Large forgings are still produced in large bodies in the steel industry in order to obtain higher quality and relaiability under severe operating conditions. In the large steel ingots, void defects exhibiting microvoid shapes are inevitably formed in the V-segregation zone of the ingots during solidification and the cast structure is formed due to long solidification time. In the hot open-die forging process, material properties are improved by eliminating internal porosity formed during ingot solidification, producing continuous flow lines and refining the grain structure. This results in increased strength, soundness and fatigue life of the forging. Internal void crushing is performed in two stages ; void closing and bonding of two closed void surface. The void closing process is the process of contacting two internal surfaces, through the deformation of internal porosity.
The scope of this study is firstly concerned with the bonding process during complete void crushing. A schematic model of the bonding mechanism and an estimation method of the bond strength in internal voids has been proposed. The estimation method of the bonding efficiency is based on the proposed bonding mechanism. It is assumed that the bonding efficiency is influenced by the working temperature, the normal surface pressure and the surface strain. In order to compare the bonding efficiency between the experiment and the calculation and to verify the proposed estimation method, semiprototype experiments with an artificial large-void were carried out.
For estimation of the bonding efficiency of an internal void by the finite element method, the thermo-viscoplastic finite element analysis including treatment of the special internal boundary condition has been developed by using the penalty method. Especially, in order to impose the internal boundary condition, the void surfaces are described by a cubic spline curve in two-dimensional deformation, or a bicubic Ferguson patch in three-dimensional deformation. Thus an arbitrary initial void and a void of deformed shape could be treated effectively and a general scheme for contact treatment could be introduced from the previous work. The contact treatment between two counter surfaces which constitute the void surface is divided into initial contacting and subsequent contacting stages in order to improve the computational convergency. In the first stage, iteration is carried out until the geometric constraint is satisfied by utilizing the nodal velocity and coordinates of two contacting surfaces. In the subsequent contacting stage, the penalty constraint in the contact treatment has been employed so that non-penetration condition is satified during the iterations.
The void size is practically very small as compared with the huge large ingot. Thus, for deformation analysis of a large ingot, a massive number of elements are needed in order to describe a void surface and to uniform mesh structure. In the present work the Global/Local scheme has been introduced in order to reduce the computational time and to easily generate the mesh system as a void module of local mesh for obtaining the accurate solution around a void.
The procedure of the global-local method consists of two steps. In the first step global analysis is carried out which seeks a reasonably good solution with a course mesh system without describing a void. Then, a local analysis is performed locally with a fine mesh system under the size-criterion of a local region, based on the global analysis results to obtain more accurate solution in the subregions of interest. Because a course mesh is employed in the global analysis and fine mesh is applied only in selected subregions in the local analysis, the computational time has been greatly reduced.
Through the FEM analysis imposing the internal contact treatment and Global/Local scheme, the bonding efficiency has been estimated and forging amount can be correctly estimated for complete bonding of two contacting void surfaces. Analysis of forging including large voids was also carried out in order to determine the cogging parameters. Through the work it has been shown that large ingot forging incorporating small voids can be effectively analyzed by using the internal contact treatment and the proposed Global/Local scheme.