Self-healing of damage in heat resisting steels

Abstract

Material damage is too fine to be detected by non-destructive tests and difficult to be repaired during use. Therefore self-healing of damage is most desirable to improve the reliability of materials and structures. In heat resisting steels, creep cavities nucleate at grain boundaries by long time use at high temperatures. These creep cavities grow along grain boundaries, from grain boundary cracks by linking up each other and cause the premature and low ductility fracture. Therefore long time creep rupture life and ductility chiefly depend upon the behavior of nucleation and growth of creep cavities. If the growth of creep cavities could be suppressed, the creep rupture properties of creep rupture life and ductility should be improved due to prevention of the premature fracture. Ordinary austenitic stainless steels contain sulfur as impurity and the sulfure segregates preferentially to creep cavity surface because of its high surface actiivty. It is possible to remove sulfur almost completely by doping cerium and adding titanium to the austenitic stainless steels. By adding boron and nitrogen, boron nitride precipitates on creep cavity surface. It was thought that the boron nitride on creep cavity surface suppresses creep cavity grwoth and improve creep rupture life and ductility by its healing effect on cavities.