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 temper ature cycle, within the range of 1200 to 1500 degrees C, during the fa brication heat treatment on the mechanical and fatigue properties of a reactor pressure vessel (RPV) steel. The tensile properties, viz., te nsile strength and reduction in area both exhibited small discontinuou s 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 en vironment (PWR water) it was observed that a high temperature cycle ca used a significant increase in fatigue crack growth behaviour. However at Delta K levels above 8 MPa root m the high temperature cycled stee l (termed overheated) and the as-received steel (termed normal) exhibi ted similar fatigue crack growth rates. The increased fatigue crack gr owth rates were caused by the occurrence of significant amounts interg ranular 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 twe nty 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 temp erature cycle treatment caused the absences of EAC growth in PWR water at 120 degrees C by causing the liquation and subsequent re-precipita tion of numerous, very small, sulphide inclusions at prior austenite g rain boundaries. Finally, at higher Delta K levels, a high temperature cycle promoted the onset of stage III fatigue, typified by the occurr ence of intergranular failure, by reducing the fracture toughness of t he pressure vessel steel.