Fracture Behaviour of Dissimilar Metal Weld Materials 304 LNSS & SA508 Cl. 3 Steel in Nuclear Piping

Abstract

Material of Nuclear Piping should have good fracture toughness to ensure safe operation of nuclear reactor. In order to be on safe side the fracture characteristics of material have to be quantified. 304 LN Stainless Steel(SS) & SA508 Cl. 3 steel are used in Dissimilar Metal Welds(DMW) in Main Heat Transport(MHT) system piping of Advanced Heavy Water Reactor (AHWR) reactor. The operating temperature of this structural component is 25 - 285°C .The pressure vessel and Primary Heat Transport (PHT) system piping of nuclear power plants are designed and operated on the basis of Leak Before Break (LBB) concept . The Leak Before Break concept has been applied for designing the Main Heat Transport(MHT) system piping of AHWR reactor. This concept aims at the application of fracture mechanics principle to demonstrate that pipes are unlikely to experience catastrophic break without prior indication of leakage. This requires detailed fracture mechanics analysis of different piping components e.g. straight pipes, elbows and branch tees .Therefore in order to implement this LBB concept in the design it is important to understand the fracture behaviour of the material in its operating conditions. The microstructures of the steels and its relevant mechanical properties such as hardness and tensile properties have been characterized. Monotonic J-R curves of the material have been determined in the temperature range of 25 - 285°C . Compact Tension(CT) specimens of Width(W) = 50 mm and thickness (B) = 20 mm have been used . J-R curves were obtained from specimens precracked to a/W = 0.5 .The single specimen unloading compliance method have been used for generating J-R curves. The results of fracture studies under monotonic loading infer : Fracture toughness values of SA508 Cl. 3 steel satisfy the validity criteria suggested in ASTM E-1820 standard , whereas of 304 LN does not satisfies validity criteria The estimated average fracture toughness values of the SA508 Cl. 3 steel at room temperature and high temperature are 612 KJ/m2 & 709 KJ/m2 respectively . The estimated average fracture toughness values of the 304 LN SS at room temperature and high temperature are 1503 KJ/m2 & 1835 KJ/m2 respectively .The magnitudes of fracture toughness values of SA508 Cl. 3 steel is much lower than 304 LN SS . Fracture toughness values of investigated steels estimated at the temperature of 285oC are higher than those at room temperature.