A New Approach in Solid State Steelmaking from Thin Cast Iron

ISIJ International, Vol. 58 (2018),ISIJ International, No. 10 Vol. 58 (2018), No. 10, pp. 1791–1800

Sheets through Decarburization in CaCO3 Pack

Ebrahim SHARIF-SANAVI, Mostafa MIRJALILI* and Jalil VAHDATI KHAKI

Department of Materials and Metallurgical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, 91775-1111 Iran. (Received on April 6, 2018; accepted on June 4, 2018)

In the solid state steelmaking process used in recent researches, pig iron was directly casted and then decarburized in an oxidizing atmosphere in order to eliminate or reduce the amount of carbon. In the pres- ent study, the feasibility of solid state steelmaking from cast iron was investigated using limestone. For this purpose, white cast iron specimens were buried in a chamber containing CaCO3 powder. Calcination of CaCO3 produces CO2 which participates in cast iron decarburization. In this technique, CO2 reacts with the carbon content of sample according to Boudouard reaction which results in carbon consumption. Furthermore, effects of temperature and time on the decarburization process were investigated. White cast iron samples were decarburized at temperatures of 800, 900 and 1 000°C for 10 and 24 h in the CaCO3 powder pack. Very fine oxide layer was also observed to form during the decarburization process. Finally, samples were studied with optical microscope and SEM to measure the depth of the decarburized layer. However, secondary graphitization was occurred during the decarburization. Results showed that decarburization at 1 000°C for 24 h has led to a completely decarburized layer of 420 μm thickness. In agreement, carbon analysis showed the reduction of carbon content from 3.16 wt% to 0.012 wt%. Kinetic studies revealed activation energy of 125 KJ/mol for decarburization of white cast iron using CaCO3 powder pack.

KEY WORDS: solid state steelmaking; decarburization; white cast iron; calcium carbonate.

observed that the rate of decarburization of cast iron strips 1. Introduction is controlled primarily by carbon diffusion in the austenite In the conventional process of steelmaking, oxidation phase. They found that dissolution rate of the cementite is process of molten pig iron makes inclusions and bubbles fast enough to maintain the local equilibrium concentra- in the liquid phase. Many efforts have made to eliminate tion between the cementite and the austenite phases at the the inclusions and bubbles. Moreover, the solubility limit interface.1,2) of oxygen in liquid iron is almost high; so during the pro- Furthermore, Sasaki2) was investigated commercial process of carbon removing, oxygen is easily dissolved in the duction of 0.5 wt% C steel by using a strip caster at Nucor3) molten iron. The advantages of solid state steelmaking (S3) to evaluate feasibility of the S3 process. According to processes are eliminating multiple steps (including BOF and McDonald4) investigations, solid state steelmaking process secondary refining), and absence of inclusions.1,2) is ideal for improvement an steel plant to reduce operating Considering environmental problems and economic cost and CO2 emissions, or to reduce the capital cost of a considerations in the steel industry, solid state steelmaking greenfield development. It is considered that the process was introduced by Sasaki et al.1,2) Sasaki’s research was needs to further researches to improve the process kinetics based on the decarburization of cast iron samples using a of solid state decarburization and enable solid state steelmak- decarburizing atmosphere. In their process, high-carbon ing process for decarburizing samples with 4 wt% carbon.4) molten iron was casted directly in form of thin sheets by Recently, production of layer integrated steel with dif- a centrifugal casting method. Then cast iron strips with ferent microstructures and properties with high strength 10 mm × 20 mm × 1 mm dimension were situated into and high ductility was investigated.5) Conventionally, layer a horizontal furnace and heated up to 1 248–1 327 K at integrated steel was produced by laminating several steel 6) H2O/H2 atmosphere. The eliminated carbon was calculated layers and using hot rolling or cold rolling. According to according to the equations of carbon diffusion in a homoge- the results obtained by Sasaki,1,2) a sheet with a three layer neous austenite plane sheet. The calculated decarburization structure can be easily produced that the surface layers have thickness then was verified by empirical investigation. They low carbon contents, in contrast with the inner layer which has initial high carbon concentration. * Corresponding author: E-mail: [email protected] The gaseous decarburization process of white cast iron DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2018-250 consists of elimination of solute carbon in austenite by sur-

1791 © 2018 ISIJ ISIJ International, Vol. 58 (2018), No. 10 face reaction, diffusion of carbon through austenite from the and high carbon iron decarburization were performed in center to the surface of solid sample and decomposition of one chamber for simplification. Moreover, white cast iron the carbon-rich phases.7) Decarburization reactions usually samples were used instead of casted pig iron for feasibility include a combination with one of the followings: study of the solid state decarburization process. For this purpose, white cast iron samples were put inside CaCO CCOC2 O ...... (1) 3 s 2()gg() powder pack for a while and the decarburized sections were characterized. CH s 2 24()ggCH () ...... (2)

CO s 22()ggCO () ...... (3) 2. Experimental In order to use an alloy with a composition close to that CH s 22OC()ggOH () 2()g ...... (4) of pig iron, cast iron samples containing 3.6 wt% carbon Many studies have been focused on the solid state decar- were used. Chemical composition of the utilized cast iron is burization of Fe–C steel which show there are many param- reported in Table 1. For this purpose, samples were casted eters affecting the decarburizing rate, such as the chemical in a shape of sequin with diameter of 4 cm and height of 6 composition,8–12) superficial coating,13) the thermal cycles of mm. Casted sequins were then solidified in a water cooling steel production14) and the atmosphere of the furnace.15–17) mold to achieve a white cast iron structure. Deng et al.15) have investigated influence of oxidizing, inert Before decarburization process, surface oxide scales were and reducing heating atmospheres such as O2, CO2, N2 and removed using SiC grinding papers until a desired surface CO on the decarburization depth of steel. According to their quality was achieved. After that, samples were placed into results, oxidizing gases, such as O2 and CO2 have increased the decarburizing chamber. The decarburization process was the decarburization depth, and the inert gas of N2 and reduc- conducted by using CaCO3 powder pack. For this purpose, ing gas of CO have strongly inhibited decarburization of samples were buried in a closed steel container full of 15) samples. CaCO3 powder, and then heated to elevated temperatures. Liu et al.16) have investigated effects of the temperature Figure 1(a) shows the schematics of decarburization cham- and oxygen concentration on the decarburization of 55SiCr ber. The height and diameter of the chamber was about 7 spring steel. They have decarburized samples in a muffle and 4.7 cm, respectively. Note that all samples were placed furnace (in ambient air) and also in a simultaneous ther- on top of the chamber and at the same position. The particle mal analyzer (in the atmosphere of 2% O2 and 98% N2). size of the used CaCO3 powder is shown in Fig. 1(b) which For samples heated in the ambient air, the decarburization indicates the average particle size of 258 nm. behavior was divided into four types: no decarburization, According to CaCO3 calcination reaction, CO2 is released only partial decarburization, only complete decarburiza- during heating: tion, and finally partial and complete decarburization. By CaCO CaOCO ...... (5) decreasing the oxygen concentration to 2%, only partial 32 (ss)()(g) decarburization was found. However, according to the CO2 then attributes in carbon removing from the speci- results of decarburizing in the thermal analysis, it was men surface, based on Boudouard reaction: impracticable to reduce the thickness of decarburized layer ...... (6) by decreasing oxygen concentration.16) CC()s OC2g 2 O()g 17) According to Baud et al. research, samples were oxi- Consequently, the weight of CaCO3 powder needed for dized at 700°C in the ambient air, dry air and moist air (31% complete decarburization of sample can be calculated using water vapor). In the ambient air, surface decarburization of stoichiometric values of above reactions. Since the weight the metal was measurable only after a period of 32 h. How- of cast iron sample was 7.5 g, the stoichiometric weight ever, in the presence of 31% water vapor, no decarburiza- of CaCO3 which is needed for total decarburization of the tion was observed, even for an oxidation time of 128 h. In sample is 2.25 g. However, CaCO3 content of chamber contrast, oxidation in completely dry air for 64 h revealed a was considered to be 50 g (almost 20 times higher than the huge decarburization; although, no surface decarburization calculated value). This overuse of CaCO3 is because of the 17) of the sample was observed for periods shorter than 8 h. probable CO2 leakage which does not participate in decarbu- 3 In the atmospheric S process, there is a competition rization process. Our measurements of CaCO3 weight decre- between oxidation and decarburization to occur. It is ment during the process and its corresponding mole of CO2 reported that at higher temperatures (above 1 200°C) oxida- indicated that most of the released CO2 has not participated tion rate is more than decarburization rate. Thus, the decar- in the decarburization reaction and left the chamber. burized layer can’t be observed because the decarburized To investigate effects of temperature and time on the layer oxidizes simultaneousy.18–21) solid state steelmaking process, buried samples were placed The aim of our study is to develop a method which can inside the furnace at temperatures 800, 900, 1 000°C for 10 be improved for solid state steelmaking. Based on the idea, when a limestone curing plant is established near a steelmaking process, it would be possible to use the outgoing gas Table 1. Chemical analysis of raw samples obtained by mass of limestone calcination. In this approach, the gas which is spectrometry (wt%). mostly CO2 can be directly used for decarburization of the Fe C Si PS Mn Cr Mo Ni Cu Ti thin high carbon iron plates which are continuously casted from pig iron. However, in this study limestone calcination 93 3.6 1.93 0.04 0.057 0.57 0.298 0.013 0.044 0.027 0.028

Fig. 1. (a) Schematics of situation of the sample in the CaCO3 Pack chamber, (b) Particle size analysis of used CaCO3 powder indicating average particle size of 258 nm. and 24 h. Since the chamber was closed, air penetration from furnace atmosphere towards the decarburization chamber was negligible. However, relatively positive pressure due to CaCO3 calcination inside the chamber can retard air contamination too. To determine thickness of the oxide layer formed during decarburization process, samples were weighed after decarburization. Then oxide layer was removed by using solution of 80% distilled water and 20% hydrochloric acid for 15 minutes at 60°C. So, samples were weighed again and according to the weight reduction, area and density of oxide compound, thickness of oxide layer was calculated by using Eq. (1).

mm12 d ...... (7) A where m1 and m2 are weight of the sample after decarburization process and acid cleaning, respectively. ρ is density of oxide compound (Fe2O3). A is the area of sample surface. Δd is thickness of the oxide layer. For measuring the decarburized thickness, samples were cut transversely. Thickness of the decarburized layer was measured using an optical microscope and image analyzing software (MIP software). To investigate the decarburization process, microstructure of samples was characterized using scanning electron microscope equipped by EDS.

3. Results and Discussion Fig. 2. Microstructure of white cast iron containing massive cementite (white) and pearlite and ledeburite (dark zones) 3.1. Primary Microstructure of White Cast Iron after etching in 2% Nital: (a) surface of sample (b) center Figure 2 shows primary microstructures of used white of sample.

cast iron. Because of high solidification rate, samples reveal illustrates PCO/PCO2 ratio versus temperature which is plotted carbide structure, which the percent of carbides increases based on the both above assumptions. Solid lines belong to from the center to the surface. In addition, ledeburite struc- decarburization equilibrium which are plotted for different ture can be seen in this figure. carbon activities. Dashed line is related to iron oxidation equilibrium (Eq. (9)); and the dotted one corresponds 3.2. Thermodynamics of Decarburization Process to FeO–Fe2O3 equilibrium. Accordingly, Fe2O3 oxide is For thermodynamic considerations, the system was thermodynamically stable at relatively low PCO/PCO2 ratios assumed to be completely sealed. Assuming that calcination (below the dotted line). This condition occurs at the initial of CaCO3 occurs fast enough to supply required CO2 for stages of the process when CO pressure is low. However, decarburization, PCO2 can be calculated based on the equi- after a while, by formation of CO and increase of its pres- librium of calcination reaction (reaction (5)). CO2 which is sure, FeO will be the most favorite oxide. Noting that when volatilized by CaCO3 calcination reacts with carbon of the iron sample is cooled to room temperature, FeO which has sample surface at high temperature according to Boudouard previously formed on the surface converts to Fe2O3. reaction (reaction (6)). Therefore, surface can be decarbu- As illustrated in Fig. 3, there are four regions of reactions. rized by reaction of surface carbon with CO2 which forms Region (a) is the area in which oxidation in form of Fe2O3 CO. Assuming the equilibrium condition for calcination is thermodynamically probable. Moreover, decarburization and Boudouard reactions, PCO/PCO2 ratio can be calculated is also possible in this region. Region (b) is the area where using Eq. (8). the ratio of PCO/PCO2 is less than both equilibrium values 2 related to oxidation in form of FeO and decarburization. In C ()PCO S GRG6 TLnD T ...... (8) this area, oxidation and decarburization are both possible ()aP() E CCO2 U and occur simultaneously. However, there is higher ther- Where, aC corresponds to the carbon activity on the speci- modynamic driving force for the decarburization rather to men surface. Additionally, sample surface is probable to be the oxidation. Region (c) is the area where the ratio of PCO/ oxidized during the decarburization process. The oxidation PCO2 is between the equilibrium values for decarburization reaction occurs according to reaction (9). Therefore, con- and oxidation reactions. Since the PCO/PCO2 ratio is more sidering CO2 which is released during CaCO3 calcination, than that of related to equilibrium of the oxidation, it is following iron oxidation can occur at elevated temperatures. concluded that no oxidation can occur in this region; however, decarburization can still take place. This condition is Fe CO2 gFeO CO g ...... (9) the most appropriate for solid state steelmaking process in Assuming the equilibrium condition for oxidation reac- which no surface oxidation occurs. Region (d) is the area tion, PCO/PCO2 ratio can be calculated using Eq. (10). In that PCO/PCO2 ratio is higher than equilibrium values needed this assumption, Boudouard reaction is supposed to be in for oxidation and decarburization. So that, raising the ratio non-equilibrium condition. to this region terminates the decarburization. This happens in the last stages of decarburization when CO2 pressure C PCO S GRG9 TLn ...... (10) drops due to CaCO3 depletion. D P T E CO2 U It is obvious that at the beginning, CO pressure is very In the ideal condition for decarburization, PCO/PCO2 ratio low; therefore, condition of the system is as mentioned for should be between the calculated ones from Eqs. (8) and region (a) and (b). After a period of time, oxidation and (10), in which equilibrium conditions for decarburization decarburization progress which leads to formation of CO. and oxidation reactions are assumed, respectively. Figure 3 By increasing the CO pressure, the condition of the system changes towards region (c). Further increase in the PCO/PCO2 ratio may result in transition to region (d) where stops the decarburization. It is believed that dissociation rate of the cementite into the austenite and carbon is fast enough to maintain the local equilibrium concentration between the cementite and the austenite phases at the interface.1,2) Therefore, when the cementite decomposes, carbon is saturated in the neighbor- ing matrix and thus carbon activity is approximately unity. At the beginning of the decarburization process, carbon activity is nearly unity even at the surface; since cementite exists in all regions. But after a period, carbon content of the specimen surface decreases; thus, surface carbon activity drops to values less than unity. Equilibrium PCO/PCO2 ratios of the decarburization reaction are illustrated in Fig. 3 for different carbon activity conditions of the specimen surface. As shown, by decreasing the carbon activity of the surface,

C PCO S the area of region (c) is more limited. Finally, region (c) Fig. 3. The D T ratio versus temperature for decarburization E PCO2 U disappears when carbon activity of the surface drops to 0.1 and oxidation reactions in different conditions of carbon at the applied temperatures above 1 000 K. In this condition, activity of the specimen surface. decarburization occurs along with the surface oxidation, in

© 2018 ISIJ 1794 ISIJ International, Vol. 58 (2018), No. 10 thermodynamics aspect. was measured using an image analyzing software (MIP software) and the graphite phase fraction was obtained as shown 3.3. Oxidation during Decarburization Process in Fig. 5. Results showed that secondary graphitization has The oxidation thickness of samples during decarburization increased as higher temperatures of decarburization. Figure was measured by weighing the decarburized samples before 6 illustrates the effect of decarburization temperature on the and after acid cleaning. Results of the oxide layer thick- secondary graphitization of white cast iron after 10 and 24 ness for white cast iron are shown in Fig. 4. The measured h. As can be seen, the maximum graphitization has occurred oxidation was almost negligible at 800°C and there was no after 24 h decarburization at 1 000°C. According to kinetics, measurable oxide layer at this temperature. As can be seen, it is reasonable that secondary decarburization increases by thickness of the oxide layer formed at the surface of samples temperature and time. has increased by temperature and time. These results are in agreement with the oxidation study of Zamanian20) and 3.5. Effect of Temperature on the Decarburization and Vourlias.21) According to previously researches,21–23) oxida- Secondary Graphitization tion rate of high carbon steel is mainly controlled by the The effect of applied temperature and time on the decar- diffusion through the oxide layer. As illustrated in Fig.4, burization of white cast iron is shown in Figs. 7 and 8. As the square of oxidation thickness shows a linear behavior by polished cross section of samples which were decarburized time which corresponds to the diffusion-controlled growth. at 800, 900 and 1 000°C for 10 and 24 h are illustrated in The slope of the line corresponds to the oxidation kinetic Fig. 7. While, Fig. 8 shows the microstructures after etching constant which has increased by temperature. in 2% Naital. As shown, no graphitization can be observed at the decarburized layer in the samples hold above 900°C 3.4. Secondary Graphitization during Decarburization (Figs. 8(c) to 8(f)). However, samples which were decar- Process burized at lower temperature (800°C) revealed secondary In order to determine secondary graphitization of sam- graphitization at the decarburized layer of the samples as ples, microstructure of not decarburized regions, particularly well at the center (Figs. 8(a) and 8(b)). Formation of the the central regions of the sample, was investigated. For this decarburized regions without any graphitization means that purpose, 20 images were captured by an optical microscope surface decarburization rate at temperatures above 900°C is from the center of each sample. Secondary graphitization

Fig. 4. Square of oxidation thickness versus time at different Fig. 6. Percentage of secondary graphitization in white cast iron decarburizing temperatures. samples after decarburization at 800, 900 and 1 000°C.

Fig. 5. MIP results of secondary graphitization for white cast iron sample which was decarburized at 800°C for 10 h.

Fig. 7. As polished optical microscope images of samples decarburized at (a) 800°C for 10 h, (b) 800°C for 24 h, (c) 900°C for 10 h, (d) 900°C for 24 h, (e) 1 000°C for 10 h, (f) 1 000°C for 24 h. more than the rate of graphitization. If the rate of graphite rate of the surface decarburization. Therefore, during cool- nucleation and growth was faster than surface decarburiza- ing when γ → α phase transformation occurs, the regions tion, it was expected to observe graphite at the surface too with carbon content lower than ferrite solubility transforms (as happened in samples decarburized at 800°C). Elongating to complete ferrite (complete decarburized region); how- the decarburization time in this condition, leads to ferrite ever, farther regions from the surface possibly transforms to formation around graphite islands and finally may yield α + Fe3C which form the partial decarburized region. On graphite destruction too which remains porosity and voids. the other hand, at decarburization temperature of ~ 800°C, Since no porosity was evident in the decarburized regions carbon content of the surface decreases in a relatively lower of samples treated at temperatures above 900°C (Figs. 8(c) rate. So that during the applied decarburization times, car- to 8(f)), it can be concluded that decarburization has been bon content of the surface does not probably reach to the completed in these regions before any graphite nucleation. ferrite solubility. Consequently, when sample is cooled As can be seen in Fig. 7, by increasing the tempera- and experiences the transformation, all regions including ture and time, higher thickness of decarburized layer was the surface reveals α + Fe3C. This finding is in contrast achieved. The increment in decarburized depth is reasonable with earlier studies24–26) which have reported that complete since carbon diffusion rate increases by temperature based decarburization is attributed to the incidence of γ → α phase on Arrhenius equation. This effect has been reported by Li transformation in the α + γ phase field. Zhang et al.19,27) et al.14) and Zamanian et al.20) in atmospheric decarburiza- showed that in the temperature range of eutectoid reaction tion process too. of spring steel, only complete decarburization is obvious; Figure 8 shows that the decarburized layer composes of however, at higher temperatures partial decarburization two regions of complete and partial decarburization. As can occurs along with complete decarburization. be seen, at the temperature of 800°C only partial decarbu- EDS analysis of the decarburized region is illustrated rized region is revealed. However, with increasing the tem- in Fig. 9(a), in comparison with the EDS results obtained perature, complete decarburized region has been appeared for not decarburized region (Fig. 9(b)). As demonstrated, too. It is due to higher diffusion and decarburization rate the carbon peak at the decarburized layer is lower than the at the enhanced temperatures. When the decarburization peak of the center of sample. It confirms that carbon has temperature increases to 900–1 000°C, carbon content of removed from the surface during decarburization process in the regions close to the surface drops to zero due to higher the CaCO3 pack.

Fig. 8. Optical microscope images of etched samples decarburized at (a) 800°C for 10 h, (b) 800°C for 24 h, (c) 900°C for 10 h, (d) 900°C for 24 h, (e) 1 000°C for 10 h, (f) 1 000°C for 24 h.

Fig. 9. SEM image and EDS analysis of the sample decarburized at 900°C for 10 h: (a) decarburized layer, (b) not decarburized layer.

Depth of the decarburized layers was measured by metal- of the decarburized layer by MIP software. Figure 10(b) lographic observation and image analyzing software (MIP indicates the thickness of total decarburized layer after 10 software). Figure 10(a) shows the depth determination and 24 h decarburizing at 800, 900 and 1 000°C. According

1797 © 2018 ISIJ ISIJ International, Vol. 58 (2018), No. 10 to Fig. 10(b), the decarburized depth has increased by the region. As can be seen, hardness numbers of around 150 HV temperature. and 200 HV were measured for the complete and partial Figure 11 shows the variation of hardness in the complete decarburized layers, respectively. However, hardness num- and partial decarburized regions relative to not decarburized ber was around 230 HV in the central region. The hardness decrement in the decarburized regions confirms that carbon content and consequently the cementite percentage in the decarburized layers have been decreased considerably. The hardness numbers obtained for the decarburized layers are in agreement with the results of Amar et al.28) and Liu et al.19)

3.6. Kinetics of the Solid State Steelmaking in CaCO3 Pack From a kinetic point of view, initially, the process is controlled probably by the diffusion of released CO2 from the CaCO3 pack towards the specimen surface. In other words, CO2 diffusion is the lowest step of the process. Therefore, it can be said that Boudouard reaction is under equilibrium condition at the specimen surface. In this condition, from the thermodynamic view, oxidation of iron is not prefer- able while enough carbon exists at the surface. By progress of the decarburization and carbon depletion at the surface, the decarburization process will be controlled by carbon diffusion in the iron substrate. Therefore, once the carbon supply rate to the surface becomes slower than the decarburization rate, Boudouard equilibrium condition falls down and thus oxidation may start. When the oxidation starts, decarburization will proceed at the iron-oxide interface. Since the iron oxide layer is a porous layer, there is a pos- sibility of CO2 penetration through the oxide layer. Hence, the decarburization reaction may occur directly with CO2 at the iron-oxide interface. Consequently, the decarburization rate is controlled by the diffusion of carbon in the iron substrate from the central regions towards the interface. The reaction is associated with CO production at the interface Fig. 10. (a) Optical microscope image showing the complete which then penetrates through the oxide layer porosities to decarburized layer (dashed region), (b) thickness of total the outer surface. decarburized layer achieved at temperature of 800, 900 and 1 000°C after 10 and 24 h. It is well known that the depth of the decarburized layer can be approximated by a diffusion-controlled power law equation as expressed below:1) Xk= tn ...... (11) where, k is kinetics constant, t is the exposure time, and X is the depth of total decarburization layer. Therefore, for determining the kinetics constant at each temperature, Ln(X) was plotted versus Ln(t). So, the slope of the plots corresponds to the n value in Eq. (11). Figure 12 illustrates the variation of logarithm of decarburization depth by logarithm of exposure time. As shown, the thickness of decarburized layer has increased considerably by the temperature. The average slope value of 0.33 was measured for the plots indicated in the figure. It is well known that factors such as grain growth, retarding compounds or alloying elements may interfere with carbon diffusion causing reduction of n to values less than 0.5. In this study, the value of 0.33 for n may return to events which have affected the kinetics of decarburization such as secondary graphitization. According to Eq. (11), the intercepts of the plots corresponds to Ln(k) at each temperature. As known, kinetics constant, k, increases exponentially by the temperature Fig. 11. Vickers hardness of the complete and partial decarbu- (Arrhenius equation). Therefore, Ln(k) has a linear behavior rized layers in comparison with not decarburized region. by the reciprocal temperature as below:

proportional to the activation energy which was calculated Ln kL nk –/QRT ...... (12) 0 to be approximately 125 KJ/mol. Since in the applied Where, k0 is constant, Q is the activation energy of decar- temperatures, austenite is thermodynamically stable, it is burization, R is the gas constant, and T is the decarburizing expected that the calculated activation energy corresponds temperature in Kelvins. Hence, by plotting the intercepts of to diffusion of carbon in austenite which is the controlling the lines, Ln(k), versus the reciprocal temperature, 1/T, the step of the process. This value is in good agreement with activation energy can be calculated. reported values for the activation energy of carbon diffusion The intercept values obtained by linear fitting of the plots in austenite (~147–148 KJ/mol).29–31) Although austenite is indicated in Fig. 12 are reported in Table 2. Figure 13 the most probable phase at the elevated temperatures, fer- indicates the Ln(k) variation versus the reciprocal tempera- rite and cementite form during cooling and hence they are ture of decarburization. According to Eq. (12), the slope is revealed in the illustrated microstructures. Lefort et al.22) has reported the activation energy of oxidation for C40E steel in CO2 atmosphere at temperatures above 900°C, which was measured about 185 KJ/mol. The relatively lower calculated activation energy in the current study may return to using specimen with high carbon content (white cast iron). However, oxidation in pack condition with varying PCO/PCO2 ratio must be definitely different from that occurred in CO2 atmosphere with a constant pressure.

3.7. Decarburization of Thin Plates of White Cast Iron To investigate feasibility of production, fully ferritic cast iron sheets with thickness of 1 mm were produced by 1) CaCO3 pack decarburization technique. Previously, Sasaki has studied decarburization of 1 mm thickness samples at decarburizing atmosphere. In this work, samples were put under decarburization process by burying the sample into CaCO3 pack. According to obtained results, decarburizing temperature of 1 000°C was applied for 24 h to decarburize the sample. Optical microscope images of the sample struc- Fig. 12. Ln(X) versus Ln(t) at different decarburizing temperature are shown in Fig. 14. As can be seen in the figure, sam- tures. ple structure is almost free of graphite. In Figs. 14(b), 14(c) and 14(d), which are captured after etching, sample structure Table 2. Intercepts of the lines illustrated in Fig. 12 which corre- is completely ferritic. Dark porosities revealed in the center spond to logarithm of the kinetics constants of the decar- of the sample are signs of elimination of the secondary burization, Ln(k). graphite which have formed during graphite dissolution and T (°C) Slope (n) Intercept, Ln(k) R2 destruction. According to micrographs, although few dark 800 2.25 0.97 voids have been appeared instead of removed graphites (Fig. 14(e)), signs of graphite eliminations are mostly obvious in 900 0.33 4.04 0.97 form of ferrite islands (Figs. 14(a) and 14(f)). These signs 1 000 4.43 0.99 have been left in the initial situations of graphite. To measure the chemical composition changes of the decarburized sample, three parts of the sample were analyzed by mass spectrometry. The average values are reported in Table 3. As can be seen, carbon content has dropped from 3.6 wt% to 0.012 wt% which confirms the reduction of carbon content of the cast iron sample during solid state steelmaking in the CaCO3 pack.

4. Conclusion (1) During decarburization of white cast iron samples in CaCO3 pack condition, thickness of oxide layer was almost negligible; however, oxide layer thickness was increased by the applied temperature. (2) Decarburized layer depth behavior indicated a power law-dependence on exposure time. Moreover, the rate of decarburization has increased exponentially by the temperature. Maximum decarburized thickness of 900 μm Fig. 13. Plot of logarithm of decarburizing kinetics constant ver- was obtained at 1 000°C after 24 h. Activation energy of sus the reciprocal temperature. 125 KJ/mol was calculated for decarburization of white cast

Fig. 14. Microstructure of 1 mm thick sample after decarburization at 1 000°C for 24 h: (a) before etching; (b), (c), (d), (e) and (f) after etching in 2% Nital.

Table 3. Chemical analysis (wt%) of 1 mm thick decarburized 11) L. J. Wang, Q. W. Cai, H. B. Wu and Y. Wei: Int. J. Miner. Metall. sample achieved by solid state steelmaking in CaCO3 Mater., 18 (2011), 543. pack. 12) A. R. Marder, S. M. Perpetua, J. A. Kowalik and E. T. Stephenson: Metall. Trans. A, 16A (1985), 1160. Fe C Si PS Mn Cr Mo Ni Cu Ti 13) X. Wang, L. Wei, X. Zhou, X. Zhang, S. Ye and Y. Chen: Appl. Surf. Sci., 11 (2012), 4977. 97.23 0.012 1.88 0.025 0.01 0.369 0.109 <0.001 0.066 0.06 0.03 14) D. Li, D. Anghelina, D. Burzic, J. Zamberger, R. Kienreich, H. Schifferl, W. Krieger and E. Kozeschnik: Steel Res. Int., 80 (2009), 298. 15) S.-H. Deng and Y. Deng: Adv. Mater. Res., 774–776 (2013), 1181. iron during solid state steelmaking in CaCO pack. 16) Y. Liu, W. Zhang, Q. Tong and L. Wang: ISIJ Int., 54 (2014), 1920. 3 17) J. Baud, A. Ferrier, J. Manenc and J. Benard: Oxid. Met., 9 (1975), (3) Cast iron samples with 1 mm thickness were com- 69. pletely decarburized in CaCO3 pack after 24 h at 1 000°C. 18) M. Hajduga and J. Kucera: Oxid. Met., 29 (1988), 419. 19) C.-L. Zhang, L.-Y. Zhou and Y.-Z. Liu: Int. J. Miner. Metall. Mater., The carbon content of the sample was reduced from 3.6 wt% 20 (2013), 720. to 0.012 wt% after pack decarburization which showed a 20) F. Zamnian: M.Sc. thesis, Ferdowsi University of Mashhad, (2015), http://ganj.irandoc.ac.ir/articles/861708, (accessed 2016-04-21). complete ferritic microstructure. 21) G. Vourlias, D. Chaliampaliasa, T. T. Zorba, E. Pavlidou, P. Psyllaki, K. M. Paraskevopoulos, G. Stergioudis and K. Chrissafis: Appl. Surf. REFERENCES Sci., 257 (2011), 6687. 22) P. Lefort, S. Valette, A. Denoirjean and D. Tetard: J. Alloys Compd., 1) J.-O. Park, T. Long and Y. Sasaki: ISIJ Int., 52 (2012), 26. 413 (2006), 222. 2) W. Lee, J.-O. Park, J.-S. Lee, J. A. de Castro and Y. Sasaki: Ironmaking 23) G. B. Gibbs: Oxid. Met., 7 (1973), 173. Steelmaking, 39 (2012), 530. 24) D. H. Rowland and C. Upthegrove: Trans. Am. Soc. Met., 24 (1936), 3) C. R. Killmore, D. G. Edelman, K. R. Carpenter, H. R. Kaul, J. G. 96. Williams, P. C. Campbell and W. N. Blejde: Mater. Sci. Forum, 25) J. L. Snoek: Physica, 8 (1941), 734. 654–656 (2009), 198. 26) W. A. Pennington: Trans. Am. Soc. Met., 37 (1946), 48. 4) C. McDonald: Ironmaking Steelmaking, 39 (2012), 487. 27) C.-L. Zhang, Y.-Z. Liu, L.-Y. Zhou, C. Jiang and J.-F. Xiao: Int. J. 5) S. Nambu, M. Michiuchi, Y. Ishimoto, K. Asakura, J. Inoue and T. Miner. Metall. Mater., 19 (2012), 116. Koseki: Scr. Mater., 60 (2009), 221. 28) A. K. De and J. G. Speer and D. K. Matlock: Adv. Mater. Process., 6) L. Li, K. Nagai and F. Yin: Sci. Technol. Adv. Mater., 9 (2008), 1. 161 (2003), 27. 7) J. Burke, J. Bell and D. R. Bourne: Acta Metall., 8 (1960), 864. 29) E. A. Brandes, G. B. Brook and C. J. Smithells: Smithells Metals 8) R. X. Shi, C. F. Yao, W. J. Hui, Q. L. Yong, Y. H. Nie and J. Z. Reference Book, Butterworth-Heinemann, Oxford, (1992), 938. Xiang: Spec. Steel, 29 (2008), 19. 30) W. D. Caliister and D. G. Rethwisch: Materials Science and 9) Y. H. Nie, S.-H. Fu, W. J. Hui, W.-T. Fu and Z.-H. Liu: J. Iron Steel Engineering: An Introduction, John Wiley & Sons; University of Res. Int., 17 (2005), 45. Minnesota, Hoboken, (2007), 119. 10) M. T. Ma, Z. G. Li and X. Y. Lu: Spec. Steel, 22 (2001), 9. 31) I. Naito: J. Jpn. Inst. Met., 5 (1940), 25.