This paper presents the results obtained, deductions made from solidification behaviour and a series of micro structural studies such as pearlite content, eu-tectic cell count and grain size of hypoeutectic gray cast iron which was sand cast (CO 2 moulding) using metallic, nonmetallic, water cooled and subzero (cryogenic) end chills. Hypo-eutectic cast irons containing C 3.42, Si 2.4 and Ni 1.5 with impurity contents (S, P, Mn etc.) were solidified unidirectionally in an American Foundrymen Society (AFS) standard mould, the end of which was provided with different end chills to study the effect of chilling during solidifi-cation. The melts were inoculated with 0.3% Fe-Si to promote graphitization. It was observed that the transition from one structure to another is more gradual than normally obtained in the structure of cast irons solidified mul-ti-directionally in a sand mould at room temperature. Austenite dendrite interactions were shown to be a major factor in determining the microstructure, in which the higher dendrite reaction leads to changes in DAS, ECC and GS. It is observed that, the number of eutectic cells is an index of graphite nucleation and the effect of these on structure, since the eutectic cells are developed on the graphite nuclei during solidification.
The ability of the chill to extract heat from the molten metal during solidification of the casting is dependent on the size of the chill and thermo-physical properties of the chill material. In other words, the capacity of the chill to absorb heat from the casting is taken as a measure of its efficiency. The volumetric heat capacity of the chill, which takes into account the volume, specific heat, and density of the chill material, has been identified as an important factor in the evaluation of the efficiency of the chill [
Chilled cast iron belongs to a group of metals possessing high strength, hardness, toughness and high wear resistance. For a metallurgist, there is sufficient information available on the solidification mode and cell size of ordinary cast iron in sand moulds. However, there is a serious lack of information on the mode of solidification of chilled cast iron and its effect on cell size, grain size, microstructure and the effects of these on mechanical properties. This prompted to embark upon a series of experiments to study the relationship among these parameters in the case of metallic, nonmetallic, water-cooled and liquid-nitro- gen-cooled chilled cast irons. The reason behind the selection of this series of chilled cast iron is that a wide range of microstructures can be obtained with different cell sizes, dendrite arm spacing and grain size.
Inoculation technique has been used for many years to improve the properties of cast iron and it is generally agreed that inoculation may affect the nucleation of the eutectic during solidification as late addition increase the number of effective nuclei [
When cast iron containing a certain amount of carbon is cooled slowly from the melt, large flake graphite appears. When cooling is more rapid, the graphite structure will be fine and under cooled graphite or white cast iron may be present depending on the cooling rate [
In general, the cooling rate during casting is largely governed by the design and thermal nature of the casting procedure, one significant factor being the mould material [
The influence of cooling rate on properties is profound because of its influence on microstructure especially the grain size. Rapid cooling causes fine grain structure and hence increase in hardness and tensile strength. Past research [
The eutectic graphite nucleation, eutectic cell number and graphite morphologies depend on the composition, cooling rate and increase in the degree of under cooling [
・ Chilled cast iron belongs to a group of metals possessing very good mechanical properties.
・ For a foundrymen, there is sufficient information available on the solidification and microstructure of cast iron, cast in sand moulds. However, there is a serious lack of information on the effect of solidification of cast iron, cast using different chills on pearlite content, eutectic cells and grain size.
・ This prompted to embark upon a series of experiments to study the relationship between these parameters in the case of cast irons, cast using different chills.
Hence, the present research work has the following purposes:
1) To obtain experimental data for pearlite content, eutectic cells, grain size and resulting microstructure for various cooling rates
2) To analyze the data in the light of the solidification process, and
3) To correlate micro-constituents such as pearlite, carbide, ferrite volume fractions with eutectic cell size and grain size.
Cast iron alloy of composition shown in
Element | C | Si | Mn | S | P | Ni | Fe |
---|---|---|---|---|---|---|---|
% composition | 3.42 | 2.4 | 0.41 | 0.04 | 0.08 | 1.5 | Balance |
Microscopic examination was conducted on all the specimens (chill end to riser end) to determine cell size, grain size, DAS and microstructure using Neophot-21 metallurgical microscope. For this, various etchants were tried but 2% Nital proved to be the best and was therefore used. Austenite grain size measurements were done on polished specimens using ASTM index method, (direct measurement using standard ASTM mesh on the microscope). Eutectic cell count measurements were carried out on polished specimens at low magnification (50X) using Stead’s reagent as the etchant.
Alloy designation: Alloy A-Sub zero copper chilled cast iron
Alloy B-Water cool copper chilled cast iron
Alloy C-Copper chilled (Metallic) cast iron
Alloy D-Graphite chilled (Non metallic) cast iron and
Alloy E-Sand cast (Without chill) cast iron.
The DAS, ECC and GS taken from the chill end of the casting cast using different chills reveal normal range of structural variations and are discussed in the following sections.
Solidification of cast iron begins with the crystallization of primary graphite. The primary graphite develops as straight graphite flakes which are totally surrounded by the liquid. The composition of the remaining liquid shifts toward the eutectic where the liquid is believed to solidify in a manner similar to that of the eutectic although the primary graphite may influence the size of the eutectic cell and the distribution of the eutectic graphite. Solidification over a temperature range is the primary requirement for dendrite growth. Primary austenite dendrites readily grow from the liquidus down to the eutectic temperature. Growth of dendrites may also continue concurrently with the eutectic as the temperature decreases through the eutectic range of the solidus. Thus, undercooling may lead to higher dendrite interaction, which was in the case of cast iron cast using different chills [
Microscopic observation of the samples reveal that, the solidification of metallic and non metallic chilled cast iron will start on a low number of nuclei as compared to the water cooled and sub zero chilled cast iron which reflected in large number of nuclei resulting in high eutectic cell count. Since the growth conditions in the liquid are favourable, these nuclei will start growing as soon as the temperature drops below the equilibrium temperature in case of metallic chilled hypo-eutectic cast iron and with maximum under-cooling in case of water cooled and sub zero chilled cast iron. A mushy type of under cooled graphite which is present apparently reverts to D-type during solidification if cooling is fast. When cooling is slow, the mushy under cooled graphite will be converted into flake graphite even at the beginning of the eutectic reaction. Past research [
Primary austenite dendrite was observed under the microscope for all the chilled hypo-eutectic cast irons developed. It was observed that lot of variations in pattern occurring as a result of differences in rate of cooling. For each specimen the structure of the dendrites was analyzed by measuring the dendrite arm spacing and observing the average dendrite length, dendrite interaction and directionality. Marked changes were also produced in graphite morphology by varying the rate of cooling using different type of chills. Primary austenite dendrites would be expected to develop within each of the castings produced in such a manner that their solidification is predicted by the rate of under cooling. For the hypo-eutectic cast iron composition selected in the present investigation and due to cooling provided during solidification, it is believed that the formation of dendrites are prior to nucleation and growth of the eutectic. The eutectic then completes solidification by filling the areas surrounding the dendrites. The resulting structures of transformed primary austenite dendrites traverse a number of eutectic cells which contain both graphite and transformed eutectic austenite. According to some investigations [
In addition, dendrite interaction which is a visual estimate of the percentage area of dendrites intersecting in a field of view was varied in the specimens tested. The largest degree of dendrite interaction was observed in cast irons chilled using higher cooling rates. The difference in dendrite length, randomness and associated graphite and matrix structure have been related and found that, faster cooling produces fine dendrites. Other factors such as inoculation, alloy content, prior melt history and carbon equivalent may also affect the dendrite morphology, but in the present investigation major factor affecting dendrite morphology is the cooling rate induced during solidification. Finally it is observed that the distribution of the primary austenite dendrites formed in the chilled cast iron cast using different chills follow the solidification behaviour which is different from others. It is found from the present research that decrement in DAS with increased cooling rate explains the fact that there is insufficient time available for diffusion of solute and also on the eutectic composition. It is also observed that the spacing of the arms is controlled by diffusion as well as heat transfer during solidification. The spacing is actually determined by the characteristic thickness of the diffusion zone around a growing dendrite.
The microstructure taken from the chill end reveal the normal range of structural variations in cast irons, viz. varying from ledeburite at the chill end in the case of cryo-cooled and water cooled chills to the random flake graphite in the case of non metallic chilled and sand cast irons. However, the transition from one structure to another was observed to be more gradual than normally obtained in the structures of cast irons solidified multi-directionally in a sand mould at room temperature. The results obtained suggest that some types of graphite in the structure form more likely as a result of the decomposition of carbides during and after the solidification process. Under the range of solidification conditions examined, the microscopic evidence suggests that the austenitic phase is first nucleated by heterogeneous nucleation. The carbide phase appears to be more likely nucleated by the austenite phase, rather than heterogeneously. The metallographic observation of the difference in the distribution of graphite which is also linked with the differences in its shape and size depends on the growth mechanism due to chilling. In the present investigation the effect of Si is twofold: firstly it minimizes D-type graphite as well as the carbide formation and secondly it promotes paralytic matrix which is desired. All the structural changes observed are due to the mould temperature (chilling) and could be readily accounted for in terms of the initial as well as subsequent solidification or growth under-cooling temperature.
The specimens of chilled cast iron cast using different chills are dictated by the structure of ledeburite, pearlite, cementite and ferrite are as shown in Figures 2-6. Severe chilling at the beginning of the eutectic reaction leads to fine supersaturated austenite in the form of willow leafs are evident in the primary austenite
which is contained in ledeburite (
graphite as indicated in
Also, if the results shown in Tables 2-4 are compared with one another, it can be seen that there exists an inverse relationship between distance from the chill with pearlite content, ECC and GS and straight forward relationship between distance from the chill and DAS along the length of the casting for the various specimens cast. It is noteworthy that specimens A contain maximum ledeburite and B contain little ledeburite with maximum cementite in pearlite matrix
Specimen location Distance from chill end (mm) | Pearlite content (vol.%) (at 500×) | ||||
---|---|---|---|---|---|
Specimen A | Specimen B | Specimen C | Specimen D | Specimen E | |
30 (chill end) | 68 | 68 | 51 | 24 | 08 |
70 | 59 | 67 | 50 | 24 | 08 |
105 | 57 | 62 | 50 | 24 | 08 |
145 | 42 | 60 | 49 | 22 | 08 |
180 | 34 | 40 | 48 | 22 | 10 |
225 (riser end) | 30 | 34 | 49 | 20 | 10 |
(Note: The remaining micro-constituents are cementite, ledeburite and ferrite).
Specimen location Distance from chill end (mm) | No. of Eutectic cells/cm2 (50× magnification) | ||||
---|---|---|---|---|---|
Specimen A | Specimen B | Specimen C | Specimen D | Specimen E | |
30 (chill end) | 190 | 168 | 142 | 84 | 72 |
70 | 174 | 150 | 130 | 78 | 74 |
105 | 106 | 108 | 104 | 76 | 74 |
145 | 106 | 96 | 96 | 76 | 74 |
180 | 96 | 96 | 90 | 74 | 74 |
225 (riser end) | 70 | 70 | 72 | 74 | 74 |
Specimen location Distance from chill end (mm) | No. of grains/inch2 (at 500×) | ||||
---|---|---|---|---|---|
Specimen A | Specimen B | Specimen C | Specimen D | Specimen E | |
30 (chill end) | 126 | 124 | 119 | 72 | 32 |
70 | 64 | 64 | 58 | 38 | 30 |
105 | 64 | 32 | 30 | 32 | 30 |
45 | 32 | 32 | 32 | 30 | 30 |
180 | 32 | 32 | 32 | 30 | 32 |
225 (riser end) | 44 | 48 | 30 | 30 | 32 |
whereas specimen C contain few cementite in pearlite matrix (Figures 2-6). Past research also showed that addition of small amount of Mn promotes the matrix in cast iron that contain both ferrite and pearlite [
Solidification of hypoeutectic cast iron begins with the crystallization of primary graphite [
It is to be noticed here that the number of nuclei available for solidification of cryo-cooled, water cooled and metallic chilled cast iron are higher than the
number of nuclei available for solidification of non metallic chilled and sand cast iron. This is due to the under-cooling effect caused by chilling. However, there are significantly fewer nuclei for solidification in the case of sand-cast iron without a chill. Solidification of flake graphite iron resulting from sand casting without a chill starts from a small number of nuclei as compared with chilled cast iron, a phenomenon indicated by eutectic cell count. Since the growth condition in the liquid is favourable, these nuclei start growing as soon as temperature below the equilibrium temperature in case of sand cast iron. Perhaps the most important fact to recognize regarding cell structure is that it develops after the precipitation of austenite dendrite structure which solidifies first. The dendritic structure hosts the cells and therefore can have a major effect on cell nucleation and growth. The solidification of flake graphite iron resulting from sand casting without a chill starts from a small number of nuclei as compared with chilled (water-cooled and liquid-nitrogen-cooled) cast iron, a phenomenon indicated by eutectic cell count. Since the growth condition in the liquid are favourable, these nuclei start growing as soon as temperature below the equilibrium temperature in case of sand cast iron and with maximum under cooling in the cases of chilled cast iron, whether water-cooled or liquid-nitrogen-cooled. The structural parameters studied include eutectic cell count, cell size, distribution and the cell rating. Eutectic cell count observations under the microscope showed that the nodule count of chilled cast iron lies between a large range of 190 cells/cm2 (for sub zero chilled) and 72 cells/cm2 (for sand cast, un chilled) the larger nodule count being generally associated with very high cooling rates or chill effectiveness and vice versa. It appears that the nodule counts are directly proportional to the bulk arrest temperature. This shows that nodule count also finds an excellent correlation with the chilling rate.
The operating mechanism according to the current solidification method is the factor that affects the growth kinetics during freezing i.e., as the speed of cooling lowers the freezing temperature more nuclei grows and thus balancing the diffusion and equating the heat loss to the chill with the gain from the heat of fusion liberated. It was observed that the changes from one recognized graphite form to another occur by a gradual change in the graphite and cell morphology which would be the characteristic and unique nucleation event required for each graphite form. This graphite morphology is perhaps one of the factors in determining the microstructure of cast iron and cell morphology for the grain size of cast iron. Grain size for cryo-chilled specimen is maximum near the chill end and decreases towards the riser end. But near the riser end there is slight increase in the grain size and this is due to the riser effect where the casting is dense. The fine grain size in the case of chilled cast iron results in the soundness of the casting (chilling effect) and hence its high strength.
Analysis of data on chilled cast iron using various types of chills showed that the cooling rate had a marked effect on solidification, DAS, microstructure, ECC and GS. It was found from the present research that:
Austenite dendrite interactions were shown to be a major factor in determining the microstructure, in which higher dendrite reaction leads to changes in DAS, ECC and GS. It is observed that, the number of eutectic cells is an index of graphite nucleation and the effect of these on structure, since the eutectic cells are developed on the graphite nuclei during solidification.
The microstructure taken from the longitudinal samples at various distances from the chill reveals the normal range of structural variations in cast irons, viz. varying from ledeburite at the chill end in the case of cryo-cooled and water cooled chills to ferrite and graphite in the case of nonmetallic chilled and sand cast irons. The results obtained suggest that some types of graphite in the structure form more likely as a result of the decomposition of carbides during and after the solidification process. Under the range of solidification conditions examined, the microscopic evidence suggests that the austenitic phase is first nucleated by heterogeneous nucleation. Then the carbide phase appears to be more likely nucleated by the austenite phase, rather than heterogeneously.
It has been found that higher cooling rate produces finer dendrite arm spacing and conversely the DAS increases as the cooling rate decreases because there is sufficient time available for diffusion of the solute. Therefore the decrement in DAS with increased cooling rate explains the fact that there is insufficient time available for diffusion of solute and also on the eutectic composition. Hence, it is concluded that diffusion controls the DAS.
It observed that eutectic cell count increased for water-cooled and liquid- nitrogen-cooled chilled cast iron due to the under-cooling effect caused by chilling. On the other hand for nonmetallic chilled and sand cast iron, the cell count is very small because there are significantly fewer nuclei available for solidification. Thus it is concluded that the number of nuclei available for solidification of cryo-cooled, water cooled and metallic chilled cast is higher than the number of nuclei available for solidification of nonmetallic chilled and sand cast iron.
Grain sizes in the case of chilled cast iron were fine but the grains were very coarse in the case of graphite (non metallic) chilled cast iron and sand-cast iron without a chill. In chilled cast iron, the experimental data show that the cell morphology is the principal factor in determining the grain size.
Hemanth, J. (2017) Effect of Metallic, Nonmetallic, Water Cooled and Cryogenic Chills on Pearlite Content (PC), Eutectic Cell Count (ECC) and Grain Size (GS) of Hypo Eutectic Nickel Alloyed Cast Iron. Modeling and Numerical Simulation of Material Science, 7, 1-18. https://doi.org/10.4236/mnsms.2017.71001