Comprehensively considering the solidification simulation results and trial production results of the two trial production processes of nodular cast iron long fork, it is found that the direct results of shrinkage and porosity obtained have a certain deviation from the reality. It is necessary to determine the correct thermophysical model in order to truly guide the process improvement and actual production.
Firstly, after the sand in the step hole of the top injection cylinder is heated during the mold filling process, it enters a quasi closed environment. In such an environment, the interface between the cylinder and the dry sand not only cannot transfer heat, but also has a certain heat preservation effect, which will lead to the continuous high temperature of the pipe; And there is a similar heat accumulation effect at the neck of the upper and lower risers, while the effect of the lower part is greater, and the upper riser is relatively cold. In this case, in the top injection scheme, the large hot spot at the lower part of the long fork cylinder needs hot metal supply to the inclined lower fork arm, so there is a great demand for feeding. For the top injection type, the hot metal feeding from the upper part of the cylinder and the feeding from the lower feeding riser can be obtained here, which has more self feeding effect. Under the guarantee of triple feeding channel, there is no pore defect. Although the large hot spot area on the upper part of the nodular cast iron long fork can be fed by the inner gate for a long time, the molten iron at this place will need to flow to the lower part to feed the large hot spot area at the lower part, and the heat dissipation condition of the upper dark riser is better, and the partial cooling will solidify earlier than this place, which will lead to significant shrinkage cavity and significant shrinkage porosity defects at this place.
For the bottom injection type, we can also get the corresponding explanation: a tube of hot sand caused by the mold filling stage can still ensure the smooth flow of the thin-wall area to a certain extent, but without two square risers (especially the lower riser) and the hot sand environment between the riser and the cylinder wall, the time of the cylinder wall as the feeding passage is obviously shortened, and the iron flow in the upper part is relatively small, Among the original three feeding channels of the large hot section at the lower part of the cylinder, the feeding from the upper part shrinks, the feeding part of the riser is directly removed, and the inner gate solidifies prematurely, leaving only self feeding to make up for the feeding demand, which finally leads to serious shrinkage problems.
In short, due to its complex structure, the cooling conditions of the ductile iron long fork in the sandbox are quite complex. Relying solely on the heat transfer setting of the EPC model based on sand mold can not completely and accurately judge the shrinkage and porosity of the part. For this case, the heat transfer setting of nodular cast iron long fork in sandbox should be adjusted according to the structure of nodular cast iron long fork and the analysis conclusion of trial production, so as to ensure that the actual heat transfer and feeding conditions can be truly reflected.
The heat transfer of nodular cast iron long fork is adjusted by dividing the sand mold into upper, middle and lower layers to ensure that the thin-wall area of nodular cast iron long fork cylinder solidifies slowly. The solidification calculation results are corrected and an accurate thermophysical model is determined. It can be estimated according to the physical condition that the inner hole of the cast iron fork is not long and the ball hole of the cast iron fork is not filled with hot liquid, but it can be estimated according to the original physical condition that the iron fork is not filled with hot liquid. The simulation results of shrinkage cavity and porosity of the modified top injection type are shown in the figure, which is relatively close to the actual situation.