Influence of C, B, and N on Creep-Rupture Strength and Structural Changes in Creep of 18 Cr-10 Ni Stainless Steel

The influence of carbon, boron, and nitrogen on the creep-rupture strength and the structural changes, especially the morphological changes of carbides precipitated during creep of 18 Cr-10 Ni austenitic steel have been studied, and the relationships between the strength and the structures in the steel have been discussed. Following results were obtained. 1) The addition of carbon up to 0.32% increases the 103 hr rupture strength of the steel at temperature of 600°C, but at 650°C and 700°C, the strength reaches the maximum at the carbon content of 0.23%. The reason is that, by the coalescence of carbides precipitated, the solid solution strengthening of carbon and the dispersion strengthening of carbides decrease with rising temperature. 2) The addition of boron up to 0.009% maintains the high creep-rupture strength, especially at high temperatures and for long time. Boron prevents the carbide precipitation on grain boundaries or the precipitation of the grain boundary reaction type and makes carbides precipitated within grains fine and uniform. 3) The addition of nitrogen up to 0.15% to the steel increases the 103 hr creep-rupture strength at 600°C, where the steel is effectively strengthened by finely dispersed precipitates, but nitrogen is not effective on the long time creep-rupture strength at 800°C, where the coalecence of the carbide is accelerated on grain boundaries and within grains.