鉄鋼圧延及び鉱物粉砕ミル用耐摩耗鋳造合金の研究開発久留米工業高等専門学校名誉教授工学博士松原安宏

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1 鉄鋼圧延及び鉱物粉砕ミル用耐摩耗鋳造合金の研究開発久留米工業高等専門学校名誉教授工学博士松原安宏 RESEARCH AND DEVELOPMENT OF ABRASION WEAR RESISTANT CAST ALLOYS FOR ROLLS OF ROLLING AND PULVERIZING MILLS YASUHIRO MATSUBARA Dr. Eng. Professor Emeritus Kurume National College of Technology INTRODUCTION For last half a century past, materials of hot work rolls in the steel industry, which are shown as an example in Fig.1, changed from low-alloyed white iron through adamite or hyper-eutectoid Ni-Cr steel and Ni-hard cast iron to high chromium cast iron. [1] On the other side, the abrasion wear resistant cast alloys for the components of cement and mineral pulverizing mills have been changed by following the transition of the roll materials. The high chromium cast iron, which was developed in the nineteen-seventies, is even now used not only for hot working mill rolls but also for pulverizing mill rolls because of its high wear resistance. A typical example of the pulverizing mill roll is shown in Fig.2. According to the demands in upgrading the productivity and the quality of products, the roll materials are being compelled to shift to a new type of material with much higher performance. The multi-alloyed white cast iron or multi-component white cast iron is a new type of alloy which contains plural kinds of strong carbide forming elements such as Cr, V, Mo, W, and resultantly, plural kind of their special carbides with high hardness and strong matrix with secondarily precipitated carbides. This iron was developed around 15 years ago in Japan and has been tried to apply to the work roll of hot finishing stands for the past 10 years. [2] Nowadays, a large portion of work rolls in hot rolling mills is replaced from high chromium and Ni-hard white cast iron rolls, and such types of rolls are now pleading over the world. It is worthy of note that this kind of trial roll was served recently to the pulverizing mills. Fig.1 Rolling mill rolls made of multi-component white cast iron. white cast iron. This paper describes the basic researches for developing the multi-component white cast irons for the hot work rolls of rolling mill roll and the abrasion wear resistant materials of pulverizing mill by taking following approach: 1) Alloy designing 2) Solidification structure 3) Solidification sequence and phase diagram 4) Phase transformation behavior 5) Heat treatment characteristics The field test results for wear resistance of this type of cast iron are also introduced in comparison with conventional roll materials. 30 Fig.2 Pulverizing mill Roll made of high chromium

2 ALLOY DESIGNING The designing of chemical compositions of the multi-component white cast iron was carried out on consideration of not only the solidification structure, i.e. the type and morphology of carbide precipitating during solidification, but also the transformation behavior of the matrix by heat treatment. The main purpose was focused on improving the abrasion wear resistance from room to elevated temperatures by availing hard eutectic carbides and matrix hardened by secondary precipitation of carbides due to heat treatment. Chromium: Cr distributed into matrix improves the hardenability of the cast iron by postponing the pearlite and bainite transformations. At the same time, eutectic carbide as M 7 C 3 contributes to good abrasive wear resistance. Therefore, Cr was selected as one of the basic alloying elements of multi-component white cast iron. However, Cr content has to be limited in order to give priority to the precipitation of other types of carbides with much higher hardness. Molybdenum and Tungsten: Mo and W are strong carbide-forming elements and they act similarly. In case that both of Mo and W are added to ferro-alloy containing carbon, complex carbides of (Mo,W,Fe) 2 C or M 2 C, (Mo,W,Fe) 6 C or M 6 C are able to form. [3] These carbides have higher hardness than chromium carbide of M 7 C 3. Even in a small amount, on the other hand, Mo and W dissolve into matrix and improve the hardenability of the iron. W and Mo also promote the secondary precipitation hardening of matrix by tempering, and they should be adopted in the multi-component white cast iron. Then, a value of tungsten equivalent (W eq ) is introduced to set up the total content of these two elements, and it is expressed by the following equation, W eq = %W + 2%Mo (1) Vanadium: V forms (V,Fe)C (simplified as MC) or (V,Fe) 4 C 3 carbide in ferro-alloy with C [4] and it greatly works upon the secondary precipitation hardening by the carbide reaction in the later stage of tempering. V carbide, VC or V 4 C 3, has higher hardness than M 2 C and M 6 C. From the viewpoints of hardness and morphology, V is expected to be the most beneficial alloying element in the multi-component white cast iron. Cobalt: It is well known that Co has not been widely used in steels because of decreasing the hardenability. However, Co improves high temperature strength due to an increase in the eutectic temperature and the suppression of grain coarsening due to the rising of temperature. An available result is expected by adopting Co to the multi-component white cast iron. Carbon: C contained in cast iron is spent at first by combining with strong carbide formers during solidification, and the remaining C dissolves into the iron matrix. In the multi-component white cast iron that contains plural carbide formers, several types of carbides are expected to precipitate. A term called carbon balance (C bal ), which is defined by the next equation, is employed as a parameter to express the behavior of C dissolved in iron matrix, [5] C bal = % C C stoich (2) where, % C is the C content of the cast iron and C stoich means the stoichiometric amount of carbon, i.e., carbon content that combines equally with all the carbide-forming elements in the cast iron. In multi-component white cast iron, C stoich can be expressed by the following equation, C stoich = % Cr % Mo % W % V (3) here, carbon is supposed to exist in the forms of Cr 23 C 6, Fe 4 W 2 C (W 2 C) or Fe 4 Mo 2 C (Mo 2 C), and VC, respectively. However, if sufficient amount of Cr 7 C 3 carbide forms in eutectics, the equation should be modified as, C stoich = % Cr % Mo % W % V (4) In the last stage of heat treatment, C bal is a very important factor for the transformation of matrix in the cast iron. It determines whether more or less carbon than the equilibrium state will dissolve to matrix, and therefore predominantly affects the property of the matrix. A positive C bal value means extra carbon remains in the iron matrix in an equilibrium condition, and a negative value means that carbon lacks in the matrix. As a basic alloy composition, approximately 5 % for each element of Cr, Mo, W and V, and 2 % C is settled in this study so that the C bal value can be around 0 %. SOLIDIFICATION STRUCTURE As-cast microstructure of multi-component white cast iron consists of carbides crystallized from liquid and 31

3 matrix. [6] Carbide Several types of eutectic carbides precipitate in the solidification structure of the cast iron and they show diversities of morphology according to the chemical composition. MC carbide: The MC carbide contains more than 50% V and it precipitates as a eutectic during solidification. This carbide can be classified into petal-like, nodular and coral-like morphologies as shown in Fig.3 to 5, respectively. Petal-like MC in Fig.3 precipitates in a low carbon cast iron where the crystallization of austenite dendrite proceeds before eutectic of (γ + MC) forms. Nodular MC carbide shown in Fig.4 precipitates in the cast iron with high carbon content. Coral-like MC carbide in Fig.5 consists of primary MC crystals in the center and eutectic MC carbides growing in radial directions from the primary MC. The coral-like MC carbides precipitate in the iron containing high vanadium content, and can co-exist with M 7 C 3 carbide or small amount of M 2 C carbide depending on the carbon content of the iron. Hardness of MC carbide is about 2800HV and MC carbide may be very effective to the wear resistance of the cast iron, and the nodular morphology of MC carbide may also improve the toughness of the cast iron because of less notch effect. When chemical composition of the iron changes, however, nodularity of MC carbide may deteriorate to chunky or flaky morphology. M 2 C carbide: Molybdenum- and tungsten-rich carbide precipitates as M 2 C type in as cast state and takes fine lamellar and/or coarse plate-like morphology as shown in Fig.6 and 7, depending on the chemical composition of the iron. However, it is reported that the M 2 C carbide transforms into M 6 C carbide by heating. [7] Lamellar eutectic M 2 C carbide exists in isolation and this indicates that the M 2 C carbide precipitates in last stage of solidification. The plate-like M 2 C carbide consists of large and thick plates. This type of carbide can be usually seen in the cast iron with high W eq value. In the iron with higher carbon content, the plate-like M 2 C carbide exists together with M 7 C 3 carbide as shown in Fig.8. M 7 C 3 carbide: Chromium carbide M 7 C 3 similar to that in high chromium white cast iron shown rod-like or ledeburitic morphology in the iron with higher W eq value and higher carbon content, and it sometimes co-exists with MC and M 2 C carbides (Fig.4 and 8). Fig.3 Petal-like MC carbide. Fig.4 Nodular MC carbide. Fig.5 Coral-like MC carbide. Fig.6 Fine lamellar M 2 C carbide. 32

4 Fig.7 Coarse plate-like M 2 C carbide. Fig.8 M 7 C 3 and M 2 C carbides. Matrix Matrix in as-cast state consists of bainite, martinsite and some retained austenite (γ R ). The volume fraction of retained austenite (V γ ) ranges widely from 5 to 50 % depending on the carbon content of the iron. However, the γ R is supposed to transform into the useful phases after heat treatment. SOLIDIFICATION SEQUENCE AND PHASE DIAGRAM Solidification Sequence The content of carbon and alloying elements were varied so that several combinations of eutectic carbides could be obtained in multi-component white cast iron and the solidification sequences were investigated by means of quenching test during thermal analysis. [8] As an example, the solidification sequence of the cast iron with basic chemical composition (Fe-5%Cr-5%Mo-5%W-5%V-5%Co-2.0%C alloy), in which the petal-like MC and plate-like M 2 C carbides co-exist, is shown in Fig.9 a, b: a thermal analysis curve in a and the quenched microstructures in b. First of all, the primary austenite dendrite forms at 1590K, and follows by precipitation of (γ+mc) eutectic at 1558K, and finally (γ+m 2 C) eutectic solidifies at 1422K. a b Fig.9 Solidification sequence of multi-component white cast iron with basic chemical composition (Fe-5%Cr-5%Mo-5%W-5%V-5%Co-2%C). a: Thermal analysis curve, b: quenched microstructures 33

5 In the case of cast iron increasing carbon content to 3%, the solidification sequence is expressed as follows, L 0 ( γ + MC) E +L 1 at 1523 K L 1 ( γ + M 7 C 3 ) E + L 2 at 1441 K L 2 ( γ + M 2 C) E at 1365 K The solidification sequences of the other kinds of multi-component white cast irons with more variation of chemical composition than the basic alloy composition are summarized in Table 1. Table 1 Solidification sequences of multi-component white cast iron varying chemical composition from the basic alloy No. Chemical composition Combination of carbide Solidification Sequence %C 5%Cr 2%Mo 2%W 9%V 5%Co 3%C 5%Cr 2%Mo 2%W 5%V 5%Co 3%C 5%Cr 2%Mo 2%W 9%V 5%Co Coral-like MC + lamellar M 2 C Nodular MC + rod-like M 7 C 3 Chunky and coral-like MC + rod-like M 7 C 3 L 0 γ P + L 1 L 1 (γ + MC) E + L 2 L 2 (γ + M 2 C) E L 0 γ P + L 1 L 1 (γ + MC) E + L 2 L 2 (γ + M 7 C 3 ) E L 0 (MC) p + L 1 L 1 (γ + MC) E + L 2 L 2 (γ + M 7 C 3 ) E at 1651 K at 1631 K at 1498 K at 1556 K at 1515 K at 1452 K at 1732 K at 1550 K at 1453 K Practical Phase Diagram Since it was found that the solidification processes of multi-component white cast iron with plural eutectic carbides were accurately revealed using the techniques of thermal analysis and quenching experiment, the practical phase diagram of the iron was considered to be possibly made by using the same method. [9] The phase diagram of quasi-binary M(Fe-Cr-Mo-W-V)-C alloy system and the liquidus surface diagrams of quasi-ternary M(Fe-Cr-Mo-W)-V-C alloy system were constructed. The quasi-binary phase diagram of the basic iron (M(Fe-5%Cr-5%Mo-5%W-5%V-5%Co)-C alloy) and the liquidus surface diagram of the iron (M(Fe-5%Cr-5%Mo-5%W-5%Co)-V-C alloy) are shown in Fig.10 a, b respectively. It is obvious that the high temperature part of the quasi-binary phase diagram (Fig.10 a) exhibits a simple eutectic system between austenite and MC carbide. As the eutectic point lies near a carbon content about 2.8%, the cast iron with the basic alloy content bears hypo-eutectic microstructure. However, this phase diagram also tells that after the eutectic reaction of (γ + MC), the melt may thereafter undergo two other eutectic reactions including M 7 C 3 and M 2 C carbides, corresponding to the carbon content. On the other side, quasi-ternary liquidus surface diagram (Fig.10 b) shows a liquidus surface construction when the vanadium content varies with accompanying carbon content. Reactions among delta-ferrite, austenite, MC carbide as well as M 7 C 3 and M 3 C carbide are displayed, while the reaction including M 2 C carbide cannot be expressed on this liquidus surface diagram. There are two peritecto-eutectic points, U 1 and U 2, and the temperature of U 1 located at high vanadium and low carbon is around 200K higher than that of U 2 at low vanadium and high carbon. Therefore, the (γ + MC) eutectic reaction progresses down to the U 2 followed by (γ + M 7 C 3 ) or (γ + M 3 C) eutectic. Anyway, these phase diagrams have been giving useful instructions to the alloy designing for practical applications. 34

6 a b Fig.10 Quasi-binary phase diagram of base alloy M(Fe-5%Cr-5%Mo-5%W-5%V-5%Co)-C alloy (a) and quasi-ternary liquidus phase diagram of M(Fe-5%Cr-5%Mo-5%W-5%Co)-V-C alloy (b). PHASE TRANSFORMATION BEHAVIOR The carbide forming elements are also distributed into matrix and so they have effects on transformation. The wear resistance and the mechanical properties of multi-component white cast iron are affected by not only the type and morphology of eutectic carbide but also the matrix structure varied depending on heat treatment. In order to get instructions of heat treatment endowing the cast iron with desired properties, it is necessary to clarify the transformation behavior of matrix. Here, the continuous cooling transformation behavior of the cast irons containing 5% of each element, Cr, Mo, W, 2% of Co, and 2.0 and 2.8% C were investigated. [10] The continuous cooling transformation (CCT) curves of low and high carbon cast irons obtained by cooling from austenitizing temperatures (T γ ) of 1273K and 1373K after annealing (1223K-18ks-FC) are shown in Fig. 11 a, b. In spite of the difference in carbon content and T γ, pearlite and bainite transformation separate to the top and the bottom individually. The nose temperatures of both the pearlite and bainite transformations range in 920K to 980K and 570K to 610K for low carbon iron, and 900K to 950K and 590K to 640K for high carbon iron, respectively. With respect to transformation time or nose time, the low carbon iron shows great difference in nose time of pearlite and bainite transformations at 1273K austenitization and the pearlite transformation is located at the long time side more than 3 times as much as the bainite transformation. When the T γ increases 100K more to 1373K, both transformations are delayed approximately twice. In the case of high carbon iron, on the other hand, the nose time of pearlite and bainite transformations are almost the same at 1273K austenitization and both are shifted to the long time side at 1373K austenitization. The critical cooling rate of the pearlite transformation (V C-P ) in 1273K austenitization is 0.12K/s in the low carbon iron and this is remarkably small in comparison with that in the high carbon iron (0.39K/s). In 1373K austenitization, V C-P of the low carbon iron decreases greatly to 0.08K/s but that of the high carbon iron is similar (0.38K/s) regardless of an increase in the T γ. This result tells that the pearlite transformation is difficult to occur in the low carbon iron and the tendency increases as the T γ rises. The critical cooling rate of the bainite transformation (V C-B ) is nearly the same in the low and high carbon irons, i.e., 0.84 and 0.77K/s at 1273K austenitization, and 0.50 and 0.48K/s at 1373 austenitization. It is understood that the direct transformation from austenite to bainite takes place in the small range of the cooling rate and usually the both of pearlite and bainite transformations occur. Therefore, the matrix structure in which pearlite and bainite co-exist will be obtained. This will give an important instruction to the practical heat treatment of multi-component white cast iron with high carbon content which will be applied to a mineral pulverizing mill roll. Next, the effect of T γ on pearlite and bainite transformations is described. Because the carbon and the other alloying elements dissolve more into matrix due to an increase of solubility at higher T γ, both the pearlite and bainite transformation can be delayed. In SEM observation of the matrix, quench from each T γ, it was confirmed that the amount of indissolved carbide is less in the matrix quenched from 1373K than that quenched from 1273K. From the fact that these dissolved carbides are considered to act possibly as nucleation sites of their transformations, it can be inferred that the matrix austenitized at high temperature oppositely lies in the situation that is difficult to transform. 35

7 a b Fig.11 Continuous cooling curves of multi-component white cast irons with basic alloy composition. a: 2.0%C iron, b: 2.8%C iron It is clear from Fig.11 that the temperature for the martinsite transformation to start (Ms) appears but that to finish (Mf) does not in this kind of cast iron. The Ms temperature increases as the cooling rate decreases down to the V C-B and then lowers gradually as the bainite transformation increases. In 1273K austenitization, the Ms temperature is 475K to 495K in the low carbon iron and 375K to 380K in the high carbon iron. The reason for Ms temperature to rise may be explained by that the alloy concentration in matrix decreases because of an increase in the precipitation of carbides, as the cooling rate reduces. The reason for Ms temperature to lower may be explained by that the enrichment of carbon in austenite and resultant stabilization of austenite take place due to the discharge of carbon with the proceeding of the bainite transformation. In the recent investigation, it is proved that the Ms temperature of the multi-component white cast iron is uniformly reduced with an increase in carbon content of the iron. HEAT TREATMENT CHARACTERISTIC Multi-component white cast irons with alloy concentration (Fe-5%Cr-5%Mo-5%W-5%V-2%Co-1.4~2.4%C) were annealed under the condition of 1123K-18ks-FC, and hardened by forced air from 1272, 1323 and 1373K austenitization. Then they were tempered in the temperature range from 623K to 873K for a constant holding time of 12ks. [11] The relationships between macro-hardness, volume fraction of retained austenite (V γ ) and tempering temperatures of the irons with different carbon content are shown in Fig. 12 a, b, c,. In each iron, the tempered hardness curve with remarkable secondary hardening is obtained and the V γ begins to reduce rapidly when the tempering temperature rises over a certain temperature. In the cast iron with 1.8%C, the hardness in as-hardened state is almost the same in the irons hardened from 1323K and 1373K but the tempered hardness curve shifted to the high hardness side as the T γ increases. Maximum tempered hardness (H Tmax ) is obtained near 773K regardless of T γ and it increases as the T γ rises. The higher is the T γ, the more is the degree of secondary hardening. The V γ in as-hardened state is less than 10% and it becomes 0% when the tempering temperature exceeds 800K. In the case of cast iron with 2.0%C, the hardness of the irons hardened from 1273K and 1323K are similar but that of the iron hardened from 1373K is lower. The tempered hardness curves show the marked secondary hardening in the irons hardened from 1323K and 1373K, and the H Tmax are over 900HV. The tempering 36

8 temperature corresponding to H Tmax is a little higher when the T γ is high. The hardness in as-hardened state differs depending on the T γ, and it increases in the order of 8%, 15% and 23% as the T γ rises from 1273K to 1373K. In each iron, the V γ starts to reduce over the tempering temperature of 723K and gets down to 0% over 823K. When the hardness is corresponded to the V γ, it is found that the H Tmax is obtained near a tempering temperature when the V γ approaches 0%. The hardness of as-hardened iron with higher carbon content of 2.4% greatly decreases as the T γ increases. In the following tempering, however, each iron shows an evident secondary hardening. The H Tmax of the irons hardened from 1273K and 1323K are 960HV at the tempering temperatures, 773K and 823K, respectively. In the iron hardened from 1373K, the H Tmax is 915HV at 823K tempering and this value is lower in spite of higher austenitization. This is because around 20% V γ is still left after tempering, and also it tells that this tempering condition is not enough to decompose completely such a large amount of γ R (65% in as hardened state). a b c Fig.12 The relationship between hardness, volume fraction of retained austenite (V γ ) and tempering temperatures. a: 1.79%C iron, b: 2.05%C iron, c: 2.36%C iron In the case that this type of cast iron is practically applied to a rolling mill roll, the γ R is undesirable because it may decompose and/or transform into martinsite and consequent cracks may be produced. Therefore, it will be useful if the optimum tempering condition to reduce the γ R effectively is shown for the cast iron with each chemical composition. Then the tempering temperatures for the V γ to be less than 2% (T 2%γ ) are obtained and they are related to carbon content and hardening condition. At every T γ, T 2%γ is proportional to the carbon content of the iron and it is expressed respectively by the following equations, For Tγ = 1273K, T 2%γ = %C (5) For Tγ = 1323K, T 2%γ = %C (6) For Tγ = 1373K, T 2%γ = %C (7) The more is the carbon content in the iron, the higher is its tempering temperature, and the higher is the T γ, the higher should the T 2%γ be chosen in all the irons with different carbon content. In order to express the T 2%γ as a function of T γ and carbon content, the multiple regression analysis is carried out and the relation is given by the next equation, T 2%γ (K) = Tγ %C (8) Fig.13 shows the comparison of T 2%γ calculated from equation (8) with the experimental values. A close agreement between them was obtained and the tempering temperature to make the γ R mostly free can be calculated from the carbon content of cast iron and the austenitinzing temperature. 37

9 Fig.13 Corresponding between experimental and calculated values for T 2%γ. WEAR RESISTANCE Rolls for the later finishing stands of steel hot rolling mill and a Loesche type slag pulverizing mill were made of multi-component white cast iron with basic chemical composition and they were supplied to the field test. Fig.14 shows the roll wear in finishing mills relative to a parameter of rolling load in which the rolling force in unit width in the direction of roll axis and the number of rolling cycles. The wear of multi-component white cast iron roll is very small compared with conventional rolls, that is, it is less than 1/5 of indefinite chilled cast iron roll and less than 1/3 of high chromium cast iron roll. Therefore, the shape of rolled products is improved and it is possible with multi-component white cast iron roll to display their shape controllability in high function rolling mill. The test results in the pulverizing mill are shown in Fig.15 compared with that of conventional high chromium cast iron roll. It is evident that the wear resistance of multi-component white cast iron roll with 2.0%C (Multi-alloy (1)) is much better than that of the high chromium cast iron roll with 27%Cr and eutectic carbon content, and that the wear of multi-alloy roll (2) containing high carbon content of 2.3% decreases more and it is two-thirds to that of high chromium cast iron roll. As to the wear performance for hot working roll made of the basic multi-component white cast iron, the test result is shown in another paper [12] that is presented later. The wear resistance is much higher than those of conventional Ni-hard and high chromium cast iron rolls, and it showed roughly one fifth and one fourth to those of rolls made of Ni-hard and high chromium cast irons, respectively. Fig.14 Comparison of wear resistance among different roll materials for hot rolling mill. Fig.15 Comparison of wear resistance among different roll materials for pulverizing mill. 38

10 CONCLUSIONS AND PROSPECTS The development processes of a new material, multi-component white cast iron, which contains several strong carbide forming elements and their special carbides with very high hardness, for roll in hot finishing stands, by researching it totally from the view points of alloy designing, solidification structure and solidification sequence and phase diagram, and phase transformation behavior and heat treatment characteristics are shown in this paper. As the results, the solidification structure, particularly type and morphology of eutectic carbides, could be controlled in a wide range by combination of alloying elements. Phase transformation behavior, obtained from the continuous cooling curves gave useful instructions to the heat treatment process. The hardness and retained austenite in matrix could be widely controlled and the remarkable secondary hardening due to precipitation of special carbides was found to make the matrix hard and strong enough for practical use. The excellent performance of hot work roll made of the multi-component white cast iron was proved by the field test and it is very popular in Japan and the good reputation has been spreading over the world. It is also noted that this type of cast iron is gradually being applied to the pulverizing mill rolls and that a good performance has been reported in the cement industry. Finally, it is convinced that the multi-component white cast iron will also find wider applications as the wear resistant materials, especially cold working mill rolls. REFERRENCES 1 Matsubara, Y., Sasaguri, N., and M. Hashimoto: The history and development of cast rolls for hot working mill, The 4 th Asian Foundry Congress-Australia, pp (1996) 2 Hashimoto, M., Otomo, S., Yoshida, K., Kimura, K., Kurahashi, R., Kawakami, T., and Kouga, K.: Development of high-performance roll by continuous pouring process for cladding, ISIJ International, vol. 32 pp (1992) 3 Raynor, G.V., and Rivlin, V.G.: Phase equilibria in iron ternary alloys, The Institute of Metals, pp and (1988) 4 Raghavan, V.: Phase diagrams of ternary iron alloys, The Indian Institute of Metals, pp (1987) 5 Steven, G., Nehrenberg, A.E., and Philip, T.V.: High-performance high-speed steels by design, Transactions of the ASM, vol. 57 pp (1964) 6 Wu, H.-Q., Sasaguri, Matsubara, Y., and Hashimoto, M.: Solidification of multi-component white cast iron: Type and morphology of carbides, AFS Transactions, vol. 104 pp (1996) 7 Karagoz, S., Riedl, R., Gregg, M.R., and Fischmeister, H.: The role of M 2 C carbides in high speed steels, Sonderbande der Praktischen Metallographie, vol. 14 pp (1983) 8 Wu, H.-Q., Hashimoto, M., Sasaguri, N., and Matsubara, Y.: Solidification sequence of multi-component white cast iron, Journal of Japan Foundry Engineering Society, vol. 68, pp (1996) 9 Wu, H.-Q., Sasaguri, N., Hashimoto, M., and Matsubara, Y.: Practical phase diagram of multi-component white cast iron, Journal of Japan Foundry Engineering Society, vol. 69 pp (1997) 10 Matsubara, Y., Sasaguri, N., Yokomizo, Y., and Wu. H.-Q.: Continuous cooling transformation behavior of multi-component white cast iron, Journal of Japan Foundry Engineering Society, vol. 71 pp (1999) 11 Matsubara, Y., Yokomizo, Y., Sasaguri, N., and Hashimoto, M.: Effect of carbon content and heat treatment condition on retained austenite and hardness of multi-component white cast iron, Journal of Japan Foundry Engineering Society, vol. 72 pp (2000) 12 Kubo, O., Hashimoto, M., and Matsubara, Y.: Influence of microstructure on wear resistance and crack propagation characteristics required for white iron rolling mill rolls, Proceedings of The Science of Casting and Solidifications, Romania (2001) 39