After molten steel enters the mold, it will produce shrinkage porosity and shrinkage cavity due to liquid shrinkage, solidification shrinkage and solid shrinkage. In order to prevent this phenomenon, a riser must be placed at the last solidification position of the steel casting to create the condition of sequential solidification and fully feed the steel casting. For large steel castings, because of the large wall thickness, only using the riser can not fully feed, and cold iron needs to be used at the thick wall. In addition to feeding, the riser also has the functions of exhaust and slag collection.
The risers used in steel castings can be divided into open and closed according to the position. Concealed risers are generally used in machine modeling. For the production of large steel castings, open risers are widely used because they require good exhaust performance and can make-up pouring or spot pouring of molten steel in the riser according to the solidification of steel castings. The shape of the riser is determined according to the shape of the steel casting or the hot joint of the steel casting. The feeding distance of the circular riser is long and the feeding efficiency is high, but sometimes the waist circular riser is used according to the requirements of continuity, while the feeding efficiency of the spherical riser is the highest, but the modeling is difficult. It is generally used in gear steel castings. The risers used in Erzhong foundry mainly include: Circular riser, waist circular riser and spherical riser, of which both circular riser and waist circular riser include thermal insulation riser and thermal insulation brick riser.
The selection of riser position is of great significance to obtain excellent steel castings. Therefore, the following points should be paid attention to when setting Riser:
(1) The riser shall be set at the thickest and hottest place to make the riser the last part of solidification. Subsidies shall be added to the hot section at the low place or cold iron shall be used to realize sequential solidification.
(2) The riser shall be placed on the upper part of the pouring position or on the top or side of the feeding part as far as possible.
(3) For risers at different heights, cold iron shall be used to separate the feeding distance of each riser.
(4) The riser shall be set on the processing surface as far as possible and convenient for cutting, so as to save the finishing man hour of steel castings and reduce the cost.
(5) The shape and size of the riser shall meet the feeding requirements of steel castings, and the volume of the riser shall ensure that there is enough liquid steel to feed the steel castings.
(6) The riser shall not be located at the stress concentration of steel castings, and attention shall be paid to reduce the shrinkage obstruction to steel castings to avoid cracks.
The design of risers mainly includes two aspects: one is to determine the position and quantity of risers, and the other is to determine the size of risers. The position and quantity of risers shall be determined according to the structure of steel castings, the position of hot joints and the pouring method. The size of the riser is calculated according to the empirical formula. The calculation methods of riser size mainly include modulus method and proportion method.