Effects of the Alloy Design and Fabrication Process on Mechanical Properties of Mo +V-Added SA508 Gr.1A Steel for Main Steam Line Piping in Nuclear Power Plants

To apply the leak-before-break (LBB) design to main steam line (MSL) piping in nuclear power plants, higher strength and relatively tough SA508 Gr.1A steel were developed by changing the alloy design and fabrication process. Two actual-size prototype types of MSL piping (M1 and M2) with different contents of C, Mo, and V were fabricated and their mechanical properties were evaluated and compared with those of commercial SA508 Gr.1A steel. In the alloy design, the content of C was decreased to reduce the degree to which cementite deteriorated the toughness, and Mo and V were added to increase the strength by forming bainite and VC precipitates. During the fabrication process, the thickness of the MSL piping was minimized by rough machining before a heat treatment to increase the strength by increasing the cooling rate during quenching. Both prototype steels had better strength and toughness than commercial steel. In particular, the mechanical properties of M2 with lower alloy contents had a good combination of strength and toughness. The yield strength, USE, and J(Ic) of M2 were 407 MPa, 393 J and 680 kJ/m(2) at 286 degrees C, respectively, which were increased by 167 MPa, 90 J, and 98 kJ/m(2) compared to the correspondingly parameters of commercial steel. The LBB margin of MSL piping using M2 steels was 1.49, which was a 30% higher than those using commercial steels.