The following points highlight the process of continuous casting of metals. The processes are: 1. Reciprocating Mould Process 2. Asarco Process.
1. Reciprocating Mould Process:
In this process molten metal is poured into a holding furnace. At the bottom of this holding furnace there is a needle valve arrangement by which the quantity of flow can be changed. Metal from this needle valve flows to a mould (generally slab shape) and is distributed all along the mould surface by a submerged horizontal cross piece.
The level of the metal in the mould is kept constant at all times. The mould is surrounded all around by cooling water. The water cooled mould is reciprocated up and down. The down-stroke is synchronized with the discharge rate of slab. As the mould moves up after its down limit, it is again filled with molten metal and again in down stroke, cooling starts and this process continues.
As messy metal comes out of mould, it is directly cooled by water sprayed on it. The rolls at the bottom keep on pulling the slab to match with the cooling rate. Below the rolls is a saw which keeps on cutting the slab to the required lengths. Slabs produced by this process are further processed by cold rolling into sheets and strips.
2. Asarco Process:
It is a modification of the reciprocating process. In this process, the forming die is an integral part of the furnace and thus there is no problem of controlling the flow of metal. The metal is fed by gravity into the mould from the furnace as it is continuously solidified and withdrawn by the rolls below.
The die is water cooled and self-lubricating and thus has excellent resistance to thermal shocks. The upper end of die is in molten metal and thus serves the function of riser and acts as path for dissipation of evolved gases. At the time of starting, a similar rod is put into the die till its upper end melts and forms continuity with molten metal and it is then withdrawn by rolls and the process becomes continuous.
By this process, rounds, tubes, squares and special shapes can be conveniently produced and it is particularly suited to alloys like phosphorised copper and standard bronzes. The physical properties of castings produced are superior to other processes.
In continuous casting, solidifying zone being relatively small, practically all problems encountered with feeding and shrinkage are overcome. Problems due to fast cooling in mould zone are faced (segregation and cracking). A greater degree of control is, therefore, required in comparison to batch casting.
Fig. 3.75 shows the working of various types of continuous casting machines.
Direct casting of sheets has also been tried but has not proved to be very successful due to some drawbacks like segregation problems (variation of metal analysis from place to place) and solidification at ends being faster than at centre results in greater thickness at the edges than desired.