VŠB - Technical University of Ostrava Faculty of Metallurgy and Materials Engineering

Transcription

1 VŠB - Technical University of Ostrava Faculty of Metallurgy and Materials Engineering Steel Casting Foundry (study supports) doc. Ing. Libor Čamek, Ph.D. Ostrava 2016

2 1. Basic parameters of electric arc furnaces and electric induction furnaces. In steel foundries, the predominant melting units today are electric arc furnace (EAF) and electric induction furnaces (EIF). 1.1 Melting units in steel foundries. [1,3] Electric arc furnaces The diagram of a three-phase electric arc furnace is shown in Fig. 1.1 The furnace is fed directly from the high-voltage cable 1, via the main switch 2, impedance coil 3 and transformer 4. The function of connecting the impedance coil is only in the stage of melting, so that its inductive resistance reduced voltage fluctuations on the arcs and in the network. From the transformer, electrical energy is conducted through copper flanges to the outer wall of transformer vault, and further through the ropes 10 to the arms of electrode holders 6. The electrodes 7 are held by the holder 8. On the arm of the electrode holder, the current is further conducted through copper flanges 6. The electrode passes through the furnace cover via the cooled electrode ring 9. Fig. 1.1 Example of an electric arc furnace The installed power of the furnace transformer in foundries usually ranges from 300 to 600 kva/t. To melt a ton of burden, the calculated theoretical consumption is 380 kwh/t. The actual consumption for melting and heating the bath to 1600 C is higher by about 80 kwh/t. Foundries operate normally EAFs with melt weight amounting to 4-20 tons. Heavy castings are produced in foundries at metallurgical steel mills, which supply the liquid metal. 2

3 EAFs have alkaline type of lining. The advantage of alkaline furnaces usually lined with magnesite and chromium-magnesite is the ability to process metal burden with nonguaranteed phosphorus and sulphur content. Temperatures in the arc exceed 3000 C. In the electric arc, dissociation of nitrogen and hydrogen, which dissolve in the bath, occurs. Electric induction furnaces The metallurgical part of electrical medium frequency furnace is shown in Fig. 2.1 In steel foundries, exclusively the electric induction crucible furnaces are used. Furnaces typically operate with medium frequency ( Hz). Induction furnaces are powered by low-voltage network through the furnace transformer. From the furnace transformer, frequency converter is usually energized by the voltage of up to 6000 V. The current is initially directed to semiconductor diodes and smoothed by an impedance coil. The required frequency is produced by power thyristors. Thyristors are controlled and frequency can be continuously varied. Inductor is powered by medium-frequency current. Fig. 1.2 Diagram of an electric induction furnace The construction of the furnace is shielded from the inductor by transformer sheet bundles which lead the electromagnetic field and reduce losses. The source of heat in induction furnaces are induced currents. Medium frequency induction furnaces operate with an input ranging from 500 to 1000 kw/t. In precision casting foundries, furnaces with melt weighing from 40 to 250 kg are installed most frequently. In other steel foundries, their capacity ranges from 0.5 to 25 t. In medium frequency furnaces, exclusively rammed lining are used, since any joints in the masonry in these furnaces could cause penetration of metal through the crucible 3

4 lining. The material for the rammed lining is acid ramming mass based on crushed quartzite (SiO2) or basic ramming mass usually based on MgO spinel Al 2 O 3 (20 % of Al 2 O 3 ), or Al 2 O 3 MgO (30 % of MgO). From the metallurgical viewpoint, induction furnaces serve as a unit for burden remelting. Except for carburizing, alloying and deoxidation, chemical composition of the steel during melting is intentionally not altered. In the induction furnace, spontaneous selfcarburization of the burden does not occur, therefore they are suitable for the production of steels with low carbon content. The induction furnace is an operational melting unit suitable for intermittent operation. Melting time may be less than one hour, depending on the type of the furnace. Then, if the time of casting is 30 minutes, two induction furnace can continuously supply the moulding line with molten metal. In many foundries, induction furnaces are the only alternative for the production of stainless steels with a low carbon content. 2. The fundamentals of thermodynamics in steelmaking processes. Solutions of molten metals, non-metals and gases in iron. [1,2] 2.1 The fundamentals of thermodynamics in steelmaking processes. All metallurgical reactions are accompanied by consumption or energy release. Chemical thermodynamics deals with interrelationships between different forms of energy and the relationship between energy changes and material properties. The course of each metallurgical process is affected both by the driving force of the process and internal and external resistance of the reacting system against the course of this process. Thermodynamic analysis enables to set the driving force, not the size of resistance against the analysed response. Thermodynamic calculations can therefore be used to determine how the monitored reaction would proceed in the case of no resistance, but it is not possible to determine the speed of the reaction. Explanation of the basic concepts The system or the set is a collection of objects that are subject to thermodynamic considerations. 4

5 From the viewpoint of energy transfer and matter transfer, we distinguish the following systems: A closed system does not receive matter from its surroundings, and it does not pass it to its surroundings, either (however, it can exchange energy with it). An open system exchanges both matter and energy with its surroundings. In terms of matter properties, we further distinguish the following systems: A homogeneous system is one whose properties are the same in all parts of the system or change only continuously (this can be e.g. the case of water or air). A heterogeneous system is composed of several homogenous sections (phases) which are separated by phase boundary surfaces (their properties are changing). Thermodynamics is interested in states in which the system is found, and in the equilibria that are established in these systems. Thermodynamic properties describe the properties of the system. These thermodynamic properties are divided into extensive and intensive properties. Extensive properties are dependent on the quantity of the substance in the system and exert an additive behaviour (their value is equal to the sum of the individual parts of which the system consists, e.g. matter, volume, material composition, energy). Intensive properties are independent of the size of the system and the amount of mass in the system (e.g. pressure, temperature, concentration, density, and all the measurement values related to the amount of substance or weight). Thermodynamic process refers to any change in the properties of the system associated with a change of at least one thermodynamic state variable. In nature, spontaneous unidirectional processes take place, and they take place with a reduction in the energy system. Gradually, they come into balance. However, perfect thermodynamic equilibrium is possible only in an isolated system. Thermodynamic state variables (p, v, T, C) are independent variables that describe the system using appropriately chosen, usually directly measurable physical quantities (temperature, pressure, volume and thermal capacity). On the basis of state variables, we can then calculate other variables (state functions) characterizing the system. Thermodynamic state functions (H, U, S, F, G) are dependent on the thermodynamic state conditions. It implies that their variation depends only on the initial and final state of the 5

6 system and does not depend on the mode of switching the system from the initial to the final state. State functions then mathematically shows a complete differential and the difference of the initial and final state is independent of the integration path. State functions are difficult to measure, but they can be expressed as a function of measurable state variables: pressure, temperature, volume, thermal capacity. State functions used in thermodynamics include e.g. the internal energy U, enthalpy H, entropy S, Gibbs energy G and Helmholtz energy F, the chemical potential. Heat and work do not exhibit thermodynamic state functions, since the transition from the initial state of the system to the final state is dependent on the initial and final state conditions, but also on the mode of system switching, i.e. the path of integration, and therefore thermodynamic functions are not status functions. 6

7 2.2 Solutions of molten metals, non-metals and gases. Oxygen in molten iron The maximum solubility of oxygen in pure iron at a temperature of 1600 C is 0.25 wt. % (2500 ppm). With increasing oxygen concentration value approximately above 0.08 %, negative deviation from Henry s law occurs due to increasing binding forces between oxygen and iron, and therefore it is necessary to take into account its effective concentration activities. Currently, oxygen is fed into steel mostly as gas in the oxidation period of melt. Melt oxidation takes place on each type of melting unit in order to reduce the concentration of undesirable elements. Thereafter, the reduction of oxygen content before or during the reduction period of melt is called deoxidation. Most authors report the solubility of oxygen in iron in the form of O 4+ cation, or for thermodynamic calculations in atomic form, and it can be described by the equation: 1 2 O 2 O The equilibrium constant K for oxygen of equation (2.1) is expressed by the equation: K O p 1 2 O 2 (2.1) (2.2) The oxygen content in the steel, depending on the oxygen partial pressure can be derived from the equation (2.2) in dependence on the oxygen partial pressure p O. O K p (2.3) O In normal steelmaking practice, its concentration usually does not exceed 300 to 400 ppm in plain steels. The oxygen content in molten steel also controls the other elements present in the steel as silicon, manganese and carbon. If deoxidizing steel with aluminium in the ladle is correctly executed, oxygen activity in plain steels is about 5-10 ppm. Nitrogen in molten iron In the melt of pure iron, nitrogen is present as an atom, or possibly as N 2+ ion. Assuming atomic nitrogen dissolution in pure iron, the nitrogen transition from the gaseous into the molten iron phase can be described by the equation: N 1 2 N 2 (2.4) Dissolution of nitrogen in pure iron precisely corresponds to Sieverts law, which can be, with respect to Henry s law, described by the equation: 7

8 % N K N. pn2 where p N2 (2.5) is partial pressure of nitrogen in the atmosphere [Pa] and K N is the equilibrium constant of nitrogen dissolution. The equilibrium content of nitrogen in molten iron corresponds to a given temperature and the partial pressure of nitrogen in the atmosphere. The solubility of nitrogen in pure iron can be calculated, for example at the temperature of 1600 C and Pa is about % of Nitrogen. p N2 In a similar manner as in iron melt, the nitrogen content equilibrium in the individual modifications of iron can also be identified in the solid state. For their calculation, it is necessary to know the temperature dependence of the solubility of nitrogen for the given modifications of iron. The simple idea of the solubility of nitrogen is shown in Fig. 2.1, which represents the equilibrium concentration of nitrogen depending on the temperature of the pure iron. The diagram indicates the concentration of nitrogen, which are in equilibrium with the atmosphere of pure nitrogen at the pressure Pa. For the endothermic nature of nitrogen dissolution, with increasing temperature, the solubility of nitrogen increases as well. Conversely, reducing the solubility of nitrogen in iron with increasing temperature is related to the formation of nitrides Fe 4 N, or possibly Fe 2 N. The formation of these nitrides is exothermic in nature, which exceeds the endothermic nature of nitrogen dissolution, which leads to an increase in solubility of nitrogen in the modification of the iron with a decrease in temperature. p N2 Fig. 2.1 The solubility of nitrogen in pure iron depending on the temperature at p N Pa 8

9 Legend: Obsah dusíku nitrogen content, dusík nitrogen, tavenina melt, teplota temperature In multicomponent systems, especially in high-alloy steels, solubility of nitrogen is also affected by action of the force with other components. For nitrogen in the case of alloyed steels, it is necessary, instead of the concentrations in equation (2.5), to use Henry s activity and thus Sieverts law in the form: K f % N N. pn 2 N In the relation (2.23), f N (2.6) represents the activity coefficient of nitrogen in steel, which can be determined on the basis of the tabulated interaction coefficients. Steels produced in an electric arc furnace contain about 80 to 120 ppm of nitrogen. Steels produced in induction furnaces usually contain less nitrogen than steels produced in the arc furnace, which is mainly due to the transfer of nitrogen to the bath in the area of an electric arc. Hydrogen in molten iron At present, it is assumed that hydrogen is present in iron melts as an atom, or possibly as a proton H +. Considering atomic dissolving of hydrogen in pure iron, it can be described in an analogous manner as in the case of nitrogen (2.7). In practical calculations, it is assumed that hydrogen forms with iron a strongly diluted solution, which is governed by Henry s Law. Dissolution of hydrogen in pure iron relatively precisely corresponds to Sieverts law, which can be, with respect to Henry s law, described by the equation (2.7): H 1 2 H 2 (2.7) % H K H. ph2 (2.8) where p H is hydrogen partial pressure in the atmosphere and K 2 H is the equilibrium constant of hydrogen dissolution. Since dissolution of hydrogen in iron has an endothermic character, its solubility increases with increasing temperature. Also for unalloyed steels, Henry s law (2.2) can be used to describe the dissolution of hydrogen in steel. The equilibrium hydrogen content in molten iron corresponds to a given temperature and the partial pressure of hydrogen in the atmosphere. In a similar manner as in iron melt, the equilibrium hydrogen content can 9

10 also be determined in various modifications of iron in the solid state. For their calculation, it is necessary to know the temperature dependence of the solubility of hydrogen in the particular modifications of iron. Fig. 2.2 gives us a simple idea of hydrogen solubility; it represents the equilibrium hydrogen concentrations depending on the temperature for pure iron. hydrogen content Fig. 2.2 The solubility of hydrogen in pure iron depending on the temperature at p H = Pa Water vapour present in the atmosphere decomposes on the metal surface into hydrogen and oxygen, which dissolve in the steel: O H 2 2 H O (2.9) K a a 2 H O H O 2 ph2o (2.10) The equilibrium constant equation shows that hydrogen activity in iron under an atmosphere containing water vapour will increase with increasing partial pressure of water vapour and with a decrease in oxygen activity. In multicomponent systems, especially in high-alloy steels, hydrogen solubility is also affected by the action of the force with other components. For hydrogen in the case of alloyed steel, it is necessary, instead of the concentrations in equation (2.8), to use Henry s activity and thus Sieverts law in the form: K [% H ] = f H. p H H 2 (2.11) 10

11 In the relation (2.11), f H represents the activity coefficient of hydrogen in steel, which can be determined on the basis of the tabulated interaction coefficients [1]. Fig. 2.3 shows the final hydrogen content in steel which is the sum of hydrogen content in metal and the hydrogen content, which passes into metal from the ladle as well as from the casting mould. In the production of steel in electric furnaces, the final hydrogen contents usually range between 4 and 6 ppm. The final hydrogen contents in steel produced in an acid induction furnace range between 3.5 and 5 ppm. Fig. 2.3 Depiction of the sum of metallurgical hydrogen and hydrogen from the mould to the probability of bubbles forming in the casting Carbon in molten iron Carbon, which is the main element in all steels, significantly alters the properties of iron, namely from very small concentrations. Carbon in steel is an important element also in terms of metallurgy. In the production of steel in an electric arc furnace, it is recommended to carburize the burden. The most commonly used carburizing agent for steel production is coke. For accurate melting or setting the carbon content in steel, crude iron and carbon-rich ferroalloys (ferromanganese, ferrochromium) are used. High carbon contents are suitable for the oxidation by gaseous oxygen. If iron ore is used for the oxidation, it is better to choose somewhat lower carbon contents after melting. The carbon content after melting should be at least 0.30 % higher than the desired carbon content at the end of the oxidation, but not less than 0.50 %. For higher levels of carbon or with alloy steels, it is therefore necessary to use the carbon activity instead of its concentration in thermodynamic calculations. 11

12 Sulphur in molten iron At higher concentrations, with more massive and heavy castings, sulphur may cause cracking and it reduces the strength of steel at temperatures below the solidus. Just below the solidus, there is a temperature zone in which the steel strength value is negligible. In this zone, low tensions arising in the casting during cooling already lead to cracks, and sulphur extends this zone even more. The sulphur source in the production of steel in electric furnaces is the burden. Elements with high affinity to sulphur form sulphides with sulphur at temperatures of 1600 C. The elements with greatest affinity to sulphur are calcium, magnesium and rare earth metals. Calcium and magnesium are in the gas phase at the temperature of 1600 C, and their solubility in steel is small. For desulphurisation of steel under alkaline slag, reducing solubility of sulphur due to the formation of sulphides, calcium is used. The reaction between sulphur and the element with a high affinity to sulphur have a great influence on morphology of sulphides that arise during solidification. Sulphur has a strong tendency to segregation and in microscale, it segregates to the boundaries of the dendrites. In the interdendritic areas, melt is enriched with sulphur to the concentrations at which sulphides of elements originate with a lower affinity to sulphur, e.g. MnS. In cast steels, for which higher strength values are required even at negative temperatures, the sulphur content is reduced below %. For high-strength cast steel, sulphur contents below % are required. To reduce the segregation of sulphur in heavy castings, the sulphur content is reduced below % and below. 3. Standards and a range of cast steels. Structural steels for castings. Special stainless steels for castings. Special wear-resistant cast steels. [1,2] 3.1 Standards and a range of cast steel. Currently, the use of standards in the Czech Republic result from commercial negotiations. The conditions of supply of castings, marking, production method, testing, or other aspects are defined by the technical delivery conditions. Currently there are valid technical delivery conditions ČSN (Czech National Standard) EN , which specifies the delivery of castings in general. 12

13 Deliveries of steel castings are particularized in the follow-up standard ČSN EN Additional requirements for steel castings. This area of standards is supplemented with Technical delivery conditions for steel castings for pressure vessels from Section 1 to Section 4. Technical conditions for individual steel grades prescribe in particular chemical composition, the weldability conditions, heat treatment, and the desired mechanical properties for different test temperatures. Marking steels is governed by standards ČSN EN Steel grades systems Section 1: The system of abbreviated grades Basic symbols ČSN EN Steel grades systems Section 2: The numerical coding System. Further classification of the range of steels into castings: Steels for general use ČSN EN Steels for castings for general use (structural steel). Stainless steels ČSN EN Steels for castings of stainless steel. Heat resistant steels ČSN EN Steels for castings of heat-resistant steels. 3.2 Structural steels for castings. Steels are intended for general use (ČSN EN 10293). For alloy steels with the Mn content lower than 1.20 %, a maximum content of P and S is specified. For the grades GE XXX, the requirement on the chemical composition is prescribed only for maximum sulphur and phosphorus contents. Other chemical composition is determined by the foundry operations so as to achieve the required mechanical values. For the grades GS XXX, only maximum levels of carbon, silicon and sulphur and also basic mechanical values are prescribed. 13

14 Unalloyed steel with the Mn content of 1.60 to 1.80 %. The quality of these grades are widespread not only in the Czech foundries. The individual grades of this group of steels differ particularly in the carbon content and in their chemical composition. Heat treatment and mechanical properties of unalloyed structural steels for castings containing 1.60 to 1.80 % of Mn can be found in the recommended literature [1]. Low alloy steels can be divided into Mo alloy steels, Cr + Mo alloy steels and steels containing Ni. In the group of Cr + Mo low alloy steels, the chromium content ranges from 0.80 to 2.50 %, and the Mo content from 0.15 to 1.20 %. The major effect on the properties of this group of steels having carbon content and the stability of carbides at higher temperatures at the molybdenum and chromium-molybdenum steels additive increases from 0.05 to 0.15% V. Heat treatment and mechanical properties of low-alloy Cr-Mo- (V) steels for castings can be found in the recommended literature [1]. The last group of cast steels for general use are high-martensitic steels with the chromium content above 12 % and with the carbon content up to 0.06 %. These steels are also classified as high-alloy steels with martensitic structure. 14

15 3.3 Stainless steels. Corrosion resistant steels are materials capable of surface passivation in the presence of chromium. The lowest concentration of chromium in the matrix which ensures the passivation is 11.5 %. Because in technical alloys, part of the chromium content may also be bound to carbides, it is necessary to increase the concentration of chromium to maintain the same corrosion resistance of steel. The corrosion resistance of steel depends on the contents of other elements, in particular, C, Ni, Mo, Mn, or possibly N and Cu. Casting of stainless steels Stainless cast steel have a low carbon content, excluding the two grades of martensitic steels, with other grades, the carbon content is less than %. The steels are divided according to the standard into martensitic, austenitic, fully austenitic and austenitic-ferritic structure. Austenitic steels contain 8-12 % of nickel and fully austenitic steels have a high content of nickel in the range of about 24 to 30.5 %. Besides chromium and nickel, an important alloying element is molybdenum. Fully austenitic grades contain up to 7 % of Mo. For alloying of some austenitic, fully austenitic and austenitic-ferritic grades, nitrogen is also used at concentrations of up to 0.25 %. Up to 4 % of copper is used for alloying some stainless steels grades. To summarize the effects austenite-forming and ferrite-forming elements, a concept of socalled equivalent of nickel Ni ekv. (3.1) and equivalent of chromium Cr ekv. (3.2) has been introduced. Their introducing allows to express the influence of chemical composition on the structure of stainless steels. The values of the individual equivalents can be determined using the relations: Ni ekv. Cr ekv. %Ni 0,5.%Mn 30.%C 30.%N %Cr %Mo 1,5.%Si 0,5.%Nb (3.1) (3.2) Martensitic steels According to the chemical composition, stainless steels with martensitic structure can be divided into chrome and chrome-nickel steels. Martensitic stainless steels contain 11.5 to 17.0 % of chromium, and the carbon content depends on the content of nickel. For grades 15

16 with less carbon (for improving weldability), austenite-forming effect of carbon must be compensated by the increased content of nickel. To determine the corrosion resistance of the martensitic steels, they should contain more than 11.5 % of chromium. To enhance the weldability of martensitic steels, it is required to reduce the carbon content which is compensated by the addition of nickel. Reducing the carbon content can be compensated according to the equation (3.1) by increasing the nickel content. Currently, ratios of 4-6% for nickel content are used, and the carbon content has decreased below 0.06 %. Martensitic steels are corrosion resistant in the hardened condition. Heat treatment and the values of mechanical properties of martensitic stainless steels can be found in the recommended literature. Due to the high strength and good weldability, martensitic steels are used in the construction of hydro turbines, compressors and components working in sea water, the food industry and in medical technology; they represent the least expensive option of stainless steels. Ferritic steels For steels with a low carbon content (up to about 0.08 % of C), this steel is already ferritic at the chromium content above 17 %. Commonly used ferrite steels contain 17 to 30 % of Cr and 0.1 to 0.20 % of C. Purely ferritic stainless steels also include special steels with a reduced carbon content below 0.01 %, known as super-ferrites. Ferritic steels have good corrosion and fire resistance, but lower notch strength and high notch sensitivity. During solidification of these steels, chromium ferrite is eliminated from melt, which is not further transformed with the decreasing temperature. The solubility of carbon in chromium ferrite is less than 0.01 %, therefore, practically all carbon present in steel is eliminated in the form of carbides. Therefore, ferritic steels with higher carbon content are fragile, and they are used as refractory steels. Steels containing around 17 % of Cr and with the carbon content below 0.08 % are used as stainless steels. These steels are applied in particular in the energy industry as parts of heat exchangers, air preheaters, recuperators and boiler components. High-alloy ferritic chromium steels are rarely used for castings. Austenitic steels 16

17 With a sufficient amount austenite-forming elements, especially Ni (Mn, C, N) the martensite start temperature is decreased below the room temperature even at a high Cr content in stainless steels. Then the steels have an austenitic structure. Currently, the Ni content of 9-12 % is used, while the content of Cr is from 18 to 20 %. The effort to replace deficient nickel led to austenitic steels, in which a part of nickel content is replaced by nitrogen, which increases the strength characteristics and nitrogen alloyed steels have excellent corrosion resistance. Compared to wrought steels, however, the nitrogen content in austenitic steels is limited to 0.20 %, namely with grades with carbon content up to 0.03 %. At high contents of nitrogen in castings, there is a risk of endogenous bubbles. Austenitic steels exhibit the greatest resistance to corrosion compared with martensite and ferritic steels. They are not suitable for environments containing sulphur oxides. Fully austenitic steels The chemical composition of steels ensures that castings have a fully austenitic structure. These steels are characterized by a high content of alloying elements and a low carbon content. With the exception of one grade, the sum of the alloying elements in the steels of this group may exceed 50 % to 60 %. Nitrogen is widely used as an alloying element in these steels, partly replacing nickel. Some steels are also relatively high alloyed with copper. Fully austenitic steels have similar strength properties as austenitic steels. However, ductility and strength of these steels is higher [1]. Austenitic-ferritic steels These steels, also called duplex steels, contain in the structure approximately the same proportion of ferrite and austenite. Of all the groups of stainless steels for castings, they contain the highest chromium content of %. Corrosion resistance of duplex steels is further increased by alloying with molybdenum and nitrogen. Austenitic-ferritic structure at the stated chromium content is achieved by nickel alloying at concentrations of from 5.50 to 8.50 %. Except of one grade of steel, austenitic-ferritic steels contain up to 0.03 % of carbon at maximum. The advantage of austenitic-ferritic steel is resistance to brittle fracture, because the ferritic region forms a barrier against propagation of cracks generated in the austenitic phase. 17

18 Refractory steels ČSN EN Steels for casting of this quality are made to a limited extent and only in specialized foundries. Standard EN classifies refractory steels according to the structure into ferritic, ferritic-austenitic and austenitic. The standard also states alloys based on nickel and cobalt. With regard to the higher carbon content, the production of refractory steels in foundries is usually easily manageable. 3.4 Special steels for wear resistant castings. High-alloy austenitic steels are not normalized in the standards ČSN EN; in operations, old ČSN or DIN standards are used. The principal alloying element in austenitic manganese steels is manganese, represented by 12 % or more, while the content of carbon is above 1.00 %, to improve mechanical properties, 0.70 to 1.20 % of Cr is also added. 4. Construction and the thermal mode of an electric arc furnace (EAF). Linings of electric arc furnaces. The development of EAF technology. [1,2] In steel foundries, the most common melting aggregate are electric arc furnaces. Currently, it is estimated that in steel foundries in the Czech Republic, there are about 50 EAF capable of operating with a burden weight of 4-18 t. 4.1 Construction and the thermal mode of an electric arc furnace The burden is melted in an electric arc furnace (EAF) by an electric arc which burns between the three graphite electrodes and the burden. The temperature of arches reaches C. The diagram of the metallurgical part of an arc furnace is shown in Fig The shell of the furnace vessel is welded from a steel plate and placed on a cradle, which allows tilting of the furnace. The furnace vessel has a charging hole and a tap hole. The charging hole is covered with working door operated mechanically in the case of older furnaces or hydraulically or electro-hydraulically in the case of modern furnaces. The working door is most often used for slagging off, charging fluxes, charging ferroalloys, and for other technological operations. The tap hole is located on the opposite side of the furnace, opposite the door. Behind the tap hole, a tapping trough is welded. A tilting device allows tilting of the furnace vessel in both directions. The furnace vessel is covered by a lid, which is controlled mechanically or hydraulically so that it is possible to deploy the furnace after opening of the lid by charging basket. 18

19 Fig. 4.1 A diagram of lining in an alkaline arc furnace 4.2 Lining of electric arc furnaces Lining of an electric arc furnace can be made in different variants of shapes and quality of refractories. For example, the hearth on magnesite blocks is tampered with grained magnesite. Vaults (covers) of electric arc furnaces in the foundry are now generally of magnesite and chromium or from monolithic alumina cast refractory concrete. For smaller types of arc furnaces (5-15 t), the lifetime of the cover is about melting processes. The lifetime of the cover depends partly on the manufactured range of materials, and also on controlling the power mode throughout the melt and the slag regime. Fig shows a diagram of lining in an arc furnace. Position no. 1 Tampered layer of magnesite mass, position no. 2 magnesite-chromium, or possibly monolithic alumina cover of the furnace, position no. 3 insulating lining, position no. 4 and no. 5 magnesite and magnesite-chromium working lining, position no. 6 steel furnace shell. The reaction between the lining and the slag Refractoriness of magnesite is reduced by oxides of iron and silicon which are commonly found in the charge. The greatest wear of the lining occurs in places that are in contact with the slag layer covering the molten metal, so called slag line. The slag saturated with MgO is unresponsive to lining. The reaction between the oxides in the slag and the lining can be affected by the composition of the slag so that the slag concentration contained preferably more than 10 % of MgO. However, a high content increased the viscosity of the slag, thereby bringing deterioration of the required metallurgical processes. The reaction between the lining and the molten metal The reaction between the lining and the molten metal can be schematically described by equation (4.1) and (4.2) 19

20 xmgo y MgO Me Me O x Mg Mg O y x Steel Casting Foundry (4.1) (4.2) According to equation (4.1), the reaction of magnesite with deoxidizing element dissolved after deoxidation in steel, generally designated as Me is given. Magnesite lining does not contains in particular after the oxidation phase pure MgO, but also other oxides that can react according to equation (4.1) with a residual concentration of deoxidation agent. These are oxides of the elements with lower activity towards oxygen than MgO, such as FeO, or SiO 2. The reaction then results in oxides which are present in steel as oxide inclusions. The reaction (4.2) may occur after deoxidation of steel in a vacuum when the oxygen activity in steel is lower than what corresponds to dissociation pressures of the oxides contained in the lining. 4.3 The development of EAF technology Electric arc furnaces are used both for the production of cast steel, and especially for the production of moulded steels. Gradual intensification of production technology in electric arc furnaces required substantial investment funds, particularly in the area of secondary metallurgy, which is one of the most important measures leading to increased productivity of arc furnaces. In this case, the arc furnace only serves to melt the burden. During melting and heating of steel, decarbonisation and dephosphorization takes place. The melting process finishes by slag-free tapping of the metal into the ladle. Other metallurgical processes (desulphurisation, fine alloying, etc.) are carried out already at one of the elements of the secondary metallurgy. Considering the productivity of Czech foundries, in the Czech conditions, there is not a realistic chance of return of investment in the entire secondary metallurgy. The purpose of potential realization of secondary metallurgical equipment in foundries is not to enhance the productivity of the furnaces, but to improve the quality of steel produced, which could not be achieved in any other way. 5. The production of unalloyed cast steel in alumina EAF. Progress of individual technological and metallurgical phases. The practice of melting. [1,2] 5.1 Technology of the production of unalloyed steels If we denote the time of the melt process τ, then we can divide it into the time of repair of the lining and charging (τ 1 ), melting time (τ 2 ), oxidation time (τ 3 ) and deoxidation (reduction) time (τ 4 ): τ = τ 1 + τ 2 + τ 3 + τ 4. From the time of melting and burden weight 20

21 (Q), we cane estimate a certain daily productivity of the furnace (G), on the assumption that the operation is continuous: Q G 24 t / den (5.1) Repair of the furnaces and charging τ1 The period of repair and charging begins at the end of tapping of the previous melt and finished by turning on the oven. After the necessary repairs to the furnace lining, the burden is prepared. The feedstock can be steel scrap, recovered material, foundry crude iron and in manufacturing alloy steels also some alloying additives (FeMo, FeW, Ni, Cu). Normally, ore and lime are added to the burden to enable decarburization and dephosphorization already during melting. The foamed slag, which in this case is formed during melting, also increases heat transfer efficiency from the arcs to the burden Melting burden τ 2 The melting period begins by turning on the furnace and finished by sending the first test for chemical analysis. Part of the electrical energy may be replaced by burning gas available today in the oxygen-fuel burners and exothermic reactions while blowing oxygen into the bath in the furnace. The energy regimen of the furnace is based on the choice of optimum arc length and the furnace input power. During melting, solid burden decreases in the furnace, thus exposing the arcs with consequent lowering efficiency of heat transfer to the bath. For this reason, it is appropriate to create foamed slag in the furnace, in which the arches are hidden. Foaming slag is achieved by addition of ore, lime and carburizing agents to the burden or the empty furnace. Melting time depends mainly on the furnace transformer input power. Manufacturers of small arc furnaces (up to about 15 t) state the actual consumption of electric energy for melting steel about 450 kwh/ton. Simplifying estimate of melting time can be made based on the installed capacity of the furnace transformer (P) according to the relation: 450 Q 2 (5.2) P k cos where Q is the burden weight in tonnes, k is the utilization coefficient of the furnace transformer indicating what parts of the installed furnace performance is used on average during melting. Product P.k.cos then determines the average power input supplied to the 21

22 furnace during melting. Under these conditions, after substituting into formula (5.2), the resulting melting time for burden of 7 t and the average installed power of the furnace transformer 3 MVA is approximately 1.5 hours Oxidative period τ 3 The oxidative period begins by the first test sampling for the chemical analysis and finished with slagging and putting in oxidation additives. The most important activity of the production phase is decarburization and dephosphorization of the bath, reducing the amount of hydrogen or nitrogen in steel, and heating the bath to the tapping temperature. For oxidation, iron ore or oxygen gas are used in steel arc furnaces. After the addition of ore and lime to the burden, mainly the oxidation of Si, Mn, P, and C takes place during melting. The order in which the individual elements will be oxidized depends on their affinity for oxygen, and their activity. The slag composition also has a significant impact both on the composition of the resulting oxides and also on the equilibrium activities. If the concentration of elements with high oxygen activity drop to trace amounts, the activities of elements with lower affinity for oxygen will be decisive for determining the oxygen activity. Decarburization reaction When using ore to carbon oxidation, the following reactions take place: I. Dissolution of ore in slag Fe2O3 Fe 3 FeO, II. The transition of oxygen from slag into steel FeO Fe O, III. Decarburization reaction at the slag metal interface C O CO, IV. Forming bubble nuclei under the interfacial slag metal interface. The partial pressure of carbon monoxide increases with a depth of the bath. In terms of the partial pressure of CO, the most favourable conditions for the formation of carbon monoxide bubbles on the surface of the bath of molten steel. Boiling may start at the surface layers of steel under the surface of slag, V. The diffusion of carbon and oxygen atoms on the surface of CO bubbles, VI. The growth of bubbles due to the course of merging of carbon and oxygen on the surface of bubbles and their flowing out of steel carbon boil, 22

23 At low activity of oxygen in steel after melting, carbon boil does not occur. During the entire period of oxidizing, if carbon boil occurs, the oxygen activity is controlled by carbon. The importance of carbon boil - degassing steel, i.e. reducing the hydrogen and nitrogen content in steel - thermal and chemical homogenization of steel - controlling the activity of oxygen during oxidation Dephosphorization of steel The customer may require the phosphorus content below % or even lower. In accordance with the molecular theory of slag, the dephosphorization reaction can be described by the equation: [ ] ( ) ( ) ( ) [ ] 2 P + 5 FeO + 4 CaO = 4CaO.P 2O5 + 5 Fe The equilibrium constant of the reaction can be expressed as: N 4CaO.P 2 O K log K 15, P N N T FeO CaO (5.3) (5.4) The concentration of oxides in slag are expressed as molar (atomic) fractions N, the phosphorus content in steel is expressed in weight percent. From the above equations it follows that the dephosphorization occurs at the slag metal interface. The oxidative period finishes with the oxidation test sampling and slagging The period of completion (deoxidation, refining, reducing period) 4 The period of completion starts by throwing deoxidizing agents into the furnace after slagging and lasts until the end of tapping. The main tasks of the period of completion: Deoxidation of steel in the furnace, i.e. reducing the oxygen activity in steel and slag to a value that is suitable for desulphurization, fine alloying and final deoxidation of steel in the ladle. Steel desulphurization. Fine alloying of steel. Adjusting the tapping temperature. Maintaining the hydrogen and nitrogen content below the required concentration. 23

24 Steel deoxidation The applied ways of technology can be divided into precipitation (deep), extraction (diffusion) deoxidation, deoxidation under reduced pressure (in vacuum) and deoxidation by synthetic slags. Precipitation deoxidation Precipitation deoxidation is a process during which the oxygen activity is reduced by adding elements with high affinity for oxygen due to their reaction with oxygen dissolved in steel. The product of precipitating deoxidation are oxides are in solid, liquid or gaseous state, which are thermodynamically stable at temperatures of metallurgical processes. A part of the solid and liquid products of precipitation deoxidation remains in steel as inclusions, some of them float out and subsequently pass into slag. In the production of cast steel, mostly aluminium and silicon are used as a strong deoxidizing element for precipitation deoxidation. In the residual content of aluminium is sufficient, the product of deoxidation in cast steel is aluminium oxide: Al 3 O Al O G , 25T (5.5) Al and Si deoxidation may be combined with the use of other elements such as Ti, Ca, rare earth elements, ferroalloys, etc. Extraction (diffusion) deoxidation In all the described processes, the product of deoxidation are inclusions, which reduce the purity of steel. The principle of extraction deoxidation of steel is deoxidation of slag and reducing the oxygen content in steel and its transfer into slag. The activity of iron oxide in slag can be usually reduced by the addition of coke or ferrosilicon. Coke is the cheapest deoxidizing slag additive. Deoxidation of slag can be described by the equations: ( FeO) + C = CO + Fe FeO + Si = SiO + 2Fe 2 2 g (5.6) (5.7) When using coke for deoxidation of slag, it is necessary to consider the possibility of steel carburization, the use of ferrosilicon in turn can increase the silicon content in steel. Deoxidation products remain in slag, therefore inclusions do not form in steel. Deoxidation of steel in vacuum In the case of vacuum processing of steel, it is beneficial to use the reaction between carbon and oxygen in the conditions of reduced pressure. For the equilibrium between carbon and oxygen, the equilibrium constant can be expressed by the equation: 24

25 K C,O pco a a C O (5.8) Reducing the partial pressure of carbon monoxide leads to a reduction in the equilibrium oxygen activity in steel. In deep vacuum, carbon at higher temperatures is a very strong deoxidization agent, and it can reduce even the most stable oxides, such as MgO or CaO. The product of deoxidation of steel is gaseous carbon monoxide practically insoluble in melt, which leaves the bath in the form of bubbles. Deoxidation of steel by synthetic slags Steel refining by synthetic slags leads to deoxidation of steel, provided that the activity of FeO in slag is lower than the oxygen activity in steel, which is a condition of diffuse deoxidation. Most used are slags based on Al 2 O 3 CaO SiO 2, which, in accordance with the ternary diagrams, form compounds with a low melting point according to the chemical composition, mostly in the range of C. Consequently, slag with high fluidity is formed, which makes prerequisites for good desulphurization of steel from a kinetic viewpoint. Desulphurization of steel In the oxidation period, the sulphur content can be reduced by %. A prerequisite for desulphurisation in the oxidation period is strongly alkaline slag. The lowest contents of sulphur are obtained in the electric arc furnace after deoxidation of steel. Desulphurization reaction can be described by the equation of molecular theory of slags: CaO S Fe = CaS FeO K S a a CaS FeO CaO S (5.15) According to the molecular theory of slags, desulphurization takes place at the slag metal interface. For desulphurization, it is necessary to work with alkaline well deoxidized slag. The FeO content in slag should preferably be below 1 %. The condition for the course of desulphurization is a low activity of sulphur in slag. However, slag must remain reduction and alkaline all the time. a a Fine alloying of steel The chemical composition of cast steel is prescribed by standards and it is contained in the purchase agreement. For each alloying element in a specific interval of concentrations and for sulphur and phosphorus, usually their maximum contents. In the case of unalloyed 25

26 steel, fine alloying means additive of carbon, manganese and silicon, or possibly any of the micro-alloying elements, for example niobium, vanadium, titanium, etc. Temperature measurement For measuring the temperature of steel, thermocouples of the type Pt-Pt-PtRh10 or PtRh13 are used most often. Measuring the temperature of liquid steel today is mainly based on the thermionic phenomenon called Seebeck effect. The hot junction of the thermocouple is formed by welding platinum wire with platinum wire containing 10 or 13 % of Rh. 6. The technology of the production of alloy cast steel in alkaline EAF. The function of the main thermodynamic conditions of metallurgical processes. [1,2,3] 6.1 Classification of cast steel For most types of wrought steel, there is also a variant of cast steel. Properties of cast steels, however, are largely influenced by the conditions of solidification, i.e. especially the cooling rate and chemical heterogeneity generated during solidification and cooling of steel. They are different from wrought steels despite the same chemical composition if a melting sample. Cast steels generally have lower values of mechanical properties (strength, ductility, etc.), which is connected not only with segregation of some elements and coarser grains, but also with the appearance of micro shrinkages and sags generated during solidification. The advantage of cast steels is that they enables virtually unlimited combination of alloying elements and their amounts with respect to the following processing of castings, and also achieving the final shape of the product (casting) by casting. The classification of cast steels is usually based on chemical composition, or according to the content of alloying elements. Standard ČSN EN divides steel into unalloyed steels, low alloy steels, and high alloy steels. 6.2 Production of low alloy steel in alkaline arc furnace The basic task of preparing burden for steel production is economical processing of return alloyed waste. The thing is that during oxidation, oxides of some elements, such as chromium and vanadium pass into slag, during slagging, the above mentioned elements 26

27 are lost. In the oxidation of steel by ore, depending on the activity of the individual elements, silicon is oxidized first, then manganese and chromium. In the presence of more than 0.50 % of Cr in the bath, the possibilities of dephosphorization are therefore limited. 6.3 Production of high alloy stainless steels in alkaline electric arc furnaces The influence of carbon on the properties of stainless steels Stainless steels are characterized by a low carbon content. Except for chromium martensitic steels, the carbon content lower than 0.07 % is increasingly required. Carbon forms carbides with chromium, which reduces the content of chromium in solid solution, it has a higher diffusion speed than chromium, and therefore the formation of carbides may be associated with heterogeneity of the solid solution. The practical result of the exclusion of chromium carbides at the grain boundaries is intergranular corrosion. Intergranular corrosion occurs especially after the welding in the heat affected zone. Austenitic steels tend to succumb to intercrystalline corrosion, depending on the carbon content of steel and the temperature. To prevent it, the carbon content is reduced to a concentration that is equal to or less than the solubility of carbon in the matrix at temperatures at which carbides are still separated. The sufficiently low carbon content in austenitic steels, which eliminates extensive intergranular corrosion, is considered the carbon content below about 0.03 %, and also alloying of steel with elements, whose affinity for carbon is higher than that of chromium, i.e. the stabilization of the melt, usually Nb, Ta and Ti. The main directions of development of stainless steels Desired development of stainless steels may be characterized by a requirement to increase resistance to corrosive environments containing chlorides decreasing the content of carbon and sulphur by alloying with nitrogen and increasing the purity of steels. The requirements for modern stainless steels: - High concentrations of elements increasing in resistance to pitting, which is indicated by the value of PRE (Pitting Resistance Equivalent), with a value equivalent PRE 40 27

28 - The carbon content below 0.03 % (or 0.02 %) in the austenitic and dual phase steels, sulphur content below %. - Nitrogen alloying up to % for cast steels. It is used to substitute nickel in austenitic steels where corrosion resistance is also increased by nitrogen. - High purity of steel achieved by secondary metallurgy processes. - Production of three-phase duplex steels, e.g. austenitic-ferritic steels. Austenitic steels, which satisfy the above mentioned requirements on PRE and the contents of C and S are also called super-austenitic. Nickel is partly replaced with nitrogen, which also suppresses the adverse effects caused segregations. In some literature, the evaluation of the resistance of steel against pitting by the value of P. I (Pitting Index) is used, in which it is essentially the same criterion as the equivalent of PRE (determination of the index value P. I is based on the same relationship as the equivalent of PRE). Austenitic-ferritic steels with 50 % of ferrite are called duplex steels. The requirements for refractory steels The properties of refractory steels must be high resistance to oxidation, corrosion and long-term stability properties in the hot gases. Oxidation resistance at high temperatures is achieved by alloying steels with Cr, Si, Al, Ni, and their composition is similar to that of stainless steels. According to the structure, they are divided into ferritic, ferritic-austenitic, and austenitic. Heat-resistant steels generally have a higher carbon content than stainless steels, usually in tenths of a percent Fundamentals of the stainless steel production The main metallurgical tasks in the production of stainless steels: - The production of steel with a low carbon content. From the viewpoint of achieving low carbon contents, it is necessary to oxidize the melt to as low carbon content as possible with chromium losses that are economically feasible, and in a later stage of melting, to minimize carburizing steel by graphite electrodes. 28

29 The smaller the ratio between the weight of melt mass and the weight of graphite electrodes and also the longer the period from the end of the oxidation to tapping, the greater carburizing the bath The function of the main thermodynamic conditions of metallurgical processes Thermodynamic conditions of oxidation of carbon in melts rich in chromium In the production of stainless chromium steels in EAF, blowing oxygen gas is used for oxidation. In the production from return waste of high alloyed steels, chromium oxidizes first, and then, at sufficiently high temperatures of the bath, carbon oxidation occurs as well. Oxidation of chromium produces oxides whose composition depends on the concentration chromium in steel. When the chromium content in the melt is up to about 9 %, oxide Cr 2 O 3 results from oxidation; at higher concentrations, oxide Cr 3 O 4 is formed. For the case of thermodynamic equilibrium and the content of chromium in the melt up to 9 wt. %, oxidation of carbon and chromium can be described by the equations (6.1) and (6.2) with the tabulated values of free enthalpy. Summation results in the relationship (6.3), which expresses the reduction of chromium oxide with carbon to form carbon monoxide and chromium dissolved in the melt. Similar to expressing the chemical reaction equation by summation, we also express the standard free enthalpy by the equation. 2 Cr 3 O Cr 2 O 3 G ,92. T 3 CO 3 O 3 C G 2 3 ( ,98. T) (6.1) (6.2) Cr 3 CO Cr O 3 C G G G ,86. T (6.3) The equilibrium constant of the reaction can be expressed as: a ln K ln a Cr 2 2 O 3. a. p 3 3 C Cr CO (6.4) During oxidation of melts with high chromium content, slags are saturated with chromium oxides, therefore their activity can be regarded as equal to one. The chromium content in the burden is governed in particular by the requirement that the quantity of ferrochromium added to fine alloying after oxidation was sufficient to cool the bath. In the case of steel having the desired composition of about 18 % of Cr, the most 29

30 frequently chosen burden composition results in the chromium content in the melt of approximately 13 %. During oxidation, the chromium content decreases by from 2 to 3 %. In the subsequent reduction, about 1 % of Cr passed from the slag back into the bath. This consideration of the technological progress is based on the assumption of production of melts rich in chromium only in the EAF, without the possibility of processing in secondary metallurgy aggregates. Influence of the temperature on thermodynamic equilibrium The preparation of burden is usually determined so that after melting the carbon content is about 0.5 %. With the increasing carbon content in the burden, the temperature at which carbon boil occurs decreases. After the start of blowing, oxygen first reacts with silicon, then chromium, and subsequently with carbon. Starting of the carbon boil as soon as possible after the beginning of blowing is important, because before the beginning of carbon boil, chromium is oxidized, which results in a loss of chromium and furthermore, it prolongs melting and increases oxygen consumption. For the chosen conditions of the beginning of oxygen blowing, i.e. 13 % of Cr and 0.50 % of C, the equilibrium temperature can be calculated, which may be a guide for determining the beginning of oxygen blowing. Below this temperature, there are no thermodynamic conditions for the reaction of oxygen and carbon dissolved in the melt. After achieving this temperature, oxidation of the components with a higher affinity for oxygen (Si) occurs, which is followed by chromium oxidation. Only when the heat released during the exothermic reaction between oxygen and the elements in the melt (Si and Cr) raises the temperature of the bath above the equilibrium temperature, the carbon reaction occurs. Practical experience confirms that carbon boil with the carbon contents of 0.50 % begins at the temperature ranging from 1600 to 1630 C. Influence of the pressure on thermodynamic equilibrium According to equation (6.4), the value of the equilibrium constant depends on the activity of carbon monoxide or on the partial pressure of carbon monoxide in the gaseous mixture, which is in equilibrium with the melt. In practice, the effect of the carbon monoxide partial pressure in bubbles emerging from the bath on the carbon equilibrium in steel with oxygen is used to achieve low carbon contents in steel. The carbon monoxide partial pressure p CO in bubbles depends both on the concentration of carbon monoxide % V CO in bubbles and the total pressure on the surface of steel p. At low pressure and high 30

31 temperatures, it can be assumed that the gas mixture which is in equilibrium with the melt, acts as a mixture of ideal gases. In this case, the activity of the carbon monoxide in the mixture can be expressed for a constant temperature by the equation: p CO n V CO p n 100 i % CO p (6.5) The partial pressure of carbon monoxide in the gaseous mixture can be reduced either by decreasing its concentration in the gaseous mixture (% V CO ), or decreasing the total pressure of the mixture (P) above the bath surface. To reduce the concentration of carbon monoxide in the gaseous phase, which is in contact with steel, the technology of blowing a mixture of inert gas and oxygen into the bath is used in practice. Reduction of the total pressure (P) above the surface of steel is achieved in vacuum metallurgy equipment. For steel foundries, secondary metallurgical equipment using reduced pressure is an investment with difficult access. 7. Intensification of steel production in EAF. Contemporary trends in steel production in EAF. Intensification of the steel production allows to reduce in particular the processing costs involved in the costs of material in the casting. Intensification of steel production in arc furnaces in foundries makes sense when all normal precautions to reduce costs are exhausted: Burden is quickly loaded to the furnace. Avoiding any downtime during the time between switching on the furnace after loading until the tapping. In terms of maintenance, special attention is paid to the adjustment of regulation and control of other elements of the furnace. Foundry operates only the smallest number of furnaces required to provide the liquid metal production. Operation of furnaces is motivated to maintain a reduction of selected technical - economic indicators. 7.1 The development of equipment and technology used in electric arc furnaces A suitable return on large investments may be in implementing the intensification measures in metallurgical steel mills at EAF furnaces in furnace equipment and technology, in their productions in a hundreds of thousands to over one million tons. 31

32 Electric arc furnaces of new construction and equipment are now metallurgical aggregate designed for the mass production of steel. Intensification of EAF was achieved, in particular, by improving the performance of the furnace transformer, increasing the supply of energy in the form of heat of exothermic reactions and heat from the oxy-fuel burners. Another measure is the use of heat from the exhaust gases from the furnace. Melt time is only determined by the time at which sufficient energy is introduced into the furnace to melt the burden and achieve the desired heating of the bath. 7.2 Contemporary trends in steel production in EAF New design elements of EAF, which have been developed in recent years, aimed to: improve the quality of steel produced reduce production costs - especially for small EAFs optimize the use of energy - to reduce the consumption of primary electricity and increase flexibility in the choice of energy reduce noise and pollution in the production of electrical steel The basic principles, which are currently used in these newly developed techniques, can be summarized in the following measures: heat recovery from outgoing exhaust gases (furnace gas) for preheating the burden (steel waste), use of carbon and oxygen, the oxidation of carbon as an additional energy source in EAF, use of CO combustion in the furnace or preheater steel waste chamber, partial replacement of electricity with the energy from the oxy-fuel burners. The introduction of modern methods of secondary metallurgy fundamentally affected the changes in the design and manufacturing technology of EAFs. Arc furnaces operating in tandem with a ladle furnace are only used as a melting unit, in which decarburization and dephosphorization is made. When tapping is started, oxidizing slag remains in the furnace. This creates favourable conditions for deep desulphurization and deoxidation of steel in ladle. Melt time is limited only by the speed of heating the metal to the tapping temperature. 32

33 Design changes made recently in modern arc furnaces completely alter the profile of the whole arc furnace. Fig. 7.1 is an example of diagram representation of the EAF contemporary design. Fig. 7.1 Schematic representation of the EAF contemporary design, on the left furnace with bottom tapping, on the right furnace with burners, equipment for foamed slag and bottom tuyeres Legend: koks - coke 8. The production of steel in acid EAFs. Advantages of acidic EAFs versus alkaline EAFs. [1,2] 8.1 The production of steel in acid electric arc furnaces Literature suggests that acid arc furnaces are used in American and Russian steel foundries, while in Europe, acidic arc furnaces are not very common. In the Czech Republic, acid EAFs are operated. Lining of acid arc furnaces Acidic linings based on silicon dioxide are thermodynamically less stable than alkaline linings based on MgO, they react with components with a higher affinity for oxygen than that of silicon (Al, Ti, and at higher temperatures also with carbon). In the case of reducing the oxygen activity below the value corresponding to equilibrium with silicon under the given conditions, it may lead to the reduction of the silicon dioxide from the lining, which is accompanied by increase in the oxygen and silicon concentration in the molten steel. Therefore, in acidic furnaces, the oxygen activity cannot be reduced to the values achievable in alkaline furnaces, which has a negative effect on the values of mechanical properties. Elements dissolved in steel that form basic oxides also react with the lining. These elements include in particular manganese. 33

34 8.2 The advantages of acid electric arc furnaces Steel production in acid EAFs is characterized by up to 20 % lower processing costs resulting from shorter time of melt, which enables an easier technological process of melting and thus the possible savings: in electricity consumption, which is also connected with lower thermal conductivity of acidic linings and electricity costs can be reduced by up to 20 %, in the consumption of graphite electrodes by up to 25 %, in labour costs, in cost of refractory material, in consumption of non-metallic additives, lower costs of transporting and depositing slag, the production technology in acidic EAFs works with smaller quantities of slag. In the production of steel in acid arc furnaces, we achieve a lower hydrogen content, therefore the volume of casting defects caused by bubbles and pinholes is generally lower. The principal disadvantage especially in today s possibilities in the field of external metal waste of acid EAFs is a techno-economic unreality of dephosphorization and desulphurization on this unit. The production range of steels for acid arc furnaces is limited to unalloyed and low alloy steel grades. 9. Construction and thermal regime of work of medium frequency electric induction furnace (EIF). Ramming EIF refractories. The practice of melting. 9.1 Production of cast steel in electric induction furnaces For the production of steel castings of lower and medium weight, which are produced on automatic moulding lines, an essential requirement is the continuous supply of molten metal. For the production of melt, electric induction furnaces are irreplaceable steelmaking units. The time of melts on a modern induction furnace may be shorter than one hour. From the viewpoint of the production assortment, a limitation in foundries is the production of alloy steels, which cannot be economically produced in EAFs. This is the case of stainless steels with a carbon content below %. From a structural point of view, electric induction furnaces can be classified as follows: Channel furnaces 34

35 Crucible furnaces mains frequency medium frequency high frequency Channel type electric induction furnaces are used in foundries of iron and non-nonferrous metals as maintenance furnaces. They are powered by the mains frequency. The principle of heating metal prevents to use in these furnaces for melting in steelmaking, and their specific electrical power is also limited. Medium frequency electric induction crucible furnaces (EIFK) are built mostly from a few dozen kilograms up to melt weight of 25 tons. Furnaces weighing kg are common in precision casting foundries. High frequency electrical induction crucible furnaces are used as laboratory furnaces operating with the charge of a few grams to several hundred grams. They are used for melting most technical metals. Medium frequency crucible furnaces are now a common melting unit in steel foundries, and other types of induction furnaces are not built any more. 9.2 Medium frequency electric induction crucible furnaces Technological advantages of EIFs compared to EAFs: They enable to the supply melts of lower weight, usually 1-6 tons, to the casting bed at intervals of 40 to 120 minutes. They almost continuously provide the moulding shop with liquid metal. The induction stirring of the melt causes thermal and chemical homogeneity of the melt. Made steels generally contain less hydrogen and nitrogen. During the melting process, no carburization of metal occurs. EIF allows to produce steel with the lowest carbon content to below 0.03 wt. %. Operational commissioning of EIFs Economic benefits EIFs compared to EAFs: Lower energy consumption for the production of liquid metal at the same productivity. Low melting loss of iron and alloying elements and their increased use from recycled material and external alloyed waste. Lower consumption of ferroalloys achieved by alloying to the lower limit of the permitted range of alloying elements. 35

36 Lower consumption of non-metallic additives. At the same productivity of aggregates, approximately half the weight of the cast, which decreases lower investment costs of auxiliary and service facilities. Lower costs of solving production and working conditions. Lower consumption of refractory material. Lower cost of depositing waste. The environmental advantages of EIFs compared to the EAFs: Less noise. Lower emissions. Less solid waste associated with the operation of the furnace The design and equipment of medium frequency electric induction furnaces Requirements for increasing the productivity of EIFs necessitated the need for mechanized loading of furnaces. Vibrating troughs are used most frequently for loading. Requirements for high performance of furnaces also led to the mechanization of making lining. When using hydraulic displacement of worn lining of the crucible and the use of vibrators in ramming a new lining, it is possible to prepare a new lining of the furnace during steel production about 2-4 hours after tapping the last melt of the preceding melt campaign. Alternating current of medium frequency is conducted to the crucible by copper strips. Between the strips and the furnace inductor, current is conducted through cooled copper cables. The inductor (coil) is formed by a copper pipe (or another profile), to which cooling water is supplied. A diagram of the furnace is shown in Fig. 2.1 Electrical power supplied to the inductor becomes an alternating electromagnetic field inducing eddy currents in burden, which heat and melt it. Outside the inductor, the magnetic field is routed through packets of transformer sheets, which also shields the furnace structure. 9.3 Ramming EIF refractories Furnace crucibles were rammed from acid, alkaline or neutral ramming masses. Lining must not have gaps or cracks. The inductor consists of a water cooled copper coil, electrically insulated. Between the individual windings of the coil the voltage drop occurs. 36

37 The electrical interconnection of the adjacent turns of the coil with metal or condensed moisture leads to inter-coil short circuit. Burning the coil through and the subsequent penetration of water into the lining can cause a leak beneath metal furnace, or even an explosion in the crucible. After each melting process, the crucible lining must be checked and in case of detection of cracks, cavities or extreme wear, the crucible is put out of operation. Acidic ramming mass of crucible electric induction furnaces In steel foundries, acidic lining of induction furnace crucibles are most frequently used. On acidic linings, it is possible to produce virtually all types of conventional stainless steels. Similar to the steel produced in acid EAFs, unalloyed and low alloy steels produced on an acidic lining are characterized by lower impact strength. Similarly, for high alloy steels, in steelmaking on acidic lining, the same strength is not achieved as in the case of the production in alkaline arc furnaces. Despite these shortcomings, steel melted in the induction furnace with acidic lining has been most frequently used in the Czech Republic so far. Alkaline and high alumina ramming masses of crucible electric induction furnaces Alkaline ramming materials are mostly produced based on magnesium oxide. Adding alumina into magnesia ramming masses decreases the lining melting point, but increases the resistance of lining to cracking. Magnesia lining contain about 10 to 25 % of corundum. High alumina linings contain 20 to 50 % of MgO. The melting point of corundum linings is lower than that of magnesia linings, but they have a greater crack resistance. 9.4 Melt management practice Melting time on electric induction furnace can be divided into the melting time and finishing: 1 2 (9.1) Where 1 melting time, 2 time of completion. 1 depends on the installed power of the converter P nom. [kw], then the useful power of the converter Puž. k.p nom. (9.2) 37

38 where k the coefficient of the converter power utilization. The coefficient k value at the beginning of melting is about 0.7 to 0.8, in the final stage of melting, its value is nearly 1. In modern induction furnaces, the convertor utilization almost unitary throughout melting burden. To calculate the melting time, the following relation can be used: i. Q 1 P už. (9.3) Where i is the energy required to melt one ton of burden [kwh.t -1 ], Q is the burden weight [t]. The energy required to melt the burden represents the heat content of metal upon melting, i.e. the change in enthalpy of metal ΔH in the interval of the starting temperature and the liquidus temperature, including the total losses of the furnace throughout the melting process. The melting phase τ 1 begins by turning on the furnace and ends by melting burden (after the first test sampling). The phase of completion 2 then includes the times required for test sampling, sending it to the lab, waiting for the results of chemical analysis, for calculating the weight of added ferroalloys and deoxidizing additives, their weighting and adding to the crucible, to adjust the temperature, tapping, or possibly repairing the lining, up to turning on the oven for the following melt. Substituting equation (9.3) into equation (9.1), we can, by its modification, get the relation for the daily productivity of the furnace in the form: Q. k. P G 24 i. Q P nom. nom... k 2 (9.4) 38

39 10. The technology of the production of unalloyed and alloyed steels in EIF [1,2] 10.1 The production of unalloyed and low alloy steels on acidic lining The manufacturing processes for steel production in EIFs cannot be managed, primarily from the perspective of the economy, in a similar way as in arc furnaces because in EIFs, it is not possible to create appropriate conditions for certain metallurgical reactions taking place under sufficiently liquid, warm and active slag of suitable composition. EIF slag is cooler than metal, and has the function of a cover (it is not active). To achieve the desired chemical composition of steel, it is particularly necessary to prepare an appropriate composition of the burden. The burden is constructed from recycled material, external purchased steel scrap and alloying. In acid EIFs, slags consist mainly of oxides of silicon and iron. Slags with a high iron oxide content are aggressive with respect to lining. Silicon in the bath should be kept at a content of around 0.3 % and above in order to reduce oxides formed. At low temperatures, silicon dissolved in steel determines the oxygen activity in steel and in slag. The temperature increase increases deoxidation ability of carbon and when reaching the critical temperature depending on the chemical composition of steel, carbon boil occurs The production of high alloy steels on acidic lining High alloy steels produced in induction furnaces are most frequently corrosion-resistant and refractory steels. Especially for stainless steels, the carbon content generally less than 0.07 % is required. On induction furnaces, it is possible to produce stainless steels with a carbon content lower than 0.03 %. Between the melt with a high chromium content and acidic lining, a reaction occurs and the following relation applies SiO 4 Cr 2 Cr O 3 Si G , 45T (10.1) Fig graphically shows thermodynamic equilibrium between chromium and silicon at the temperature of 1500, 1600 and 1700 C, provided that the slag is saturated with chromium trioxide and the lining consists of pure silicon dioxide. Thermodynamically, the reaction between chromium dissolved in steel and acidic lining at Henry s activity of chromium equalling to 18 (corresponding to approximately 18 wt. % of Cr) and the temperature of 1500 C can take place, if Henry activity of silicon in steel is less than 0.5 (approximately 0.3 wt. % of Si). Practical experience confirms these theoretical assumptions. During melting and maintain lower temperatures of metal, these steels generally contain about 0.40 % of Si at tapping temperatures of up to 1600 C and 39

40 reactions between acidic chromium and lining do not occur. At higher tapping temperatures of these steels of 1620 to 1670 C, it is no longer necessary to count with increasing silicon content in the steel. chromium activity silicon activity Fig Thermodynamic equilibrium between Cr and Si in an acidic crucible at the temperature of 1500, 1600 and 1700 C under slag saturated with Cr 2 O Production of steel with alkaline and high aluminate lining Alkaline linings contain more than 50 % of MgO. High aluminate (corundum - neutral) lining, containing more than 50 % of Al 2 O 3 and a remainder of MgO. Linings of this type are formed by oxides having higher thermodynamic stability than SiO 2. Steel produced in these types of linings has lower oxygen activity and higher strength after deoxidation in the ladle. The reaction between the lining and the molten metal is therefore negligible. Economic benefits of the production of high alloyed steels alloyed with chromium are connected with achieving lower melting losses of chromium. 11. Deoxidation of steel in the ladle and casting. Final deoxidation of steel in the ladle and its effect on steel properties. [1,2,3] 11.1 Deoxidation of steel in the ladle and casting During casting, properties of castings are affected more than during the previous manufacturing operations in the melting unit. One of the major influences on the properties of castings is that of a final deoxidation of steel in the pan with aluminium, or possibly modification of steel with calcium or rare earth metals. Casting quality is also influenced by the method of preparation of the foundry ladle, its drying, heating, possible maintenance of the lining and removal of residual slag. The casting speed is to be controlled by the diameter of the nozzle. Reoxidation of metal during the casting stream 40

41 during mould filling can affect the formation of casting defects such as, in particular, sand inclusions, bubbles and pinholes. Foundry ladles used for transport and casting of steel Transport of liquid metal and casting is ensured by means of the foundry ladle (FL). In steel foundries, pans with bottom outlet and the plug cap are usually used. Steel is cast from FL nozzle, which closes by the stopper rod, and it has a graphite head in the lower part. Fig shows a view of a stopper mechanism with the cap and FL lining. Economically undemanding equipment is a major advantage of the plug stopper. Due to its thermal vulnerability, the plug system cannot be used for secondary metallurgy equipment, with metal heating or during vacuum degassing. For these devices, slide fasteners are used. Fig shows a diagram of the slide closure and tuyere blocks located in the bottom of the FL. These devices represent an expensive investment and their use in steel foundries has not been usual so far. Fig View of a plug stopper of the Fig A diagram of a slide closure and a foundry ladle tuyere block For lining FLs, acidic refractories with SiO 2 content, which is usually higher than 85 %, and a residual content of Al 2 O 3 are often used in foundries. Refractory material is mostly supplied in the form of blocks or ramming masses. For lining alkaline FLs, magnesite and high aluminous mass of a similar composition as for lining induction furnaces Final deoxidation of steel in the ladle and its effect on steel properties Metallurgical quality of steel affects the notch strength test and the transition temperature. These material characteristics are influenced by variations in chemical composition, purity of steel, the grain size and heat treatment. The achieved values of notch strength of optimally heat treated steel is mainly affected by the morphology, size and distribution of 41

42 inclusions. For the final deoxidation of steel in the LPs, aluminium is used. The result of the deoxidation reaction are aluminium oxides. If the residual aluminium is higher than %, which is necessary for most of unalloyed and low alloy steels to prevent pinholes in castings, stable oxide Al 2 O 3 is formed as a product of the reaction between aluminium and oxygen. Therefore, at the temperature of liquid steel, it will exist as a solid phase (melting point 2030 C), which will, according to Stokes Law, float from steel at a certain velocity, depending on the composition and size of the inclusions and the dynamic viscosity of the melt v 2 ρ g 9 2 ρ η 1 r 2 (11.1) v floating velocity of inclusions [cm.s -1 ], r - particle radius [cm], 1, 2 specific weight of inclusions, molten steel [g.cm -3 ], - viscosity of molten steel [P], a g gravity acceleration (981 cm.s -2 ). Non-metallic inclusions have considerable influence on the properties of steel. Morphology of inclusions depends on their chemical composition, which is determined by the conditions of deoxidation. According to the origin, inclusions can be divided into exogenous, whose origin is related to the formation of erosion and corrosion actions throughout the production process and casting. Endogenous inclusions are mainly the product of deoxidizing, desulphurizing reactions, but also reoxidation processes taking place throughout the course of steel production, including crystallization of cast steel. In terms of chemical composition, inclusions are mainly oxidic, sulphidic, which predominate, then, in lower quantity, oxi-sulphidic inclusions, nitrides, carbides, and a limited amount of silicates and aluminates. It is very important to classify inclusions according to the shape, as square, spheroidal and dendritic type. From the viewpoint of the temperature of formation of inclusions, they are divided into primary ones, which arise within the range of steelmaking temperatures, secondary ones, formed just above the liquidus temperature, and inclusions that arise between the liquidus and solidus temperatures are tertiary. Precipitation inclusions are formed below the solidus temperature. 42

43 Sulphide inclusions, which normally arise during steel solidification, are directly linked to the oxygen activity in steel and hence the oxygen content in the molten steel. Their morphology is determined by the type of deoxidizing agent. For precipitating deoxidation, pure aluminium is usually used, so its residual content determines the chemical composition of inclusions and the shape, and influences the mechanical properties of steel. Influence of the residual aluminium content dissolved in steel on the morphology of sulphide inclusions is shown in schematic Fig Type I, type II, type III, type Ib, type IV Fig Influence of the residual content of aluminium dissolved in steel on the morphology of sulphide inclusions Oxide inclusions are formed in each period of steel making and casting, while the largest amount is produced during deoxidation. If the strongest deoxidizing agent is Al, their morphology and the decomposition products resulting from deoxidation may be represented according to the diagram in clusters of Al 2 O 3 Fig Schematic representation of the effect of chemical composition of steel, depending on the method of deoxidation on the morphology of inclusions and the resulting deoxidation products 43

44 If the aluminium content of steel is less than 0.01 %, inclusions are eliminated in the liquid state, they are globular, and during solidification they disintegrate into oxides of the nmn.msio 2.pAl 2 O 3 type. If the aluminium content in steel is greater than 0.01 %, small crystals of Al 2 O 3 of dendritic character are formed. In the case of excessive content of aluminium, crystals of Al 2 O 3 form clusters. Reoxidation of steel The term reoxidation of the steel is used for oxidation of melt during tapping metal from the furnace and the subsequent period when steel remains in the ladle, casting, until solidification of steel. In steel oxidation, Czech terminology uses the term secondary and tertiary oxidation according to their chronology and temperature. Secondary oxidation of steel means processes which are associated with increasing concentration and activity of oxygen in steel after primary deoxidation in the furnace aggregate. Secondary oxidation takes place during tapping of steel, when the stream of molten steel is in contact with 20 to 30 times larger volume of air, and during the time when the rest of metal remains in the ladle. Tertiary oxidation is oxidation of steel between the liquidus and solidus temperature. 12. The possibilities of secondary metallurgy in steel foundries. Using various methods and principles of individual processes in secondary metallurgy. [1,3] The term secondary metallurgy (SM) includes a considerable number of options and types of technological processes that take place outside the melting unit, which is usually an electric arc furnace or electrical induction furnace in steel foundries. In this case, the melting phase, or possibly the oxidation phase takes place in EAFs or EIFs. Another phase of the reduction and completion occurs in some secondary metallurgy equipment. The objective is the ability to increase the productivity of the melting unit and create better conditions for example for deep deoxidation, modification of steel, or its desulphurization Methods of secondary metallurgy There are many different technological elements of secondary metallurgy. Some are already overcome by other modern developments in this field. The basic classification of the most frequently applied secondary metallurgy methods taking place outside the melting unit, whose processes are realized: 44

45 Processes taking place at atmospheric pressure AP (Argon Pouring) IP (Injection Process) SL (Scandinavian Lancers) LF (Ladle Furnace) AOD (Argon Oxygen Decarburisation) Processes taking place in a vacuum VD (Vacuum Degassing) VOD (Vacuum Oxygen Decarburisation) VAD (Vacuum Arc Degassing) ASEA-SKF RH (Rührstahl Heraeus) Processes without reheating metal Process IP, AP, SL, VD Procedures with reheating metal Process LF, VAD, AOD, VOD, ASEA-SKF The melt in the ladle or the converter is heated either by exothermic chemical reactions, or electrical heating. In chemical heating, reaction heat of aluminium, silicon and carbon oxidation is most frequently used. When steel is alloyed into the ladle or in the case of higher heat losses, metal is generally heated by electricity, with an electric arc most frequently used as a heat source, which is same principle as the three-phase electric arc furnace. The above mentioned classification is now rather formal and it is based on the history of the individual processes. Currently, various methods are combined in the existing facilities, and they are further developed. Metallurgical possibilities of some secondary metallurgy methods Applying secondary metallurgy in steel foundries is associated with the desired range of castings and also the foundry producibility. For certain foundries, only basic elements of secondary metallurgy may be suitable. In contrast, for some foundries, technically and 45

46 financially more demanding components of secondary metallurgy may be more preferable. SM processes currently include a large number of individual processes and their various combinations, which make it possible to achieve very low levels of undesirable elements, such as sulphur, hydrogen, nitrogen, etc., or which guarantee economic processing and the production of high alloy steels. Using the elements of SM is crucial especially in steel mills and metallurgical plants that process large volumes of material. In steel foundries, usually only technically and economically less demanding elements of SM are introduced Applying the individual methods and principles of the individual secondary metallurgy processes The aim of substantially all of the elements of SM is to convert all, or some of the operations taking place in the reduction period outside the melting unit, which is usually EAF in steel foundries. This can be achieved by a significant increase in its productivity and a simultaneous improvement of some metallurgical parameters compared to the usual process of steel production in EAFs. Secondary metallurgy methods operated at atmospheric pressure Secondary metallurgy methods at atmospheric pressure are carried out either in the ladle or the converter. The most common methods carried out in the ladle include refining metal with inert gas, or a combination with blowing dust ingredients or injection using filled profile, ladle and converter AOD and CLU processes. Refining metal with inert gases Blowing inert gas into the melt, referred to as AP (Argon Pourging) is the simplest SM process. The inert gas used is argon and in some cases, also nitrogen is blown. The said gases are under the circumstances considered to be inert. Blowing inert gas primarily results in thermal and chemical homogeneity of metal in the ladle. Inert gas is blown through a ceramic block. Argon in the form of bubbles represents atmosphere with the zero partial pressure of hydrogen and nitrogen in the melt. Gas, such as hydrogen in the bath and in the gaseous atmosphere (argon bubble) tries to get to a steady state by the process in which hydrogen diffuses from steel into the argon bubbles where it associates to molecules and along with 46

47 the argon bubble, it is floated out from the melt. This process is terminated when the hydrogen partial pressure in the gaseous atmosphere will correspond to a given concentration of hydrogen in the melt, i.e. at the moment of equilibrium between the gas mixture and the melt, provided that the Ar bubble with hydrogen remains in the melt for a sufficient time. The floating velocity of the bubbles depends on their size. The conditions are less favourable for reducing the nitrogen content in the melt than that of hydrogen due to impaired diffusion of nitrogen in the melt. Injecting dust additives with a nozzle SM components using injection of dust additives include, e.g. the SL method (Scandinavian Lancers), TN process (Thussen Niederrhein), or IP process (Injection Process). A nozzle is usually used to inject ground ferro-alloys containing calcium, silikocalcium, pulverized lime, and possibly also carburizing agents, or ordinary ferroalloys into steel in the ladle. The device is primarily intended for desulphurization, which uses mainly injecting pulverized lime. In this SM process, the content of gases, especially hydrogen increases. Injecting using a filled profile A filled profile is a thin-walled tube from a sheet steel with low carbon, typically with a diameter of 6-20 mm filled with dust, for example, SiCa, FeCaAl, Al, C, milled ferroalloys, etc. The diameter of the filled profile is chosen according to the weight of the melt, and also according to the type of the feeding device of the filled profile to the foundry ladle. For ladle with liquid metal weighing 4-8 tons, a profile diameter of about 8 mm is used. The profile is guided over driving pulley of the feeder and the steel guide tube into the steel ladle. The most common use of filled profiles is to modify inclusions to type Ib by SiCa, wherein the calcium utilization is not greater than 50 %, and also for the injection of aluminium. The ladle intended for injection of filled profiles is adjusted to homogenization blowing of inert gas. Injecting the filled profile that contains calcium is preceded by deoxidation of steel with aluminium. In the production of carbon steel, oxygen activity of up to 2 ppm can be achieved in the foundry ladle. 47

48 Profiles filled with other ferroalloys or alloying elements are used to correct the chemical composition of steel and for alloying elements having high affinity for oxygen, possibly also to correct the carbon content. Summarising the above mentioned SM processes implies that during this secondary metallurgy steel processing in the foundry ladle under atmospheric pressure without heating, steel temperature rapidly decreases. Even at high tapping temperatures, the described procedures of treatment outside the furnace usually cannot be performed longer than for 5-7 minutes. 13. Defects of steel castings. The causes of the various types of defects and ways of eliminating them. [1,2] 13.1 Casting defects Defects are often the result of imperfect and poorly controlled technologies. Checking during certain production phases is difficult also because there are no affordable and sufficiently accurate methods for monitoring of all parameters of the applied technologies. A casting defect means any variation in appearance, shape, size, weight, structure and properties detected by laboratory or other tests from the agreed technical specifications or standards associated with the manufacture the type of casting. Casting defects can thus be apparent and hidden. Under the current convention, the same deviation from the agreed quality of the casting between the manufacturer and the customer may be still admissible defect, or inadmissible, repairable or removable defect. The applicable ČSN (Czech national standard) distinguishes seven categories of defects in iron alloy castings. Defects of iron alloy castings, whose division into seven classes of defects and defect categories, along with the classification, explaining the main causes and suggestions for their prevention in the foundry production, are also described by Elbel [5] The causes of the various types of defects and ways of eliminating them The frequent defects in foundries for steel castings are classified as follows: surface defects, impressions, macroscopic inclusions and defects in macrostructure, microstructure defects, and defects in chemical composition including properties of castings. 48

49 Surface defects Defects of this type can be considered apparent defects, which can be detected during the inspection after blasting and cleaning the surface, or possibly using the magnifying glass. In most cases, defects are removable. In some cases, this requires negotiations between the purchaser and the manufacturer of the casting, including the determination of repair method. This class includes the majority of defect types. Fig shows a surface defect a flash. Flash Fig A fin in oil passage in the cast steel piece of a crankshaft Disrupted continuity Defects of the disrupted continuity type are divided into four types of defects: fissures, cracks, disrupted continuity due to mechanical damage and disrupted continuity related to unconnected metal. The cause of these defects is a number of parameters related to the manufacturing technology of casting design, foundry mould design, physical and material parameters of the produced steel in the solid and liquid phase, and also to the conditions of melting, casting and solidification. In this class there are most kinds of defects. Impressions Defects of this type belong mainly to the apparent, but also among hidden defects. The impressions in a casting may be either open (apparent), or sealed under the surface of the casting (hidden). Open impressions can be detected relatively easily. For the identification of sealed impressions, it is necessary to use special methods. Most often an ultrasound test, radiography using X-ray radiation or gamma radiation. Fig shows an impression a sealed endogenous gas holes. This defect was created from melt in an electric arc furnace base with an increased concentration of hydrogen and nitrogen. 49

50 Fig Shapes of sealed endogenous gas holes Macroscopic inclusions and defects in macrostructure Defects of this type include those that are hardest to identify and also most difficult to remove. The group of slug inclusion defects involves two types of defects: exogenous slug inclusion and secondary slug inclusion, and the group of defects called non-metallic inclusions simultaneously comprises six most frequent defects, such as sand inclusions, floated sand, coating falling off, oxidic covers, carbon covers and black spots. The group of defects called macrosegregation and exudations includes such types of defects as gravitational segregation, macrosegregation physical segregation, A segregations and V segregations [5]. Microstructural defects These defects usually involve limiting deviations of parameters of properties of casts from the agreed technical specifications and standards, and is not the case of standard casting defects. These are defects of the type microscopic impressions, which are further divided into sags, microbubbles, and micro cracks. However, for their identification, is necessary to adjust the casting surface at the site of the defect and to use reasonable enlargement a magnifying glass, or possibly stereoscopic or metallographic microscope. Another group includes defects called inclusions, wrong grain size, wrong content of structural components, hard spot, turbidity, reversed turbidity, surface decarbonisation and other deviations from the microstructure. Defects chemical composition and properties of castings 50