Cast iron is melted in the atmosphere, and a variety of oxides will be brought into various charges. However, the amount of W (c) and w (SI) in various cast iron is very high, and the casting defects caused by oxygen are very rare. Only when the smelting operation is extremely abnormal and the process principle is seriously violated, the serious oxidation will lead to a large amount of burning loss and a high amount of W (FeO) in the slag, resulting in the formation of reactive pores.
Under the condition of induction furnace melting, this kind of problem generally does not occur. Under the condition of cupola melting, although the molten charge passes through the high temperature oxidation zone in the form of droplets after melting, the interface between molten iron and atmosphere is very large, and generally there is no problem due to too much w (o) in cast iron. In recent 20 years, high pressure oxygen injection through tuyere of large cupola has become a normal process. Even under this smelting condition, there is no report about the problem caused by too high W (o) content in molten iron.
In cast steel, oxygen is harmful and unhelpful. Sufficient deoxidation during melting is an important condition to ensure the quality of steel castings. However, oxygen is an indispensable and beneficial element in various cast irons with graphite in microstructure and high W (c) and w (SI) contents.
A variety of fine oxides are important components in the heterogeneous core on which graphite precipitates in cast iron. The main reason is that the undercooling of gray cast iron melted by induction furnace is high and the micro oxide in molten iron is low. During smelting in induction furnace, with the prolongation of holding time of molten iron in the furnace, the fine oxides (including various sulfides and silicates) contained in molten iron are easy to gather and float due to the stirring effect of induced current, and the fine oxides suspended in molten iron will be gradually reduced by C, so the undercooling of cast iron during eutectic transformation is gradually enhanced.
It can be seen that oxygen in the form of oxide is very important for the nucleation of graphite in all kinds of cast iron containing free graphite. Even for as cast black core malleable cast iron without graphite, oxide is needed as heterogeneous nucleus of graphite during graphitization annealing.