Effect of Boron on Discontinuous Precipitation in JBK-75, a Modified A-286 Type of Austenitic Stainless Steel

The morphology and growth kinetics of discontinuous precipitation in JBK-75 super stainless steel containing 0.0000%, 0.0015% and 0.0030% boron have been investigated at temperatures ranging from 775 to 850 C by light and electron microscopy. Discontinuous precipitation was observed in these steels at all aging temperatures. It formed as lamellar cells at austenite grain boundaries and grew into the age hardening austenitic matrix containing spherical formula precipitates. The cells resulting from discontinuous precipitation have a lamellar structure consisting of formula precipitate phase, Ni3 Ti, and a solute depleted austenitic matrix phase. The cell growth rates and lamellar spacings increased with increasing aging temperature for all boron contents, and for each aging temperature increasing the boron content resulted in a decrease in both cell growth rate and lamellar spacing. Analysis of the cell growth kinetics under the assumption that the diffusion of titanium in the cell reaction front was rate controlling showed that increasing the boron content decreased the diffusivity for all aging temperatures. The activation energy for diffusion, however, remained unchanged