SOLIDIFICATION RATE OF WHITE CAST IRON

Abstract

The solidification rate of white cast iron and the effect of casting temperature, mould tem perature and casting size upon it were studied by means of the "pour-out method". By investigation of the roughness and microstructure in the solidification-front surface, the- mechanism of solidification was discussed.
The results obtained were summarized as follows:-
(1) In the solidification of white cast iron, the linear relation between the thickness of solidified layer and the square root of solidification time was satisfied only in the early stage of solidification and it deviated from the above relation as time proceeded. Then, the solidification curve was the form of S letter which had two points of inflexion, in other words, . white cast iron was solidified through three stages.
(2) From the linear relation in the early stage, the following equation, what was called an "equation of rate of skin formation" could be derived,
where, d: solidified layer (in inch)
t: solidification time (in mn)
k: solidification constant
a: constant
k or "rate of skin formation" decreased as the casting temperature and mould temperature became higher and the casting size became larger.
(3) As solidification proceeded, the roughness in solidification-front surface became larger according to its solidification curve. And it becames larger with increase of the casting temperature, mould temperature and casting size.
(4) The first point of inflexion in solidification curve was the beginning point of crystallization of a "bee-hive" like ledeburite and the second one was the end point of crystallization of dendritic cementite. With increase of the casting temperature, mould temperature and casting size, these points became later and their time interval became longer, and consequently the microstructure became rough.
(5) From the results above mentioned, the schematic diagram of solidification of white cast iron was drawn and the reason for the appearance of two points of inflexion was explained.