Characteristics of heat exchange of 2 iron sand covered casting
After the liquid metal is poured into the iron mold, the casting - sand layer - iron type is an unstable heat exchange system. In order to simplify the problem, it is assumed that the casting is half yuan, and it is assumed that the temperature field of each element in the system is distributed in a straight line. Figure 1 represents a part of the system, and it is clear that the same heat flow Q passes through each component of the system:
Two heat transfer criteria for heat exchange between casting and sand layer, iron and sand layer, respectively. K1 is the ratio of thermal resistance of casting to sand layer; K2 is the ratio of thermal resistance of iron to thermal resistance of sand coating. Taking K1 and K2 into consideration, with the change of the thickness of the overlying sand layer, there are three kinds of heat transfer conditions, namely, casting - sand layer - iron type, which may actually happen:
When k = 1, K2 = 1, the sand layer within the normal thickness of the casting cooling speed increases with the decrease of the thickness of sand layer.
When the thickness of the sand layer exceeds a certain thickness, the iron mould has no effect on the cooling of the casting, and this is equivalent to the ordinary sand casting or resin sand casting. Because the thermal conductivity of the overlying layer is much smaller than that of the iron type, the cooling of the casting is slow.
When k = 1, K2 = 1, the thickness of sand layer is too thin, then the equivalent of the metal casting.
The above heat exchange characteristics have been confirmed by experiments, when the crankshaft (CT Max -14) iron coated sand foundry sand thickness from 4 to 32mm gradually changes, cementite in crankshaft continues to decrease, the amount of pearlite and ferrite volume increasing. When the thickness of the sand layer is less than 4mm, the cooling strength of the casting is similar to that of the metal mold (thick coating). When the sand layer is larger than 32mm, its cooling strength is equivalent to that of ordinary resin sand.