Self-Healing of Creep Cavities by Boron Segregation and Boron Nitride Precipitation Autonomously Developed during High Temperature Use

Abstract

In heat resisting steels, creep cavities are formed at grain boundaries through long time use at high temperatures and stresses. These creep cavities grow along grain boundaries, lead to grain boundary cracks by linking up each other, and cause the low ductility and premature fracture. The long time creep rupture properties of heat resisting steels depend mainly on growth of creep cavities. As creep cavities are thought to grow by diffusive transport of matter from the cavity surface to the grain boundary, a surface control of creep cavities may be very effective for suppression of the creep cavity growth. In this study, the chemical composition of austenitic stainless steels have been modified with the addition of minute amount elements such as boron and cerium with the aim to suppress creep cavitation by segregation of boron and precipitation of boron nitride onto creep cavity surface. Chemistry of creep cavity surface was analyzed by Auger electron spectroscopy. Extensive sulfur segregation was observed on creep cavity surface of the steels without boron and cerium addition. In the modified steels, the segregation of elemental boron and the precipitation of boron nitride compound were observed on the creep cavity surfaces. The segregation and the precipitation were thought to suppress the surface diffusion of creep cavity, since both boron and boron nitride are very stable at high temperatures. Cerium acted as a getter for soluble sulfur in the steels by the precipitation of ceriumoxysulfide (Ce 2 0 2 S) to facilitate the segregation of boron and precipitation of boron nitride. The segregation of boron and the precipitation of boron nitride reduced creep cavity growth rate substantially, and improved markedly long time creep rupture strength coupled with long time ductility. It may be said that the segregation and the precipitation provide the steels with the self-healing function for cavitation, since they are autonomously developed during the high temperature use.