Weldability of a Microalloyed Carbon-Manganese Steel for Nuclear Power Plants: The Hardness Criterion and Fracture Toughness on the HAZ

Abstract Microalloyed carbon-manganese steels are utilized in specific applications within the energy industry where stringent mechanical properties are essential to endure service conditions, including temperature and loadings. However, it has been observed that under certain welding parameters, particularly in the low range, the hardness in the heat affected zone (HAZ) near the fusion line can hardly comply with the criterion of 320HV imposed after post-weld heat treatment (PWHT) by the applicable specifications. This is particularly true in the parent metal HAZ induced by the last weld beads of the weld joint if the microalloying elements such as V and Nb approach the upper-bound wt.% contents. This paper deals with the weldability of a microalloyed carbon-manganese steel from the hardness and fracture toughness stand points. This work also proposes a method for predicting the maximum hardness level in the HAZ after PWHT by using an artificial neural networks (ANN) technique. It is demonstrated that despite the presence of local hardness values in the HAZ higher than the hardness requirement imposed by the applicable specification (ISO 15614-1), the component design requirements for fracture toughness level and ductile initiation behavior at the operating temperatures are not compromised. Finally, this work highlights some inconsistencies between the applicable specification and the use of modern micro-alloyed carbon-manganese steels.