Implementation of Cored Wire Treatment Technology in Nodular Cast Iron Foundries ^†

Abstract

Nodular cast iron is one of the most widely used materials in the machine building industry. The main reasons for this are its strength, elongation, and competitive price compared to other steels and metals. The possibility to have a high strength and elongation together is thanks to the spheroidal shape of the graphite inserts in the metal structure of the iron. To exploit these advantages, special treatments such as adding magnesium are used after the melting process but before pouring the metal in the casting mold. Classic technology is called tundish/sandwich technology when ferrosiliconmagnesium alloy in bulk is placed at the bottom of a ladle before filling it with liquid cast iron. In the present article, an alternative technology will be presented where a fesimg alloy is filled in a steel wire and inserted automatically into a ladle. The advantages of this technology will be described in detail.

1. Introduction

The evolution of cast iron alloys has been pivotal to the advancement of modern metallurgy and engineering. Among these, cast iron—an iron–carbon alloy known for its excellent castability and wear resistance—has undergone significant transformations since its early use in ancient China over 2000 years ago. Originally prized for its strength and ease of production, traditional gray cast iron found widespread use during the Industrial Revolution in the construction of bridges, engines, and machinery. However, its relatively brittle nature posed limitations in more demanding mechanical applications.

A breakthrough occurred in the mid-20th century with the invention of nodular cast iron (also known as ductile iron or spheroidal graphite iron) by Keith Millis in 1943. By introducing a small amount of magnesium into molten iron, graphite flakes—which typically weaken cast iron—were transformed into spherical nodules. This change in microstructure significantly enhanced the material’s ductility, tensile strength, and impact resistance, while maintaining its excellent castability and vibration damping properties [1]. As a result, nodular cast iron quickly found applications in the automotive, construction, and heavy machinery industries, replacing steel in many structural and functional components.

To produce nodular iron reliably and efficiently, various magnesium treatment methods have been developed. Among these, the cored wire treatment has emerged as a modern, precise, and controlled approach. This method involves injecting a magnesium-containing wire into molten iron, ensuring a uniform magnesium distribution while minimizing environmental hazards and production inconsistencies. The implementation of cored wire technology represents a significant advancement in the production of nodular cast iron, offering enhanced quality control, safety, and process automation [2,3].

2. Materials and Methods

Nodular cast iron, also known as ductile iron or spheroidal graphite iron, is a type of cast iron renowned for its superior mechanical properties—particularly its excellent ductility, strength, and toughness. These properties are achieved through a controlled metallurgical process that modifies the shape of graphite within the iron matrix. Unlike grey cast iron, where graphite forms as flakes, in nodular cast iron, the graphite appears as spheroids or nodules, which significantly reduces the points of stress concentration and enhances the metal’s structural integrity [4,5,6,7].

The production of nodular cast iron begins with a base iron, typically low in sulfur, which is then treated with nodulizing agents—most commonly magnesium or magnesium-containing alloys such as FeSiMg (ferrosilicon magnesium). These agents are introduced to the molten iron to promote the spheroidal formation of graphite during solidification [8,9,10,11]. The magnesium reacts with sulfur and oxygen in the melt, cleaning it and creating the necessary conditions for nodule formation. There are various techniques for introducing magnesium into the melt, including the tundish cover method and the more modern cored wire injection method.

Nodular cast iron is widely used in industries requiring components with high strength-to-weight ratios, wear resistance, and toughness. Common applications include automotive parts (such as crankshafts, gears, and suspension components), water and sewage pipes, heavy machinery, and wind turbine hubs [12,13,14,15]. Its adaptability and cost-effectiveness make it a preferred material for both standard and high-performance engineering applications.

2.1. Classic Tundish (Open Ladle) Treatment for Nodular Cast Iron

The classic method of producing nodular cast iron involves treating the molten base iron in an open ladle—commonly known as the tundish cover method or open ladle method. This process typically uses a special ladle with a reaction chamber or a pocket for holding the nodulizing alloy, usually FeSiMg [16,17,18,19].

The general procedure is as follows:

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Preparation: A specific amount of FeSiMg alloy is placed at the bottom of the ladle, often in a pocket or chamber designed to partially enclose the alloy and direct the reaction gases.

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Pouring the Metal: The molten base iron is poured into the ladle, where it comes into contact with the FeSiMg. The violent reaction between magnesium and elements such as sulfur and oxygen causes significant fuming and spattering. The magnesium acts as the nodulizing agent, promoting the formation of spheroidal graphite.

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Covering: To minimize the loss of magnesium from oxidation and ensure a more stable reaction, the ladle is often partially covered with steel plates or refractory material. However, the process still exposes the reaction to the atmosphere to some extent, which can lead to inconsistent results.

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Slag Removal and Inoculation: After the reaction is complete, slag and reaction residues are removed from the surface of the molten metal. Inoculants are often added afterward to refine the graphite structure and control solidification.

While this method has been widely used for decades and is relatively simple in setup, it has several drawbacks, as follows:

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High magnesium loss due to oxidation.

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Fuming and spattering, which can pose safety hazards for operators.

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Limited process control, leading to variability in quality.

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A lower yield of nodulizing agents, requiring excess additions.

2.2. Cored Wire Treatment Technology with FeSiMg

The cored wire method represents a modern and more controlled approach to nodularizing molten iron. This technique involves the use of a continuous, steel-sheathed wire, filled with FeSiMg and other possible alloying or inoculating materials [20,21].

The process uses the following steps:

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Cored Wire Composition: The wire is made by tightly packing powdered or granular FeSiMg inside a mild steel tube, which is then coiled for easy handling and feeding.

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Injection into the Ladle: The cored wire is fed into the bottom of a ladle filled with molten iron using a wire-feeding machine. The wire is injected at a controlled speed and depth, allowing the nodulizing alloy to react within the molten metal under a protective slag layer or inert environment.

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In Situ Reaction: As the wire melts, the FeSiMg reacts with the sulfur and oxygen in the iron, similar to the classic method, but the reaction occurs subsurface, minimizing exposure to air.

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Post-Treatment Inoculation: Additional inoculants can also be introduced via cored wire, ensuring a fully integrated treatment.

Advantages of the cored wire method include the following:

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Precise and repeatable control of magnesium addition.

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A higher yield of FeSiMg due to reduced oxidation losses.

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Improved safety by eliminating surface reactions and fuming.

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Cleaner metal with lower slag and fewer inclusions.

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The potential for automation and process optimization.

Even with these advantages of cored wire technology, it is considered as a technology requiring a lot of parameters, such as temperature, sulfur, and time for pouring, to be precise in order to achieve all the benefits described above. Within the present article, it will be shown how, with simple steps, much better nodularization results can be achieved via cored wire treatment, with sufficient economic benefits.

3. Results

In order to prove the advantages of cored wire technology, an experiment is performed by comparing the classic tundish technology with cored wire treatment technology. A 900 kg capacity ladle is used in both methods, with three melts created by each technology. The samples are analyzed considering microstructure, chemical analysis, tensile strength, and elongation.

For the classical technology, 18 kg of FeSiMg alloy is inserted in the ladle and covered by steel chips. The chemical analysis of the FeSiMg alloy composition is as follows:

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Mg—6.9%, Si—43%, Ca—1.01%, and rare earths—0.96 %, 2–15 mm grain size.

Compared with the tundish technology, the same ladle was used together with special installation for the cored wire treatment (Figure 1) by adding 23.4 m of cored wire (Figure 2). The following chemical analysis was performed, which is equal to 9,36 gross material and 6.37 kg filling material:

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Mg—25.3%, Si—45.7%, Ca—4.92%, and rare earths—0.96 %,13 mm diameter of the wire.

Comparison: Classic Tundish vs. Cored Wire Treatment Technology

The results of the treatment are summarized below in

Table 1. Results of the treatment with tundish metal and cored wire technology.

Parameter	Tundish—Ladle 1	Tundish—Ladle 2	Tundish—Ladle 3	Cored Wire—Ladle 1	Cored Wire—Ladle 2	Cored Wire—Ladle 3
Final Mg (%)	0.039	0.033	0.356	0.051	0.046	0.048
Tensile strength (MPa)	432	458	428	495	489	488
Elongation (%)	9.6	8.2	5.4	19.4	20.9	20.03
Brinell hardness (HB)	143	139	135	144	141	136
Graphite size	4, 5, 6	4, 5, 6	4, 5, 6	5, 6, 7	5, 6, 7	5, 6, 7
Graphite shape	V, VI	V, VI	V, VI	V, VI	V, VI	V, VI
Number of graphite spheres per mm2	75	100	100	150–200	150–200	150–200
Metal base	P9	P7	P9	P1	P1	P1

and also in Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8 in microstructure photos with a magnification of ×100.

4. Discussion

In this study, the effects of different treatment methods on the properties of nodular cast iron were examined. The analyzed treatments included the use of tundish ladle processing (Tundish—Ladle 1, Tundish—Ladle 2, and Tundish—Ladle 3) and cored wire additions (Cored Wire—Ladle 1, Cored Wire—Ladle 2, and Cored Wire—Ladle 3). The key parameters discussed include the final Mg content, tensile strength, elongation, Brinell hardness, graphite morphology (size and shape), and the number of graphite spheres per mm².

4.1. Magnesium Content (Mg%)

Magnesium content is a crucial factor in the production of nodular cast iron, as it directly influences the formation of graphite nodules, which, in turn, affect the mechanical properties of the cast iron. From our results, we observe a notable increase in Mg content in the cored wire treatment compared to the tundish ladle process.

• Tundish—Ladle 1 (0.039%) and Tundish—Ladle 2 (0.033%) exhibit relatively low Mg contents, which is typical for conventional treatments. However, Tundish—Ladle 3 (0.356%) shows a significantly higher Mg content. This may suggest an improvement in the ladle’s ability to maintain the Mg concentration during the transfer process, potentially due to a more optimized process flow or temperature control.
• The cored wire ladle treatments show Mg levels ranging from 0.046% to 0.051%. These values are consistently higher than those for the tundish ladle process. The increased Mg content in the cored wire treatments is likely due to the more controlled addition of magnesium through the cored wire, which provides a more localized and efficient method for alloying.

This indicates that cored wire treatment is more effective at achieving the desired Mg levels compared to tundish ladle processing, contributing to the formation of better-quality nodules.

4.2. Tensile Strength

Tensile strength is an important mechanical property of cast iron, reflecting the material’s resistance to breaking under tension. Our results demonstrate that the cored-wire-treated samples (Cored Wire—Ladle 1, 2, and 3) have a superior tensile strength compared to those treated via the tundish ladle process.

• Tundish—Ladle 1 (432 MPa), Tundish—Ladle 2 (458 MPa), and Tundish—Ladle 3 (428 MPa) show lower tensile strengths when compared to the cored-wire-treated samples, which range from 495 MPa to 488 MPa.
• The cored wire ladle treatments consistently show improvements in tensile strength, likely due to the more homogeneous and refined microstructure promoted by the increased and more controlled Mg addition. This results in stronger nodules that improve the overall mechanical properties of the cast iron.

This highlights the effectiveness of the cored wire treatment in enhancing the tensile strength of nodular cast iron.

4.3. Elongation

Elongation provides insight into the ductility of a material, indicating how much the material can stretch before breaking. The cored wire treatments show significant improvements in elongation compared to the tundish ladle samples, as follows:

• The elongation values for the tundish ladle samples are relatively low, ranging from 5.4% to 9.6%. In contrast, the cored wire ladle treatments show markedly higher elongation values, ranging from 19.4% to 20.9%.
• This improvement is likely linked to the finer, more uniform distribution of graphite nodules formed by the cored wire process, which promotes better ductility and a more balanced microstructure.

The enhanced elongation values for the cored wire treatments suggest that this method not only improves strength, but also provides better plasticity, which is a desirable trait for certain applications requiring a higher formability.

4.4. Brinell Hardness

Brinell hardness is a measure of a material’s resistance to indentation and gives an indication of its wear resistance. Our hardness results show the following:

• Tundish—Ladle 1 (143 HB), Tundish—Ladle 2 (139 HB), and Tundish—Ladle 3 (135 HB) exhibit relatively lower hardness values, which can be attributed to the coarser graphite structure typically produced by this treatment method.
• The cored wire ladle samples show similar hardness values, ranging from 136 HB to 144 HB, which can be explained by the finer graphite nodules and more refined matrix created by the cored wire process. This more refined microstructure contributes to a greater resistance to wear and improves overall hardness.

The results suggest that cored wire treatment do not change the hardness of nodular cast iron, even increasing the elongation.

4.5. Graphite Morphology

The size and shape of the graphite nodules have a significant impact on the mechanical properties of cast iron. In this study, the graphite size ranges from four to seven across all treatments, with no significant difference in size between the tundish ladle and cored wire methods. The shape of the graphite remains consistent as well, with the nodules predominantly exhibiting V and VI shapes, which are characteristic of high-quality nodules. However, it is noteworthy that the cored wire— ladle treatments show a higher number of graphite spheres per mm² (150–200) compared to the tundish ladle treatments, where the values are closer to 75–100. The increased number of nodules in the cored-wire-treated samples is likely a key factor in the improved mechanical properties (tensile strength, elongation, and hardness).

4.6. Metal Base

The metal base varies across treatments, with the tundish ladle treatments showing a mix of P7 and P9, while the cored wire treatments all show P1 as the metal base. This change in the base material could influence the overall properties of the cast iron, particularly in terms of its alloy composition and behavior during processing. However, since the differences in the base metal are not drastically large, it is likely that the cored wire treatment’s effect on magnesium addition and graphite formation plays a more dominant role in the observed results

The findings of this study present significant insights into the comparative evaluation of two nodular cast iron treatment technologies—the classic tundish ladle method and the more modern cored wire treatment. This research was aimed at understanding the impact of both methods on the critical properties of the cast iron, including magnesium content, tensile strength, elongation, Brinell hardness, and graphite morphology. Based on the data and analysis, several conclusions can be drawn, which not only highlight the advantages of the cored wire technology, but also underscore areas for further optimization.

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Enhanced Magnesium Control and Efficiency

A key observation from the comparison between the two methods is the superior control of magnesium (Mg) content in the molten iron when utilizing the cored wire treatment. In the traditional tundish ladle method, the magnesium content is relatively low, with values ranging from 0.033% to 0.039%, typical of processes that experience higher oxidation losses. This is contrasted with the cored wire treatments, which show consistent magnesium levels between 0.046% and 0.051%. The ability to more efficiently control the magnesium content in the cored wire method can be attributed to the precise and localized addition of FeSiMg, which significantly reduces oxidation losses. This greater efficiency in magnesium usage enhances the overall metallurgical quality of the nodular cast iron, ensuring that the desired properties are achieved with reduced material waste.

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Superior Mechanical Properties: Tensile Strength and Elongation

The mechanical properties of cast iron are crucial for determining its suitability for various industrial applications. The results clearly demonstrate that the cored-wire-treated samples exhibit a superior tensile strength compared to the tundish-ladle-treated samples. The tensile strength for the cored wire treatments ranges from 488 MPa to 495 MPa, while the tundish ladle method results in values ranging from 428 MPa to 458 MPa. This improvement can be attributed to the more homogeneous and refined microstructure promoted by the cored wire treatment, which allows for better bonding between the graphite nodules and the iron matrix.

Similarly, elongation values are notably improved in the cored-wire-treated samples, with elongation ranging from 19.4% to 20.9%, compared to the tundish ladle samples, which have elongation values between 5.4% and 9.6%. The increased elongation reflects a better ductility and formability, which are critical for applications that require a material capable of undergoing plastic deformation without fracturing. The cored wire treatment effectively balances strength and ductility, making it more versatile for a wider range of engineering applications.

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Brinell Hardness and Wear Resistance

The hardness of the nodular cast iron, as measured by Brinell hardness (HB), also sees similar results to the cored wire treatments. The values range with both technologies are the very similar and it is again benefit for the cored wire treatment technology, because with the same hardness much better elongation can be reached.

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Graphite Morphology: Size, Shape, and Distribution

Graphite morphology plays a pivotal role in determining the mechanical properties of nodular cast iron. In this study, although the size and shape of the graphite nodules are relatively consistent across both methods (with a predominant V and VI shape), the number of graphite spheres per mm² is significantly higher in the cored-wire-treated samples. The cored wire treatments achieve values between 150 and 200 spheres per mm² compared to the tundish ladle treatments, which have values closer to 75–100 spheres per mm². The increased number of graphite nodules in the cored-wire-treated cast iron directly correlates with improved mechanical properties, including a higher tensile strength and better elongation. The more refined graphite structure enhances the material’s ability to absorb stress and deform plastically, further contributing to its improved performance.

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Cost-Effectiveness and Process Efficiency

While the cored wire method demonstrates clear improvements in material properties, it is also worth noting the potential economic benefits of this technology. The reduced oxidation losses and higher magnesium efficiency can lead to cost savings, as less FeSiMg alloy is required to achieve the same or superior results. Additionally, the potential for process automation in the cored wire treatment offers further advantages in terms of repeatability, precision, and overall production efficiency. In contrast, the traditional tundish ladle method, with its higher losses and more manual handling, can be less efficient and may require more frequent adjustments to maintain consistent results.

The initial setup cost of the cored wire treatment may be higher due to the need for specialized equipment, such as the wire-feeding machine, but the long-term savings in material consumption, reduced waste, and improved safety (due to the elimination of fuming and spattering) make this method a viable option for improving both the quality and economics of nodular cast iron production.

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Reduced Silicon Addition and Carbon Retention Leading to Lower Scrap Generation

An important advantage of the cored wire treatment method over the traditional tundish ladle process is the reduced need for high amounts of silicon in the molten iron. In the tundish ladle process, significant quantities of silicon are often added to the iron to aid in the nodulizing process. However, the presence of excessive silicon can lead to unwanted consequences, including a loss of carbon from the molten metal. This loss of carbon can negatively affect the cast iron’s properties, particularly its strength and hardness, and may lead to increased scrap production due to inconsistencies in the material [22].

In contrast, the cored wire method requires less silicon to achieve the desired magnesium content, which helps to retain a higher level of carbon in the metal. This retention of carbon is crucial for maintaining the desired mechanical properties of the nodular cast iron, while also minimizing the loss of valuable materials during the process. As a result, the cored wire treatment leads to a more efficient utilization of raw materials, with less scrap generated during production. This reduction in scrap not only lowers material costs, but also improves the overall yield of usable cast iron, contributing to more sustainable manufacturing practices.

Additionally, the lower silicon addition in the cored wire method means that less effort is required to recover and return silicon to the melting process. In the tundish ladle method, excess silicon often needs to be recovered and recycled to avoid wastage. In contrast, the more controlled and localized addition of silicon through the cored wire process helps to eliminate the need for extensive recovery efforts, further streamlining the production process.

This efficient use of silicon and carbon retention is a significant advantage of the cored wire treatment, making it a more cost-effective and environmentally friendly option compared to traditional methods.

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Recommendations for Future Research

Despite the promising results of the cored wire treatment, there are still several areas where future research can help to further optimize this technology. Future studies should focus on the following:

• Optimizing the wire composition: The current study used FeSiMg with 25.3% Mg, but exploring different alloying combinations, including varying the amounts of rare earth elements, could lead to even more efficient processes.
• Temperature and process control: The effects of variations in ladle temperature and pouring time on the performance of the cored wire treatment should be further explored to ensure optimal conditions for each batch.
• Long-term performance studies: More long-term, real-world testing is needed to assess how cored-wire-treated nodular cast iron performs under various service conditions, such as high stress, temperature fluctuations, and wear.
• Automation and process scaling: Investigating the scalability of the cored wire method for large-scale industrial applications and its integration with automated systems would further enhance its competitiveness in the industry.

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Final Remarks

This study clearly demonstrates that cored wire treatment offers significant advantages over the traditional tundish ladle method in the production of nodular cast iron. The cored wire method provides a more efficient and controlled way to introduce magnesium into the molten metal, resulting in superior mechanical properties, including tensile strength, elongation, hardness, and improved graphite morphology. The findings suggest that adopting cored wire treatment could lead to an improved material performance and cost-effectiveness, particularly for high-performance applications in the automotive and heavy machinery industries.

As the technology matures and further optimization takes place, the cored wire method could become the standard for producing high-quality nodular cast iron with a more consistent and reliable output. With its benefits in terms of both quality and cost, it has the potential to revolutionize the way nodular cast iron is produced, offering better results for manufacturers and consumers alike.

5. Conclusions

The present research proves the advantages of cored wire treatment technology, which are as follows:

1. Reducing the consumption of FeSiMg alloy in the production process by 30–50%.

2. Adding more Mg content with a smaller amount of FeSiMg alloy, which helps to reach higher grades of nodular cast iron with lower costs.

3. Adding less Si content, which reduces carbon losses, decreases shrinkage defects, and optimizes the use of the gating systems in furnaces thanks to the lower silicon content.

4. Reduces the influence of the human factor, improves the monitoring of the inserted alloy in the melt, and gives constant and repeatable results.