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 transforma- tion 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 ma- trix structures. Fig. 2. Effect of the second phase on tensile prop- erties 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 per- formed 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.