Preparation of White Heart Malleable Cast Iron in Na {2}

Materials Transactions, Vol. 47, No. 2 (2006) pp. 263 to 266 #2006 The Japan Institute of Metals

Preparation of White Heart Malleable Cast Iron in Na2O–SiO2 Oxide Molten Salt

Hiroaki Yamamoto, Kenji Shinzaki* and Koichiro Koyama

Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, Himeji 671-2201, Japan

The preparation process of white heart malleable cast iron in Na2O–SiO2 oxide molten salts was studied. We investigated the eﬀect of the molten salt composition on the decarburization of white cast iron. A specimen of white cast iron (8 mm 8 mm) containing 3.34 mass% carbon was immersed in a Na2O–SiO2 molten salt at 1323 K for 24–72 h. After the heat treatment of white cast iron in Na2O–SiO2 (64–36 mol%) at 1323 K for 48 h, low-carbon surface layer had a thickness of about 100 mm and contained 0.53 mass% carbon. This surface layer was formed as a result of the reaction of carbon in white cast iron with free oxygen in the oxide molten salt. However, no decarburized layer was observed on the specimen surface after the heat treatment in the Na2O–SiO2 (38–62 mol%) molten salt. As the Na2O composition increased within the limit of 41 < Na2O (mol%) 5 65, the thickness of the decarburized layer also increased. It was found that white heart malleable cast iron could be prepared from white cast iron in Na2O–SiO2 oxide molten salts, and the degree of decarburization depended on the composition of the molten salts.

(Received October 21, 2005; Accepted December 22, 2005; Published February 15, 2006) Keywords: oxide molten salt, basicity, decarburization of cast iron, heat treatment, white heart malleable cast iron

1. Introduction 1400 AB C

Cast iron has a low melting temperature, and its melt L

exhibits good fluidity; further, it has been used for casting for /K a long time. Cast iron is an Fe–C alloy containing more than T 2.0 mass% carbon, 1–3 mass% silicon, and other elements to 1000 5 7 3 4 O control specific properties. It is harder than low carbon steel, O 2 2 Si Si SiO SiO 2 6 2 but has lower ductility. This problem is overcome in 4 Temperature, Na Na Na malleable cast iron by heat treatment. White heart malleable Na cast iron is a malleable cast iron that comprises a decarburized low-carbon zone at the surface and an internal zone that 800 Na2O 20 40 60 SiO2 contains pearlite, ferrite, and temper carbon. White heart SiO (mol%) malleable cast iron has excellent properties in terms of 2 5,6) ductility, corrosion resistivity, weldability, and the possibility Fig. 1 Na2O–SiO2 phase diagram. of plating. It has been widely used as mechanical materials for automobiles, scaffolding, and so on.1) Usually, white heart malleable cast iron is prepared by the heat treatment of white cast iron enveloped in decarburization agents such as 2. Experimental iron oxide at 1173–1373 K for 40–100 h.2–4) From the viewpoint of energy conservation, this process should be Commercial sodium carbonate (Na2CO3), silicon dioxide conducted at low temperature and for a short duration. (SiO2), and sodium metasilicate anhydrous (Na2SiO3) were Many types of molten salts including oxides, chlorides, used to prepare the Na2O–SiO2 oxide molten salts. The phase 5,6) fluorides, and so on have been studied for electrodeposition, diagram for the Na2O–SiO2 system is shown in Fig. 1. The synthesis, surface finishing of materials, and metal refine- decarburization of white cast iron was conducted at or near ment. In this study, Na2O–SiO2 oxide molten salts were used the eutectic compositions of A, B, and C indicated in Fig. 1. for the decarburization of white cast iron in order to prepare Na2SiO3 and Na2CO3 for Na2O-rich compositions, or white heart malleable cast iron. The production process of Na2SiO3 and SiO2 for SiO2-rich compositions were weighed white heart malleable cast iron is expected to be simplified. In in appropriate amounts and mixed and milled in a mortar. this paper, the preparation process of white heart malleable This mixture was heated to 973 K and maintained in this cast iron using Na2O–SiO2 oxide molten salt was studied and condition for 24 h in order to prepare Na2O by extracting CO2 the effect of the molten salt composition on the decarburiza- from Na2CO3. The mixture was then heated to 1323 K to tion of cast iron was investigated. prepare the oxide molten salts. In this study, the chemical composition (mass%) of the white cast iron used for the preparation of white heart malleable cast iron is as follows: C: 3.34, Si: 0.65, Mn: 0.35, *Graduate Student, University of Hyogo. Present address: TOCALO CO., P: 0.073, and S: 0.095. The size of the white cast iron was LTD., Kobe 658-0013, Japan 8 mm 8 mm. It was immersed in Na2O–SiO2 oxide 264 H. Yamamoto, K. Shinzaki and K. Koyama molten salt at 1323 K for 24–72 h and cooled to room and cementite (bright parts in the photograph), as shown in temperature after the heat treatment. The obtained specimens Fig. 2(a). On the other hand, a white-colored layer—with a were cut into two parts, polished by waterproof abrasive thickness of about 100 mm thickness—on the surface of the papers and a buff, etched by a solution of 4% picral, and specimen, as shown in Fig. 2(b), and black precipitations observed by an optical microscope. surrounded by a white-colored phase in the inner part of the specimen, as shown in Fig. 2(c), are observed. White heart 3. Results and Discussion malleable cast iron generally consists of a decarburized low- carbon zone at the surface and an internal zone that contains 3.1 Preparation of white heart malleable cast iron pearlite, ferrite, and temper carbon. It is considered that the The decarburization of white cast iron was conducted in white surface layer shown in Fig. 2(b) is the decarburized Na2O–SiO2 (64–36 mol%) oxide molten salt at 1323 K for layer, and the black precipitations and white phases shown in 48 h. The results of the optical microscopic observation of the Fig. 2(c) are the temper carbon and ferrite phases, respec- specimens (a) before and (b, c) after the heat treatment are tively. To confirm the decarburization of white cast iron, the shown in Fig. 2. The white cast iron contains 3.34 mass% change in the carbon concentration in the cross-section of the carbon and consists of pearlite (gray parts in the photograph) specimen was measured by EPMA line analysis. Figure 3 shows the result of the EPMA line analysis. The carbon concentration near the surface of the specimen is lower than a) that near the inner zone. The surface carbon concentration was about 0.53 mass%, and it was lower than that of the white Intensity b)

0 100 200 Distance from Surface,d/µm

Fig. 3 Change in the carbon concentration of the specimen heat treated at 1323 K for 48 h in Na2O–SiO2 (64–36 mol%) molten salt measured by EPMA line analysis.

m 300 µ / d

200

100

100µm 0 Thickness of Surface Decarburized Layer, 021 3 Fig. 2 Optical microscopic photographs of the cross-section of the Time,t/day specimens (a) before and (b, c) after the heat treatment at 1323 K for 48 h in Na2O–SiO2 (64–36 mol%) molten salt. (a), (b): surface of the Fig. 4 Thickness of the surface decarburized layer after heat treatment in specimens. (c): almost center of the specimen. Na2O–SiO2 (64–36 mol%) molten salt at 1323 K. Preparation of White Heart Malleable Cast Iron in Na2O–SiO2 Oxide Molten Salt 265

a) Bacisity Parameter B 1.2 1.4 1.6 1.8 2.0 80 m µ / d

20 b) Thickness of Surface Decarburized Layer, 0 30 40 50 60 70 Na2O (mol%)

Fig. 6 Thickness of the surface decarburized layer after heat treatment at 1323 K for 24 h in various compositions of Na2O–SiO2 molten salts.

after heat treatment for 72 h is shown in Fig. 5(a). The thickness of the surface decarburized layer is about 260 mm. The surface carbon concentration was about 0.11 mass%, c) which was lower than that of the specimen obtained after heat treatment for 48 h, as mentioned above. Furthermore, the decarburization of the inner part of the specimen that was not observed in Fig. 2(b) can be observed in Fig. 5(a). This suggests that the decarburization reaction (diﬀusion of carbon at the surface) proceeds to the inner part after 48 to 72 h. It is observed that the surface decarburized layer can be obtained within 1 day; however, it takes 2–3 days to decarburize the inner part of white cast iron in Na2O–SiO2 oxide molten salt.

3.2 Effect of the molten salt composition on the decarburization of white cast iron 100µm The decarburization of white cast iron was conducted in (a) Na2O–SiO2 (64–36 mol%), (b) Na2O–SiO2 (54–46 mol%), Fig. 5 Optical microscopic photographs of the cross-section of the and (c) Na2O–SiO2 (38–62 mol%) molten salts at 1323 K for specimens after heat treatment at 1323 K for 72 h in Na2O–SiO2 molten 24 h. Figure 5 shows the optical microscopic photographs of salts ((a) 64–36 mol%, (b) 54–46 mol%, (c) 38–62 mol%). the cross-sections of the specimens. The thicknesses of the surface decarburized layers obtained after the heat treatment in molten salts (a) and (b) were about 260 and 170 mm, cast iron before the heat treatment. The thickness of the low- respectively. However, no surface decarburized layer was carbon zone at the surface, as shown in Fig. 3, is about found in the specimen obtained from the SiO2-rich molten 100 mm, and it coincides with the thickness of the white- salt (c). It is considered that the decarburization reaction colored layer shown in Fig. 2(b). Thus, the surface layer is a proceeds and the thickness of this layer increases with the low-carbon layer formed as a result of the reaction of carbon composition of Na2O. in the white cast iron with free oxygen in the oxide molten The degree of the decarburization of white cast iron has a salt. These results suggest that white heart malleable cast iron certain relationship with the composition of Na2O–SiO2 can be prepared by heat treatment in the Na2O–SiO2 oxide molten salts, that is, to the basicity, which is an index for the molten salt. activity of oxygen ions in the molten salt. Although there are Figure 4 shows the thickness of the surface decarburized several methods to define basicity,7–11) we adopt the basicity layer after heat treatment in Na2O–SiO2 (64–36 mol%) parameter B that is based on the coulomb force between a molten salt at 1323 K for 24–72 h. The thickness of this cation and an oxygen ion and can be calculated from the layer increases with the duration of the treatment. The optical molten oxide composition.7) However, it should be noted that microscopic photograph of a cross-section of the specimen this parameter almost depends on the Na2O composition in 266 H. Yamamoto, K. Shinzaki and K. Koyama the Na2O–SiO2 binary system. In order to investigate the 4. Conclusions effect of the composition (basicity) on the decarburization, experiments were carried out with different compositions of The preparation process of white heart malleable cast iron the Na2O–SiO2 system including those mentioned above at in Na2O–SiO2 oxide molten salts was studied, and the effect 1323 K for 24 h. The thickness results of the surface of the molten salt composition on the decarburization of cast decarburized layer as a function of the Na2O composition iron was investigated. The following conclusions were and the basicity parameter B are shown in Fig. 6. At low obtained: Na2O compositions below 41 mol% (at low basicity), the (1) White heart malleable cast iron can be obtained with decarburization of white cast iron did not occur. As the Na2O various compositions of Na2O–SiO2 oxide molten salts. composition increased within the limit of 41 < Na2O For example, a surface decarburized layer with a (mol%) 5 65 (1:37 < B 5 1:85), the thickness of the decar- thickness of about 100 mm was obtained after the heat burized layer increased and reached the maximum thickness treatment in Na2O–SiO2 (64–36 mol%) molten salt at of about 75 mm at 65 mol% Na2O. However, the thickness of 1323 K for 48 h. the surface decarburized layer suddenly decreased at (2) The thickness of the surface decarburized layer increas- 70 mol% Na2O (higher than B ¼ 1:9). It is considered that ed with the duration of the treatment. This layer could most of the surface decarburized layer, with a thickness be obtained within 1 day, but it took 2–3 days to greater than 75 mm, dissolved in Na2O–SiO2 molten oxide decarburize the inner part of the white cast iron. because the diameter of the specimen decreased after the (3) At low Na2O compositions below 41 mol%, the experiment. decarburization of white cast iron did not occur. Let us discuss the relationships between the basicity and As the Na2O composition increased within the limit the decarburization on the basis of the structure of Na2O– of 41 < Na2O (mol%) 5 65 (1:37 < B 5 1:85), the SiO2 molten salts. The SiO2 melt has a –Si–O–Si– network thickness of the decarburized layer increased and structure formed by sharing the oxygen of the SiO4 reached the maximum thickness of about 75 mm at tetrahedral units. According to Lux’s acid-base definition, 65 mol% Na2O. Na2O is a basic substance that gives an oxygen ion, and SiO2 (4) At a high-basicity composition (B > 1:9), the thickness is the acidic substance that accepts the oxygen ion.12,13) When of the surface decarburized layer decreased because it Na2O is added to the SiO2 melt, SiO2 accepts the oxygen ions was oxidized to iron oxide and dissolved into molten given by Na2O and parts of the –Si–O–Si– networks are salt. broken.14–17) At low-basicity compositions (B < 1:37), most of the oxygen ions given by Na O are used to break the –Si– 2 REFERENCES O–Si– network. Further, the activity of the free oxygen ion 2 (O ) in the oxide molten salts is too low to decarburize the 1) S. Tanaka, M. Isakawa and H. Nagasaki: Kensagijutsu 10 (2005) 50– white cast iron. 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