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Article

Vacuum Carbon Reducing Iron Oxide Scale to Prepare Porous 316 Stainless Steel

by
Fang Zhang
,
Jun Peng
*,
Hongtao Chang
and
Yongbin Wang
School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China
*
Author to whom correspondence should be addressed.
Metals 2022, 12(12), 2118; https://doi.org/10.3390/met12122118
Submission received: 25 August 2022 / Revised: 26 November 2022 / Accepted: 29 November 2022 / Published: 9 December 2022
(This article belongs to the Special Issue Metal Recovery and Separation from Wastes)
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Abstract

:
In order to improve the added value of iron oxide scale and reduce the manufacturing cost of porous stainless steel, steel rolling iron oxide scale as an iron-containing raw material was used to prepare porous 316 stainless steel by high-temperature sintering under vacuum conditions, while carbon was used as a reducing agent and pore-forming agent, and the necessary metal powders were added. In our work, the specific reduction system was confirmed, including the sintering temperature, sintering time, vacuum degree and carbon amount, through thermodynamic calculation combined with experiments. Thermodynamic analysis results showed that the transformation process of the chromium element in the raw materials at 10−4 atm and 300~1600 °C was FeCr2O4 + Cr3O4→Cr2O3 + Cr3O4 + Cr23C6→Cr23C6 + Cr7C3 + FCC→FCC + Cr23C6→FCC→FCC + BCC→Cr(liq). The FCC phase with qualified carbon content could be obtained at 10−4 atm and 1200 °C, while 90.88 g iron oxide scale, 17.17 g carbon, 17.00 g metal chromium, 12.00 g metal nickel and 2.5 g metal molybdenum were necessary to produce 100 g porous 316 stainless steel. The porous 316 stainless steel with a carbon content of 0.025% could be obtained at 10−4 atm and 1200 °C for 180 min, while the chromium element underwent the transformation of metal, Cr→FeCr2O4→Cr23C6→Austenite. The porosity of the porous 316 stainless steel was 42.07%. The maximum size of impurity particles was 5 μm when the holding time reached 180 min. Magnetic separation was an effective method to reduce impurities in the porous stainless steel.

1. Introduction

Iron oxide scale is a by-product of continuous casting billet or steel ingot and its rolling process, also known as iron scale, which accounts for about 1.5% of annual steel production [1]. For example, global crude steel production in 2021 was 1.95 billion tons, and the output of iron oxide scale was about 29.27 million tons, which is quite considerable [2]. Compared with other solid wastes, iron oxide scale has the advantages of high total iron content (more than 70%), low impurity content and easy purification [3]. At present, the recycling method of iron oxide scale is mainly concentrated on the production of a slagging agent, reduced iron powder and iron red pigment, and as an auxiliary iron-containing raw material for sintering, pelleting or powder metallurgy [4,5]. Thus, the above methods of iron oxide scale have a low utilization level. In order to efficiently utilize the metal components in iron oxide scale, it is necessary to carry out multi-angle research on the reduction mechanism of iron oxide scale so as to provide theoretical basis and technical support for improving the added value of its products.
Porous stainless steel has unique properties that are different from dense materials due to the presence of holes, such as its small density, large surface area, good sound absorption performance, low thermal conductivity, excellent permeability and so on [6,7]. Therefore, porous metal materials are widely used in the manufacture of filter purification materials [8], energy conversion devices [9], catalyst supports [10], sound absorbers [11] and biological transplantation materials [12,13,14]. Among them, porous 316 stainless steel has the advantages of high-temperature resistance, corrosion resistance and oxidation resistance, as well as good comprehensive mechanical properties, excellent biocompatibility, easy processing, et al. [15,16]. Therefore, it can be widely used as a structural material and functional material for medical-drug-carrying implant devices [13,17,18], fuel cells [19], filters [20], heat exchangers [21] and so on.
The usual preparation methods of porous stainless steel mainly include powder sintering technologies [22,23,24], a physical dealloying process [18], fiber felt [25,26] and so on. Most of them use stainless steel powder or stainless steel fiber as the main raw material, which is mixed with pore-forming agent, then sintered, casted or deposited in a protective gas or vacuum [27]. In addition, the stainless steel powder is produced by atomizing a molten metal with water or inert gas in centrifugal equipment or with a plasma rotary electrode [13,28]. Moreover, the production of the molten 316 stainless steel undergoes EAF (electric arc furnace)→AOD (argon oxygen decarburization furnace)→LF (ladle furnace) [29,30], which is complex, high polluting and high energy-consuming [31].
If the preparation of stainless steel powder can be combined with the molding process of the products, the process of porous stainless steel will be efficiently shortened with the raw material costs being reduced and the production efficiency being improved at the same time. In this work, steel rolling iron scale was used as an iron-containing raw material to prepare porous 316 stainless steel by high-temperature sintering under vacuum conditions, while carbon was served as a reducing agent and pore-forming agent, and other metal powders, such as alloy elements, including chromium, nickel and molybdenum. The chemical composition of 316 stainless steel is shown in Table 1. The price of iron oxide scale was 1.06~1.13 CNY per kilogram, and the average price of metal chromium powder, metal nickel powder, metal molybdenum powder was 79.2, 148.5, 275.0 CNY per kilogram, respectively [32]. The raw material cost was 39.1 CNY per kilogram according to the median component of 316 stainless steel in Table 1. The price of high-purity graphite powder was 0.6 CNY per kilogram. The commercial price of 316 stainless steel powder was 64.3~96.6 CNY per kilogram. Furthermore, the process of vacuum carbon reduction sintering provided in this paper is simpler because the stainless steel powder production is merged with the process of pore forming. Therefore, it will significantly reduce the production cost of porous 316 stainless steel. Meanwhile, the added value of the product made from iron oxide scale will been increased.
However, the metal chromium powder will be oxidized during iron oxide scale is reduced by carbon, and the reduction temperature, system pressure and carbon proportion are very crucial [33,34]. In this paper, in order to confirm the optimal preparation process of porous 316 stainless steel with iron oxide scale and metal powders, FactSage thermodynamic database combined with experimental research was implemented. In addition, the morphology and porosity of the porous 316 stainless steel were analyzed, and the occurrence state of impurities from iron oxide scale was studied to improve the purity of the product.

2. Materials and Methods

2.1. Experimental Raw Materials

The raw materials used in this paper include steel rolling iron oxide scale, metal powders and high-purity graphite powder. The chemical composition of the treated iron oxide scale is shown in Table 2. The total iron content (TFe) of the iron oxide scale was 73.17%, and the sum of oxide impurity content was 1.28%. Because the iron oxide scale comes from ordinary carbon steel, the compositions of chromium oxide, nickel oxide and molybdenum oxide were not detected.
The composition and morphology of the iron oxide scale before being treated were analyzed with a JSM-6510 scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS) (JEOL, Beijing, China), and the results are shown in Figure 1. In Figure 1(a), the surface of the iron oxide scale is scaly, in which the content of iron and oxygen is the highest, and that of silicon, aluminum, calcium and other impurity elements is very low, and their distributions are uneven. Meanwhile, the elements of silicon and aluminum or aluminum and calcium coexist in impunity particles on the surface of the iron oxide scale.
The mineral composition of the iron oxide scale was analyzed by X-ray diffraction (XRD) on MiniFlex600 X-ray Diffractometer (Rigaku, Beijing, China), and the particle size of the treated iron oxide scale was tested with LS230 Laser Particle Size Analyzer (BECKMANCOULTER, Suzhou, China). The analysis results are shown in Figure 2 and Figure 3, respectively. As can be seen from Figure 2, the main components of the iron oxide scale are FeO and Fe3O4, in addition to a small amount of Fe2O3 and Fe. The average particle size (mean) of the iron oxide scale was 9.25 μm, the surface particle size (S.D) was 6.04 μm, 10% of the particles were smaller than 0.308 μm, and 90% of the particles were smaller than 11.58 μm.
The raw materials for the preparation of the porous 316 stainless steel also included metal chromium powder, metal nickel powder, metal molybdenum powder, as well as high-purity graphite powder. Among them, the metal powders were used to adjust the alloy compositions of the porous 316 stainless steel, and the graphite powder was used as a reducing agent and a pore-forming agent. The basic information of the above raw materials is shown in Table 3.

2.2. Methods

During the preparation of the porous 316 stainless steel, firstly, the iron oxide scale was crushed, sieved, cleaned for oil removal and underwent wet magnetic separation followed by being dried at 120 °C for 2 h. Then, the iron oxide scale was mixed with the appropriate amount of metal powders and high-purity graphite powder. Additionally, the mixture was press into ф15 × 3 mm pills. Lastly, the pill samples were sintered in a controllable atmosphere vacuum tube furnace. The preparation process of the porous 316 stainless steel is shown in Figure 4.
The specification composition of the 316 stainless steel and the target composition of porous the 316 stainless steel prepared in this paper are shown in Table 1. Additionally, the median composition in Table 1 was the target component of the porous 316 stainless steel. Because metal chromium powder is bound to be oxidized by iron oxides at high temperature, it is necessary that the reduction process should be implemented under a suitable vacuum degree and sintering temperature. The specific reduction system, including the sintering temperature, vacuum degree and carbon amount, was determined through thermodynamic calculation combined with experiments. In our work, thermodynamics calculations were performed using FactSage8.1. The Equilib module was selected, and the product databases selected were FactPS, FToxid-SPINA, FToxid-CORU, FToxid-MeO, FSstel-FCC, FSstel-BCC, FSstel-M23C, FSstel-M7C3, FSStel-CEME and FSstel-Liqu. In the process of thermodynamic calculation, all of the compositions in Table 1 and Table 4 were applied except sulfur and phosphorus.
In addition, in order to reveal the carbon reduction steps of the mixture of all raw materials, the major phase components of the intermediate products at different temperatures during the reduction process was analyzed by XRD with a scanning angle (2θ) from 10° to 90°, step size of 0.02° and scanning speed of 2 (°)/min. The morphology of the porous 316 stainless steel was investigated with a SEM-EDS for point and area scanning. The porosity of the porous stainless steel was measured with the water immersion method three times, and the average value of the three measurements was taken as the final result.

3. Results and Discussion

3.1. Determination of the Reduction Sintering System

The carbon addition was the most important for the preparation of quality porous 316 stainless steel, in which the carbon composition required was less than 0.03%. In addition, in order to guarantee the carbon addition was enough, it was assumed that iron oxides in the iron oxide scale were completely reduced to carbon monoxide by carbon because it was not clear that the reduction product was CO or CO2. The dosages of iron oxide scale and alloys for the porous 316 stainless are shown in Table 4, which was confirmed with FactSage 8.1 thermodynamic database according to Table 1 and Table 3. In detail, 90.88 g iron oxide scale could be reduced by 16.93 g carbon. Meanwhile, 17.00 g metal Cr, 12.00 g metal Ni and 2.50 g metal Mo also were needed for 100 g 316 stainless steel.
The function of carbon in the raw materials was to reduce the iron oxide scale, but chromium metal powder was likely oxidized by the ferric oxides from iron oxide scale before the ferric oxides were reduced by carbon. Furthermore, chromium oxides could be reduced by graphite only when the sintering temperature was raised above a certain value; that is, the intersection temperature of the oxygen potential lines of chromium oxides and of carbon oxides. However, the sintering temperature of the porous material was required to be lower than its melting point in order to effectively control its porosity. The occurrences of chromium element at 300~1600 °C and 1 atm were predicted with FactSage 8.1 thermodynamic database in order to obtain the optimal sintering conditions, and the results are shown in Figure 5. The results show that the chromium element in FeCr2O4 was the only form at 300~800 °C, and FeCr2O4 was always present in the reduction products. Then, part of FeCr2O4 converted to Cr3O4, as the temperature was higher than 800 °C. Most of the FeCr2O4 significantly transformed to Cr2O3 and a small amount of FCC as the temperature rose to 1080 °C. Then, Cr2O3 disappeared completely and converted to Cr7C3, and the liquid metal started to generate in large quantities at 1126 °C, while Cr3O4, Cr7C3 and FeCr2O4 were not reduced completely. Therefore, it could be understood that the transition sequence of the Cr-containing phase was FeCr2O4→FeCr2O4 + Cr3O4→Cr3O4 + Cr2O3→Cr7C3 + Cr3O4→Cr(liq) + Cr3O4. Furthermore, it was impossible to obtain the 316 stainless steel with a porous structure under 1 atm because the chromium element in Cr3O4, Cr7C3 and FeCr2O4 did not thoroughly transform into FCC before the liquid phase generated.
According to decarburization and chromium preservation theory, the chromium element in FeCr2O4, Cr3O4, Cr2O3 and Cr7C3 can gradually convert into FCC at lower temperatures by decreasing the system pressure, and a liquid phase does not appear at the same time [35]. Because 316 stainless steel belongs to austenitic stainless steel and the carbon specification composition in 316 stainless steel is required to be less than 0.03%; the carbon content in the FCC and the mass of FCC under 10−5~1atm and 300~1600 °C were also calculated according to Table 1 and Table 4, and the calculation results are shown in Figure 6. Figure 6 shows that the carbon content in FCC significantly declined at the same temperature with the decrease in the system pressures. Moreover, the equilibrium temperatures corresponding to a carbon content of 0.03% under 10−5~10−1 atm also successively rose, and these temperatures were 793, 1080, 1226, 1385 and 1390 °C, respectively. Fortunately, the carbon content in FCC could be reduced to 0.03% when the equilibrium system was below 10−3 atm, which is possibly feasible for the preparation of porous 316 stainless steel.
In addition, Figure 7 shows the temperature ranges in which FCC accounted for more than 98% of the total product under 10−5 atm, 10−4 atm, 10−3 atm, 10−2 atm and 10−1 atm, which were 964~1100 °C, 1037~1200 °C, 1097~1228 °C, 1168~1300 °C and 1245~1300 °C, respectively. The results in Figure 6 and Figure 7 indicate that the carbon content in FCC met the requirements of 316 stainless steel prepared at 10−4 atm and 1080~1200 ℃. Therefore, it was also necessary to confirm the transition sequence of the Cr-containing phase under 10−4 atm through thermodynamic calculation.
Figure 8 shows the chromium content in the Cr-containing phase at different temperatures and under 10−4 atm. As can be seen from Figure 8, the transition sequence of the Cr-containing phase was FeCr2O4 + Cr3O4→Cr2O3 + Cr3O4 + Cr23C6→Cr23C6 + Cr7C3 + FCC→FCC + Cr23C6→FCC→FCC + BCC→Cr(liq) with the increase in the equilibrium temperature, and the liquid phase generated at 1427 °C. Meanwhile, most of the chromium element mainly existed in the form of FCC with a small amount of spinel at 1037~1200 °C. Therefore, it can be determined that the porous 316 stainless steel could be obtained under 10−4 atm and at 1080~1200 °C. However, Fe2Cr2O4 always coexisted with FCC under the above conditions, which may be due to the insufficient addition of carbon. Therefore, the amount of carbon in Table 4 needs to be adjusted to ensure that Fe2Cr2O4 is reduced completely.
In order to reduce FeCr2O4 completely, the amount of FeCr2O4 and the carbon content in the FCC at 10−4 atm and 1200 °C were calculated when the carbon addition was increased from 16.50 g to 17.20 g, and the results are shown in Figure 9a,b, respectively. Figure 9a shows that the amount of FeCr2O4 gradually declined with the increase in the carbon addition. It reduced to zero when the carbon addition was 17.12 g, which means that FeCr2O4 was completely reduced at this moment. In contrast, in Figure 9b, the carbon content in the FCC continuously increased with the rise of the carbon addition. It increased to 0.006% when the carbon addition was 17.12 g, which met the specified carbon content of the 316 stainless steel. Furthermore, the appropriate carbon addition should be less than 17.17 g, while the carbon content in FCC was lower than 0.03%. Therefore, a carbon addition of 17.17 g was more reasonable, while FCC was 98.95 g; the chromium content in FCC was 17.11%, and the yield of the chromium element was 99.59%.
Through the above thermodynamic analysis, it can be determined that 90.88 g iron oxide scale could be reduced to obtain the 98.95 g 316 stainless steel with 17.17 g carbon under 10−4 atm and 1200 °C. However, the optimal sintering time needed to be confirmed by the actual sintering experiments. A vacuum reduction sintering system is shown in Figure 10, in which the sintering samples were kept at 10−4 atm and 1200 °C for 120, 150, 180, 210 and 240 min, respectively.
By means of the vacuum reduction sintering experiments, the yield of the metal powders was confirmed, as shown in Table 5. The yield of metal chromium powder and metal molybdenum powder was 98.71% and 97.20%, respectively. The losses were caused by the evaporation of Cr2O3 and MoO3 [36,37]. Every sintering sample was 5 g and held at 4 MPa for 2 min to make a sample of ф15 × 3 mm, and then the sintering process was carried out according to the reduction schedule in Figure 10. The weight of every sample was weighed before and after the sintering process, and the weight-loss rate and the carbon content of the sample held at 10−4 atm and 1200 ℃ is shown in Figure 11.
In Figure 11, the weight loss rate and the carbon content of the sintering sample were stable after being held for 180 min under 10−4 atm and 1200 °C, which were 29.27% and 3.71 × 10−3%, respectively. The carbon content met the requirement of the 316 stainless steel. Therefore, the vacuum reduction sintering system was determined to be 10−4 atm and 1200 °C for 180 min.
The chemical composition of porous 316 stainless steel prepared at 10−4 atm and 1200 °C for 180 min is shown in Table 6. The content of carbon, sulfur and phosphorus was 0.025%, 0.010% and 0.020%, respectively. Meanwhile the content of the alloy element was also within the specification range of the target steel.
In order to reveal the actual transformation of the chromium element, the samples being sintered at 10−4 atm and 700, 900, 1100 and 1200 °C for 180 min were analyzed with XRD, and the results are shown in Figure 12. The chromium element underwent the transformation of metal Cr→FeCr2O4→Cr23C6→Austenite at 700 °C→900 °C→1100 °C→1200 °C and 10−4 atm. In detail, metal chromium was oxidized to FeCr2O4 by the iron oxide scale at lower than 700 °C, FeCr2O4 changed to Cr23C6 at 1100 °C, while iron oxide scale was reduced to metal iron.
Figure 13 shows the carbon and oxygen content in the sample at 10−4 atm and different temperatures. As the temperature increased, the carbon and oxygen contents showed a continuous downward trend; the fast stage was at 1100~1150 °C, while the carbon in Cr23C6 was oxidized and removed by the oxygen in the residual ferrous oxide [38]. In the stage of 1150~1250 °C, the decline of the carbon and oxygen content in the sample was getting slower, while the carbon was dissolved into austenite, and the oxygen in impurities, such as CaO, SiO2 and Al2O3, could not be removed.

3.2. Microstructure of Porous 316 Stainless Steel

Figure 14 shows the micromorphology of the porous 316 stainless steel prepared at 10−4 atm and 1200 °C for 180 min. The porous stainless steel consisted of sintering necks and pores, the size and shape of the pores were irregular, and the porosity measured by the immersion medium method was 42.07%. Zhang W.P. et al. [27] prepared porous stainless steel with a porosity of 28.21~60.16% by the vacuum melting method with 30, 40, 50 and 60 vol.% ammonium bicarbonate (NH4HCO3) as a pore-forming agent. In Figure 14, point ① is the matrix in which the chemical compositions included iron, chromium, nickel, molybdenum and manganese, and there was no impurity element in the matrix. At the same time, the roughly spherical particles were observed on the surface of the sintering neck, with a radius of 1~2 μm, as shown by point ②. The components of the particles mainly included Al2O3, SiO2 and CaO, while the content of iron, chromium and manganese element were obviously lower than that in the matrix. It was confirmed that the particles came from the impurities in the iron oxide scale.
Figure 15 shows the sintering neck interior microstructure of the porous 316 stainless steel. As can be seen from Figure 15a, the size of the austenite grains in the sintering neck was uneven, and they were all less than 10 μm. The bright white bands or particles shown in Figure 15b were confirmed by energy spectroscopy analysis as a precipitated phase with a relatively high content of chromium and molybdenum. This precipitated phase was TCP phase, namely, σ phase, which is a hard and brittle intermetallic compound with a square lattice and is mainly composed of iron, chromium, molybdenum and other elements [39,40]. The content of molybdenum in the precipitated phase was 9.35% and much higher than that of the stainless steel at 2.49%. Meanwhile, the chromium content of the poor chromium area between the two white bands in the precipitated phase was 7.67% and far below that of the stainless steel at 16.60%.

3.3. Growing up of Impurity Particles

In order to prove the growing up of impurity particles in the iron oxide scale during reduction sintering, the morphology and composition of the particles sintered at 10−3 Pa and 1200 °C for 60, 120 and 180 min were analyzed with SEM-EDS. The back-scattered electronic images of the sintering neck and the particles are shown in Figure 16. The content of iron, manganese, calcium and chromium in the roughly spherical particles in Figure 16a is higher than that in Figure 16b,c. The content of iron and chromium decreased with the extension of the holding time, while the size of the impurity particle gradually increased. The maximum size of the impurity particles reached 5 μm, and their color became deepest when the holding time reached 180 min, indicating that the content of iron, manganese, chromium and other elements gradually decreased because the atomic number of these elements is greater than that of calcium, silicon and aluminum.
Because the iron oxide scale reduction process was carried out step by step, ferrous oxide was an intermediate product. In order to reveal the influence of ferrous oxide on the melting properties of impurities, the CaO-SiO2-Al2O3 ternary phase diagram containing MgO and FeO was calculated and analyzed with FactSage 8.1 database, as shown in Figure 17. In Figure 17, there is liquid slag coexisting with several mineral phases, including Ca2SiO4, monoxide, Ca3MgAl4O10, melilite, CaAl2Si2O8, peridot, spinel, clinopyroxene and so on. In the calculation, the content of FeO was 20%, while that of iron oxide scale declined from 58.63% to zero. In fact, FeO has the function of reducing the melting point [41,42]. The liquid phase is a benefit to the separation of impurity oxides gradually from the iron matrix because the shrinkage of the iron phase is greater than that of the impurity particles [43]. As a result, the particles enriched on the austenitic grain boundary as roughly spherical particles.
In order to reduce the impurities in the porous 316 stainless steel, the magnetic field strength was increased from 3000 Oe to 5000 Oe, and the non-magnetic substances in the iron oxide scale were further separated and removed. The SEM back scatter image of the surface and inside of the porous 316 stainless steel prepared with new raw materials and the original reduction sintering system is shown in Figure 18. Figure 18a is the surface micromorphology of the porous 316 stainless steel, and Figure 18b is that of the inside. Therefore, the impurities in the sample were significantly reduced, and magnetic separation was an effective method to reduce impurities in the porous stainless steel.
The main goal of this paper was to develop high-value-added metal materials and products with an iron oxide scale as a raw material, but the impurity in the iron oxide scale was the main factor that impacted the product quality. Therefore, it is necessary to explore effective ways to reduce the impurity content or find the appropriate application field of the above metal materials and products.

4. Conclusions

Porous 316 stainless steel was prepared by carbon reduction under vacuum with iron oxide scale as the main raw material in this study. The specific reduction system was confirmed, including the sintering temperature, sintering time, vacuum degree and carbon amount through thermodynamic calculation combined with experiments. The characters of the intermediate products and final product were analyzed and measured. Thermodynamic analysis results showed that the conversion process of the chromium element in the raw materials was FeCr2O4→FeCr2O4 + Cr3O4→Cr3O4 + Cr2O3→Cr7C3 + Cr3O4→Cr(liq) + Cr3O4 at 1 atm and 300~1600 °C. The liquid phase began to generate at 1126 °C, so porous stainless steel could not be prepared at 1 atm. The transformation process of the chromium element in the raw materials at 10−4 atm and 300~1600 °C was FeCr2O4 + Cr3O4→Cr2O3 + Cr3O4 + Cr23C6→Cr23C6 + Cr7C3 + FCC→FCC + Cr23C6→FCC→FCC + BCC→Cr(liq). The FCC phase with qualified carbon content could be obtained below 10−4 atm and 1200 °C, while 90.88 g iron oxide scale, 17.17 g carbon, 17.00 g metal chromium, 12.00 g metal nickel and 2.50 g metal molybdenum were necessary to produce 100 g porous 316 stainless steel. The sintering experiment results showed that porous 316 stainless steel with a carbon content of 0.025% could be obtained at 10−4 atm and 1200 °C for 180 min, while chromium element underwent the transformation of metal, Cr→FeCr2O4→Cr23C6→Austenite. The porosity of the porous 316 stainless steel was 42.07%. Additionally, the size of the austenite grains in the sintered neck was uneven, and they were all less than 10 μm. The σ phase appeared in the porous 316 stainless steel, in which the content of molybdenum was 9.35% and much higher than that of the stainless steel at 2.49%. Meanwhile, the chromium content of the poor chromium area was 7.67%, which was far below that of the 316 stainless steel at 16.60%. The maximum size of the impurity particles was 5 μm when the holding time reached 180 min. Magnetic separation was an effective method to reduce the impurities in the porous stainless steel.

Author Contributions

Investigation, F.Z., H.C. and Y.W.; methodology, F.Z.; data curation, F.Z., H.C. and Y.W.; writing—original draft, F.Z.; conceptualization, J.P.; supervision, J.P.; writing—review and editing, J.P. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (51864041, 51874186, 51664066).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Microscopic morphology and elemental distribution on the surface of iron oxide scales before treatment.
Figure 1. Microscopic morphology and elemental distribution on the surface of iron oxide scales before treatment.
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Figure 2. X-ray pattern of iron oxide scale.
Figure 2. X-ray pattern of iron oxide scale.
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Figure 3. Particle size distribution of iron oxide scale.
Figure 3. Particle size distribution of iron oxide scale.
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Figure 4. The preparation process of porous stainless steel.
Figure 4. The preparation process of porous stainless steel.
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Figure 5. Occurrence of Cr element at 1 atm and different temperatures.
Figure 5. Occurrence of Cr element at 1 atm and different temperatures.
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Figure 6. Carbon content in FCC under different system pressure.
Figure 6. Carbon content in FCC under different system pressure.
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Figure 7. Temperature range of FCC existing under 10−5~10−1 atm.
Figure 7. Temperature range of FCC existing under 10−5~10−1 atm.
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Figure 8. Content of Cr-containing phase at 10−4 atm and different temperature.
Figure 8. Content of Cr-containing phase at 10−4 atm and different temperature.
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Figure 9. Effect of carbon addition at 10−4 atm and 1200 °C; (a) amount of FeCr2O4; and (b) carbon content in FCC.
Figure 9. Effect of carbon addition at 10−4 atm and 1200 °C; (a) amount of FeCr2O4; and (b) carbon content in FCC.
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Figure 10. Vacuum reduction sintering system of porous 316 stainless steel.
Figure 10. Vacuum reduction sintering system of porous 316 stainless steel.
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Figure 11. Rate of weight loss and carbon content in sample reduced at 10−4 Pa and 1200 °C.
Figure 11. Rate of weight loss and carbon content in sample reduced at 10−4 Pa and 1200 °C.
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Figure 12. XRD pattern of sintering sample at 10−4 atm and different temperatures for 180 min; (a) 700 °C; (b) 900 °C; (c) 1100 °C; and (d) 1200 °C.
Figure 12. XRD pattern of sintering sample at 10−4 atm and different temperatures for 180 min; (a) 700 °C; (b) 900 °C; (c) 1100 °C; and (d) 1200 °C.
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Figure 13. Carbon and oxygen content in samples at 10−4 atm and different temperatures.
Figure 13. Carbon and oxygen content in samples at 10−4 atm and different temperatures.
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Figure 14. Micromorphology of porous 316 stainless steel at 10−4 atm and 1200 °C for 180 min; (a) macro morphology; (b) microscopic morphology of the matrix and impurity particles; ① matrix; and ② impurity particle.
Figure 14. Micromorphology of porous 316 stainless steel at 10−4 atm and 1200 °C for 180 min; (a) macro morphology; (b) microscopic morphology of the matrix and impurity particles; ① matrix; and ② impurity particle.
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Figure 15. Microstructure of porous 316 stainless steel; (a) austenite grain; and (b) σ phase; the energy spectrum analysis is on the right side of the back-scattered electronic image.
Figure 15. Microstructure of porous 316 stainless steel; (a) austenite grain; and (b) σ phase; the energy spectrum analysis is on the right side of the back-scattered electronic image.
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Figure 16. Back-scattered electronic image and composition analysis of particles at 10−4 atm and 1200 °C; (a) 60 min; (b) 120 min; and (c) 180 min; the energy spectrum analysis is on the right side of the back-scattered electronic image.
Figure 16. Back-scattered electronic image and composition analysis of particles at 10−4 atm and 1200 °C; (a) 60 min; (b) 120 min; and (c) 180 min; the energy spectrum analysis is on the right side of the back-scattered electronic image.
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Figure 17. CaO-SiO2-Al2O3 ternary phase diagram.
Figure 17. CaO-SiO2-Al2O3 ternary phase diagram.
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Figure 18. Back-scattered electronic image of porous 316 stainless steel with stronger magnetic separation iron oxide scale; (a) surface; (b) inside; ① the energy spectrum analysis of point ①; and ② the energy spectrum analysis of point ①.
Figure 18. Back-scattered electronic image of porous 316 stainless steel with stronger magnetic separation iron oxide scale; (a) surface; (b) inside; ① the energy spectrum analysis of point ①; and ② the energy spectrum analysis of point ①.
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Table
Table 1. Chemical composition of 316 stainless steel, wt %.
Table 1. Chemical composition of 316 stainless steel, wt %.
ComponentCCrNiMnMoSPFe
Steel specification≤0.0316~1810~14≤2.02~3≤0.030≤0.035Bal
Median≤0.031712≤2.02.5≤0.030≤0.035Bal
Table
Table 2. Chemical composition of iron oxide scale, wt %.
Table 2. Chemical composition of iron oxide scale, wt %.
TFeFeOFe2O3SiO2CaOAl2O3MgOSPLOI
73.1758.6339.380.300.360.230.390.020.010.68
Table
Table 3. Basic information about auxiliary materials.
Table 3. Basic information about auxiliary materials.
Auxiliary MaterialsPurityManufacturer
Chromium metal powder≥99.9%Zhongmai Metal Materials Co., Ltd., Nangong, China
Nickel metal powder ≥99.9%Zhongmai Metal Materials Co., Ltd., Nangong, China
Molybdenum metal powder≥99.9%Zhongmai Metal Materials Co., Ltd., Nangong, China
Graphite powder≥99.9%Kermel Chemical Reagent Co., Ltd., Tianjin, China
Table
Table 4. Ingredient list of raw materials for porous 316 stainless steel, g.
Table 4. Ingredient list of raw materials for porous 316 stainless steel, g.
Iron Oxide ScaleC Cr Ni Mo
90.8816.9317.0012.002.50
Table
Table 5. Yield of alloy in vacuum reduction sintering process, wt %.
Table 5. Yield of alloy in vacuum reduction sintering process, wt %.
Raw MaterialMetal ChromiumMetal NickleMetal Molybdenum
Yield98.71100.0097.20
Table
Table 6. Chemical composition of porous 316 stainless steel, wt %.
Table 6. Chemical composition of porous 316 stainless steel, wt %.
FeNiMnMoCrCOSP
66.8912.001.982.4916.600.0250.200.0100.020
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Zhang, F.; Peng, J.; Chang, H.; Wang, Y. Vacuum Carbon Reducing Iron Oxide Scale to Prepare Porous 316 Stainless Steel. Metals 2022, 12, 2118. https://doi.org/10.3390/met12122118

AMA Style

Zhang F, Peng J, Chang H, Wang Y. Vacuum Carbon Reducing Iron Oxide Scale to Prepare Porous 316 Stainless Steel. Metals. 2022; 12(12):2118. https://doi.org/10.3390/met12122118

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Zhang, Fang, Jun Peng, Hongtao Chang, and Yongbin Wang. 2022. "Vacuum Carbon Reducing Iron Oxide Scale to Prepare Porous 316 Stainless Steel" Metals 12, no. 12: 2118. https://doi.org/10.3390/met12122118

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