Procedia Environmental Science,

Engineering and Management

http://www.procedia-esem.eu

Procedia Environmental Science, Engineering and Management 7 (2020) (4) 497-504

International Conference on Agriculture, Environment and Allied Sciences (AEAS),

December 24th-25th, 2020, Istanbul, Turkey

INFLUENCES OF CAST IRON MICROSTRUCTURE ON THE ENVIRONMENT AND THERMAL CONDUCTIVITY

Alexey Gennadyevich Panov1, Irina Faridovna Shaekhova1, Mahmut Maskhutovich Ganiev1, Gurtovoj Dmitrij Andreevich2, Anastasia

Alexandrovna Trubkina1

1Kazan Federal University, Russia 2KAMAZ Publicly Traded Company

Abstract Foundry industry is a branch of industry which generates a huge quantity of wastes. Increase of raw materials prices (cast iron scrap, sand, energy resources) and cost for dumping load, it follows that the economic factors become very important arguments in the process of minimizing of wastes generation. Cast irons are among the most widely used materials due to their unique combination of mechanical and physical properties, while their cost is relatively low. According to the reports of "Census of world casting production" for the period 2004-2017 it can be concluded that the production volumes of gray cast iron with lamellar graphite (CILG) are significantly higher than the production of nodular graphite cast iron (NGCI), despite the higher mechanical characteristics of the latter. Gray cast iron (GCI) is used widely under conditions of prolonged and cyclic exposure to high temperatures due to its increased thermal conductivity. GCI is used in the manufacture of brake discs, cylinder blocks and cylinder heads. Due to the increased thermal conductivity, heat is quickly removed, thereby preventing thermomechanical fatigue, deformation and crack propagation, which allows cast irons to be used in more severe conditions, while extending the service life of a product. As the temperature rises, the strength and thermal conductivity of GCI decreases. Therefore, one should pay attention to compacted graphite iron (CGI) as a replacement for the traditional GCI. Although its thermal conductivity is lower, the increased strength justifies replacement. Consequently, the lower value of thermal conductivity observed in CGI requires improvement and additional research, which is the subject of this work. Keywords: cast iron, ductile iron, gray cast iron, mechanical properties, microstructure, the environment, thermal conductivity, vermicular graphite, wastes generation.

Selection and peer-review under responsibility of the AEAS Conference Author to whom all correspondence should be addressed: AlGPanov@ksu.ru

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1. Introduction

Over the past half century, the situation in the field of alloy production has changed dramatically. According to the graphs (Fig. 1), gray cast iron is still the most widespread alloy, however, production volumes have decreased by more than a quarter (Bondarenko and Gladky, 2006; Census of world casting production, 2004; Pierce et al., 2019). At the same time, the production of ductile iron increased significantly. This became possible, among other things, due to the development of serial production of cast billets from a new class of cast irons with a vermicular form of graphite, uniquely combining physical, mechanical, technological and operational properties (Dawson, 2008; Dawson et al., 2018; Girshovich, 1978; Stefanescu et al., 1988).

Fig. 1. Comparative structure of alloy production in the world for 1966 and 2016 General characteristics of the physical and mechanical properties for the main types

of cast iron are shown in Table 1.

Table 1. Properties of cast iron with different shapes of graphite inclusions at room temperature

Iron Type Gray Iron Compacted Graphite Iron Ductile Iron Hardness, (BHN) 170-220 140-250 150-300 Density, gm/cc 6.8-7.1 7.0-7.2 7.3

Ultimate tensile strength, ksi 201 248-583 269-727 Thermal conductivity, W/m.K 39.0-52.5 31-50.0 25-38 Ultrasonic in velocity, /µsec 0.17-0.20 0.20-0.21 0.21-0.23

Heat transfer plays a significant role in many structural components, especially in

those which operate at elevated temperatures (Cochran, 2015; Guo et al., 2014; Shao et al., 1998): brake discs, engine blocks and cylinder heads. The ability to dissipate heat efficiently increases the resistance to the effects of such negative factors as thermal fatigue and deformation. Structural elements that are exposed to high temperatures undergo dimensional changes. If a product is heated unevenly, hotter areas will expand more than colder areas. Thus, internal parts of the structure can be subjected to significant thermal stresses.

Thanks to the use of CGI in the production of critical parts that operate under increased mechanical loads (pressure, friction, temperature drops), it is possible to achieve the maximum technical and economic effect. However, for this, it is necessary to have as

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much information as possible on the CGI properties, in particular, on thermal conductivity, depending on the features of their structure.

2. Materials and methods

In this work, we studied the thermal conductivity indices of cast iron specimens with

vermicular and nodular graphite with approximately the same metallic ferrite-pearlite matrix with a pearlite fraction of about 10% and differing by fraction in the microstructure of inclusions with different graphite shapes.

The measurement of thermal conductivity indicators was carried out on 11 cylindrical samples (diameter - 10 ± 0.5 mm, height - 2.5 ± 0.5 mm) using a specialized device "Netzsch LFA 457 - Thermophysical Parameter Meter for Solids" by the laser flash method.

Proof test conditions: • Ambient temperature: 22 ± 3 0C. Ambient temperature measurement accuracy: 1

0C; • Atmospheric pressure: 84 - 107 kPa. Accuracy of atmospheric pressure

measurement: 0.1 kPa; • Relative humidity of ambient air: 40-60%; • Measurement accuracy of relative humidity: 1%; • Rated voltage in the power grid: 220 V; • Rated frequency of voltage in the mains: 50 Hz. The sample, mounted on a special holder, was placed in a silicon carbide electric

furnace with an inert atmosphere. The lower part of the sample was heated by a laser pulse during 0.5 ms. The temperature change in the upper part of the sample was recorded by InSb IR detector. The exposure at the temperature of measurements between shots was 2 minutes, 3 measurements on each sample. The density of cast iron d is determined at room temperature using a standard geometric method. The thermal conductivity λ(T) of the alloys was calculated using the well-known relation λ = aꞏdꞏCp. The laboratory error in thermal conductivity determination was 4.6%.

Before microstructural analysis of the experimental samples, sample preparation was carried out, which consisted of three stages: pouring into resin, grinding and polishing. For ease of use, the samples were fixed by pouring into resin (Fig. 2).

Fig. 2. Pouring samples into resin

Grinding and polishing of the samples was carried out on a specialized BUEHLER MetaServ250 machine (Fig. 3) using a sanding paper glued to a rotating disk. The sample was processed sequentially, passing from a coarser abrasive grain to a finer one with abrasive particle dispersion increase. During grinding with each abrasive paper, we tried to maintain the same position of the sample, controlling the parallelism of the scratches. Polishing was

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carried out using glue-based discs covered with a cloth to eliminate marks left after sanding with the finest sandpaper.

Fig. 3. Grinding - BUEHLER MetaServ polishing machine

The quantitative analysis to determine the proportion of graphite inclusions in the microstructure of cast iron was carried out using a metallographic unit for the automatic study of the material microstructures (Fig. 4). The complex consists of an inverted metallographic microscope Neophot - 32, a color digital microscope video camera SIMAGIS BS-4CU (resolution 4 Mpix) and the software SIAMS 800 (the analyzer of solid microstructure fragments) with automatic modules that allow to evaluate the parameters of material microstructure according to various standards, including GOST and ISO.

Fig. 4. Metallographic complex for microstructure analysis

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Digital images of microstructures were obtained using the innovative software for panoramic microscopy SIAMS 800. Application of the standard technique ISO 16112: 2006 (E) in automatic mode. Compact (vermicular) graphite in cast iron. Classification allows to determine the share of VG in the microstructure of cast iron with a minimum error (no more than 5%) (Biswas et al., 2015; Shaehova et al., 2019). Fig. 5 shows the examples of the initial and processed digital images of the experimental sample microstructures.

Fig. 5. Digital images of microstructures in PMHS: a) the original digital photo; (b) Image after application of the tool "ISO 16112:2006 (E). Compact (vermicular) graphite in cast iron.

Classification" AAI SIAMS 800

3. Results and discussion Table 2 shows the results of 11 experimental samples on thermal conductivity (λ)

obtained at room temperature, as well as the data on the fraction of vermicular graphite in their microstructure. For each sample, they obtained 10 digital images of individual fields of the microstructure, which allow to obtain more objective information on the VG fraction.

The data array was subjected to statistical processing. Thus, the values were found that were used to plot the dependence of cast iron thermal conductivity on VG fraction (Fig. 6).

Table 2. Results of microstructural analysis and thermal conductivity values of CGI samples with

different fraction of graphite

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Fig. 6. Dependence of the influence of VG fraction in the microstructure of cast iron for its thermal conductivity

There is an obvious difference in thermal conductivity between three most common

types of cast iron: GCI, CILG and CGI. Cast iron with lamellar graphite has the best thermal conductivity of the listed materials, cast iron with nodular graphite has the lowest thermal conductivity (Velichko, 2007). The diagram of the heat transfer process in iron-carbon alloys is shown on Fig. 7.

Cast iron is a typical metal composite. Different structural components of cast iron conduct heat differently. The thermal conductivity of the cast iron matrix is lower than the thermal conductivity of graphite along its basal planes. The inclusions in the GCI, and also, to a large extent, in the CGI, are a three-dimensional coupled system with a large length of basal planes (Holmgren, 2010). In contrast to this, CILG has larger areas of the matrix between graphite inclusions, which have a relatively small length of closed basal planes, therefore, heat spreads over long distances in it at a lower speed.

The constructed 3D models of various types of graphite particle distribution in cast iron are shown on Fig. 8.

As compared to GCI, the thermal conductivity of CGI is 25% lower on average at room temperature, but only 10-15% lower at 400 ºC. Thus, the differences between the indicators of thermal conductivity for these classes of cast iron are decreased with temperature increase.

Steel Ductile iron Gray iron

(0001)

Flake graphite particies

Spheroidal graphite particies

(0001)

Hea

t flo

w

Hea

t flo

w

Hea

t flo

w

а) b) c)

Fig. 7. Scheme of the heat transfer process in steel (a), high strength (b) and grey cast iron (c)

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а) b) c)

Fig. 8. 3D reconstruction of graphite particles: a) spherical graphite; b) vermicular graphite; c) lamellar graphite

4. Conclusions

The analysis of the literature has shown that in the process of VG structure formation,

the hexagonal lattice of graphite grows in interconnected eutectic cells in a certain dominant direction, thereby increasing the number of interconnected graphite inclusions as in GCI, which leads to a noticeable increase in thermal conductivity as compared to CILG.

An experimental study of thermal conductivity within the framework of these studies showed that with the increase of vermicular graphite proportion in the structure from 11% to 80%, the thermal conductivity of cast iron at room temperature increases by 17%, from 27.7 W/m*K to 33.06 W/m*K.

Due to its unique physical and mechanical properties, cast iron with compacted graphite has recently been considered by designers for use in parts operating at increased loads and temperatures. However, there is currently insufficient public information on the thermophysical properties of this promising material for reliable design. In the present work we studied the thermal conductivity of CGI with approximately the same ferrite-pearlite matrix and different contents of vermicular and nodular graphite fractions. It has been determined that with 10% of pearlite in the CGI matrix and the increase of vermicular graphite fraction from 11% to 80%, the thermal conductivity of CGI at room temperature grows according to the law λ = 27.812*VG0.0558 W/m*K.

Acknowledgements The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.

References Biswas S., Monroe C., Prucha T., (2015), Analysis of Published Cast Iron Experimental Data, In:

Proceedings of the 3rd World Congress on Integrated Computational Materials Engineering (ICME 2015), Poole W., Christensen S., Kalindini S.R., Luo A., Madison J.D., Raabe D., Sun X. (Eds.),Springer Nature, Cham, Switzerland, 293-303.

Bondarenko S.I., Gladky I.P., (2006), Influence of graphite shape on thermal stability of cast iron, Bulletin of KhNADU, 33, 78-88.

Census of world casting production: Modern Casting, (2019), On line at: http://www.thewfo.com/census.

Panov et al./Procedia Environmental Science, Engineering and Management, 7, 2020, 4, 497-504

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Cochran J.K., (2015), Morality, rationality and academic dishonesty: A partial test of situational action theory, International Journal of Criminology and Sociology, 4, 192-199.

Dawson S., (2008), Compacted Graphite Iron – A Material Solution for Modern Diesel Engine Cylinder Blocks and Heads, 68th WFC - World Foundry Congress 7th - 10th February, 93-99.

Dawson S., Panov A.G., Gumerov I.F., Panfilov E.V., Gurtovoy D.A., Dibrov I.A., Anikin S.A., (2018), Experience of large-scale production of high-quality automotive cast iron castings with vermicular graphite, M: Foundry of Russia, 4, 8-16.

Girshovich N.G., (1978), Handbook of Iron Casting, 3rd ed., Mashinostroenie, Leningrad, Russia. Guo Y., Wang C.Y., Yuan H., Zheng L.J., (2014), Milling forces of compacted graphite iron (CGI) and

gray iron (GI), Materials Science Forum, 800-801, 32-36. Holmgren D., (2010), Modelling the thermal conductivity of various cast irons, Key Engineering

Materials, 457, 318-323. Shao S., Dawson S., Lampic M., (1998), The mechanical and physical properties of Compacted

Graphite Iron, Materialwissenschaft Und Werkstofftechnik, 29, 397-411. Pierce T., Haynes A., Hughes J., Graves L., Maziasz J., (2019), High temperature materials for heavy

duty diesel engines: Historical and future trends, Progress in Materials Science, 103, 109-179. Shaehova I.F., Panov A.G., Degtyaryova N.G., Ivanova V.A., (2019), Problems of Using Computer

Control of Vermicular Graphite in the Microstructure of Cast Iron, In: Proc. of the Int. Conf. on Computer Graphics and Vision «Graphicon», 260-264.

Stefanescu D.M., Hummer R., Nechtelberger E., (1988), Compacted Graphite Irons, In: Metals Handbook, vol. 15, ASM International, 9th ed., 667-677,

https://doi.org/10.31399/asm.hb.mhde2.9781627081993. Velichko M., (2007), 3D Characterization of Graphite Morphologies in Cast Iron, Advanced

Engineering Materials, 9, 39-45.