Carburizing is one of the common casting defects in the lost foam casting process of steel castings. The carburizing casting defects of steel castings are shown in the figure. The foam pattern material is mainly composed of carbon and hydrogen, which decomposes rapidly under high-temperature molten steel to produce hydrogen and free carbon. Due to the strong affinity between hydrogen and oxygen than carbon, the high-temperature decomposed hydrogen is first combined with the oxygen between the pattern gaps to discharge water vapor, and a large amount of decomposed free carbon remains in the mold, which is soaked and corroded with the surface of molten steel, resulting in Carburization on the casting surface, that is, carburization. According to relevant literature and experiments, it is found that there is a certain regularity of carburization, that is, carburization mainly occurs on the surface of the casting, there is almost no Carburization in the heart, and there is no carburization near the inner gate. The farther away from the inner gate, the more serious the carburization is. According to the mechanism of carburization, the following measures can be taken to basically control the carbon content of castings within the process requirements:
(1) Select high-quality foam plastic
The quality of foamed plastics has a direct impact on the gasification rate of patterns and the morphology of pyrolysis products during pouring. High quality foams have low carbon content, high molecular weight and lower pattern density under the same foam strength. It is the first choice for the production of steel castings by lost foam process to select the original beads with less gas generation and low carbon content, and the formed foam with low bulk density and light weight. It is also the most effective way to solve the problem of carburization of steel castings at present.
(2) Select reasonable pouring process
The design of lost foam casting pouring process should be able to accelerate the gasification of mold material, reduce and stagger the contact and reaction time between liquid phase and solid phase in decomposition products and liquid steel, and reduce or avoid carburization of steel castings. It mainly includes controlling pouring temperature and pouring speed, good coating permeability and dry sand mold permeability, appropriate mold wall negative pressure, the use of anti carburizing coating and so on.
(3) Taking advantage of the characteristics that the farther away from the inner gate, the more serious the carburization is, a riser is set at the farthest end from the inner gate or at the highest point of the casting, so that the molten steel with serious carburization pollution entering the mold cavity first can enter the riser, and the casting body can obtain relatively pure molten steel.
(4) “Burn first and pour later”, that is, burn the foam in the mold cavity to form a cavity, and then pour the molten metal.