Investigation on sand burn-on formed on steel casting while imposing vacuum (depression) in sand mould

摘要

Green sand mould remains the most popular process for the production of steel castings. However nature of sand and moulding operations renders the process prone to the appearance of variety of casting defects, and one of these is sand burn-on. In the present paper the relevant work done on sand burn-on in steel castings has been briefly collated and it is further extended to nature and causes of appearance of sand burn-on in sand mould (having some bonding material, molasses) on which vacuum ("depression" or negative pressure) has been imposed. It has been done by specially designed mould box, which is connected with vacuum pump (-1 kgf/cm2). The analysis of sand burn-on collected from the surface of test bar and its macro study show that it contains almost 2% oxides of iron and other oxides such as SiO2, MgO, CaO, Na2O, K2O. The material is loosely adhered with casting wall. This is due to the weak fayalite reaction (2FeO + SiO2 = 2FeOSiO2) because of continuous circulation of air from the pores of the mould and hot sand grains adjacent to the casting wall, and the temperature of the mould is lowered by 20%, decelerating the liquid steel - sand mould reactions. Further impregnating the active surface of mould with ethyl silicate, the hot strength of the mould is increased by 200% - 300% and reduces the volume of pores up to 60%. It decreases the possibility of metal penetration.