STUDY OF DUCTILE IRON PRODUCTION FOR CASTINGS DESIGNATED FOR EXTREME CONDITIONS

This paper studies the possibility of producing ductile iron castings intended for extreme conditions on an industrial scale. The preparation of charge and its melting conditions, modification, primary inoculation and main inoculation were studied within extensive series of experimental melts. In the scope of charge evaluation, especially the ratio of sorel type raw iron to steel charge was studied in order to reduce the raw iron portion while maintaining the castings qualitative requirements. The modifiers FeSiMg621 and FeSiMg731 were evaluated during modification. During primary inoculation the inoculants Inocast100 and SB10 were compared. The inoculation blocks Germalloy were used during main inoculation. The implementation of experimental melts was followed by chemical composition analysis, metallographic evaluation and a study of microstructure and mechanical testing performance of samples. The chemical composition was determined based on optical emission spectrometry and combustion analysis. The metallographic analysis and the microstructure evaluation were made using an optical microscope and image analysis. Testing of mechanical properties was focused on the tensile test, impact test and hardness test. It was proved that the foundry was able to produce the required quality ductile iron made of different combinations of charge materials, modifiers and inoculants. All contemplated combinations of the production technology meet the standard-defined requirements for this type of material.


INTRODUCTION
Cast iron is a material with good mechanical and physical properties. Its relatively low production costs increase an interest in its application in various industries [1,2,3]. Cast iron with spheroidal graphite is a modern material with excellent mechanical properties whose tensile strength is even comparable to that of steel. Considering its lower production price, cast iron with spheroidal graphite thus replaces the more expensive steel in some industries [4,5]. Spheroidal graphite is obtained in cast iron by inoculation. The best structure of graphite is achieved using 2-step inoculation [6]. The current trend in cast iron production is to reduce the percentage of raw iron in the charge and to replace this raw iron with the cheaper steel scrap material and thus decrease the production costs [7]. A research has been ongoing at KOVOSVIT MAS Foundry, a.s. focused on studying possible ways of producing castings of cast iron with spheroidal graphite intended for extreme conditions. A SCHAFT 1 type casting was chosen for the tests; this casting is designed for manufacturers of naval manipulators for medium-sized ships. Superior material quality is required for this type of casting. The casting is made of EN-GJS-400-18-LT, i.e. cast iron with spheroidal graphite and a ferrite matrix with a guaranteed value of impact energy at -20 °C [8,9].
The effect of the composition of charge materials and the choice of the modifying and inoculating agent on utility properties of the material were evaluated in the scope of experimental melts. Together with the casting, testing samplesthe so-called Y-Blockswere added to the inlet system of the mould, used to produce test bodies for the metallographic analysis and for the testing of mechanical properties [9,10,11,12]. The purpose of the study was to define the most appropriate parameters for the production of castings made of cast iron with spheroidal graphite with a special focus on optimization of the raw iron and steel charge ratio, and the used modifying and inoculating agents.

TECHNOLOGICAL PROCEDURE OF SCHAFT 1 CASTING PRODUCTION
Based on operating experience, chemical composition of the melt before tapping was proposed at KOVOSVIT MAS Foundry a.s. This composition is shown in Table 1 to give an idea; modification and inoculation will follow. Furthermore, final chemical composition of the melt for the casting was proposed as presented in Table 2.  A series of 9 melts was performed in the scope of the operating experiments, testing 3 combinations of charge materials in various ratios. Table 3 provides the composition of the charge for three selected melts identified as TS87, TM11 and TD75. A basic technological production process was prepared for the series of the experimental melts; this process is illustrated in Figure 1 and is composed of the following steps: Figure 1 Schematic illustration of the basic technological production process o LLG cutsto delay the reaction; ✓ Besides modification, primary inoculationthe so-called preinoculation using inoculantsis performed in the ladle at the time of tapping. To give an idea, Figure 2 shows a photograph from melt tapping into the ladle with concurrent modification and primary inoculation; Tapping into the ladle with concurrent modification and primary inoculation Figure 3 Pit detail at the top part of the mould with prepared inoculation blocks ✓ Inoculation → is performed in two phases; initially, partially in the course of tapping into the ladle, the so-called preinoculation, and subsequently, the so-called main inoculation, performed in the pit using inoculation blocks in the course of making the casting. Figure 3 shows the detail of the inoculation pit with prepared inoculation blocks; ✓ Inoculants Inocast100 × SB10 are used to optimize the structure of the cast iron with spheroidal graphite. The selection of the type and amount of the inoculant was performed based on the amount of liquid metal and the casting duration; ✓ Table 4 shows the modifiers and inoculants and their amounts used in the selected melts;  Figure 4 shows a photograph of the casting process and Figure 5 shows the photograph of a casting after its removal from the mould and blasting;

ANALYSIS OF SAMPLES FROM EXPERIMENTAL MELTS
Melt samples were taken from selected points with concurrent temperature measurement in the course of the above mentioned series of experimental melts. Additionally, Y-Blocks placed in the inlet system were cast at the same time. The collected samples were identified and gradually analysed; the specifications of individual analyses are described below: ✓ Chemical composition analysis → the analysis was performed using optical emission spectroscopy (OES) with excitation using a high-energy spark discharge, using the Q4 TASMAN device.

RESULTS AND DISCUSSION
After the experimental melts, evaluation was performed to determine the mechanical properties, as well as metallographic evaluation and evaluation of the microstructure [9,10,11,12]. The results are summarized in the so-called metallographic cards that provide an idea of the mechanical properties and of the resulting structure of EN-GJS-400-18-LT used to cast a SCHAFT 1 type casting. An example of such a card for TM11 melt is provided in Tables 5 to Table 8 and in Figure 6.     Evaluation of results of the experimental melts → based on a complex assessment of the so-called metallographic cards of EN-GJS-400-18-LT for SCHAFT 1 castings. Table 9 gives chemical composition of the melt after modification and preinoculation and the carbon equivalent CE and the saturation degree (eutecticity) SC for melts TS87, TM11 and TD75. The achieved chemical composition shown in Table 9 corresponds to the chemical composition proposed in Table 3 except several deviations up to a few tenths of a percentage for C, Mn, P and Mg. Nevertheless, these minor deviations have no effect on material quality as discussed below. The charge preparation process and charge melting have been thus managed well.  It should be mentioned that ČSN EN 1563 [9] determines the minimum impact energy value KV2 for EN-GJS-400-18-LT only for the temperature of -20 °C; given the complexity of evaluation of individual melts, impact energy was determined also at 23 °C and -40 °C. Table 11 provides a summary of parameters describing the achieved structure of the cast iron and graphite. As follows from Table 11, the highest number of graphite particles per mm 2 was achieved in melt TS87, namely 336 with nodularity 89 % and graphite particle sizes 15-60 μm, which corresponds to the size range of 6/7. For melt TD75, 298 graphite particles per mm 2 were obtained with nodularity 93 % and graphite particle sizes 15-60 μm, which corresponds to the size range of 6/7. For melt TM11, 256 graphite particles per mm 2 were obtained with nodularity 71 % and graphite particle sizes <15-30 μm, which corresponds to the size range of 6/7.
Thus, it can be noted that all melts comply with the requirements for the microstructure of EN-GJS-400-18-LT. They contain mostly the required type VI graphite, type V graphite to a limited extent, and also type III graphite only in sporadic cases and up to low percentage values as per ČSN EN ISO 945 [10]. The perlite content is up to 2 %. Furthermore, it can be stated that graphite particle sizes decrease with an increasing number of these particles.

CONCLUSION
The paper describes the study of the production of cast iron with spheroidal graphite for castings intended for extreme conditions. Operating tests were implemented for this purpose at KOVOSVIT MAS Foundry a.s., focused on verifying the possibility of producing castings of EN-GJS-400-18-LT designed for manufacturers of naval manipulators for medium-sized ships. The following conclusions were drawn based on the test results: ✓ For the future, the production process can be enhanced using the modern technology of a filled profile in a twin-core arrangement for inoculation and modification where one core will contain the modifier and the other one will contain the inoculant.