Compared with cast iron, the pouring temperature of cast steel is higher, which is more than 1560 ~ 1620 ℃ to prevent cold insulation of steel castings; However, the high pouring temperature has long thermal action time on the sand mold, large scouring force on the mold, and the liquid steel is easy to oxidize to form oxide, which is easy to react with the sand mold and cause serious sand sticking.
(1) In order to solve the problem of rough and unsmooth surface, raw sand with relatively fine particle size should be selected. If a single sand molding is used, the clay content in the molding sand must be strictly controlled and cannot exceed the specified amount. Because the molding sand does not contain pulverized coal and the mud content is low, the air permeability is easy to be high. If it is a single sand, the air permeability should not exceed 120, and the air permeability of surface sand should be lower, so as to produce sand castings with smooth surface and no mechanical sand sticking. The raw sand with particle size of 75 ~ 150 mesh (or 55 ~ 100 mesh) is selected. It is best to use high-purity quartz sand instead of 45 ~ 75 mesh coarse quartz sand.
(2) In order to prevent chemical sand sticking on the surface of sand castings, the SiO2 content of raw sand should be higher than 96% for bucket tooth steel castings with high pouring temperature. Due to the high SiO2 content of raw sand and the lack of buffer materials such as pulverized coal in molding sand, steel castings have a greater tendency to produce sand inclusion and scab defects. Due to the low sintering temperature of fine sand and molding sand, it is easy to burn the dead sand and molding sand, so it can not be reused. Therefore, new sand should be added in time.
(3) In order to improve the surface air drying strength and toughness of molding sand and prevent sand hole defects caused by sand washing, an appropriate amount of starch materials (such as dextrin, a starch, calcium lignosulfonate powder, water glass and other diluted aqueous solutions, or phenolic resin alcohol solution) can be added to molding sand. If necessary, spray surface enhancer on the surface of sand mold to improve the surface strength of molding sand.
(4) The compactness of the sand mold shall be high, and the sand mold at the sand sticking part of the sand mold casting is mostly not tight. It is necessary to enhance and ensure the compactness and compactness uniformity of the mold. Special compaction shall be carried out for the parts that are difficult to be compacted and the corners. The compactness of the sand mold shall be high (usually ≥ 85). It is required to have good uniformity and reduce the sand particle gap; In particular, it should be noted that the repair should be in place after the core and core are damaged, and there should be no local looseness due to the repair.
(5) Anti sticking sand coating can be used. The coating shall be evenly applied to the parts prone to sand sticking. The thickness of the coating can also be appropriately increased in the parts with serious sand sticking. It is appropriate to use 0.75 ~ 1.0 mm and 0.3mm for each application. It is required to apply multiple times to ensure the dry thickness. Coating can change the surface tension between mold and molten metal and reduce the mechanical penetration of molten metal to the mold surface, so as to effectively solve the sand sticking defect.
(6) Pay attention to the setting of gating system and riser, and avoid local overheating of sand castings and molds. The gating system is designed to avoid the internal sprue directly punching the mold wall.
(7) Properly reduce the pouring temperature, pouring speed and pouring height. The so-called “appropriate reduction” means to reduce the thermal effect of liquid metal on the sand mold on the premise of ensuring that it can be poured into sand mold castings. Pay attention to reducing the pouring height, upper mold height and gate cup height to reduce the metal hydrodynamic pressure, static pressure and thermal impact on the mold. For bucket tooth sand castings, although the wall is not thick, the pouring temperature is high, which correspondingly prolongs the thermal action time between the liquid steel and the cavity surface. During pouring, the dynamic and static pressure of the liquid steel on the cavity is large, which creates conditions for mechanical penetration and sand adhesion.