Causes and Prevention of Local Ma Defects in Castings
Production of the use of a total of three types of sand, that is, small molding machine modeling wet sand, semi-automatic molding line (FBM) with wet sand and resin sand production line with self-hardened resin sand. It has been found that local defects are mostly found on the Castings produced by the FBM production line. The small molds are rarely present in the Castings, even if they are relatively light, and such defects are made from the Castings produced by the resin sand Not seen. Obviously, this phenomenon shows that the local hemp defects and Castings used in the sand, in the production conditions, it only appears in the wet sand molding Castings, and FBM modeling line is the most serious.
FBM modeling line used by the sand and small molding machine used sand are quartz sand, bentonite and coal mixed with water from the molding sand, but the use of the two conditions are very different. First, FBM modeling line of sand for a single sand, that is, no part of the sand and back sand, used together after the regeneration process. Small molding machine with sand and sand of the sand, the used sand is only a small part of the regeneration treatment, making it a surface sand. FBM modeling line of sand because of the large amount of regeneration, the amount of new sand is relatively small, a large number of old sand repeated use and regeneration, continue to withstand high temperature iron liquid burning and mixing, the proportion of ashes continue to increase, thereby reducing the sand of the fire Sex. The actual detection, the sintering degree of about 1050 ℃; second, FBM modeling line production of Castings, the use of larger core. The core is made of overhanging sand, and the proportion of the overlay sand that resists the resin and the curing agent in the molding sand is increasing, resulting in the acidity of the molding sand. After testing, the sand of the pH between 7 to 8, slightly acidic.
FBM modeling linear sand of low fire resistance and which contains more acidic, is likely to lead to the local occurrence of Castings an important reason.
(2) melting temperature
Production of Castings of many varieties, the size of the difference between the poor, and each time the number of each Castings orders ranging. Sometimes, a furnace iron liquid to be cast at the same time several large differences in thickness and thickness of the Castings, which is difficult to determine the melting temperature. In order to ensure that the pouring of thin pieces, and sometimes had to melt the temperature increased to 1550 ~ 1570 ℃. After the melting temperature has been improved, although it has ensured the Castings of the thin pieces, it also brings some drawbacks. One of them is that with the increase of melting temperature, the slag becomes thinner and the mobility increases, which is not easy to separate from the liquid metal, resulting in a large number of silicate melt entering the cavity with the molten iron. In addition, as the melting temperature increases, the molten iron increases in the melting, spheroidizing and subsequent Castings processes, thereby increasing the amount of oxide in the molten iron.
The analysis shows that the melting temperature is high, the amount of slag residue and the number of metal oxides in the molten iron increases, and it is also an important condition for the local incense in the Castings.
2.2 Forming mechanism
According to the above analysis, it can be deduced that the formation process of the local hemp defects in the Castings is as follows: the liquid slag remaining in the molten iron and the various metal oxides are floating up with the molten iron into the mold, and finally the concave angle Obstructed and stayed here. Here, the liquid slag solidifies to form slag. Alkaline with a variety of metal oxides, under the action of high temperature and the sand in the acidic substances react, the reaction products to form slag holes and pores and other defects. In addition, the concave corner of the Castings is the sand at the sharp corners, where surrounded by iron liquid, the most serious heat. Under the action of high temperature, the low melting point of the sand surface is melted, and the distance between the grit is increased, and the molten iron enters it, thus forming the hot clay sand defect. These defects together, where the formation of local pimple.