Abstract
Heat-to-heat variation in long-term creep rupture strength and ductility for a SUS347H austenitic stainless steel has been investigated on the basis of microstructure observations. The long-term creep rupture tests up to about 105h were carried out at 600, 650, 700 and 750°C. The microstructural changes during creep were observed for three different heats of the steel after interrupting creep tests at various testing times. M23C6 and NbC carbides precipitate during the creep tests. The major controlling factor for the creep rupture strength and the ductility was found to be the formation and the propagation of grain boundary cracks, resulting from grain boundary sliding. The size distribution of M23C6 particles precipitated along grain boundaries strongly depends on the content of boron in the heats with 4 to 27 ppm boron. There is an optimum boron content of 12 ppm to minimize the propagation of grain boundary cracks. It is concluded that the heat-to-heat variation is caused by the difference in the boron content which affects the size distribution of M23C6 precipitates and hence the formation and the propagation of grain boundary cracks.
