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If the carbon in cast iron is partly in the form of carburized body and partly in the form of graphite, then the top of the fracture can see the white and gray parts mixed with each other. And the gray cast iron manufacturer usually calls this kind of pockmarked organization, which is the white-out organization, and sometimes it also appears in the transition part between the peripheral white-out and the central gray-out.
If the cooling rate of the casting is very high, then the iron will soon be cold to below the carburizing eutectic temperature, and there will be carburizing and austenite precipitation in the thin-walled areas and corners of the casting. At the same time, graphite can also nucleate and grow. The heat released by two eutectic reactions will make the liquid phase temperature rise back to above the carburizing eutectic temperature, and the iron-carburizing eutectic reaction will stop, which will limit the precipitation of free carburizing body, and under such conditions, the cast iron can be completely solidified between the two eutectic temperatures, and the pockmarked organization is obtained, which is more commonly known as white-out organization by gray cast iron manufacturers.
1. High cooling rate
Before the eutectic reaction occurs, the iron is cooled to below the carburizing eutectic temperature. The process factors that cause a high cooling rate are as follows.
(1) The walls of the casting are relatively thin.
(2) The pouring temperature of gray cast iron manufacturers is too low, the role of the poor heating cavity before solidification, the temperature difference between the casting and the mold.
(3) the castings have a strong thermal conductivity.
2. The carbon content of cast iron is too low
Gray cast iron in the solidification process precipitation of the primary austenitic dendrites, the remaining eutectic composition of the liquid phase is not much, when the eutectic reaction occurs, the heat released is not enough to allow the temperature to rise above the carburizing body temperature.
3. Influence of alloying elements
Most of the alloying elements affect the eutectic carbon content, causing a change in the carbon equivalents and thus contributing to the whiteout. In addition, alloying elements will also change the eutectic temperature, affecting the interval between the eutectic temperature of the graphene and carburizing system. Some carburizing body stabilizing elements, while lowering the eutectic temperature of iron-graphite, raise the eutectic temperature of the carburizing body, so that the iron is certainly easier to subcool below the eutectic temperature of the carburizing body. There are also some alloying elements, for gray cast iron manufacturers, they can reduce both the graphite eutectic temperature and the carburizing body eutectic temperature so that there is no significant effect on the white-out tendency.