The figure shows the solidification of cast steel valve body castings at different times.
It can be seen from the figure that during the solidification and cooling process, the liquid metal basically solidifies from the thin part to the thick part. The temperature of the hot joint at the connection between the larger rotating torus and the cylinder is still very high, and the temperature of the contact part between the riser and the casting is still very high, while the temperature of the surrounding liquid metal has been very low and tends to solidify.
It can be seen from figure C that when the solid rate solidifies to 70%, the upper and lower parts of the casting have gradually begun to solidify respectively. Until 80%, the sprue has been completely solidified and the upper half of the riser has been completely solidified. At this time, the casting is divided into upper and lower isolated liquid phase areas, and the liquid metal feeding at the gate and riser cannot be obtained. It shows that an isolated liquid phase zone is formed at the large rotating ring surface of the casting and the root of the riser, where shrinkage cavity and porosity defects are easy to form.