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A study of feeding distance of SG iron castings.

INTRODUCTION

Ductile iron is not a single material but is part of a group of materials which can be produced to have a wide range of properties through control of the microstructure. The common defining characteristic of this group of materials is the shape of the graphite. In ductile irons, the graphite is in the form of nodules rather than flakes as it is in grey iron. The sharp shape of the flakes of graphite create stress concentration points within the metal matrix and the rounded shape of the nodules less so, thus inhibiting the creation of cracks and providing the enhanced ductility that gives the alloy its name. The formation of nodules is achieved by the addition of nodulizing elements, most commonly magnesium (note magnesium boils at 1100[degrees]C and iron melts at 1500[degrees]C) and, less often now, cerium (usually in the form of Misch metal).

In casting most metals and alloys shrink as the material changes from a liquid state to a solid state. Therefore, if liquid material is not available to compensate for this shrinkage a shrinkage defect forms. When progressive solidification dominates over directional solidification a shrinkage defect will form. The geometrical shape of the mould cavity has direct effect on progressive and directional solidification.

At the end of tunnel type geometries divergent heat flow occurs, which causes that area of the casting to cool faster than surrounding areas; this is called an end effect. Large cavities do not cool as quickly as surrounding areas because there is less heat flow; this is called a riser effect.

Also note that corners can create divergent or convergent (also known as hot spots) heat flow areas. In order to induce directional solidification chills, risers, insulating sleeves, control of pouring rate, and pouring temperature can be utilized.

[FIGURE 1 OMITTED]

Many papers have discussed about the feeding distance of steel material in terms of modulus of casting. Fig 1 represents one of the estimations of feeding distance for steel castings under various circumstances. But the same is not applicable to ductile iron because of the presence of carbon in the range of 3.5% to 4.0% in the form graphite which creates volumetric expansion of metal.

Solidification of Cast Iron:

The metallographic structure, soundness and consequently the properties and service performance of castings depend on their solidification behaviour.

During the solidification of hypoeutectic gray cast iron, two main events can be differentiated, the precipitation of the primary phase and the eutectic phase. The primary phase begins at the liquidus temperature with the nucleation of austenite that grows in the form of dendrites, which develop and form the solidification units named primary grains or crystals. Nucleation of the crystals takes place at the mold wall and in the inner melt originating the columnar and equiaxed grains respectively.

Each grain is composed of one dendrite and has the same crystallographic orientation all over. The dendrites grow mainly in extension until they collide or impinge with each other producing a solid network that work as the skeleton of the material. After this point coarsening is the main growth mechanism.

During the dendritic growth, segregation modifies the composition of the interdendritic melt. When the eutectic composition is reached, carbon precipitates as graphite and the eutectic solidification starts. The dendritic growth is still present during part of the eutectic solidification in such a way that their temperature intervals overlap. The eutectic solidification unit is the eutectic cell (EC), which is formed through a cooperative growth of austenite and graphite. The eutectic cells overgrow and envelope the dendritic network.

Their volumetric growth cease when coherence is achieved between cells. During the growth of the eutectic phase the graphite flake morphology characteristic of gray iron is defined. In the current work the development of a method that combines interrupted solidification, natural solidification, DAAS treatment, different etching techniques and Fourier Thermal Analysis (FTA) to study the solidification of hypoeutectic gray cast iron is described. The method permits the characterization and quantification of the primary phase in terms of the primary grain structure, evolution of secondary dendrite arm spacing (SDAS) and evolution of solid fraction of primary austenite.

The eutectic phase is characterized and quantified in terms of the development of eutectic cells size and number of eutectic cells. The method provides a good scenario for the relation of all these parameters.

[FIGURE 2 OMITTED]

Design of appropriate riser is very much essential. Riser design is made based on modulus of the casting. Modulus of the casting (Mc) is the ratio between volume of the casting to the cooling surface area of the casting. Modulus of casting is directly related to cooling time (T) of the casting. It is calculated as follows

Modulus of the casting

= Volume of the casting / Cooling surface area of the casting (1)

or

= cross sectional area / perimeter of the cooling surface (2)

Experimental Work:

Experimental work has been planned with a main objective of studying the solidification behaviour of ductile iron under various conditions and the dependence of other parameters on feeding distance. Feeding distance, as said earlier it is the maximum distance that riser can feed the liquid metal to the casting during solidification so as to get the shrinkage free casting. Feeding distance is an important parameter which decides the riser size and number of risers. Feeding distance of the riser is measured from the edge of the riser to the maximum distance up to which sound casting is realized.

To study the feeding distance of the riser it has been decided to develop a proto type pattern as shown in fig

3. The following things have been considered in designing the pattern.

1. Bars with same thickness and width, i.e. same modulus and only with varied lengths.

2. Risers are with same size to satisfy the modulus of casting.

3. All the patterns and risers are connected with a single sprue.

4. Varied ingate thickness according to individual casting weight to ensure uniform metal flow rate to all the bars.

[FIGURE 4 OMITTED]

The photograph of the casting is shown here. Length of the bar is fixed as 200mm, for the first few set of trials and extended to 270mm and 300mm but the width and thickness of the bar remains same for all the lengths as 25mm. Riser design has been made based on the modulus method as per the calculation explained earlier. Modulus of casting (M) decides the cooling rate of the casting. The below tabulated modulus has been calculated based on the first method.

RESULTS AND DISCUSSIONS

Various trials were conducted and the results and other process parameters were tabulated. Table is given below. Up to 200mm bar length there was no shrinkage found even with high and low designed levels of Carbon and Silicon combinations. In 270mm and 310mm bar shrinkage was noticed irrespective of chemical composition range. So it can be said that for a given conditions below the affordable feeding distance can be said as 200 mm.

The shrinkage defects have been noticed by using Radiography testing.
                                                      Tapping
Length                             Resi               Temperature
in mm     C       Si      Mn       Mg        CE       [degrees]C

200       3.68    2.66    0.349    0.0414    4.58     1508
200       3.53    2.49    0.372    0.0466    4.37     1512
200       3.61    2.55    0.317    0.0461    4.47     1503
200       3.6     2.65    0.371    0.0353    4.49     1508
200       3.68    2.59    0.367    0.0397    4.56     1505
270       3.68    2.59    0.367    0.0397    4.56     1505
310       3.68    2.59    0.367    0.0397    4.56     1505
200       3.68    2.29    0.329    0.0465    4.45     1507
270       3.68    2.29    0.329    0.0465    4.45     1507
310       3.68    2.29    0.329    0.0465    4.45     1507
200       3.68    2.29    0.329    0.0465    4.45     1507
270       3.68    2.29    0.329    0.0465    4.45     1507
310       3.68    2.29    0.329    0.0465    4.45     1507
200       3.67    2.45    0.357    0.0402    4.498    1508
270       3.67    2.45    0.357    0.0402    4.498    1508
310       3.67    2.45    0.357    0.0402    4.498    1508
200       3.46    2.27    0.308    0.0322    4.227    1500
270       3.46    2.27    0.308    0.0322    4.227    1500
310       3.46    2.27    0.308    0.0322    4.227    1500
200       3.68    2.25    0.31     0.0404    4.441    1507
270       3.68    2.25    0.31     0.0404    4.441    1507
310       3.68    2.25    0.31     0.0404    4.441    1507
200       3.7     2.3     0.31     0.0286    4.474    1505
270       3.7     2.3     0.31     0.0286    4.474    1505
310       3.7     2.3     0.31     0.0286    4.474    1505
200       3.64    2.82    0.311    0.0297    4.591    1503
270       3.64    2.82    0.311    0.0297    4.591    1503
310       3.64    2.82    0.311    0.0297    4.591    1503

          Pouring       Melt
Length    Temperature   Duration    Soundness
in mm     [degrees]C    in Hrs      Report        Hardness

200       1391          1.35        No defect     285
200       1399          1.2         No defect     255
200       1400          1.3         No defect     255
200       1400          1.15        No defect     255
200       1412          1.7         No defect     229
270       1412          1.7         Level--I      229
310       1412          1.7         Level--I      229
200       1372          1.15        No defect     229
270       1372          1.15        Level--I      229
310       1372          1.15        Level--II     229
200       1372          1.15        No defect     229
270       1372          1.15        Level--I      229
310       1372          1.15        Level--I      229
200       1396          1.2         No defect     207
270       1396          1.2         Level--III    207
310       1396          1.2         Level--III    207
200       1399          1.6         No defect     229
270       1399          1.6         Level--I      229
310       1399          1.6         Level--I      229
200       1398          1.15        No defect     255
270       1398          1.15        Level--III    255
310       1398          1.15        Level--III    229
200       1394          1.25        No defect     229
270       1394          1.25        Level--III    229
310       1394          1.25        Level--I      229
200       1410          1.25        No defect     255
270       1410          1.25        Level--III    255
310       1410          1.25        Level--II     269

* Level I, II, III = SHRINKAGE DEFECTS


REFERENCES

[1.] Nilsson KarlFre drik, Blagoeva Darinaand Moretto Pietro, 2005. An experimental and numerical analysist correlate variation on ductility to defects and microstructure in ductile cast iron component, Engineering fracture mechanics, 73(9): 1133-1157.

[2.] Lacaze, J., M. Castro and G.L. Esoult, 1998. Solidification of spheroidal graphit cast iron-IInumerical simulation, Actamater, 6: 997-1010.

[3.] Javaid, A., K.G. Davis and M. Sahoo, 2000. Effect of chemistry and processing variables on the mechanical properties of thin wall DIcastings, AFSTrans., 108: 191-200.

[4.] Labre, C. And M. Gagnj, 2000. Development of carbide free thin wall ductile iron castings, AFSTrans., 108:31-38.

[5.] Schrems, K.K., J.A. Hawk, N. Doan And A.P. Druschitz, Statistical analysis of the mechanical properties of thin walled DI castings.

[6.] Prdersen, K.M. and N. Tiedje, 2007. Temperature measurement during solidification of thin wall ductile cast iron. Part 1: Theory and experiment., doi:10.1016/j.measurement.

D. Sabarish, R. Ayyappan, M. Karthick

Department of Mechanical Engineering, Knowledge Institute of Technology

Received 25 April 2016; Accepted 28 May 2016; Available 5 June 2016

Address For Correspondence:

D. Sabarish, Department of Mechanical Engineering, Knowledge Institute of Technology E-mail: sabarishjhn@gmail.com
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Author:Sabarish, D.; Ayyappan, R.; Karthick, M.
Publication:Advances in Natural and Applied Sciences
Date:May 30, 2016
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