Abnormal influence of impurity element phosphorus on the hot ductility of SA508Gr.4N reactor pressure vessel steel

Abstract

The effect of phosphorous on the hot-ductility of SA508Gr.4N reactor pressure vessel (RPV) steel has been studied. The investigation is done with the use of a Gleeble machine in conjunction with optical metallography, scanning electron microscopy, and transmission electron microscopy. It is observed that both the undoped and P-doped steels exhibit hot-ductility troughs between 750 and 900 °C. However, the trough for the P-doped steel is much shallower than that for the undoped steel. At around 850 °C, the reduction of area (RA) of the undoped steel is quite low (~50%), in comparison to the P-doped steel (~75%). Since there is no grain-boundary precipitation of ferrite and carbides in both undoped and P-doped samples over this temperature range, it can be inferred that the enhancement in hot-ductility for the P-doped steel is independent of these two factors. Grain-boundary microanalysis shows the presence of grain-boundary segregation of P in the tested P-doped samples, implying that the P-induced hot-ductility improvement of this RPV steel may arise from the suppression of grain-boundary sliding by P grain-boundary segregation. Undoubtedly, the present effect of P is unusual and the underlying mechanism for this effect has been briefly discussed. It needs to be underlined that a minor addition of P can play a positive role in the hot-ductility improvement of SA508Gr.4N RPV steel.