Mechanical Property and Microstructural Variation in Semi-Solid Processed High Chromium Cast Iron

Mechanical Property and Microstructural Variation in Semi-Solid Processed High Chromium Cast Iron

1Nazir Hussain, 2Amitesh Kumar, 3P. Vijayanand

1. Cambridge Institute of Technology, Ranchi, India,

2National Institute of Foundry & Forge Technology, Hatia, Ranchi

3Research Scholar in Ranchi University, Ranchi

Abstract – High chromium cast irons (HCCI) find its application in industries like mining, earth-handling and areas where exceptional wear resistance and hardness is required. HCCI show these properties because of the presence of chromium carbides in a higher amount. The carbide percentage and chromium percentage in the matrix control the mechanical properties & microstructure of HCCI. In the present work comparative study of slope casting and conventional gravity mold casting were carried out. Fe- 10wt% chromium was cast in both cases from the same melt. In this work attempts have been made to investigate the microstructure and hardness of cast samples. The results show that semi solid processed 10% Cr alloys possess better hardness than conventionally cast 10% Cr alloys due to change in microstructure.

INTRODUCTION

High chromium cast irons (HCCIs) are mainly used in mining, milling, earth-handling and manufacturing industries which require materials with exceptional wear and corrosion resistance. High-chromium cast iron since shows good wear and corrosion resistance properties and are widely used in making components that demands these properties. The exceptional wear resistance of high chromium cast irons is due to the high volume fraction of hard chromium carbides, although the toughness of the matrix also contributes to the wear resistance [1-3]. The high controlled percentage of chromium helps to retard the formation of graphite and stabilize the carbides. In the mean time copper, nickel, manganese and molybdenum are normally added to suppress the formation of pearlite during cooling [4-5]. High chromium cast irons wear behavior has been well investigated [6,7] and the microstructure and properties relationship has also been extensively studied [2,8,9].However, the comprehensive explanation regarding the resistances of HCCI, one of the most important industrial materials, to its remarkable wear, corrosion resistance is still incomplete and the information provided in literature is limited [4,10,11].In the present work semi-

solid processing ; in which molten metal is cooled and solidified partially before it is poured in the mould has bright future in many industrial fields [12].Semi-solid processed alloys do not contain dendritic structures as in conventional castings. In this kind of metal alloys processing the solidifying metals contain primary phase which is nearly spherical. This leads to the creation of a slurry-like form that allows the semi-solid alloy to behave as a fluid of low viscosity when flowing under shear during the casting operation. The microstructure features of semi- solid processed high chromium cast irons such as 10 wt% chromium are of concern so as to improve the lives of wear parts in mining and mineral industry, especially when combined erosion and corrosion resistance is required. Although the effects of heat treatments on general microstructure and some information on properties of semi- solid processed 27wt% chromium irons have been reported [13-14], a detailed examination of microstructure has not previously been attempted. Nowadays interest is being shown in developing the semi-solid process and making it technologically simple and viable process. One such route suggested is processing the alloys in inclined controlled heated plates or cooling slopes [15-28]. The sloping plate can provide very strong under-cooling and cause a large quantity of nuclei to form, which creates fundamental conditions for fine spherical grain formation [29-30]. Final microstructure is being controlled by various parameters involved in the process like melt superheat, slope angle, mold material, slope length. The studies did so far have been helpful to some extent in understanding the complicated microstructure evolution during the shear processing of the alloys. But detailed explanation of micro- structural evolution during low shear inclined slope semisolid processing is still not available. However, the inclined slope process is attractive due to flexibility and the ease of processing that has been the limiting factor to great extent in the development of semisolid processing. In the current study the cooling system used is of 1 meter in

length inclined at an angle to provide sufficient amount of shear force so as to avoid dendritic grains in the final structure. The 1 meter length channel is of mild steel on which a definite amount of alumina coating is applied , the thermal properties of alumina and mild steel is also taken into account to study the effect of sloped channel on the morphology of 10wt% chromium iron melt when it passes through the channel. Then optimizing the operating parameters of the setup such as angle of the slope, superheat, slope temperatures to ensure non-dendritic morphologies. The effect of modification on the structure and mechanical properties of low shear semisolid high chromium cast iron has to be studied. The experimental result is then validated using simulation software platform, and the results obtained after simulation were in good agreement with the experimental ones.

Experiment

Methods & Experimental details

Sand mould was made silica sand of AFS 70 grain fineness number, 5% bentonite and water were added. The mixture was prepared in sand Muller. After the molding the pattern was resealed and allowed to dry for 5 hours. 3-D solid model of bar is shown in the Figure 1.Dimensions of each stepped are given in Table 1 and table 2.

Table 1: Dimension of stepped bar (semisolid cast)

Table 2: Dimension of stepped bar (conventionally cast)

Fig.1: 3D model of Stepped bar.

The composition of melt is investigated and is shown in Table 5. As soon as the melt reaches to a temperature above liquidus temperature, the melts were taken out of the furnace and poured under the different conditions; conventionally poured sample and semisolid processed sample were made. For making semisolid melt the molten metal was poured in a semi-circular channel made of cast iron and a specific amount of alumina was applied over it, the channel was inclined at an optimum angle so that a sufficient amount of shear stress acts on the flowing metal. The alumina layer is painted with graphite in its inner radius to prevent sticking of the liquid metal being poured through it. The paint is air dried well before pouring.

In conventional method the melt was directly poured in the mold but in the semisolid method the melt traverse a length of 1 meter till it reaches the pouring basin made in the mold.

The physical and thermal properties of materials used in modeling of the slope setup are given in Table 3 and Table 4 .

Table 3: Thermal properties of Alumina.

Table 4: Thermal properties of Air wall.

RESULTS & DISCUSSION:-

Chemical Composition: The chemical composition of cast iron was determined using a spectrometer. The chemical composition of cast sample is given in table 5.

Table 5: Chemical composition cast samples

Microstructure:

In this work, four different thickness of plate of high chromium cast iron (HCCI) containing 10 wt. % of chromium were cast by conventional method as well as semi solid process. Semi solid process was carried out on an inclined slope. Their microstructures were characterized using image analyzer. Microstructues are shown in the Figure 2. The carbides formed in the HCCI with carbon content (3.5 wt. %) were identified as cubic-face centred M23C6.For the conventional casting the structure was found to be dendritic and the percentage of carbide formed was not appreciable to overcome the properties obtained in semisolid casting samples. In semisolid samples the structure can been seen clearly in the microstructure (Fig. 2

(b) & (d)) which is more rounded than conventionally cast samples.

Hardness testing:

Average hardness of both conventionally cast and inclined slope casting of samples were measured using Vicker Hardness tester. The sample prepared by semisolid casting methodology showed higher hardness than the conventionally cast samples due to the markedly change in morphology of carbide of semi solid process casting samples compared to conventionally cast samples. The hardness values of samples are given in Table 6. Thinner section of stepped in both the process showed higher hardness due to higher rate of cooling compare to thicker section of stepped bar. The high cooling rate produced very fine microstructure than slow cooling rate.

Fig.2: Microstructure of (a) & (c) Conventionally cast, (b) & (d) Semisolid Sample of 10wt% Cr cast iron

Table 6: Hardness (Vicker Hardness)

CONCLUSON:-

1. Spheroidal structure can be obtained by Slope casting method.

2. The hardness of thinner sample in each type of casting process has shown higher hardness value. This is due to faster cooling rate.

3. Semisolid casting process for high chromium cast iron proved to be a better manufacturing method as it improves the mechanical properties of the materials and also lessen the defect caused due to turbulence or higher pouring temperature.

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