During the production of bolster and side frame by green sand casting, sand inclusion defects are found, which are mainly concentrated near the gate and the large plane of the upper mold, and metal nodules will be formed on the surface of steel castings; Some have only irregularly shaped grooves or creases.
(b) Sand inclusion around the big triangle of side frame
(c) Sand inclusion around the pan surface of shaking pillow
When there is sand inclusion defect in steel castings, many sand holes can be found around them. Sand inclusion in the side frame mainly occurs at the upper cross beam (Fig. a) and around the big triangle (Fig. b), and the sand inclusion defect of the bolster occurs near the inner sprue and around the center plate surface (Fig. C).
The following factors may cause the defects of green sand in cast steel castings.
(1) The water content of sand mold is too high, the sand mixing time is too short, the mixing and grinding of molding sand is uneven, and the gas or silica sand expands under the action of high temperature of liquid steel.
(2) The compactness of the sand mold is low or uneven, the air permeability of the sand mold is insufficient, the expansion of the molding sand at the compact part is different from that at the loose part, arching and cracking occur under the action of high temperature and air pressure, and the molten steel infiltrates into the arching and cracking part to form sand inclusion defects.
(3) The pouring speed is slow, and the speed of liquid steel filling the mold cavity is too slow, so that the sand mold is partially subjected to high temperature, the liquid steel is baked for too long, resulting in arching deformation and cracking, and the liquid steel infiltrates into the arching cracking part to form sand inclusion defects.
(4) The hot and wet tensile strength of molding sand is low.