The Behavior Of Internal Crack Healing And The Restoration Of Mechanical Properties For Heavy Forgings

Heavy forgings,the most key parts of large complete sets of equipment,play important roles in national economic construction,defense development and national security.The heavy forging manufacturing capacity and technology level are the important symbols of the comprehensive national power.Cavity-type defects such as cracks and voids are inevitable during the manufacturing process of heavy forgings.These defects will severely deteriorate the mechanical properties of heavy forgings.The ultrasonic flaw detection results have revealed that cavity-type defects are the major cause of waste products.Therefore,for the problems of internal crack healing in heavy forgings,the behaviors and mechanisms of internal crack healing and the restoration of mechanical properties after crack healing treatment have been investigated systematically in order to provide reference and basis for quality control and improvement in heavy forgings,which have great theoretical significance and practical value.Three healing experiments have been conducted through physical simulation.The effect of healing temperature,holding time,cooling methods,reduction ratio,strain rate and hot micro-pressure on crack healing and microstructure of the matrix has been studied systematically.The results show that the degree of crack healing increases with increasing healing temperature,holding time,reduction ratio and hot pressure,and with decreasing coolding speed and strain rate.Hot pressure in the high temperature elastic zone does not only promote the rapid crack healing but also prevents the grain coarsening of the matrix.A new healing method of hot pressure in the high temperature elastic zone is proposed to eliminate interal cavity-type defects in heavy forgings.The crack healing temperature of 30Cr2Ni4 MoV steel should not exceed 1100?.Two crack healing mechanisms exist in 30Cr2Ni4 MoV steel.The crack healing mechanism is atomic diffusion at lower temperatures(?900?),while crack healing depends mainly on recrystallization and grain growth mechanisms at higher temperatures(?1000?).Atomic diffusion provides materials for recrystallization and grain growth in crack healing zone.Recrystallization leads to the rapid crack healing,and grain growth facilitates the elimination of crack healing zone and microstructural homogenization.The process of internal crack healing can be divided into five stages.New pre-crack models are proposed to investigated the restoration of static and dynamic mechanical properties of crack healing zone.The results show that the tensile strength recovery rate is higher than that of the plasticity.When internal cracks are disappeared completely,tensile properties of crack healing zones can be restored completely but their impact properties are only partially achieved.Therefore,the degree of crack healing should be based on the percentage recovery of comprehensive mechanical properties.Recrystallization in the crack healing zone promotes the rapid restoration of static and dynamic mechanical properties.