Ductile iron with nodular and compact graphite as a rational alternative to malleable cast iron

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Introduction
Malleable iron has been successfully used for a long time for medium-and heavy-loaded parts in the machine and automotive industry, in the production of railway products and agricultural equipment [1][2][3][4][5]. Subsequently, malleable iron was almost completely replaced by high-strength spheroidal graphite iron (ductile iron). The exception is small and/or thinwalled castings with a wall thickness of up to 3 mm, while ductile iron castings are made with a minimum wall thickness of 6 mm [6][7][8][9].
It is obvious that flaky graphite is a somewhat stronger stress concentrator compared to globular graphite when comparing the range of grades KCh (malleable iron) (GOST 1215) and VChShG (ductile iron) (GOST 7293), as a result, the strength and plastic properties of malleable iron (KCh) are lower than those of ductile iron (VChShG). At the same time, technologically, obtaining blanks from malleable iron is a more expensive process than obtaining castings from ductile iron. The energy and labor intensity of the malleable iron production technology significantly prevails over the material consumption of the ductile iron production technology.
For a long time in the iron foundry, it was stereotypically considered that ductile iron with a content of vermicular graphite in its structure of more than 20% is an unequivocal defect. The influence of a change in the content of compact graphite in the range from 20 to 60% on the strength and plastic characteristics of cast iron has not been considered or studied. However, a more rational approach to setting the properties of an alloy for a specific part or group of parts is being implemented in practice based on the use of mathematical modeling in modern conditions. This raises the need for alloys with a combined combination of properties. This can be provided by the so-called ductile iron with nodular and vermicular graphite (DINVG), which is essentially a hybrid cast iron in terms of microstructure [10,11]. According to the ratio of nodular and compacted graphite, it cannot be strictly attributed to ductile iron due to the content of compacted graphite more than 20%, but it is not compacted graphite iron (CGI), since the content of compacted graphite in it is less than 60%.
A new rational approach to the use of this type of cast iron is being implemented in various countries. For example, according to information from the report of specialists from the Japanese company Toyo Denka Kogyo, which specializes in the production of modifying materials for cast iron, it is this type of cast iron without lamellar graphite and with a content of vermicular graphite of 20-60% that is called "ductile iron/ compacted graphite iron" (DI/CGI) [2] and has a certain distribution in Japan. Castings from this type of cast iron are manufactured by URALaz from the enterprises of the Russian Federation. The name of the material in the technical documentation of the enterprise is VCh40-50. This type of cast iron has found its application, in particular, in the iron foundry of AVTOVAZ JSC for medium-loaded (including thermally loaded) chassis and engine parts of a car [10,11], such as: exhaust manifold; rear axle gear housing; rear axle differential bearing cover; right engine mount bracket (both 8-valve and 16-valve); engine mount bracket; rear brake bracket.

Review
There are rarely articles on the use of hybrid cast iron (DINVG) in the scientific and technical literature [10][11][12][13][14]. Most of the work is devoted to certain types of cast iron, such as malleable cast iron, ductile cast iron with spherical graphite, compact graphite cast iron. The article [15] considers alternative methods for the production of cast iron with compact graphite and the available possibilities for controlling its structure. Modern ideas about the mechanism of formation of nodular graphite in cast iron are considered from the point of view of their explanatory power in [16]. It is determined that a complex model covering the characteristics of both the nucleation and growth of graphite is the most plausible. The mechanisms of growth of graphite nodules in ductile iron were studied using highresolution three-dimensional tomography in papers [17,18]. For this, the methods of twobeam scanning electron microscopy (FIB-SEM) were used. The observations are consistent with established theories of graphite nodule growth and iron entrapment/absorption between graphite sectors during ductile iron solidification. The behavior of solidification of cast iron with nodular graphite was studied in the article [19]. The solidification mode was determined. The results are consistent with current knowledge of crystallization morphology with the help of cooling curves and expansion measurements.
There are works on the use of DINVG. The works [10,11] give an example of the use of DINVG for various medium-loaded car parts. The papers concluded that a wider range of mechanical properties of DINVG can be obtained by varying the pearlite/ferrite and nodular/vermicular graphite ratios in the microstructure.

Methods, materials and results
Two castings were chosen as the object of study. The chemical composition and mechanical properties of DINVG castings obtained during their certification (Fig. 1, 2) are presented in tables 1 and 2.
The chemical composition of cast iron was determined according to GOST 7565-81 (Iron, steel and alloys. Sampling for determination of chemical composition).
The content of pearlite and ferrite in unalloyed and low-alloyed cast iron according to the images of its pickled surface using the SIAMS 700 industrial software and hardware image analysis complex SIAMS. The proportion of graphite determined in advance is used to calculate the area occupied by perlite. The structure of cast iron is evaluated by a score in accordance with scale 6 according to GOST 3443-87.

No.
Casting ( fig.1-2 A comparative analysis of the actual characteristics of cast iron with nodular and vermicular graphite was carried out with their standard ranges for KCh45-6, given in GOST 1215-79. Grade KCh45-6 has a pearlite-ferrite metal base, i.e. with a predominance of the pearlite component over the ferritic, and cast iron with nodular and vermicular graphite, on the contrary, is ferrite-pearlitic. The peculiarity of the hardness range of cast iron with nodular and vermicular graphite (177-207 HB) is that it falls mainly on the upper limit for the grade KCh45-6 (150-207 HB). The strength and ductility of cast iron with nodular and vermicular graphite is provided by a ferrite-pearlite metal base with high hardness. The strength and ductility of ductile iron is provided by a pearlite-ferritic metal base with a lower hardness. Therefore, the following conclusion can be drawn: the ratio of nodular/vermicular graphite (70…95/5…30%) has a greater incising, softening effect than flaky graphite.

Conclusion
Thus, the considered type of cast iron (DINVG) has a rational application for a certain niche of machine building parts and in the future should be included either in the revised GOST 28394-89 for CGI, or its own autonomous standard should be developed for it.