Through the observation of the defects, it is found that the defects are shrinkage porosity and shrinkage cavity, which are concentrated in the final solidification part of the product and are caused by insufficient feeding of riser.
ZHY casting uses electric heating riser to improve feeding efficiency. Electric heating riser is a kind of production mode in which graphite electrode is placed at the top of product riser to make molten steel in liquid state for a certain period of time by arc heating, so as to extend the solidification time at the top of riser to achieve riser feeding. This kind of riser steel can achieve 100% feeding.
In order to solve the above problems, if the riser height is reduced and the feeding distance is shortened, the electric heating molten pool will extend to the finished roll neck. For the solidification characteristics of semi steel material with wide crystallization area, it is easier to cause the molten steel superheat at the molten pool position, and the casting defects such as coarse structure, porosity and shrinkage cavity will be formed at the finished roll neck. Therefore, in order to ensure the quality of the product, A certain height of riser allowance must be reserved, but in this way, the feeding distance of riser is generally long, and the tendency of axis looseness is large.
When the feeding distance of the riser in the vertical direction is too long to meet the feeding requirements, the subsidy can be set to form the sequential solidification in the riser direction, so as to increase the feeding distance of the riser. At present, in the production of products, it is generally used to add insulation materials in the sand mold to achieve the effect of subsidy.
In order to further increase the feeding effect, the experimental gradient riser was designed. The riser near the roller body solidified first, and the solidification time of the riser gradually extended with the increase of height, so as to ensure the smooth feeding channel and improve the feeding effect. The specific setting of the riser is as follows: heat preservation material is added to the sand mold in the middle of the riser, electric heating is used for heat preservation in the upper part of the riser, and ordinary sand mold riser is used in the lower part of the riser, and the heat preservation time of the riser is gradually extended from bottom to top, as shown in the figure.