Materials Processing
Carburizing Effect of Cast Iron in High-Frequency Induction Electric Furnace Melting by Bio-Coke
2025 Volume 66 Issue 5 Pages 645-653
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2025 Volume 66 Issue 5 Pages 645-653
In order to achieve sustainable development goals (SDGs) in the casting process and become a decarbonized society by 2050, decarbonization in the manufacturing process, such as the utilization of converted biomass resources, should be promoted. Recently, using recycled steel scraps that contain small amount of carbon tends to increase as a cheap iron source. In this case, the total amount of carburizer has to increase. Therefore, it is necessary to examine the characteristics of biomass, which is a sustainable carbon source and carbon-neutral, as a substitution for coal coke and to examine the casting method that is friendly to the environment.
Biocoke is a solid product to utilize biomass effectively. Biocoke can be produced from herbaceous biomass. The possibility to promote the domestic production of the carburizer can be expected by using bamboo as a raw material. The carburizing effect of biocoke as a carburizer in the melting process using a high frequency induction melting furnace has been confirmed. However, the carburizing process and mechanism of carburizer made from biomass have not been studied.
In this study, Bio-coke with different degrees of carbonization was carried out in the casting process to clarify the carburizing process. The graphitization degree was used for the evaluation of the carbon on the contact surface between bio-coke and molten metal using the graphitization degree. The consideration of the carburizing process was carried out from the carburizing effect and graphitization degree. The lower the degree of crystallinity of carbon in bio-coke, the faster the carbonization was carried out. It was found that the carburizing was performed when the R-value, which indicates the degree of graphitization, reached around 0.8.
This Paper was Originally Published in Japanese in J. JFS 95 (2023) 9–15.
The casting process uses fossil resources, such as coal coke, and the environmental impact of carbon dioxide emissions has become an issue. In order to achieve the Sustainable Development Goals (SDGs) and realize a carbon-free society by 2050 [1], the decarbonization of manufacturing, including the conversion to biomass resources, should be promoted along with the decarbonization of the manufacturing process [2]. In recent years, recycled steel scrap with a low carbon content has been on the rise as an inexpensive iron source, and the use of a carburizer is required. Therefore, it is necessary to consider the use of biomass, which is a sustainable carbon source and carbon-neutral, as a substitute for the carburizer, and to study an environmentally friendly casting process. Casting processes using low-grade carbon, such as charcoal, have been attempted, and Manabe et al. used it as a substitute for the carburizer and found it to be effective [3, 4]. However, carbonizing of the biomass leads to an increase in the costs.
Bio-coke (BIC) is a next-generation zero-emission fuel with a relative weight yield of almost 100%, high density and high strength, and low-energy loss during production. BIC is expected to improve environmental and economic problems. BIC can also be produced from a herbaceous biomass, and by using bamboo, which is widely distributed in Japan but has become a problem due to bamboo damage caused by neglected bamboo forests and bamboo encroachment into surrounding forests [5], as a raw material, it is expected to contribute to the local production and consumption of resources and domestic production of a carburizer.
Research studies using BIC have reported the characteristics of the BIC [6, 7] and its effect as a substitute for heavy oil A and kerosene used for heating in plastic greenhouses [8]. Casting research has confirmed that BIC is effective in substituting for coal coke in cupolas [9–11] and in melting using a high-frequency induction furnace with BIC as a substitute for the carburizer [12, 13]. However, there are no reports discussing the carburizing process from biomass or the mechanism of carburizing. In this study, as a method for clarifying the carburizing process, casting using BIC with different degrees of carbonization was conducted, and the carburizing effect was investigated. The carbon status at the interface between BIC and molten metal was also evaluated using the degree of graphitization. Based on these results, the carburizing process of the BIC was discussed.
The schematic of the BIC manufacturing equipment is shown in Fig. 1. Bamboo was used as raw biomass. Chipped bamboo was crushed in a cutter mill, humidified, and filled with biomass in a cylinder (Diameter 48 mm) in an electric furnace. The temperature was raised to a set temperature under pressure, held for a set time, then removed after furnace cooling. Table 1 shows the molding conditions for the BIC.
Schematic of molding device of BIC.
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The diameter of the molded BIC was 48 mm, which is the same as the cylinder inner diameter, while the height was 41 mm and the apparent density was 1.36 g/cm3. Since the higher apparent density leads to insufficient gas diffusion and insufficient surface combustion during the combustion process, it was classified as a flame-retardant. The compressive strength of the BIC at an ambient temperature was 130 MPa. Since the compressive strength required for coal coke used in the melting process of cupola furnaces is 20 MPa or higher, the bamboo BIC formed in this study is strong enough to be used as substitute for coal coke.
To confirm the carburizing effect of bamboo BIC, 1 kg of the BIC (ϕ48 mm × h40 mm, weight 100 g), 84 g of Fe-75 mass%Si alloy, and 12 g of Fe-28.7 mass%S alloy were placed in a graphite crucible, and 5 kg of steel scrap (S45C) was placed above the BIC to prevent its floating. The temperature was raised in a high-frequency induction melting furnace (Fuji Denpa Kogyo Co., Ltd., 30 kW, 3 kHz), and molten metal was cast into a mold for analysis every 300 s after melting to measure contact area. Table 2 shows the melting materials used.
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An optical emission spectrometer (AMETEK Co., Ltd., SPECTROLAB M12) was used for the analysis of the contact area. Figure 2 shows the transition of the contact area in the molten metal from the start time of the temperature increase. The raw materials started to melt at 1773 K 1.2 ks after the start. BIC remained suspended as a solid on the molten metal after melting and disappeared 2.4 ks after the BIC contact with the molten metal.
Transition of carbon content in cast irons with elapsed time.
The BIC burned before the material started to melt, and the contact area exceeded that of the S45C at 1.2 ks from the start of melting, indicating the effect of carburization. The carbon content reached 3.53 mass% at 1.8 ks, then a slight decrease was observed. Figure 3 shows a photograph of the microstructure of a sample etched with 3% nital at the elapsed time of 1.8 ks. Although graphite with graphite with a particle size over 100 µm is observed, the matrix mainly consists of pearlite and a little ferrite. And the bamboo BIC also contains trace elements, such as nitrogen and calcium, this has no effects on the mechanical properties or microstructure. The carbon content was a maximum at 1.8 ks, which can be attributed to the fact that the BIC was carbonized by the heat of molten metal and the subsequent carburization effect. After 1.8 ks, the amount of carburization from the BIC remaining on the molten metal surface as solid and the amount of decarburization from the oxidation reaction between molten metal and air are in balance, and at 2.4 ks, the BIC has already disappeared. The yield of contact area of the BIC was 33 mass%. Based on these results, it was hypothesized that the BIC was carbonized in the furnace under the conditions of a low oxygen content, and that carburization occurred from the carbonized BIC. In order to clarify the fact that BIC is carbonized and then carburized, BIC with different degrees of carbonization were produced, and the relationship between the degree of carbonization and the effect of carburization was investigated by casting experiments.
Microstructure of cast iron (3 mass% nital etched) (elapsed time; 1.8 ks). (online color)
Before conducting casting experiments using BIC with different degrees of carbonization, the pyrolysis characteristics of the bamboo biomass were investigated to determine the heat treatment temperature as a criterion for the degree of carbonization. The differential thermal and thermogravimetric analyzer (SHIMADZU Corp., DTG-60A) was used to measure the weight yield (TG) and endothermic properties (DTA). The analysis conditions are shown in Table 3, and the TG and DTA curves are shown in Fig. 4.
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Thermal decomposition curves for bamboo (TG and DTA).
TG(N) and TG(Air) in Fig. 4 show the rapid decrease in weight from around 540 K. This is due to the progress of the pyrolysis reaction of the bamboo biomass and the rapid gasification of the biomass. Subsequently, the weight loss at TG(N) slowed down to around 660 K, and the discharge of volatile matter was completed. The DTA (Air) results show two peaks. The first peak was detected around 620 K. This is due to the combustion reaction caused by the gasification of the bamboo biomass. The second peak was observed at around 730 K. This is an exothermic reaction due to char combustion. Semi-carbonization is defined as the “semi-carbonized region” between 553 K and 653 K [14]. In this study, the BIC was heat-treated at 623 K to be semi-carbonized (torrefied) and at 873 K to be carbonized.
3.3 Semi-carbonization and carbonization of the BICThe sand bath method was used for the heat treatment of the BIC in an electric furnace. Pre-manufactured BIC was wrapped in four layers of aluminum foil to prevent it from coming into contact with air during the treatment. The BIC wrapped in the aluminum foil was buried in a stainless-steel container with alumina sand, and a thermocouple was placed on top. The temperature was increased at 0.17 K/sec. in the electric furnace, and once the surface temperature of the BIC reached the target temperature, the BIC was held for 3.6 ks and then naturally cooled in the furnace and removed.
Figure 5 shows the appearance of each BIC. The BICs became smaller, and their weights decreased due to the release of volatile matter by the heat treatment. The torrefied BIC had a 33 mass% weight yield, and the carbonized BIC had a 25 mass% weight yield. The TG(N) curve of the bamboo biomass in Fig. 4 shows that the weight yield at 623 K was about 33 mass%, which means that the sample met the requirements for being torrefied. The standard weight yield of the carbonized BIC was also 25 mass%, which is consistent with the yield after the end of weight loss in the TG(N) curve.
Overview of BIC, torrefied BIC, and carbonized BIC. (online color)
As shown in the results of the previous casting experiments, the experiments were started after molten metal was placed in a graphite crucible (Nippon Crucible Co., Ltd.: #15). As a result, it is considered that the melting time, chemical composition of the molten metal, contact area of the BIC with molten metal, and carburization from the graphite crucible are factors that cause variations in the measurement results. Therefore, melting experiments were conducted using the method shown in Fig. 6 so that the amount of carburization could be quantitatively measured. In order to prevent carburization from the graphite crucible and to ensure a uniform contact area between molten metal and BIC, an alumina crucible (Nippon Crucible Co., Ltd.: White Phoenix) with a wall thickness of 10 mm was installed in the graphite crucible and the surrounding area was covered with coal coke. Moreover, a disk of coal coke was placed at the bottom of the thick-walled alumina crucible, and an alumina crucible with a wall thickness of 5 mm (NIKKATO Co., Ltd., Special refractory crucible CP) was placed on top. In addition, 30 kg of the base alloy was prepared in advance and cut into 500 g pieces to ensure a uniform melting time and chemical composition of the melted materials. The chemical composition of the base metal is shown in Table 4.
Schematic of quantitative measurement apparatus of carbon content in the melts. (online color)
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A 500 g sample of the base metal was placed in an alumina crucible and the temperature raised to 1773 K to melt it. After the molten metal reached 1773 K, each BIC was dropped from the top. The contact areas of the BIC, torrefied BIC, and carbonized BIC are 18 cm2, 15 cm2, and 9 cm2, respectively. After a certain period, only the central alumina crucible was removed and cast into the analysis mold. The alumina crucible was then returned to its original position, the base metal was placed in the crucible again, and the procedure was repeated. The sample recovered from the analysis mold was subjected to emission spectrometry to determine the contact area. Since BIC decreased with contact time in this experiment, an additional BIC was dropped over contact time so that the weight of the BIC did not fall below 25 g. Although the weight of the first BIC was reduced, it remained until the outflow of the molten metal, so it can be judged that there was no effect on the contact area in the BIC that was additionally dropped. In addition, in experiments with contact times of 5 sec. to 60 sec., if the BIC dropped was bulky, the runner would be blocked when pouring, so the weight of each BIC was 25 g.
4.2 Experimental resultsTable 5 shows the chemical compositions of the five elements in the cast iron contacted with BIC. The carbon content remained unchanged up to the contact time of 30 s. After 60 s, the contact area increased with contact time, but there was no change in the elements other than carbon. The same tendency was shown for the torrefied BIC and carbonized BIC. The detection limit of the contact area in the emission spectrometer used in this study was 5 mass%. The results of pouring the base metal into the analysis mold after holding it for 180 sec. after remelting at 1773 K showed that the contact area decreased to 2.06 mass%. Without BIC, decarburization occurred after holding for 180 sec.
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Table 6 shows the relationship between contact time of the BIC, torrefied BIC, and carbonized BIC with molten metal and the amount of carbon content in order to focus on the carbon amount, which is the amount of added carbon. The transition of the carbon content of the molten metal as a function of the contact time is shown in Fig. 7. In the case of all the BICs, the carbon content once slightly decreased between the contact time of 5 sec. and 10 sec., followed by a carburization effect starting at 15 sec. This is because the amount of decarburization of the molten metal is larger than the amount of carburization from the BIC. In other words, it is considered that it is difficult to come into contact with the molten metal while the volatile matter is released from each BIC. In casting experiments using the BIC, a rapid increase in the carbon content was observed at the contact time of 60 sec., exceeding 4.3 mass% at 420 sec. In the case of the torrefied BIC and carbide BIC, a rapid increase in the contact area was observed at contact time of 30 sec. The carbon content of the carbonized BIC exceeded 4.3 mass% at about 180 sec. and exceeded the detection limit of 5 mass% at 300 sec. Although contact time at which a rapid increase in the contact area was observed, it was the same for both the torrefied BIC and carbide BIC. The difference in the contact area between the torrefied BIC and carbide BIC was 0.19 mass% at the contact time of 60 sec. and 0.13 mass% at a contact time of 180 sec.
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Transition of carbon content in the melt with contact retention time.
The results using the torrefied BIC and carbonized BIC showed a rapid increase in contact area after contact time of 30 sec., while the results using BIC showed an increase after 60 sec. This suggests that the BIC was in close proximity to the molten metal in the furnace and the volatile matter was rapidly released, causing BIC to be covered with volatile matter and not in contact with molten metal. This is thought to have inhibited carburization because BIC was covered with volatile matter and was not in contact with molten metal. At contact time of 60 sec., the contact area increased as the carbonization of the BIC progressed, and the contact area proportionally increased. On the other hand, since the volatile matter of the torrefied BIC and carbonized BIC were removed by prior heat treatment, the amount of carbon contained increased rapidly at the same contact time of 30 s. This result suggests that the BIC acts as the carburizer after being carbonized in the furnace. In addition, the difference in the carbon content between the torrefied BIC and carbonized BIC at the time of reaching 1773 K for a contact time of 60 sec. was 0.19 mass%. This may indicate that there is a difference in the carburizing effect depending on the carbonized environmental temperature of the BIC. When BIC is used, volatile matter is released in the furnace, and then carbonization proceeds rapidly at the furnace environment temperature of 1773 K. The torrefied BIC released about half of its volatile matter through the previous heat treatment, and complete carbonization had not yet been completed. The BIC is dropped into the furnace and is completely carbonized at 1773 K, which is the environmental temperature inside the furnace. However, the carbonized BIC is not carbonized again in the furnace because it has been completely carbonized during the previous heat treatment. This difference in the heat treatment temperature at which the final carbonization process is applied is the cause of the difference in the formation of carbon bonds in the BIC and difference in the amount of carbonization between the torrefied BIC and carbonized BIC at the contact time of 60 sec.
Based on these results, we considered that the difference in the structure of carbon in the BIC was a factor and performed a structural analysis by Raman spectroscopy.
Many research studies have been conducted about the graphitization process of carbon, which is known to be a physical change involving an increase in three-dimensional regularity and crystal growth in the temperature range above 1773 K [15]. It is also known that graphite crystallization significantly increases between 1773 K and 3273 K. In this study [16], the molten metal temperature was between 1773 K and 1823 K, and it is assumed that the carbon in the biomass carbonized, and a part of the contact surface became graphitized. Therefore, an evaluation was attempted based on the degree of graphitization.
Raman spectroscopy was used to evaluate the degree of graphitization. Raman spectroscopy is used to investigate the structure of various carbon materials, and it is known that when the structure of graphite is disturbed, Raman bands are observed at 1360 cm−1 and 1620 cm−1 in addition to the Raman band at 1580 cm−1 in the Raman spectrum, and as the structural disturbance becomes larger, the relative intensity of these bands to the Raman band at 1580 cm−1 increases as the structural disorder increases, and the overall band shape becomes broad [17]. The D band (1360 cm−1) is expressed in contrast to the original graphite G band (1580 cm−1), and the ratio of the intensities of these two Raman bands (I1360/I1580) is known as R [18, 19], which evaluates the degree of graphitization. It is also indicated that the narrower the width at half maximum of the G band (ΔV1580), the higher the degree of crystallization [17].
In this study, a Raman spectrometer (Horiba Co., Ltd., LabRAM HR evolution) was used, and the analysis was performed using LabSpec6 software dedicated to spectrometry instruments. A light source with a wavelength of 633 nm was used to detect the scattered light by a CCD detector. A 50× objective lens was used, the exposure time was 60 sec., and the number of integration times was 2. Smoothing and baseline correction were performed on the Raman spectrum output to LabSpec6 to determine the ΔV1580 and R values.
The BIC was brought into contact with molten metal under the conditions shown in Table 7 using the experimental method described in Section 4.1. Therefore, the contact time at which flaming combustion weakened and the volatile matter on the contact surface decreased was set to 120 sec. The contact time was also set to 30 sec. for the torrefied BIC and carbonized BIC. Since the torrefied BIC and carbonized BIC do not contain volatile matter, only contact time of 30 sec. was investigated. After contact, the coal coke was buried in sand to stop the combustion, and after natural cooling, only the contact surface was scraped off and pulverized to less than 1 mm. Contact tests with coal coke were also conducted as reference material.
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Figure 8 shows the Raman spectrum of the BIC after contact with molten metal, the Raman spectrum of the torrefied BIC without contact or after contact with molten metal, the Raman spectrum of the carbonized BIC without contact or after contact with molten metal, and the Raman spectrum of coal coke without contact or after contact with molten metal. These data were smoothed and the baseline corrected.
Raman spectrum of BICs and coal coke.
Due to the nature of the Raman spectroscopy, it is difficult to measure the Raman spectrum of materials with a high fluorescence, and no carbon spectrum was observed in the biomass. However, BIC in contact with molten metal showed a D band around 1360 cm−1 and G band at 1580 cm−1 for both at 30 sec. and 120 sec. Raman shifts, confirming that the BIC was carbonized by contact with molten metal. Although the baseline of the uncontacted torrefied BIC was corrected, there was a lot of noise in the spectrum itself. This is due to the fact that the torrefied BIC was not completely carbonized. In addition, crystallization of the torrefied BIC was promoted by the contact with molten metal. Since the carbonized BIC and coal coke have already been carbonized by pretreatment, crystallization can be confirmed even before contact with molten metal. Figure 9 shows a comparison of the width at half maximum (ΔV1580) of the G-bands of the BIC in contact with molten metal and those of the uncontacted torrefied BIC and carbonized BIC. Comparing ΔV1580 in Fig. 9, the crystallinity of the carbon in the torrefied BIC is lower than that in the carbonized BIC, indicating that the lower crystallinity of carbon in the BIC results in a higher carburization rate. This is postulated to be because the bonds of carbon decompose and dissolve into the molten metal when the molten metal is carburized, and thus the lower crystallinity, the easier it is to decompose. It was also confirmed that BIC carbonizes when it comes into contact with molten metal, but the longer the contact time, the lower the crystallinity. This suggests that carbon decomposition occurs at the same time as when the BIC is carbonized by the heat of the molten metal.
ΔV1580 of BICs and coal coke.
Figure 10 shows the R-values of the test pieces. As a result of comparing the R-values of the test pieces, the R-values of the torrefied BIC, carbonized BIC, and coal coke converged around 0.8 when in contact with molten metal, suggesting that this graphitization level is the threshold value for adding coal coke to the molten metal. The graphitization degree of the BIC in contact with molten metal for 30 sec. was lower than that of the other BICs. This is because carbon is easily carburized into the molten metal due to inhibition by the release of volatile matter from the BIC during 30 sec., and it is assumed that crystallization has not yet occurred. The R value, which is an index of graphitization, reached around 0.8 after 120 sec. of contact time, indicating that the molten metal has been carburized, which is the same trend as the low carburization rate in the experimental results.
Relationship between contact retention time and degree of graphitization (R values) of BICs and coal coke.
In this study, the graphitization degree was evaluated by the intensity ratio of the G and D bands of Raman spectroscopy. However, various evaluation methods have been used to evaluate the graphitization of carbon [20], including the evaluation of the graphitization degree from XRD profiles [21] and evaluation by the graphitization degree P1 [22, 23]. We will clarify the relationship between the graphitization degree and the amount of carburization by further clarifying the graphitization degree, leading to the elucidation of the carburization mechanism from BIC.
In this study, we aimed to improve environmental conservation and economic problems by replacing the carburizers and coal coke used in the casting process with BIC. We produced BIC from bamboo, which is widely distributed throughout Japan, devised a method for quantitatively measuring contact area using BIC with different degrees of carbonization, and considered the carburization process of the BIC during the casting process. As a result, we obtained the following results.
The author gratefully acknowledges the advice provided by Prof. Kazunori Asano, Faculty of Science and Engineering at Kindai University and members of the Biocoke Research Institute at Kindai University in preparing this paper.
