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

Materials Transactions, Vol. 47, No. 8 (2006) pp. 1878 to 1881 #2006 The Japan Institute of Metals

Preparation of White Heart Malleable Cast Iron in Na2O-K2O-SiO2 Oxide Molten Salt

Hiroaki Yamamoto, Kenji Shinzaki*, Masao Morishita 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 by heat treatment of white cast iron in Na2O-K2O-SiO2 oxide molten salts at lower temperature than usual production method was studied. The liquidus temperature of the K2O added Na2O-SiO2 (64–36 mol%) system was measured by the hot-thermocouple method and the eﬀect of the treatment temperature on the degree of decarburization of white cast iron was investigated. The result of measuring liquidus temperature indicated that the Na2O-K2O-SiO2 (38.4–40–21.6 mol%) ternary oxide melted at about 893 K. After heat treatments of white cast iron (8 mm 8 mm) containing 3.34 mass% carbon in this oxide molten salt at 1123–1223 K for 72 h, the white heart malleable cast iron could be obtained. Although, the thickness of the surface decarburized layer decreased with the decrease of the treatment temperature, the white heart malleable cast iron having about 30 mm surface layer was obtained even at low temperature of 1123 K. The basicity of this molten salt was 2.53, which was too high to be suitable for the decarburization of white cast iron at 1323 K. It is found that white heart malleable cast iron can be prepared by the heat treatment in high basisity oxide molten salt at lower temperature than 1323 K. [doi:10.2320/matertrans.47.1878]

(Received March 31, 2006; Accepted May 19, 2006; Published August 15, 2006) Keywords: oxide molten salt, basicity, decarburization of cast iron, heat treatment, white heart malleable cast iron

1. Introduction 2. Experimental

White heart malleable cast iron generally consists of a Commercial sodium carbonate (Na2CO3), potassium car- surface decarburized low-carbon zone and an internal zone bonate (K2CO3), silicon dioxide (SiO2), and sodium meta- that contains pearlite, ferrite, and temper carbon. It has silicate anhydrous (Na2SiO3) were used to prepare the Na2O- excellent properties in terms of ductility, corrosion resistiv- SiO2 and Na2O-K2O-SiO2 oxide molten salts. These reagents ity, weldability, and the possibility of plating, and has been were weighed in appropriate amounts, and mixed and milled widely used as mechanical materials for automobiles, in a mortar. This mixture was heated to 1023 K and scaffolding, and so on.1) The usual production process of maintained in this condition for 24 h in order to prepare white heart malleable cast iron requires the heat treatment at Na2O and K2O by extracting CO2 from Na2CO3 and K2CO3, higher temperature than 1273 K for long duration.2–4) From respectively. The mixture was then heated to the experimen- the viewpoint of energy conservation, this process should be tal temperature (1073–1323 K) to prepare the oxide molten conducted at low temperature and for a short duration. We salt. have proposed and reported about the preparation of white In this study, the chemical composition (mass%) of the heart malleable cast iron by the decarburization of white cast white cast iron used for the preparation of white heart 5) iron in Na2O-SiO2 binary oxide molten salt. It was clarified malleable cast iron is as follows: C: 3.34, Si: 0.65, Mn: 0.35, that the thickness of the surface decarburized layer increased P: 0.073, and S: 0.095. The size of the white cast iron was with the Na2O composition within the limit of 41 < Na2O 8 mm 8 mm. It was immersed in the oxide molten salt for (mol%) 5 65 in the Na2O-SiO2 oxide molten salt at 1323 K. 24–72 h and cooled to room temperature after the heat 6,7) According to the phase diagram of the Na2O-SiO2 system, treatment. The obtained specimens were cut into two parts, however, there is no composition in the above Na2O limit polished by waterproof abrasive papers and a buff, etched by where the Na2O-SiO2 oxide mixture melts below 1233 K. a solution of 5% nital, and observed by an optical micro- Since the liquidus temperature of Na2O-K2O-SiO2 ternary scope. The carbon content and its change in the cross-section oxide is comparatively low,8) it is expected that the melting of the specimen were measured by an electron probe micro- temperature of the K2O added Na2O-SiO2 (64–36 mol%) analyzer (EPMA, JEOL, JXA-8900R). ternary oxide system decreases. In order to prepare white heart malleable cast iron at lower temperature than 1323 K, 3. Results and Discussion the liquidus temperature of the K2O added Na2O-SiO2 (64– 36 mol%) system was measured by the hot-thermocouple 3.1 Relationship between basicity and decarburization method and the effect of the treatment temperature of the at 1323 K oxide molten salt on the degree of decarburization of white The thickness of the surface decarburized layer of the cast iron was investigated in this paper. specimens obtained after heat treatment of white cast iron at 1323 K for 24 h in various compositions of the Na2O-SiO2 binary oxide molten salts as a function of the basicity, which is based on the coulomb force between a cation and an oxygen ion and can be calculated from the molten oxide 9) *Graduate Student, University of Hyogo. Present address: TOCALO CO., composition, is shown in Fig. 1 with additional data to the LTD., Kobe 658-0013, Japan figure shown in the previous paper.5) It was found that the Preparation of White Heart Malleable Cast Iron in Na2O-K2O-SiO2 Oxide Molten Salt 1879

80 1300 m µ / d 1200 60

1100 /K T 40 1000

20 Temperature, 900 Thickness of Surface Decarburized Layer, 0 800 1.2 1.4 1.6 1.8 2.0 0 20 40 60 80 Bacisity Parameter B K2O (mol%) Fig. 1 Thickness of the surface decarburized layer after heat treatment at Fig. 2 Liquidus temperature of the K O added Na O-SiO (64–36 mol%) 1323 K for 24 h in various compositions of Na O-SiO oxide molten salts. 2 2 2 2 2 system measured by the hot-thermocouple method. thickness of the surface decarburized layer increased with the basicity within the limit of 1:37 < B 5 1:85 (41 < Na2O (mol%) 5 65). The lowest melting temperature of Na2O- Surface Decarburized Layer SiO2 system in this Na2O limit is about 1233 K at the 6,7) composition of Na2O-SiO2 (64–36 mol%), which is the (a) eutectic composition and the basicity is B ¼ 1:83. Although the melting temperature of the K2O added Na2O-SiO2 (64– 36 mol%) ternary oxide system is unknown, it is expected to decrease, since the known liquidus temperature of Na2O- 8) K2O-SiO2 ternary oxide is comparatively low.

3.2 Liquidus temperature of K2O added Na2O-SiO2 (64–36 mol%) oxide molten salt The liquidus temperature of the K2O added Na2O-SiO2 (64–36 mol%) system was measured by the hot-thermocouple method. The apparatus for the hot-thermocouple method was mentioned in the previous paper.10) The melting temper- atures of Na2O-K2O-SiO2 oxides are shown in Fig. 2. It is carbon pearlite found that the lowest melting temperature of the K2O added Na2O-SiO2 (64–36 mol%) system is about 893 K at the (b) composition of Na2O-K2O-SiO2 (38.4–40–21.6 mol%). Con- sequently, the decarburization of white cast iron was attempted to prepare white heart malleable cast iron in this oxide molten salt at lower temperature than 1323 K.

3.3 Preparation of white cast iron in Na2O-K2O-SiO2 oxide molten salt Figure 3(a) shows the result of the optical microscopic observation of the cross-section near the surface of the specimen obtained after heat treatment at 1173 K for 72 h in Na2O-K2O-SiO2 (38.4–40–21.6 mol%) oxide molten salt. The white cast iron used in this study contains 3.34 mass% carbon and consists of pearlite and cementite, as shown in 100 µm previous paper.5) On the other hand, a white-colored layer— with a thickness of about 60 mm thickness—on the surface of Fig. 3 Optical microscopic photographs of the cross-section near the the specimen is observed in Fig. 3(a). It is considered that surface of the specimens after heat treatment in Na2O-K2O-SiO2 (38.4– this white-colored surface layer is a low-carbon layer formed 40–21.6 mol%) molten salt at (a) 1173 K for 72 h and (b) 1323 K for 24 h. 1880 H. Yamamoto, K. Shinzaki, M. Morishita and K. Koyama

100

(a): C m µ / d 80

surface 60

Intensity (a. u.) 40

(b): Si 20

Thickness of Surface Decarburized Layer, 0 1050 11001150 1200 1250 surface Temperature, T/K Intensity (a. u.) Fig. 5 Thickness of the surface decarburized layer after heat treatment at 1073–1223 K for 72 h in Na2O-K2O-SiO2 (38.4–40–21.6 mol%) molten salt. 0 100 200 300 Distance from Surface,d/µm be figured out. The detection of carbon is comparatively high Fig. 4 Change in the detected intensity of (a) carbon and (b) silicone near the boundary as shown in Fig. 4(a). This is caused by the measured by EPMA line analysis (beam diameter: 1 mm) in the cross- fact that the remained silicon in the decarburized layer section near the surface of the specimen after heat treatment at 1173 K for inhibits the diffusion of carbon.11) 72 h in Na2O-K2O-SiO2 (38.4–40–21.6 mol%) molten salt. Figure 5 shows the thickness of the surface decarburized layer of the specimen obtained after heat treatment of white as a result of the reaction of carbon in the white cast iron with cast iron at 1073–1223 K for 72 h in the Na2O-K2O-SiO2 free oxygen in the oxide molten salt. In order to confirm the (38.4–40–21.6 mol%) oxide molten salt. The thickness of the decarburization of white cast iron, the distribution of carbon surface decarburized layer decreases with the decrease of the in the cross-section near the surface of the specimen was treatment temperature. This is caused by the decreases of the measured by EPMA line analysis (beam diameter: 1 mm). The diffusion rate of carbon in the specimen and/or the result is shown in Fig. 4(a). The detected intensity of carbon decarburization reaction on the surface. However, the white near the surface of the specimen is lower than that of the heart malleable cast iron having about 30 mm surface layer inner zone. The surface carbon concentration was about 0.49 was obtained even at low temperature of 1123 K. The basicity mass%, and it was lower than that of the white cast iron of this Na2O-K2O-SiO2 (38.4–40–21.6 mol%) oxide molten before the heat treatment. The thickness of the low-carbon salt is B ¼ 2:53, and it is too high to be suitable for the zone at the surface of the specimen, as shown in Fig. 4(a), is decarburization of white cast iron at 1323 K as shown in about 60 mm, and it coincides with the thickness of the white- Fig. 1, because the excess oxygen ion in the molten salt colored layer shown in Fig. 3(a). Thus, the white-colored oxidizes the iron in the decarburized layer.5) The heat surface layer is a low-carbon layer formed as a result of the treatment was conducted actually at 1323 K in this molten decarburization reaction of white cast iron. These results salt for 24 h, and the result of the optical microscopic suggest that white heart malleable cast iron can be prepared observation of the cross-section near the surface of the by heat treatment in the Na2O-K2O-SiO2 oxide molten salt. specimen is shown in Fig. 3(b). The white-colored surface Although the detected intensity of carbon in the surface layer was not observed and the surface of the specimen was decarburized layer is almost the same, that of silicon has irregular. It is considered that almost all the surface gradient as shown in Fig. 4(b). It is considered that the decarburized layer was oxidized by oxygen in the molten diffusion rate of carbon is higher than that of silicon in the salt and dissolved in Na2O-K2O-SiO2 oxide molten salt, white cast iron and the carbon near the surface is removed because the diameter of the specimen decreased after the rapidly and some amount of silicon remained in the surface experiment. From these results, it is found that the basicity of decarburized layer. An inner part of the surface decarburized oxide molten salt for the preparation process of white heart layer of white heart malleable cast iron consists of pearlite, malleable cast iron at lower temperature than 1223 K is ferrite, and temper carbon. Thus, the intensity of carbon required to be higher than that of 1323 K. depends on the structure under the measurement line. Although the intensity change of carbon shown in Fig. 4(a) 4. Conclusions does not show the average concentration change exactly, the overview of carbon distribution tendency near the boundary The preparation process of white heart malleable cast between the inner part and the surface decarburized layer can iron in Na2O-K2O-SiO2 oxide molten salts at lower temper- Preparation of White Heart Malleable Cast Iron in Na2O-K2O-SiO2 Oxide Molten Salt 1881 ature than 1323 K was investigated. The following conclu- REFERENCES sions were obtained: 1) S. 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