STUDY ON VOID HEALING BEHAVIOR DURING FORGING PROCESS FOR 25Cr2Ni4MoV STEEL

Based on the measured stress-strain curves and thermo-physical data of 25Cr2Ni4MoV steel, a FEM model of void closure in heavy forging process was established through ABAQUS software. The void closure behaviors under different hot plasticity conditions have been investigated. The FEM results showed three distinct stages during the void closure. The voids with the same position in a cylindrical specimen close up at a similar height reduction ratio (Delta H/H-0) around 25% at different temperatures, which indicates that the void closure is not sensitive to the deformation temperature. On basis of the FEM results, a physical-simulated experiment for void bonding process has been performed through compression tests on a hollow cylindrical specimens, with deformation temperatures ranging from 900 V to 1200 degrees C, Delta H/H-0 from 25% to 45% and a constant strain rate 0.01 s(-1). The experimental results have shown that the Delta H/H-0 for the void completely bonded at 1200, 1100 and 1000 degrees C are all about 35%, but increases to 45% when the deformation temperature decreased to 900 degrees C, which confirms that void bonding is a diffusion controlled process. Through the experimental results, it can be further demonstrated the high temperature combined with severe deformation can enhance the ability of atoms transition and decrease the micro-gap between contacted surfaces. Finally, the influencing factors on the bonding efficiency were investigated through SEM observation on the fractured surface of the tensile specimen with internal closed void.