‘Vari-Morph’ (VM) Cast Iron with Several Forms of Graphite: Technology, Properties, Application

Abstract

Cast iron with mixed-shape graphite and controlled fractions of individual shapes, known as VM cast iron (‘Vari - Morph’), can become material for castings with special requirements. The name of the cast iron ‘Vari Morph’ (VM) was first proposed by the authors in 2018 at the World Congress of Foundries in Krakow. VM cast iron displays physical and mechanical properties, which cannot be achieved with homogeneously shaped graphite. Cast iron with (L – flake) + (V – vermicular) graphite is characterised by good thermal conductivity and better A5 (elongation) and Rm (tensile strength) (than grey cast iron. What is of particular interest is cast iron with a mixed form: (S – spheroidal) + (V – vermicular). Currently, research is being carried out to achieve cast iron with a high-quality index (QI) defined as Rm/HB. This paper presents the results of research of physical (thermal conductivity), mechanical (Rm and A5) and functional properties (thermal fatigue) of VM cast iron. The ultrasound technique was applied for assessing the graphite compactness degree (ξ): ultrasonic wave speed CL = f(ξ), damping factor α = f(ξ). The article also presents the correlations between the above-mentioned parameters, as well as describes the technology used to produce VM cast iron and possible areas of application of the material.

1. Introduction

Cast iron with carbon in the form of graphite, known as grey cast iron, is the most commonly used material in the casting industry around the world. Cast iron, in terms of graphite morphology, can be divided into three groups: cast iron with flake graphite (EN ISO GJL- ..), cast iron with vermicular graphite (EN-ISO GJV-..) and cast iron with spheroidal graphite (EN ISO GJS- ..). Currently, the production process of cast iron does not allow for using mixed forms of graphite in the same castings. Only a small percentage of a different form of graphite is acceptable (for SG cast iron—up to 5.0%, for VG cast iron—up to 20%). Production technologies for individual types of cast iron have been developed to achieve such homogeneous graphite structures. In many cases, the introduced restrictions, and the need to maintain a highly homogeneous graphite morphology (flake, vermicular or spheroidal), considerably limit the possibility to take advantage of the numerous beneficial properties of cast iron. Graphite morphology affects all properties and characteristics of cast iron, including physical, mechanical, technological, functional and operational properties. However, such limitations have no reasonable justification. The conducted research [1, 2, 3, 4, 5, 6] points to the possibility of producing cast iron with mixed graphite morphology and has confirmed the possibility of controlling this morphology in a deliberate and predictable manner. Figure 1 presents cast iron with a mixed graphite form, which has been named Vari Morph (VM). The EN ISO 945 standard provides for only three types of graphite forms in raw castings that have not been heat-treated. The others, crossed out in Figure 1a, include the forms of graphite after heat treatment of cast iron. VM cast iron includes both the groups of graphite forms shown in Figure 1b and c. The idea of VM cast iron is to create a smooth transition of graphite forms from flake, through mixed: flake + vermicular (Figure 1b) and mixed vermicular + spheroidal, up to fully spheroidal. This smooth transition creates the possibility of taking advantage of the whole range of physical and mechanical properties. In the case of standardised cast iron grades, the changes of properties have a step-like nature.

Figure 1.

Types of graphite morphology:/a – According to EN ISO 945, b/ cast iron with mixed forms of flake and vermicular graphite, c/cast iron with mixed forms of vermicular and spheroidal graphite [7, 8].

The research described in this paper has found a significant impact of graphite morphology on the whole range of cast iron properties and has confirmed the possibility of controlling these properties. One of the most important results that can be achieved is the possibility to produce grey cast iron with relatively good strength properties (R_m - 300 ÷ 400 MPa) and, at the same time, low hardness (HB - 160 ÷ 180). In conventional cast iron with homogeneous flake graphite (EN GJS), to achieve the described strength, the pearlitic form of metal matrix is used, which increases hardness to HB - 230÷240. Contemporary methods of machine castings, with the use of high-speed CNC machines, have been adapted to materials with reduced hardness. In the case of iron castings, hardness below 200HB is required. If only flake graphite is used, it is very difficult to achieve these parameters (Rm and HB), while for higher Rm values, it is impossible. With hardness values significantly exceeding 200HB, the machining speed of CNC machines must be reduced, which is not profitable. This has been confirmed empirically by research [9] in a foundry that performs CNC machining of its own castings. Cast iron quality is often determined by the Quality Index (QI), defined as: IQ = Rm/HB. Grey cast iron has a relatively low QI value of 1.0÷1.4, while for ductile cast iron, it is approx. 2.5÷3.0. By controlling graphite morphology and allowing for mixed forms in the structure of castings, it is possible to fill the QI range between these two extremes, obtaining values in the range 1.0 ÷ 2.5. Although vermicular cast iron partly fits in between grey and spheroidal cast iron, the requirement for 80% vermicular graphite, imposed by the European standard, on the one hand, complicates the production technology, while, on the other, limits the freedom to take advantage of numerous cast iron properties, including achieving a better QI value.

Producing cast iron with other than flake graphite forms requires out-of-furnace secondary processing of liquid metal, to which small amounts of additives (inoculants, nodularisers) have been introduced. The possibility to control graphite morphology in a wide range has been confirmed by such authors as [1, 2, 3, 4, 5, 6]. The mentioned research concerned the increased resistance of cast iron to thermal fatigue by controlling the wide range of graphite morphology: from flake to spheroidal [10, 11, 12, 13]. In addition, as evidenced by the existing analytical research, cast iron with non-homogeneous graphite morphology is characterised by above-standard properties that cannot be achieved with homogeneous forms. Studies on the impact of the graphite shape factor ξ (degree of compactness) on the resistance of cast iron to thermal fatigue indicate other close correlations between other properties of cast iron and the shape factor ξ, including: Rm = f(ξ), A = f(ξ), E = f(ξ), α (vibration damping) = f(ξ), λ (thermal conductivity) = f(ξ). The shape factor has been defined in numerous ways. According to one definition, it can be explained as the area occupied on a metallographic section by the graphite precipitate to its perimeter when squared. The authors of this article propose to define the graphite shape factor ‘f’ for VM cast iron as a weighted average of the values of the indexes for the flake, vermicular and spheroidal forms of graphite, considering the percentage share of each form in the total set of particles visible on the metallographic section. Because most cast iron properties are determined as a function of the graphite shape factor, the correct determination of this value is of key importance for the accurate description of Vari Morph (WM) cast iron characteristics.

2. Technologies of Vari Morph (WM) cast iron production

VM cast iron can be produced in several ways, similar to the production of vermicular or ductile cast iron. The technologies for producing cast iron with mixed form of graphite are similar to those for vermicular/spheroidal cast iron and include the following solutions:

1. Production of cast iron with a limited sulphur content (S < 0.01%) in the initial state, by introducing a strictly controlled amount of nodulariser (Mg < 0.03%). The introduction of the controlled quantity is achieved using three methods:

   1. Using the PE (flexible core wire) method with FeSiMg low magnesium alloy. An experimental test stand for this method will be constructed as part of the project at AGH University of Science and Technology.

   2. Using low-magnesium alloys dosed at the bottom of sealed ladles (slender ladles – Tundish method).

   3. Inmold technology, with placement of low magnesium alloys dedicated to Inmold technology in reaction chambers.

2. Production of cast iron using a constant amount of magnesium alloy and varying amounts of sulphur. Technology adapted to the production of cast iron in cupola-type furnaces. Testing will be carried out in Tundish-type slender ladles.

3. Cast iron production using anti-spheroidising elements, mainly titanium (Ti), introduced in strictly controlled amounts. Testing in Tundish-type ladles.

4. Production of cast iron using alloys containing additives of rare earth elements (Mischmetal), currently dedicated to the production of vermicular cast iron.

Methods most widespread in industrial practice include:

1. Controlled magnesium content method. Pure magnesium or magnesium alloy is introduced into the liquid metal in an amount less than required to obtain spheroidal graphite. In vermicular cast iron, the final Mg content varies between 0.01 and 0.03% (Figure 2). After the addition of magnesium, inoculation is carried out, mostly with ferrosilicon-based inoculants, similar to the production of ductile iron.

   The method is quite difficult in actual industrial practice, as the range of magnesium content in cast iron, at which graphite crystallises in vermicular form, is very narrow. To produce cast iron with flake + vermicular graphite, the magnesium content must be kept below 0.02%. In turn, to produce cast iron with vermicular + spheroidal graphite, the magnesium content should be kept in the range Mg = 0.02–0.03%. The above-mentioned values apply to cast iron with low sulphur content (S < 0.01%), whereby magnesium is not used for its desulphurisation.

   An excessive amount of Mg results in the formation of spheroidal graphite, while an insufficient amount Mg creates flake graphite. Such cast iron is also more prone to the fading of the vermiculisation effect compared with cerium-treated cast iron. To obtain vermicular graphite in thick cast sections, the time from magnesium inoculation to pouring must be kept to the minimum. In castings with different wall thickness, the method is impracticable.

2. Simultaneous introduction of magnesium and titanium. The method consists of introducing into cast iron an anti-spheroidising element, mainly titanium, with a small amount of cerium, simultaneously with magnesium. The addition of titanium extends the range of magnesium content in cast iron, at which vermicular and mixed-form graphite is formed.

3. To facilitate application of this method, a special alloy has been developed and patented: 4–5% Mg; 0.4÷0.7% MZR; 4.0 ÷ 4.5% Ca; 8.5 ÷ 10.5% Ti; 1÷ 1.5% Al.; 48÷52% Si. This alloy melts at a temperature of 1100°C. The alloy is introduced into cast iron in the same way as in the case of ductile cast iron production. Subsequently, inoculation of cast iron in the ladle with FeSi is required. The composition of the cast iron used as a starting material should be close to eutectic, and the sulphur content should not exceed 0.035%. Cast iron temperature at the time of alloy introduction should be 1450–1500°C. At this temperature and with a sulphur content of 0.035%, the amount of alloy added should be in the range of 1.5–1.7% of cast iron weight. At temperatures below 1430°C, the amount of spheroidal graphite in cast iron increases. A disadvantage of the method is the introduction of 0.1 ÷ 0.15% Ti into cast iron, which remains in the foundry circulation scrap. In addition, the machinability of castings deteriorates.

4. The introduction of Mishmetal alloy. The method consists in introducing a mixture of rare earth elements into cast iron with low-sulphur content (<0.02%S), e.g. Mishmetal alloy based on cerium (30 ÷ 50%) with the addition of: yttrium, lanthanum, praseodymium, neodymium, gadolinium and other elements. This has been developed for the production of vermicular cast iron but can also be used for the production of VM cast iron. The cast iron used as a starting material should have a chemical composition similar to that of ductile cast iron. The carbon content should not exceed 3.7%. Depending on the wall thickness of the casting, the silicon content can be up to 3.0%. Typically, the Sc eutectic saturation coefficient (the degree of eutectiveness (S_C)) of this type of cast iron is approx. 1.0 or slightly above 1.0. To obtain a ferritic structure in the cast state, pig iron with low contents of manganese (<0.3%), phosphorus (<0.05%) and sulphur (<0.02%) should be used in the smelting process. Mishmetal alloy can be added to the liquid metal stream in the form of rods. Due to the low melting point of the alloy (790÷850°C) and the high boiling point of the components (approx. 3000°C), it easily dissolves in cast iron without pyrotechnic effect and the emission of fumes. In turn, a disadvantage of this method is the significant tendency of the cast iron to form cementite (chill).

5. PQ-CGI Inmold process This method has been developed and patented by NovaCast. The PQ-CGI Inmold process is based on precise control of the oxygen content (in cast iron) and the formation of crystallisation nuclei. The process uses the PQ – CGI system, which is based on a modern thermal analysis method.

6. Sinter – Cast technology: the method takes advantage of the effects of vermiculisation (with controlled quantity of magnesium) and inoculation treatment. The use of two flexible wires for the procedure in industrial conditions, one of which contains a magnesium core and the other an inoculation core, ensures the efficiency and repeatability of this technology.

7. Elkem method (COMPACTMAG alloy). The method uses a new alloy developed by the Elkem company to produce vermicular cast iron. Its composition is listed in Table 1. The alloy can also be used in the production of VM cast iron with mixed-form graphite. This alloy has a magnesium content of 5÷6% and an addition of cerium (5–7%). It is used for vermiculisation in the treatment ladle, using the Sandwich method and its variations.

Figure 2.

Principle of obtaining vermicular graphite and Vari morph cast iron by the control magnesium content method [14].

2.1 Proprietary technologies to produce Vari morph cast iron

A method for producing VM cast iron was selected based on preliminary tests. The tests were initially conducted in the experimental foundry of the Faculty of Foundry Engineering of the AGH University of Science and Technology in Kraków and then in a selected industrial foundry. The controlled Mg content method was selected, and the nodularisation process was carried out using a ladle with a tight lid (Tundish technology) and a flexible core wire (PE) method. Schematic illustration of the process is presented in Figure 3. In industrial conditions, the nodularisation process was also carried out using the Inmold method. Using this technology, cast iron with mixed-form graphite (Vari Morph) is obtained by introducing a lower amount of Mg compared with the amount used in ductile iron production. Therefore, the secondary (out-of-furnace) treatment itself, consisting of the introduction of strictly controlled amount of Mg, can be treated as cast iron nodularisation process. This term is used to describe it in this paper. In the case of VM cast iron, nodularisation is incomplete, and the graphite morphology is variable, covering three basic forms in which it occurs in cast iron: flake (L), vermicular (V) and spheroidal (S).

Figure 3.

Schematic diagram of the Vari morph cast iron production method with the introduction of strictly controlled amounts of Mg.

Figures 4 and 5 show the test stands in the experimental foundry, while Figures 6 and 7 show the industrial version. Figure 4 shows a drawing of the experimental Tundish ladle and the pouring of samplers for thermal derivation analysis (TDA).

Figure 4.

Tightly covered ladle (Tundish): a/ drawing: 1 – Cover, 2 – Ladle, 3 – Thermocouple, 4 – Liquid metal; b/ pouring samplers from the ladle for TDA analysis.

Figure 5.

Experimental stand for cast iron nodularisation with the PE method): a/ drawing; b/ processes performed by cored wire method – Precision dispensing Mg.

Figure 6.

Experimental technology of VM cast iron production using the Inmold process on a DISA MATIC line: a/ general concept of the technology, b/ view of model plate [(according to patent [16]).

In the second technological solution for the production of Vari Morph cast iron, nodularisation of cast iron was carried out by the PE (flexible core wire) method.

The experimental test stand is shown in Figure 5. The cast iron nodularisation process involves the introduction of a steel wire (thin-walled tube) filled with FeSiMg alloy containing magnesium, usually in the amount of 16–23%, into the liquid metal. The process was carried out with full control over the rate of rod immersion in the metal and the amount of alloy, or more precisely Mg, introduced into cast iron. Production of VM cast iron requires strict control of the amount of Mg introduced into cast iron.

Experimental production of VM cast iron has also been carried out under industrial conditions as part of NCBiR projects [9]. Test ingots and castings were made in moulds with a vertical parting line, on the DISA MATIC line (Figure 6), and moulds with horizontal parting line on the FBO automatic moulding line (Figure 7).

Figure 7.

Model plate and mould for research on the production of VM cast iron using the Inmold technology on the FBO line; a/ upper model plate, b/ lower mould [9].

The purpose of the tests conducted in the experimental foundry as well as under industrial conditions was the assessment of the possibility to produce VM cast iron in a manner ensuring repeatability and the required structure of graphite in the casting, as well as the assessment of the possibility to take advantage of selected physical, mechanical and functional features and properties of cast iron. Comparison of the three methods of cast iron nodularisation allowed us to verify them in terms of efficiency of the process of magnesium introduction from FeSiMg alloys into cast iron (degree of magnesium assimilation). The highest degree of Mg assimilation is achieved with Inmold technology (65–75%), followed by Tundish ladle nodularisation technology (55–60%), while the lowest was with the PE flexible core wire technology (30–35%). The Mg assimilation degree for each technology depends on several factors, including the temperature of the cast iron, sulphur (S) and oxygen (O2) content. Nevertheless, the following estimates can be accepted as a guideline.

The authors of this paper compiled the results of research on plain cast iron with a chemical composition close to eutectic composition (Sc ~1.0; CE ~ 4.30) with reduced manganese content (Mn < 0.25). In most cases, cast iron had a ferritic matrix, which allowed us to assess the impact on physical and mechanical properties of graphite morphology rather than of the type of matrix.

The results allowed determining several empirical correlations between the graphite shape factor and selected properties.

A group of cast iron grades, the chemical composition of which was close to the eutectic value, was subjected to analysis (C = 3.3÷3.6%, Si =2.6÷2.95%, Mn < 0.25; P < 0.02%; S < 0.01%; Mg =0.005÷0.040%).

2.2 Determination of the graphite shape factor

The graphite shape factor, a key parameter describing the structure of Vari Morph cast iron with mixed-form graphite, requires more detailed analysis. The literature on the subject [1, 2, 3, 4, 5] proposes various factors describing numerically the shape of a single precipitate. Some authors [10, 11] used the ξ factor (formula 1). Regarding VM cast iron, the arithmetic or weighted average of this actor was used after counting the percentage share of individual graphite forms.

The graphite shape classification was based on the indicator shape of graphite ξ, which was defined by formula 1 [17]:

$$\xi =\frac{A_i}{P_i^2}$$E1

where:

A_i, P_i – surface (m²) and diameter (m), respectively, of a single precipitate, i – number of graphite precipitates. Indicator ξ takes the following values:

0÷0.03 (ξ < 0.03) for flake graphite, 0.035÷0.065 for vermicular graphite, 0.065÷0.08 (ξ > 0.065) for nodular graphite.

The computer-aided methodology of determining the shape indicator was developed, with using the Image J program. This is a new method especially suitable in quantitative determinations of this indicator of cast iron grades having different graphite forms. This method is based on the concept of calculating the mean weighted value of the shape indicator, and all visible separations on the microstructure are included in the calculation, regardless of their size and shape.

Microscopic images were subjected to the stereological analysis by means of the program Image J for pictures analysis. The microstructure image in digital recording and the image processed by the Image J program are shown in Figure 8, as an example.

Figure 8.

Microstructures of cast iron (VM) – With a mixed graphite form. a/ microstructure image. b/ image processed by the image J.

Each graphite precipitate is recorded by the program by means of the graphite shape indicator included in Eq. (2) [5]. Graphite single precipitation shape index ‘f’ is defined as the ratio of the graphite precipitation area to the circle area with a diameter equal to the largest particle size (graphite) (Table 2).

Master alloy	Si	Mg	Ce	Ca	Al
COMPACTMAG	44–48%	5–6%	5–7%	0.8–1.2%	1.0% max

Table 1.

Chemical composition of the COPACTMAG master alloy [15].

Table 2.

Graphite shape index values ξ, i ‘f’ calculated according to various equations.

$$f=\frac{A_V}{A_c}$$E2

The assessment of the shape index in VM cast iron, in which different forms of graphite occur next to each other, is always simplified and incomplete, regardless of the calculation methodology. Before making the selection, the authors made a number of comparisons, trying to correlate the value of the determined shape index with the selected physical and mechanical properties of cast iron. Better correlation of the results was obtained with the use of the ‘f’ index described by Eq. (2). The shape indices are determined for a flat 2D structure image, and the cast iron properties are related to the three-dimensional image of the 3D graphite particles. At the present stage of the description of graphite particles, such methodology is commonly used. Eq. (2) describes, on the one hand, the level of separation compactness in a simpler way, and on the other hand, it accentuates the scale of the distance between the shape and the spherical shape. As indicated in Table 2, the value of this indicator (f) changes nearly eight times when switching from the flaky to the ball form. In the case of the shape indicator (…), with a similar change in the shape of the graphite particles, the indicator value changes only threefold. The conducted analysis and the comparisons made prompted the authors to choose the ‘f’ index to describe the sought dependencies for VM cast iron.

Graphite shape	Number of precipitates	%; Fraction of the range	Mean; ‘f’; of the range	Indicator; ‘f’
Flake	31	12.0	0.27	0.62
Vermicular	106	41.1	0.49
Spheroidal	121	46.9	0.81

Table 3.

Analysis of the shape indicator.

The procedure of determining the mean shape indicator was performed in subsequent steps in which the written below values were determined.

• Number of graphite precipitates per mm² (with omitting precipitates being on the picture edges and the ones which surface is smaller than 0.0785 μm), Figure 9;

• Surface percentage fraction of graphite;

• Surface of each precipitate;

• Diameter of the circle on the precipitate corresponding to its highest dimension.

Figure 9.

Scheme of shapes of graphite precipitates cross sections [10].

The graphite shape indicator f was determined for each precipitate, acc. to [12], in accordance with Eq. (2), where:

A_v — surface of the graphite precipitate [mm²],

A_c — area of the circle of a diameter equals the highest particle dimension [mm²].

The shape indicator ‘f’ theoretically changes from 0.0 to 1.0, while under real conditions from app. 0.20 to 0.95.

It was assumed that the shape indicator of graphite is changing within ranges:

• 0.00–0.34 for flake graphite,

• 0.35–0.64 for vermicular graphite,

• 0.65–1.00 for spheroidal graphite.

In the next step, the number of precipitates of the shape indicator ‘f’ being in individual ranges was calculated, and their percentage fraction in relation to all precipitates was determined. For each range, the separate mean indicator f was determined. They constituted the bases for calculating the main shape indicator f, based on the weighted mean. Individual results of graphite shape analyses obtained in the assumed calculation procedure are listed in Table 3.

It was indicated on the basis of the performed calculations that the mean indicator of the graphite shape f = 0.62, which classifies precipitates as vermicular graphite. Simultaneously the spheroidal degree is N = 46.9%. Fractions of individual kinds of graphite shapes are shown in Figure 10. One of the results of the complex assessment of graphite precipitates is placing them within ranges of the shape indicator ‘f’, shown in Figure 11. In the analysed case of VM cast iron precipitates, shape indicator ‘f’ of which is within the range 0.22 ÷ 0.98, corresponding to the top range of the vermicular graphite, are dominating (Table 4). The shape indicator for each cast iron from successive melts was determined in the same way.

Figure 10.

Number of graphite precipitates in individual surface classes.

Figure 11.

Fractions of individual graphite kinds.

Table 4.

Typical metallographic structures and calculated ‘f’ values.

3. Results of own investigations

3.1 Tests of physical and performance properties

Applying the described melting methodology, the material for tests was prepared in forms of test coupons Y type, from which samples for individual tests were made. The following physical properties were assessed: specific density, ability to propagate longitudinal waves and thermal conductivity of VM cast iron. These properties of ‘Vari Morph’ (VM) cast iron depend on the graphite form ‘f’, which allows to determine dependencies: ρ = f (f); C_L = f(f) and λ = f (f). The limited Mn content in cast iron allowed obtaining purely ferrite cast iron or ferrite with a small (to app. 15%) pearlite fraction. In this way, the influence of the metallic matrix kind on the tested properties of cast iron was eliminated.

The ultrasound wave speed in cast iron depends on the graphite form and changes within a wide range [16, 17, 18, 19, 20, 21]. This is confirmed by the results of investigations presented in Figure 12. In investigations we are striving to the development of the empirical dependence f = (C_L) for the whole variability range of the shape indicator. If such dependence is characterised by a high correlation coefficient (high certainty), the ultrasound technique will be used for the non-destructive assessment of the graphite shape indicator in ready castings. The results of tests, performed on a relatively not numerous group, indicate a good correlation between these two values.

Figure 12.

Number of graphite precipitates as a function of the shape indicator.

Specific density (ρ) of VM cast iron was determined by the immersion method with using a strain gauge adapted to these measurements. The procedure of the sample density measuring was the standard one.

The results of investigations of the influence of the graphite precipitates shape on the density of VM cast iron are presented in Figure 13. It was found in metallographic tests that the graphite amount, measured as the surface taken on the polished section, was similar in all tests. This was due to maintaining the stabilised chemical composition of cast iron. It can be noticed that the transfer from flake via compact (vermicular) to the spheroidal form of graphite causes the density increase by approximately 0.15 g/cm3. The so-called compactness of material increases, which influences its certain properties such as: strength, tightness, plasticity.

Figure 13.

Dependence of the mean shape indicator ‘f’ and the ultrasound wave speed C_L.

In the future, VM cast iron is supposed to be the material, structure of which—related to graphite precipitates shapes—will be ‘adjusted’ to the destination of the product and the character of its operation under real conditions. One of the features, which will be controlled, is the thermal conductivity of cast iron. Investigations of the samples group obtained in successive melts were carried out on the prototyped research set-up, shown in Figure 10. The thermal conductivity was determined under conditions of a stable heat flow in cylindrical samples of diameter ø20mm and length 80 mm. The conductivity was determined within the temperature range T = 25 ÷ 500°C. The original research site is shown in Figure 14, and the obtained results are presented in Figure 15.

Figure 14.

Influence of the graphite form (indicator ‘f’) on the VM cast iron density.

Figure 15.

Research set-up for testing the thermal conductivity of materials, including foundry metals and alloys; a/view of the stand, b/preparation of the sample for testing.

It is generally known that the change of the graphite form from flake (f ~ 0.25) to spheroidal (f > 0.75) leads to decreasing of the thermal conductivity. The quantitative dependence, in which the graphite shape is described by the mean value of indicator ‘f’, is determined in the hereby paper.

The functional property of cast iron, which depends on the graphite form, is the heat fatigue resistance. The heat fatigue resistance tests were performed by means of the L.F. Coffin method [10, 18, 22], in which cylindrical samples are cyclically heated by resistance method. The results obtained for VM cast iron are presented in Figure 16. Increasing the graphite shape indicator ‘f’ value, increasing the compactness degree of graphite precipitates, favours increasing the heat fatigue resistance. However, it should be noticed that under real operational conditions of cyclically heated structure elements, the structures made of cast iron with more compact graphite (higher ‘f’ indicator) will be heated to higher temperatures, due to their low conductivity (Figure 17).

Figure 16.

Dependence between the graphite shape indicator and thermal conductivity of ferrite cast iron.

Figure 17.

Influence of the graphite shape indicator on thermal fatigue resistance of VM cast iron.

3.2 Tests of mechanical properties

The results of tests of mechanical properties on samples cut from the test ingots cast in each sample are listed in Table 5. Rm and A5 were determined in tensile testing, while Brinell hardness was measured on the heads of the specimens after they were broken using a 10 mm ball. The quality index (QI = Rm/HB) was then calculated based on the determined values. All the values are presented in Table 5. Quite interesting results, especially in terms of the QI, were obtained for a group of seven casts in which the graphite form was intentionally modified using secondary out-of-furnace treatment. QI is defined as the ratio of tensile strength Rm to hardness HB. By leaving the ferritic matrix (HB < 185) and increasing cast iron strength by way of ‘compacting’, high values of the QI index were obtained (significantly higher than in the case of cast iron with flake graphite form).

No.	f	Rm	A5	HB	IQ
1	0.31	117.6	1.3	173	0.680
2	0.62	430.0	9.3	183	2.350
3	0.64	341.8	13.5	141	2.424
4	0.66	372.9	12.5	149	2.503
5	0.70	522.1	13.8	181	2.885
6	0.43	363.1	6.1	183	1.984
7	0.85	609.8	13.1	200	3.049

Table 5.

Properties of VM cast iron with a dominant ferritic matrix.

Some of the most important mechanical properties, which change depending on the value of the graphite shape factor, are tensile strength (R_m) and plasticity (A5). When analysing the correlation between the graphite shape factor and R_m and A5 values, the impact of the metallic matrix should be eliminated. Thus, tests should be performed on samples with the same type of matrix. The result obtained for ferrite cast iron is presented in Figure 13. Although investigations are at their initial stage, it is already possible to determine the correlation between the shape factor ‘f’ and the strength Rm of VM cast iron. By changing the graphite form within the range 0.30 ÷ 0.80, it is possible to increase cast iron strength from approximately 100 MPa to more than 500 MPa.

Similarly, the strong influence of the graphite shape factor is observed at tests of cast iron elongations and A5 determining. The results are shown in Figure 18. Changes of the shape factor ‘f’ within the range 0.30 ÷ 0.80 cause the elongation increase of VM cast iron from approx. 1.5% to approx. 14%. The direction of changes is known; thus, the tests involve the determination of empirical correlations in the form of mathematical dependencies.

Figure 18.

Correlation between the graphite shape factor and tensile strength R_m of ferrite matrix VM cast iron.

The QI index is a measure of the quality of grey cast iron with mixed-form graphite, including the VM cast iron in question. The value of the quality index QI increases proportionally to the increasing value of the graphite shape factor ‘f’. The empirical correlation between these values, describing the characteristics of the Vari Morph cast iron, is shown in Figure 19. The definition of the QI index suggests that an increase in its value means a greater increase in strength than in hardness, which is caused by changes in the metallographic structure (Figure 20). This trend of increasing Rm, while keeping HB constant, is particularly advantageous in terms of cast iron machining. Ferritic matrix castings, despite their high strength achieved by increasing the ‘f’ factor (compacting graphite precipitates), can be very easily machined with the use of CNC machining devices.

Figure 19.

Correlation between the graphite shape factor and plasticity A5 of ferrite matrix VM cast iron.

Figure 20.

Correlation between graphite shape factor ‘f’ and cast-iron quality index QI.

4. Examples of cast iron castings with mixed-form (Vari Morph) graphite

In industrial practice, the presence of mixed-form graphite in castings is a very common phenomenon, especially in the production of vermicular or spheroidal cast iron. Differences can be observed within a single casting, in its walls solidifying at different rates, or between castings that use cast iron with different physical and chemical properties. Naturally, the variation in the structure and form of graphite applies to those cast iron grades that have been produced using out-of-furnace nodularisation treatment in their liquid state.

However, in the authors’ opinion, the mentioned cases should not be classified as Vari Morph cast iron. VM is a cast iron type with mixed-form graphite, the structure of which has been intentionally varied using the technological process of nodularisation. The characteristics of cast iron obtained by creating non-standard cast iron grades are attractive from the point of view of their practical application. Thanks to the new structure of graphite precipitates, it is possible to obtain such desirable properties of cast iron as: increased resistance to temperature changes and thermal shock, increased tightness of cast iron, increased quality index QI and better machinability with comparable or higher strength R_m.

Several examples of castings that are/should be produced from Vari Morph cast iron are presented below. Figure 21 shows a heating furnace element operating under cyclic heating and cooling conditions. The component is subject to wear due to thermal fatigue.

Figure 21.

Heating furnace element: Casting made of MV cast iron operating under conditions of cycling heating.

Other examples of castings subject to wear as a result of thermal shock include cinder pot, steel ingot moulds (Figure 22) and moulds for pouring AL alloys into ingot moulds (Figure 23). In existing practice, these castings have been made of both spheroidal and vermicular cast iron, and metallurgical ingot moulds have also been made of grey cast iron. The spheroidal form of graphite hinders heat dissipation leading, on the one hand, to a slower casting process of the ingots, and, on the other, to greater overheating of the moulds and cinder pots, which is also an undesirable feature that accelerates their wear. In all these cases, Vari Morph cast iron with mixed-form graphite—vermicular + spheroidal in similar proportions—would be a better material. Such cast iron should be characterised by the graphite shape factor ‘f’ in the range of f = 0.65–0.75.

Figure 22.

Metallurgical equipment (a/ cinder pot, b/ casting mould) made of ductile or vermicular cast iron operating under cyclic heat shock conditions.

Figure 23.

Mould for pouring AL alloys and casting ingots (a/ casting technology, b/ working surface). Material for casting moulds—Ductile cast iron or, more rarely, vermicular cast iron.

5. Conclusions

The conducted research has confirmed the technological possibility of producing Vari Morph cast iron with mixed-form graphite in a controlled manner. It can be produced in several ways, both in experimental foundries and under industrial conditions. VM cast iron has several advantageous properties, including a high QI. It allows us to fill the gaps between the existing standardised cast iron grades with homogeneous graphite morphology: flake, vermicular or spheroidal. VM cast iron can be a great material for structures and castings that require above-standard properties that cannot be achieved with homogeneous graphite, such as application for thermal shock operating conditions, increased tightness of castings, etc. In the authors’ opinion, VM cast iron is a promising material of the future, filling a gap in the offer of the existing cast iron grades.

All properties of cast iron with a homogeneous ferritic matrix have been described in this paper as a function of the graphite form factor, the determination procedure of which has also been developed by the authors and verified by means of the described tests.

The research conducted has allowed us to determine empirical correlations between the graphite shape factor and the selected properties and functional features of VM cast iron.

These correlations involve:

• physical properties (specific density, thermal conductivity, ultrasound wave speed): λ = f(f); CL = f(f); ρ = f(f),

• mechanical properties (strength (Rm; A5)): Rm = f (f); A5 = f(f), IQ = f(f),

• functional properties (thermal fatigue resistance) N = f(f)

• At the current stage of work on the Vari Morph cast iron production technology, it can be stated that in the case of medium-size castings, the easiest way to process the process is to use a tightly covered Tundish ladle and a controlled amount of added Mg

• In the case of small castings produced on machine moulding lines, the Inmold technology is a favourable solution for the production of VM cast iron, especially in the case of serial production of castings with the use of spheroidising automotive dedicated to this technology.

Acknowledgments

This research was conducted within the project of: POIR 01.01.-00-0042/17.