Mechanical Properties of Ductile with Duplex Matrix* Cast Iron

UDC 669.131.7:539.42:669.112.24:539.537

Mechanical Properties of Ductile Cast Iron with Duplex Matrix*

By Noboru WADE** and Yoshisada UEDA***

Synopsis ferrite-austenite duplex mixture of various propor- The intention of the paper was to improve the mechanicalproperties of tions can readily be produced practically in a rela- ductile cast iron by a duplex matrix which is used in steels. tively wide range of austenitizing temperature and Frritic ductile cast iron was heat-treated to produce thefollowing duplex time, and (2) austenite forms mostly around graphite matricesof various proportions;ferrite-bainite, ferrite pearlite and ferrite- nodules at higher temperatures but forms at grain tempered troostite. boundaries at lower temperatures. The tensile and impact tests wereperformed on the irons with a duplex The improvement of mechanical properties will be matrix. The 0.2% proof stress, tensile strength and hardness increase expected14~ by using these duplex matrices obtained with increasing volumefraction of the secondphase, but there is no linear relationship known as the law of mixture. The harder the secondphase from the transformation of ferritic ductile cast irons. is, the higher the strength becomes. In the elongation and impact energy The present study was, therefore, performed to of the alloy with higher silicon content, two peaks appear at volumefrac- examine the mechanical properties of ductile cast tions of upper bainite of about 50 and 95%, and the transition tempera- iron with duplex matrices of various proportions, tures drop to minimums; the elongation values are 18 and 12%, the and to obtain the optimum condition of improving absorbed and upper shelf energies are 14.5 to 15 kg . m/cm2 and the the strength and toughness. Moreover, an attempt transition temperaturesare - 45° to - 47°C in the un-notchedspecimen. was made to interpret the improvement of the mechan- Thus, the strength and toughness of ductile iron can be improved by the ical properties in terms of the observed microstruc- proper secondphase of a proper volumefraction in ferritic structure. The tural change. improvementcomes from the fine duplex matrix structure and low carbon content of the secondphase, which is a characteristic in the austenitizing II. Experimental Procedures of ferritic ductile cast iron, and it also comesfrom the secondphase with high strength and high ductility, such as upper bainite, formed mainly Pig iron for ductile cast iron and commercial pure around the graphite nodules,probably because of preventinga crack initia- iron were melted in an induction furnace, and com- tion at the graphite-secondphase interface. mercial metallic silicon and copper were added into the melt. The melt was then treated with a Fe- 45%Si-10%Mg alloy and cast into the C02 molds I. Introduction having Y-type block of 25 mm thick. The heat treatment of producing fine duplex struc- The chemical composition of the specimens is given tures has been given attention as an effective method in Table 1. To obtain the ferritic structure, the speci- of improving the toughness of steels, and intensive mens were annealed at 900°C for 2 hr, furnace-cooled studies have been carried out systematically for to 720°C, held at 720°C for 20 hr and then air-cooled. 9%Ni,l~ Ni-Cr-Mot-4) and stainless steels.5~ The specimens were then heat treated, as shown As for ductile cast iron, however, there are only in Fig. 1, in molten salts (BaC12plus KCl for austeni- few studies,s''~ in which it is reported that ductile tizing, and NaN03 for austempering and tempering) cast iron with a ferrite-pearlite duplex matrix pro- to produce the following duplex matrices of various duced by an isothermal heat transformation from the proportions; ferrite-bainite, ferrite-pearlite and ferrite- pearlitic structure is improved in tensile strength by tempered troostite. All the specimens were pre- 15 kg/mm2 and in elongation by 2 to 3 % compared heated at 700°C for 20 min to obtain the homo- with those of the conventional bull's-eye ductile cast geneous heating temperature. The austempering iron. However, more detailed studies will be required in practice about kinds and proportions of the second Table 1. Chemical composition of specimens. (wt%) phases. In the previous works8 13) concerned with the heat transformation of ductile cast iron, it was eluci- dated that the heat transformation characteristics of ferritic ductile cast iron such as, the austenitizing mechanism, rate of transformation, heat transformation temperature (Ac1) and volume change during transformation are considerably different from those of steels and pearlitic ductile cast irons, and that (1)

* Originally published in Imono (J. Japan Foundrymen's Society), 50 (1978), 305 and 51 (1979), 480, in Japanese. English version received February 15, 1980. ** Department of Iron and Steel Engineering, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 465. Department of Metallurgy, Faculty of Engineering, Nagoya University.

Research Article (117) (118) Transactions ISIJ, Vol. 21, 1981

Fig. 1.

Heat treatments for obtaining duplex matrix structures.

Fig. 2. Effect of the second phase on tensile properties and hardness. (All the specimens were austenitized at 900°C.) Fig. 3. Effect of the second phase on tensile properties and hardness. was performed at 400° and 300°C to obtain upper 111. Results and lower bainite, respectively. The holding time of austenitizing in obtaining ferrite-austenite mixtures 1. Tensile Properties and Hardness was pre-determined by metallographic observations. Figures 2 and 3 show the relation of the tensile The volume fraction of the second phase (bainite, properties and hardness to the volume fraction of pearlite and troostite) was measured by an automatic various second phases. As indicated in Fig. 2(a), scanning microscope and photographs. the values of 0.2% proof stress, tensile strength and After the heat treatment, the specimens of 8 mm hardness increase with increasing volume fraction of diameter and 50 mm gage length for tensile test were the second phases, and the increase is more remarkable finished by polishing with a fine sand paper and in cases of harder second phases of larger proportions. sticked on a foil-type strain gage. The linear relationship between these values and Tensile test was carried out with an Instron-type the volume fraction of the second phase known as the universal testing machine of 10 ton capacity at a law of mixture does not exist in this case. The loading speed of 0.3 cm/min. similar phenomenon is also recognized in steels,l6~ The instrumented Charpy impact tests (10 kg. m and it is suggested that the deviation from the linear capacity) on the specimens with V-notch and un- relationship will come from the difference between the notch, 5 X 10 X 55 mm in size, were carried out in strains in the first and the second phases. the temperature range of -196°C to +90°C, and on the other hand, the elongation, reduction of a load-deflection curve was recorded. The test area and tensile energy (the energy absorbed by the temperatures were adjusted with an appropriate specimen up to failure, calculated from the stress- mixture of liquid nitrogen, methyl alcohol and iso- strain curve) show considerably complicated variations pentane, an ice water and a warm water. as indicated in Fig. 2 (b). For instance, the elonga- Fractographic and microscopic observations, hard- tion fairly decreases at volume fraction of the second ness measurement and calorimetric analysis8'15~were phase of 10 to 20%, then recovers gradually and performed. passes through a maximum value with an increase in Transactions ISII, Vol. 21, 1981 (119) the volume fraction of the second phase. The maxi- The values of 0.2% proof stress, tensile strength and mum value in elongation appears when the second hardness indicate an increasing tendency to the phase is upper or lower bainite, or pearlite except volume fraction of the second phase approximately troostite. Similarly, the tensile energy exhibits a similar to the unalloyed iron of higher silicon content maximum value suggesting the improvement of the (No. 2). There appear also two peaks in the elonga- toughness at room temperature. The ductile cast tion and tensile energy at volume fractions of upper iron with a ferrite-upper bainite duplex structure is bainite of 50 and 95%, where the elongations are 18% the most excellent one in ductility and toughness. (tensile strength is 65 kgfmm2) and 12% (tensile Figure 3 shows the tensile behavior of ductile cast strength is 82 kg/mm2), respectively. iron of higher silicon content. Tensile test was performed about the iron with a ferrite-bainite duplex 2. Evaluation of Tensile Strength and Elongation matrix, and some specimens were austenitized at The relationship between the tensile strength and 850°C as well as 900°C in order to find the effect of elongation of ductile cast iron with a duplex matrix austenitizing temperature on the tensile properties. is summarized in Fig. 4 in comparison with the Japa- The values of 0.2% proof stress, tensile strength nese Industrial Standard (JIS) requirements. These and hardness indicate an increasing tendency to the plots are used conventionally for the evaluation of volume fraction of bainite approximately similar to the tensile properties of metals.7"7~ It is apparent the cast iron of lower silicon content shown in Fig. 2, that a considerably wide range of tensile strength and but these are generally higher. For example, the elongation can be obtained by the duplex matrices tensile strength of the iron with higher silicon content even if the chemical composition is the same, and is about 10 kg/mm2 higher than that of lower silicon that the iron with a ferrite-upper bainite duplex content in both fully upper and lower bainitic struc- matrix has an optimum combination of strength and tures. The effect of austenitizing temperature on elongation, where the elongations at volume fractions the tensile strength is more apparent when the amount of upper bainite of 50 and 95% are about two times of the bainite is large. higher than those of the iron with a ferrite-pearlite In the values of elongation and tensile energy of duplex matrix at the same level of tensile strength. the iron of higher silicon content, two peaks appear at volume fractions of upper bainite mixture of 50 3. Impact Properties at Room Temperature (in the Case and 95% as shown in Fig. 3(b); the elongation is 18% of the Un-notchedSpecimen) and the tensile energy is 27 kg. m (the tensile strength Figures 5 and 6 show the results of impact test is 68 kg/mm2) at volume fraction of 50%, and the performed at room temperature as a function of the elongation is 12% and the tensile energy is 24 kg. m volume fraction of bainite. (the tensile strength is 93 kg/mm2) at 95% volume Where, the maximum load (Pm), yield load (Pa) fraction. and deflection (o) were obtained from the load- The iron of higher silicon content, therefore, has deflection curve and are defined as illustrated in higher values in both strength and ductility at opti- Fig. 5. mum ferrite-upper bainite mixtures. In the impact energy (or called the absorbed A further research was performed in order to ex- energy), E1, was evaluated from the swing angle of amine the effect of copper on the tensile properties. hammer in the conventional test, and Eo from the

Fig. 4. The relationship between tensile Fig. 5. Fig. 6. strength and elongation of ductile Impact properties at room temperature. Impact properties at room temperature. cast iron with a duplex matrix. (In the un-notched specimen) (In the un-notched specimen)

Research Article (120) Transactions ISIJ, Vol. 21, 1981 integrated area of the load-deflectioncurve. the volume fraction of bainite. In the upper shelf It is apparent from Fig. 5 that there are two peaks energy of the un-notched specimen, there appear two in the impact energy and deflection at volume frac- peaks at volume fractions of the upper bainite of 55 tions of the upper bainite of 50 and 95%, produced and 95% produced by austenitizing at 900°C and by austenitizing at 900°C and austempering at 400°C. austempering at 400°C, where the values are 15 and But no apparent peak exists in the upper bainite 14.5 kg • mf cm2, respectively. This is the similar mixture produced by austenitizing at 850°C and tendency to the impact energy at room temperature austempering at 400°C. in Fig. 5. The maximum load and yield load increase with In the upper bainite mixture produced by austeni- increasing volume of bainite, and particularly become tizing at 850°C, however, the upper shelf energy higher at volume fractions over 90%, and there exists decreases simply with the upper bainite volume , and a maximum in Pm at volume fraction of 95%. is somewhat lower than that produced by austenitizing From Fig. 6 about the lower bainite, it is also at 900°C. Such a dependence on the austenitizing evident that the impact energy and deflection decrease temperature is a characteristic of the impact behavior considerably with increasing volume fraction of the of the iron with a ferrite-bainite duplex matrix. bainite at first and at volume fraction of about 80%, In the V-notched specimen, however, the upper and are lower than those in the upper bainite mixture, shelf energy changes scarcely with the upper bainite , while the maximum load and yield load increase to and is considerably lower than that in the un-notched a great extent with increasing amount of lower bainite specimen. and are higher than those in the upper bainite The transition temperature shows a complicated mixture. variation with the upper bainite volume as indicated Some of those values show complicated variations in Fig. 9. There are two minimum values at volume to the bainite volume fraction. This phenomenon fractions of the upper bainite of 55 and 95% produced is similar to the results from the tensile test. The by austenitizing at 900°C, where the values are almost 0.2% proof stress, tensile strength, elongation and the same as or only a few degrees higher than that tensile energy correspond to the yield load, maximum of the ferritic, and are lower than those of the upper load, deflection and impact energy, respectively. bainite mixture produced by austenitizing at 850°C It is consequently demonstrated that a remarkable and also lower than those of pearlitic iron. improvement of toughness in the ductile cast iron can Consequently, the optimum conditions for the be achieved with ferrite-upper bainite duplex mixture, improvement of the toughness of the iron with a as well as ductility, at optimum volume fractions. ferrite-upper bainite duplex matrix were revealed.

4. Transition Temperature 5. Microscopicand Fractographic Observations Figure 7 shows the impact energy-temperature The characteristic mechanical behavior of the iron curves for various matrix structures. Where, the with a duplex matrix, especially in the ductility and impact energy value was obtained from the swing toughness as mentioned above, appears to be at- angle of hammer. It is evident that the impact tractive in practice. It is generally recognized18~that energy of the iron with a proper ferrite-upper bainite the ductility and toughness of the steel with a fine duplex matrix is superior to those of the fully ferritic duplex structure is favorable, because that fine dis- and bainitic, and also to pearlitic irons, in most range persed second phases have a little restriction to the of test temperatures. deformation of the first phase (or matrix). Micro- The upper shelf energy and transition tempera- scopic observations were, therefore, conducted to ture ( TrE) evaluated from the impact energy-tempera- examine the morphology and distribution of duplex ture curve are shown in Figs. 8 and 9 as a function of matrix. Typical duplex matrix structures are given

Fig. 8. Effect of upper bainite volume Fig. 9. Effect of upper bainite volume Fig. 7. Impact energy-temperature curve. onthe upper shelf energy. on the transition temperature. Transactions ISIJ, Vol. 21, 1981 (121) in Photos. 1 and 2 with various matrices and pro- Photograph 4 is the fracture profile of the ferritic portions. It is evident that the ferrite-upper bainite iron tested at various temperatures, which shows duplex structures are relatively finer, and the fine typical dimple patterns including a graphite nodule phases exist around graphite nodules at both volume in each dimple, and cleavage facets with river pat- fractions of 50 and 95%, where the ductility and terns. Photograph 5 shows the fracture profile of toughness are improved (see Photos. 1(b) and (c)). the iron with volume fraction of the upper bainite of However, when the upper bainite of a small portion 95%, where the toughness and ductility are the best. formed along ferrite grain boundaries, a minimum Dimple patterns and cleavage facets with cleavage elongation is obtained (see Photo. 1(a)). steps and river patterns are also observed. But they Photograph 2 shows, furthermore, that the ferrite- are smaller than those in ferritic iron. Moreover, pearlite and ferrite-troostite duplex structures are lots of cleavage steps are seen on the fracture at relatively coarse. -98°C . It is apparent from Photo. 3 that the formation site of bainite depends on austenitizing temperatures, Iv. Discussion and that the upper bainite forms mostly around the graphite nodules when it is austenitized at 900°C, 1. Ratio of 0.2% Proof Stress to Tensile Strength while it forms along the ferrite grain boundaries when Figure 10 shows the variation of the ratio of the it is austenitized at 850°C. This phenomenon comes 0.2% proof stress to the tensile strength (o.2/aT) with from the change of the preferential precipitation site, the volume fraction of various duplex matrices. I t is as previously observed,9,1o,12~of austenite at different observed that there exists a minimum in all duplex austenitizing temperatures. It is therefore suggested matrices at an intermediate proportion and that the that an excellent toughness of the iron is caused by ratios are in a relatively wide range of 0.53 to 0.86. the fine bainite formed around graphite nodules. The lower value of the ratio is a characteristic of the

Photo. 1. Ferrite-upper bainite duplex matrix structures. (Specimen No. 2. Austenitized at 900°C and austempered at 400°C)

Photo. 2. Various duplex matrices of cast irons. (Specimen No. 1)

Photo. 3. Effect of austenitizing temperature on the formation site of upper bainite. (Specimen No. 4) The specimens were austempered 20 min at 400°C after austenitizing. (122) Transactions ISIJ, Vol. 21, 1981

Photo. 4. Fracture profile after impact test at various temperatures. (fully ferritic iron with un-notched)

Photo. 5. Fracture profile after impact test at various temperatures. (in the un-notched specimen) The specimen was austenitized at 900°C and austempered at 400°C, and the volume fraction of upper bainite of 95% was obtained.

2. Exponent of Strain Hardening, n An evaluation for the mechanical properties of metals is frequently performed by means of the exponent of strain hardening.4,20) The n values, which are obtained from the slope of the curve showing the relation between the logarithm of stress and the logarithm of strain, are shown in Figs. 2(b) and 3(b) with a ferrite-bainite duplex matrix. In the case of the iron of lower silicon content as shown in Fig. 2(b), the tendency of the variation of

Fig. 10. Variation of the ratio of 0.2% proof stress to tensile the n value to the volume fraction of the second phase strength with volume fraction of the second phase. is similar to that of elongation in both cases of upper- and lower bainite. In the case of the iron of higher silicon content, cast iron with a duplex matrix, and will contribute to however, while the n value shows only an increase an excellent workability. The similar phenomenon with increasing amount of lower bainite, it varies is also recognized in a steel,19~ which has been given similarly to elongation with increasing amount of attention as a dual phase steel sheet. upper bainite. A decrease in the ratio up to an intermediate pro- It is generally accepted that the increase in the n portion of the second phase may suggest that the value scarcely affects on ductility loss in ductile plastic deformation of the iron will be accelerated by materials, but affects considerably in brittle ones.21> increasing interface between ferrite and second phases. From this respect, ductile cast irons with a ferrite- An increase in the ratio beyond the intermediate upper bainite duplex matrix produced by austenitizing proportion, on the other hand, will come from an at 900°C and then austempered at 400°C are excel- increase in the harder second phase which has less lent in ductility. deformability and restricts the plastic deformation of ferrite, resulting in the enhanced proof stress. 3. Relationship betweenthe Impact and Tensile Properties Therefore, the ratio will mainly be affected by the at Room Temperature deformability of ferrite in the former and that of As described above, the impact values showed a second phase in the latter. similar tendency to the tensile values as a function of Transactions ISIJ, Vol. 21, 1981 (123)

Fig. 13. Relation of impact energy to tensile energy. (The tensile energy is obtained from a stress- Fig. 12. Relation of deflection to elongation. strain curve in the tensile test.)

Fig. 11. Relationship between the impact and tensile properties.

Fig. 14. Fig. 15. Relationship between the upper shelf energy Relationship between the transition tempera- and tensile strength. ture and tensile strength. (Percent numbers indicate the volume fraction (Percent numbers indicate the volume frac- of upper bainite.) tion of upper bainite.) the volume fraction of the second phase. The rela- 4. Relation betweenthe Strength and Toughness tions of these values at the same volume fraction of In this section, the tensile strength and toughness bainite are illustrated in Figs. 11 to 13. will be related directly. Figure 11 shows that there is an approximately Figures 14 and 15 show the relations of the upper linear relation between the maximum load (and yield shelf energy and transition temperature to the tensile load) and tensile strength (and 0.2% proof stress) strength. In the upper shelf energy of the un- at room temperature. Figure 12 shows a parabolic notched specimen austenitized at 900°C and aus- relation between the deflection and elongation ac- tempered at 400°C, there are two peaks at tensile companied with somewhat scattered values. The im- strengths of 68 and 93 kg/mm2, where the energies are pact energy exhibits also a parabolic relation with the 15 and 14.5 kg2mf cm2 and the volume fractions of tensile energy as shown in Fig. 13, but the tendency upper bainite are 55 and 95%, respectively. The is different between the upper and lower bainite. upper shelf energy is higher than those of the fully Thus, fairly simple relationships are present be- upper bainitic and of the pearlitic, and is also higher tween the impact and tensile properties. than that produced by austenitizing at 850°C. In the impact test, the specimen is deformed dy- The desirable characteristic can also be seen in namically under complicated loading conditions the transition temperature shown in Fig. 15 at the with three-dimentional stress and strain, while in the same tensile strengths of 68 and 93 kg/mm2. Thus, tensile test, it is deformed statically under a simple both the toughness and strength of the iron are im- uni-axial loading condition.22~ From this respect, proved considerably at these volume fractions of the the relationship seems to be complicated, but it is upper bainite. relatively simple in fact. Therefore, the instrumented Charpy impact test provides more effective informa- 5. Effectof the Site of Bainite Formation on the Toughness tion on the impact properties than the conventional As mentioned above, the improvement of toughness un-instrumented impact test, so that a qualitative or can be achieved only by the duplex matrix of proper to some extent quantitative evaluation of tensile prop- volume fraction of the upper bainite produced by erties will be possible from the impact test. austenitizing at 900°C and austempering at 400°C. This suggests that the formation site of upper bainite (124) Transactions ISIT, Vol. 21, 1981

will have some important effects on the toughness. From Figs. 5, 8, and 9 and Photo. 3, it is apparent that the austenitizing temperature affects on the formation site of austenite, which results in bainite by austempering. From these experimental results, therefore, it is suggested that the quality of the matrix structure, especially around graphite nodules, may play an important role in the impact and tensile properties. The graphite is generally a source of a crack initiation site since the brittle and low strength graphite acts as an inner crack in cast iron. A bending test of the specimen, which was polished and picral- etched for microstructure examination, was carried out to examine the cracking behavior. Photograph 6 shows that the crack propagation occurs by the linking of graphite nodules ahead of the main crack (Photo. 6(a)), and is prevented by the bainite around the graphite nodule, so that the crack propagates preferably in ferrite (Photo. 6(b)). Thus, the toughness can be improved by the upper bainite with high strength and toughness around graphite nodules since the most preferential site of crack initiation is the graphite-ferrite interface.

6. The Relation between Mechanical Properties and Carbon Content Dissolved in Austenite To obtain more satisfactory explanation for the reason of the complicated mechanical behaviors, a Photo. 6. Crack propagation profile in bending test specimen calorimetric analysis was conducted by using the iron of ductile cast irons. specimen of higher silicon content in order to examine (The arrow indicates the direction of main crack the carbon content dissolved in austenite, because it propagation.) is supposed that the carbon content will give some effect on the mechanical behavior. A small specimen of 4 mm diameter and 12 mm long was isothermally austenitized for a short period of time at 900°C or 850°C, quenched to room temperature and tempered for 2 hr at 475°C, where no graphitization occurred, and then analyzed calorim- etrically. The results are shown in Fig. 16 giving the total amount of carbon dissolved (C) and the average carbon content in austenite (CAF) to the volume fraction of austenite (F). The total carbon content increases with increasing volume fraction of austenite, showing a fairly similar increasing tendency to the tensile strength and 0.2% proof stress. It is therefore suggested that the cabon dissolved in austenite will exert a considerable influence on the tensile strength and proof stress. Fig. 16. Variation of carbon content dissolved in austenite On the other hand, the average carbon content in during austenitizing. austenite decreases at first with increasing volume fraction of austenite up to about 40 to 50%, then carbon in austenite continues to attain the equilibrium stays constantly or increases slightly, and after that carbon content at a given temperature. increases again considerably. Such a tendency of From the results of the average carbon content in the average carbon content can also be derived by austenite, the complicated variations in ductility and a theoretical analysis.23'24) Carbon dissolved super- toughness for the iron of higher silicon content with saturately in austenite at an unsteady state decreases a ferrite-upper bainite duplex matrix can be explained rapidly to a required value at a steady state of aus- as follows tenitizing, and it increases again because of more Since a small amount of harder bainite with higher rapid carbon diffusion than increasing volume of carbon content acts like a source of stress concen- austenite. After 100% austenitizing, diffusion of tration, the ductility and toughness are lowered up

Research Article Transactions Is", Vol. 21, 1981 (125) to about 10% in volume fraction of the bainite, and more effective information than the conventional un- then they become higher in spite of an increase in the instrumented test, and a qualitative assumption for volume fraction of bainite up to 50% because of a the tensile properties is also possible. decrease in the average carbon content. From the correlated impact and tensile properties, In the latter stage from 50% in volume fraction of a considerable improvement in both toughness and bainite, however, they become lower again with in- strength can be achieved when the iron is austenitized creasing fraction of higher strength bainite and de- at 900°C and austempered at 400°C, and the volume creasing fraction of ductile ferrite. However, there fractions of upper bainite of about 50 and 95% are appears a maximum in the ductility and toughness at obtained. Particularly at the volume fraction of about 95% in volume fraction of the upper bainite, 95%, the tensile strength is 93 kg/mm2 (it is about which comes from the fine dispersed bainite with two times higher than that in ferritic), the transition considerably low carbon content mostly around temperature is -45°C (in the un-notched specimen) graphite nodules. The bainite around graphite nod- and the upper shelf energy is 14.5 kg. mf cm2 (it is ules will also contribute to the matrix strengthening. higher than that in ferritic). At this volume fraction, in addition, the toughness is considerably higher than V. Conclusions that in fully bainitic at the same tensile strength, and Mechanical properties of ductile cast iron with also in pearlitic structures. duplex matrices of various proportions were examined The improvement of mechanical properties comes systematically by means of the tensile and instru- from the fine duplex matrix structures and low carbon mented Charpy impact tests. content of the second phase, and also comes from the The 0.2% proof stress, tensile strength and hardness second phase with high strength and ductility, such increase with an increase in the volume fraction of as upper bainite, formed mainly around graphite the second phase, and the harder the second phase is, nodules, probably because of preventing a crack the higher the values become. But there exists no initiation at the graphite-second phase interface. linear relationship known as the law of mixture. The tensile strength of the iron of higher silicon Acknowledgements content (2.5%Si) with lower bainite matrix arrives The authors wish to express their appreciation to to the maximum value of 141 kgfmm2 (with elonga- Asahi Malleable Iron Co., Ltd. for the providing of tion of 3%). ductile cast irons, and Y. Kondo, Nagoya Govern- A considerable increase in the elongation and ment Industrial Research Institute, and S. Aoki, tensile energy occurs at intermediate volume fractions a student of Nagoya University, now Nichiden Sei- of the second phase, while they decrease at small mitsu Co., Ltd., and I. Ogasawara, a student of fractions. The tendency is more apparent in a ferrite- Nagoya University, now Howa Kogyo Co., Ltd., bainite duplex matrix. for the experimental assistance. 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Research Article