EFFECTS OF HIGH-TEMPERATURE EXCURSION ON MECHANICAL AND FATIGUE PROPERTIES OF NUCLEAR PRESSURE-VESSEL STEEL DURING FABRICATION

Presented is study concerning the effects of introducing a high temperature cycle, within the range of 1200 to 1500 degrees C, during the fabrication heat treatment on the mechanical and fatigue properties of a reactor pressure vessel (RPV) steel. The tensile properties, viz., tensile strength and reduction in area both exhibited small discontinuous decreases over the temperature range of 1200 to 1500 degrees C while the room temperature Charpy impact energy was dramatically reduced by 50%. At room temperatures in a deoxygenated, demineralised aqueous environment (PWR water) it was observed that a high temperature cycle caused a significant increase in fatigue crack growth behaviour. However at Delta K levels above 8 MPa root m the high temperature cycled steel (termed overheated) and the as-received steel (termed normal) exhibited similar fatigue crack growth rates. The increased fatigue crack growth rates were caused by the occurrence of significant amounts intergranular failure facets of the fatigue surfaces of the overheated steel. In PWR water at a temperature of 120 degrees C it was shown that, at initial Delta K levels the overheated pressure vessel steel exhibited fatigue crack extension characteristics which were around ten and twenty times slower (depending upon the overheating temperature) than the normal pressure vessel steel data. Such differences were explained in terms of the fractographic features. It was argued that the high temperature cycle treatment caused the absences of EAC growth in PWR water at 120 degrees C by causing the liquation and subsequent re-precipitation of numerous, very small, sulphide inclusions at prior austenite grain boundaries. Finally, at higher Delta K levels, a high temperature cycle promoted the onset of stage III fatigue, typified by the occurrence of intergranular failure, by reducing the fracture toughness of the pressure vessel steel.