Self-Healing of Creep Cavities Formed in Austenitic Stainless Steel
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In austenitic stainless steels creep cavities nucleate at grain bourdaries by long time use at high temperatures. These creep cavities grow along grain bounrdaries, form grain boundary cracks by linking up each other, and cause premature and low ductility fracture. Therefore long time creep rupture life and ductilites chiefly depend upon the behavior of nucleation and growth of creep cavities. If the growth of creep cavities could be suppressed, creep rupture life and ductilites should be improved remarkably. The present work is intended to propose a self-healing process for the cavitation by utilization of thin filmy precipitation at creep cavity surface and improve the creep rupture properties by the self-healing process. In ordinary austenitic stainless steels, sulfur is contained as impurity and segregates to creep cavity surface because of its high surface activity. It is thought that a stable compound can precipitate at creep cavity surface if the sulfur is completely removed. Therefore in order to remove sulfur completely and precipitate a stable compound at creep cavity surface, a modified 304 stainless steel added with cerium, titanium, boron and nitrogen was laboratory-melted. The modified steel showed high rupture strength and high rupture ductilities, and it was confirmed that boronnitride precipitates at creep cavity surface in the steel. It was thought that the boronnitride at creep cavity surface suppresses the creep cavity growth and improves creep rupture properties by the self-healing effect on creep cavities.