mass fractions for two elements, namely, europium (Eu) and ytterbium (Yb), deviate sharply downwards (so-called negative Eu and Yb anomalies). The magnitude of an REE anomaly can be quantified as the E/E* ratio, where E is the mass fraction of the REE in question and E* is the value calculated by linear interpolation between the mass fractions of the two neighboring elements (e.g., Sm and Gd in case of a Eu anomaly). Negative anomalies have E/E* < 1, while positive anomalies have E/E* > 1, and no anomaly is assumed for E/E* values around 1 [33,34].
For reference samples, no comparable REE patterns were found. In contrast, covering powder and top slag show a positive Eu anomaly, while casting powder 1 shows a distinct positive Er anomaly (Table 4). The cause of the latter remains unclear, since anomalies of this kind do not occur in nature.
The unused runner brick sample analyzed using LA-ICP-MS shows a pattern that closely resembles those of the refractory samples analyzed using ICP-MS after digestion, runner brick, and mold entrance. The used runner brick sample, in contrast, demonstrates a reduction in the concentration of REE due to the superficial dilution of REE-containing phases with solidified metal during the casting process. Furthermore, no significant alterations in the REE profile could be identified. The slight zigzag pattern visible for the heavier REE (holmium to lutetium) can most likely be attributed to increasing measurement uncertainty at low REE content.

3.3. Sr Isotope Ratios

To expand the data available from trace element measurements and to corroborate them with a parameter that is more robust towards fractionation processes, 87Sr/86Sr ratios of the inclusion specimens and reference samples were determined. For 87Sr/86Sr ratio measurements in the five auxiliary material samples, two refractory samples and the top slag, MC-ICP-MS after Na2O2 sintering digestion and chromatographic matrix separation was used. For the isotope analysis of the inclusions and the unused runner brick, the MC-ICP-MS was coupled with a laser ablation system. Results are summarized in Figure 7b. For the used runner brick sample, no results could be obtained, since the Sr content of the boundary layer was too low due to dilution with solidified metal.
Based on their 87Sr/86Sr ratios, the reference samples can be categorized into three distinct groups: the casting powders exhibiting the lightest isotopic composition, the refractory materials showing the heaviest isotopic signatures, and the blower and covering powders, along with the top slag primarily composed of covering powder, positioned in between these extremes. The results of the LA-MC-ICP-MS measurements of the inclusions and the unused runner brick are inherently associated with a heightened level of uncertainty attributable to both the measurement technique and the properties of the samples. Nevertheless, despite this inherent uncertainty, the measurements consistently reveal Sr isotope ratios that on average align with those characteristic of the refractory materials.

4. Discussion

4.1. Morphology and Main Constituents

The similarity in the morphology (Figure 5) and composition (Table 4) of the inclusions, with both inclusions consisting mainly of calcium–aluminum oxides, points to a common origin or mechanism of formation. However, based on the main constituents and morphology only, it is not possible to identify their origin or to distinguish clearly between endogenous and exogenous formation. Apparently, none of the reference samples matches the composition of the inclusions. In any case, the possibility of chemical alterations of the source material during the subsequent process steps must also be taken into consideration. Therefore, a more in-depth investigation of the inclusions is required.

4.2. REE Fingerprinting

From the results of trace element measurements using (LA-)ICP-MS (Table 4, Figure 7a), it is evident that both inclusions contain considerable quantities of rare earth elements (REEs). This observation is incompatible with endogenous formation from aluminum metal, which is typically devoid of REEs [20,36].
While the general trend of these curves is consistent with silicate materials of crustal origin, negative Eu anomalies are commonly observed in nature. However, negative Yb anomalies of the magnitude observed are unknown in natural terrestrial materials. It can therefore be assumed that they were caused by redox processes at high temperatures in the molten steel. While the REEs generally form trivalent ions, the electron configurations of Eu and to a lesser extent Yb favor the formation of divalent ions [33]. Such fractionation processes are conceivable under strongly reducing conditions in the presence of elements such as metallic aluminum, silicon, or calcium.
In contrast, the REE profiles for the two refractory samples analyzed, from the runner brick and the mold entrance, do not exhibit any distinctive features, with the exception of a very slight negative Eu anomaly. The trend of the curves also follows the aforementioned pattern typical of silicate materials of crustal origin. This is to be expected, since it can be assumed that such materials were used in the production of the refractory materials. In general, the refractory materials contain the highest amounts of REEs of all reference samples.
REE profiles with similar characteristics have been obtained for the casting powders and the blower powders, although with lower REE abundance, with the notable exception of the distinct positive Er anomaly in casting powder 1 mentioned above. The profile of the top slag sample closely resembles that of the covering powder, which is to be expected, given that this was the auxiliary material applied to the ingot top at the conclusion of the casting process. It appears that minimal chemical alteration occurred during this stage of the process.
Given the apparent importance of interactions between the steel melt and oxide materials in inclusion formation, the inner surface of a used runner brick was investigated by means of LA-ICP-MS in order to identify any alterations in REE profiles. For purposes of comparison, an unused, otherwise identical runner brick was subjected to analysis. Figure 7a depicts the chondrite-normalized REE profiles in the unused and used runner brick samples. As anticipated, the unused sample exhibits the unmodified REE pattern, accompanied by a slight negative Eu anomaly. For the used sample, no significant alterations in the REE profile could be identified, except that REE content was lower by a factor of about 20 to 30.
In conclusion, the acquired REE profiles do not permit a direct correlation between the inclusions and any of the reference samples investigated as potential sources. This is due to the negative Eu and Yb anomalies, which were likely generated (and in the case of Eu, probably intensified) during the deoxidizing, casting, or vacuum arc remelting processes. It is similarly not possible to definitively rule out any of the samples, with the exception of casting powder 1, which exhibits a distinctive erbium anomaly. Nevertheless, the striking resemblance in the REE profiles for inclusions 1 and 2 lends support to the hypothesis of a shared origin. The profiles point to a silicate-bearing material as the probable source. Furthermore, the data indicate that the source material must have undergone profound non-isochemical alteration during the process.

4.3. Sr Isotope Ratios

The 87Sr/86Sr ratios determined in inclusions 1 and 2 (Table 4, Figure 7b) are within a range in which values for refractory materials also lie consistently. Therefore, it can be assumed that fragments of the refractory lining were involved in the formation of the inclusions, even if they cannot be considered their single source.
The deviating results of laser analysis in inclusion 1_2 can be explained by the fact that the analysis was performed at the outer boundaries of the inclusion. Therefore, the Sr signal was generally low, and the outer shell of the inclusion can be expected to be possibly influenced by secondary sources of Sr, such as e.g., trace amounts of Sr contained in Ca added for the removal of Al oxides (see below), or casting or blower powder.

4.4. Hypothesis

The remarkable resemblance in both morphology and composition of the inclusions, despite originating from ingots made of distinct steel types, strongly suggests a shared source. However, none of the reference samples perfectly aligns with the composition of these inclusions, implying potential chemical alterations to the source material and/or the involvement of multiple source materials. Namely, both inclusions contain mainly aluminum and calcium oxides (with roughly two to three times as much Al2O3 as CaO by mass, see Table 4) and minor amounts of silicon dioxide. The low silicon oxide content in the inclusions compared to most of the reference samples can be explained in terms of decomposition and volatilization, which are known to occur under the conditions of vacuum arc remelting, according to Equation (1) [37] (according to the designation method used here, substances dissolved in the metal phase are enclosed in brackets, substances in the slag phase in parentheses, and gaseous substances in braces [38]):
(SiO2) → {SiO} + [O]
However, such a reaction would lead to enrichment of both aluminum and calcium oxide without altering the Al2O3/CaO ratio.
Hence, our assumption posits the existence of two primary source materials: an aluminum-rich source, likely an aluminosilicate, and a calcium-rich source. The origin of aluminum, likely responsible for the majority of the detected REE and Sr in the inclusions, is presumed to be particles eroded from the refractory lining. This assumption is supported by their high 87Sr/86Sr ratios, relatively high REE content (speaking against deoxidation products as a source material), and the trend of the REE patterns (indicating a silicate material).
The identification of the calcium source is less straightforward. It is known that calcium metal readily reacts with solid aluminum oxide in steel melts, i.e., in a temperature range of 1550 °C to 1650 °C, to form calcium aluminates (Equation (2)):
[Ca] + (x + 1/3) (Al2O3) → (CaO∙xAl2O3) + 2/3 [Al]
Calcium treatment is widely used to transfer solid aluminum oxide inclusions in aluminum-killed steel into calcium aluminates, which can be fully or partly liquid depending on their calcium oxide content, in order to improve castability [39]. However, this form of calcium treatment after aluminum deoxidation was applied in the casting of ingot 1, but not ingot 2 (Table 1). In addition, it can be assumed that at the stage when exogenous inclusions are likely to be formed, i.e., during casting, most of the added calcium has already reacted with aluminum oxide. Alternatively, the blower powders also contain calcium metal, which exothermically reacts with any oxidants present in order to heat the top of the ingot. Therefore, a reaction analogous to Equation (2) may have happened at this point of the process.
Calcium oxide may also have reacted with an aluminum source, forming calcium aluminum oxides, a reaction that has been shown to occur relatively quickly at high temperatures [40]. Therefore, blower powders as well as casting powders are candidates for the calcium source. This is underlined by the measurement results for Sr isotopes of inclusion 1_2, which can be interpreted as residual powder material in the outer boundary of the inclusion. However, the fact that the calcium oxide content in casting powder 2 is significantly lower than in casting powder 1 (which were used during casting ingots 2 and 1, respectively, see Table 1) while it is about equal in the inclusions, rather speaks against the casting powders. In addition, the heat and turbulent mixing of the steel melt on application of the blower powders might favor such reactions, which also supports blower powder 1 being the most likely candidate for a calcium source material.

5. Conclusions

Within the present study, two individual specimens of macroscopic non-metallic inclusions found in vacuum arc remelted steel ingots were thoroughly characterized with regard to morphology and main constituents, rare earth element content, and 87Sr/86Sr ratios using SEM/EDX and laser ablation (MC-)ICP-MS to test the potential of a combined fingerprinting approach to source the origin of NMIs. This combination of several complementary measurement approaches allowed for a more comprehensive examination of individual inclusions than each of these methods alone. Therefore, a maximum amount of information could be extracted from the individual specimens and compared to the corresponding data for the reference samples in order to identify potential source materials.
The two inclusion specimens were found in ingots consisting of different steel grades that had undergone the same process chain. This included the application of various auxiliary materials, vacuum arc remelting, a process with the potential for profound chemical modification of the inclusions, and forging of the ingots, affecting the morphology of the inclusions and causing a loss of structural information. Therefore, any attempts to identify the source of non-metallic inclusions were severely complicated, especially since the chemical processes taking place during vacuum arc remelting, such as volatilization phenomena and reactions at the slag/metal interface, are still poorly understood.
Despite the complications, the analytical approach yielded consistent results for both specimens, indicating a common mechanism of formation and supporting the validity of the analytical approach. Both inclusions are likely exogenous in nature, consisting mainly of calcium–aluminum oxides, and must have undergone profound chemical modifications during the casting and remelting processes. The investigation of morphology and main constituents using SEM/EDX, a technique commonly employed in this field, proved inconclusive with regard to the question of the starting material. However, the application of LA-ICP-MS and LA-MC-ICP-MS in combination with other techniques allowed for the determination of both trace elements and isotope ratios, thereby providing valuable insights. These findings indicate that the inclusions most likely originated in particles eroded from the refractory lining of the casting system. Consequently, potential improvements to the process could include modifications to the refractory materials used.
This study represents an important initial attempt to employ a comprehensive array of analytical techniques for the examination of non-metallic inclusions. It is crucial to acknowledge that each inclusion is inherently distinctive, necessitating the integration of multiple analytical techniques. Given these limitations, the nature of the sample itself already poses constraints. Moreover, although further systematic investigations are planned, the inherent complexity and singularity of each inclusion highlight the difficulties in comprehensive analysis.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/met15010067/s1. Table S1: Element content in ingots 1 and 2; Table S2: Experimental parameters; Table S3: m/z and integration times; Table S4: Mass fractions; Table S5: Laser ablation parameters.

Author Contributions

Conceptualization, J.C.K., A.G. and T.P.; methodology, J.I., T.C.M. and T.P.; validation, S.W., J.I. and T.C.M.; formal analysis, S.W. and J.I.; investigation, C.W., G.M., S.W., J.I. and T.C.M.; resources, J.C.K., A.G. and T.P.; data curation, C.W., G.M., S.W. and J.I.; writing—original draft preparation, C.W. and G.M.; writing—review and editing, C.W., G.M., S.W., J.C.K., J.I., T.C.M. and T.P.; visualization, C.W., G.M., S.W. and J.I.; supervision, J.C.K., A.G., J.I., T.C.M. and T.P.; project administration, J.C.K., A.G. and T.P.; funding acquisition, J.C.K., A.G. and T.P. All authors have read and agreed to the published version of the manuscript.

Funding

Zukunftsfonds Steiermark (project PN 1115) provided financial support for setting up the infrastructure.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

We thank Tamara Cwioro and Christine Opper for their support during sample preparation, as well as Bernhard Sartory for performing SEM/EDX analyses. We would also like to thank Kathrin Thiele for reviewing the manuscript and providing valuable suggestions for improvement.

Conflicts of Interest

Author Jörg C. Korp was employed by the company Dr. Korp Technological-Consulting. Author Andreas Graf was employed by the company Breitenfeld Edelstahl AG. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

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Figure 1. Macroscopic NMIs investigated in the present study. The spots visible in the vicinity of inclusion 1 are results of laser shots used to adjust laser energy and analyze the composition of the steel matrix. The holes visible above and below inclusion 2 were drilled in order to mark its location.
Figure 1. Macroscopic NMIs investigated in the present study. The spots visible in the vicinity of inclusion 1 are results of laser shots used to adjust laser energy and analyze the composition of the steel matrix. The holes visible above and below inclusion 2 were drilled in order to mark its location.
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Figure 2. Schematic of the steps in the production of the investigated materials.
Figure 2. Schematic of the steps in the production of the investigated materials.
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Figure 3. Schematic representation of the ingot casting process.
Figure 3. Schematic representation of the ingot casting process.
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Figure 4. Microscope images of unused (left) and used runner brick (right).
Figure 4. Microscope images of unused (left) and used runner brick (right).
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Figure 5. SEM images across sections of inclusions 1 and 2.
Figure 5. SEM images across sections of inclusions 1 and 2.
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Figure 6. SEM/EDX element mapping showing the distribution of O, Al, Si, and Ca in sections of inclusion 1 (left) and inclusion 2 (right).
Figure 6. SEM/EDX element mapping showing the distribution of O, Al, Si, and Ca in sections of inclusion 1 (left) and inclusion 2 (right).
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Figure 7. (a) Chondrite-normalized REE mass fractions determined for inclusions 1 and 2 and reference samples. For better visibility of the REE patterns, the value for promethium (*), which does not occur naturally, is interpolated between Nd and Sm. (b) 87Sr/86Sr ratios for inclusions 1 and 2 and reference samples. For both NMIs, results of the measurements of two lines each are shown separately (inclusion 1_1 and 1_2, and inclusion 2_1 and 2_2). Error bars represent combined uncertainties at a 95% confidence level. Please note that on the present scale, error bars for results of solution-based ICP-MS measurements are typically smaller than the respective data points and therefore poorly visible. The inclusions and the used and unused runner brick samples were analyzed using LA-(MC-)ICP-MS. All other reference samples were analyzed using (MC-)ICP-MS after Na2O2 sintering digestion.
Figure 7. (a) Chondrite-normalized REE mass fractions determined for inclusions 1 and 2 and reference samples. For better visibility of the REE patterns, the value for promethium (*), which does not occur naturally, is interpolated between Nd and Sm. (b) 87Sr/86Sr ratios for inclusions 1 and 2 and reference samples. For both NMIs, results of the measurements of two lines each are shown separately (inclusion 1_1 and 1_2, and inclusion 2_1 and 2_2). Error bars represent combined uncertainties at a 95% confidence level. Please note that on the present scale, error bars for results of solution-based ICP-MS measurements are typically smaller than the respective data points and therefore poorly visible. The inclusions and the used and unused runner brick samples were analyzed using LA-(MC-)ICP-MS. All other reference samples were analyzed using (MC-)ICP-MS after Na2O2 sintering digestion.
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Table 1. Overview of the three ingot castings covered in this work and the obtained samples. Inclusions 1 and 2 were found in ingots 1 and 2, respectively, while several reference samples were taken from the casting of ingot 3.
Table 1. Overview of the three ingot castings covered in this work and the obtained samples. Inclusions 1 and 2 were found in ingots 1 and 2, respectively, while several reference samples were taken from the casting of ingot 3.
ParameterIngot 1Ingot 2Ingot 3
SamplesInclusion 1Inclusion 2Top slag
Mold entrance
Runner brick
Runner brick used
Steel grade1.45451.27381.4545
Deoxidizing agentAlAlAl
Calcium treatmentYesNoYes
Auxiliary materialsCasting powder 1Casting powder 2Casting powder 1
Blower powder 1Blower powder 1Blower powder 2
Covering powderCovering powderCovering powder
Table 2. Selected element content specifications for steel grades 1.4545 and 1.2738, shown as mass fractions w and expressed as percentage by mass, adapted from [25].
Table 2. Selected element content specifications for steel grades 1.4545 and 1.2738, shown as mass fractions w and expressed as percentage by mass, adapted from [25].
Steel GradeCSiMnPSCrMoNiFe
1.4545≤0.07≤1.00≤1.00≤0.030≤0.01514.00–15.50<0.503.50–5.50Bal.
1.27380.35–0.450.20–0.401.30–1.60≤0.030≤0.0301.80–2.100.15–0.250.90–1.20Bal.
Table 3. Overview of the analytical methods applied to the samples in the present study.
Table 3. Overview of the analytical methods applied to the samples in the present study.
SampleSEM/EDXICP-MSMC-ICP-MSLA-ICP-MSLA-MC-ICP-MS
Inclusion 1✓ ✓✓
Inclusion 2✓ ✓✓
Casting powder 1 ✓✓
Casting powder 2 ✓✓
Blower powder 1 ✓✓
Blower powder 2 ✓✓
Covering powder ✓✓
Top slag ✓✓
Mold entrance ✓✓
Runner brick ✓✓
Runner brick unused ✓✓
Runner brick used ✓✓
Table 4. Mass fractions w expressed as percentage by mass of main constituents (calculated as oxides, determined using SEM/EDX, ICP-MS, or LA-ICP-MS) and lanthanum (as a measure of overall REE content, determined using ICP-MS or LA-ICP-MS). REE anomalies calculated for europium, erbium, and yttrium (E/E*, from data determined using ICP-MS or LA-ICP-MS), and 87Sr/86Sr ratios (determined using MC-ICP-MS or LA-MC-ICP-MS; for inclusion 1 and inclusion 2, results for two lines analyzed using LA-MC-ICP-MS on each inclusion are given) for all samples analyzed in the present work.
Table 4. Mass fractions w expressed as percentage by mass of main constituents (calculated as oxides, determined using SEM/EDX, ICP-MS, or LA-ICP-MS) and lanthanum (as a measure of overall REE content, determined using ICP-MS or LA-ICP-MS). REE anomalies calculated for europium, erbium, and yttrium (E/E*, from data determined using ICP-MS or LA-ICP-MS), and 87Sr/86Sr ratios (determined using MC-ICP-MS or LA-MC-ICP-MS; for inclusion 1 and inclusion 2, results for two lines analyzed using LA-MC-ICP-MS on each inclusion are given) for all samples analyzed in the present work.
SampleMethodMass Fraction w, Percentage by MassREE Anomalies87Sr/86Sr
MgOAl2O3SiO2CaOLaEu/Eu*Er/Er*Yb/Yb*
Inclusion 1LA-(MC-)ICP-MS0.0945 160.06450.21.00.20.7117 0.7083
SEM/EDX 49323
Inclusion 2LA-(MC-)ICP-MS0.1149 140.07730.11.00.20.7118 0.7121
SEM/EDX 51624
Casting powder 1(MC-)ICP-MS1.488.047.621.40.00260.86.11.00.7081
Casting powder 2 (MC-)ICP-MS1.0510.125.04.80.00310.71.01.00.7077
Blower powder 1(MC-)ICP-MS0.9032.220.024.20.00110.91.11.10.7096
Blower powder 2(MC-)ICP-MS0.542.753.816.40.00470.91.01.00.7098
Covering powder(MC-)ICP-MS3.2483.82.21.20.00034.60.91.60.7093
Top slag(MC-)ICP-MS2.2350.51.71.60.00034.30.81.30.7097
Mold entrance(MC-)ICP-MS0.5460.934.40.10.00630.71.01.00.7119
Runner brick(MC-)ICP-MS0.4946.736.30.10.00830.61.01.00.7116
Runner brick unusedLA-(MC-)ICP-MS 36.523.00.20.00520.61.01.00.7116
Runner brick usedLA-(MC-)ICP-MS 37.229.50.30.00020.61.11.4
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MDPI and ACS Style

Walkner, C.; Mukhametzianova, G.; Wagner, S.; Korp, J.C.; Graf, A.; Irrgeher, J.; Meisel, T.C.; Prohaska, T. Tracing the Origin of Oxide Inclusions in Vacuum Arc Remelted Steel Ingots Using Trace Element Profiles and Strontium Isotope Ratios. Metals 2025, 15, 67. https://doi.org/10.3390/met15010067

AMA Style

Walkner C, Mukhametzianova G, Wagner S, Korp JC, Graf A, Irrgeher J, Meisel TC, Prohaska T. Tracing the Origin of Oxide Inclusions in Vacuum Arc Remelted Steel Ingots Using Trace Element Profiles and Strontium Isotope Ratios. Metals. 2025; 15(1):67. https://doi.org/10.3390/met15010067

Chicago/Turabian Style

Walkner, Christoph, Gulnaz Mukhametzianova, Stefan Wagner, Jörg C. Korp, Andreas Graf, Johanna Irrgeher, Thomas C. Meisel, and Thomas Prohaska. 2025. "Tracing the Origin of Oxide Inclusions in Vacuum Arc Remelted Steel Ingots Using Trace Element Profiles and Strontium Isotope Ratios" Metals 15, no. 1: 67. https://doi.org/10.3390/met15010067

APA Style

Walkner, C., Mukhametzianova, G., Wagner, S., Korp, J. C., Graf, A., Irrgeher, J., Meisel, T. C., & Prohaska, T. (2025). Tracing the Origin of Oxide Inclusions in Vacuum Arc Remelted Steel Ingots Using Trace Element Profiles and Strontium Isotope Ratios. Metals, 15(1), 67. https://doi.org/10.3390/met15010067

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