In many steel-industry applications, a forging technique for large ingots is required because the major parts are needed as one large body in order to obtain higher quality. In the large forgings, internal voids have to be eliminated as defects. The present work is concerned with the elimination of the internal voids in large ingots so as to obtain sound products. Internal void crushing is obtained by two stages: void closing and bonding of the two closed void surfaces. The present work is focused on the bonding process. The modeling of the bonding mechanism and the estimation method for the bonding efficiency are explored. Semi-prototype experiments are also carried out in order to verify the proposed modeling. Observation of the microscopic structure elucidated the proposed mechanism of hot pressure bonding. An inspection test is also carried out to distinguish whether a bonded workpiece has internal defects or not. The proposed bonding mechanism will help to analyze the crushing of the internal voids. The effect of each parameter on the bonding efficiency is discussed.