The following points highlight the four main steps involved in the casting process. The steps are:- 1. Preparation of Pattern and Mould 2. Melting and Pouring of the Liquefied Metal 3. Cooling and Solidification of Liquid Metal 4. Defects and Its Inspection.
Steps involved in the Casting Process # 1. Preparation of Pattern and Mould:
A pattern is the replica of the part to be cast and is used to prepare the mould cavity. Patterns are made of either wood or metal. A mould is an assembly of two or more metal blocks, or bonded refractory particles (sand) consisting of a primary cavity.
The mould cavity holds the liquid material and essentially acts as a negative of the desired product. The mould also contains secondary cavities for pouring and channeling the liquid material into the primary cavity and to act as a reservoir, if necessary.
A four-sided frame in which a sand mould is made is referred to as a flask. If the mould is made in more than one part, the top portion is called the cope and the bottom one is termed as the drag.
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For producing hollow sections, the entry of the liquid metal is prevented by having a core in the corresponding portion of the mould cavity. The projections on the pattern for locating the core in the mould are called core prints. There are diverse types of patterns and moulds depending on the material, the job, and the number of castings required.
Pattern Allowances:
A pattern is always made somewhat larger than the final job to be produced. This excess in dimensions is referred to as the pattern allowance. There are two categories of pattern allowances, namely, the shrinkage allowance and the machining allowance.
The shrinkage allowance is provided to take care of the contractions of a casting.
The total contraction of a casting takes place in three stages, and consists of:
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(i) The contraction of the liquid from the pouring temperature to the freezing temperature,
(ii) The contraction associated with the change of phase from liquid to solid,
(iii) The contraction of the solid casting from the freezing temperature to the room temperature.
It must be noted, however, that it is only the last stage of the contraction which is taken care of by the shrinkage allowance. Obviously, the amount of shrinkage allowance depends on the linear coefficient of thermal expansion αl of the material. The higher the value of this coefficient, the more the value of shrinkage allowance.
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For a dimension l of a casting, the shrinkage allowance is given by the product αll(θf– θ0), where θf is the freezing point of the material and θ0 is the room temperature. This is normally expressed per unit length for a given material. Table 2.1 gives some quantitative idea about the shrinkage allowance for casting different materials.
Usually, a cast surface is too rough to be used in the same way as the surface of the final product. As a result, machining operations are required to produce the finished surface. The excess in the dimensions of the casting (and consequently in the dimensions of the pattern) over those of the final job to take care of the machining is called the machining allowance.
The total machining allowance also depends on the material and the overall dimension of the job, though not linearly as the shrinkage allowance. Table 2.1 gives also an idea of the machining allowance for various materials. For internal surfaces, the allowances provided should obviously be negative, and normally the machining allowances are 1 mm more than those listed in the table.
There is another deviation from the original job dimensions and is intentionally provided in the pattern; this is called draft. It refers to a taper put on the surface parallel to the direction of withdrawal of the pattern from the mould cavity. A draft facilitates easy withdrawal of the pattern. The average value of the draft is between 1/2° and 2°.
Preparation of Mould:
Moulds are made by hand if the number of moulds to be prepared is small. If a large number of simple moulds are required, moulding machines are then used.
In this article, we shall briefly discuss some important features of mould making; also, some typical moulding machines will be outlined.
To facilitate an easy removal of the pattern, a parting compound, e.g., non- wetting talc, is dusted on the pattern. Fine grain facing sand is used to obtain a good surface on the casting. Normally, a dead weight is placed on the cope flask to prevent die cope flask from floating due to hydrodynamic forces of the liquid metal.
For a large mould, care should be taken to prevent the sand from falling off the cope flask when it is lifted to remove the pattern. This can be done by providing extra supports, called gaggers, within the cope flask. For a casting with re-entrant surfaces, e.g., a wheel with a groove at the rim, the mould can be made in three parts (Fig. 2.3). The part between the cope and the drag is termed as the cheek. For an easy escape of the gases, vent holes are provided in the cope flask.
The moulding machines operate on one or a combination of the principles explained in Fig. 2.4. In jolt ramming, the mould is lifted through a height of about 5 cm and dropped 50-100 times at a rate of 200 times per minute. This causes somewhat uneven ramming, but is quite suitable for horizontal surfaces. On the other hand, squeezing is found satisfactory for shallow flasks. The sand slinging operation is also very fast and results in uniform ramming. This, however, incurs high initial cost.
Steps involved in the Casting Process # 2. Melting and Pouring of the Liquefied Metal:
Melting:
A proper care during melting is essential for a good, defect-free casting. The factors to be considered during melting include gases in metals, selection and control of scrap, flux, furnace, and temperature. We shall now give a short discussion on these.
The gases in metals normally lead to faulty castings. However, the presence of a controlled amount of specific gases can be beneficial in imparting certain desirable qualities to the castings.
In metal castings, the gases-
(i) May be mechanically trapped (in such situations, proper venting arrangements in the mould prevent their occurrence),
(ii) May be generated due to the variation in their solubility at different temperatures and phases, and
(iii) May be produced due to chemical reactions.
The gases most commonly present are hydrogen and nitrogen. Metals are divided into two groups so far as the solubility of hydrogen is concerned. One group is called endothermic; this includes common metals such as aluminum, magnesium, copper, iron, and nickel.
The other group, called exothermic, includes, amongst others, titanium and zirconium. Endothermic metals absorb less hydrogen than exothermic metals. Further, in endothermic metals, the solubility of hydrogen increases with temperature. The reverse is true for exothermic metals.
In both cases, the solubility (5) can be expressed as-
S = C exp [-Es/(kθ)], (2.1)
where Es (positive for endothermic) is the heat of solution of 1 mol of hydrogen and 6 is the absolute temperature with C and k as constants. Equation (2.1) clearly shows that gas precipitation during cooling cannot take place in exothermic metals for which Es is negative.
Hydrogen is believed to dissolve interstitially in exothermic metals, thus causing lattice distortion. In endothermic metals, hydrogen dissolves in lattice defects and produces no distortion. Table 2.2 shows the solubility of hydrogen in the solid and liquid phases at solidus temperature for various metals. The difference in these solubilities is responsible for the evolution of the gases.
It should be noted that hydrogen solubility is an acute problem in ferrous casting. Here, although the amount of hydrogen by weight appears negligible, the volume evolved during solidification is quite large. Sievert’s law states that the amount of hydrogen dissolved in a melt varies as –
The primary sources of hydrogen in a melt are furnace dampness, air, oil, and grease. There is no simple dehydrogenating addition to eliminate hydrogen in the form of slag. So, care should be taken to maintain the hydrogen level to a minimum.
Most hydrogen removal techniques are based on equation (2.2), i.e., reducing the partial pressure of hydrogen by bubbling some other dry insoluble gas through the melt. For nonferrous metals, chlorine, nitrogen, helium, or argon is used. Nitrogen cannot be used for ferrous and nickel based alloys since it is soluble in these, and also it may form nitrides which affect the grain size; therefore, in ferrous alloys in particular, an accurate control of the nitrogen is necessary. In such situations, carbon monoxide bubbles are used. This removes not only hydrogen but also nitrogen; the carbon content is controlled by subsequent oxidation and recarburization.
For ferrous metals, a marked decrease in the solubility of nitrogen during the change of phase may give rise to porosity in the casting. The re-entry of nitrogen from the air is prevented by the impermeable slag at the top of the melt.
Currently, vacuum melting is increasingly being used for preventing the solution of gases in metals and the combination of reactive elements in the melt. Additions in the ladle, rather than in the melt, have been found to be more effective for controlling the gases and chemical compositions.
Furnaces:
The furnaces used for melting metals differ widely from one another. The selection of a furnace depends mainly on the metal chemistry, the maximum temperature required, and the metal delivery rate and mode. The other important factors in making a selection are the size and shape of the available raw materials.
The metal chemistry decides not only the control of standard elements but also some important mechanical properties, e.g., machinability.
The optimum temperature after melting is decided by a property, called fluidity, of the metal. Fluidity refers to the relative ability of the liquid metal to fill in the mould at a given temperature. Normally, the lower the viscosity, the higher the fluidity. The fluidity of a metal can be checked as follows.
A spiral of standard dimensions is poured with the liquid metal at various temperatures. The length of the spiral which can be fed in this way before the solidification starts gives the measure of fluidity. If we examine the temperature-fluidity curves for various metals, we find that the higher the fluidity of a metal, the lower the difference needed between the pouring temperature (furnace temperature) and the melting temperature.
For completely filling up the intricate, thin sections of the mould, this difference should be a minimum. A large difference implies higher cost and more gas solubility.
The rate and mode of liquid metal delivery are largely decided by the process—batch or continuous melting used.
Pouring (Gating Design):
After melting, the metal is poured or injected into the mould cavity. A good gating design ensures distribution of the metal in the mould cavity at a proper rate without excessive temperature loss, turbulence, and entrapping gases and slags.
If the liquid metal is poured very slowly, then the time taken to fill up the mould is rather long and the solidification may start even before the mould has been completely filled up. This can be avoided by using too much superheat, but then gas solubility may cause a problem. On the other hand, if the liquid metal impinges on the mould cavity with too high a velocity, the mould surface may be eroded. Thus, a compromise has to be made in arriving at an optimum velocity.
Steps involved in the Casting Process # 3. Cooling and Solidification of Liquid Metal:
A clear understanding of the mechanism of solidification and cooling of liquid metals and alloys is essential for the production of successful castings. During solidification, many important characteristics such as crystal structure and alloy composition at different parts of the casting are decided. Moreover, unless a proper care is taken, other defects, e.g., shrinkage cavity, cold shut, misrun, and hot tear, also occur.
Riser Design and Placement:
The solidification time depends primarily on the ratio VIA, where V is the volume of the casting and A is the surface area of heat dissipation (i.e., of the casting). This is also to be expected intuitively since the amount of heat content is proportional to volume and the rate of heat dissipation depends on the surface area. This information is utilized when designing a riser to ensure that the riser solidifies after the casting.
However, the information on the amount of liquid metal needed from the riser is used only to compensate for the shrinkage that takes place from the pouring temperature till solidification. Depending on the metal, the percentage of this shrinkage varies from 2.5 to 7.5. Thus, the use of a large riser volume (to ensure large solidification time) is uneconomical. So, a riser should be designed with the minimum possible volume while maintaining a cooling rate slower than that of the casting.
It may be noted that a casting with a high surface area/volume ratio requires a riser larger than that determined by considering only the cooling rate. This is shown clearly by the example that follows.
Let us consider a steel plate of dimensions 25 cm x 25 cm x 0.25 cm. The casting then has the A/V ratio as –
The riser we have considered has the volume 1.95 cm3 only. Therefore, a much larger riser is required.
For a given shape of the riser, the dimensions of the riser should, however, be chosen so as to give a minimum A/V ratio, and the minimum volume should be ensured from the shrinkage consideration. It must be remembered that a liquid metal flows from the riser into the mould only during the early part of the solidification process. This necessitates the minimum volume of the riser to be approximately three times that dictated by the shrinkage consideration alone.
To check the adequacy of the riser size for a steel casting, Caine’s relationship is normally used. The solidification time is proportional to the square of the ratio volume/surface area. Caine’s relationship, however, is based on the assumption that the cooling rate is linearly proportional to the ratio surface area/volume.
Here, the ordinate of a point on the curve shows the volume ratio and the abscissa the freezing ratio; also, the subscripts c and r refer to the casting and the riser, respectively. For a given casting-riser combination, if the point in Fig. 2.31 falls to the right of the curve, the adequacy of the riser is ensured. The equation for a rise ring curve is of the form
When a is the freezing constant for the metal, b is the contraction ration from liquid to solid, and c is a constant depending on the different media around the riser and the casting. The value of c is unity if the mould material around the casting and the riser is the same. For steel, the typical values are a = 0.1 and b = 0.03.
The tedious calculation of (A/V)c for a complex casting has given rise to another method where a rise ring curve of the type shown in Fig. 2.32 is used. In this method, the shape factor (l + w)/h, instead of (A/V)c, is plotted along the x-axis, where l, w, and h denote, respectively, the maximum length, the maximum width, and the maximum thickness of the casting. This method and Caine’s relationship give almost identical results for a casting of simple shape. If the appendages to the main body (of a simple, regular shape) of a casting are thin, then the solidification time does not alter significantly.
As a result, a marginal increase in the calculated volume (on the basis of the main body) of the riser performs the job satisfactorily. As the appendages become heavier, the riser volume required is calculated on the basis of a modified total volume of the casting. The total volume of the casting is taken as the volume of the main section plus the effective percentage of the appendage volume, called the parasitic volume.
The effective percentage is estimated from curves of the type shown in Fig. 2.33. A shape is called plate-like or bar-like depending on whether the width of the cross-section is more or less than three times the depth.
No special means of controlling the cooling rate (and hence the solidification time) of either the casting or of the riser. In practice, however, chill blocks or thin fins are used on the casting to increase its cooling rate. Chilling is less effective for a metal having a thermal conductivity higher than that of the chill. Similarly, to increase the solidification time of the riser, some exothermic compounds are added in the riser to keep it molten for a longer period.
So far, we have restricted our discussion to the adequacy of the riser size from the points of view of shrinkage and cooling rate. Another important aspect of rise ring is to ensure that the available liquid metal in the riser can be fed to the desired locations within the casting.
In fact, the thermal gradient, within the casting, during the last stage of cooling is the most important factor. The minimum allowable gradient depends on the shape and size of the cross-section. Normally, for a casting with low (A/V) ratio (e.g., cube and sphere), one central riser is able to feed the entire casting. On the other hand, for a casting with high (A/V) ratio (e.g., for a bar and a plate), usually more than one riser is necessary. In such a case, a proper location of the riser has to be decided.
For a steel plate of up to 100 mm thickness, one central riser is satisfactory if the maximum feeding distance is less than 4.5 times the plate thickness. The feeding distance should be measured from the edge of the riser, as explained in Fig. 2.34a. It should be noted that, of the total distance 4.5t, the riser gradient prevails up to a distance 2t, whereas the end-wall gradient prevails in the remaining distance 2.5t. Thus, the maximum distance between the edges of two consecutive risers is 4t and not 9t (see Fig. 2.34b).
A bar of square cross-section with sides measuring 50-200 mm can be fed satisfactorily from a single riser, up to a maximum distance of 30 √s, where s is the side of the square expressed in mm. The maximum distance between the edges of two consecutive risers is found to be 1.2s (and not 60√s).
The presence of a chill in the mould increases the feeding distance of the riser. This is achieved by providing a sharp thermal gradient with consequent decrease in the feeding resistance. It is obvious that the chill should be placed at the ends if a single riser is used. For more than one riser, the chill should be placed midway between the two risers. Figure 2.35 schematically explains the proper placement of risers and chills. The maximum permissible distances for various cases are also indicated in this figure.
Steps involved in the Casting Process # 4. Defects and its Inspection:
Defects in Casting:
The treatment is restricted essentially to the sand mould castings.
The defects in a casting may arise due to the defects in one or more of the following:
(i) Design of casting and pattern.
(ii) Moulding sand and design of mould and core.
(iii) Metal composition.
(iv) Melting and pouring.
(v) Gating and rise ring.
The following defects are most commonly encountered in the sand mould castings:
(i) Blow- It is a fairly large, well-rounded cavity produced by the gases which displace the molten metal at the cope surface of a casting. Blows usually occur on a convex casting surface and can be avoided by having a proper venting and an adequate permeability. A controlled content of moisture and volatile constituents in the sand-mix also helps in avoiding the blow holes.
(ii) Scar- A shallow blow, usually found on a fiat casting surface, is referred to as a scar.
(iii) Blister- This is a scar covered by the thin layers of a metal.
(iv) Gas holes- These refer to the entrapped gas bubbles having a nearly spherical shape, and occur when an excessive amount of gases is dissolved in the liquid metal.
(v) Pin holes- These are nothing but tiny blow holes, and occur either at or just below the casting surface. Normally, these are found in large numbers and are almost uniformly distributed in the entire casting surface.
(vi) Porosity- This indicates very small holes uniformly dispersed throughout a casting. It arises when there is a decrease in gas solubility during solidification.
(vii) Drop- An irregularly-shaped projection on the cope surface of a casting is called a drop. This is caused by dropping of sand from the cope or other overhanging projections into the mould. An adequate strength of the sand and the use of gaggers can help in avoiding the drops.
(viii) Inclusion- It refers to a nonmetallic particle in the metal matrix. It becomes highly undesirable when segregated.
(ix) Dross- Lighter impurities appearing on the top surface of a casting are called dross. It can be taken care of at the pouring stage by using items such as a strainer and a skim bob.
(x) Dirt- Sometimes sand particles dropping out of the cope get embedded on the top surface of a casting. When removed, these leave small, angular holes, known as dirts. Defects such as drop and dirt suggest that a well-designed pattern should have as little a part as possible in the cope. Also, the most critical surface should be placed in the drag.
(xi) Wash- A low projection on the drag surface of a casting commencing near the gate is called a wash. This is caused by the erosion of sand due to the high velocity jet of liquid metal in bottom gating.
(xii) Buckle- This refers to a long, fairly shallow, broad, vee-shaped depression occurring in the surface of a flat casting of a high temperature metal. At this high temperature, an expansion of the thin layer of sand at the mould face takes place before the liquid metal at the mould face solidifies. As this expansion is obstructed by the flask, the mould face tends to bulge out, forming the vee shape. A proper amount of volatile additives in the sand-mix is therefore essential to make room for this expansion and to avoid the buckles.
(xiii) Scab- This refers to the rough, thin layer of a metal, protruding above the casting surface, on top of a thin layer of sand. The layer is held on to the casting by a metal stringer through the sand. A scab results when the upheaved sand is separated from the mould surface and the liquid metal flows into the space between the mould and the displaced sand.
(xiv) Rat tail- It is a long, shallow, angular depression normally found in a thin casting. The reason for its formation is the same as that for a buckle. Here, instead of the expanding sand upheaving, the compressed layer fails by one layer, gliding over the other.
(xv) Penetration- If the mould surface is too soft and porous, the liquid metal may flow between the sand particles up to a distance, into the mould. This causes rough porous projections and this defect is called penetration. The fusion of sand on a casting surface produces a rough, glossy appearance.
(xvi) Swell- This defect is found on the vertical surfaces of a casting if the moulding sand is deformed by the hydrostatic pressure caused by the high moisture content in the sand.
(xvii) Misrun- Many a time, the liquid metal may, due to insufficient superheat, start freezing before reaching the farthest point of the mould cavity. The defect that thus results is termed as a misrun.
(xviii) Cold shut- For a casting with gates at its two sides, the misrun may show up at the centre of the casting. When this happens, the defect is called a cold shut.
(xix) Hot tear- A crack that develops in a casting due to high residual stresses is called a hot tear.
(xx) Shrinkage cavity- An improper riser may give rise to a defect called shrinkage cavity, as already detailed.
(xxi) Shift- A misalignment between two halves of a mould or of a core may give rise to a defective casting. Accordingly, this defect is called a mould shift or a core shift.
Inspection of Castings:
Nondestructive inspection techniques are essential for creating a confidence when using a cast product. In this article, we shall briefly outline some of these techniques for testing the various kinds of defects.
1. Visual Inspection:
Common defects such as rough surfaces (fused sand), obvious shifts, omission of cores, and surface cracks can be detected by a visual inspection of the casting. Cracks may also be detected by hitting the casting with a mallet and listening to the quality of the tone.
2. Pressure Test:
The pressure test is conducted on a casting to be used as a pressure vessel. In this, first all the flanges and ports are blocked. Then, the casting is filled with water, oil, or compressed air. Thereafter, the casting is submerged in a soap solution when any leak will be evident by the bubbles that come out.
3. Magnetic Particle Inspection:
The magnetic particle test is conducted to check for very small voids and cracks at or just below the surface of a casting of a ferromagnetic material.This done, the powdered ferromagnetic material is spread out onto the surface.
The presence of voids or cracks in the section results in an abrupt change in the permeability of the surface; this, in turn, causes a leakage in the magnetic field. The powdered particles offer a low resistance path to the leakage. Thus, the particles accumulate on the disrupted magnetic field, outlining the boundary of a discontinuity.
4. Dye-Penetrant Inspection:
The dye-penetrant method is used to detect invisible surface defects in a nonmagnetic casting. The casting is brushed with, sprayed with, or dipped into a dye containing a fluorescent material. The surface to be inspected is then wiped, dried, and viewed in darkness. The discontinuities in the surface will then be readily discernible.
5. Radiographic Examination:
The radiographic method is expensive and is used only for subsurface exploration. In this, both X- and y-rays are used. With y-rays, more than one film can be exposed simultaneously; however, X-ray pictures are more distinct. Various defects, e.g., voids, non-metallic inclusions, porosity, cracks, and tears, can be detected by this method. On the exposed film, the defects, being less dense, appear darker in contrast to the surrounding.
6. Ultrasonic Inspection:
In the ultrasonic method, an oscillator is used to send an ultrasonic signal through the casting. Such a signal is readily transmitted through a homogeneous medium. However, on encountering a discontinuity, the signal is reflected back. This reflected signal is then detected by an ultrasonic detector. The time interval between sending the signal and receiving its reflection determines the location of the discontinuity.
The method is not very suitable for a material with a high damping capacity (e.g., cast iron) because in such a case the signal gets considerably weakened over some distance.