The Effect of Carbide-Stabilizing Modification of Melt with Tellurium-Containing Powder Briquettes on the Structure and Properties of Cast Iron

The article presents the investigation results of the effect of melt treatment with powder briquetted modifiers containing tellurium on the structural features and mechanical characteristics of cast iron produced from this melt. The melt was processed using in-mold modification with briquettes consolidated from a mixture of tellurium (10%), copper (40%) and iron (50%) powders. The casting was produced with use of vertical casting arrangement with siphon gating system. The modifier briquettes were installed in the area of the lower part of the сasting mold. As the research results showed, carbide-stabilizing modification with tellurium leads to a decrease in the concentration of graphite inclusions in cast iron compared to cast iron without tellurium, corresponding to an increase in the content of the cementite phase in the alloy structure from 20-26% for cast iron without tellurium to 28-32% with tellurium and an increase level of the bleaching criterion. With an increase in the concentration of tellurium in cast iron, the value of material strength and hardness as well as its wear resistance has increased too.

Keywords:Cast iron; Modification; Powder; Tellurium; Structure; Graphite inclusions; Cementite; Hardness; Strength

To increase the strength and wear resistance of grey cast iron; used; in particular; for the manufacturing of rolling rolls; various methods of melt processing (modification) are used; changing the morphology of graphite inclusions and also increasing the bleachability of cast iron [1-5]. Such processing often makes it possible to exclude the use of significant amounts of expensive alloying elements in its composition. One of the most effective modifiers that influences the structure of cast iron and improves its mechanical properties is tellurium. The effect of tellurium on the properties of gray cast iron is mainly realized through the modification of the matrix structure and graphite morphology of cast iron [6].

Thus; in particular in [6-8] it has been revealed that the tellurium could influence the graphite morphology and matrix structure of cast irons and caused increase of mechanical properties of cast irons. When adding relatively little amount of tellurium; it restricts the growth of graphite and causes the graphite become finer; shorter; curved; causes some lump-shaped graphite appearing and increases the graphite count that results in increasing of ferrite content. At the same time; the authors [9] consider that by adding tellurium; the iron melt would solidify according to the quasi-stable system and lead to the formation of the white structure; providing a significant increase in the hardness and wear resistance of the casting. The reason is that tellurium is a strong anti-graphitization element; which can effectively promote the formation of white hole structure in gray cast iron. Its exact mechanism is still under discussion at present; but it is generally believed that tellurium has a low melting point and can be melted in molten iron; which affects the crystallization and solidification process of molten iron [6].

Tellurium lowers the liquidus temperature slightly and the solidus temperature greatly. This leads to austenite grain growth. The ferrite border is formed in the solid state. Secondary cementite occurs during cooling as the result of the reduced solubility. The higher tellurium concentration in cementite stabilizes it [10]. With increasing tellurium concentrations, the quantity of graphite inclusions decreases; but their size increases. The increase in the size of graphite inclusions by means of authors [10] is explained by the rapid coalescence of fine graphite inclusions formed near tellurides in the process of solidification. Meanwhile; the decreasing amount of graphite observed with increasing tellurium concentration is due to the stabilization of cementite [10]. The carbide-stabilizing property of tellurium is used; in particular; in the manufacture of cast iron rolling rolls. To improve the bleaching of roller cast irons; modification of melts is used with tellurium; as one of the strongest carbide stabilizers; suppressing graphitization during the solidification of cast iron even with very small doses of its introduction (0.003 - 0.005%).

However; despite its high bleaching ability; it has so far had limited use due to the difficulties of its introduction and stabilization in liquid cast iron. This is due to the fact that the most common technology of modifying molten cast iron today; namely; its processing with piece-melted master alloys and modifiers in a ladle [1-3,6]; being quite simple from a technological point of view; has; however; a number of significant drawbacks. One of the most important among these shortcomings is a relatively low and unstable level of assimilation of modifiers (in particular; tellurium) due to significant loss of the latter to evaporation; since their evaporation temperature is; as a rule; lower than the melting temperature of cast iron [5-8].

In this sense; the process of in-mold modification (In-mold process) is more effective; economically beneficial and relatively environmentally friendly for obtaining high-quality cast iron with a given structure and set of properties [2-5]. With this method; the modification process is carried out directly at the moment of filling the mold cavity with liquid cast iron; for which the sprue system provides an intermediate reaction chamber on the path of the melt to the casting; in which the estimated amount of crushed modifier is placed before assembling the mold. In the process of pouring the mold; cast iron; passing through the reaction chamber; dissolves the modifier and is subjected to modifying treatment. In such conditions; on the one hand; oxidation of modifier elements is excluded or significantly reduced; and on the other hand; the time between modification and crystallization of the melt is maximally shortened; which increases the efficiency of graphitizing modification [2]. With the optimal combination of temperature and speed of filling the mold with metal; the charge of the reaction chamber should dissolve evenly and completely in the jet of cast iron; which leads to a drastic change in the microstructure; mechanical and functional properties of the base alloy.

Among the known methods of in-mold modification; the method of modification with the use of powder briquetted modifiers; which are obtained using powder metallurgy technology; appears to be the most promising [6-10]. This technological approach makes it possible to widely adjust the chemical composition and density of briquettes by varying the content of individual components of the powder charge; as well as their pressure of consolidation. This; in turn; affects the characteristics of contact interparticle strength and melting temperature; as well as the thermal conductivity of the briquette material; which determines the intensity of their dissolution in liquid metal. Taking into account the above; the purpose of this work was to study the effect of processing the melt with powder briquetted modifiers containing tellurium on the structural features and mechanical characteristics of the cast iron obtained from this melt for the production of rolling rolls.

The following composition of pig iron was used as the initial alloy for producing roller cast iron (wt. %): 3;1 С; 0;6 Sі; 0;5 Мg; 0;8 Cr; 3;6 Ni; 0;12 P; 0;03 S; Fe - the rest. Experimental melts were carried out in an induction furnace. Powders of iron (-160μm); copper (-100μm) and tellurium (-100μm) were used to produce modifying briquettes. When developing the compositions of the briquettes; we sought to ensure that their specific gravity was not lower than the specific gravity of liquid cast iron (6.9g/cm3). In this case; the floating of briquettes on the surface of liquid cast iron and their slagging are excluded. To ensure such a specific gravity of the briquettes; their component composition was formed from 5-15 % (wt.) tellurium powder; 30-50 % copper powder and iron powder - the rest. The density of briquettes was 7.0-7.3g/cm³.

The investigation of the tellurium effect on the structure and properties of the cast iron was carried out by means of casting of the rolling rolls in a mold with a vertical casting arrangement (Figure 1) which included a siphon gating system with a tangential supply of metal according to the “vortex pouring” scheme. This scheme provides the uniform treatment of the entire volume of the casting with the modifier material; that makes it possible to achieve high stability of the casting results.

Figure 1:Roller mold filling diagram: 1-feeder; 2-feeder mounts; 3-chill mold; 4-modifier briquettes.

The modifier briquettes were installed in the area of the lower neck of the roll casting. To increase the area of interaction of the liquid metal with the surface of the briquetted modifiers (in order to increase the intensity of their dissolution); the latter were collected in a stack 4 alternately from preforms of different diameters; which ensured the formation of a stepped surface (Figure 1). The number of modifying briquettes installed in the roller mold was selected in such way as to ensure the tellurium content in the cast iron in one of the following ranges: I - (6-10)×10^-3; II - (11-16)×10^-3 и III = (17- 21)×10^-3 % (wt.). The temperature of the melt before pouring the mold was 1380 °С.

The mechanical properties of the produced cast irons were assessed based on the results of bending tests and determination of Shore hardness (HSD) in accordance with ASTM D2240-15. When studying the microstructure of the cast irons; the percentage content of carbides in the structure; as well as the microhardness of carbides and the matrix phase were determined. The quality of the bleaching was assessed according to the A_k criterion:

where x is the depth of the pure bleaching zone; z is the depth of the transition zone (mm).

The wear resistance of the cast irons was evaluated based on the results of tri biotechnical tests during dry friction of cast iron samples against roller-bearing steel ShX15 as a counter body with a hardness of 56 HRC. Sliding speed v=1m/s; load p=100N. To evaluate the efficiency of using briquetted modifiers for melt processing; a comparison was made between the data obtained as a result of the experiment and the data for serially smelted roller cast irons produced using the technology of ladle modification of the melt without the use of tellurium in the modifier.

As it was shown by the results of a comparative study of the microstructure of the produced cast irons; in cast irons produced using melt modification by powder modifiers with tellurium; the number of graphite inclusions significantly decreased compared to cast iron without tellurium. Modification causes the graphite particles become finer; shorter; curved. Some lump-shaped graphite inclusions appear too. The noted effect intensifies with an increase in the content of tellurium in cast iron (Figure 2). Metallographic studies have established that the structure of the working layer of rolls cast without tellurium is a coarse conglomerate of cementite and austenite decomposition products. The metal matrix consists of lower bainite; martensite colonies and individual sections of troostite (Figure 3a).

Figure 2:The effect of carbide-stabilizing modification of the melt on the concentration of graphite inclusions in cast iron with different tellurium content: a – 0 %; b – 8·10^-3 %; in – 13·10^-3 %; g – 19·10^-3 % (wt.)

After carbide-stabilizing modification with tellurium; a change in the concentration of graphite inclusions is accompanied; in turn; by an increase in the cementite phase in the structure of the alloy from 20-26 % for cast iron without tellurium; to 28-32 % with tellurium (Figure 4). The purity of the bleached working layer of the rolls by graphite inclusions increases and the level of the criterion for the bleach increases; right up to the maintenance of a clean bleach over the entire surface of the casting with Te of 19·10^{- 3} % (Table 1). In the structure of the metal matrix; the amount of bainite increases due to a decrease in troostite (Figure 3b). At the same time; the size of the cementite plates decreased on average from 21 to 14 microns.

Figure 3:The microstructure of the working layer of series smelting rolls (a) and cast iron modified with tellurium (19×10^-3 %) (b).

Figure 4:Effect of the tellurium concentration in cast iron on the content of carbides.

Table 1:The Effect of the tellurium content in the cast iron on the Ак whitewash quality criterion.

Noticeable increase in the content of cementite in the structure of modified cast iron also led to a corresponding increase in the level of mechanical properties. Thus; the hardness increased from HSD 60-64 for unmodified cast iron to HSD 70-79 after carbidestabilizing modification of the melt (Figure 5a); and the bending strength - from 420-435 to 440-510 MPa respectively (Figure 5b).

The results of the research on the tribotechnical characteristics of cast irons with different tellurium concentrations showed that the modification of the melt with tellurium-containing powder briquettes also makes it possible to significantly increase the wear resistance of the alloy in comparison with unmodified cast iron (Figure 5c). This is logically explained by an increase in the hardness of the alloy as a result of carbide-stabilizing modification.

Figure 5:Dependence of the main mechanical characteristics (a,b) and the amount of wear of cast iron during dry friction (c) on the content of tellurium.

a. Modification of cast iron melt by powder modifiers with tellurium leads to changes in morphology and decrease in the concentration of graphite inclusions compared in its structure to cast iron without tellurium.
b. Decrease in the concentration of graphite inclusions is accompanied by an increase in the content of cementite phase in the alloy structure and increase in the level of the whiteness criterion.

With the increase in the concentration of tellurium in cast iron; the value of material strength and hardness as well as its wear resistance has increased.

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