Influence of Mo and W on High Temperature Hardness of M 7 C 3 Carbide in High Chromium White Cast Iron +1

Transcription

1 Materials Transactions, Vol., No. 4 (214) pp. 684 to Japan Foundry Engineering Society Influence of Mo and W on High Temperature Hardness of M 7 C 3 Carbide in High Chromium White Cast Iron +1 Kaoru Yamamoto 1,+2, Sudsakorn Inthidech 2, Nobuya Sasaguri 1 and Yasuhiro Matsubara 3 1 Department of Materials Science and Engineering, Kurume National College of Technology, Kurume 83-8, Japan 2 Department of Production Engineering, Mahasarakam University, Mahasarakam 44, Thailand 3 Fukuoka Jo Gakuin, Fukuoka , Japan The influence of Mo and W addition on the high temperature hardness of M 7 C 3 carbide was investigated using unidirectionally solidified hypereutectic cast iron with 2 mass%. Concentrations of alloying elements in primary M 7 C 3 carbide were measured by EDS. As Mo or W content of the cast iron increases, the concentration of Mo and W in the carbide increased and that of decreased. However, the content was almost the same as about 6 mass% in spite of increasing the Mo and W contents. It was found from the XRD results that the lattice constant of M 7 C 3 carbide changed and its attendant volume of a unit cell increased as the Mo or W content was increased. From this point of view, it can be considered that atoms in the M 7 C 3 carbide were substituted by Mo or W which has larger atomic radius than. In all the specimens, the hardness of M 7 C 3 carbides are about 16HV.3 at the room temperature, and it decreases gradually with a rise of the test temperature. The decreasing ratio of carbide hardness becomes smaller at high temperature as the Mo or W concentration in M 7 C 3 carbide increases. Thus, the dissolution of Mo or W atom into M 7 C 3 carbide is very effective to maintaining the high temperature hardness of carbide. However, the increment of the hardness became smaller at higher Mo and W contents, so that an excess addition of both elements gave less effect than expected. The fracture toughness of M 7 C 3 carbide at the room temperature was measured using an indentation fracture method, and the values were very similar among the carbides regardless of Mo and W concentration in the M 7 C 3 carbide. [doi:1.232/matertrans.f-m21481] (Received September 3, 213; Accepted December 13, 213; Published bruary 21, 214) Keywords: high chromium cast iron, M 7 C 3 carbide, high temperature hardness, alloying element, fracture toughness 1. Introduction Table 1 Target chemical conditions for Cast iron specimens (mass%). The microstructure of high chromium white cast irons consists of a matrix and eutectic chromium carbide of M 7 C 3. The mechanical properties of this series of alloys, like hardness and strength, can be controlled by heat treatment. They have been widely used for their abrasion wear resistance in hot rolling mill rolls and rollers or tables of mineral pulverizing mills. Depending on service condition, there are many cases where high cast irons are exposed to high temperature. There, the wear resistance of the cast iron may deteriorate by oxidation and changing matrix structure. This behavior is manifested by declines of the strength and hardness at higher temperature. It is important for heat resistant materials to retain their strength at high temperature. In alloyed steels, the addition of certain elements provides resistance to softening at elevated temperatures. 1) These elements promote the precipitation of secondary carbides during heat treatment. 2 ) Deterioration of cast iron at high temperature is due to softening not only of the matrix but also the carbide itself. Therefore, it is also important to increase the carbide hardness at high temperatures. In this study, improvement of carbide strength at high temperature was sought by adding strong carbide formers such as Mo and W to a hypereutectic high cast iron. Since both elements distribute preferentially to the M 7 C 3 carbide during solidification, 6,7) their addition could improve the hardness of the carbide. They were added individually to the 2% cast iron, and their effects on the high temperature hardness of M 7 C 3 carbide were systematically evaluated. +1 This Paper was Originally Published in Japanese in J. JFS 84 (212) Corresponding author, yamamoto@kurume-nct.ac.jp Specimen C Mo W Mo-specimens bal. W-specimens bal. 2. Experimental Procedures 2.1 Test specimens High cast irons with compositions from hypoeutectic to near eutectic have been usually used according to ASTM standards. 8) In this range of composition, it is difficult to characterize the M 7 C 3 carbide particles themselves because the eutectic carbides have fine and rod-like or lamellar morphology. On the other hand with hypereutectic cast irons, the primary M 7 C 3 carbides crystallize in mass shapes and the alloy composition of primary M 7 C 3 carbide is approximately same as that of eutectic M 7 C 3 carbide. In this study, the characterization was carried out using the primary M 7 C 3 carbides crystallized in hypereutectic high cast irons. The target chemical compositions of cast iron specimens are shown in Table 1. According to the liquidus surface diagram of C system proposed by R. S. Jackson, 9) as shown in Fig. 1, M 7 C 3 carbides in 2 mass% iron precipitate as the primary phase when C content get over 3.2 mass%. The amounts of Mo and W added were each varied from to mass% in a base alloy with 4.%C and 2% where large sized primary carbides are expected to crystallize. Raw materials used for charge calculations in test specimen preparation were pig iron, ferroalloys and pure metals. The charge materials were melted, super-heated over 1773 K inside a Tammann furnace and then poured into a sand mold set on a water cooled copper chill plate as shown

2 Influence of Mo and W on High Temperature Hardness of M7C3 Carbide in High Chromium White Cast Iron 4 φ6 3 φ3 3 Exothermic powder Exothermic lining CO2 mold Content (mass%) δ 2 68 M7C3 2 γ 1 Test piece Fig. 1 φ3 M3C C Content (mass%) 1 Copper Chill Plate Water Diagram of liquidus surface for C system.9) Fig. 2 Schematic cross-sectional view of unidirectional solidification mold. Eutectic M2C %Mo 3%Mo %Mo 1μm %W 3%W Fig. 3 Influence of Mo or W content on as-cast microstructure of 2% 4.%C Mo and 2% 4.%C W alloy. in Fig. 2. The exothermic lining prevented the melt solidifying from the side of the mold. In this mold, unidirectional solidification took place from bottom to top. The primary M7C3 carbide with hcp crystal structure grows in the h1i direction. Each cast specimen was cut at mm from its bottom, and the cross-sectional surface was used for characterization experiments. Observation of solidification structures was carried out using an optical microscope and the quantitative analyses of alloy concentration within M7C3 carbide particles were performed using EDS. The lattice constants of the M7C3 carbides were determined by the XRD method. The M7C3 carbide particles, with diameters more than 1 µm, were subjected to Vickers micro-hardness measurements at elevated temperatures. Each specimen was held at prescribed temperature for min in vacuum of 1 1¹2 Pa, and the hardness of the {1} planes of the primary M7C3 carbide were measured with a load of 2.9 N (3 gf ) and the loading time of 2 s. These measurements were performed at room temperature and intervals of 1 K from 373 to 873 K. Finally, the fracture toughness of the M7C3 carbide at room temperature was estimated by Indentation Fracture Method Results and Discussions Effects of Mo and W contents on as-cast microstructure of specimens The microstructures of several 2% 4.%C alloys with Mo from % to %Mo are shown in Fig. 3. In all specimens, microstructures of hexagonal primary M7C3 carbide particles, over 1 µm in diameter, and fine lamellar eutectic M7C3 carbides growing from the vicinity of primary

3 686 K. Yamamoto, S. Inthidech, N. Sasaguri and Y. Matsubara Mo content in M7C3 carbide (mass%) 1 Mo Mo 1 2 Mo content of specimen (mass%) Fig. 4 Relationship between concentration of alloying elements in primary M 7 C 3 carbide and Mo content of specimen or content in M7C3 carbide (mass%) W content in M7C3 carbide (mass%) 1 W W 1 2 W content of specimen (mass%) Fig. Relationship between concentration of alloying elements in primary M 7 C 3 carbide and W content of specimen or content in M7C3 carbide (mass%) Partition coefficient, ka (A=, Mo, ) (a) Mo k (b) W k k kmo kmo 2. k kmo Partition coefficient, ka (A=, W, ) kw. 1 2 Mo content of specimen (mass%). 1 2 W content of specimen (mass%) Fig. 6 Effect of Mo or W content on partition coefficient of alloying elements to primary M 7 C 3 carbide in 2% 4.%C cast iron. carbide, were obtained. It was found that primary M 7 C 3 carbide grew along heat flow direction. Even if the Mo content increased to %, the microstructure of as-cast specimen did not change. However, M 2 C carbides form as a eutectic at the final stage of solidification when the Mo content increases to 1%. In the series of 2% 4.%C cast irons with W additions, the microstructures were similar to those of the cast irons with Mo. Moreover, eutectic M 2 C carbides are also observed in the specimens with W more than 1%. It is clear from these results that the morphology of primary M 7 C 3 in 2% cast irons with various Mo or W content are almost the same. The concentrations of alloying elements in primary M 7 C 3 carbide were analyzed using EDS. The relationships between the alloy concentration in primary M 7 C 3 carbide and Mo or W content of the specimens are shown in Figs. 4 and, respectively. The Mo content in the M 7 C 3 carbide increased linearly with an increase in Mo added to the specimen. When the Mo content of specimen is %, Mo dissolved in the carbide is approximately 1%. Accordingly, the content in the M 7 C 3 carbide decreased from 4 to 3%. The content in the M 7 C 3 carbide was originally around 6%. It changed slightly when the Mo content increased to 1%Mo, but reduced a little at %Mo. The variation of alloying element concentrations in the M 7 C 3 carbide for the W-bearing cast irons is similar to that in the Mo series. As the W content in the M 7 C 3 carbide increases, the content decreases. When the W content of the alloy reaches 17%, W dissolved in the carbide is 9.7% and content is 32%. However, concentration in the M 7 C 3 carbide is not affected by increasing W content. It remains around 6%. The partition coefficients of alloying elements to the primary M 7 C 3 carbide were calculated by Thermo-Calc. 1) The influence of Mo or W content on the partition coefficient is shown in Fig. 6, (a) for Mo-bearing specimens and (b) for W-bearing specimens, respectively. The partition coefficient of Mo (k Mo ) is about.4, that of W (k W ) is around.92. k

4 Influence of Mo and W on High Temperature Hardness of M 7 C 3 Carbide in High Chromium White Cast Iron Lattice constant of unit cell, a, c / nm Mo (a axis) Mo (c axis) W (a axis) W (c axis).4 1 Mo or W content in M7C3 carbide (mass%) Fig. 7 Effect of Mo and W contents on unit cell lattice constants (a axis and c axis) of M 7 C 3 carbide. Fig. 8 3 Volume of unit cell, V /nm Mo W Alloy content in M7C3 carbide (mass%) Effect of Mo and W contents on unit cell volume of M 7 C 3 carbide. is and k is about.. It is clear that each partition coefficient to the primary M 7 C 3 carbide does not changed even if the Mo or W content of carbide varies greatly. From these results, it is concluded that the concentrations of Mo and W in the primary M 7 C 3 increases in proportion to the initial compositions of alloying elements in the specimens. The unit cell lattice constants for the M 7 C 3 carbides were determined by XRD analysis. The relationship between the lattice constants of the hexagonal unit cell (a axis and c axis) and the alloy contents in the carbide are shown in Fig. 7. Though the data show scatter, a axis increases and c axis decreases with an increase in Mo content. On the other hand, the a axis decreases and c axis increases with increasing W content. The unit cell volume is calculated by the lattice parameter data. The effects of Mo and W contents on the volume of the M 7 C 3 carbide unit cell are shown in Fig. 8. The unit cell volume of the M 7 C 3 carbide increases as the content of each element increases. The proposed change of atomic layout in the M 7 C 3 carbide is shown in Fig ) When either an atom of Mo or W dissolves into the carbide, it replaces an atom. The atomic --C alloy --Mo or W-C alloy Fig. 9 ystal system of M 7 C 3 carbide. 11) radii of, Mo and W are.124,.136 and.137 nm, 12) respectively. With those of Mo and W being larger than that of, the lattice constants of a and c change with alloying element addition depending on the type of atom. In each case, the volume of the unit cell is expanded. 3.2 The effect of alloying elements on high temperature hardness Hardness of primary M 7 C 3 carbide in all cast iron specimens was measured from room temperature to 873 K. The relationships between the hardness of M 7 C 3 carbide and test temperature of Mo-bearing specimens and W-bearing specimens are shown in Figs. 1 and 11, respectively. In Fig. 1, over the range of Mo content, the hardness of M 7 C 3 carbide ranges from 4 to 168HV.3 at room temperature. Sharp declines in hardness occur for each Mo containing alloy with rising test temperature. However, the extent of hardness decline with temperature gradually slows with higher Mo content. Between room temperature and 873 K, the hardness of the Mo-free alloy declines by over 8 HV. By comparison, for the %Mo cast iron, a drop in hardness of only HV occurs. The addition of Mo gives the obvious effect of resisting softening to the high cast iron at high temperature. The carbide hardness versus temperature profile for the W-bearing cast irons is similar to that of Mo-specimens. The hardness decreases as the test temperature is elevated. The change in this property is quite sharp for the W-free cast iron. However, the decline becomes slower with rise in W content of the alloy. This trend attests that higher W addition imparts the cast iron to increase softening resistance at high temperature. The effects of both Mo and W content on the M 7 C 3 carbide hardness at various test temperatures are shown in Fig. 12. The hardness of M 7 C 3 carbide is little affected by alloy concentration in the carbide at low temperature. At high temperatures, however, the hardness increases with an increase sharply up to % Mo or W and then levels off. There may exist a critical concentration of elements to make the hardness saturate. The influence of Mo or W content on high temperature hardness of the M 7 C 3 carbide may be discussed using a misfit parameter. The creep rate of metal material is commonly evaluated using the misfit parameter ( m ) expressed in eq. (1). 13) m ¼ r s r r : : : Mo or W : C ð1þ

5 688 K. Yamamoto, S. Inthidech, N. Sasaguri and Y. Matsubara Vickers hardness, HV %Mo 1%Mo 3%Mo %Mo 1%Mo %Mo Vickers hardness, HV %W 1%W 3%W %W 1%W %W Temperature, T/K Fig. 1 Effect of Mo content of specimen on relationship between hardness of M 7 C 3 carbide and test temperature Temperature, T/K Fig. 11 Effect of W content of specimen on relationship between hardness of M 7 C 3 carbide and test temperature. Vickers hardness, HV (a) 73K K 6 1 Mo content in M7C3 carbide (mass%) Vickers hardness, HV (b) 14 73K K 6 1 W content in M7C3 carbide (mass%) Fig. 12 Effect of test temperature on relationship between Mo or W content in M 7 C 3 carbide and hardness of M 7 C 3. where, r s : atomic radius of solute, r : atomic radius of solvent. Matsuo et al. reported the relationship between the misfit parameter and the creep rate of 17% 14%Ni austenitic stainless steels. 14) It shows that the misfit parameter increases in the presence of Mo or W, and the creep rate reduces linearly. Umemoto et al. also reported the effect of alloying element on the high temperature hardness of cementite. ) When those alloying elements that increase the misfit parameter exist in cementite, the decreasing ratio of hardness corresponding to elevating of the test temperature is smaller than that of the specimen without alloying element. It is considered that the strengthening mechanism is similar to the creep behavior. In the carbides of this study, an atom is replaced by Mo and W, and the misfit parameters for Mo and W to are.9 and.1, while the misfit parameter of to is.1. The larger these values are the greater lattice distortion. The high temperature hardness of the M 7 C 3 carbide was improved due to the same reason as the creep resistance. Another reason for Mo and W to promote the resistance to softening at an elevated temperature could be due to the high melting point and large atomic radius of these elements. When Mo and W are added more than critical content, these elements can be substituted for and the influence on high temperature strength decreases. Therefore, the amount of alloying element affecting on high temperature hardness could be limited. 3.3 Influence of Mo and W on fracture toughness of M 7 C 3 carbide A fracture toughness measurement was made on primary M 7 C 3 carbide at room temperature by the indentation fracture method. 16) Figure 13 illustrates a schematic drawing of indentation fracture method. When the indenter contacts a carbide particle, as shown in the figure, cracks propagate from the corners of indentation on the carbide. The fracture toughness is obtained with the following eq. (2). K c ¼ ðepþ 1=2 ðd=2þa 3=2 ð2þ

6 Influence of Mo and W on High Temperature Hardness of M 7 C 3 Carbide in High Chromium White Cast Iron 689 Fig. 13 where, K c : fracture toughness (Pa m 1/2 ), E: Young s modulus of M 7 C 3 carbide (Pa), d: diagonal length of indenter (m), : calibration factor, P: indentated load (N), a: half length of crack (m). The fracture toughness on the {1} plane of M 7 C 3 carbide is calculated using the applied load of 2.9 N (3 gf ) and the loading time of s. The diagonal length of indentation (d) and the half-length of crack (a) were measured and K c was obtained by the eq. (2). In addition, the calibration factor ( ) and Young s modulus of M 7 C 3 carbide (E) used were.26 and 31 GPa, 17) respectively. Table 2 presents values for the fracture toughness of the M 7 C 3 carbide (K c ) with different Mo or W content. Regardless of kind and concentration of alloying element in the carbide, the K c values are similar to one another. These results could occur because the carbide is very brittle and the K c values are low. Therefore, it can be concluded that the effect of Mo or W on fracture toughness is very small at the room temperature. 4. Conclusions d 2a indentation crack induced by indentation Schematic drawing of indentation fracture method. Table 2 Fracture toughness values of M 7 C 3 carbides with different Mo and W contents of specimens. Specimen Fracture toughness K c, MPa m 1/2 2% 3.7 2% 1%Mo 3. 2% 1%W 3.3 In order to improve the mechanical properties of M 7 C 3 carbide at high temperature, Mo and W were added from to mass% to a 2% hyper-eutectic cast iron. The effects of contents of these elements on the high temperature hardness, lattice parameter of unit cell and fracture toughness of M 7 C 3 carbide were investigated. The results can be summarized as follows: (1) Mo and W added to a cast iron dissolve into the M 7 C 3 carbide phase depending on each partition coefficient. As either Mo or W content of the alloy increases, the Mo or W content in the carbide increases and on the contrary the content decreases. (2) The lattice parameters of the M 7 C 3 carbide changes with addition of Mo or W in the carbide. The volume of a unit cell increases and then the lattice distortion is increased. (3) The hardness of M 7 C 3 carbide decreases gradually for each W and Mo containing alloys as the test temperature rises. The influences of Mo and W on the carbide hardness are small at low temperature. Both elements resist significantly the softening of carbides above 73 K. There might be critical concentrations of Mo and W to make the hardness saturate at high temperature. (4) The fracture toughness of M 7 C 3 carbide at the room temperature in Mo and W-free, 1%Mo and 1%W specimens are 3.7, 3. and 3.3 MPa m 1/2, respectively. The dissolution of Mo and W in the M 7 C 3 carbide has little effect on the fracture toughness of the carbide at room temperature. REFERENCES 1) The Japan Institute of Metals and Materials: Tekkouzairyo, (The Japan Inst. Metals, Sendai, 198) p. 48 (in Japanese). 2) E. Yajima, R. Ichikawa and K. Yoshizawa: Wakaigizyutsusyanotameno-kikai-kinzokuzairyo, (Maruzen, Tokyo, 1996) p. 17 (in Japanese). 3) M. Kuwano, K. Ogi and K. Matsuda: J. JFS 4 (1982) ) C. Tong, T. Suzuki and T. Umeda: J. JFS 62 (199) ) S. Inthidech, P. Sricharoenchai, N. Sasaguri and Y. Matsubara: Trans. AFS 112 (24) ) Y. Ono, N. Murai and K. Ogi: ISIJ Int. 32 (1992) ) K. Yamamoto, M. M. Liliac and K. Ogi: Int. J. Cast Met. Res. 16 (23) ) ASM International: Metals Handbook, Vol. 1, (ASM International, 2) p ) R. S. Jackson: J. Iron and Steel Inst. 28 (197) ) B. Sundmann, B. Jansson and J.-O. Andersson: CALPHAD 9 (198) ) K. Ichino, T. Toyooka, H. Hiraoka and Y. Kataoka: Abrasion 22 (JFS Kyushu, 22) pp ) The Japan Institute of Metals and Materials: Kinzoku Data Book, (Maruzen, Tokyo, 24) p ) K. Maruyama and E. Nakashima: Kouonnkyoudo-no-zairyoukagaku, (Uchida Roukakuho, 1997) p. 17 (in Japanese). 14) T. Matsuo, T. Shinoda and R. Tanaka: Tetsu-to-Hagane 63 (1977) ) M. Umemoto and K. Tuchiya: Tetsu-to-Hagane 88 (22) ) JIS R 167:21: Testing method for fracture toughness of fine ceramics at room temperature, (Japan Industrial Standards, Japan, 21). 17) M. G. Di V. Cuppari, R. M. Souza and A. Sinatora: Wear 28 (2)