Effect of Carbide Precipitation on the Creep-Rupture Characteristics of Austenitic Heat-Resisting Steels

摘要

In some austenitic heat-resisting steels, specimens of solution-treated or insufficiently aged state show marked decrease in creep-rupture elongation and deterioration in notch toughness, in a certain range of stress or creep-rupture life. The rupture takes place along grain boundaries and the specimens become very sensitive to intergranular corrosion in etching. Investigations into these phenomena were performed in details on LCN 155 and other austenitic heat-resisting steels and the following points have been made clear: (1) In solution-treated or insufficiently aged austenitic steels, chromium carbides precipitate at grain boundaries during creep-rupture test and the chromium depleted zones are produced around the grain boundaries. In an extreme case, the chromium concentration in this region falls to less than 1wt % approximately. (2) The high sensitivity to intergranular corrosion may be due to the chromium depleted zone. (3) The high notch-rupture sensitivity and the intergranular rupture are ascribed to the formation and growth of voids at grain boundaries. The piling up of excess vacancies produced by Kirkendall effect in the diffusion of chromium atoms attending to chromium-carbide precipitation at grain boundaries induces the formation of voids and takes part in their growth, and stress promotes these phenomena. (Author)