In the gradual forming process of die forging, the softening process is mainly based on dynamic recovery, and its structure will also change to some extent. So, in what order and in what way will die forging change, and what characteristics will they eventually exhibit? There will be further requirements for reaming the finished die forgings. What is the approach to this?
In the initial stage of die forging deformation, high density misaligned substructures are formed. These dislocations can be uniformly distributed or become subgrain boundaries of brittle substructures. Also observed in cold deformation, when the softening process is not obvious, this stage of hot deformation can be called the hot work hardening stage. Then in the second stage of die forging structure change, due to the strengthening of the softening process, the polygonal sub-grain boundary is formed, and the sub-grain boundary region has a higher free dislocation density. In the process of deformation, the polygonal substructure gradually replaces the thermal working structure. The multilateral substructure itself is also changing, leading to the formation of near-equiaxed subgrains.
At the end of the change of die forging structure, the isoaxial polygon substructure remains unchanged, corresponding to the rising part of the deformation diagram, and the stress and metal substructure are constantly changing. In the next stage of thermal deformation, the stress and the resulting polygon structure do not change.
There are many ways to enlarge the hole of die forgings, including punch expanding, mandrel expanding and slot expanding. Punch hinges are made by using a small punch to punch a hole in the blank first and then passing a larger punch through it, which can slightly enlarge the hole and gradually enlarge the hole to the desired size. It is mainly used for reaming holes less than 300 mm in diameter. Mandrel reaming is mainly used in the forging process of ring die forging. It is necessary to insert the core rod into the hole and support it on the horse frame. In forging, the blank is fed as it is hammered so that the blank is repeatedly forged around the circumference and extended between the mandrel and the anvil until the inner diameter reaches the desired size.
Die forgings are split and reamed by stamping two small holes in the blank, then cutting metal between the two holes, and then expanding the incision and reamed holes with punches to achieve the desired size of the forgings. This method is suitable for forging large diameter thin - wall forgings or thin - wall forgings with irregular holes.