Study On The Crack Self-Healing In Metals By Heat Treatment

This subject is studied in order to probe the mechanism and influence factors of inner crack healing in structural steel by heat treatment only, at the same time to explore the preferable technology healing the crack. The results will be significant practically to increase the speed of manufacturing metal with intelligent and prolog its useful life. It will establish the foundation of theory and experiment for manufacturing the metal materials with intelligent capability, high efficiency, high performance and long-life.The materials used to test are 20 steel, 45 steel and 20CrMnTi steel. Inner crack is prepared in the materials by compressing drilled hole, and then the samples are heated at different temperature for different time and cooled in air. Microscope and SEM was employed to examine the effect of crack healing. The results indicate that inner cracks healing will happen in the three materials when the healing temperature is higher than 850℃. A banded structure, which mainly consists of fine grain ferrous, is observed in healing area. The healing band gradually narrowed down with increasing the heating temperature and heat retaining time. The voids produced during the process of crack healing contain impurity element more than other area. It is found that three obvious stages in the process of crack healing, that is the long crack transformed into a row of short crack; the short crack healed and transformed gradually into void; the size of the void reduce and change gradually into spheroid, the number of spherical void reduce and the microstructure and elements well distribute.The effect of crack healing depends mainly on the heating temperature rather than the heat retaining time. A critical temperature exists in the crack healing. The critical temperature should be higher than the recrystallization temperature of the material. The alloy elements Cr, Mn, Ti, which can combine with carbon into carbide, resist the diffusion of carbon; in the meal time the carbide resist the recrystallization grain become coarse, so that there is the upper content of ferrite and the fine recrystallization grain in crack healing area.The raise of strain energy (lattice distortion energy) caused by crack is internal power healing the crack. There should be a critical energy factor of crack healing. When sum of the internal power and external power is higher than the critical healing energy, the crack heal, inthe else, the crack cannot heal. The main mechanism of crack healing in metal materials is recrystallization and its grain growing up. The substance foundation of crack healing is the diffusion of atoms.