Theoretical And Experimental Study On Internal Crack Healing Of Heavy Forgings

Heavy forgings, as the fundamental parts of large equipments, are very important to the national economy, defense and the momentous equipments related to advanced science and technology. Their production capacity and quality represent the level of nation's heavy industry. Internal crack defect is a common technical problem in heavy forgings. Internal crack healing and its character and mechanism were studied thoroughly by experimental method and theoretical method.The experiment of internal crack healing proves that internal crack healing presents healing crystallization on crack surfaces or growth of matrix grains near by crack surfaces. Both healing crystallization and of matrix grains growth are mode of internal crack surface migration, and the material source of them is atom diffusion in matrix.There must be a driving force that makes internal crack healing process possible. So samples with different inner crack models were treated by different ways,such as heating and holding,thermoplastic deformation and thermoplastic deformation with heating and holding. Experiments prove that the internal crack healing behavior is related to heat treatment temperature and time which are external factors,but is controlled by self-factors of the crack.The macroscopic process of crack healing is crack surface migration to center of the crack. According to thermodynamics principle, the start-up criterion of surface migration was set up, and the concept of critical radius of surface migration starting was put forward and obtained.The microcosmic process of crack healing is transition of atom from substrate to crack surface. According to the law of thermodynamics, the thermodynamic model which could be used to estimate healing degree of the crack, and the result accords with the experimental rule approximately.On the basis of diffusion theory, model of internal crack healing whose driving force is distortion energy near crack surface was set up, and the calculation results of it are close to experimental results.