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Computation of the Eff ect of Alloying Elements on the Physical Properties of SiMo Ductile Cast Iron

Gülşah Aktaş Çelik¹, Mariginna I. T. Tzini², Ş. Hakan Atapek¹, Şeyda Polat¹, Gregory N. Haidemenopoulos²,³

¹Kocaeli University, Department of Metallurgical and Materials Engineering, Kocaeli, Turkey²University of Th essaly, Department of Mechanical Engineering, Laboratory of Materials, Volos, Greece

³Khalifa University of Science and Technology, Department of Mechanical Engineering, UAE

Abstract

High silicon and molybdenum ductile cast iron (SiMo) is a common manifold material due to its low cost and good castability as well as high oxidation resistance and high-temperature strength. However, the trend in increasing power density of engines limits the use of commercial SiMo alloy at increased temperatures. Instead of this alloy, stainless steels are preferred, however they have higher cost and are more difficult to cast. This brings the need for modification of SiMo by using several alloying elements. In this study, the effect of alloying elements like Al, Si, Cr, Mo, V, Ti, W and Nb on the A1

temperature, mole fraction of graphite and thermal expansivity of ferritic matrix were calculated by ThermoCalc software using TCFE6 database. The A1

temperature is important since it determines the maximum temperature at which the alloy can be used. Due to the thermal cycles that the material exposed during service low thermal expansivity is required. Mole fraction of graphite is also important because it effects the ductility as well as thermal conductivity.

Simulation studies revealed that all these alloying elements except for Mo increased the A1 temperature whereas the effect of Al and Si was the highest. All alloying elements except for Si decreased the mole fraction of graphite. Thermal expansion property of the SiMo was decreased by the addition of Cr, Mo, W, while it was increased by the addition of others.

1. Introduction

Cast irons have been used as exhaust manifolds since the first exhaust manifold was manufactured. According to literature, unalloyed, high carbon grey cast irons withstand operating temperatures up to 540 °C were used as the first exhaust manifold materials [1]. Since then, exhaust gas temperature exceeds 750 °C and reaches 1000 °C in some applications due to the stringent emission standards, environmental

protection and fuel economy regulations [1-5]. Therefore, in the near future it will be impossible to use cast irons even SiMo which is commonly used as manifold material today.

Although the maximum operating temperature of SiMo cast iron is limited to 820 °C [6], its casting process provides to obtain compact and complex geometry with a lower process and material cost [1]. Besides, ferritic ductile cast irons like SiMo has high thermal conductivity, low coefficient of thermal expansion and good mechanical properties which are vital for long service life of exhaust manifolds [7]. High thermal conductivity provides fast cooling thus the material is subjected to high temperatures for shorter periods of time. Its lower coefficient of thermal expansion will help to avoid cracks that may form due to thermal fatigue [5-7]. However, in order to increase its service temperature, it is necessary to modify SiMo composition using some alloying elements which have increasing effect on A1. It is known that elements such as Si, Al, Ti, W, Nb, etc. that expand the ferrite area will increase the A1

temperature [8].

In this study, the effect of alloying elements like Al, Si, Cr, Mo, V, Ti, W and Nb on the A1 temperature, mole fraction of graphite and thermal expansivity of ferritic matrix were calculated by ThermoCalc software using TCFE6 database.

2. Computational Procedure

The effect of alloying elements like Al, Si, Cr, Mo, V, Ti, W and Nb on the (i) A1 temperature, (ii) mole fraction of graphite and (iii) thermal expansivity of ferritic matrix of SiMo ductile cast iron were calculated by ThermoCalc software using TCFE6 database. The SiMo composition used for calculations is given in Table 1.

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Table 1. Chemical composition of the SiMo alloy (wt. %)

C Si Mo Mn Ni Fe 3.4 3.6 0.8 0.15 0.05 Bal.

3. Results and Discussion

3.1. Computational results of SiMo ductile cast iron

Figure 1 shows the isopleth section identifying both phases and critical temperatures of SiMo composition given in Table 1. A1 temperature, which is considered the limit of the maximum service temperature of SiMo, was computed as 823.91 °C.

Figure 1. Isopleth section of SiMo

3.2. Evaluation of the effect of alloying elements

A diagram showing the effect of alloying elements on A1 temperature of SiMo is given in Figure 2. In order to understand the effect of Si, it is necessary to consider values above 3.6 % Si in the diagram since SiMo already has this amount in its composition. Diagram shows that, Si is the most effective element in increasing A1 temperature, 8 % Si addition rises the A1 temperature to 1150 °C. Al addition in SiMo has also increasing effect on A1 temperature. With 8 % Al addition in SiMo, A1 temperature reaches to 1025 °C. Carbide forming elements except Mo keep A1

temperature almost stable.

Figure 2. Effect of alloying elements on A1

temperature of SiMo.

Figure 3 shows the effect of alloying elements on mole fraction of graphite in SiMo. Above 3.6 % Si, mole fraction of graphite decreases slowly with increasing Si content. Al has more decreasing effect on mole fraction of graphite than Si. Among the carbide forming elements, only W almost has no effect. On the other hand, the effect of Nb in decreasing mole fraction of graphite is much less weaker compared to the elements which keep the A1

temperature almost stable (Cr, Ti, V).

Figure 3. Effect of alloying elements on mole fraction of graphite of SiMo.

Effect of alloying elements on thermal expansivity of ferrite at room temperature (RT) is shown in Figure 4. Matsushita et. al. showed that for fully ferritic spheroidal graphite cast iron, coefficient of thermal expansion decreases with increasing graphite [9]. In this study a similar effect is observed; Al increases the thermal expansivity more than Si and only W has a reducing effect on thermal expansivity of SiMo.

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Figure 4. Effect of alloying elements on thermal expansivity of ferrite of SiMo at RT.

Figure 5 and 6 show the isopleths of elements having increasing effect on A1 temperature (Al, Si) and carbide forming elements (W, Nb), respectively. For the calculations, SiMo composition given in Table 1 is used. In the isopleths by the addition of alloying elements, only Fe amount in the SiMo composition descreases since it is the balance element. These carbide forming elements are chosen mainly because they do not have decreasing effect on A1 temperature (Fig. 2). Their effect on mole fraction of graphite is also important since it affects thermal conductivity [10]. W increases the mole fraction of graphite and the decreasing effect of Nb is much weaker than other carbide forming elements (Fig. 3). As for the thermal expansivity of ferrite, W has a decreasing effect and Nb has no significant effect on this property (Fig. 4).

Al and Si dissolve in ferrite and provide solute solution strengthening of ferritic matrix [11]. Therefore both elements increase A1 temperature without any carbide precipitation (Fig. 5a and b). Si provides delta ferrite transformation from liquid above % 8 Si (Fig. 5b). Figure 6a shows that W provides eutectic M6C carbide formation like Mo and has small increasing effect on A1 temperature. Nb forms primer MC type carbide (Fig. 6b). Studies indicate that addition of Nb and W in Fe-based cast alloys increases the mechanical properties due to precipitation strengthening by primary carbides. These carbides also prevent grain boundary motion and provide grain size strengthening [12, 13].

(a)

(b)

Figure 5. Isopleths of (a) Al, (b) Si elements in SiMo composition.

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(a)

(b)

Figure 6. Isopleths of (a) W, (b) Nb elements in SiMo composition.

4. Conclusion

In this study, effect of Al, Si, Cr, Mo, V, Ti, W and Nb on A1 temperature, mole fraction of graphite and thermal expansivity of ferrite in SiMo composition were studied by ThermoCalc software. Results revealed that alloying elements increased the A1

temperature except for Mo. Increase in A1

temperature brings the possibility of material use at higher temperatures and among these elements the effect of Al and Si was the highest. All alloying elements except for Si decreased the mole fraction of graphite. Thermal expansion property of the SiMo is

decreased by the addition of Cr, Mo, W, whereas is increased by the addition of others.

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