Healing Technique And Healing Mechanism Of Crack For Low Carbon Steel

In this paper,the internal cracks of the most widely used low carbon steel are taken as the object of research.The effects of healing technique and parameters of the cyclic phase transformation heat treatment and isothermal heat treatment on crack morphology,matrix structure,crack healing process,crack healing mechanism and mechanical properties are studied systematically.The results can enrich the metallic material bonding theory and provide theoretical basis for the repair of metallic material components,which has important theoretical significance.At the same time,it can provide technical guidance and reference for the repair of important metal components in service,which has theoretical significance,great practical value and social benefits.First,the isothermal heat treatment healing experiment is carried out to study the effects of healing time,healing temperature,crack width,and plastic deformation of the matrix on crack healing and matrix structure.Based on this,the cyclic phase transformation heat treatment healing experiment is made,and the effect of cycle parameters on crack healing and matrix structure is studied.Finally,the comparison of the difference between two healing technique is made in such aspects as the evolution of the crack morphology,the healing mechanism,and the recovery of the mechanical properties of the specimens.The results indicate:The degree of crack healing increases with the increase of healing time,healing temperature,amount of plastic deformation of the matrix and the number of cycles.The grain coarsening of matrix is the disadvantage of the isothermal heat treatment.However,the cyclic phase transformation heat treatment not only has finer effect on the matrix grains but also has higher degree of crack healing,which are conducive to the recovery of the mechanical properties of metal materials.In addition,the cyclic phase transformation heat treatment has higher healing speed than the isothermal heat treatment.Both of crack healing process of the cyclic phase transformation heat treatment and the isothermal heat treatment can be divided into three stages.In addition,the metallurgical healing mechanism is atomic diffusion and recrystallization according to the evolution of diffusion microvoids and crack morphology.Atomic diffusion palys an important role throuhtout crack healing process,while recrystallization occurs only during the first stage of crack healing.Atomic diffusion provides the material basis for recrystallization,and recrystallization promotes the further diffusion of atoms.The microhardness of the crack healing zone and the ultimate tensile strength of the specimen increase with the increase of the healing area.When the healing area is similar,increasing the healing time and temperature will lead to coarsening of the grains,which will cause the microhardness of the crack healing zone and ultimate tensile strength to decrease.The isothermal heat treatment will cause the microhardness of the matrix to decrease,while the cyclic phase transformation heat treatment has little effect on the microhardness of the matrix.Besides,the ultimate tensile strength of cyclic heat-treated is higher than that of isothermal heat-treated specimen even if two kind of healing techniques have similar healing area.In other words,the crack healing degree of the cyclic phase transformation heat treatment healing technique is higher than that of the isothermal heat treatment healing technique.