Procedia Environmental Science,

Engineering and Management

http://www.procedia-esem.eu

Procedia Environmental Science, Engineering and Management 7 (2020) (4) 515-521

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

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

ENVIRONMENTAL IMPACT OF CAST IRON PRODUCTION

Alexey Gennadyevich Panov, Irina Faridovna Shaekhova, Makhmut Maskhutovich Ganiev, Anastasia Alexandrovna Konogorskikh,

Tagir Ildarovich Shiapov

Kazan Federal University, Kremlyovskaya St, 18, Kazan, Republic of Tatarstan, Russia

Abstract Although cast iron is one of the oldest materials in engineering, it is seeing something of a rebirth of fortunes in the modern world regarding environmental impact and sustainability. This is based on the fact that Cast Iron does not rust in the traditional sense, it oxidises very slowly. This process gives off iron ashes which are carbon based and good for the environment. They promote green growth on plants and increase planktonic life in the oceans, thus combating global warming. Modern production control of a bainite structure refers to a qualitative method of analysis and is based on a visual assessment of parameters, which limits the possibility of obtaining objective results. Quantitative methods of production control do not exist today. The main objective of this study is to develop a technique for preparing micro sections for controlling the bainite component in the microstructure of cast iron and evaluating its environmental effects. An algorithm was developed in the course of the work for obtaining etched micro sections for quantitative analysis of bainite. Due to the fact that there are no personal methods for etching microstructures to identify the bainite component, it was decided to conduct research work on the selection of a composition for chemical etching and also use it as a reagent for thermal etching. The reagent given in Appendix 2 "Composition of reagents for etching thin sections and revealing the general structure of cast iron and individual structural components" GOST 3443-87 was used representing 4% picric acid solution (4 g picric acid and 96 cm3 of ethyl alcohol) for etching the surface of a thin section. The composition of the etchant and the modes of thermal etching of the material under study were selected as a result of the literature review conducted by us. Keywords: bainite, environmental impact, metallographic analysis, micro section, microstructure, thermal etching.

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

Recent studies have shown that adding iron dust to the oceans could have a dramatic positive effect on their health, increasing the oxygen output of our seas and oceans and reducing greenhouse gasses. In recent decades in our country and abroad, there has been an increase in the production and use of high-strength bainitic nodular cast irons, which differ from other grades of high-strength cast irons (VCh) by a significant increase in strength, toughness, and wear resistance. In world practice, austenitic-bainitic cast iron (ADI) is already widely used in the automotive and other engineering industries for the manufacture of gears, heavy-loaded gears, spring holders for trucks, railway couplings and other critical parts, successfully replacing carbon and low-alloy steels (Dawson et al., 2018; Kornienko et al., 2004; Psyrkov, 2012).

The environmental footprint of the iron and steel industries are associated with their consumption of energy and high levels of production. Manufacturing iron and cast iron products is one of the major sources of CO2 and toxic materials released to the environment (Psyrkov, 2012).

However, despite the growing interest in these cast irons, today there are no regulatory documents that would spell out clear requirements for the preparation of microsection samples to control bainite in the microstructure of cast iron. And even with the maximum use of the modern capabilities of an optical microscope, it is difficult to provide reliable identification of this intermediate structural component in high-strength cast irons. In this regard, the need to create a methodology that makes it possible to objectively recognize bainite in the structure of cast iron is urgent.

Today, bainitic cast irons are considered as the most promising material for the manufacture of such heavily loaded automotive parts as front suspension arms, crankshafts, gears, etc. The main reason for the increased attention of some companies to bainitic cast iron is a significant reduction in energy consumption in the production of such parts as gear wheels, connecting rods, camshafts, friction-braking elements. In addition, they have a high complex of both mechanical and operational properties, and a lower tendency to form cavities (in comparison with steel). In addition, their advantages include a decrease in the allowances for machining, a decrease in the mass of castings by ~ 10%, and also a small change in dimensions during heat treatment (Psyrkov, 2012).

The worldwide popularity of bainitic ductile iron began with the release of crankshafts by the automobile company Ford, then such companies as General Motors, Mazda and others began to produce parts from this cast iron. Currently, hundreds of standard sizes of parts are produced on an industrial scale, and according to the information of the authors (Kornienko et al., 2004; Psyrkov, 2012), gear wheels are the most massive products abroad. General Motors also tests and practices gears, gearboxes and suspension arms made from bainitic nodular cast irons (BNCIs). As noted by the author (Psyrkov, 2012), thanks to the use of gear wheels made of this cast iron, the gears operate very quietly, and the graphite present in its base acts as a lubricant during operation. Thus, the operational characteristics of BNCIs are as high as, if not higher than in many types of traditional structural materials. 2. Materials and methods

Currently, most of the enterprises carry out control of the cast iron structure mainly by

destructive methods, including metallography, tensile testing, hardness etc. One of the most widespread methods for studying the structure of metals and alloys is the metallographic method, which is based on the study of the microstructure during chemical etching of a flat polished section surface. To identify the microstructure of alloys, the following methods are used: chemical etching, electrolytic etching, magnetic method, thermal etching, etching in

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molten salts, cathodic etching, and the microstructure relief enhancement after volume transformations.

Optical microscopy is one of the first and most widely used methods for studying the structure of metals and alloys. However, the capabilities of this method are restricted by the limiting magnification of 1500-2000, at which details of the structure with a size of at least 0.2 μm can be observed. In addition, as applied to cast irons, the metallographic method is not always informative, since graphite inclusions in a metal matrix have a different degree of branching, which is why the study of arbitrary sections on flat samples does not allow an objective assessment of the spatial arrangement of graphite inclusions and their volumetric appearance. To clarify the results of the analysis, the method of repeated polishing of a thin section and the study of the same section of the structure under a microscope are sometimes used (Pokrovsky and Khrol, 2015).

Another problem of metallographic control of the microstructure of cast irons is the visually difficult differentiation of some products of decomposition and transformation of austenite, such as troostite/sorbitol or martensite/bainite. One of the methods for controlling the microstructure of the cast iron matrix in such situations is the identification of structural components by their microhardness. The method for measuring microhardness is regulated by GOST 9450-76 and differs from the Vickers method only in that the indenter is smaller and is pressed in at lower loads from 5 to 500 gf.

Methods of electron and atomic force microscopy are also used to study the morphology of bainite. Unlike optical microscopy, scanning electron microscopy has a higher resolution and depth of field, due to which it allows us to obtain better images, to trace the modification of the shape and morphology of inclusions, the metal matrix and the nature of surface destruction using their volume representation. With regard to atomic force microscopy, it should be noted that this method is currently used primarily for quantitative assessment of the sample surface, for determining the height of the peaks characterizing the bainite phase, and is being observed both on an un-etched surface and after chemical etching. In addition, the method can be used to determine the parameters of bainite plates and subgrains for some steel grades (Kovensky and Neupokoeva, 2013).

In addition, the use of X-ray structural analysis of the bainite structure is also known (Nesterenko, 2010; Yudin et al., 2018). This method is universal and is widely used for various studies of materials. It is used to study the crystal structure, qualitative and quantitative phase analysis, to study the type, concentration and distribution of lattice defects, etc. The method is based on the study of the structure of a substance by diffraction patterns arising from scattering of X-rays on the analysed object (Pokrovsky and Khrol, 2015).

Another modern diffraction method for studying bainite structures is the EBSD (Electron Backscatter Diffraction) method. This method consists in obtaining diffraction patterns of reflected electrons using a scanning electron microscope. The diffraction patterns obtained using this method contain information on the orientation of the crystals and help in identifying phases with different crystal structures, and are also capable of providing information on local mechanical stresses. In particular, using this method, researchers (Morito, 2015) obtained colour maps of orientations, on the basis of which scientists argue that the thickness of martensite plates is less than that of bainite, and lower bainite has smaller blocks than upper bainite. At the same time, the paper notes that the identification of bainite and martensite using the visual method is human-dependent; therefore, in order to exclude the "human factor", the authors proposed to identify the boundaries of the recognized structures by automatically and numerically determined frequencies and densities.

Samples for EBSD analysis are prepared similarly to samples for metallographic analysis, but more carefully. The final operation of sample preparation for this method can be electric polishing.

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Nevertheless, despite the fact that the electron microscopy method is very effective in identifying phases in a sample, it is not widely used in the practice of research laboratories, since it has some difficulties arising in the process of sample preparation, which are described in more detail in the paper (Lukashova, 2016), and also in production practice due to additionally high cost and duration of control.

In this work, we investigated the possibility of increasing the information content of metallographic control through the use of the method with the thermal etching of a microsection. The thermal etching method is based on the difference between the oxidation rates of structural components having different chemical structures, for example, ferrite, cementite, phosphide, as well as on the difference in the orientation of the precipitated crystals (Beckert, 1988).

For example, if a polished metal sample is heated in air at a relatively low temperature, its surface will oxidize. The oxidation rate of various phases in a multiphase alloy can vary depending on their composition causing variations in oxide film thickness and characteristic colours. The clearest colour picture of the microstructure is obtained when a certain optimal film thickness is reached, usually of the order of 30 nm. Thinner oxide films are invisible without special research methods. The colours produced by a film of suitable thickness are caused by interference light rays reflecting off the inner and outer surfaces. With a further increase in the film thickness, it more and more absorbs the ray reflected from the inner surface of the film, and the interference pattern fades, loses its strength, brightness and becomes less clear, just as in the case of critical thinning of the film with insufficient oxidation.

In the laboratory of the Department of Materials, Technologies and Quality at the Naberezhnye Chelny Institute (branch) of K (P) FU, a sample of high-strength nodular cast iron was prepared for the study of the microstructure. The sample preparation process for an experimental microsection included the following stages: grinding, polishing, and etching. Grinding was carried out manually on a special BUEHLER MetaServ 250 grinding and polishing machine with automatic water supply. To obtain a high-quality microsection surface, we successively passed from one abrasive paper to another (the size of abrasive particles was 400, 800, 2000). When replacing the paper, the sample was washed and wiped off with filter paper, and rotated at 90° so that the scratch marks during subsequent processing were perpendicular to the marks from the previous treatment.

The polishing process was carried out on the same machine using felt. The cloth was moistened with a polishing liquid, which was mixed in the following proportion: 5 grams of alumina for 1 litre of water. After polishing, the sample was washed with water; the polished surface was wiped and dried with filter paper; if necessary, the surface was cleaned with an alcohol solution.

The etching of the polished sample surface was carried out in two ways (Beckert, 1988). The first method consists in short-term etching (6-10 sec.) of a microsection with a reagent at room temperature. The second method includes an additional operation in the form of holding in a muffle furnace at a constant temperature for a certain amount of time. A 4% solution of picric acid was used as a reagent for both methods (Mayorov et al., 2006). The holding time and temperature for thermal etching were selected on the basis of the authors' works (Beckert, 1988; Kovensky and Neupokoeva, 2013; Pokrovsky and Khrol, 2015). The holding mode in the MIMP-3P muffle furnace was 5.5 hours at a temperature of 260 °C. Cooling was carried out at room temperature in air. 3. Results and discussion

Metallographic analysis was carried out using an optical microscope Neophot-32 and

an automatic computer metallographic image analyser SIAMS 800. Due to the presence of a

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mark, it was possible to capture the same site of a microsection before etching, and after etching by methods No. 1 and No. 2, respectively. The visual difference between the sample images after standard and thermal etching is that after the second etching method the section acquired a blue-violet tint (Fig. 1).

a b c

Fig. 1. Microstructure of ductile iron (X100), analysis area - 10.2 sq. mm: a) before etching; b) etching # 1; с) etching # 2

However, despite the clear separation of some phase components by colour, the

identification of the bainite component in automatic mode using the standard SIAMS 800 software method turned out to be difficult. Application in automatic mode of the standard method "Multiphase Analysis" of the SIAMS 800 program showed that there are inaccuracies in the identification of some fragments of structures. For example, analysis of the sample image after etching by the method No. 1 showed that a relatively large amount of bainite is automatically recognized by the program as martensite and some darkened areas of the needle-like structure of the metal base are identified by the program as graphite (Fig. 2).

a) b)

Fig. 2. The result of analysis for digital images of section microstructures after etching by method #1 in

automatic mode: a) without filters; b) with filters At the same time, there are much less such distortions for the image obtained by

etching according to method No. 2. When analysing digital images of the sample after etching using the method No. 2, graphite inclusions are taken into account by the program as retained austenite (Fig. 3).

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a) b)

Fig. 3. Result of analysis for digital images of section microstructures after etching by method #2 in

automatic mode: a) without filters; b) with filters

Thus, there is an improvement in the situation concerning the identification of various phases in the microstructure of austenitic-bainitic cast iron, including the bainitic phase. In addition, on the basis of the results obtained, it seems possible to create a method for the quantitative determination of the bainitic component in austenitic-bainitic cast iron.

4. Conclusions

In this survey, we examined the environmental impact of producing cast iron. it was found that bainite is a key ingredient in iron casting processes. The production of bainite and the amount of its usage have significant environmental impacts because of the associated coke consumption. Hence it can be concluded that in order to tackle today’s environmental problems, preliminary research and practice should focus on both preventive and remedial approaches at the same time. On-site carbon sequestration at current power plants, steel mills and foundries should be promoted. For the new generation of power plants, investment in green energy sources should be prioritized. For the iron industries in specific, recycling and scrap usage should be promoted by educating the public and the industry leaders. Here are some findings:

1. Austenitic-bainitic cast irons (ADI) are promising materials, and one of the reasons for holding back the development of the use of these cast irons in industry is the lack of production control methods for the microstructure of this type of cast iron, in particular, the quantitative control of the bainitic component in the matrix of this cast iron.

2. There are a fairly large number of methods for the qualitative and quantitative assessment of bainite, however, the metallographic method remains the most promising for production control at present.

3. One of the key problems in the application of the optical microscopy method to control the bainite component in the structure of cast iron is the difficult differentiation of bainite and martensite using traditional methods of preparing microsections.

4. The results of the use of thermal etching, which allows the significant improving the differentiation of the matrix structures of austenitic-bainitic cast iron, make it possible to create a technique for the quantitative assessment of the bainitic component by optical microscopy.

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Thus, as a result of the work, it became possible to create a production technique for controlling bainite in the microstructure of cast iron, based on thermal (colour) etching to ensure sufficient differentiation of structures, in particular, to identify bainite in the microstructure of cast iron.

Acknowledgements The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. The authors would like to thank Vladimir Ivanovich Astashchenko and Nadezhda Georgievna Degtyareva for valuable advice.

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