The cooling rate of a casting affects its microstructure, quality, and properties. The cooling curve illustrates the way in which molten metals solidify. There is a fundamental difference between the cooling curve observed during the solidification of a pure metal and that of an alloy.
a) Pure Metal
The temperature of metal at its pouring point is higher than its solidification temperature. The solidification temperature stays constant throught the period of time until all the liquid metal become solid.
The actual freezing of metal takes time, the local solidification time in casting during which the metal’s latent heat of fusion is released into the surrounding mold. The total solidification time is the time taken between pouring temperature and complete solidification. After complete solidification, the solidified metal, called the casting, is taken out of the mold and allowed to cool to ambient temperature. Figure 1.4 shows the cooling curve for a poured metal during casting.
Fig. 1.4 Cooling curve for a poured metal during casting.
The solidification process begins at the interface of mold and metal and over the entire outer skin of the casting. This rapidly cooling action causes the grains in the skin to become fine and randomly oriented. As cooling continues and the energy transfer continues through the solid thin layer of metal toward the mold material, energy travels in one direction while the energy of the solidification process travels in the opposite direction. Since the heat transfer is through the skin wall, the grains continually grow as dendritic growth until complete solidification has been achieved. The grains resulting from this dendritic growth are coarse and columnarly oriented toward the center of the casting. Schematic illustration of grain formation is shown in Fig. 1.5.
Fig. 1.5 Schematic illustration of grain structure in a casting of a pure metal.
b) Alloys
Solidification in the case of an alloy begins when the temperature of the molten metal drops below the liquidus and is completed when the solid form is reached. A phase diagram and cooling curve for alloys during casting is shown in Fig. 1.6.
Fig. 1.6 Phase diagram and cooling curve for alloy composition during casting.
While pure metals have a well-defined melting point temperature, for alloys there is a melting temperature range, over which liquid and solid co-exist. The melting range can be quite large, which is the case for cobalt–chromium alloys, resulting in a phenomenon known as coring. Coring means that individual grains do not have the same chemical composition from the center to the outer edge of the grain. The importance of coring is that it produces a microstructure that is in general more likely to be attacked by galvanic corrosion. However, in the case of cobalt–chromium, the metal is protected from corrosion by the formation of a very thin metal oxide on the surface, so signs of corrosion are rarely observed.
Solidification begins with the formation of solid nuclei and the growth of these nuclei into the liquid by the addition of atoms at the advancing interface. If the latent heat that evolves during solidification is not removed, the solid nuclei will heat back up to the melting temperature and solidification will stop. Consequently, the rate of solidification is determined primarily by the rate of the latent heat of melting.
On a macroscopic scale, the structure of casting depends on the rate of nucleation and the rate of heat removal from the casting, e.g., on the temperature of the molten liquid when it is poured into the mold and on the temperature of the mold walls. If the wall is cold and the casting is large, solidification will begin in the chill zone at the mold wall and proceed inward, parallel to the flow of heat out from the melt, shown in Fig. 1.7 as zone a.
Fig. 1.7 Grain structure in a casting: (a) chill zone, (b) columnar zone, (c) equiaxial grain structure.
The grains so formed are therefore elongated, or columnar, in shape. In Fig. 1.7, this is shown as zone b. If zone b can reach the center of the casting before the temperature there drops low enough for nucleation to occur, the entire casting structure will be composed of columnar grains. Usually this is not the case, since the center part of the casting begins to solidify before the columnar grains arrive there. Because the grains in the center of the casting are not forced to grow in any particular macroscopic direction, heat being removed isotropically, they are more equiaxial in shape. In Fig. 1.7 this is shown as zone c.
The relative amount of columnar versus equiaxial grains depends on such macroscopic factors as the rate of heat removal and the uniformity of cooling, as well as the presence of solid nuclei in the liquid region. A uniform but slow cooling rate, accompanied by a quiescent liquid pool, leads to a coarse grain structure in the interior of the casting, whereas a fast cooling rate and turbulent pool produce a fine grain structure, which is more desirable for good mechanical properties. In fact, a general rule about any solidification process is this: the faster the cooling rate, the finer the microstructure.
The total solidification time is the time required for the casting to solidify from molten metal after pouring. This time is a function of the volume of a casting and the surface area that is in contact with the mold.
According to Chvorinov’s rule, the mathematical relationship can be written as
| (1.7) |
where
| t | = | solidification time, s (min) |
| V | = | volume of the casting, m3(in.3) |
| A | = | surface area of the casting, m2(in.2) |
| c | = | mold constant |
| n | = | exponent (1.5 < n ≤ 2, but usually taken as 2). |
The mold constant c depends on the properties of the cast metal (heat of fusion, specific heat, and thermal conductivity), the mold material, and the pouring temperature. The value of c for a given casting operation can be based on experimental data from previous operations carried out using the same mold material, metal, and pouring temperature, even though the shape of the workpiece might be very complex.
The rule simply states